Borataalkenes, Boraalkenes, and the η²-B,C Coordination Mode in Coordination Chemistry and Catalysis

Abstract

Borataalkenes and boraalkenes are the boron-containing isoelectronic analogues of alkenes and vinyl cations respectively. Compared with alkenes, the borataalkene and boraalkene ligand motifs in transition metal coordination chemistry are relatively underexplored. In this review, the synthesis of borataalkene and boraalkene complexes and other transition metal complexes featuring the η²-B,C coordination mode is described. The diversity of coordination modes and geometry in these complexes, and the spectroscopic and structural evidence supporting their assignments is disclosed as well as computational analysis of bonding. The applications of the borataalkene ligand motif in synthetic organic homogeneous catalysis, especially those involving geminal bis(pinacolatoboronates) and 1,4-azaborines, are discussed.

Introduction

Alkenes are among the most common ligand classes in organometallic chemistry.¹ In fact, the first isolated organometallic compound is Zeise’s salt, a platinum complex bearing an ethylene ligand (Figure 1).² Borataalkenes are the boron-containing isoelectronic and isostructural analogues of alkenes, and are typically synthesized by α-deprotonation of alkyl boranes.^3,4 Borataalkenes are nucleophilic species, and find use in boron-Wittig reactions. As suggested by the two resonance forms (alpha-boryl carbanion and borataalkene), borataalkenes can undergo both nucleophilic substitutions and cycloadditions.³ On the other hand, the coordination chemistry of borataalkene ligands is relatively underexplored. Due to the prevalence of alkene ligands in transition metal complexes and homogeneous catalysis, the development of borataalkenes as ligand motifs has attracted increasing attention in recent years.⁵

Figure 1.

Zeise’s salt and borataalkenes.

In contrast to alkenes which typically bind to transition metals via a symmetrical η²-C,C coordination mode,⁶ borataalkene ligands can bind in unsymmetrical η¹-B, η¹-C, and η²-B,C coordination modes (Figure 2).⁵ This phenomenon results from the highly unsymmetrical electronic structure of borataalkenes in comparison with alkenes, in which the C 2p orbital has a much higher contribution to the HOMO, and the B 2p orbital has a much higher contribution to the LUMO. Due to this asymmetry, metal to boron interactions will be favored for electron rich transition metals, and metal to carbon interactions will be favored for electron-deficient transition metals. Perturbation of the typical borataalkene electronic structure via the introduction of electron-donating or -withdrawing groups also will affect coordination tendency. The correct assignment of coordination mode in borataalkene complexes is therefore largely dependent on X-ray crystal structure analysis.^3a

Figure 2.

Electronic structure and binding modes of borataalkenes and alkenes.

Besides borataalkenes, several other ligand motifs feature η²-B,C binding, namely boraalkenes,^7,8 arylboranes (ambiphilic ligands) and borylferrocenes (Figure 3).^3a,9,10 Ambiphilic ligands containing the arylborane motif have seen increasing development in recent years. These systems have been applied to achieve catalytic small molecule activation often through metal-ligand cooperation.^9,10

Figure 3.

Other types of complexes featuring η²-B,C coordination.

The Liu group has recently disclosed a series of platinum and palladium complexes of azaborine-containing ligands which also feature η²-B,C coordination resulting from the iminium-borataalkene resonance structure of the 1,4-azaborine.¹¹ The phosphino-1,4-azaborine ligands have been applied as supporting ligands for numerous palladium-catalyzed regio- and stereo-selective functionalizations of 1,3-enynes, the selectivity of which is controlled by the unique electronic structure and η²-B,C coordination mode of the azaborine ligand.

This review highlights the recent advances in the synthesis and applications of transition metal borataalkene complexes and other transition metal complexes containing η²-B,C interactions. This review will outline: (a) the synthesis of transition metal borataalkene complexes and their typical coordination modes, especially the unique η²-B,C coordination mode; (b) other boron-carbon multiply bonded π ligands such as boraalkenes, borylferrocenes, arylboranes, and 1,4-azaborines and their η²-B,C bound complexes; (c) the stoichiometric reactions and catalytic applications of η²-B,C-bound transition metal complexes; (d) the generation of transition metal borataalkene complexes as reactive catalytic intermediates in coupling reactions. As Emslie et al. published a review on this topic in 2012,⁵ this review will focus mainly on subsequent discoveries. Some older publications are included as introductory material.

η¹ -C Borataalkene Complexes

The first reported transition metal borataalkene complex was isolated in 1999 by Piers and coworkers by reaction of Schrock’s methyl methylene tantalum(V) complex with Piers borane HB(C₆F₅)₂ at −78 °C (Scheme 1).¹² The resulting intermediate reductively eliminates methane upon warming to −20 °C to generate a green, paramagnetic solution which consists of an equilibrium mixture of singlet, η² 1 and triplet, η¹ 2 borataalkene complexes as suggested by DFT calculations. Complex 1 can be trapped by oxidative cyclization with 2-butyne to generate 3, whereas complex 2 can be trapped with π-acidic ligands as the neutral adducts 4a and 4b.

Scheme 1.

η¹ and η² tantalum borataalkene complexes.

Crystallographic evidence for the η² formulation for complexes 4a and 4b include Ta–B distances of 2.728(6) and 2.738(6) Å, and B-C distances of 1.508(8) and 1.525(7) Å which are intermediate between typical B−C_sp3 bond distances (1.578 Å for BMe₃)¹³ and B=C_sp2 (1.444 Å in [Mes₂B=CH₂]⁻[Li)12-crown-4]⁺).^3b Additionally, the ¹¹B-NMR chemical shifts of 8.5 and 7.2 ppm are more consistent with typical four-coordinate boron species. Molecular orbitals of 4a obtained using Kohn-Sham DFT calculations correspond well with a Dewar-Chatt-Duncanson model of olefin coordination. The highest occupied molecular orbital could be characterized as the Ta-CO backbonding orbital. The authors note that due to the naturally higher contribution of carbon to the borataalkene’s occupied orbitals, “when there are no d electrons present… η¹ bonding is the natural mode for this ligand type”.

Several other transition metal borataalkene complexes which apparently lack any metal to boron interaction have been reported. Ti(IV), having the d₀ electronic configuration, is unable to engage in any stabilizing metal d -> borataalkene LUMO interaction, which disfavors any coordination to boron. Thus, many examples of Ti(IV) η¹-C borataalkene complexes have been reported.¹⁴ These complexes were universally synthesized by treatment of dimethyl titanium(IV) compounds 5a-d with tris(pentafluorophenyl)borane (B(C₆F₅)₃) resulting in methyl abstraction to generate cationic titanium intermediate 6 (Scheme 2). σ-Bond metathesis and loss of methane generates α-boryl alkyltitanium intermediate 7, which then undergoes an intramolecular transmetalation of a C₆F₅ group generating η¹-C borataalkene complexes 8a-d. In all cases, the M-B bond distance was determined to be too long for any substantial bonding interaction, and M-C-B bond angles were >110°. Interestingly, although there is no appreciable M-B interaction, the borataalkene B-C bond lengths in 8a-d are all quite short at around ~1.50 Å, suggesting significant double bond character and supporting contribution from η¹-C borataalkene resonance forms 8a’−8d’.

Scheme 2.

Synthesis of η¹-C borataalkene Ti(IV) complexes.

A series of group 6 transition metal η¹-C borataalkene complexes are also reported (Scheme 3).¹⁵ Hydroboration of group 6 metal carbyne complexes result in either boryl metal alkylidene 9 or hydrido metal alkylidene 9’ depending on the regioselectivity of this step. α-Migratory insertion of the boryl or hydride ligand results in the formation of complexes 10, in which the metal forms an additional η² arene interaction. Evidently, in these group 6 complexes, the arene interaction is more stabilizing than the potential interaction with boron. As in complexes 8, the experimentally determined borataalkene B-C bond distances in 10 are quite short, suggesting partial double bond character.

Scheme 3.

Synthesis of Group 6 η¹-C borataalkene complexes.

Sadighi also reported the synthesis of a Cu(I) borataalkene complex whose coordination mode is not so straightforwardly assigned (Scheme 4).¹⁶ Treatment of borylcopper complex 11 with styrene affords benzylcopper complex 12 after β-migratory insertion of the boryl ligand. Upon heating, 12 rearranges to borataalkene complex 13. This reaction is believed proceed through a β-hydride elimination to generate olefin complex 14, which can subsequently undergo a β-migratory insertion into the benzylic position to generate 13. Evidence in favor of this pathway is the successful synthesis of 13 from copper hydride precursor 15 and alkenyl boronate 16. The Cu-B distance in 13 of 2.608(3) Å indicates some degree of copper-boron interaction, although it is significantly longer than all of the other copper η²-B,C borataalkene complexes which will be mentioned later in this review. Additionally, the Cu-C-B angle of 96.3(2)° is significantly more acute than one would expect for a purely Cu-Csp₃ bond. Further evidence in favor of an η²-B,C borataalkene ligand formulation is the fact that the Cu-C bond axis is perfectly orthogonal to the O-B-O plane of the Bpin group. However, in solution the assignment is less clear. The ¹¹B NMR chemical shift of 13 is 33.4 ppm is only very slightly upfield from that of 12 (34.7 ppm) which argues against a significant copper to boron interaction in solution. Likely, the presence of two resonance-donating heteroatoms attached to boron in Bpin raises the LUMO energy of the borataalkene ligand making any metal to boron interaction energetically less favorable.

Scheme 4.

Sadighi’s synthesis of Cu(I) borataalkene complexes.

Transition metal η¹-C borataalkene complexes have been used as nucleophiles and catalytic intermediates extensively.³ Indeed, before Sadighi’s report of the structurally characterized copper(I) borataalkene complex 13 (Scheme 17), Knochel reported a general route to copper/zinc stabilized α-boryl anions by insertion of zinc into α-halo boronic esters 17 (Scheme 5).¹⁷ Zinc borataalkene compounds 18 are generated, which can be converted to the more reactive copper compounds 19 by reaction with copper(I) cyanide dilithium chloride complex (CuCN•2LiCl) in THF. Compounds 18 and 19 undergo addition and substitution reactions with electrophiles including aldehydes, ketones, enones, and alkyl halides to furnish functionalized products 20a-c.

Scheme 17.

Structural, spectroscopic, and computational analysis of 65–67.

Scheme 5.

Reactions of zinc and copper borataalkene compounds.

More recently, geminal bis(boronates) 21 have emerged as readily accessible, bench stable precursors to α-boryl organometallic compounds.¹⁸ These compounds are known to engage in reactivity with a diverse array of electrophiles and coupling partners under transition metal catalysis. Aryl,^19a,b vinyl,^19c benzylic,^19d and allylic^19d electrophiles can be coupled under palladium catalysis (Scheme 6). The reaction follows a typical cross-coupling mechanism with the bis(boronate) being activated by base, typically hydroxide, prior to transmetalation.

Scheme 6.

Pd-catalyzed couplings of geminal bis(boronates).

Similarly, geminal bis(pinacolatoboronates) 24 undergo copper-catalyzed coupling with a somewhat complementary array of electrophiles (Scheme 7, top) including alkyl halides,^20a allylic halides/pseudohalides in an S_N2’ fashion,^20b imines,^20c epoxides and aziridines.^20d These reactions involve a copper(I) source, typically copper(I) chloride (CuCl), and an alkoxide base, most commonly lithium tert-butoxide (LiOt-Bu). A simplified mechanism is presented in Scheme 7 (bottom). The copper halide complex first undergoes a salt metathesis with the alkoxide base to generate a copper(I) alkoxide. The copper(I) alkoxide then undergoes a transmetalation with the bis(boronate) 24 to generate a nucleophilic copper(I) borataalkene complex, which attacks the corresponding electrophile 25 to afford the product 26.

Scheme 7.

Cu-catalyzed couplings of geminal bis(boronates).

η² -B,C Borataalkene Complexes

Late transition metal borataalkene complexes were relatively unexplored until recently, and there are now numerous examples of η²-B,C bound late transition metal borataalkene complexes. Late transition metals, having available d-electrons for backbonding, are more amenable to η²-B,C binding than earlier transition metals such as tantalum and titanium. A deprotonation strategy for the synthesis of a rhodium(I) borataalkene complex was reported by the Erker group (Scheme 8).²¹ Treatment of phosphino-borane 27 with LiTMP (lithium 2,2,6,6-tetramethylpiperidine) afforded anionic borataalkene lithium salt 28. Complexation with rhodium precursors yields chelated borataalkene complexes 29 featuring η²-B,C coordination. Attempts to prepare iridium analogue 30 only afforded Ir(III) complex 31 due to cyclometallation via C-H insertion into the mesityl group. Complex 29 served as an effective catalyst for hydrogenation of olefins.

Scheme 8.

Synthesis of Rh(I) borataalkene complex 29 via deprotonation of 27.

Owen and colleagues synthesized an η²-B,C borataalkene complex of Rh(I) by reaction of norbornadiene Rh(I) chloride dimer with the anionic borane-bound 2-mercaptopyridine ligand 32 followed by treatment of the resulting complex 33 with excess triphenylphosphine (Scheme 9).²² Ligand association and a series of reactions of the bridging hydrides with the norbornadiene ligand furnishes borataalkene complex 34, the η²-B,C coordination of which was confirmed by X-ray diffraction analysis.

Scheme 9.

Owen’s synthesis of Rh(I) η²-B,C borataalkene complex.

After phosphine association with 33 and loss of chelation by norbornadiene, a boron to rhodium hydride transfer occurs to give 35. β-Migratory insertion give complex 36, which can undergo another phosphine association causing alkyl migration to the borane to yield 37. A second β-migratory insertion yields alkyl bridged intermediate 38, which undergoes β-alkyl elimination to deliver borataalkene complex 34. Dihydrogen undergoes rapid addition to the B=C bond of 34 to give hydride bridged complex 39.

The Ozerov group observed a metal η²-B,C borataalkene coordination in iridium complexes derived from boryl PBP pincer ligated iridium complex 40 (Scheme 10).²³ In the presence of excess terminal olefin, conversion to borataalkene complex 41 is observed. The proposed mechanism is outlined as follows: Olefin association and loss of CO leads to complex 42, which then undergoes C-H insertion with the terminal C_sp2-H bond to give Ir(III) complex 43. Alkenyl migration from iridium to boron yields intermediate 44, which undergoes β-migratory insertion to afford borataalkene complex 41. The overall process is reversible, and thermolysis of 41 at elevated temperatures leads to partial conversion back to 40.

Scheme 10.

Ozerov’s synthesis of Ir(I) η²-B,C borataalkene complex.

Yamashita and coworkers utilized a unique silyl migration reaction involving first migratory insertion of bis(trimethylsilyl)acetylene into Au(I) complex 45 to generate the unexpected trans-alkenyl gold complex 46 (Scheme 11).²⁴ Further heating 46 in Et₂O for 12 hr effects isomerization to η²-B,C borataallene complex 47. DFT calculations were carried out to elucidate the likely mechanism of these transformations and revealed that β-migratory insertion first occurs to generate the expected syn-insertion product 48. A retro-1,2-metallate shift then occurs to generate alkynyl borate complex 49. B-Si bond rotation and subsequent 1,2-metallate shift affords complex 50. A 1,2-silyl migration occurs to afford η²-C,C borataallene complex 51, which can then isomerize to the more thermodynamically stable η²-B,C borataallene complex 47. The borataallene formulation for complex 47 is corroborated by structural and computational data, namely the WBI (B-C = 1.19) and experimentally determined bond distances (Au-C = 2.164(4) Å, Au-B = 2.441(4) Å). Notably, 47 is the first reported example of a true transition metal borataallene complex.

Scheme 11.

Synthesis of Au(I) borataallene complex via silyl migration.

Group 11 transition metal borataalkene complexes have seen rapid development in recent years, and many examples are now reported with Au(I) complexes being especially common. Breher and coworkers reported an α-borylated phosphorus ylide 52, which was synthesized by “trans-ylidation” of triphenylethylphosphonium bromide with diethyl chloroborane (Scheme 12).²⁵ The calculated Wiberg bond indices (WBI) around the ylidic carbon of 52 were determined to be C-B = 1.42 and C-P = 1.25 indicating significant borataalkene character (WBI for C_ethyl-B = 0.97). Reaction of 52 with gold(I) precursor (SMe₂)AuCl afforded Au(I) borataalkene complex 53 which was structurally characterized.²⁶ Reaction of 52 with Cu(I) and Ag(I) precursors gave oily products which could not be characterized structurally, precluding any assignment of coordination mode. Complex 53 features an unsymmetrical η²-B,C coordination mode having a significantly shorter Au-C bond.

Scheme 12.

Synthesis of Au(I) boryl phosphorus ylide complexes and computational analysis of bonding.

Although synthesis of the complete coinage metal series failed, DFT calculations were used to predict the structures and orbital interaction energies of the complete series of group 11 transition metal complexes of ligand 52. All complexes are bound more closely to carbon than boron. The copper complex is predicted to have the shortest metal-ligand bond distances, followed by gold and silver. A Dewar-Chatt-Duncanson model of bonding is considered, with the orbital interactions decomposed into ΔE_orb(ρ1) (B=C HOMO -> M LUMO) and ΔE_orb(ρ2) (M HOMO -> B=C LUMO+7). Donation from the B=C pi-bonding orbital into the metal LUMO is the dominant interaction providing most of the stabilization, with backdonation playing a lesser but still significant role. Both orbital interactions are strongest with gold, followed by copper and silver.

Martin and coworkers recently disclosed the synthesis of a 9-borataphenanthrene anion by deprotonation of neutral 9,10-dihydro-9-boraphenanthrene 54 (Scheme 13).²⁷ The 9-borataphenanthrene anion 55 can be regarded as having both boratabenzene and borataalkene character, although Clar’s rule predicts the borataalkene resonance form to be the dominant contributor the overall structure. Indeed, DFT calculations predict the HOMO to be localized mostly to the B-C unit, suggesting the possibility for η²-B,C coordination to transition metals.

Scheme 13.

Synthesis of 9-borataphenanthrene Cr(0) and Au(I) complexes.

In line with this prediction, anion 55 reacts with Au(PPh₃)Cl to generate η²-bound borataalkene complex 56, which is biased towards carbon and away from boron (Au−C = 2.188(7) Å vs. Au−B = 2.427(8) Å). This reflects the high electronegativity of Au(I) and thus its affinity for the more electron rich position in the ligand. Notably, this is the first reported example of a group 11 η² borataalkene complex. In contrast, Cr(0) precursor Cr(MeCN)₃(CO)₃ reacts with 55 to form η⁶-B,C₅-boratabenzene complex 57 which is isolated as the crown ether-solvated potassium salt (crown = dibenzo-18-crown-6). The preference of Cr(0) for η⁶ coordination is likely a consequence of its higher electron richness relative to Au(I). This would make the metal d to ligand LUMO (to which the boron and C1 contribute very little) interaction more important for the overall stabilization of the complex vs the ligand HOMO to empty metal d orbital interaction.

Building upon this work, Martin subsequently reported the complete series of group 11 borataalkene complexes derived from boratabenzene and borataphenanthrene anions (Scheme 14).²⁸ Boratabenzene 58 was reacted with group 11 metal chlorides to give the corresponding η²-B,C borataalkene complexes 59–61. In all cases the M-C bond distances were shorter than the M-B bond distances. Interestingly, Ag(I) complex 60 exhibits almost an almost symmetrical binding mode where the Ag-C distance is only slightly shorter than the Ag-B distance (Ag–C = 2.3586(14) Å, Ag–B = 2.3839(16) Å). When ligation of 58 to ClCu(PPh₃) instead of ClCu(IPr) was attempted, the resulting complex had η⁶ coordination. Borataphenanthrene 55 could also be ligated to Cu(I) and Ag(I) to generate η²-B,C borataalkene complexes 62 and 63.

Scheme 14.

Martin’s synthesis of group 11 borataalkene complexes.

Structural parameters obtained from X-ray diffraction data and some computed parameters for the series of borataphenanthrene complexes are summarized in Scheme 15. Complex 63 exhibits the longest M-C and M-B bonds, and has the longest B=C bond distance. While no clear trends can be observed from the X-ray diffraction data, the calculated data show some straightforward trends. The predicted B=C bond distances and binding energies increase with coordination to the heavier metals. The charge is predicted to be localized more to the B=C carbon in the heavier congeners.

Scheme 15.

Structural and calculated parameters for complexes 62, 63, and 56.

CAAC-stabilized boranes have been used as borataalkene precursors. These borataalkenes are more polarized towards boron than a typical borataalkene, and metal coordination to boron is favored.

Braunschweig and coworkers synthesized the CAAC-stabilized parent boryl anion 64 (Scheme 16).²⁹ Complexation of 64 with group 11 metals yields borataalkene complexes 65-67.³⁰ Complexes 65-67 all contain shorter M-B distances compared to the corresponding M-C distances. Au(I) complex 67 is very nearly an η¹-B complex, with a very short Au-B distance (2.23(1) Å) and long Au-C distance (2.68(1) Å). However, they can all be described as η²-B,C borataalkene complexes. Interestingly, the structural parameters of zinc analogue 68 are fully consistent with an η¹-B coordination mode, which provides a useful benchmark (Zn-B = 2.139(2) Å). The tendency of CAAC-supported borataalkenes for stronger boron coordination highlights the unique steric and electronic effects of the CAAC moiety as compared with more traditional borataalkenes.

Scheme 16.

Synthesis of stabilized parent boryl anion 64, and ligation to coinage metals.

Structural and spectroscopic parameters of complexes 65–67 are summarized in Scheme 17. Coordination bias shifts from carbon to boron moving from Cu to Ag (Scheme 17a). This is coupled with an increase in both the M-B-C bond angle and an upfield shift in the ¹¹B-NMR. As can be seen from the B=C bond distance and the change in computed Wiberg bond indices, a stronger coordination to boron results in a lengthening in the B-C bond. This can be interpreted as representing a stronger donation from the B=C HOMO to the vacant metal s orbital, and stronger backbonding from the filled metal d orbital to the B=C LUMO. This Dewar-Chatt-Duncanson model of bonding in these complexes is corroborated by DFT calculations. Natural bond orbitals (NBOs) of the free borataalkene ligand show a slight asymmetry in the orbital contributions (bonding NBO 66% pAO carbon, 34% pAP boron; antibonding NBO 34% pAO carbon, 66% pAO boron, Scheme 17b). ETS-NOCV calculations suggest that B=C HOMO to metal 3d donation is mostly responsible for the stabilizing metal-ligand interaction in complex 65 (28.4 kcal/mol), with backdonation providing modest stabilization in comparison (5.3 kcal/mol, Scheme 17c).

η¹-B Borataalkene Complexes

Due to the natural charge distribution of borataalkenes, and as seen in the previous sections of this review, borataalkenes have a tendency for coordination through carbon. Thus, η¹-B borataalkene complexes are quite rare. However, through perturbation of the borataalkene electronic structure, coordination to boron can be facilitated. Namely, the placement of an electron-donating heteroatom on carbon and electron-withdrawing substituents on boron results in a polarization of the B=C unit towards boron.

Bertrand and coworkers reported a new strategy for generating borataalkenes via B-H deprotonation of an electron deficient, CAAC-stabilized borane (CAAC = cyclic alkyl aminocarbene).^31a This strategy is in contrast with the more common alpha C-H deprotonation utilized by Erker and Martin. Deprotonation of stabilized borane 68 with KHMDS affords anion 69 (Scheme 18). Anion 69 is described by two resonance forms, a Lewis base-stabilized boryl anion, and a borataalkene. The significant shortening of the CAAC C-B bond vs. 68 and the planar geometry around the boron atom in 69 suggests significant contribution of the borataalkene resonance form. However, complexation with Au(PMe₃)Cl affords η¹-B Au(I) complex 70.

Scheme 18.

Bertrand’s borataalkene synthesis via B-H deprotonation strategy.

The same B-H deprotonation strategy was used with CAAC-supported cyanoborane 71 (Scheme 19).^31b Even with only one cyano group, the boron center of 71 can be deprotonated with n-BuLi to generate borataalkene 72. Salt metathesis with (Ph₃P)AuCl generates η¹-B borataalkene complex 73, which may also be represented by the boryl gold resonance form 73’. Anion 72 reacts with main group electrophiles such as trimethylsilyl chloride at boron, suggesting charge localization on boron, although sterics likely also play a role.

Scheme 19.

Synthesis of Au(I) borataalkene complex 73.

Boraalkene Complexes

Boraalkenes are the neutral, boron-containing species isoelectronic with vinyl cations. Due to their being charge neutral, they are less polar than borataaalkenes and tend to coordinate via the η²-B,C mode to transition metals. Some of the η²-B,C bound boraalkenes compounds, specifically azaborataallene complexes are reported by Nöth³² and Braunschweig³³ (Scheme 20, top). Rhodium vinylidene complex 75 undergoes a molybdenum borylene exchange with [(OC)₅Mo=B=N(SiMe₃)₂] to give 76. The ligand motif is best described as a 1-aza-2-bora-cumulene. Interestingly, although there are many different possible coordination modes due to the number of pi bonds, the metal selectively coordinates to the B=C bond. η²-B,C coordination was calculated to be ~16 kcal/mol more favorable than coordination to the C=C unit, highlighting the affinity of late transition metals for more polar B=C unit. The bent geometry of the ligand as well as the long B-C distance in 76 (1.489(12) Å) are indicative of significant back-bonding from the metal.

Scheme 20.

Azaborataallene complexes.

Coordination of azaborataallene 77 with various first-row transition metal carbonyl complexes results in complexes 78, which also feature η²-B,C coordination preferentially in all cases, as opposed to coordination to the B=N bond. DFT calculations demonstrated that back-bonding to the B-C π* contributes considerably to the stabilization of such complexes. A qualitative MO depiction of key stabilizing interactions in a simplified iron azaborataallene (boraalkene) complex is illustrated in Scheme 20 (bottom).

Braunschweig and coworkers recently reported the synthesis of an unsymmetrical, doubly CAAC-stabilized diborene 79 which served as a versatile precursor to borataalkene and boraalkene complexes (Scheme 21).³⁴ Reaction of 79 with half an equivalent of bis(2-phenylethynyl)mercury (Hg(CCPh)₂) resulted in insertion of phenylacetylide into the B=B bond and formation of η¹-C borataalkene complex 80. No Hg-B interaction is observed in 80, consistent with the inability of the low energy Hg d-orbitals to engage in backbonding. Upon heating to 80 °C, diborene 79 undergoes insertion of the carbonic carbon of the non-tethered CAAC ligand into the B=B bond and a ring expansion affording boraalkene 81. In addition to the boraalkene resonance form, 81 can also be described by an iminium-borataalkene resonance structure 81’. Complexation of 81 with AgOTf affords η²-B,C boraalkene complex 82. Complex 82 bears relatively short and symmetric Ag-C and Ag-B contacts (Ag-C = 2.263(3) Å, Ag-B = 2.364(4)).

Scheme 21.

CAAC-stabilized diborane 79 as a precursor to borataalkene complexes.

Erker and coworkers reported a deprotonation strategy for the synthesis of boraalkene complexes (Scheme 22).⁷ From NHC-supported borane 83a or 83b, protonolysis of the B-H bond with bistriflimidic acid (HNTf₂) generates a loosely ion- paired cationic borenium intermediate followed by deprotonation of the acidic α-C-H bond using IMes as a bulky base to afford neutral NHC-supported boraalkenes 84a and 84b. The base used for the deprotonation had to be selected carefully, presumably to not interact with the highly electrophilic boron center of the borenium-type intermediate. Treatment of 84a and 84b with (SMe₂)AuCl affords η²-B,C boraalkene complexes 85a and 85b. Complex 85a was structurally characterized, confirming the slightly unsymmetrical η² coordination mode (Au-C = 2.125(3) Å, Au-B = 2.291(3) Å). The ¹¹B NMR chemical shift of 85a (δ 14.1) is shifted significantly upfield from 84a (δ 19.0), suggestive of pyramidalization and/or an increase in electron density at boron due to gold coordination. Reaction of 84a with [RhCl(CO)₂]₂ afforded single crystals of rhodium complex 86a. Although 86a could not be spectroscopically characterized, the structural information derived from its crystal structure is informative. The boraalkene B-C bond in 86a is significantly lengthened (B-C = 1.502(2) Å) compared to complex 85a (B-C = 1.476(4) Å) and boraalkene 84a (B-C = 1.453(3) Å). The B-C bond lengthening is indicative of stronger backbonding in 86a compared with 85a.

Scheme 22.

Deprotonation pathway to boraalkene complexes.

Erker subsequently reported the synthesis of a series of boraalkene complexes derived from NCS-substituted boraalkene 87 (Scheme 23).⁸ Complexation with group 11 metal precursors yield η²-B,C boraalkene complexes 88 and 89. In both complexes the metal-carbon bond distance is significantly shorter than the metal-boron distance. The coordination is more symmetrical in 89, and the long boraalkene B-C bond and upfield ¹¹B NMR signal are both indicative of a higher degree of backbonding than in 88 which can be rationalized as being due to the higher radial projection of Au’s d orbitals relative to Cu. Complexation with [(η³-allyl)PdCl]₂ affords Pd boraalkene complex 90 which also features a relatively symmetrical η²-B,C coordination mode. The B-C bond in 90 is only slightly elongated compared to 87, and the metal-boraalkene contacts are the longest in the series. In general, a trend can be recognized with respect to the B=C bond distances and ¹¹B NMR chemical shift which is similar to complexes 65–67. That is, coordination bias towards boron results in shielding (or, a more pyramidalized boron center) and lengthening of the B=C bond. This could be attributed to stronger backbonding to the B=C LUMO by the heavier transition metals with more radially extended d orbitals.

Scheme 23.

Boraalkene complexes and selected structural and spectroscopic data.

Bora- and dibora-butadienes are a relatively underexplored class of boraalkene ligands. In 1992 Siebert described an unusual route to borabutadiene complexes which involves the ring opening and isomerization of diborolane 91 promoted by cyclopentadienyl bis(ethylene)cobalt(I) to give complex 92 (Scheme 24).³⁵ The borabutadiene bonding description is supported by the relatively short B=C bond length in 92 (1.493 Å).

Scheme 24.

Synthesis of 1-borabutadiene complex 92.

Upon treatment of bis(borylene) iron complex 93 with one equivalent of 2-butyne under photochemical conditions, diboracyclohexadiene complex 94 could be obtained via two sequential 3+2 and 4+2 cycloaddition (Scheme 25).³⁶ Analogous treatment of 93 with bis(trimethylsilyl)acetylene affords diborabutadiene iron complex 95, which may also be represented as formally iron(II) diboryl complex 95’ with pendant alkene coordination. The central borabutadiene C-C bond distance in is 1.452 Å, which while quite short for a carbon-carbon single bond, is significantly longer than the endocyclic C-C bonds in 94 (mean 1.42 Å). Additionally, the C=B(Dur) bond distance in 95 of 1.512 Å is significantly shortened compared to the endocyclic B-C bonds in 94 (mean 1.56 Å), indicative of C-B multiple bonding. This suggests that both resonance structures 95 and 95’ are contributors to the overall bonding situation.

Scheme 25.

Diborabutadiene complex 95.

Synthesis of a 1-borabutadiene complex was accomplished by reaction of NHC-stabilized borabutadiene 96 with 0.5 equiv of bis(ethylene)rhodium chloride dimer affording complex 97, from which crystals were obtained allowing structural characterization, but which eluded spectroscopic characterization (Scheme 26).³⁷ Thermolysis of complex 97 led to the formation of 1-boraallyl complex 98. Complex 98 is hypothesized to form through C-H activation of an IMe N-methyl group followed by migratory insertion of the resulting rhodium hydride into the 1-borabutadiene ligand.

Scheme 26.

Synthesis and reactivity of 1-borabutadiene complex 97.

Other Complexes Featuring η²-B,C Coordination

The metal η²-B,C coordination exists not only in complexes with borataalkene and boraalkene ligands, but also in complexes with B-C_aryl moiety-containing ligands such as borylferrocene³⁸ and boron-containing ambiphilic ligands.^9,10 Borylferrocenes can be synthesized by electrophilic aromatic substitution of ferrocene with BBr₃, followed by functionalization of the mono- or di-borylated complexes with various nucleophiles to afford complexes 99 or 100 (Scheme 27a). X-ray diffraction analyses indicate the Cp (cyclopentadienyl) rings are not coplanar with the boron groups resulting from “bending” of the B-C_ipso bond towards the iron center. The degree of bending increases with the Lewis acidity of the boron atom. Additionally, the Cp rings are slightly deformed. The contribution of boron to the ligand binding is supported by DFT calculations which suggest that it is a combination of the p orbitals of the Cp ring and the boron atom which form a bonding interaction with the iron d orbitals, rather than a more localized Fe-B interaction (Scheme 27b).

Scheme 27.

η²-B,C_ipso and η¹-C_ipso interactions in borylmetallocenes a) Synthesis of borylferrocenes. b) Qualitative depictions of selected stabilized MOs from a bent, C_s-symmetric borylferrocene. c) Synthesis of Group 4 borylindenyl complexes.

On the other hand, Group IV borylindenyl complexes show no corresponding metal-boron interaction (Scheme 27c).³⁹ Hydroboration of dimethylbenzofulvene 101 with Piers’ borane (HB(C₆F₅)₂) followed by deprotonation with LiTMP affords borataalkene 102.^39b Complexation with CpTiCl₃ yields 103, wherein one of the chloride ligands is bridging. Zirconium analogue 104 features a tricoordinate boron group (¹¹B δ=55.7 ppm), and X-ray diffraction analysis does not indicate there is any significant Zr-B interaction (Zr-B > 3.6 Å). The absence of d-electrons in these systems disfavors bending of the Cp-B bond, as the resulting stabilized MOs with high metal d character are likely unfilled and therefore do not contribute to stabilizing the overall structure.

Pioneered by Bourissou, phosphine-borane ambiphilic ligands are multi-dentate ligands containing both Lewis base(s) and Lewis acid(s) within the same ligand framework. As several recent reviews have covered the synthesis and applications of ambiphilic ligands in catalysis, this review will focus more narrowly on those complexes and catalysts involving η²-B,C coordination. To date, a variety of transition metals have been reported to form the η¹-B coordination with boron-containing ambiphilic ligands. In some cases such coordination is augmented by an aryl η¹-C_ipso or η²-C_ipso,C_ortho coordination without significantly perturbing the ring aromaticity. These coordination arrangements can be considered as formal η²-B,C binding.

A collection of η²-B,C_ipso or η³-B,C_ipso,C_ortho bound ambiphilic ligand complexes of Fe,⁴⁰ Co,⁴¹ Rh,⁴² Pd,⁴³ Ni,⁴⁴ Cu,⁴⁵ and Pt⁴⁶ (105-114) are shown in Scheme 28. Among them, iron shows great coordination diversity. For example, in complex 105 an η²-B,C_ipso coordination is displayed, whereas in bimetallic iron complex 106, both η²-B,C_ipso and η³-B,C_ipso,C_ortho are displayed within the same complex. In solution, only the η²-B,C_ipso coordination mode in 106 is observed, highlighting the dynamic nature of these ligands. Complex 107 even features an η⁷ coordination mode.

Scheme 28.

Collection of ηⁿ-bound complexes bearing ambiphilic ligands

Due to the Lewis-acidic nature of the arylborane ligands’ boron center, they can act as non-innocent ligands. Most commonly the boron group aids in stabilizing a bridging hydride ligand (Scheme 29). When complex 115 is exposed to dihydrogen, the η²-B,C_ipso-M bonding is inert against cleavage by H₂,^44a however, the more bulky mesityl substituent in 111 and the resulting η³-B,C_ipso,C_ortho coordination is labile enough for facile activation of H₂ at room temperature furnishing complex 111-H₂. This result highlights that although the coordination mode in these complexes can be fluxional, it has a substantial influence on the reactivity of the metal particularly in cases with ligand cooperation. Complex 111 also serves as a catalyst for the hydrogenation of olefins under mild conditions. Likewise, diphenyl silane (Ph₂SiH₂) undergoes oxidative addition with ligand cooperation to give silylnickel(II) complex 111-SiH.^44b Complex 111 also serves as a catalyst for hydrosilylation of aldehydes.

Scheme 29.

Reversible H₂ activation by complex 111.

Bourissou and coworkers reported the diphosphine-borane ligated palladium(0) complex 116 (Scheme 30).^43a The structural parameters of complex 116 are consistent with η²-B,C_ipso coordination to the arylborane moiety (Pd-B = 2.294(2) Å, Pd-C_ipso = 2.638(2) Å). Although complex 116 does not activate dihydrogen or silanes, it does react with potassium hydride (KH) in the presence of [2.2.2]-cryptand to furnish anionic palladium complex 117. When reacted with chlorobenzene, 116 is reformed along with elimination of benzene (PhH) and potassium chloride (KCl). A plausible mechanism supported by experimental and computational evidence is shown in Scheme 30.

Scheme 30.

Hydrodechlorination of chlorobenzene catalyzed by 116.

Reduction of complex 116 with KH in the presence of [2.2.2.]-cryptand first gives 117. Dissociation of PPh₃ and subsequent oxidative addition of chlorobenzene (PhCl) gives complex 118 with concomitant release of potassium chloride. Reductive elimination and association of PPh₃ regenerates 116 and releases the reduced arene. Interestingly, this catalytic cycle is an “inversion” of a typical palladium-catalyzed cross coupling reaction mechanism with oxidative addition and transmetalation steps having been switched. The cooperative action of the borane ligand allows for transmetallation directly to palladium(0) prior to oxidative addition, likely facilitating the oxidative addition by creating a more nucleophilic metal center.

Peters’ iron dinitrogen complex 106 displays rich and diverse reactivity with E-H bonds (E = O, S, C, N) (Scheme 31).⁴¹ Reaction of 106 with two equivalents of phenol (PhOH) results in the formation of Fe(I) phenolate complex 119 along with release of H₂ (Scheme 31, top). This process can be considered as a one-electron oxidation of each iron center in 106. The authors suggest as a plausible mechanism the formation of an iron phenolate borohydride intermediate, followed by bimolecular reductive elimination of H₂. This mechanism is supported by the isolation of 120 upon reaction of 106 with two equivalents of thiophenol (PhSH). Such a complex could also undergo bimolecular reductive elimination with release of H₂, but this was not observed under the reaction conditions. When reacted with 8-aminoquinoline, formation of Fe(II) oxidative addition product 121 is observed. Similarly, with benzo[h]quinoline, C-H activation occurs yielding complex 122. X-ray diffraction analysis indicates that complex 122 retains an η²-B,C_ipso coordination mode although with a different aryl ring than in 106. The Fe-B and Fe-C_ipso contacts are relatively short at 2.3342(9) and 2.3739(9) Å respectively. No such coordination is observed in the crystal structure of 121. Complex 106 also catalyzed the hydrosilylation of aldehydes (Scheme 31, bottom). The Co analogue (complex 108) displays similar reactivity with respect to both E-H bond activation and hydrosilylation catalysis.

Scheme 31.

E-X activation by Fe complex 106.

Complexes of 1,4-Azaborine Ligands

Although not strictly borataalkenes, 1,4-azaborines can be described by the iminium-borataalkene resonance structure which suggests partial B=C double bond character in the overall electronic structure. Due to this electronic structure, 1,4-azaborine ligands tend to exhibit η²-B,C binding in their transition metal complexes, reminiscent of borataalkene ligands.^11a These complexes can be considered as formal borataalkene complexes. This phenomenon was first observed in 2014 when the first chelating 1,4-azaborine ligand was synthesized and complexed with various platinum(II) precursors (Scheme 32).^11a

Scheme 32.

Synthesis of pyridine-1,4-azaborine platinum(II) complexes.

1,4-azaborine ligand 123 was complexed with trans-[{PtCl(μ-Cl)(PEt₃)}₂] to generate pyridine adduct 124, in which there is no interaction of the platinum center with the azaborine ring. Chloride abstraction with silver triflate (AgOTf) affords cationic platinum complex 125, in which the azaborine ligand adopts a κ²-N-η²-B,C coordination mode. The same coordination mode results from the reaction of 123 with [{PtMe₂(μ-SMe₂)}₂] in the neutral platinum complex 126.

The changes in the intra-ring bond distances of the 1,4-azaborine ring are illustrative of the changes in the electronic structure of the ligand upon coordination with a transition metal. Reference bond distances are provided by the X-ray structure of 124 (Figure 4). Upon coordination to the cationic platinum center of 125, the N1-C2 distance shortens from 1.365(4) Å in 124 to 1.318(8) Å in 125. Concomitantly the C2-C3 distance lengthens from 1.361(5) Å to 1.415(8) Å. These changes indicate a stronger contribution of the iminium-borataalkene resonance structure to the overall structure of the 1,4-azaborine moiety. The η²-B,C coordination in 125 is asymmetrical, with a much shorter Pt-C distance (2.179(6) Å) than Pt-B distance (2.374(6) Å). Complex 126 follows a similar trend, although the iminium-borataalkene resonance contribution appears less prominent than in 125. Complex 126 features a much longer Pt-C2 distance (2.296(3) Å) and shorter Pt-B distance (2.337(4) Å) vs. 125, likely due to the more electron rich platinum center (charge-neutral complex), which favors backdonation. Attempts to prepare the corresponding carbonaceous Pt complex with analogous naphthylpyridine ligand failed.

Figure 4.

ORTEP illustrations of 124, 125, and 126 with 50% thermal probability ellipsoids. Hydrogens omitted for clarity. Counteranion in 125 omitted for clarity.

Given the unique binding mode of the benzofused 1,4-azaborine ligands, it can be inferred that they may confer unique properties upon the transition metal to which they are ligated. These unique properties could then be leveraged in transition metal catalysis to develop new reactivity patterns. To probe the possible effects of the newly discovered coordination mode in transition metal-catalyzed reactions, the selectivity of palladium-catalyzed 1,3-enyne hydroboration was investigated as a function of ligand structure (Scheme 33). There are many possibilities for regio-, stereo-, and site selectivity in this reaction but three products predominate. The use of a bisphosphine ligand results in exclusive selectivity for cis-hydroboration product 127. Monodentate phosphines such as triphenylphosphine (PPh₃) and biarylphosphine 130 furnish allenylborane 129 as the major product. On the other hand, when the 1,4-azaborine analogue 131 was used as the ligand, trans-hydroboration product 128 is furnished as the major product, and in higher overall yield compared with 130.

Scheme 33.

Selectivity in palladium-catalyzed 1,3-enyne hydroboration.

This discovery was used as the basis for developing a general palladium-catalyzed trans-hydroboration reaction of 1,3-enynes (Scheme 34).^11b In the presence of Pd₂dba₃ and phosphino-1,4-azaborine ligand 132, nick-named “SenPhos” after its inventor Senmiao Xu, a wide range of terminal and internal 1,3-enynes could undergo highly selective trans-hydroboration to yield dienyl boronates 133 after quenching with pinacol. The origin of the high trans selectivity was later revealed by combined computational and experimental mechanistic studies.^11f The mechanism most consistent with the evidence from these studies is illustrated in Scheme 34 (top). After the precatalyst reacts with ligand 132 to generate a monoligated palladium(0) complex (L_nPd(0)), this complex could then bind enyne 134, which was used as the model substrate for these studies, to generate Pd olefin complex 135. The alkyne moiety of 135 is then attacked electrophilically by the Lewis acidic catecholborane (HBCat) anti to the palladium center in a so-called “outer-sphere” oxidative addition process furnishing zwitterionic π-allylpalladium adduct 136. Adduct 136 was determined to be the resting state of the catalytic cycle. Rate-limiting hydride abstraction by a second equivalent of HBCat would then generate ion pair 137, and the borohydride ion could then transfer a hydride back to palladium to generate neutral palladium(II) complex 138. Inner-sphere reductive elimination at the allenic carbon yields the product 133 and regenerates the free Pd(0) catalyst.

Scheme 34.

Mechanism of trans-hydroboration of 1,3-enynes.

As the most unusual aspect and defining feature of this mechanism is the outer-sphere oxidative addition process, an analogue of adduct 136 was prepared independently to verify the feasibility of such an intermediate. In the presence of a strong boron-based electrophile (B(C₆F₅)₃) and with no external nucleophile to promote reductive elimination, compound 139 could be isolated upon mixing (1,5-cyclooctadiene)bis(trimethylsilylmethyl)palladium(II) with ligand 132 and 1,3-enyne 134. Complex 139 is stable enough for structural characterization and features the same η²-B,C coordination mode observed with the pyridyl 1,4-azaborine ligands in their platinum complexes.

Based on this mechanism, the first general trans-selective cyanoboration of 1,3-enynes was developed (Scheme 35).^11c Ligand 140, a dialkyl biarylphosphine reminiscent of Buchwald-type ligands,⁴⁷ was found to be the optimal ligand. Copper(I) cyanide (CuCN) was used as the cyanide source, and a mechanism like the trans-hydroboration reaction is assumed. That is, after outer-sphere oxidative addition and transmetallation resulting in elimination of CuCl, inner sphere C-C reductive elimination would yield tetrasubstituted alkenyl nitriles 141. The efficiency and stereoselectivity of the reaction are highly dependent on the structure of the ligand. While dicyclohexylphenylphosphine (PhPCy₂) is ineffective to promote the transformation, phosphino-1,4-azaborine 140 delivers the product in 92% yield and >95:5 trans selectivity under the reaction conditions. The all-carbon analogue CC-140 is significantly less efficient and selective, giving only 6% yield of the product and lower stereoselectivity under otherwise the same conditions. This result illustrates the importance of the η²-B,C coordination mode, and its substantial influence on reactivity.

Scheme 35.

trans-Cyanoboration of 1,3-enynes.

Utilizing a rare class of organoboron compounds, C-boron enolates 144, a cis-carboboration of 1,3-enynes was developed (Scheme 36).^11d The reaction affords highly substituted alkenylboronates 145. In stark contrast to the Pd/Senphos catalyzed trans hydroboration and cyanoboration reactions, the carboboration is highly cis selective. This surprising result prompted mechanistic investigations, which revealed a unique syn-outer sphere oxidative addition mechanism illustrated in scheme 36 (top).^11e Free palladium(0) complex 146 first binds enyne 134 to generate enyne complex 147 which was identified as the resting state of the palladium catalyst. Complex 147 is attacked electrophilically by the C-boron enolate in a syn fashion, i.e., with the palladium and boron groups syn to each other, to generate π-allylpalladium intermediate 148. Intermediate 148 then undergoes a B-C to Pd-O enolate isomerization process to 149 enabled by the syn relationship of the Pd and boron groups. The Pd-bound enolate group of 149 is then transferred back to boron to generate the O-boron enolate complex 150. Complex 150 can then undergo an intramolecular, outer-sphere reductive elimination to afford the product 145 and regenerate free palladium catalyst 146.

Scheme 36.

cis-Carboboration of 1,3-enynes.

The performance of ligand 140 and its all-carbon analogue 140-CC were compared using 134 and 144a as model substrates. It was found that while 140 could carry the reaction to completion, the reaction stalls at around ~50% conversion after 4 hours for 140-CC (Scheme 36, bottom). This result is consistent with the stabilizing effect of the η²-B,C coordination imparted by 140 that gives a longer-living catalyst.

Building on the previously discussed revelations with respect to the mechanisms of Pd/Senphos-catalyzed 1,3-enyne functionalizations, a cooperative catalyst system consisting of Pd/Senphos, tris(pentafluorophenyl)borane (B(C₆F₅)₃), copper(I) bromide, and a stoichiometric amound of amine base (2,2,6,6-tetramethylpiperidine, TMP) was devised to effect the trans-hydroalkynylation of internal 1,3-enynes (Scheme 37).^11g A variety of alkynes 151 could undergo selective trans-addition to 1,3-enynes to generate cross-conjugated dienynes 152. Whereas previous Pd/Senphos-catalyzed 1,3-enyne functionalizations have relied on an electrophilic borane for activating a palladium enyne complex prior to reductive elimination, the hydroalkynylation catalyst system uses a catalytic Lewis acid to promote outer-sphere oxidative addition followed by quenching of the resulting adduct with an electrophile, here a proton.

Scheme 37.

trans-Hydroalkynylation of 1,3-enynes.

A proposed mechanism is shown in Scheme 37 with enyne 134 and phenylacetylene (151a) as model substrates. Enyne complex 147 is activated by B(C₆F₅)₃ in an anti-outer-sphere oxidative addition process to generate 153. Meanwhile, a combination of copper(I) bromide and TMP generates a copper acetylide and protonated TMPH⁺ (2,2,6,6-tetramethylpiperidinium) from phenylacetylene (151a) in the same manner as in Sonogashira couplings.⁴⁸ TMPH⁺ can then protodeboronate complex 153, liberating free B(C₆F₅)₃ and TMP, and generating cationic π-allylpalladium complex 154. Copper to palladium transmetalation and concomitant η³ to η¹ allyl isomerization then affords intermediate 155. Intermediate 155 then undergoes reductive elimination yielding the product 152 and regenerating the free palladium catalyst.

Conclusions

The recently developed strategies for synthesizing transition metal borataalkene and boraalkene complexes as well as other complexes featuring the rare η²-B,C coordination mode have been summarized in this review along with their role in transition metal catalysis. The diversity of coordination modes and the unique reactivity induced by bora- and borataalkene ligands and other boron-containing π ligands are leveraged to develop new catalytic activity. In particular, this review points to two emerging areas of borataalkene coordination chemistry with promising synthetic organic applications in homogenous catalysis: 1) The ready access to geminal bis(pinacolatoboronates) establishes the corresponding transition metal borataalkene complexes (following a transmetallation) as key intermediates in cross coupling chemistry to generate versatile alkylboron building blocks with a stereogenic center. 2) The Senphos family of ligands containing a masked borataalkene motif (an iminium borataalkene embedded in the 1,4-azaborine structure) has demonstrated that new reaction selectivity in 1,3-enyne activation can be achieved that is uniquely enabled by the electronic structure of the borataalkene component of the Senphos ligands. We anticipate that the foundations provided by the fundamental synthesis, characterization, and reactivity studies of bora- and borataalkene compounds highlighted in this review will inform the development of future synthetic catalytic applications. It is also clear from the survey that more general methods/strategies for the synthesis of stable, tunable ligands with B=C units are needed to further advance this field. Additionally, a deeper understanding of the effects of B=C coordination on the fundamental reactivity of transition metals and vice versa, how the electronic structure of transition metals will influence B=C coordination, will be useful for those utilizing bora- and borataalkenes in reaction development.