Organocatalyzed Phospha‐Michael Addition: A Highly Efficient Synthesis of Customized Bis(acyl)phosphane Oxide Photoinitiators

Abstract

Addition of the P−H bond in bis(mesitoyl)phosphine, HP(COMes)₂ (BAPH), to a wide variety of activated carbon–carbon double bonds as acceptors was investigated. While this phospha‐Michael addition does not proceed in the absence of an additive or catalyst, excellent results were obtained with stoichiometric basic potassium or caesium salts. Simple amine bases can be employed in catalytic amounts, and tetramethylguanidine (TMG) in particular is an outstanding catalyst that allows the preparation of bis(acyl)phosphines, R−P(COMes)₂, under very mild conditions in excellent yields after only a short time. All phosphines RP(COMes)₂ can subsequently be oxidized to the corresponding bis(acyl)phosphane oxides, RPO(COMes)₂, a substance class belonging to the most potent photoinitiators for radical polymerizations known to date. Thus, a simple and highly atom economic method has been found that allows the preparation of a broad range of photoinitiators adapted to their specific field of application even on a large scale.

The phospha‐Michael addition of bis(mesitoyl)phosphine to a variety of activated C−C bonds was investigated. Simple amine bases were employed as catalysts, with tetramethylguanidine showing an outstanding performance and allowing the preparation of bis(acyl)phosphines under very mild conditions in excellent yields and short time. Oxidation of the bis(acyl)phosphines led to bis(acyl)phosphane oxides, a substance class belonging to the most potent photoinitiators.

Introduction

Acylphosphane oxides and especially bis(acyl)phosphane oxides (RPO(COR¹)₂; BAPOs) are highly reactive photoinitiators that are employed for a variety of radical polymerizations. ^[1] Under irradiation with visible light, these molecules decompose in Norrish‐type I reactions to give phosphonyl and acyl radicals according to RPO(COR¹)₂+hν→RP⋅O(COR¹)+OC⋅R¹. ^[2] Because these photoproducts are colorless, acylphosphane oxides are popular photoinitiators that have been applied in large‐scale industrial coating processes for more than 40 years. More recently, BAPOs were also discovered as photo‐reductants to obtain metal nanoparticles from the corresponding Cu ⁿ⁺ (n=1, 2), Ag⁺, Au⁺ or Pd²⁺ salts. ^[3] Related to this, Cu⁺ complexes were produced in a BAPO initiated photo‐reduction from Cu²⁺ precursor complexes; this allowed a photochemical variant of the azide‐alkyne click‐cycloaddition to be developed. ^[4] Given this widespread application of acylphosphane oxides, it is remarkable that only very few derivatives are commercially available; they mainly carry a phenyl group as phosphorous bound substituent R in R _n PO(COR¹)_3−n (n=1, 2; Scheme 1a) for commercially available examples).[ ^2a , ⁵ ]

Scheme 1.

a) Some examples of commercially available acylphosphane oxides as photoinitiators: Irgacure® 819, Lucirin® TPO, Lucirin® TPO−L, Li‐TPO and Irgacure® 1700. b) Tautomeric structures of BAPH (1).

The phospha‐Michael addition of RP(O)H₂ or R₂P(O)H (R=alkyl or aryl group) as Michael donors to acceptor molecules with an unsaturated carbon‐carbon bond is 100 % atom‐efficient ^[6] and an outstanding synthetic tool for P−C bond formation. ^[7] This reaction is promoted by a variety of catalysts and in many cases the addition of simple organic bases is sufficient. However, the phospha‐Michael addition with primary or secondary phosphines requires transition metal compounds–frequently platinum complexes–as catalysts or harsh reaction conditions.[ ⁸ , ⁹ , ¹⁰ ] A noteworthy exception is the reaction of lithium diphenylphosphide, LiPPh₂, with γ‐butenolides or crotonates as Michael acceptors. The phosphide salt is generated in situ in a separated reaction step by addition of n‐butyl lithium to the phosphine HPPh₂.[ ⁸ , ¹¹ , ¹² , ¹³ ]

Recently, guanidine derivatives such as tetramethylguanidine {(Me₂N)₂C=NH; TMG} were found to be especially efficient organocatalysts for the phospha‐Michael addition of P(V)−H compounds.[ ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ ] It is proposed that the guanidinium ion, [(Me₂N)₂C(NH₂)]⁺, which is generated in the catalytical cycle, behaves either as a mono‐functional (electrophilic) or a bifunctional (electrophilic and nucleophilic) catalyst (see Schemes S1 and S2 in the Supporting Information for relevant examples).[ ^7b , ^19d , ^19f , ^19g ]

While numerous examples of phospha‐Michael additions to alkenes as Michael acceptors have been reported, the use of alkynes as Michael acceptors is not well investigated and only very few examples are known. ^[8] Burdaga et al. employed dimethyl acetylenedicarboxylate as electrophile for the synthesis of vinylphosphoranes. ^[20] In 1983, Konstyanovskii et al. and more recently Trofimov et al. investigated the addition of hydrido phosphoranes, HP(O∩O)₂ (O∩O=glycolate) or phosphines, R₂PH, to propionitrile ^[21] or cyanoacetylenes. ^[22] In cases where the phospha‐Michael addition was successful, the corresponding vinyl organophosphorus compound was formed with Z‐stereoselectivity.

Bis(acyl)phosphine, BAPH 1, (Scheme 1b) is easily obtained on large scale from [Na(PH₂)×(Na(OtBu)_2.4] and MesCOCl. ^[23] In a 4 : 1 mixture of 2‐methoxyethanol and water, 1 is rather acidic [pK _a=5.8] ^[24] and exists in form of two tautomers HP(COMes)₂ A [δ(³¹P)=4.4 ppm] and MesCO−P=C(OH)Mes B [δ(³¹P)=91.4 ppm], which are in a solvent dependent equilibrium. In the crystalline state, 1 forms yellow needles which only contain tautomer B and are air stable. We observed that 1 can undergo phospha‐Michael additions with acrylates, H₂C=CR−COOR¹, as acceptors and in presence of a base under relatively mild reaction conditions.[ ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ ]

Here, we report a detailed study on the reactivity of BAPH 1, as a Michael donor, with particular focus on the optimization of the reaction conditions. As a result, an organocatalyzed addition of BAPH to various multiple bonds has been developed, which allows the easy preparation of photoinitiators that can be fine‐tuned and adapted to fulfil specific needs.

Results and Discussion

Optimization of reaction conditions

In order to investigate the scope of the organocatalyzed phospha‐Michael addition (PMA) of 1 to a wide range of multiple bonds, the reaction conditions were first optimized using methyl acrylate as a model substrate. The PMA product 2 (Scheme 2) can be easily detected by ³¹P NMR spectroscopy [δ=50.5 ppm (t, ² J _PH=12.8 Hz)] and without further purification is oxidized with H₂O₂ to give the corresponding bis(acyl)phosphane oxide 3 as yellow crystals in about 90 % yield [δ(³¹P)=24.8 ppm (m, ² J _PH=10.3 and ³ J _PH=9.9 Hz)].

Scheme 2.

Reaction scheme of the phospha‐Michael addition (PMA) of 1 to methyl acrylate followed by oxidation to BAPO (3).

As promotors, simple alkali metal carbonates M₂(CO₃) (M=Li, Na, K), salts such as KCl, KOH, M(OtBu) (M=Na, K), and amine bases such as trimethylamine, NEt₃, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), and TMG (see above) were tested. The PMAs were performed either in anhydrous ethereal solvents like 1,2‐diethoxyethane (DEE) or 1,2‐dimethoxyethane (DME) as well as in a biphasic system consisting of DEE and water (Table 1). Reactions without base or under acidic conditions were performed as blank tests. In both cases, no conversion was observed (Table 1, entries 1 and 2). The performance of M₂(CO₃) (M=Li−Cs) was studied in a biphasic DEE/water mixture. One equivalent of the carbonate was used in all reactions and as Table 1 shows there is a remarkable increase in the reaction rate in the order Li⁺≪Na⁺<K⁺≈Cs⁺ (entries 3–6). Specifically, reactions with K⁺ and Cs⁺ are about 60 times faster than with Li⁺. However, as the result in entry 7 shows, basic conditions are required (KCl does not promote the PMA). Under anhydrous conditions, the PMA reaction was carried out in DME with either 5 mol% Na(OtBu) or 5 mol% K(OtBu). While in the reaction with Na(OtBu) no conversion was observed (entry 8), the one with K(OtBu) gave about 80 % conversion after 16 h (entry 9), thus demonstrating that the PMA reaction can indeed be catalyzed by K⁺ ions. When toluene is used as a solvent, the reaction is slightly slower (60 % in 1 d, entry 10). When ten equivalents of water are added deliberately to DME as solvent and 5 mol% K(OtBu) are used as catalyst, the reaction is complete after 3 h (entry 11). Also, with 5 mol% KOH (added as 20 M aqueous solution) and ten equivalents of water the reaction runs to completion within 6 h in DME as solvent (entry 12). Remarkably, when the reaction is performed without the additional 10 equiv. of water, only 11 % conversion is reached after 24 h, thus indicating that a sufficient amount of water is essential.

Table 1.

Phospha‐Michael addition of BAPH 1 to methyl acrylate promoted by base and acid catalysts.

	Solvent	Catalyst	Conversion
1[a]	DEE/H2O	no catalyst	no conversion
2[a]	DEE/H2O	HCl	no conversion
3[a]	DEE/H2O	Li2CO3 (1 equiv.)	100 % in 5 d
4[a]	DEE/H2O	Na2CO3 (1 equiv.)	100 % in 24 h
5[a]	DEE/H2O	K2CO3 (1 equiv.)	100 % in 2 h
6[a]	DEE/H2O	Cs2CO3 (1 equiv.)	100 % in 2.5 h
7[a]	DEE/H2O	KCl (1 equiv.)	no conversion
8[b]	DME	NaOtBu (5 mol%)	no conversion
9[b]	DME	KOtBu (5 mol%)	77 % in 16 h
10[b]	Toluene	KOtBu (5 mol%)	60 % in 24 h
11[b]	DME/10 equiv. of H2O	KOtBu (5 mol%)	100 % in 3 h
12[b]	DME/10 equiv. of H2O	KOH (5 mol%, 20 M in H2O)	100 % in 6 h
13[b]	DME	NEt3 (5 mol%)	38 % in 24 h 63 % in 48 h
14[b]	DME	DBU (5 mol%)	79 % in 1 h 100 % in 2 h
15[b]	DME	TMG (5 mol%)	95 % in 45 min 100 % in 1 h
16[b]	Toluene	TMG (5 mol%)	32 % in 1 h 90 % in 24 h

[a] Biphasic system DEE/H₂O. General reaction conditions: degassed DEE (15 mL) and H₂O (3 mL), 1 (0.02 M, 1 equiv.), methyl acrylate (1 equiv.) distilled and stored under argon. [b] Dry and degassed solvent. General reaction conditions: 1 (0.3 M, 1 equiv.), 5 % mol catalyst, methyl acrylate (1 equiv.) distilled and stored under argon.

Simple commercially available amine bases were likewise tested as catalysts (5 mol%) for the PMA. Only 63 % conversion is reached with NEt₃ in DME as solvent after two days (entry 13). But the bases DBU and TMG, which both contain a N−C−N motif, give quantitative conversion within few hours (entries 14, 15); TMG being slightly more efficient. Hence, the efficiency of the amine bases as PMA catalysts follows the order NEt₃≪DBU<TMG, which correlates roughly with their Brønsted basicities [pK _a(NEt₃ in THF)=12.5, pK _a(DBU in THF)=16.9 and pK _a(TMG in THF)=15.5].[ ³⁰ , ³¹ ] Again a non‐polar solvent such as toluene slows down the reaction rate and only 32 % conversion is reached after 1 h (90 % in 1 d; entry 16).

The results listed in Table 1 allow us to conclude that an ethereal solvent and 5 mol% TMG as catalyst gives the best results in the PMA between the bis(acyl)phosphine 1 and the Michael acceptor methyl acrylate and full conversion is reached at room temperature after 1 h reaction time. Alternatively, the PMA can be efficiently catalyzed by adding KOH or K(OtBu) as catalyst and performing the reaction under basic conditions in a DME/10 equiv. of H₂O solvent system. This procedure has the advantage that the work‐up of the reaction mixture is rather simple because the catalyst remains in the aqueous phase while the product is extracted into the organic phase. In the Supporting Information, an example for a one‐pot synthesis is given where, starting from elemental phosphorus and sodium, the photoinitiator 3 is prepared in over 70 % overall yield on a multigram scale (110 g, 0.257 mol).

Substrate scope

Having optimized reaction conditions at hand, various functionalized alkenes were tested as substrates in the PMA. Selected examples are listed in Table 2. ^[24] In all cases, full conversion was reached when BAPH 1 was reacted with an alkene in presence of TMG as catalyst. The primary reaction products, namely the bis(acyl)phosphines (BAPs) can be isolated but are conveniently oxygenated with aqueous hydrogen peroxide in a one‐pot procedure to give the corresponding bis(acyl)phosphane oxides as green‐yellow substances. All compounds are light‐sensitive but otherwise stable against moisture and air. With α,β‐unsaturated carbonyl compounds, the reaction is stereospecific and exclusively the terminal carbon center of the polarized RHC^δ+=C^δ−−FG double bond is attacked by the phosphorus center (R=H, Me; FG=functional group). As expected, the reaction is sensitive to steric effects. Substituents either at the α‐ or β‐carbon center of the C=C unit slow down the reaction rate (see entries 1–3 for examples). Very short reaction times (15 min) are reached with highly activated C=C double bonds such as in itaconic anhydride (entry 6) or methylenedihydrofuran‐2,5‐dione or maleinimide (entry 11). With hydrogen peroxide as oxidation agent the ring‐opened β,γ‐bis(carboxyl) derivative 13 is obtained while with oxygen as oxidation agent under anhydrous conditions the cyclic anhydride 14 is formed. Apart from a variety of α,β‐unsaturated carbonyl compounds such as acroleins, acids, esters, and amides (see entries 1–13), also other activated alkenes such as acrylonitrile (entry 14), vinyl sulfones (entries 15 and 16), vinyl phosphonates (entry 18), and even vinyl pyridine (entry 17) can be used. In all cases, 10 mol% TMG was employed as catalyst and the reactions were run at room temperature. Especially noteworthy is the possibility to prepare multiple photoinitiators in an atom‐economic and simple way (entries 12, 13 and 16). These compounds have proven to be especially useful because they do not act only as initiators for radical polymerizations but also as crosslinkers that allowed materials with unique properties to be prepared. ^[27]

Table 2.

Synthesis of BAPO photoinitiators by phospha‐Michael addition of BAPH to various Michael acceptors followed by oxidation. General reaction conditions: dry and degassed DME, degassed Michael acceptor, 10 mol% TMG catalyst and, if not otherwise stated, room temperature. BAPs were, if not otherwise stated, oxidized with 30 % (w/w) H₂O₂ (aq). Isolated yields are given with respect to the BAP, yields marked with * are determined via ³¹P{¹H} NMR.

	Michael acceptor	Obtained BAPO respectively yield	Full conversion time to BAP
1			30 min
2			2 h[a]
3			3 h
4			6 h
5			1 h
6			15 min
7			30 min[a]
8			8 h
9			16 h[a]
10			6 d[a]
11			15 min[a]
12			5 d
13			1 h
14			18 h
15			1 h
16			1 h
17			2 h[a]
18			10 h at 100 °C with 1 equivalent of K2CO3 in 5 : 1 DEE/H2O

[a] Tert‐butyl hydroperoxide was used as oxidant to oxidize the BAP to BAPO.

Only with vinyl phosphonate as Michael acceptor the conditions needed to be modified. Very slow conversion even at elevated temperature was observed with TMG as catalyst. But with 1 equivalent of K₂(CO₃) and heating the reaction mixture up to 100 °C, almost quantitative conversion was also reached in this case (entry 18).

The addition of P−H bonds across a C≡C triple bond has rarely been investigated.[ ²¹ , ²² , ³² ] The reaction between BAPH 1 and 3‐phenylpropiolic acid methyl ester, which contains a polarized Ph−C^δ+≡C^δ−−COOMe triple bond as acceptor (Scheme 3), was investigated and optimized under various reaction conditions (Table 3).

Scheme 3.

Phospha‐Michael addition of BAPH (1) to 3‐phenylpropiolic acid methyl ester promoted by bases followed by oxidation to BAPO 40.

Table 3.

Phospha‐Michael addition of BAPH 1 to 3‐phenylpropiolic acid methyl ester promoted by bases. General reaction conditions: degassed solvent, 1 (0.2 M, 1 equiv.), base additive, 3‐phenylpropiolic acid methyl ester (1 equiv.).

	Solvent	Base	Conversions[a]
1	DME	TMG (5 mol%)	20 % in 24 h[b]
2	DEE/H2O	1 equiv. of TMG	77 % in 5 h[b]
3	DEE/H2O	1 equiv. of K2CO3	90 % in 5 h[b]
4	DEE/H2O	1 equiv. of Cs2CO3	100 % in 5 h[b]

[a] Calculated by integration of the signals of the reagent and product in ³¹P NMR. [b] Product obtained as 65 : 35 ratio of E/Z isomers.

In this case as well, with 5 % TMG as catalyst and DME as solvent, the reaction is slow (entry 1). But performing the PMA in DEE/H₂O significantly increases the rate by about 20 times (entry 2). Performing the PMA in presence of one equivalent of K₂(CO₃) or especially Cs₂(CO₃), leads to quantitative conversion within 5 h (entries 3 and 4). The PMA is regioselective, and the P(COMes)₂ group is added in the α‐position to the phenyl substituent exclusively to give vinyl phosphines (E)‐39 and (Z)‐39 as a mixture of E/Z isomers in a ratio of about 65 : 35 (determined by ³¹P NMR spectroscopy taken of an aliquot of the crude reaction mixture [(E)‐39: ³¹P NMR: δ=65.1 ppm (d, ³ J _PH=11 Hz); (Z)‐39: ³¹P NMR: δ=68.9 ppm (d, ³ J _PH=24 Hz)] (Figure S4). The crude mixture containing (E)‐39/(Z)‐39 was oxidized using an aqueous 30 % H₂O₂ solution. The oxidation reaction proceeded slowly at room temperature to furnish a mixture of the corresponding BAPOs (E)‐40/(Z)‐40 identified by ³¹P NMR [(E)‐40: ³¹P NMR: δ=8.8 ppm (d, ³ J _PH=18 Hz); (Z)‐40: ³¹P NMR: δ=18.3 ppm (d, ³ J _PH=31 Hz)] (Figure S10). Yellow single crystals suitable for X‐ray diffraction experiments of the major stereoisomers, that is bis(acyl)phosphine (E)‐39 and the corresponding phosphorus oxide (E)‐40, were obtained and plots of the structures are shown in Figure 1.

Figure 1.

Left: ORTEP plot of the experimentally determined structure of (E)‐39. Displacement ellipsoids are depicted at 50 % probability. Hydrogen atoms (apart from the one of the alkene moiety) are omitted for clarity. Selected bond lengths [Å] and angles [°]: P1−C1 1.882(1), P1−C11 1.875(1), P1−C21 1.838(1), C1−O1 1.216(2), C11−O2 1.212(2), C21−C22 1.344(2), O1−C1−C11−O2 17.6(2). Right: ORTEP plot of the experimentally determined structure of (E)‐40. Displacement ellipsoids are depicted at 50 % probability. Hydrogen atoms (apart from the one of the alkene moiety) are omitted for clarity. Selected bond lengths [Å] and angles [°]: P1−C1 1.881(2), P1−C11 1.895(2), P1−C21 1.823(2), P1−O3 1.483(2), C1−O1 1.213(3), C11−O2 1.209(3), C21−C22 1.339(3), O1−C1−P1−O3 171.83(15), O2−C11−P1−O3 79.39(19), O1−C1−C11−O2 −82.8(3).

The P−C bond lengths in phosphine (E)‐39 (1.865 Å) are at the upper limit of bonds between a P^III center and a carbon atom in a sp² valence electron configuration (1.836 Å). ^[33] Surprisingly, these bond lengths do not change much upon oxidation and in the vinyl BAPO derivative (E)‐40 all P−C bonds are unusually long, especially the P−C_acyl bonds P1−C1 and P1−C11 (1.881 and 1.895 Å). While bond distances between P^V and C(sp²) are usually below 1.8 Å. Such long P−C distances have also been observed for other BAPO derivatives.[ ²³ , ³⁴ ] One of the mesitoyl substituents in (E)‐40 shows an almost anti‐planar arrangement of the C=O unit with respect to the P=O group (O1−C1−P1−O3 171.83°) while the other COMes group resides in a gauche‐like conformation (O2−C11−P1−O3 79.39°). The O−C−C−O torsion angle significantly changes upon oxidation, from 17.6° in (E)‐39 to −82.8° in (E)‐40. Thus, the acyl substituents in (E)‐40 are in an almost orthogonal alignment to each other. These torsion angles are comparable with the structure parameters of previously characterized BAPOs with torsions of 176.0° and 170.6°, −88.6° and −102.6° (O−C−P−O) and 80.1° and 81.7° (O−C−C−O). ^[23] Similarly to the commercially available Irgacure® 819, both acyl substituents are aligned in a syn, syn conformation. ^[34]

The photoactivity of BAPO (E)‐40 was tested in preliminary reactions. Upon irradiation with light of λ=405 nm in pure methyl methacrylate, instantaneous polymerization occurred. Photobleaching was investigated by irradiating a solution of (E)‐40 in [D₆]benzene. After 10 min the ³¹P resonance signal of (E)‐40 has disappeared completely to give several products which were not further investigated. A detailed investigation of the optical properties and efficiency as photoinitiator of (E)‐40 is beyond the scope of this report but these results indicate that (E)‐40 behaves very likely comparable to other BAPO photoinitiators.

Structures of alkali metal and ammonium bisacylphosphides

In order to gain some insight into the reactivity trends observed in the reactions with the alkali metal carbonates M₂CO₃ (M=Li, Na, K, Cs) and the amines NEt₃ and TMG, the salts M[P(COMes)₂], [M(BAP)] (41: M=Li, 42: M=Na, 43: M=K, 44: M=Cs), [HNEt₃](BAP) 45 and [H₂NC(NMe₂)₂](BAP) 46 were prepared. These compounds are simply made in quantitative yields either by reacting the phosphine BAPH with one equivalent of an alkali metal tert‐butoxide, M(OtBu) (M=Li, Na, K), or Cs₂(CO₃) (Scheme 4), or the amines (see the Supporting Information for details). Single crystals suitable for X‐ray diffraction experiments were obtained in all cases (Table 4).

Scheme 4.

Synthesis of M(BAP) salts (M=Li (41), Na (42), K (43), Cs (44)) from 1 in DME.

In [D₈]THF as solvent, all species show singlet resonances in the ³¹P NMR spectra at δ=86.0 (41), 81.1 (42), 80.8 (43), 80.1 (44), 85.0 (45), 80.2 (46) ppm (Figures S15, S19, S22, S25 and S28). The structure of Na(BAP) 42 was previously determined and discussed. ^[23] Both compounds, 41 and 42, contain mesitoyl groups in the cis position such that six‐membered M−O‐C−P−C−O rings reminiscent of metal acetylacetonate complexes, [M(acac)], are formed. These MO₂C₂P rings aggregate to form dimers in the solid state with a folded central M₂O₂ ring [fold angle θ=40.6(4)° (41), θ=57.8(5)° (42); see Figure 2a for a structure plot of 41].

Figure 2.

ORTEP plots of the solid‐state structures of M(BAP) salts. Displacement ellipsoids are depicted at 50 % probability. Hydrogen atoms (apart the ones involved in hydrogen bonding) are omitted for clarity. For selected bond lengths and angles, see Table 4. a) Lithium bis(mesitoyl)phosphide Li(BAP)×0.5 DME (41×0.5 DME). One co‐crystallized DME molecule is omitted for clarity. b) Potassium bis(mesitoyl)phosphide K(BAP)×0.5 DME (43×0.5 DME). One co‐crystallized DME molecule is omitted for clarity. c) Caesium bis(mesitoyl)phosphide Cs(BAP)×0.5 DME (44×0.5 DME). One co‐crystallized DME molecule is omitted for clarity. d) Triethylammonium bis(mesitoyl)phosphide [HNEt₃](BAP) (45), one of the possible HNEt₃ ⁺ orientations is depicted. e) Tetramethylguanidinium bis(mesitoyl)phosphide [TMGH](BAP) (46). Two co‐crystallized acetone molecules are omitted for clarity.

While in the sodium salt 42 the coordination at the Na⁺ cations is completed by oxygen centers from coordinated THF and DME molecules such that either a trigonal‐bipyramidal coordination or a severely distorted octahedral sphere is obtained, one DME molecule bridges the two Li⁺ ions in 41 such that a tetrahedral coordination sphere at each Li⁺ is obtained.

In the structures of K(BAP) 43 and Cs(BAP) 44, six‐membered MO₂C₂P are also observed and these likewise form dimeric units with folded M₂O₂ rings [θ=47.38(6)° (43), θ=59.98(6)° (44)]. But these dimeric units further aggregate such that three M₂O₂ rings are obtained which form a zigzag arrangement (staircase architecture) in which only the two terminal K⁺ or Cs⁺ ions are further coordinated by a DME molecule. The central planar M₂O₂ ring is planar and contains a center of symmetry (Figure 2b, c).

In [HNEt₃](BAP) (45), the triethylammonium cation interacts slightly asymmetrically with only one BAP unit through two long hydrogen bonds of different length with each of the oxygen atoms of the two carbonyl units [d(O1⋅⋅⋅H) ca. 2.2 Å; d(O2⋅⋅⋅H) 2.0 Å] (Figure 2d). This structural feature is also observed in the high temperature (298 K) solid state structure of BAPH 1 ^[24] while in the low temperature structure (100 K) the H atom is located on the C ₂ axis of the molecule and hence the O⋅⋅⋅H⋅⋅⋅O bridge is symmetric (O−H 1.22 Å). ^[35] In the solid state, the TMG salt 46 forms a dimeric structure (Figure 2e). Notably, each of the two [TMGH]⁺ ions bridges two BAP⁻ units via hydrogen bridges between the NH₂ group and the carbonyl oxygen centers such that a coplanar arrangement of the [TMGH]⁺ cations is achieved [d(N−H⋅⋅⋅O1), d(N−H⋅⋅⋅O2), d(N−H⋅⋅⋅O3) and d(N−H⋅⋅⋅O4) ca. 2.0 Å]).

The BAP⁻ anion shows very similar metric parameters in all compounds. The P−C bonds are slightly shorter than standard σ²‐λ ³ P−C bonds (1.84 Å) ^[33] and range in between 1.78 and 1.82 Å indicating – in combination with the elongated C−O bonds (ca. 1.25 Å; cf 1.21 Å in ketones) ^[33] – some delocalization of the negative charge within the almost planar O−C−P−C−O moieties (O−C−C−O torsion angles ≤11°). The C−P−C angle varies very little between 102.7° and 103.4°. Note that in all compounds 41–46, from the three rotamers, which can be defined with respect of the mutual orientations of the C=O groups as E,E, Z,E, and Z,Z isomers, only the one with the C=O units in Z,Z conformation (i. e., cis orientation of the C=O groups, see below Figure 3) is observed in the solid state. In the computational part it will be shown that different conformations might prevail in solution.

Figure 3.

a) Possible isomers of M(BAP) salts (M=Li, Na, K, Cs). b) NCI plot of the [TMGH](BAP) ion pair.

Calculations

In order to get a deeper insight into the reaction mechanism, the phospha‐Michael addition between BAPH as donor and methyl acrylate as acceptor was calculated by using different methods [ωB97X‐D and M06‐2X functionals, second‐order Møller‐Plesset (MP2) method, 6‐311G** basis set]. Solvent effects were simulated by using the polarizable continuum model (PCM). As the results at different levels of theory show the same trends, we only discuss the findings obtained with the ωB97X‐D/6‐311G** (PCM) method.

For BAPH several isomers were optimized (Table S1), and the lowest‐energy conformer of the phosphine form HP(COMes)₂ was found to be slightly more stable than the tautomer with an intramolecular O⋅⋅⋅H hydrogen bond (d(O⋅⋅⋅H)=1.556 Å, ΔG=1.7 kcal mol⁻¹). This is in agreement with the observation that both tautomeric forms coexist in solution and are detected by NMR spectroscopy.

The relative energies of possible rotational isomers of the close ion pair [K(dme)][P(COMes)₂], [K(dme)(BAP)], were calculated. These are shown in Figure 3 with their denotation. In contrast to the solid‐state structure 43, the E,E conformation of the BAP⁻ is the most stable, although the energy differences to the other isomers (≤2.4 kcal mol⁻¹) are within the accuracy of the method used in the calculation. The same observation is made for the different rotamers of BAP⁻ in the guanidium salt (E,Z<E,E<Z,Z; ΔE=0.7 kcal mol⁻¹; see the Supporting Information for details). This is due to the fact that non‐negligible attractive dispersion forces exist between the mesityl substituents in a parallel arrangement (see the NCI plot in Figure 3b) and compensate for attractive interactions between K⁺ (or hydrogen bonds in case of TMGH⁺) and the chelating C=O units in BAP⁻ in an E,E conformation. These results show that BAP⁻ salts likely exhibit a very high conformational flexibility in solution.

Based on this flexibility of BAPH and the ion pairs containing BAP⁻, many possible minimum‐energy reaction pathways (MERPs) exist, of which we have calculated several (see Tables S6–S9 and Figures S29–S31 for details). Figure 4 shows the MERP (given in ΔG) with the lowest overall activation barrier (ΔG ^#=14.2 kcal mol⁻¹), which was found. Note that the energy differences to other possible MERPs are small.

Figure 4.

The most favorable MERP (pathway 1) for the reaction of BAPH and H₂C=CH‐COOMe (methyl acrylate) catalyzed by TMG (C: black, O: red, P: magenta, N: blue, H: gray). The relative Gibbs free energies refer to the three separated starting materials and were obtained at the ωB97X‐D/6‐311G** (PCM=Et₂O) level of theory. Hydrogen atoms (except for the ones that are directly involved in the reaction) have been omitted for clarity. Inset: NCI plot of TS2.

In the first reaction step, BAPH is deprotonated by the organocatalyst TMG resulting in the ion pair (IP) which contains the guanidinium cation, [TMGH]⁺, and the delocalized phosphide anion, BAP⁻. This step was calculated both in the presence and in the absence of methyl acrylate, MA, and it was found that the additional Michael acceptor MA does not affect the activation barrier. It is statistically not likely that three reactants meet at the same time (for entropy reasons), and therefore the possibility that TMG, BAPH, and MA interact simultaneously can be ruled out. Instead, an equilibrium according to BAPH+TMG⇆[TMGH](BAP) (IP) is assumed, which has a low activation barrier of only ΔG ^≠=6.1 kcal mol⁻¹ (TS1). Consequently, this equilibrium state is the starting point for the PMA.

In the second step of the reaction, on the way to the activated complex at TS2, MA may interact with IP to give the reactant complex RC, in which the terminal carbon center of the activated C=C double bond is held in proximity to the nucleophilic phosphorus center by hydrogen bonds between the NH₂ group of [TMGH]⁺, the P center of BAP⁻ [d(P⋅⋅⋅H) =2.536 Å], and the C=O unit of methyl acrylate [d(O⋅⋅⋅H)=1.871 Å]. An atom‐in‐molecule (AIM) calculation shows bond critical points on the paths for all these hydrogen bonds (see the Supporting Information for details). Note that this state shows resemblance to the solid‐state structure of [TMGH](BAP) 46. Passing the overall barrier at TS2 of ΔG ^≠=14.2 kcal mol⁻¹ is the most energy requiring step on the MERP (Figure 4) and assumed to be the rate determining step that leads to the intermediate IM, in which the P−C bond has been formed and the anionic charge is delocalized within the terminal −C⁻H−C(=O)OMe⇄−CH=C(O⁻)(OMe) unit. The guanidinium counter‐cation remains bonded via a NH⋅⋅⋅O hydrogen bond to the carbonyl unit of the methyl ester moiety [(d(O⋅⋅⋅H)=1.750 Å]. In the next step, the carbanion is protonated by the guanidinium cation, a process with a very low energy barrier of ΔG ^≠=1.8 kcal mol⁻¹ at TS3 (relative to IM), leading to the product complex PC. In the final step, TMG dissociates to give the product in an overall exergonic reaction (ΔG=−15.6 kcal mol⁻¹). In addition to the reaction catalyzed by TMG, the PMAs with [K(dme)]⁺ or NMe₃ as catalysts were inspected (see Figures S48 and S49 for details). While the overall course of these reactions is similar to the one catalyzed by TMG, higher barriers for the phospha‐Michael addition step by 0.8 or 6.4 kcal mol⁻¹, respectively, were calculated: [K(dme)]⁺: ΔG ^≠=15.0 kcal mol⁻¹; NMe₃: ΔG ^≠=20.6 kcal mol⁻¹, for details see the Supporting Information. These results are consistent with the experiments that show an increase in reaction rate following the order TMG>K⁺≫NEt₃. The difference between the activation barriers in the PMA with TMG or [K(dme)]⁺ is rather small and within the accuracy of the employed theoretical method. Nevertheless, a plausible explanation for the slower reaction with potassium cations can be given. The formation of the Michael adduct IM is rather endergonic and in the absence of a proton donor may be reversible. With TMG as catalyst, the protonation proceeds intra‐molecularly and is therefore straightforward. However with [K(dme)]⁺ as catalyst, an additional proton source is needed. This is in line with the observed acceleration of the reaction rate by the addition of water. Furthermore, the low activation barrier calculated with TMG as catalyst is due to the presence of dispersion interactions between the methyl groups in [TMGH]⁺ and the BAP⁻ and MA units as shown in the NCI plot of the activated complex at TS2 (Figure 4, inset). The relatively high activation barrier computed for the PMA with NMe₃ is simply due to the fact that this catalyst forms only one H‐bond, which does not allow an efficient alignment for the nucleophilic attack of the phosphorus center in BAP⁻ onto the terminal carbon atom of MA.

Finally, calculations were performed for the addition of BAPH to the C≡C triple bond of the β‐carbon in 3‐phenylpropiolic acid catalyzed by TMG. The activation barrier (ΔG ^≠=16.9 kcal mol⁻¹) of this reaction is higher (ΔΔG ^≠=2.7 kcal mol⁻¹) compared to the reaction with MA as substrate. Again, this result is in good agreement with the experimental observations, that is longer reaction times are observed with an alkyne as Michael acceptor.

Conclusions

As stated in the introduction, BAPOs are among the most efficient photoinitiators known, achieving quantum yields in the α‐cleavage process up to Φ _α=0.7, ^[36] curing speeds in thin films of up to 80 m min⁻¹, ^[37] and curing depths of up to 3.6 mm ^[38] at low weight concentrations (typically 1–3 % w/w), under irradiation with low‐energy light at about 400 nm. Therefore, they are key components for photopolymerization processes, especially in 3D printing, and very likely will become even more essential in the future .[ ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ ] Specifically, in the fabrication of medical devices and implants, acylphosphane oxides may play a prominent role.[ ⁴⁴ , ⁴⁵ , ⁴⁶ ] Given the broad range of possible applications of acylphosphane oxides as photoactive components, it is surprising that only few derivatives have been synthesized or commercialized. The phospha‐Michael addition (PMA) between bis(mesitoyl)phosphine, BAPH, and a number of activated olefins can be promoted either by potassium or caesium salts under basic conditions in a biphasic ether/water solvent system. Alternatively, the PMA can be catalyzed, especially by tetramethylguanidine (TMG), under anhydrous conditions; this is especially noteworthy because in most cases the addition of a P−H bond in a phosphine requires either the use of a noble metal catalyst or harsh reaction conditions. Under all the conditions reported here, high, or even excellent yields were obtained, and the reaction conditions are applicable for large‐scale preparations. DFT calculations were used in order to reveal possible reaction mechanisms. In particular, the reaction catalyzed by TMG shows the typical characteristics of an organocatalyzed reaction in which substrate and reagent are hold in close proximity by hydrogen bridges. This insight should allow catalysts of even higher efficiency to be developed. This is especially important for the development of BAPOs applicable in biomedical formulations where all possible contaminations must be reduced to a minimum (in particular, the photoinitiator must not itself be toxic).[ ⁴⁷ , ⁴⁸ ] The PMA reaction is a highly atom‐economical way to generate a phosphorus–carbon bond and has here been optimized and exploited for the synthesis of BAPOs, which can now be designed to suit the demands of an envisioned photopolymerization process.

Experimental Section

All air‐ or moisture‐sensitive reactions were carried out under dry argon using either standard Schlenk techniques or working inside a glovebox (M‐Braun Lab‐Master MB 150 B−G). The glassware is stored in an oven at 130 °C for at least 16 h and cooled under vacuum prior to use. Solvents are purified using an Innovative Technology PureSolv MD 7 solvent purification system and stocked over activated molecular sieves. When degassed solvents were used, nitrogen was bubbled for a minimum of 15 min. Deuterated solvents are purchased from Cambridge Isotope Laboratories. [D₈]THF and [D₆]benzene are distilled from sodium benzophenone, whereas [D₂]DCM is dried over CaH₂ before use. All reagents are used as received from commercial suppliers unless otherwise stated. Air sensitive compounds are handled and stored in a M‐Braun Lab‐Master MB 150 B−G glovebox. Light sensitive compounds are handled with exclusion of light.

NMR spectra were recorded on Bruker Avance 200, 250, 300, 400 and 500 spectrometers operating at room temperature if not otherwise specified. Chemical shifts δ were measured according to IUPAC and are given in parts per million (ppm) relative to TMS (¹H NMR and ¹³C NMR) and 85 % H₃PO₄ in D₂O (³¹P NMR). Absolute values of coupling constants J are given in Hertz (Hz).

Single‐crystal diffraction experiments are performed on a Rigaku Oxford Diffraction XCalibur S (Molybdenum X‐ray source λ=0.71 Å), Rigaku Oxford Diffraction Synergy S (molybdenum X‐ray source λ=0.71 Å; copper X‐ray source λ=1.54 Å), Bruker D8 Venture (molybdenum X‐ray source λ=0.71 Å; copper X‐ray source λ=1.54 Å), Bruker Smart APEX II (molybdenum X‐ray source λ=0.71 Å) and Bruker Smart APEX (molybdenum X‐ray source λ=0.71 Å). The data are processed using the corresponding diffractometer software (CrysAlisPro 40_64.16b, CrysAlisPro 41_64.110a or APEX3), and are corrected for absorption with a multi‐scan method. The structures are solved with the Olex2‐1.3 software using the ShelXS or ShelXL method and refined using the ShelXL method. The plots of the structures are generated using Ortep3 and the thermal ellipsoids are shown at 50 % probability level, unless otherwise stated.

Elemental analyses were carried out by the Micro‐Laboratory of the Organic Chemistry Department (D‐CHAB, ETH Zürich).

Methyl‐3‐(bis(mesitoyl)phosphaneyl)propanoate (2)

Phospha‐Michael addition in DME or toluene: BAPH (0.50 g, 1 equiv., 1.53 mmol) was loaded into a 50 mL Schlenk flask under an inert argon atmosphere, followed by 5 mL of the dry solvent. The corresponding catalyst was added (0.05 equiv., 0.08 mmol, 5 mol%), followed by methyl acrylate (0.14 mL, 1 equiv., 1.53 mmol). The progress of the reaction was followed by ³¹P NMR, and the NMR sample was put back into the reaction flask after the measurement.

Phospha‐Michael addition in DEE/H₂O: BAPH (0.10 g, 1 equiv., 0.30 mmol) was loaded into a 50 mL Schlenk flask under an inert argon atmosphere. 15 mL of degassed DEE were added, followed by 3 mL of degassed water. The corresponding amount of base (1 equiv.) was added under stirring at room temperature, followed by methyl acrylate (28 μL, 1 equiv., 0.30 mmol). The the progress of the reaction was followed by ³¹P NMR, and the NMR sample was put back into the reaction flask after the measurement. ¹H NMR (300 MHz, [D₈]THF): δ (ppm): 6.82 (s, 4H, H _{arom, Mes},), 3.60 (s, 3H, OCH₃ ), 2.49 (m, 2H, −CH₂ −), 2.31 (m, 2H, −CH₂ −), 2.24 (s, 6H, p‐CH₃ ), 2.18 (s, 12H, o‐CH₃ ). ¹³C{¹H} NMR (76 MHz, [D₈]THF): δ (ppm): 214.1 (d, ¹ J _CP=48 Hz, P−C=O), 170.1 (d, ³ J _CP=11 Hz, MeO−C=O), 137.3 (s, C ⁴ Mes), 137.1 (d, ² J _CP=28 Hz, C ¹ Mes), 131.5 (s, C ^2,6 Mes), 126.8 (s, C ^3,5 Mes), 48.8 (s, OCH₃), 28.6 (d, ² J _CP=18 Hz, −CH₂−(CO)−OMe), 18.3 (s, p‐CH₃), 17.5 (s, o‐CH₃), 14.0 (d, ¹ J _CP=8 Hz, −CH₂−P). ³¹P NMR (122 MHz, [D₈]THF): δ (ppm): 50.5 (t, ² J _PH=13 Hz, PR₃). T _m=69 °C.

Methyl‐3‐(bis(mesitoyl)phosphaneyl)‐3‐phenylacrylate (39)

Phospha‐Michael addition in DME: BAPH (350 mg, 1 equiv., 1.07 mmol) was loaded into a 50 mL Schlenk flask under an inert argon atmosphere. Degassed DME (5 mL) was added, followed by TMG (7 μL, 0.05 equiv., 0.05 mmol) and 3‐phenylpropiolic acid methyl ester (160 μL, 1 equiv., 1.07 mmol). The progress of the reaction was followed by ³¹P NMR, and the NMR sample was put back into the reaction flask after the measurement.

Phospha‐Michael addition in DEE/H₂O: BAPH (1.20 g, 1 equiv., 3.68 mmol) was loaded into a 50 mL Schlenk flask under an inert argon atmosphere. Degassed DEE (15 mL) was added, followed by 3 mL of degassed water. The corresponding base (1 equiv. or 0.05 equiv. depending on the performed test) was added followed by 3‐phenylpropiolic acid methyl ester (549 μL, 1 equiv., 3.68 mmol). The the progress of the reaction was followed by ³¹P NMR, and the NMR sample was put back into the reaction flask after measurement.

The trans isomer of (E)‐39 was isolated as single crystals (400 mg, 22 %) suitable for X‐ray analysis by separating the organic phase and layering it with hexane at 5 °C. The NMR spectra were recorded using the collected crystals.

An E/Z mixture of the products can be obtained in quantitative yield as an oily bright yellow residue by evaporating the solvent after the reaction completion. Without any further purification (like the above‐described crystallization) the E/Z ratio was 65 : 35.

E isomer (E)‐39 : ¹H NMR (500 MHz, CDCl₃): δ (ppm): 7.19 (d, ³ J _HH=8 Hz, 1H, H _{arom, Ph}), 7.10 (t, ³ J _HH=8 Hz, 2H, H _{arom, Ph}), 6.88 (d, ³ J _HH=8 Hz, 2H, H _{arom, Ph}), 6.73 (d, ³ J _PH=10 Hz, 1H, C=C−H), 6.67 (s, 4H, H _{arom, Mes}), 3.52 (s, 3H, OCH₃ ), 2.24 (s, 6H, p‐CH₃ ), 2.10 (s, 12H, o‐CH₃ ). ¹³C{¹H} NMR (126 MHz, CDCl₃): δ (ppm): 214.5 (d, ¹ J _CP=50 Hz, P−C=O), 165.0 (d, ³ J _CP=14 Hz, MeO−C=O), 147.5 (d, ¹ J _CP=22 Hz, C=C), 139.7 (s, C ⁴ Mes), 139.1 (d, ² J _CP=29 Hz, C ¹ Mes), 138.4 (d, ² J _CP=12 Hz, C¹ Ph), 136.1 (s, C⁴ Ph), 133.6 (s, C ^2,6 Mes), 131.9 (d, ² J _CP=20 Hz, C=C), 128.9 (s, C ^3,5 Mes), 128.0 (d, ³ J _CP=6 Hz, C^2,6 Ph), 127.4 (s, C^3,5 Ph), 51.5 (s, OCH₃), 21.1 (s, p‐CH₃), 19.87 (d, ⁴ J _CP=5 Hz, o‐CH₃). ³¹P NMR (203 MHz, CDCl₃): δ (ppm): 64.4 (d, ³ J _PH=10 Hz, PR₃). ³¹P{¹H} NMR (203 MHz, CDCl₃): δ (ppm): 64.4 (s, PR₃). ¹H NMR (300 MHz, [D₆]benzene): δ (ppm): 7.26‐7.18 (m, 3H, H _{arom, Ph}), 7.05‐7.00 (m, 3H, H _{arom, Ph}, C=C−H), 6.50 (s, 4H, H _{arom, Mes}), 3.19 (s, 3H, OCH₃ ), 2.23 (s, 12H, o‐CH₃ ), 2.05 (s, 6H, p‐CH₃ ). ¹³C{¹H} NMR (75 MHz, [D₆]benzene): δ (ppm): 214.4 (d, ¹ J _CP=50 Hz, P−C=O), 164.9 (d, ³ J _CP=14 Hz, MeO−C=O), 147.3 (d, ¹ J _CP=26 Hz, C=C), 140.0 (d, ² J _CP=30 Hz, C ¹ Mes), 139.6 (s, C ⁴ Mes), 139.4 (d, ² J _CP=12 Hz, C¹ Ph), 139.0 (s, C⁴ Ph), 134.0 (s, C ^2,6 Mes), 133.2 (d, ² J _CP=21 Hz, C=C), 129.2 (s, C ^3,5 Mes), 128.8 (d, ³ J _CP=6 Hz, C^2,6 Ph), 51.0 (s, OCH₃), 21.0 (s, p‐CH₃), 20.1 (d, ⁴ J _CP=5 Hz, o‐CH₃). ³¹P NMR (121 MHz, [D₆]benzene): δ (ppm): 63.8 (d, ³ J _PH=10 Hz, PR₃). ³¹P{¹H} NMR (121 MHz, [D₆]benzene): δ (ppm): 63.8 (s, PR₃).

Z isomer (Z)‐39 : ³¹P NMR (122 MHz, DEE): δ (ppm): 68.9 (d, ³ J _PH=24 Hz, PR₃). ³¹P{¹H} NMR (122 MHz, DEE): δ (ppm): 68.8 (s, PR₃). ³¹P NMR (121 MHz, [D₆]benzene): δ (ppm): 68.8 (d, ³ J _PH=24 Hz, PR₃). ³¹P{¹H} NMR (121 MHz, [D₆]benzene): δ (ppm): 68.7 (s, PR₃).

Methyl‐3‐(bis(mesitoyl)phosphoryl)‐3‐phenylacrylate (40): The crude DEE phase from the synthesis of 39 was separated, dried using anhydrous Na₂SO₄, and filtered, and the solvent was evaporated under high vacuum. The remaining yellow solid was dissolved in 10 mL of toluene. The mixture was protected from light, and hydrogen peroxide (35 % w/w in water, 1 equiv., 92 μL) was added at room temperature. The mixture was stirred for 16 h, and the solvent subsequently removed under high vacuum. Single crystals were obtained by recrystallization in ethanol (5 mL), by warming the mixture at 60 °C and cooling it down to room temperature. The crystals were suitable for X‐ray analysis, and the product, (E)‐40, was obtained in 16 % yield (90 mg). The remaining product is recovered in quantitative yield as mixture of E and Z isomers.

E isomer (E)‐40 : ¹H NMR (500 MHz, CDCl₃): δ (ppm): 7.40–7.30 (m, 4H, H _arom and C=C−H), 6.95 (d, J _HH=8 Hz, 2H, H _{arom, Ph}), 6.89 (s, 4H, H _{arom, Mes}), 3.66 (s, 3H, OCH₃ ), 2.35 (s, 6H, p‐CH₃ ), 2.24 (s, 12H, o‐CH₃ ). ¹³C{¹H} NMR (126 MHz, CDCl₃): δ (ppm): 213.0 (d, ¹ J _CP=56 Hz, P−C=O), 164.2 (d, ³ J _CP=21 Hz, MeO−C=O), 145.6 (d, ¹ J _CP=54 Hz, C=C), 141.5 (s, C ⁴ Mes), 135.8 (d, ² J _CP=44 Hz, C ¹ Mes), 135.7 (d, ² J _CP=9 Hz, C¹ Ph), 136.0 (s, C ^2,6 Mes), 132.2 (d, ² J _CP=7 Hz, C=C), 129.3 (s, C ^3,5 Mes), 128.8 (d, ³ J _CP=4 Hz, C^2,6 Ph), 128.5 (s, C⁴ Ph), 127.7 (s, C^3,5 Ph), 52.0 (s, OCH₃), 21.2 (s, p‐CH₃), 19.9 (s, o‐CH₃). ³¹P NMR (203 MHz, CDCl₃): δ (ppm): 8.3 (d, ³ J _PH=18 Hz, OPR₃). ³¹P{¹H} NMR (203 MHz, CDCl₃): δ (ppm): 8.2 (s, OPR₃).

Z isomer (Z)‐40 : ³¹P NMR (122 MHz, DEE): δ (ppm): 18.3 (d, ³ J _PH=31 Hz, OPR₃). ³¹P{¹H} NMR (122 MHz, DEE): δ (ppm): 18.2 (s, OPR₃). C, H analysis (%) for 40: calcd. C: 71.70, H: 6.22; found C: 69.17, H: 6.16.

Lithium bis(mesitoyl)phosphide, Li(BAP)×0.5 DME (41): BAPH (500 mg, 1 equiv., 1.53 mmol) was loaded into a Schlenk flask under an inert argon atmosphere and dissolved in 15 mL of dry DME. Lithium tert‐butoxide (123 mg, 1 equiv., 1.53 mmol) was added under vigorous stirring. After 1 h, the solvent was evaporated, and the Li(BAP) salt isolated as bright yellow powder in quantitative yield. A single crystal suitable for X‐ray analysis was grown from DME by slow evaporation of the solvent at room temperature. ¹H NMR (500 MHz, [D₈]THF): δ (ppm): 6.71 (s, 4H, H _{arom, Mes}), 2.31 (s, 12H, o‐CH₃ ), 2.20 (s, 6H, p‐CH₃ ). ¹³C NMR (126 MHz, [D₈]THF): δ (ppm): 236.0 (d, ¹ J _CP=91 Hz, P−C=O), 145.4 (d, ² J _CP=37 Hz, C ¹ Mes), 136.7 (s, C ⁴ Mes), 133.8 (d, ³ J _CP=1 Hz, C ^2,6 Mes), 128.7 (s, C ^3,5 Mes), 21.3 (s, p‐CH₃), 20.0 (d, ⁴ J _CP=2 Hz, o‐CH₃). ³¹P NMR (203 MHz, [D₈]THF): δ (ppm): 86.0 (s, ⁻ PR₂). ³¹P{¹H} NMR (203 MHz, [D₈]THF): δ (ppm): 86.0 (s, ⁻ PR₂).

Sodium bis(mesitoyl)phosphide, Na(BAP)×0.5 DME (42): BAPH (500 mg, 1 equiv., 1.53 mmol) was loaded into a Schlenk flask under an inert argon atmosphere and dissolved in 15 mL of dry DME. Sodium tert‐butoxide (147 mg, 1 equiv., 1.53 mmol) was added under vigorous stirring. After one hour, the solvent was evaporated, and the Na(BAP) salt isolated as bright yellow powder in quantitative yield. Single‐crystal structure and NMR characterization were in agreement with the data reported in the literature. ^[49]

Potassium bis(mesitoyl)phosphide, K(BAP)×0.5 DME (43): BAPH (500 mg, 1 equiv., 1.53 mmol) was loaded into a Schlenk flask under an inert argon atmosphere and dissolved in 15 mL of dry DME. Potassium tert‐butoxide (172 mg, 1 equiv., 1.53 mmol) was added under vigorous stirring. After 1 h, the solvent was evaporated, and the K(BAP) salt isolated as bright yellow powder in 91 % yield. A single crystal suitable for X‐ray analysis was grown from DME by slow evaporation of the solvent at room temperature. ¹H NMR (500 MHz, [D₈]THF): δ (ppm): 6.65 (s, 4H, H _{arom, Mes}), 2.32 (s, 12H, o‐CH₃ ), 2.18 (s, 6H, p‐CH₃ ). ¹³C{¹H} NMR (126 MHz, [D₈]THF): δ (ppm): 231.7 (d, ¹ J _CP=89 Hz, P−C=O), 146.8 (d, ² J _CP=40 Hz, C ¹ Mes), 135.7 (s, C ⁴ Mes), 133.8 (d, ³ J _CP=2 Hz, C ^2,6 Mes), 128.5 (s, C ^3,5 Mes), 21.3 (s, p‐CH₃), 20.1 (d, ⁴ J _CP=3 Hz, o‐CH₃). ³¹P NMR (203 MHz, [D₈]THF): δ (ppm): 80.8 (s, ⁻ PR₂). ³¹P{¹H} NMR (203 MHz, [D₈]THF): δ (ppm): 80.9 (s, ⁻ PR₂). C, H analysis (%) for K(BAP)×DME: calcd. C: 63.41, H: 7.10; found C: 63.24, H: 6.33.

Caesium bis(mesitoyl)phosphide, Cs(BAP)×0.5 DME (44): BAPH (500 mg, 1 equiv., 1.53 mmol) was loaded into a Schlenk flask under an inert argon atmosphere and dissolved in 15 mL of dry DME. Caesium carbonate (749 mg, 1.5 equiv., 2.30 mmol) was added under vigorous stirring. After one hour, the suspension was filtered under inert atmosphere using a por. 3 glass frit. The solvent of the filtrate was evaporated, and the Cs(BAP) salt was isolated as bright yellow powder in quantitative yield. A single crystal suitable for X‐ray analysis was grown from DME by slow evaporation of the solvent at room temperature. ¹H NMR (500 MHz, [D₈]THF): δ (ppm): 6.67 (s, 4H, H _{arom, Mes}), 2.31 (s, 12H, o‐CH₃ ), 2.18 (s, 6H, p‐CH₃ ). ¹³C{¹H} NMR (126 MHz, [D₈]THF): δ (ppm): 231.0 (d, ¹ J _CP=88 Hz, P−C=O), 146.5 (d, ² J _CP=40 Hz, C ¹ Mes), 135.8 (s, C ⁴ Mes), 133.8 (s, C ^2,6 Mes), 128.5 (s, C ^3,5 Mes), 21.2 (s, p‐CH₃), 20.1 (d, ⁴ J _CP=2.0 Hz, o‐CH₃). ³¹P NMR (203 MHz, [D₈]THF): δ (ppm): 80.1 (s, ⁻ PR₂). ³¹P{¹H} NMR (203 MHz, [D₈]THF): δ (ppm): 80.1 (s, PR₂). C, H analysis (%) for Cs(BAP)×0.5 DME: calcd. C: 70.02, H: 7.21; found C: 69.00, H: 6.59. C, H analysis (%) for Cs(BAP): calcd. C: 52.42, H: 4.84; found C: 52.49, H: 4.87.

Triethylammonium bis(mesitoyl)phosphide, [HNEt₃](BAP) (45): Procedure reported previously by Dr. Georgina Simone Müller. ^[50] BAPH (134 mg, 1 equiv., 0.41 mmol) was dissolved in 5 mL of dry, degassed toluene. Triethylamine (57 μL, 1 equiv., 0.41 mmol) was added, and the solution was stirred for 2 h at room temperature. The solvent was removed under reduced pressure to obtain 172 mg of the desired salt 45 as a yellow solid (98 %). Single crystals suitable for X‐ray analysis were grown from a saturated toluene solution. ¹H NMR (200 MHz, [D₈]THF): δ (ppm): 6.79 (s, 4H, H _{arom, Mes}), 2.87 (q, ³ J _HH=7 Hz, 6H, CH₂ ), 2.31 (s, 12H, o‐CH₃ ), 2.23 (s, 6H, p‐CH₃ ), 1.13 (t, ³ J _HH=7 Hz, 9H, CH₃ ). ¹³C NMR (50 MHz, [D₈]THF): δ (ppm): 233.5 (br, P−C=O), 142.1 (d, ² J _CP=30 Hz, C ¹ Mes), 138.1 (s, C ⁴ Mes), 133.9 (d, ³ J _CP=3 Hz, C ^2,6 Mes), 128.8 (s, C ^3,5 Mes), 46.8 (s, CH₂), 21.0 (s, p‐CH₃), 19.6 (d, ⁴ J _CP = 3 Hz, o‐CH₃), 10.7 (s, CH₃). ³¹P NMR (81 MHz, [D₈]THF): δ (ppm): 84.8 (s, ⁻ PR₂). ³¹P{¹H} NMR (81 MHz, [D₈]THF): δ (ppm): 85.0 (s, ⁻ PR₂).

Tetramethylguanidinium bis(mesitoyl)phosphide, [TMGH](BAP) (46): BAPH (500 mg, 1 equiv., 1.53 mmol) was loaded into a Schlenk flask under an inert argon atmosphere and dissolved in 15 mL of dry DME. Tetramethylguanidine (176 mg, 0.19 mL, 1 equiv., 1.53 mmol) was added under vigorous stirring. After 1 h, the solvent was evaporated and the [TMGH](BAP) salt was isolated as bright orange‐yellow powder in quantitative yield. A single crystal suitable for X‐ray analysis was grown from a saturated solution of dry acetone at room temperature. ¹H NMR (500 MHz, [D₈]THF): δ (ppm): 9.09 (s, 2H, −NH ₂), 6.63 (s, 4H, H _{arom, Mes}), 2.91 (s, 12H, N−CH ₃), 2.29 (s, 12H, o‐CH₃ ), 2.17 (s, 6H, p‐CH₃ ). ¹³C{¹H} NMR (126 MHz, [D₈]THF): δ (ppm): 230.7 (d, ¹ J _CP=83 Hz, P−C=O), 164.0 (s, C TMG), 147.3 (d, ² J _CP=36 Hz, C ¹ Mes), 135.4 (s, C ⁴ Mes), 133.7 (d, ³ J _CP=2 Hz, C ^2,6 Mes), 128.3 (s, C ^3,5 Mes), 39.9 (s, N−CH₃), 21.3 (s, p‐CH₃), 20.1 (d, ⁴ J _CP=2 Hz, o‐CH₃). ³¹P NMR (203 MHz, [D₈]THF): δ (ppm): 80.2 (s, ⁻ PR₂). ³¹P{¹H} NMR (203 MHz, [D₈]THF): δ (ppm): 80.2 (s, ⁻ PR₂). C, H analysis (%) for [TMGH](BAP)×acetone: calcd. C: 67.31, H: 8.47; found C: 67.20, H: 8.20.

Deposition Numbers 2181239 (for 3), 2181238 (for 11), 2181194 (for 12), 2181203 (for 13), 2181195 (for 15), 2181202 (for 20), 2181204 (for 22), 2181196 (for 23), 2181201 (for 30), 2181197 (for 33), 2181200 (for 34), 2181198 (for 35), 2181205 (for (E)‐39), 2181211 (for (E)‐40), 2181210 (for 41), 2181209 (for 43), 2181207 (for 44), 2181206 (for 45), and 2181208 (for 46) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Conflict of interest

The authors declare no conflict of interests.

Supporting information

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Supporting Information

Conti R., Widera A., Müller G., Fekete C., Thöny D., Eiler F., Benkő Z., Grützmacher H., Chem. Eur. J. 2023, 29, e202202563.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information.