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. 2024 Feb 2;146(6):4252–4259. doi: 10.1021/jacs.3c14281

Vinylic C–H Activation of Styrenes by an Iron–Aluminum Complex

Nikolaus Gorgas †,‡,*, Benedek Stadler †, Andrew J P White †, Mark R Crimmin †,*
PMCID: PMC10870711  PMID: 38303600

Abstract

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The oxidative addition of sp2 C–H bonds of alkenes to single-site transition-metal complexes is complicated by the competing π-coordination of the C=C double bond, limiting the examples of this type of reactivity and onward applications. Here, we report the C–H activation of styrenes by a well-defined bimetallic Fe–Al complex. These reactions are highly selective, resulting in the (E)-β-metalation of the alkene. For this bimetallic system, alkene binding appears to be essential for the reaction to occur. Experimental and computational insights suggest an unusual reaction pathway in which a (2 + 2) cycloaddition intermediate is directly converted into the hydrido vinyl product via an intramolecular sp2 C–H bond activation across the two metals. The key C–H cleavage step proceeds through a highly asynchronous transition state near the boundary between a concerted and a stepwise mechanism influenced by the resonance stabilization ability of the aryl substituent. The metalated alkenes can be further functionalized, which has been demonstrated by the (E)-selective phosphination of the employed styrenes.

Introduction

Our ability to selectively activate and functionalize C–H bonds in organic molecules is fundamental to countless chemical processes.1 Despite notable advances in this field,2,3 strategies for the selective functionalization of sp2 C–H bonds of alkenes are underdeveloped.4−12 This limitation can be traced back to fundamental selectivity issues that emerge in the reaction of alkenes with single-site transition-metal complexes. π-Coordination of the alkene to the metal is often kinetically accessible and nonreversible (Figure 1). The dominance of this pathway can effectively inhibit available mechanisms for vinylic C–H activation, including oxidative addition.13 This selectivity contrasts the rich chemistry of aromatic sp2 C–H or aliphatic sp3 C–H bonds where substrate binding (π-coordination or σ-complex formation) is typically reversible and a prerequisite for C–H bond breaking by oxidative addition.14

Figure 1.

Figure 1

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Single site vs bimetallic reactivity in vinylic C–H activations.

For example, Bergman and co-workers have studied the reaction of ethylene with the 16-electron reaction intermediate [IrCp*(PMe)3]. They found that π-complexation of the alkene is thermodynamically favored with respect to the oxidative addition of vinylic sp2 C–H bonds. Moreover, the π-complex was found to be not an intermediate in the lowest energy C–H activation process.3,4 Computational studies support the conclusions and suggest that π-coordination and C–H activation of the alkene are separate and competitive pathways.15−18

We recently reported a well-defined Fe–Al complex (1) that is capable of selectively breaking the sp2 and sp3 C–H bonds of pyridine substrates as well as acetonitrile.19−21 Herein, we present C–H activation in the vinylic position of styrenes using the same Fe–Al system. These reactions are highly selective, resulting in a rare (E)-β-metalation of the alkenes. In contrast to single-site systems, alkene binding appears to initiate C–H activation and is essential for the reaction to take place. An unusual reaction pathway in which a (2 + 2) cycloaddition intermediate is directly converted into the hydrido vinyl product is proposed. This new mechanism results in the net oxidative addition of an alkenyl sp2 C–H bond across the two metal centers and opens up new possibilities for selective alkene functionalization by C–H activation using a bimetallic approach.22−26

Results and Discussion

Nonreversible (2 + 2) Alkyne Binding

Addition of 1 equiv of a terminal alkyne RC≡CH (R = Ph, SiMe3 or n-Bu, 2-Py) to a solution of 1 in C6D6 at room temperature resulted in an immediate color change from dark to bright orange, in each case leading to the quantitative formation of the cycloaddition products 2a–d (Figure 2). These reactions appear to be nonreversible. 2a–d were all isolated in yields of >95% and are stable in both solution and in the solid state. 2b was characterized by single-crystal X-ray diffraction (Figure 6a).

Figure 2.

Figure 2

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Nonreversible addition of terminal alkynes to 1.

Figure 6.

Figure 6

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X-ray structures of 2b (a), 3c (b) 4a (c), and 5e (d). Bond lengths are given in Å.

2a–d all give rise to very similar and characteristic NMR signals for the coordination of the alkyne. For example, the 31P{1H} NMR spectrum of 2a exhibits a mutually coupled spin system comprising a doublet at δP = 29.0 ppm (2P) and a triplet resonance at 19.9 ppm (1P), consistent with the chemical nonequivalence of the axial and equatorial phosphine ligands. In the 1H NMR spectrum, the bridging hydrides appear as a broadened virtual triplet at δH = −14.71 ppm, and a doublet of triplets at δH = 10.02 ppm (3JHP = 13.4 and 5.6 Hz) can be found for the ArC≡CH proton of the coordinated alkyne.27

Reversible (2 + 2) Alkene Binding

Styrene substrates were found to bind reversibly to 1 (Figure 3). The stepwise addition of excess styrene (10–40 equiv) to 1 in C6D6 at room temperature led to the gradual appearance of a new species 3a, suggesting an equilibrium established immediately after each addition. A Van’t Hoff analysis of the reaction of 1 with excess styrene (21.6 equiv) was conducted in toluene-d8 over a temperature range of 248–298 K. The formation of 3a was found to be slightly exergonic: ΔH°298 = −12.5 kcal mol–1 and ΔG°298 = −0.4 kcal mol–1.

Figure 3.

Figure 3

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Reversible cycloaddition of styrenes to 1.

The formation of 3c appeared to be more energetically favorable. Addition of 4-(trifluoromethyl)styrene (1.5 equiv) to a solution of 1 in toluene-d8 at −35 °C resulted in an immediate color change from dark red to bright orange, and the NMR spectra recorded at the same temperature revealed that the equilibrium between 1 and 3c has been completely shifted to the product side. 3c could be crystallized from n-pentane at −35 °C, and the solid-state structure was analyzed by single-crystal X-ray diffraction (Figure 6b).

In the 1H NMR spectrum of 3c recorded at −35 °C, two broad apparent triplets at δH = −14.92 and −15.93 ppm can be found for the bridging Fe–(μ-H)2–Al hydrides as well as another three broad signals at δH = 2.74, 1.03, and 0.64 ppm for the ArCH=CH2 protons of the coordinated alkene group. In the 31P{1H} NMR spectrum, the three PMe3 ligands of 3c appear as a well-resolved ABX spin system: the AB part centered at δP = 36.5 ppm (JAB = 41.5 Hz) and the X part at 22.2 ppm.

Vinylic C–H Activation

Over the course of 14 days, the room-temperature reaction of 1 with styrene (1 equiv) in C6D6 afforded the vinylic C–H activation product 4a in 78% NMR yield (Figure 4).284a was isolated in a pure form and crystals suitable for X-ray diffraction were grown, confirming the (E)-β-alumination of styrene (Figure 6c). The new species exhibits a broadened hydride resonance at δH = −15.58 ppm in the 1H NMR spectrum integrating to 3H and a singlet at δp = 29.0 ppm in the 31P{1H} NMR spectrum characteristic for a (PMe3)3Fe–(μ-H)3–Al motif that resulted from the C–H activation reaction.19−21 The PhCH=CH protons resonate at δH = 7.86 and δH = 7.34 ppm, showing a large coupling constant of 3JHH = 19.9 Hz diagnostic for an (E)-configuration of the C=C double bond.

Figure 4.

Figure 4

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Vinylic C–H activation of styrene substrates.

Styrene derivatives containing electron-withdrawing substituents reacted much more quickly than those with electron-donating groups. For example, upon addition of 1 equiv of 4-(trifluoromethyl)styrene 4c was formed nearly quantitatively within 3 h. In the case of 4a and 4b, an excess of the respective styrenes (10 equiv) was necessary to obtain reasonable reaction rates and full conversion of 1 at room temperature. The formation of the products in these cases was found to follow first-order kinetics, with t1/2 ≈ 24 h (4a) and t1/2 ≈ 71 h (4b).29

The highest reaction rates at room temperature were observed in the reaction of 1 with 2-vinylpyridine which affords 4d almost instantly when carried out in the presence of catalytic amounts (1–2 mol %) of MgBr2. MgBr2 appears to act as a Lewis acid catalyst preventing coordination of the pyridine nitrogen to Al and activating the substrate for the C–H activation reaction (vide infra).

Nonreversible (2 + 4) Addition

In the absence of the Lewis acid additive, 2-vinylpyridine forms a (2 + 4) cycloaddition product with 1 (Figure 5). Addition of 2-vinylpyridine to a solution of 1 in toluene-d8 at −35 °C resulted in the formation of 5d in ca. 85% NMR yield alongside 4d as a minor side product. The 1H NMR spectrum of 5d recorded at −40 °C shows a sharp triplet resonance at δH = 3.81 ppm, diagnostic for the α-CH group of the (2 + 4) bound substrate. Warming the reaction solution to room temperature resulted in the slow decomposition of 5d and did not convert it into 4d (see Supporting Information). These results suggest that the (2 + 4) addition represents a competitive pathway preventing the C–H activation reaction. The analogue product 5e was obtained from the reaction of 1 with methyl acrylate at room temperature, with the (E)-C–H activation product 4e also being formed in a 15% NMR yield.

Figure 5.

Figure 5

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(2 + 4) additions to 1.

In the case of 5e, crystals suitable for X-ray diffraction could be grown, confirming the structure of the (2 + 4) cycloaddition product with the β-CH2 attached to Fe and the oxygen of the former carbonyl group bound to Al (Figure 6d).

Structure and Bonding

2b, 3c, 4a, and 5e were characterized by single-crystal X-ray diffraction, and their solid-state structures are depicted in Figure 6. In 4a, the Fe–Al (2.368(1) Å) as well as Al–C (2.002(4) Å) distances are very similar for the C–H activation products of 1 reported previously.19−21 The Fe–Al separation in 2b (2.423(1) Å) and 3c (2.425(1) Å) is longer than in 4a and 1 (2.217(6) Å) but comparable to the parent dibromide complex [(PMe3)3(Br)Fe-(μ-H)2-Al(Br)(MesBDI)] (2.453(1) Å, BDI = bis(β-diketiminate)).19 The C–C distances of the alkene/alkyne substrate get elongated upon binding (3c: 1.544(5) Å; 2b: 1.362(6) Å) and are consistent with the formulation as a C–C single (ethane: 1.535 Å) or C=C double bond (ethylene: 1.339 Å).30 Similarly, the C–C bond lengths of the bound substrate in 5d (C1–C2: 1.504(3) Å; C2–C3: 1.336(3) Å) are in accordance with an enolate tautomeric structure depicted in Figure 6.

DFT calculations were conducted to gain further insight into the alkene/alkyne binding to 1.31 Calculations on the thermochemistry reveal that the cycloaddition products are significantly more stable for alkynes than for alkenes. For example, the formation 2a is exergonic by ΔG = −31.2 kcal mol–1, whereas the binding of styrene in 3a only results in a moderate stabilization of ΔG = −2.3 kcal mol–1.

Alkene binding to 1 was further investigated by ETS-NOCV (extended transition state-natural orbital for chemical valence) calculations (see Figure S14). Donation of electron density from the former Fe–Al bond into the π* orbital of the substrate (3a: Δρ1 = −420.7 kcal mol–1) accounts for >90% of the total orbital stabilization energy (3a: ΔEorb = −421.0 kcal mol–1). Natural bond orbital (NBO) analysis identified a σ-bond between iron and the β-carbon of the substrate, whereas bonding of Al to the α-carbon is defined as the donor–acceptor interaction between a C-centered lone pair and empty s/p orbitals in Al possessing a partial ionic character. This is underpinned by the charges from NPA (natural population analysis), revealing that the Al–C bond is more polarized (3a: Al + 1.80, C −0.88) in comparison to the Fe–C bond (3a: Fe −0.52, C −0.80).

Mechanism of the Vinylic C–H Activation

DFT calculations were also undertaken on the mechanism of vinylic C–H activation of styrenes.32 A low-energy pathway was identified involving direct intramolecular C–H activation of the bound styrene in 3a (Figure 7a).

Figure 7.

Figure 7

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(a) Calculated free-energy profile for the vinylic C–H activation of styrene. Energies are in kcal mol–1. B3LYP-D3/Def2-TZVPP/SDDAll (Fe,Al)/PCM (benzene)//B3PW91-D3/6-31G**/SDDAll (Fe,Al)/PCM (benzene). (b) Visualization of bond rearrangements in TS-3 using LMO centroids (CLMOs).

The mechanism is initiated through the concerted but asynchronous addition of the C=C double bond to 1. This first step has a low activation barrier of ΔG⧧298K = 15.8 kcal/mol (TS-1a) and is moderately exergonic by −2.3 kcal/mol (INT-3a), in accordance with the experimentally observed reversibility of the reaction. The resulting alkene complex was found to adopt two different conformations (INT-2a and INT-3a) with respect to their distortion along the Fe–Al vector. These conformers appear to be close in energy (ΔΔG°298K = 2.7 kcal/mol) and are separated by barriers of just 2.1 and 4.0 kcal/mol, respectively. From INT-3a, vinylic C–H activation proceeds intramolecularly through TS-3a to directly give the final product INT-4a. This step represents the highest barrier (22.4 kcal/mol) of the entire pathway. The formation of INT-4a is exergonic by −19.4 kcal/mol, consistent with a nonreversible process.

TS-3a appears to be a highly asynchronous transition state near the boundary between a concerted and a stepwise mechanism (vide infra). The low imaginary frequency (−92.3 cm–1) of TS-3a refers to the reorientation of the bound alkene in which the α-C is moving away from the Al center, while the β-C is transferred from Fe to Al. There is no stationary point for the actual cleavage of the C–H bond, which occurs along the intrinsic reaction coordinate en route to INT-4a (see the intrinsic reaction coordinate (IRC) in Figure 8b). The (E)-stereospecific nature of the C–H activation is a direct consequence of the orientation of the substituents in the transition state TS-3a (vide infra).

Figure 8.

Figure 8

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a–c) IRCs around TS-3 employing different styrene substrates. (d) Gibbs free-energy profile for the stepwise C–H activation process with 2-vinylpyridine in presence of a Lewis acid catalyst (free energies in kcal mol–1). Optimized structures of the transition states TS-3d (e) and TS-3d’ (f).

In order to get better insight into the nature of TS-3a, an analysis of the key localized molecular orbitals (LMOs) along the IRC was carried out. LMOs were calculated following the Pipek-Mezey criterion,33 and a procedure34 described by Vidossich and Lledóss was used to generate centroids of these LMOs (CLMOs). The CLMOs were used to follow the bond rearrangements around TS-3a (Figure 7b).35 For the sake of clarity, the depiction of the overall process was separated into pre- and post-TS stages. From INT-3a to TS-3a (pre-TS), migration of the β-C of the bound alkene goes along with an electron transfer from the Fe–C bond to Al, while the electrons from the Al–C are shifted toward the α-C and get delocalized through participation of the arene π-system (vide infra). In the post-TS stage, the most significant rearrangement of the electronic structure occurs during C–H bond cleavage. Electrons from the C–H σ-orbital are shifted toward Fe, forming the Fe–H bond. This process is consistent with a hydride transfer and distinct from the reaction of 1 with pyridines19,20 and CH3CN,21 which are found to proceed via a proton transfer (reductive deprotonation). Simultaneously, the delocalized lone pair at the α-C is shifted back toward the β-C to re-establish the π-system of the C=C double bond in the vinyl ligand.

The proposed mechanism is supported by deuterium labeling studies. A comparison of the reaction rates of 1 with styrene and styrene-d8 at 323 K from two separate experiments resulted in a kinetic isotope effect (KIE) of 2.64 ± 0.04. The overall KIE is likely affected by an additional equilibrium isotope effect (EIE) caused by the reversible alkene binding, which, however, appeared to be relatively small at room temperature (EIE = 1.06 at 298 K). This contrasts with the ortho C–H activation of pyridine for which an unusually large kH/kD value of 14.0 ± 0.2 at 298 K was obtained, likely caused by a quantum tunneling and thus diagnostic for a proton-transfer reaction.19

The C–H activation step was calculated for the entire series of 2a–d (Table 1). The obtained activation barriers correlate well with the Hammett parameters36 of the respective substituents and reflect the relative reactivities of these substrates observed experimentally. Across the series, the imaginary frequencies of TS-3a–c are getting lower with more positive Hammett parameters, indicating flattening of the potential energy surface around the transition state.

Table 1. Substituent Effect on the Activation Energies for INT-3 to INT-4 in the Reaction of Styrenes with 1.

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More insight was gained from the analysis of the IRCs (Figure 8a–c). While the IRC for 4-methoxystyrene is almost symmetrical around the transition state, it forms a plateau for 4-(trifluoromethyl)styrene flanked by a steep decay, marking the onset of the C–H bond cleavage. Computationally, the series was expanded to 4-nitrostyrene37 due to the large positive Hammett parameter of the NO2 substituent (σ– = +1.27).38−40 In this extreme case, complete deconvolution of the rearrangement and C–H activation of the bound substrate into two separate transition states were obtained (TS-3e, ΔG⧧ = 9.4 kcal/mol, and TS-3e′, ΔG⧧ = 2.1 kcal/mol; see Supporting Information).41 This trend as well as the calculated activation barriers can be traced back to the resonance stabilizing effect of the para-substituent. A comparison of TS-3 down the series shows that the C(α)–C(Ar1) bond distances are getting shorter, indicating an increasing double bond character (TS-3b: 1.412 Å and TS-3e: 1.384 Å). At the same time, NBO analysis of TS-3 reveals the negative charge accumulation at the α-C decreases in the same order (TS-3b: −0.59 and TS-3e: −0.48). This ability to stabilize the negative charge at the α-C position appears to be the key factor for the reactivity. To some extent, these findings resemble recent studies on the transition from a stepwise to a concerted behavior of SNAr reactions.42,43

The same effect was observed for the MgBr2-promoted reaction of 2-vinylpyridine with 1 (Figure 8d–f). Binding of the Lewis acid to the pyridine nitrogen allows for an extreme resonance structure, resulting in a stepwise process and an even lower barrier than for 4-nitrostyrene. The emergent intermediate (INT-3d′, ΔG = −3.7 kcal/mol) is just slightly lower in energy than the adjacent barriers (TS-3d: ΔG = 2.5 kcal/mol and TS-3d′: ΔG = 0.4 kcal/mol).

NBO analysis provided further insight into the nature of this intermediate. In INT-3d′, the former π-system of the alkene remains broken resulting in two separated p-orbitals. The β-carbon appears to be sp3-hybridized carrying a negative partial charge (−1.10) stabilized through coordination to aluminum. The α-C appears to be sp2-hybridized, forming a resonance structure with the adjacent pyridyl group and thus carries a much lower negative partial charge (−0.46).

Binding of the substrate in INT-3d′ was further stabilized through an agostic interaction between one of the β-C–H bonds and the iron center. In TS-3d′, this C–H bond is cleaved (vide supra). With the lone pair on the α-C, the alkene π-system is re-established forming a C=C double bond which, as a consequence of the antiperiplanar conformation of the C–C bond in INT-3d′, adopts the final (E)-configuration.

C–H Phosphination

The alkenyl group in the C–H aluminated products can be further functionalized. This is demonstrated with a small scope of styrene substrates that gave C–H activation products 4a–c. Reacting these compounds with chlorodiphenylphosphine at 60 °C for 18 h in toluene leads to the quantitative conversion of 4a–c to afford the chlorinated complex 6 and the (E)-β-vinylphosphines 7a–c with excellent stereoselectivity (Figure 9). 7a–c are highly sensitive to air and were isolated as air-stable thiophosphines 8a–c. Even deuterated vinylphosphine 8a–d7 can be obtained as exemplified by the phosphination of 4a–d7. This method represents a rare example of a direct vinylic C–H phosphination.3,44

Figure 9.

Figure 9

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Stoichiometric C–H phosphination of styrenes.

Conclusions

In summary, we report the vinylic C–H activation of styrenes with a bimetallic Fe–Al complex. These reactions were found to proceed via a novel mechanism involving binding of the alkene across the Fe–Al bond, followed by an intramolecular transition state for the C–H bond cleavage.

For monometallic systems, the direct oxidative addition of the sp2 C–H bond in an unactivated alkene is hard to achieve. This is well documented and can be rationalized by the competing π-coordination of the alkene over the formation of a weakly bound σ-C–H complex vital to the bond cleavage at the transition-metal center.5

By contrast, our findings suggest that alkene binding is essential for the C–H activation to take place in the present bimetallic system. The (2 + 2) cycloaddition of the styrene substrates is weak and appears to initiate a low-energy pathway.

We identified an unusual transition state connecting the alkene complex with the final hydrido vinyl product. This key step proceeds through a highly asynchronous transition state near the boundary between a concerted and a stepwise mechanism influenced by the resonance stabilization ability of the aryl substituent. Moreover, the geometry of the transition state results in the selective metalation of the (E)-β-C–H bond of the substrate. Our preliminary results on the C–H phosphination of styrenes demonstrate the potential of this concept for C–H functionalization reactions and might stimulate future developments toward catalytic processes.

Acknowledgments

N.G. is grateful to the Austrian Science Fund (FWF) for the provision of an Erwin Schrödinger Fellowship (Project no. J-4399). We are also grateful to the European Research Council for funding (101001071). Peter Haycock and Dr. Stuart Eliott are thanked for assistance with NMR experiments. The computational results presented have been achieved in part using the Vienna Scientific Cluster (VSC-4).

Data Availability Statement

Raw NMR and computational data are available at the following repository: https://doi.org/10.14469/hpc/13502.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c14281.

  • Synthetic procedures, kinetic experiments, NMR spectra of all compounds, crystal structures of 4d, S1, and S2, crystallographic data, and computational methods (PDF)

  • Cartesian coordinates of the DFT-optimized structures (XYZ)

Open access was funded by the Austrian Science Fund (FWF).

The authors declare no competing financial interest.

Supplementary Material

ja3c14281_si_001.pdf (9.7MB, pdf)
ja3c14281_si_002.xyz (219.5KB, xyz)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ja3c14281_si_001.pdf (9.7MB, pdf)
ja3c14281_si_002.xyz (219.5KB, xyz)

Data Availability Statement

Raw NMR and computational data are available at the following repository: https://doi.org/10.14469/hpc/13502.

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