Article pubs.acs.org/Organometallics
Si−H Bond Activation at the Boron Center of Pentaphenylborole Holger Braunschweig,* Alexander Damme, Christian Hörl, Thomas Kupfer, and Johannes Wahler Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany S Supporting Information *
ABSTRACT: Si−H bond activation is usually considered a domain of transition-metal complexes, and only few metal-free systems have proven suitable for this task. We have now found that Et3SiH readily reacts with pentaphenylborole to afford 1bora-3-cyclopentenes as the syn and anti addition products. Here, Si−H bond cleavage is accomplished at a single boron center, a reactivity that is facilitated by a combination of high electrophilicity and loss of antiaromaticity. The mechanism of this transformation most likely involves a sequence of adduct formation, σ-bond metathesis, and conrotatory ring closure, similar to that observed for H/D exchange between H2 and silanes mediated by HB(C6F5)2 and heterolytic H2 splitting by boroles, respectively.
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INTRODUCTION The latest developments in the chemistry of boroles1 have unveiled remarkable reactivity patterns associated with reduction,2 Lewis base coordination,3 or Diels−Alder-type transformations.4 Recent highlights include H2 activation,5 coordination of CO,6 and formation of stable radical species.7 It was shown that the unique electronic structure of these 4-πelectron antiaromatic boracycles plays a crucial role in most of these transformations. On the basis of the facile splitting of H2 by pentaphenylborole (PPB; 1) and its perfluorinated analogue,5 we wondered if E−H bonds are also amenable to activation. Consequently, we set out to study the activation of Si−H bonds by PPB. Indeed, 1 easily activated the Si−H linkage of Et3SiH to afford the syn and anti hydrosilylation products syn-2 and anti-2 under mild conditions. These results become even more relevant on keeping in mind that Si−H bond activation is still a domain of transition-metal complexes. Thus, the oxidative addition of hydrosilanes to vacant coordination sites of transition-metal centers represents a key step in the catalytic hydrosilylation of unsaturated substrates to form Si−C bonds and is of outstanding significance in organic chemistry, as well as for industrial processes.8 In contrast, the activation of Si−H bonds by (single) nonmetal centers is still limited to a few carbenes,9 “frustrated Lewis pairs” (FLP),10 and some highly electrophilic species.11 Both N-heterocyclic carbenes (NHC) and cyclic (alkyl)(amino)carbenes (CAAC) have been shown to react with hydrosilanes either by direct 1,1-addition to the carbene carbon9a or by C−N bond cleavage.9b While in these cases Si−H bond splitting occurs at a single nonmetal center, synergistic effects are responsible for the heterolytic Si−H bond activation by FLPs to afford zwitterionic borate species.10 The first evidence for the high potential of electrophilic boranes to activate Si−H bonds was provided in the 1970s with the observation that BF3·OEt2 is an effective mediator of the hydrosilylation of carbonyl functions.12 These initial reports © XXXX American Chemical Society
resulted in the development of new catalytic procedures for the hydrosilylation of ketones and imines on the basis of B(C6F5)3 catalysis.13 B(C6F5)3 also acts as an efficient precatalyst for the H/D exchange between H2 and hydrosilanes mediated by HB(C6F5)2.11d However, these transformations usually rely on the presence of an additional substrate, and direct Si−H bond cleavage at a single boron center has only been observed upon thermolysis of Et3SiH with B(C6F5)3 (60 °C, 3 days) to afford HB(C6F5)2 and Et3Si(C6F5).11b,d It was also the high electrophilicity of the trityl cation that enabled catalytic approaches for the hydrosilylation of CC double bonds via silyl cation intermediates.11a,c
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RESULTS AND DISCUSSION The reaction of Et3SiH with 1 occurred readily in benzene solution at room temperature to afford the kinetic product anti2 (Scheme 1) quantitatively within 24 h. Complete conversion Scheme 1. Reactivity of PPB (1) toward Et3SiH
was indicated by the decolorization of the deep blue solution, and anti-2 was isolated in 83% yield as colorless crystals. No evidence for the formation of syn-2 or any side products was evident in the NMR spectra of the reaction mixture. The boracyclopent-3-ene anti-2 has been clearly identified on the basis of multinuclear NMR spectroscopy and X-ray diffraction. Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Received: June 21, 2013
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Figure 1. Molecular structures of anti-2 (left) and syn-2 (right) in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Ellipsoids and hydrogen atoms of the phenyl and ethyl groups are omitted for clarity. The asymmetric unit of anti-2 contains four independent molecules, which differ only marginally from each other, for which reason only one molecule is depicted.
Scheme 2. Proposed Mechanism for the Formation of anti-2 and syn-2
C14 carbon atoms (anti-2, 100.6(3)−114.4(2)°; syn-2, 101.9(1)−119.5(1)°). A plausible mechanism for the formation of 2 and the kinetic preference of anti-2 is shown in Scheme 2. Recent studies on the facile H/D exchange between dihydrogen and Et3SiH mediated by B(C6F5)3 strongly indicated that Si−H bond activation at a single boron center proceeds via the initial formation of a Et3Si−H···B(C6F5)3 adduct followed by σ-bond metathesis.11d Accordingly, the first step of the reaction of 1 with Et3SiH most likely entails adduct I, which subsequently undergoes σ-bond metathesis via transition state ITS to afford the 1-bora-2,4-pentadienes II. At this point, the mechanism merges into that effective for the H2 splitting by boroles.5 Thus, B−C bond rotation might afford rotamers IIanti and IIsyn, each leading to one of the isomeric hydrosilylation products anti-2 and syn-2 by rapid conrotatory ring closure to the zwitterions IIIanti/IIIsyn and straightforward 1,2-hydride migration steps. Therefore, why does H2 activation produce a kinetic mixture of syn and anti products (1:2 in solution), while the reaction with Et3SiH is highly selective and only generates anti-2 under ambient conditions? Unlike the related rotamers of H2 activation, IIanti and IIsyn do not interconvert rapidly because of significant steric repulsion between the Ph and SiEt3 substituents in IIsyn. Consequently, rotamer IIanti is strongly preferred and the kinetic product anti-2 is formed exclusively. The barrier imposed by this unfavorable steric interaction can only be overcome by heating solutions of anti-2 at 60 °C, which slowly results in the formation of the thermodynamic product syn-2.
Thus, the observation of only one 29Si NMR signal (δ 3.67) and only one singlet resonance for the C4B-bound hydrogen atom (δ 4.35) emphasizes the high selectivity of the reaction. The 11B NMR spectrum of anti-2 shows a broad resonance at δ 81, which is consistent with a three-coordinate borane center. Moreover, the chemical shift is comparable to the related H2 addition products (δ 78.5 ± 1)5 and significantly shifted to lower field with respect to 1 (δ 66).1 When a benzene solution was heated to 60 °C for 3 days, anti-2 was irreversibly converted into its syn isomer syn-2, which was obtained as a colorless solid in 90% isolated yield after workup (Scheme 1). Concomitantly, the characteristic 29Si NMR and 1H NMR signals of anti-2 gradually evolved into new singlet resonances for syn-2 (δ(29Si) 1.36; δ(1H) 4.87), while the 11B NMR chemical shift (δ 83) did not change significantly and the signal remained rather broad. Obviously, syn-2 is the thermodynamically favored hydrosilylation product, which was further supported by theoretical studies. Density functional theory shows that syn-2 is energetically favored over anti-2 by 12.3 kJ mol−1. The molecular structures of both species have been verified in the solid state by X-ray diffraction (Figure 1). Basically, the structures strongly resemble each other, and anti-2 and syn-2 only differ in their relative orientations of the H and SiEt3 substituents at the carbon atoms in positions α to the trigonalplanar boron centers (anti-2, ∑ = 360°; syn-2, ∑ = 359.9°). Both boracyclopent-3-enes feature essentially planar C4B backbones (torsion angles between −5.06(4) and 5.9(4)°) with CC double bonds between C12 and C13 (anti-2, 1.367(5) Å; syn-2, 1.353(3) Å) and pyramidalized C11 and B
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126.30, 126.56, 127.04, 127.61, 127.86, 127.97, 128.59, 129.20, 129.83, 130.31, 130.90, 131.48, 132.95, 135.68 (CH), 63.74, 139.24, 139.87 (br), 141.23, 141.98, 143.91, 144.92, 147.21 (C). 29Si{1H} NMR (99 MHz, C6D6, 296 K): δ 1.36; Anal. Calcd for C40H41BSi (560.66): C, 85.69; H, 7.37. Found: C, 86.04; H, 7.46. Crystal Structure Determinations. The crystal data of anti-2 and syn-2 were collected on a Bruker X8 APEX diffractometer with a CCD area detector and multilayer mirror monochromated Mo Kα radiation. The structures were solved using direct methods, refined with the ShelX software package, and expanded using Fourier techniques.15 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned to idealized positions and were included in structure factor calculations. Crystal data for anti-2: C40H41BSi, Mr = 560.63, colorless block, 0.53 × 0.30 × 0.23 mm3, monoclinic, space group Cc, a = 21.6102(14) Å, b = 19.7073(12) Å, c = 30.4940(18) Å, β = 101.515(2)°, V = 12725.4(14) Å3, Z = 16, ρcalcd = 1.171 g cm−3, μ = 0.101 mm−1, F(000) = 4800, T = 100(2) K, R1 = 0.0529, wR2 = 0.1388, 12523 independent reflections (2θ ≤ 52.04°), 1525 parameters. Crystal data for syn-2: C40H41BSi, Mr = 560.63, colorless block, 0.20 × 0.08 × 0.07 mm3, monoclinic, space group P21/c, a = 9.09(2) Å, b = 19.06(3) Å, c = 18.03(2) Å, β = 91.52(4)°, V = 3124(10) Å3, Z = 4, ρcalcd = 1.192 g cm−3, μ = 0.103 mm−1, F(000) = 1200, T = 100(2) K, R1 = 0.0737, wR2 = 0.1068, 6632 independent reflections (2θ ≤ 53.5°), 382 parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. CCDC 941623 (syn-2) and CCDC 941624 (anti-2). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. The geometries of anti-2 and syn-2 were fully optimized without symmetry restraints using the Gaussian09 program package, while crystallographic coordinates were used for the starting geometry.16 The computations were performed using DFT methods, applying the hybrid functional BPE using 6-31G(d,p) basis sets for H, B, C, and Si.17 Frequency calculations were used to verify that the final geometries represent energy minima.
CONCLUSIONS In summary, we have demonstrated facile metal-free Si−H bond activation at the single boron center in PPB (1). Thus, the combination of high electrophilicity with the loss of antiaromaticity not only facilitates H2 splitting but is also capable of cleaving Si−H bonds, a reactivity that is usually associated with transition-metal complexes. In addition, the reaction with Et3SiH proceeds highly selectively to afford the kinetically favored addition product anti-2 exclusively at room temperature, while its thermodynamic analogue syn-2 is only generated upon heating at 60 °C. The background for this observation is of steric nature and becomes evident from mechanistic considerations, which are based on recent findings from H2 activation by boroles5 and H/D exchange reactions between dihydrogen and silanes catalyzed by HB(C6F5)2.11d
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EXPERIMENTAL SECTION
General Considerations. All manipulations were conducted either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Solvents were purified by distillation from potassium under dry argon and stored over molecular sieves. C6D6 was dried by distillation from potassium, degassed by three freeze−pump−thaw cycles, and stored over molecular sieves. Pentaphenylborole 1 14 was prepared according to published procedures. Et3SiH was purchased from Sigma Aldrich and purified by distillation. NMR spectra were acquired on a Bruker Avance 500 (1H, 500.1 MHz; 11B, 160.4 MHz; 13C, 125.7 MHz; 29Si{1H}, 99.4 MHz). 1H and 13C{1H} NMR spectra were referenced to external TMS via the residual protons of the solvent (1H) or the solvent itself (13C). 11B{1H} NMR spectra were referenced to external BF3·OEt2. 29 Si{1H} NMR spectra were referenced to external TMS. Microanalyses (C, H, N) were performed on a Leco Instruments elemental analyzer, type CHNS 932. Preparation of anti-2. Et3SiH (52.3 mg, 450 μmol) was added to a solution of pentaphenylborole (0.20 g, 450 μmol) in benzene (10 mL) at room temperature and stirred for 24 h. The reaction was accompanied by the disappearance of the deep blue color to eventually produce a colorless solution. All volatiles were removed in vacuo, and the residue was extracted into hexanes (10 mL) and filtered. The filtrate was reduced in volume (4 mL) and stored at −30 °C to afford anti-2 (0.21 g, 374 mmol, 83%) as a colorless solid, which was washed with pentane (3 × 1 mL) and dried in vacuo. 1H NMR (500 MHz, C6D6, 296 K): δ 0.64−0.80 (m, 6H, SiCH2CH3), 0.90 (t, 3JH−H = 7.27 Hz, 9H, SiCH2CH3), 4.35 (s, 1H, CH), 6.56−6.58 (m, 1H, C6H5), 6.80−6.83 (m, 1H, C6H5), 6.84−6.92 (m, 7H, C6H5), 6.93−6.98 (m, 1H, C6H5), 7.09−7.12 (m, 1H, C6H5), 7.14−7.19 (m, 7H, C6H5), 7.22−7.26 (m, 2H, C6H5), 7.30−7.34 (m, 4H, C6H5), 7.36−7.39 (m, 2H, C6H5), 7.63−7.66 (m, 2H, C6H5). 11B NMR (160 MHz, C6D6, 297 K): δ 81.0 (br). 13C{1H} NMR (126 MHz, C6D6, 297 K): δ 9.19 (CH3), 6.05 (CH2), 61.62, 125.99, 126.47, 126.62, 127.05, 127.13, 127.78, 127.92, 128.66, 129.42, 129.44, 130.11, 130.48, 130.52, 131.65, 131.70 (CH), 61.52, 139.80, 140.12, 140.34, 141.19, 141.34 (br), 144.57, 150.31 (C). 29Si{1H} NMR (99 MHz, C6D6, 296 K): δ 3.67. Anal. Calcd for C40H41BSi (560.66): C, 85.69; H, 7.37. Found: C, 85.44; H, 7.67. Preparation of syn-2. A solution of anti-2 (76.1 mg, 136 μmol) in benzene (1 mL) was heated at 60 °C for 3 days and subsequently filtered. Slow diffusion of hexanes (1 mL) into the filtrate at room temperature afforded syn-2 (68.5 mg, 122 mmol, 90%) as colorless crystals, which were washed with pentane (3 × 1 mL) and dried in vacuo. 1H NMR (500 MHz, C6D6, 296 K): δ 0.63−0.75 (m, 6H, SiCH2CH3), 0.89 (t, 3JH−H = 7.75 Hz, 9H, SiCH2CH3), 4.87 (s, 1H, CH), 6.66−6.69 (m, 1H, C6H5), 6.76−6.80 (m, 1H, C6H5), 6.85−6.98 (m, 12H, C6H5), 7.15−7.19 (m, 1H, C6H5), 7.28−7.34 (m, 6H, C6H5), 7.39−7.42 (m, 2H, C6H5), 7.74−7.76 (m, 2H, C6H5). 11B NMR (160 MHz, C6D6, 297 K): δ 83.0 (br). 13C{1H} NMR (126 MHz, C6D6, 297 K): δ 9.01 (CH3), 5.45 (s, 1JC−Si = 52 Hz, CH2), 56.43, 125.20,
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ASSOCIATED CONTENT
S Supporting Information *
Tables giving crystallographic and computational details and CIF files giving crystallographic data of syn-2 (CCDC-941623) and anti-2 (CCDC-941624). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*H.B.: e-mail,
[email protected]; fax, (+49) 931-31-84623. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the DFG for financial support. REFERENCES
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