DOI: 10.1002/chem.201200244
Enantioselective Hydrosilylation with Chiral Frustrated Lewis Pairs Dianjun Chen,[a] Valeri Leich,[a] Fangfang Pan,[b] and Jrgen Klankermayer*[a] Dedicated to Dr. Christian Bruneau on the occasion of his 60th birthday
In recent years, the chemistry of frustrated Lewis pairs (FLP) has been greatly developed, and the carefully chosen combination of sterically hindered Lewis acids and bases has shown unprecedented reactivity towards the activation of small molecules.[1] This FLP approach rapidly found application in catalysis and effective systems for catalytic hydrogenation without the use of transition metals were developed.[2] Subsequently, chiral borane-based, FLP-enabled, enantioselective hydrogenation of prochiral imines in up to 84 % enantiomeric excess (ee) was demonstrated.[3] Thus, hydrosilylation as a comparably attractive process based on the use of cheap and stable hydridic silanes is gaining increasing interest.[4] In most cases, and especially in asymmetric transformations, effective hydrosilylation processes are mediated by transition-metal catalysts.[4–5] As early as 2000, Piers and co-workers reported the highly effective BACHTUNGRE(C6F5)3catalyzed hydrosilylation of imines, and spectral evidence strongly supported Si H bond activation through a FLP mechanism.[6] Recently, Alcarazo and co-workers extended the FLP concept for silane activation, and in their study the FLP, incorporating hexaphenylcarbodiphosphorane and BACHTUNGRE(C6F5)3, was shown to cleave the Si H bond heterolytically.[7] Interestingly, FLPs could also catalyze the deoxygenative hydrosilylation of CO2 to CH4 with triethylsilane as reducing agent, indicating the scope of this metal-free transformation.[8] In our continued interest in the application of FLP reactivity to catalysis, the enantioselective hydrosilylation by using chiral FLPs as catalysts is reported herein. The frequently used Lewis acidic component in FLPs, BACHTUNGRE(C6F5)3, is a highly active catalyst for hydrosilylation of carbonyl compounds and imines under ambient conditions.[6, 9] Based on systematic mechanistic studies, Piers and co-workers proposed a mechanism for this transformation that involves the formation of a counterion pair through abstraction of hydride from silane by BACHTUNGRE(C6F5)3 in the presence of [a] Dr. D. Chen, V. Leich, Prof. Dr. J. Klankermayer Institut fr Technische und Makromolekulare Chemie RWTH Aachen University Worringerweg 1, 52074 Aachen (Germany) E-mail:
[email protected] [b] F. Pan Institut fr Anorganische Chemie RWTH Aachen University Landoltweg 1, 52074 Aachen (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200244.
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Scheme 1. Proposed mechanisms of BACHTUNGRE(C6F5)3-catalyzed hydrosilylation of carbonyl compounds.
Lewis basic substrates (Scheme 1).[10] Recently, Oestreich and Rendler reinvestigated the BACHTUNGRE(C6F5)3-catalyzed hydrosilylation of acetophenone with an enantiomerically enriched chiral silane, and in their study, a diastereomeric silicon ether was preferentially produced.[11] After stereospecific cleavage of the Si O bond with retention of configuration at the Si atom, the chiral silane was recovered with complete inversion of configuration at the silicon center. The observation of Walden inversion in this catalytic experiment suggested an SN2-Si mechanism (Scheme 1). More interestingly, the product 1-phenylethanol could be obtained with a moderate, but promising enantioselectivity of already 38 % ee.[11] However, the corresponding hydrosilylation of imines with the chiral silane resulted in only racemic products.[12] Rather than solely using BACHTUNGRE(C6F5)3 as a catalyst for metalfree hydrosilylation, we recently became interested in the catalytic activity of frustrated Lewis pairs for this reaction. Initially, mixtures of dimethylphenylsilane (PhMe2SiH) and typical FLPs were studied by NMR spectroscopy in solution. When the FLPs, including Mes3P/BACHTUNGRE(C6F5)3 and PhMe2SiH, were stoichiometrically dissolved in [D2]-dichloromethane, the 31P, 19F, and 11B NMR experiments revealed no obvious evidence of silane cleavage (Scheme 2). Carrying out the same experiment with tBu3P, a new species formed immediately in dichloromethane solution. Multinuclear NMR spectroscopy corroborated the product as the ionic complex [tBu3PSi(Me)2Ph][HBACHTUNGRE(C6F5)3] (1; Scheme 2). The 31P NMR spectrum of the cation exhibits a resonance at d = 34.2 ppm,
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Chem. Eur. J. 2012, 18, 5184 – 5187
COMMUNICATION
Scheme 2. Reaction of FLPs with dimethylphenylsilane in dichloromethane solution.
which is significantly high-field shifted in comparison to the cation [tBu3PH] + (d = 56.6 ppm),[13] whereas the 19F NMR (d = 133.6 (o), 164.5 (p), 167.3 ppm (m-C6F5)), and 11 B NMR (d = 25.4 ppm) signals confirmed the [HBACHTUNGRE(C6F5)3] structure of the anion.[13] Single crystals could be obtained from pentane–dichloromethane solutions at 30 8C, and X-ay diffraction analysis unambiguously confirmed the formation of 1 (Figure 1 and the Supporting Information).
Figure 1. Crystal structure of 1. Hydrogen atoms are omitted for clarity, except for the hydrogen atoms bonded to boron. Thermal ellipsoids are set at 50 % probability.
loading of 4 mol %. In addition, an NMR investigation showed that the [HBACHTUNGRE(C6F5)3] anion was the only boron species detectable in the solution. The observed significant reaction-rate difference and exclusive formation of hydridoborate in the reaction solution corroborated the assumption that the hydrosilylation reaction rate strongly depends on the activation of the Si H bond. In the absence of the Lewis acid BACHTUNGRE(C6F5)3, the Si H bond remains inactivated, and consequently no hydrosilylation of imines takes place. Addition of BACHTUNGRE(C6F5)3 to the silane solution results in the formation of a B H dative bond and weakening of the Si H bond. This activation is sufficient to render the addition of silane to the imine in a fast and concerted pathway. However, heterolytic cleavage of the Si H bond in the presence of the FLP (tBu3P/BACHTUNGRE(C6F5)3), results in the rapid formation of the silylium and [HBACHTUNGRE(C6F5)3] ions, and the hydride transfer to the imine becomes rate determining, resulting in a slow hydrosilylation reaction. Therefore, the catalytic reactions with only the Lewis acid BACHTUNGRE(C6F5)3 and with the FLP (tBu3P/BACHTUNGRE(C6F5)3) represents the two activity boundaries of the hydrosilylation reaction pathways. Consequently, the influence of weaker Lewis bases for this transformation should be investigated, and indeed the FLP with the weaker Lewis base component Mes3P provided 65 % conversion after 8 min. Consequently, by careful tuning of the Lewis acidity or basicity of FLP partners, these experimental findings could help to optimize the catalytic system with a focus on enantioselectivity. Based on these findings and the successful synthesis of chiral FLPs in a previous study,[3b] the extension of the chiral FLP concept to enantioselective catalytic hydrosilylation was attempted. For this purpose, a new stable chiral borane 4 (Figure 2) could be synthesized by reaction of the Piers
The interesting observation of Si H bond splitting by FLP straightforwardly directed the investigation towards catalytic hydrosilylation reactions. For the initial catalytic experiments, N-(1-phenyl-ethylidene)aniline (2 a) was chosen as model substrate (Scheme 3). Figure 2. Chiral borane 4 and phosphonium hydridoborates 5 and 6.
Scheme 3. Catalyzed hydrosilylation of imine by BACHTUNGRE(C6F5)3 and FLP.
As was already illustrated by Piers, BACHTUNGRE(C6F5)3 was found to be a very active catalyst for the hydrosilylation of various imines.[6] In the presence of 0.35 mol % of BACHTUNGRE(C6F5)3, imine 2 a was converted to the corresponding secondary amine 3 (after hydrolysis) in only 4 min. However, reactions with the FLP incorporating tBu3P and BACHTUNGRE(C6F5)3 only resulted in 25 % conversion after four days of reaction time with a catalyst
Chem. Eur. J. 2012, 18, 5184 – 5187
borane[14] with chiral alkene (1R,4R)-1,7,7-trimethyl-2-(2naphthyl)-bicycloACHTUNGRE[2.2.1]hept-2-ene. The resulting diastereomerically pure borane 4 was simply isolated quantitatively after removing the solvent in vacuo. Compound 4 featured a set of 19F NMR signals at d = 130.7 (o), 150.0 (p), and 161.1 ppm (m-C6F5), and a broad 11B NMR signal at d = 75.2 ppm, indicating a neutral boron center. The absolute configuration of 4 was determined as bis(perfluorophenyl)[(1R,2R,3R,4S)-4,7,7-trimethyl-3-phenylbi-cycloACHTUNGRE[2.2.1]heptan-2-yl]borane by X-ray diffraction analysis of its dihydrogen activation product 5 (see the Supporting Information). With only the enantiopure chiral borane 4, the hydrosilylation of imine 2 a with dimethylphenylsilane in toluene at
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Table 1. Hydrosilylation of imine 2 a with chiral borane or FLP catalysts.
Entry[a]
Catalyst
t
Yield [%][b]
ee [%][c]
1 2 3 4
4 4/Mes3P 1:1 4/tBu3P 1:1 5
2h 4d 4d 4d
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