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Hydrosilylation of Olefins Catalyzed by Well-Defined Cationic Aluminum Complexes: Lewis Acid versus Insertion Mechanisms Kayla Jakobsson, Terry Chu, and Georgii I. Nikonov* Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, Canada L2S S Supporting Information *

ABSTRACT: The cationic aluminum complex [NacNacAlH]+ (2; NacNac = CH{C(Me)N(2,6-Pri2C6H3)}2) can be easily generated from NacNacAlH2 by hydride abstraction and functions as a catalyst for the hydrosilylation of olefins and alkynes. Mechanistic studies suggest that, although olefin insertion into the Al−H bond is very facile, the catalysis does not proceed by an insertion/metathesis mechanism but likely by Lewis acid activation. Stoichiometric reactions of 2 with alkynes furnished unexpected products of CC addition across the NacNacAl moiety to give tripodal aluminum cations, which are also potent catalysts for the hydrosilylation of alkynes. KEYWORDS: aluminum, homogeneous catalysis, hydrides, alkene, hydrosilylation

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INTRODUCTION The development of more economical and environmentally benign surrogates for common catalysts based on precious metals is an intensive area of current research.1 In this regard, main-group compounds have attracted significant attention due to the low cost and toxicity of many s- and p-block elements.2−5 At 8.1%, aluminum is the third most abundant element in Earth’s crust,6 which when coupled with its low toxicity makes it a very attractive candidate for catalyst design. Applications of aluminum in catalysis find some spectacular classical examples in the Meerwein−Ponndorf−Verley (MPV) reduction of carbonyls7 and Friedel−Crafts alkylation and acylation of aromatics.8 Aluminum complexes have also been applied as catalysts for the ring-opening polymerization of cyclic esters.9 More recently, Berben and co-workers have used aluminum complexes stabilized by the bisiminopyridine ligand to catalyze the dehydrogenative coupling of benzylamine,10 dehydrogenation of formic acid,11 and electrocatalytic production of hydrogen.12 The Wright laboratory has shown that aluminum complexes can catalyze the dehydrocoupling reaction of amine−borane13 as well as amines and silanes,14 while an αdiimine complex of aluminum was demonstrated by Graves to be a competent catalyst for the epoxidation of alkenes.15 Frustrated Lewis pairs16 containing aluminum as the Lewis acidic partner have been utilized as polymerization catalysts,17 phase-transfer catalysts,18 and catalysts for amine−borane dehydrocoupling.19 In the field of catalytic reduction, simple aluminum compounds, in particular AlCl3, have been used as catalysts for hydrosilylation since 1978.20−23 Very recently, several well-defined molecular aluminum catalysts have been developed (Chart 1). Thus, Roesky et al. showed that the neutral aluminum hydride NacNacAl(OTf)H (NacNac = CH{C(Me)N(2,6-Pri2C6H3)}2) catalyzes the hydroboration of © 2016 American Chemical Society

Chart 1. Well-Defined Aluminum Catalysts for Hydrosilylation24−27

aldehydes and ketones with pinacolborane as the reducing agent,24 whereas the related complex EtNacNacAlH2 (EtNacNac = CH{C(Me)N(2,6-Et2C6H3)}2) catalyzes the hydroboration of alkynes.25 More relevant to the current study is the very small class of cationic aluminum complexes represented by Bergman’s tripodal complex [Tp*AlMe]+ (Tp* = hydrotris(1,3-dimethylpyrazol-1-yl)borate)26 and Wehmschulte’s cation [Et2Al]+ used in the catalytic hydrosilylation of carbonyls (aldehydes, ketones, aldimines, and lactone) and CO2,27 respectively. We have recently reported that NacNacZnH (1) is an active catalyst for the chemoselective hydrosilylation of Received: June 15, 2016 Revised: September 19, 2016 Published: September 23, 2016 7350

DOI: 10.1021/acscatal.6b01694 ACS Catal. 2016, 6, 7350−7356

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ACS Catalysis aldehydes, ketones, and nitriles.28 We then supposed that the isolobal aluminum complex [NacNacAlH]+ (2) could show superior activity due to its increased Lewis acidity (Chart 2).

Table 1. Optimization of Hydrosilylation Conditions

Chart 2. Isolobal Relationship between Zinc Compound 1 and Aluminum Cation 2

Therefore, we opted to prepare this compound and study its activity as a hydrosilylation catalyst. This endeavor resulted in the discovery of aluminum-catalyzed hydrosilylation of alkenes and alkynes, which appears to proceed via a Lewis acid mechanism. This finding is important, as hydrosilylation catalyzed by s- and p-block metals is little studied29,30 and its application to alkenes and alkynes is particularly scarce.21,22,31,32

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RESULTS AND DISCUSSION Jordan et al. have previously reported that methyl abstraction from NacNacAlMe2 by the trityl cation leads to the isolable cation [NacNacAlMe]+.33 Similar treatment of the dihydride NacNacAlH2 with [Ph3C]+[BArF4]− (ArF = C6F5) afforded the cation 2. The choice of solvent happened to be crucial for this reaction, because 2 precipitates in the form of an oil from nonpolar solvents (e.g., aromatics, hexanes) and decomposes in polar solvents such as CH2Cl2 and THF. Eventually, we found that 2 can be cleanly generated in chlorobenzene and is stable for at least 1 day in this solvent. Initial substrate screening for catalytic hydrosilylation showed that hard donors, such as nitriles, pyridines, and ketones, coordinate to the Lewis acidic cation 2 too tightly, resulting in significantly decreased activity in comparison with catalysis by related calcium,31a,b,34 magnesium,35 and zinc NacNac complexes.28 We therefore shifted our goals toward softer substrates: alkenes and alkynes. Gratifyingly, when 3,3-dimethyl-1-butene was reacted with methylphenylsilane in the presence of 5 mol % of 2, full conversion of the alkene occurred in less than 10 min at room temperature. However, a mixture of products was formed due to the very facile redistribution of the starting and final silane products (Table 1, entry 1). Changing the silane to dimethylphenylsilane also produced mixtures of products (Table 1, entry 2). Finally, use of the more robust triethylsilane allowed formation of only one product with the same conversion in the same time span (Table 1, entry 3), but interestingly this product was not the predicted (3,3dimethylbutyl)triethylsilane but rather (2,3-dimethylbutyl)triethylsilane, likely formed as a result of methyl migration in an intermediate silylated carbocation (vide infra). With this choice of silane, the conditions were further optimized by varying the catalyst loading from 5% to 0.1%. While the reaction is still very fast with a 1% catalyst load, it slows down significantly at 0.5% and stops at 0.3% likely due to catalyst poisoning by impurities in the substrate. To probe the sensitivity of this catalytic system toward oligomerization of olefins, a control experiment was conducted. Carrying out the reaction in the absence of silane did result in oligomerization; however, the reaction took much longer (18 h; Table 1, entry 8). Thus, our optimal conditions were set at 1 mol % catalyst loading with triethylsilane as the reducing agent.

a Conversions were determined by 1H NMR spectroscopy. bMixtures of products due to parallel redistribution. cAfter 4 days at room temperature. dAfter 18 h at room temperature, oligomerization occurred.

Next, the scope of the catalysis was explored. Terminal alkenes and cyclohexene all showed quantitative conversion to the alkylsilanes in less than 10 min at room temperature (Table 2, entries 1−4). For tri- and tetrasubstituted alkenes, a higher catalyst loading (5 mol %) was required to achieve similar conversions within the same time span (Table 2, entries 5−7). Lower catalyst loadings resulted in only minor reactivity or none at all. For the trisubstituted cyclic alkene 1-methylcyclohexene (Table 2, entry 5), 1H NMR data obtained in chlorobenzene were very close to the spectral data obtained in C6D5Br for the same product prepared in a phosphoranecatalyzed hydrosilylation and a relative cis orientation of the Me and SiEt3 groups was assigned, consistent with trans hydrosilylation.32 Trans addition was also observed previously by Yamamoto21b and Jung21c for the AlCl3-catalyzed hydrosilylation of cyclic alkenes and by Gevorgyan for B(C6F5)3catalyzed hydrosilylation.36 The trans addition is easily explained by the stabilization of an intermediate carbocation as a result of conjugation with the β-silyl group,21b which should favor the addition of hydride from the opposite, less sterically encumbered side of the substrate. Although conversions into hydrosilylation products were high, the yield of the target silanes was rather moderate due to the formation of various redistribution products.37 This fact alone suggests the intermediacy of solvated silylium ions, e.g. [R3Si]+*ClPh, as intermediates in the reaction (vide infra). In the case of styrene, partial polymerization (up to 15%) was observed. We then turned to applying this catalyst toward the hydrosilylation of more robust substrates, alkynes. Aluminumcatalyzed hydrosilylation of alkynes is known, but limited examples are found in the literature.22a−d Heating a reaction mixture with phenylacetylene for multiple days resulted in a 7351

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ACS Catalysis Table 2. Scope of Alkene Hydrosilylation Catalyzed by 2

a Conversions into hydrosilylation products and yields were determined by 1H NMR spectroscopy using ferrocene as an internal standard. bThe other hydrosilylation product is (PhCH2CH2)2SiEt2 (21%). cCatalyst loading could be decreased to 0.75 mol % to achieve the same conversion in 15 min. dCatalyst loading of 5 mol %.

Table 3. Scope of Alkyne Hydrosilylation Mediated by 2

a

Conversions and yields were determined by 1H NMR spectroscopy using ferrocene as an internal standard. bReaction mixture heated to 70 °C.

milder conditions, i.e. room temperature. Gratifyingly, we observed the hydrosilylation of some terminal and internal alkynes. Since the signals of the substrate generally overlapped with the signals of the product in the 1H NMR spectrum, conversions were determined on the basis of the consumption of silane. For terminal alkynes, 24 h at room temperature was

polymeric product (Table 3, entry 1). We hypothesized that the substrate had initially undergone hydrosilylation to give a styrene derivative which was then polymerized under these relatively harsh conditions. This observation limited our choice of substrates to alkyl-substituted acetylenes and internal alkynes without the presence of aromatic groups. We also chose to use 7352

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ACS Catalysis required to achieve high conversions, albeit with low yields (Table 3, entries 2 and 3). For diethylacetylene, 25 h at room temperature was required to attain quantitative conversion (entry 4). In the case of cyclohexylacetylene and cyclohexylmethylacetylene, hydrosilylation resulted in cis addition of the Si−H bond, as evidenced by the observation of large trans 3 JH−H couplings between the alkene hydrogens. Only one major alkenyl silane product for each reaction was observed; however, various other silane products, formed as a result of silane redistribution, were identified by 1H−29Si HSQC correlation. Divergent mechanistic scenarios were proposed for the AlCl3-catalyzed hydrosilylation of alkenes. Thus, Oertle and Wetter proposed in situ formation of HAlCl2 which then hydroaluminates olefins to give an alkyl aluminum followed by a Si−H/Al−C metathesis with the hydrosilane.21a In contrast, Yamamoto21b and Jung21c favored the Lewis acid promoted formation of a silylium ion and its addition to the alkene to give a carbocation culminated by hydride transfer from the hydrosilane or aluminum hydride, respectively. On the other hand, previous studies on related NacNac calcium- and magnesium-catalyzed reactions suggested substrate insertion into the M−H bond as the key mechanistic event.29a,35 Likewise, Roesky et al. suggested substrate insertion into the Al−H bond followed by metathesis with the B−H bond in the catalytic hydroboration mediated by NacNacAlHX (X = OTf, H).24,25 To elucidate the mechanism of catalysis by the aluminum cation 2, the stoichiometric reactivity toward alkenes and alkynes was studied. For 1-hexene and 3,3-dimethyl-1-butene, the products observed were indeed the alkyl derivatives resulting from CC insertion into the Al−H bond (Scheme 1), but remarkably for the latter substrate, the alkyl product was

Scheme 2. Reversible Reaction between 1-Hexene and 3

(1.73 ppm) protons of 1-hexene fragment and the γ-carbon (60.3 ppm) in the 1H−13C HMBC spectrum. In addition, the two methyl groups in the ligand framework resolved into two separate singlets at 1.41 and 1.38 ppm due to the generation of a new chiral center. Similar cycloaddition of alkenes with [NacNacAlMe]+ has been observed previously by Jordan38 and Harder.39 Warming the sample to room temperature results in complete dissociation of 1-hexene and regeneration of 3, in accord with the recent report by the Harder group.39 Given the fact that hydroalumination of alkenes by 2 was very fast, we deemed that both aluminum cations 2 and [NacNacAlR]+ could mediate this reaction by means of Lewis acid catalysis. Indeed, alkyl complex 3 catalyzes the hydrosilylation of 1-hexene as rapidly as 2 (Scheme 3). Further Scheme 3. Hydrosilylation of 1-Hexene Mediated by 3

corroborating this suggestion was the observation that in catalytic (5 mol %) and stoichiometric reactions of 3 with cyclohexene and HSiEt3, the only products observed were (C6H11)SiEt3 and the intact cation 3 (Scheme 4). These labeling experiments allow us to rule out the alkene insertion/ metathesis pathway in the catalytic reactions.

Scheme 1. Reactivity of 2 toward 1-Hexene (Top) and 3,3Dimethyl-1-butene (Bottom)

Scheme 4. Hydrosilylation of Cyclohexene by 3 at 5 mol % (Top) and 100 mol % Loadings (Bottom)

Since mixtures of silane products were invariably formed in catalytic runs, the ability of cation 2 to redistribute silanes was studied in a separate set of experiments. Reactions with both HSiEt3 and H2SiEt2 resulted in fluxional systems containing H4Si, H3SiEt, H2SiEt2, and HSiEt3. The exchange could be frozen at about −20 °C, and the identity of silane products could be established in the 29Si INEPT+ spectrum. While redistribution of silanes is, in general, a well-established phenomenon, it is usually observed for halo-, alkoxy-, amido-, hydrido- and aryl-substituted silanes and is relatively rare for alkylsilanes. These stoichiometric and labeling experiments present a very divergent mechanistic picture. The lack of skeletal rearrangement of the alkyl group during the formation of 4 and its inability to regenerate the hydride 2 upon reaction with hydrosilanes allows us to rule out the usual insertion/bond metathesis pathway suggested for related hydrosilylation and

the expected neohexyl derivative 4 and not a 2,3-dimethylbutyl product observed in the catalytic reaction. This disparity suggested that alkene insertion is not relevant to the actual catalytic process. The mechanism based on alkene insertion followed by Si−H/Al−C metathesis was ultimately ruled out when 3 failed to react with HSiEt3 even after heating at 70 °C for 24 h. Interestingly, it was noted that addition of a slight excess of 1-hexene to 2 resulted in broadened signals for 3 in the 1H NMR spectrum. Cooling a chlorobenzene solution of 2 with 2 equiv of 1-hexene to −21 °C gave well-defined signals for a single product, 5 (Scheme 2). Complex 5 is the product of alkene addition across the aluminum atom and the γ-carbon in the NacNac ligand of 3, confirmed by the correlation observed between the methylene (0.11 and −0.64 ppm) and methine 7353

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ACS Catalysis

Scheme 5. Possible Lewis Acid Catalyzed Mechanisms for Alkene Hydrosilylation by [NacNacAlR][BArF4]: (a) Piers−Oestreich Type Mechanism and (b) Yamamoto Type Mechanism

tripodal cation [Tp*AlMe]+ reported by Koller and Bergman26 required much harsher conditions (100 °C), suggesting that tripodal systems are less reactive than 2 and 3, both operating at room temperature. To summarize, there are strong indications that the catalysis mediated by complexes 2 and 3 proceeds via a variation of Lewis acid activation but the exact mechanistic details are unclear at this moment. We favor the scenario outlined in Scheme 5a, which is consistent with all the experimental data available to date. Stoichiometric reactivity of alkynes was probed next. Unexpectedly, addition of 1 equiv of phenylacetylene to 2 did not result in insertion into the Al−H bond but rather gave a product of CC addition across the NacNacAl moiety, namely to the methine carbon and aluminum atom, to form the tripodal product 6 (Scheme 6). Similar reactivity with various

hydroboration reactions catalyzed by group 2 NacNac complexes.29b,35 Formation of the rearranged product (2,3dimethylbutyl)triethylsilane in the hydrosilylation of 3,3dimethyl-1-butene and the ease of silane redistribution by this catalytic system may suggest that these aluminum cations act as Lewis acid catalysts. A catalytic cycle promoted by the Lewis acid may proceed through two mechanisms, distinct from each other by the initial activation of silane or alkene (Scheme 5). Activation of silanes by electrophilic boranes has been well established by Piers40 and Oestreich,41 and the related silane complex [Et3Si−H···Al(C6F 5)3] has been very recently documented by Chen et al.42 In the second step of this pathway (Scheme 5a), the alkene attacks the incipient silylium ion, creating a carbocation intermediate which undergoes hydride abstraction from NacNacAlRH to give the alkylsilane and regenerates the aluminum cation. In the case of 3,3dimethyl-1-butene, the corresponding secondary carbocation would rearrange into a more stable tertiary carbocation, which would account for the formation of (2,3-dimethylbutyl)triethylsilane. Analogous reaction pathways have been previously suggested for hydrosilylation reactions catalyzed by AlCl3,21b,c B(C6F5)3,36 and [FP(C6F5)3]+ Lewis acid catalysts.32 The alternative mechanism was suggested by Yamamoto et al.21a,b This reaction pathway involves an aluminum cation coordinating to the olefin followed by hydride transfer from the silane (Scheme 5b), which can account for the structural rearrangement of 3,3-dimethyl-1-butene but contradicts the chemistry depicted in Scheme 4. In this case, the mechanism predicts the formation of NacNacAl(Hex)(Cyh) (Hex = hexyl, Cyh = cyclohexyl), which is expected to react with the incipient silylium ion (or its solvate with chlorobenzene) to give preferentially Et3SiHex for steric reasons. In contrast, we observed the exclusive formation of Et3SiCyh. Given the facile, reversible formation of the tripodal compound 5, one can argue that it too could be a catalyst. While its involvement in the catalytic cycle cannot be tested directly at this point, there are two arguments that it is not the true (or most active) catalyst. The first point is that 5 features a more shielded, four-coordinate aluminum center, which is likely less electrophilic. Second, catalytic hydrosilylation by the

Scheme 6. Coupling of Cation 2 with Alkynes

unsaturated substrates has been previously documented for other NacNac complexes.38,39,43 The diagnostic features of the 1 H NMR spectrum of 6 are a broad signal for the intact Al−H group at δ 3.86 and a signal for the backbone CH (5.66 ppm), observed as a doublet (5JH−H = 1.5 Hz) due to coupling to the o-Ph signal at 7.13 ppm confirmed by 1H−1H COSY, 1H−13C HSQC, and NOESY spectra. Stoichiometric reactions of 1phenyl-1-propyne and diphenylacetylene furnished similar addition products. The catalytic potency of the alkyne-bridged product 6 was then investigated in the catalysis of a reaction between 3-hexyne and HSiEt3. Comparable catalytic activity, with 98% conversion within 25 h, was observed, which compares well with the >99% conversion achieved for the reaction catalyzed by 2. When 7354

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(3) (a) Harder, S. Chem. Rev. 2010, 110, 3852−3876. (b) Piers, W. E.; Marwitz, A. J. V.; Mercier, L. G. Inorg. Chem. 2011, 50, 12252− 12262. (4) Revunova, K.; Nikonov, G. I. Dalton Trans. 2015, 44, 840−866. (5) Stahl, T.; Klare, H. F. T.; Oestreich, M. ACS Catal. 2013, 3, 1578−1587. (6) Lutgens, F. K.; Tarbuck, E. J. Essentials of Geology; Prentice Hall: Upper Saddle River, NJ, 1999. (7) (a) Meerwein, H.; Schmidt, R. Justus Liebigs Ann. Chem. 1925, 444, 221−238. (b) Verley, A. Bull. Soc. Chim. Fr. 1925, 37, 537−542. (c) Ponndorf, W. Angew. Chem. 1926, 39, 138−143. (d) Ooi, T.; Miura, T.; Maruoka, K. Angew. Chem., Int. Ed. 1998, 37, 2347−2349. (e) Campbell, E. J.; Zhou, H.; Nguyen, S. T. Angew. Chem., Int. Ed. 2002, 41, 1020−1022. (f) McNerney, B.; Whittlesey, B.; Cordes, D. B.; Krempner, C. Chem. - Eur. J. 2014, 20, 14959−14964. (g) Yeagle, K. P.; Hester, D.; Piro, N. A.; Dougherty, W. G.; Kassel, W. S.; Graves, C. R. Aust. J. Chem. 2015, 68, 357−365. (8) Friedel, C.; Crafts, J. M. Compt. Rend. 1877, 84, 1392−1450. (9) (a) Ikpo, N.; Flogeras, J. C.; Kerton, F. M. Dalton Trans. 2013, 42, 8998−9006. (b) Jianming, R.; Anguo, X.; Hongwei, W.; Hailin, Y. Des. Monomers Polym. 2014, 17, 345−355. (c) Wei, Y.; Wang, S.; Zhou, S. Dalton Trans. 2016, 45, 4471−4485. (10) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2013, 135, 9988− 9990. (11) Myers, T. W.; Berben, L. A. Chem. Sci. 2014, 5, 2771−2777. (12) Thompson, E. J.; Berben, L. A. Angew. Chem., Int. Ed. 2015, 54, 11642−11646. (13) Less, R. J.; Simmonds, H. R.; Wright, D. S. Dalton Trans. 2014, 43, 5785−5792. (14) Allen, L. K.; Garcia-Rodriguez, R.; Wright, D. S. Dalton Trans. 2015, 44, 12112−12118. (15) Koellner, C. A.; Piro, N. A.; Kassel, W. S.; Goldsmith, C. R.; Graves, C. R. Inorg. Chem. 2015, 54, 7139−7141. (16) (a) Stephan, D. W. Acc. Chem. Res. 2015, 48, 306−316. (b) Stephan, D. W. J. Am. Chem. Soc. 2015, 137, 10018−10032. (c) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2015, 54, 6400− 6441. (17) (a) Zhang, Y.; Miyake, G. M.; Chen, E. Y. X. Angew. Chem., Int. Ed. 2010, 49, 10158−10162. (b) Zhang, Y.; Miyake, G. M.; John, M. G.; Falivene, L.; Caporaso, L.; Cavallo, L.; Chen, E. Y. X. Dalton Trans. 2012, 41, 9119−9134. (c) He, J.; Zhang, Y.; Chen, E. Y. X. Synlett 2014, 25, 1534−1538. (18) Appelt, C.; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Angew. Chem., Int. Ed. 2012, 51, 5911−5914. (19) Appelt, C.; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Angew. Chem., Int. Ed. 2013, 52, 4256−4259. (20) Finke, U.; Moretto, H. Process for the addition of dialkylhalohydrosilanes to unsaturated hydrocarbons and the silanes obtained. German Patent DE2804204, August 2, 1979. (21) AlCl3-catalyzed alkene hydrosilylation: (a) Oertle, K.; Wetter, H. Tetrahedron Lett. 1985, 26, 5511−5514. (b) Yamamoto, K.; Takemae, M. Synlett 1990, 1990, 259−260. (c) Song, Y.-S.; Yoo, B. R.; Lee, G.-H.; Jung, I. N. Organometallics 1999, 18, 3109−3115. (22) AlCl3-catalyzed alkyne and cyclopropane hydrosilylation: (a) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61, 7654−7655. (b) Sudo, T.; Asao, N.; Gevorgyan, V.; Yamamoto, Y. J. Org. Chem. 1999, 64, 2494−2499. (c) Sudo, T.; Asao, N.; Yamamoto, Y. J. Org. Chem. 2000, 65, 8919−8923. (d) Kato, N.; Tamura, Y.; Kashiwabara, T.; Sanji, T.; Tanaka, M. Organometallics 2010, 29, 5274−5282. (e) Nagahara, S.; Yamakawa, T.; Yamamoto, H. Tetrahedron Lett. 2001, 42, 5057−5060. (23) Nakajima, Y.; Shimada, S. RSC Adv. 2015, 5, 20603−20616. (24) Yang, Z.; Zhong, M.; Ma, X.; De, S.; Anusha, C.; Parameswaran, P.; Roesky, H. W. Angew. Chem., Int. Ed. 2015, 54, 10225−10229. (25) Yang, Z.; Zhong, M.; Ma, X.; Nijesh, K.; De, S.; Parameswaran, P.; Roesky, H. W. J. Am. Chem. Soc. 2016, 138, 2548−2551. (26) Koller, J.; Bergman, R. G. Organometallics 2012, 31, 2530−2533. (27) Khandelwal, M.; Wehmschulte, R. J. Angew. Chem., Int. Ed. 2012, 51, 7323−7326.

DSiEt3 was employed, most of the deuterium was found in the product with partial incorporation in the acidic backbone methyls of the NacNac ligand (Scheme 7). On the other hand, like the cation 2, complex 6 can mediate the redistribution of HSiEt3 and releases hydrosilylated phenylacetylene very slowly. Scheme 7. Alkyne Labeling Experiment with DSiEt3 and 6 as the Catalyst

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CONCLUSION In conclusion, we have demonstrated that the cationic aluminum complex [DippNacNacAlH][BArF4] is a catalyst for the hydrosilylation of various alkenes and alkynes. Stoichiometric reactions between the aluminum complex and alkene and alkyne substrates revealed divergent reactivity. While alkenes inserted into the Al−H bond to give alkyl products, labeling experiments ruled out the common insertion/metathesis pathway for this system. On the other hand, stoichiometric reactions with alkynes revealed addition to the backbone methine carbon and aluminum to form new bridged tripodal species. Both of these products are competent catalysts for hydrosilylation. Reactions of hydride complex 2 with trialkyl- and tetraalkylsilanes showed facile silane redistribution, accounting for the low yields of the target hydrosilylation products. These observations suggest a mechanism based on Lewis acid activation by the cationic aluminum center.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01694. Details of syntheses, characterizations, and catalytic studies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for G.I.N.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Petroleum Research Fund, administered by the American Chemical Society. K.J. further thanks the Natural Sciences and Engineering Research Council of Canada for an Undergraduate Student Research Award. T.C. is grateful to the Government of Ontario for an Ontario Graduate Scholarship. We thank Razvan Simionescu for help with NMR experiments.

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REFERENCES

(1) Bullock, R. M. Catalysis Without Precious Metals; Wiley-VCH: Weinheim, Germany, 2010. (2) (a) Power, P. P. Nature 2010, 463, 171−177. (b) Power, P. P. Acc. Chem. Res. 2011, 44, 627−637. (c) Power, P. P. Chem. Rec. 2012, 12, 238−255. (d) Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354−396. 7355

DOI: 10.1021/acscatal.6b01694 ACS Catal. 2016, 6, 7350−7356

Research Article

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DOI: 10.1021/acscatal.6b01694 ACS Catal. 2016, 6, 7350−7356