Communication pubs.acs.org/Organometallics
Merging Platinum-Catalyzed Alkene Hydrosilylation with SiH4 Surrogates: Salt-Free Preparation of Trihydrosilanes Polina Smirnov and Martin Oestreich* Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 115, 10623 Berlin, Germany S Supporting Information *
ABSTRACT: The monosilane (SiH4) surrogate di(cyclohexa-2,5dien-1-yl)silane is shown to be compatible with platinum-catalyzed hydrosilylation of α-olefins. The cyclohexa-2,5-dien-1-yl substituents in the monohydrosilylation adducts serve as protecting groups, and treatment with catalytic amounts of B(C6F5)3 liberates the Si−H bonds along with benzene. By this, trihydrosilanes become accessible in two steps without the formation of salt waste.
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The preparation of trihydrosilanes 5 is generally challenging, and the literature-known methods rely on the reduction of either AlkylSiCl36 or AlkylSi(OEt)3.7 As part of our earlier work,3c we had already demonstrated that solid surrogate 1 and its liquid congener 6 are compatible with transition-metalcatalyzed alkene hydrosilylation (one example each). Platinumcatalyzed hydrosilylation of oct-1-ene had worked equally well with 1 (69%) and 6 (58%), but the intermediate derived from 6 (quantitative) was far superior to that of 1 (30%) in B(C6F5)3catalyzed release of the Si−H bonds. This two-step sequence consisting of transition-metal-catalyzed hydrosilylation and Lewis-acid-promoted deprotection offers a straightforward entry into the salt-free preparation of trihydrosilanes (2 → 7 → 5, Scheme 1, bottom). In this Communication, we summarize the elaboration of this methodology to access this useful class of compounds.8 We continued testing various α-olefins (Table 1). The initially obtained yield of 58% in the hydrosilylation of oct-1ene3c was further improved to 74% in excess alkene9 (2a → 7a, entry 1). Likewise, an α-olefin with a cyclohexyl group in the homoallylic position reacted in 56% yield within 24 h (2b → 7b, entry 2). However, more sterically hindered systems required longer reaction times for full consumption of surrogate 6 (2c/2d → 7c/7d, entries 3 and 4). Interestingly, when 2c was used in excess, 2-fold hydrosilylation involving both Si−H bonds in 6 was observed, furnishing 8c in 51% isolated yield (not shown; see the Supporting Information for characterization data). Reducing the loading of 2c to 1.2 equiv led to the chemoselective formation of adduct 7c. Allylbenzene was also suitable for this reaction (2e → 7e, entry 5), although the yield was diminished due to the formation of β-methylstyrene as byproduct. That yield was improved when allylpentafluorobenzene was used instead (2f → 7f, entry 6). Moreover, C(sp3)−Br and C(sp3)−Si bonds as well as a silyl ether were tolerated in the hydrosilylation step (2g−i → 7g−i, entries 7−
he development of different methods for alkene hydrosilylation continues to attract attention.1 However, the smallest member of the hydrosilane family, monosilane (SiH4), is essentially not used, as its handling comes along with inherent hazards of flammability and toxicity. To provide a safer alternative to this dangerous gas, a new type of stable hydrosilane surrogate was recently introduced by us, enabling the installation of SiH4−n moieties (n = 1−4).2,3 For example, solid cyclohexa-2,5-dien-1-yl-substituted silane 1 reacts with various terminal and internal alkenes 2 in the presence of tris(pentafluorophenyl)borane, B(C6F5)3,4 in an ionic transfer hydrosilylation (2 → 3−5, Scheme 1, top).3c This one-pot Scheme 1. SiH4 Surrogates in Alkene Hydrosilylation
process involves two consecutive catalytic cycles where the same borane catalyst promotes liberation of the Si−H bonds and then mediates the hydrosilylation of the C−C double bond through Si−H bond activation.5 This strategy was effective for the chemoselective synthesis of mono- (3; n = 3) and dihydrosilanes (4; n = 2)3c but not trihydrosilanes (5; n = 1). © XXXX American Chemical Society
Received: June 20, 2016
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DOI: 10.1021/acs.organomet.6b00505 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Preparation of Trihydrosilanes from α-Olefinsa
Wilkinson’s catalyst (Ph3P)3RhCl. In contrast, the same protocol applied to Ph3SiH afforded the hydrosilylation adduct in good yield. Also, 1,1- and 1,2-disubstituted alkenes did not undergo hydrosilylation. The newly developed two-step synthesis provides an alternative route for the preparation of synthetically useful trihydrosilanes.8 The use of the liquid monosilane surrogate avoids handling of gaseous SiH43c but also AlkylSiCl36 and AlkylSi(OEt)3.7 The cyclohexa-2,5-dien-1-yl substituents serve as an easy-to-remove protecting group at the silicon atom, only releasing benzene as waste. Moreover, these groups could also be engaged in transfer hydrosilylation2 to access heteroleptic diand triorganosilanes (or dihydro- or monohydrosilanes).3a,b
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00505. Synthetic procedures and figures giving NMR spectra of the compounds synthesized in this paper (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. a
Unless otherwise noted, the platinum-catalyzed hydrosilylations were performed on a 0.4 mmol scale (based on 6) in alkene 2 (0.4 mL) at room temperature under an inert atmosphere in a glovebox. bIsolated yield after flash chromatography on silica gel. cDetermined by 1H NMR spectroscopy using mesitylene as an internal standard. d1.2 equiv of 2c used. eAlkene migration observed. fDecomposition.
Notes
9). All hydrosilylation adducts 7a−h except for 7i underwent the B(C6F5)3-catalyzed deprotection in CD2Cl2 quantitatively (7a−h → 5a−h, entries 1−8). The silyl ether in 7i was not stable toward B(C6F5)3. To demonstrate the scalability of this methodology, we performed the two-step synthesis of n-octylsilane on a 4 mmol scale based on surrogate 6 (cf. 2a → 7a → 5a, entry 1). After 20 h, intermediate 7a was obtained in 82% yield after flash chromatography on silica gel. Purified 7a was then subjected to deprotection, and trihydrosilane 5a was isolated by distillation after 5 h. Attempts to turn this two-step transformation into a one-pot procedure were unsuccessful. Several other linear α-olefins, 2j−m, functionalized at the aliphatic terminus as well as α,β-unsaturated acceptors 2n−p were subjected to platinum-catalyzed hydrosilylation employing surrogate 6 (Chart 1). Unfortunately, none of them were
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.S. gratefully acknowledges the Minerva Foundation for a postdoctoral fellowship (2016−2017). M.O. is indebted to the Einstein Foundation (Berlin) for an endowed professorship. REFERENCES
(1) Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluć, P. In Advances in Silicon Science; Marciniec, B., Ed.; Springer: Berlin, 2009; Vol. 1, pp 3−51. (2) For a review of transfer hydrosilylation, see: Oestreich, M. Angew. Chem., Int. Ed. 2016, 55, 494−499. (3) (a) Simonneau, A.; Oestreich, M. Angew. Chem., Int. Ed. 2013, 52, 11905−11907. (b) Keess, S.; Simonneau, A.; Oestreich, M. Organometallics 2015, 34, 790−799. (c) Simonneau, A.; Oestreich, M. Nat. Chem. 2015, 7, 816−822. (4) For a recent review of B(C6F5)3-catalyzed Si−H and H−H bond activation, see: Oestreich, M.; Hermeke, J.; Mohr, J. Chem. Soc. Rev. 2015, 44, 2202−2220. (5) Sakata, K.; Fujimoto, H. Organometallics 2015, 34, 236−241. (6) (a) Finholt, A. E.; Bond, A. C., Jr.; Wilzbach, K. E.; Schlesinger, H. I. J. Am. Chem. Soc. 1947, 69, 2692−2996. (b) Sakurai, H.; Shoji, M.; Yajima, M.; Hosomi, A. Synthesis 1984, 598−600. (7) (a) Hosomi, A.; Iijima, S.; Sakurai, H. Chem. Lett. 1981, 243− 246. (b) Mucha, N. T.; Waterman, R. Organometallics 2015, 34, 3865− 3872. (8) For the use of trihydrosilanes in surface modification, see for example: (a) Fadeev, A. Y.; McCarthy, T. J. J. Am. Chem. Soc. 1999, 121, 12184−12185. (b) Owens, T. M.; Nicholson, K. T.; Banaszak Holl, M. M.; Süzer, S. J. Am. Chem. Soc. 2002, 124, 6800−6801. (c) Pelzer, K.; Hävecker, M.; Boualleg, M.; Candy, J.-P.; Basset, J.-M. Angew. Chem., Int. Ed. 2011, 50, 5170−5173. (9) The large excess of the alkene is not necessary. For example, 70% yield was achieved in the model reaction when using 2.0 equiv of the alkene substrate; surrogate 6 was fully consumed within 20 h.
Chart 1. α-Olefins and α,β-Unsaturated Acceptors Not Compatible with the Hydrosilylation Step
compatible with the reaction setup, and only small amounts of the desired adducts 7j−p (not shown) were detected despite complete conversion of 6. The hydrosilylation of styrene with 6 furnished a complex mixture along with traces of the expected adduct (2q → 7q; not shown). A similar outcome was found when the platinum catalyst (cod)PtCl2 was replaced by B
DOI: 10.1021/acs.organomet.6b00505 Organometallics XXXX, XXX, XXX−XXX
