Generation of Silyl Radicals via Photocatalyzed Si-H Bond Activation
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Photocatalytic reactions are those processes that take place by using a catalyst activated by light.1 Photocatalysis by polyoxoanions, in particular by tetrabutyl ammonium decatungstate (n-Bu4N)4[W10O32] (TBADT)2 is emerging as a powerful method for the formation of valuable chemical bonds under environmental friendly conditions. This is based on the capability of TBADT to activate a variety of aliphatic derivatives including amides, aldehydes, oxygenated compounds (ethers, acetals) and even alkanes, via direct C-H activation (X = CR3, see Scheme).2,3 The corresponding radicals (X●) are then trapped by electron-poor olefins to give a radical adduct that accepts a hydrogen atom from the reduced photocatalyst (●P-H), to afford the end product with the concomitant P regeneration. In this work we shifted our attention to Si-H bonds since none of the methods so far described for the generation of silyl radicals is of general applicability. Indeed, the direct silylation of electrophilic -unsaturated nitriles, esters, sulfones and even ketones) has been achieved in the presence of 2% mol TBADT. The reaction was limited to trisubstituted alkyl or aryl silanes (X = SiR3). The presence of at least one phenyl group was mandatory to achieve a clean reaction (36%-90%). By contrast, the reaction on trialkylsilanes gave a mixture of products in moderate yields (40-60%) arising from the competitive Si-H/ C-H bonds activation.4 References: 1. D. Ravelli, D. Dondi, M. Fagnoni, A. Albini. Chem. Soc. Rev., 2009, 38, 1999. 2. M. D. Tzirakis, I. N. Lykakis, M. Orfanopoulos Chem. Soc. Rev., 2009, 38, 2609. 3 For recent examples by our group, see: H. Qrareya, D. Ravelli, M. Fagnoni, A. Albini. Adv. Synth. Catal., 2013, 355, 2891 and references therein. 4. H. Qrareya, D. Ravelli, M. Fagnoni, A. Albini, manuscript in preparation.