Anti-Hydrosilylation Reactions of Alkynes Catalyzed by Palladium Nitrate

Abstract

Hydrosilylation of alkynes catalyzed by transition metal complexes is the most effective method for the preparation of vinylsilanes that might be the excellent precursors to silicon polymers. Although thermodynamically stable synhydrosilylation products are generally observed in this type of reactions, changing catalyst and reaction conditions can lead to an unusually high degree of anti-hydrosilylation. We performed the hydrosilylation reactions of RC≡CH (R = Ph, C10H21) with R'3SiH (R'3 = Et3, t-BuMe2, Me2Ph) in the presence of 2 mol % palladium nitrate at room temperature. Initially, Pd(NO3)2-phosphine was used as the catalyst in order to expand our previous works since it had been found to be an effective catalyst for the activation of both aromatic and aliphatic carbon-hydrogen bonds in our laboratory. However, palladium(II) was reduced to palladium(0) in the presence of phosphine in this work and the reaction did not proceed further. Here we report the unusual anti-hydrosilylation reactions of alkynes with R3SiH using Pd(NO3)2 as the catalyst without phosphine, which has rarely been reported in this kind of reactions previously. The cis and trans products (1 and 2, respectively) were obtained along with dialkynylated silanes (3) as described in Scheme 1 and the results are summarized in Table 1. As shown in Table 1, it is unusual that thermodynamically less stable cis isomers 1 derived from anti-hydrosilylation were obtained almost exclusively over trans isomers 2 for the entries 1, 2, 4 and 5 with selectivity of 96-99%. More surprisingly, only anti-hydrosilylation product 1 was observed at the beginning of the reaction in GC analysis and it was slowly converted to 2, of which ratios reached 1 to 4% at most and the ratios were unchanged since then. Usually, the reaction mechanism has been reported to proceed through the initial syn-addition of the hydrosilanes to alkynes giving the trans product, followed by isomerization to the cis product, even in the case of the other predominant antihydrosilylation reactions. In fact, the isomerization of cis product to trans product was first observed by Watanabe et al., but the selectivity was not high enough. The investigation of the reaction mechanism in this work is in progress and will be published later. It is noteworthy that the above results were observed only for the aliphatic silanes. In the case of aromatic silane, Me2PhSiH, the reactions with PhC≡CH and C10H21C≡CH gave 62 : 38 and 49 : 51, respectively, in the ratios of the