Substituent effects in the ruthenium catalyzed hydrosilylation of para-substituted phenylacetylenes

The RuCl2(PPh3)3 catalyzed hydrosilylation of various phenylacetylenes (p-XC6H4Ctriple bond; length of mdashCH, X = H, Cl, Me, OMe, CF3) with HSiCl2Me was investigated to determine the effect of para-substitution on phenylacetylene reactivity. A series of competition experiments were performed to determine the reactivity of the substituted phenylacetylenes which followed the trend (relative reactivity): CF3(2.21) > Cl(1.99) > F(1.85) > OMe(1.04) > H(1.00) > Me(0.85). A plot of log (phenylacetylene relative reactivity) versus Hammett σp exhibited two separate, but parallel, trendlines. The positive slopes (ρ = 0.60) of these lines indicated that electronegative substituents increased phenylacetylene reactivity. In addition to the electronic effect, a conjugation effect was present that resulted in the second trendline for X = OMe, Cl and F.