Lewis Acid Catalyzed trans-Allylsilylation of Unactivated Alkynes

The addition of different substituted allylsilanes 2 to unactivated alkynes 1 in the presence of catalytic amounts of HfCl 4 or the EtAlCl 2 −TMSCl catalyst system produced in high yields the silylated 1,4-dienes 3 regio- and stereoselectively. The exclusive trans manner of addition was confirmed by analysis of crude reaction mixtures by 1 H NMR and capillary GLC methods. Good agreement of relative reactivities of reaction of various allylsilanes 2a − e toward phenylacetylene ( 1a ) in the presence of HfCl 4 with the relative reaction rates of 2a − e with carbenium ions supported the involvement of cationic species 11 as a reaction intermediate. The mechanisms for the HfCl 4 and EtAlCl 2 −TMS catalyzed trans -allylsilylation of alkynes are proposed.