Hydrosilylation of Alkynes Catalyzed by an Iron(II) PCP Pincer Alkyl Complex

Abstract

Vinylsilanes are very useful building blocks in organic synthesis and have widespread applications in life sciences and materials chemistry. Here we describe the potential of complex cis-[Fe(PCP-iPr)(CH₂CH₂CH₃)(CO)₂] as an effective catalyst for the hydrosilylation of both terminal and internal alkynes with SiPhH3 to give vinylsilanes. The reactions were typically performed with a catalyst loading of 1 mol% for 24 h at 70 °C. The catalytic reaction is initiated by migratory insertion of a CO ligand into the Fe─alkyl bond to yield an acyl intermediate, which reacts with silanes to form the 16e⁻ Fe(II) silyl catalyst [Fe(PCP-iPr)(SiPhH₂)(CO)]. In the case of aliphatic terminal alkynes good regioselectivity (anti-Markovnikov addition) toward the thermodynamically more stable β-(E)-vinylsilanes in ratios of up to 10:90 was achieved, while for aromatic alkynes the selectivities were poor with ratios of β-(Z)- to β-(E)-vinylsilanes of about 40:60. With internal unsymmetrical alkynes, the two possible regioisomers of the syn-addition of SiPhH₃ were obtained in different ratios with no clear trend toward one regioisomer. Internal symmetrical alkynes yielded exclusively the respective syn-products in high yields. Mechanistic investigations including deuterium labelling studies were undertaken to provide a reasonable reaction mechanism.