Gas-Phase Thermal Rearrangements of Potential Vinylidene Precursors to Silylbenzofurans and Silylbenzopyrans

Article abstract of DOI:10.1021/jo00202a004

In an attempt to utilize the considerable migratory aptitude of silicon in the synthesis of 3-silylbenzofurans, the flash vacuum pyrolysis (FVP) of o-<(trimethylsilyl)ethynyl>phenol was found to provide not only the furan expected from vinylidene cyclization but two isomers resulting from initial 1,5-hydrogen migration from oxygen to form an intermediate allenyl ketone.FVP of 2-(trimethylsilyl)4,5-dihydrofuran produced an unprecedented gas-phase reductive elimination to a vinylidene. o-Ethynyl- and o-propynylanisoles did not afford benzopyrans through vinylidene/C-H(Me) insertion but underwent radical transformations.However, o-<(trimethylsilyl)ethynyl>anisole unexpectedly extruded Me2Si to form 2-ethylbenzofuran as the only product.Various mechanisms for this remarkable decomposition are considered.The acyclic analogue 1-(trimethylsilyl)-4-methoxybut-1-yn-3-ene pyrolytically extruded not Me2Si but carbon monoxide! This is rationalized as proceeding through an initial 1,5-methyl migration from oxygen to carbon.