Mechanistic study of the thermal rearrangement of silylheterocumulenes and their propargyl isomers

The formation of bis(silyl)ketenes on reaction of 2,3-dihydrofuran (and 2,3-dihydro-2-phenylfuran) with n-BuLi and quenching with chlorosilanes was investigated and found to occur by the rearrangement of siloxyacetylene intermediates. 2,3-Dihydro-thiophene did not undergo similar decomposition;4,5-Dihydro-3-methyl-2-(trimethylsilyl)furan and 2,3-dihydro-5-(trimethylsilyl)thiophene were both subjected to flash vacuum pyrolysis (FVP) and found to undergo reductive elimination to form vinylidenes, which then formed the corresponding acetylenes. When 2,3-dihydro-2-phenyl-5-(trimethylsilyl)furan was subjected to FVP only fragmentation occurred. These results were suggestive of a concerted mechanism, versus a homolysis mechanism, for the formation of silylketenes via a siloxyacetylene on pyrolysis of 5-silyl-2,3-dihydrofurans;Several siloxyacetylenes were subjected to FVP and found to undergo quantitative conversion to the corresponding silylketenes. Retro-Wolff rearrangement of the silylketenes produced was observed at temperatures higher than those required for the siloxyacetylene-silylketene rearrangement;Extension of the studies to silylthioacetylenes produced isomerization to the corresponding silylthioketenes, but instead of quantitative conversion, the systems were found to establish an equilibrium. At increasingly higher pyrolysis temperatures the silylthioketenes were found to decompose to acetylenes which could be accounted for by 1,2-alkyl and 1,2-silyl shift mechanisms. Silylated heteroacetylenic ethers with an acetylenic carbon [superscript]13 C-enriched were prepared and pyrolyzed. The resulting silylheteroallenes were found to have only a single heteroallenic carbon [superscript]13 C-enriched, that which one would expect if the rearrangement occurred via a 1,3-silyl shift. Arrhenius parameters for both systems were obtained utilizing a stirred-flow reactor (SFR). These results, combined with theoretical calculations, were shown to be consistent with a 1,3-silyl shift mechanism for the formation of silylheterocumulenes from their propargyl isomers