THERMALLY-INDUCED 1,2-SHIFTS TO CONVERT OLEFINS TO CARBENES - DOES SILICON DO IT - IF SO, WHY NOT CARBON

Thermal isomerization of olefins to carbenes via a 1,2-silyl shift was examined by both experiment and theory. No evidence of this rearrange ment was found for acyclic vinylsilanes, nor could electronic assistan ce by silicon be identified in cis, trans isomerizations. Serendipitou s synthesis of a 2,4-dimethylene-1,3-disilacyclobutane allowed a kinet ic examination of its gas-phase, thermal ring expansion to a 2-methyle ne-1,3-disilacyclobutane. The Arrhenius parameters (log A = 12.48, E(a ct) = 54.09 kcal/mol) are the first to be reported for an olefin-to-ca rbene rearrangement. The analogous all-carbon system failed to ring ex pand. Ab initio calculations revealed that this was opposite to any pr edictions which would be made from ring strain considerations. Calcula tions showed that for silyl migration the transition state was late an d was actually the carbene, while for carbon migration the TS was earl y and considerably higher in energy than the resulting carbene. The 2- methylene-1-silacyclobutane rearrangement (ref 5) was reexamined to fi nd that reversible ring opening to a 1,4-diradical occurred at tempera tures below those required to ring expand via a carbene TS.