SILAHETEROCYCLES .14. REGIOSPECIFIC CYCLOADDITION REACTIONS OF DICHLORONEOPENTYLSILENE WITH CYCLOHEXA-1,3-DIENE - A NOVEL 7-SILABICYCLO[4.2.0]OCT-2-ENE-]2-SILABICYCLO[2.2.2]OCT-5-ENE REARRANGEMENT

In the presence of cyclohexa-1,3-diene (2), dichloroneopentylsilene (1), generated in situ from trichlorovinylsilane and t-BuLi, favors the regioselective formation of the anti/syn isomeric [2+2] cycloadducts (3, 74%) over the [4+2] addition, leading to the endo/exo isomer Diels-Alder compounds 4 (26%). On standing, the thermodynamically less stable silacyclobutane derivatives 3 completely isomerize to give the [4+2] products within several weeks. Possible reaction pathways, including a zwitterionic intermediate A, are discussed. In contrast, the dimethyl- and diphenyl-substituted [2+2] adducts (7, 8), available from 3 by substitution reactions, show different isomerization behavior. They cannot be transformed into the Diels-Alder compounds, but the syn [2+2] isomers slowly from the thermodynamically more stable anti derivatives at room temperature. On thermolysis they yield open-chain products by a retro ene reaction. The high synthetic potential of strongly electrophilic 1 is demonstrated by the comparison of the cycloaddition behavior with diorgano-substituted neopentylsilenes R2Si = CHCH2Bu-t: Reaction of Me2Si = CHCH2Bu-t with 2 favors the [4+2] product formation, while Ph2Si = CHCH2Bu-t yields the Diels-Alder adducts exclusively.