SOLVENT-DEPENDENT STEREOSELECTIVITY IN THE ADDITION OF P-CH3OC6H4OH TO (E)-1,2-DI-TERT-BUTYL-1,2-DIMESITYLDISILENE - EVIDENCE FOR ROTATION AROUND THE SI-SI BOND IN THE ZWITTERIONIC INTERMEDIATE

The diastereoselectivity of the addition of p-CH3OC6H4OH to (E)-1,2-di -tert-butyl-1,2-dimesityldisilene (2) in benzene and in THF is reporte d, and the mechanistic implications of the results are discussed. In b oth benzene and in THF the stereochemistry of the products is independ ent of the phenol concentration (in the range of phenol excess of 2-30 0 equiv). The syn:anti product ratio is 90:10 in benzene and of 20:80 in THF, i.e., with stereochemical preference opposite to that in benze ne. The finding that the stereochemistry of the products is independen t of the phenol concentration indicates that in the second step of the addition reaction intermolecular proton transfer (from a second molec ule of phenol) does not occur. Thus, the anti-addition product results from rotation around the Si-Si bond in the zwitterionic intermediate followed by intramolecular proton transfer. The very different product ratios in benzene and in THF support the formation of zwitterionic in termediates and suggest a longer lifetime of the zwitterionic intermed iates in THF than in benzene.