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 reported, and the mechanistic implications of the results are discussed. In both benzene and in THF the stereochemistry of the products is independent of the phenol concentration (in the range of phenol excess of 2-300 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 benzene. The finding that the stereochemistry of the products is independent of the phenol concentration indicates that in the second step of the addition reaction intermolecular proton transfer (from a second molecule 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 intermediates and suggest a longer lifetime of the zwitterionic intermediates in THF than in benzene.