Mechanistic studies of the addition of carbonyl compounds to tetramesityldisilene and tetramesitylgermasilene

The mechanism for the addition of carbonyl compounds to disilenes and germasilenes was investigated by examination of the structure of the products obtained from the reaction of the modified mechanistic probe, trans,trans-2-methoxy-3-phenylcyclopropane carbaldehyde, with tetramesityldisilene and tetramesitylgermasilene. The aldehyde was found to undergo characteristic ring-opening reactions depending on whether a radical or a cation develops at the carbinyl carbon. Thus, reaction of the mechanistic probe under ionic conditions (p-TsOH) yielded a mixture of cis- and trans-2-methoxy-3-phenyl-2,3-dihydrofuran (6) from ring-opening toward the methoxy substituent. In contrast, reaction of the mechanistic probe under radical conditions (HSi(SiMe33), AIBN) yielded a mixture of cis- and trans- 3-methoxy4-phenyl- 1-[tris(trimethylsilyl)siloxy] but-1-ene (7) from regioselective ring-opening toward the phenyl substituent. 2,2,3,3-Tetramesityl-4-phenyl-5-methoxyoxa-2,3-disilacyclohept-6-ene (8) was formed from the reaction between the aldehyde and tetramesityldisilene, and a mixture of three isomers of 2,2,3,3-tetramesityl(phenyl)(methoxy)oxagermasilacyclohept-6-acyclohept-6ene (10a-c) and [dimesityl(1-oxa-4-methoxy-5-phenylpentadienyl)silyl] germane (10d) was formed in the reaction of the aldehyde with tetramesitylgermasilene. The formation of 8, 10a, 10b, and 10d provides unequivocal evidence for the presence of radical intermediates in the addition of carbonyl compounds to the disilene and germasilene.