No abstract

The mechanism of the addition of aldehydes to disilenes and germasilenes was investigated by examining the reaction between trans-(2-phenylcyclopropyl)carboxaldehyde and tetramesityldisilene or tetramesitylgermasilene. [Dimesityl(2-cis-4-trans-1-oxa-5-phenylpentadienyl)silyl]dimesitylsilane (1) and 2,2,3,3-tetramesityl-4-phenyloxa-2,3-disilacyclohept-6-ene (2) were formed in the reaction between the aldehyde and the disilene, and the analogous [dimesityl(2-cis-4-trans-1-oxa-5-phenylpentadienyl)silyl]dimesitylgermane (4) and 2,2,3,3-tetramesityl-4-phenyloxa-2,3-silagermacyclohept-6-ene (5) were formed in reaction with the germasilene. It is proposed that compounds 1, 2, 4, and 5 are the result of either disproportionation or ring closure of an intermediate biradical derived from initial bond formation between the oxygen atom of the aldehyde and the doubly bonded silicon atom of the dimetallene.

The addition reactions of tetramesitylgermasilene with 1-methoxybutadiene, ethyl vinyl ether, vinyl acetate, or styrene were studied. When tetramesitylgermasilene was allowed to react with 1-methoxybutadiene or styrene, formal [2+2] addition products were isolated. The addition of styrene to tetramesitylgermasilene was determined to be completely regioselective. In the presence of ethyl vinyl ether or vinyl acetate, tetramesitylgermasilene undergoes a 1,2-mesityl shift yielding a silylgermylene, at a faster rate than addition to either alkene. Tetramesityldisilene was also found to yield a formal [2+2] adduct with styrene. However, tetramesityldigermene rearranges to a germylgermylene at a faster rate than styrene addition.

It has been demonstrated that the same Pd catalyst can be used to effect allylic substitution or vinylic cross-coupling reactions selectively and interchangeably on polyfunctionalized olefin building blocks despite the differences in reaction mechanism. This was achieved by altering the pK(a) of the conjugate acid of the allylic leaving group while keeping the vinyl coupling partner constant. In the case of 2,3-dibromo-1-propene, Pd-catalyzed allylic ionization with malonate nucleophile proceeded selectively and quantitatively in the presence of the Suzuki reaction components necessary for cross-coupling. Conversely, the bromide of 2-bromo-1-(4-ethylphenoxy)-2-propene could be cross-coupled selectively without activation of the allylic phenoxy substituent. In both reactions, the same catalyst could then be used to complete the sequence, which typically involved heating as the trigger to promote the second, more reluctant reaction. Mechanistic considerations as well as synthetic applications demonstrating the value of this interchangeable catalyzed sequence are presented.