Alkynylsilanes silyl migration

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

Ring expansion accompanied by 1,2-silyl migration was observed in the reactions of a cyclobutenedione monoacetal 103 with alkynylsilanes (equation 70)180. 

Scheme 4-42) [134]. Werner s group has shown that alkynylsilanes also undergo 1,2-silyl migration in the same system via an intermediate 7i-complex [135]. Alternatively, the rearrangement may be catalyzed by base [136] or induced by sequential deprotonation/protona-tion [137]. The relative stability of the alkyne and vinylidene complexes is dependent on the electron density and the d-electron count of the metal, as illustrated by the behavior of the d -Mo complexes 84 in which the alkyne is a four-electron donor addition of CO causes the 84 85 conversion whereas tautomer 84 is favored with the phosphite ligand (Scheme 4-43)... 

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