Double cyclic silylenes

Surprising thermal and chemical stability of the cyclic silylene (29) may be explained by an enhanced aromaticity. This can be seen by comparison with its percursor (30) the difference in length between the single N—C (1.415 A) and double C=C (1.322 A) bonds in (30) decreases in (29) (N—C 1.400 A, =C 1.347 A) to imply an increased delocalization. Also the Si—N bond length increases in (29) only by 0.056 A (from 1.697 A to 1.753 A) instead of the 0.080-0.100 A typical for transition from tetra- to dicoordinate silicon this may indicate developing partial Si= N double bond character in (29) <94JA269l>. 

The formation of silaneselone 57 was evidenced by the trapping reaction with mesitonitrile oxide leading to the corresponding cycloadduct 58 and was also supported by the observation of a remarkably downfield 29Si chemical shift (8Si = 174) indicative of the Si=Se double bond of 57. Although this direct selenation of silylene 55 with an equimolar amount of selenium was not reproducible, the use of excess amount of elemental selenium resulted in the formation of a new cyclic diselenide, diselenasilirane 59, as a stable compound (8Si = -44 and... 

Seven-membered ring trani-alkenes have been accessed diastereoselectively from dienes and aldehydes by the silver-catalysed transfer of silylene (Scheme 13). The trans geometry of the double bond has been assigned from cleavage of the silicon-oxygen bond (path (a)). The high strain of these cyclic trawi-alkenes can... 

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