Extremely Regioselective Intramolecular Silylformylation of Bis(silylamino)alkynes

Intramolecular silylformylation of ω-bis(dimethylsilylamino)-i-alkynes is investigated. This process affords exo-(1-formylalkenylidene)azasilacycloalkane as the initial product, which is converted to the more stable exo-(1-hydroxymethylalkenylidene)azasilacycloalkane through NaBH₄ reduction in methanol in high overall yield. Both exo-alkenylidene azasilacycloalkanes are literature unknown compounds and would be versatile polyfunctionalized intermediates in organic syntheses. It should be noted that only the exo-SiMe₂H group is removed in this reduction, and the resulting cyclic silylamine is stable against methanolysis and hydrolysis, which is unexpected and unprecedented. The observed stability of the Si-N bond in the cyclic silylamine is found to be ascribed to the existence of the exo-alkylene group at the C-5 position. Thus, the hydrogenation of this double bond leads to a facile Si-N bond cleavage by methanol. In a similar manner, the reactions of 1-bis(dimethylsilyl)amino-2-(1-hexynyl)-cycloalkane, followed by NaBH₄ reduction, give the corresponding exo-(1-formylalkyl-1-ene)-azasilabicyclo[x.3.0]alkanes in high yields. When the diphenylsilyl group is employed in place of the dimethylsilyl group, the reaction gives the corresponding intramolecular silylformylation product in good yield. The exo-HPh₂Si-N bond of this product is found to be stable for the reduction with NaBH₄, giving the corresponding allylic alcohol.