SILICON-COMPOUNDS WITH STRONG INTRAMOLECU LAR STERIC INTERACTIONS .50. REACTIONS OF SILYLENES AND DISILENES WITH 2,2'-BIPYRIDYL, PYRIDINE-2-ALDIMINES AND ALPHA-KETOIMINES - CYCLOADDITIONS VERSUS C-H INSERTION

Treatment of 2,2'-bipyridyl or its 4,4'-dimethyl derivative with the s terically encumbered silylenes tBu2Si: and MeS2Si: gives violet, very air-sensitive Z2'-bipyridyl(N,N')silanes in which the heteroaromatic r ings are converted into conjugated double-bond systems. Reaction of th ese silylenes with N-t-butylpyridine-2-aldimine proceeds also in a (4 + l)-fashion to provide the N-t-butylpyridine-2-aldimine-(N,N')silanes 11 and 12. Subsequent addition of tetra-t-butyldisilene across the CC double bond of 12 in the 3,4-position gives the tricyclic compound 13 . Replacement of the N-t-butyl by the sterically more crowded N-2,6-di isopropylphenyl group leads to an unprecedented intermolecular inserti on of dimesitylsilylene into the CH bond of the acyclic CN group. The structure of the insertion product 1,1-dimesityl-2-pyridine-2-yl-1-sil aethane-2-imine (15) was confirmed by X-ray crystallography. Photolysi s of hexa-t-butylcyclotrisilane in the presence of alpha-ketoimines be aring isopropyl or cyclohexyl groups at the nitrogen atom gives the co rresponding silylene cycloaddition products to the N- and 0-atoms. Wit h the N-methyl derivative, however, the (4 + 2)-cycloaddition product 21 of tetra-t-butyldisilene to these atoms is isolated. The X-ray stru cture analysis of the 1-oxa-2,3-disila-4-aza-5-cyclohexane derivative 21 shows that the unsaturated six-membered ring has the half chair con formation, which minimizes the repulsive interactions between the bulk y substituents and leads to a hardly stretched Si-Si bond length (237. 9 pm).