2-(2,4-Di-tert-butylphenyl)-1,1-bis(trimethylsilyl)silene and 2-(2,4,6-tri-tert-butylphenyl)-1,1-bis(trimethylsilyl)silene two new sterically protected but still unstable silaethenes

(2,4-Di-tert-butylphenyl)tris(trimethylsilyl)silylmethanol (1a), prepared by the reaction of tris (trimethylsilyl)silylmagnesium bromide with 2,4-di-tert-butylbenzaldehyde, was deprotonated by treatment with methyllithium in ether at -78 degrees C to give the transient 2-(2,4-di-tert-butylphenyl)-1,1-bis(trimethylsilyl)silene (3a), which dimerizes in a head-to-head fashion with the formation of (E)-/(Z)-3,4-bis (2,4-di-tert-butylphenyl)-1,1,2,2-tetrakis(trimethylsilyl)-1,2-disilacyclobutane (5). Besides 5 an unstable compound 4 was obtained, which was preliminarly assigned as 5,7-di-tert-butyl-1-(2,4-di-tert-butylphenyl)-1,2,3,8a-tetrahydro-2,2,3,3-tetrakis(trimethylsilyl)-1,2-disilanaphthalene (4), the formal [2 + 4] cyclodimer of 39. Compound 4 gradually decomposes to give (E)-/(Z)-5, and is considered to be the kinetically preferred dimer of 3a, which is converted into the thermodynamically stable 5. 2,4,6-Tri-tert-butylbenzaldehyde reacts with tris(trimethylsilyl)silyllithium resulting in the formation of 6,8-ditert-butyl-1,2,3,4-tetrahydro-4,4-dimethyl-2,2-bis(trimethylsilyl)-2-silanaphthalene (6). Compound 6 is the product of the insertion of the Si=C bond into the C-H bond of an o-tert-butyl group of the intermediate 2-(2,4,6-tri-tert-butylphenyl)-1,1-bis(trimethylsilyl)silene (3b), which despite extreme steric shielding proved to be still unstable. For compounds (Z)-5 and 6 the results of the X-ray analyses are given.