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 deproto‐nated by treatment with methyllithium in ether at –78°C to give the transient 2‐(2,4‐di‐ tert ‐butylphenyl)‐1,1‐bis(trimeth‐ylsilyl)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 ‐bu‐tylphenyl) ‐1,2,3,8a‐tetrahydro‐2,2,3,3‐tetrakis(trimeth‐ylsilyl)‐1,2‐disilanaphthalene ( 4 ), the formal [2 + 4] cyclodimer of 3a . 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(tri‐methylsilyl)silyllithium resulting in the formation of 6,8‐di‐ tert ‐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.