Head-to-head versus head-to-tail dimerizations of transient 2-[(dimethylamino)phenyl]-1,1-bis(trimethylsilyl)silenes

2-[2-(Dimethylamino)phenyl]-1,1-bis(trimethylsilyl)silene (2), generated by deprotonation and trimethylsilanolate elimination from [2-(dimethylamino)phenyl][tris(trimethylsilyl)silyl]-methanol (1), dimerizes in a head-to-tail mode with formation of an E/Z mixture of 2,4-bis[2-(dimethylamino)phenyl]-1,1,3,3-tetrakis(trimethylsilyl)-1,3- disilacyclobutane (3), while 2-methyl-2-[4-(dimethylamino)phenyl]-1,1-bis(trimethylsilyl)silene (8), prepared by treatment of [4-(dimethylamino)benzoyl]tris(trimethylsilyl)silane (6) with methyllithium, affords the head-to-head cyclodimer 3,4-dimethyl-3,4-bis[4-(dimethylamino)phenyl]-1,1,2,2-tetrakis(trimethylsilyl)- 1,2-disilacyclobutane (9) in addition to the linear dimer 1-[1-(4-(dimethylamino)-phenyl)ethenyl]-2-[1-(4-(dimethylamino)phenyl)ethyl]-1, 1,2,2-tetrakis(trimethylsilyl)di-silane (10). 2-[2-((Dimethylamino)methyl)phenyl]-1,1-bis(trimethylsilyl)silene (12), prepared similarly by trimethylsilanolate elimination from [2-((dimethylamino)methyl)phenyl]tris-(trimethylsilyl)silylmethanol (11), gives under the same conditions [2-((dimethylamino)-methyl)benzyl](trimethylsiloxy)bis(trimethylsilyl)silane (14).