Metalation and silylation of allene: A convenient synthesis of tetrakis(trimethylsilyl)allene

Abstract

Allene is metalated by n-butyllithium at −50° in tetrahydrofuran/hexane ( 1 1 ) to produce allenyllithium. When allene was mixed with two or more equivalents of n-butyllithium and trimethylchlorosilane subsequently was added to the mixture, silylated derivatives (Id), (II), (III), (IV) and (V) were formed in varying amounts. A competitive reaction demonstrated that metalation of allene and the subsequent reaction sequence leading to (V) was competitive with the coupling of n-butyllithium with trimethylchlorosilane. Rapid formation of a dilithiated species in the metalation step appeared likely since metalation of allene with exactly two equivalents of n-butyllithium followed by addition of two equivalents of trimethylchlorosilane suppressed formation of (I) and produced a high yield (63%) of (II). By mixing one equivalent of allene with five equivalents of n-butyllithium followed by five equivalents of trimethylchlorosilane, a 70% isolated yield of (V) was obtained. A similar yield (63% isolated yield) was obtained by treating one equivalent of allene with two equivalents of n-butyllithium followed by two equivalents of trimethylchlorosilane and then adding, in succession, two equivalents more of each of the latter two reagents. A mechanism for formation of (V) was postulated involving (1) formation of lithioallene and 1,3-dilithiopropyne in the metalation step followed by (2) formation of (II) and stepwise sequential metalation and silylation to produce (III), (IV), and (V) in the silylation step.