A Surprisingly Stable 1‐(Chlorosilyl)‐2‐phosphaethenyllithium Compound, RCl 2 SiC(Li)=PMes*

RCl 2 Si–C(Li)=PMes* ( 1 ) (R = 9‐methylfluoren‐9‐yl, Mes* = 2,4,6‐tri‐ tert ‐butylphenyl), considered as a key intermediate in the synthesis of the heteroallene RClSi=C=PMes*, was obtained from RCl 2 Si–C(Cl)=PMes* by lithium/chlorine exchange with n BuLi and evidence for its identity was obtained by hydrolysis. Remarkably, however, it failed to undergo a lithium salt elimination to form the corresponding chlorophosphasilaallene, RClSi=C=PMes*, even under prolonged heating in toluene. As shown by theoretical studies (RHF/3‐21G* and RHF/6‐31G**//RHF/3‐21G* level), interactions between the lithium ion and the π‐systems of the substituents at the phosphorus and silicon atoms could partly account for this stability. The lithium compound 1 reacts with methanol or chlorotrimethylsilane leading to new silylphosphaalkenes. The reaction of the lithium derivative 1 with benzaldehyde gave a four‐membered heterocycle with an exocyclic P=C bond which virtually corresponds to the product of a [2+2] cycloaddition between the Si=C and C=O double bonds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)