Carbenoids halogen-stabilized

Reactions of halogen-stabilized carbenoids with imines have been carried out using preformed lithium species e.g. equation or via a carbenoid generated from diiodomethane utilizing zinc-copper... 

Reactions of halogen-stabilized carbenoids with imines have been carried out using prefoimed lithium species (e.g. equation 36), or via a carbenoid generated from diiodomethane utilizing zinc-copper couple (Simmons-Smith conditions). A stereospecific ring closure is observed after the addition of lithiodichloromethane to a benzaldimine (equation 37).The addition of lithiochloro(phenylsulfon-yl)methane to aromatic imines affords 2-phenylsulfonyl-substituted aziridines, which can be deproton-ated and alkylated in excellent yield (Scheme 20). 

The halogen-metal exchange of one of a geminal pair of bromine atoms is readily achieved -a carbenoid is formed which may have sufficient stability to react as an organolithium before decomposing to a carbene by a-elimination.. m in cyclic systems, diastereoselectivity in this process has long been known,114116 though the outcome of reactions such as that of 137 is under a mixture of kinetic and thermodynamic control.117... 

A lato modirication of the original Kdbrich procedure entails the generation of caibenoids in the presence of an added equivalent of lithium bromide salt (equation 22). Apparently, the Lewis acid character of the added salt helps to stabilize the carbenoid by coordination wiA the halogen atom. The lithium salt is also implicated in Lewis acid complexation to the oxygen atom of the G=>0 bond, and seems to assist in the cyclization step as well. 

Organolithiums bearing a halogen on the a-carbon are termed carbenoids. Bromine and chlorine are sufficiently acidifying to allow preparation of the halocarbenoids, but fluorine requires additional stabilization, for example by phosphorus (yide supra). Lithium halocarbenoids are thermally unstable species whose chemistry must be performed at low temperatures and this feature has hindered their use in preparative synthetic organic chemistry. 

The ease of the halogen displacement is attributed to mesomeric stabilization of the transition state, which has some SN1 character. Neither addition of HMPT nor replacement of Li by K is necessary both could even have an adverse effect, since the chloroether could be deprotonated to the carbenoid ClCH(M)OR (the RCH2CH(Cl)OR homologues also could undergo 1,2-elimination of HC1 under more polar conditions). 

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