Dichlorocarbenes, equilibria

During the next fifty years the interest in derivatives of divalent carbon was mainly confined to methylene (CHg) and substituted methylenes obtained by decomposition of the corresponding diazo compounds this phase has been fully reviewed by Huisgen. The first convincing evidence for the formation of dichlorocarbene from chloroform was presented by Hine in 1950. Kinetic studies of the basic hydrolysis of chloroform in aqueous dioxane led to the suggestion that the rate-determining step was loss of chloride ion from the tri-chloromethyl anion which is formed in a rapid pre-equilibrium with hydroxide ions ... 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

Hi) By formation of seven- from six-membered rings The expansion of a di- or a tetra-hydropyran ring fused to a three-membered ring has been used as a synthetic approach to oxepins. Thus the synthesis of oxepin (193) was attempted by thermal dehydrochlorination of a tetrahydropyran (equation 57) obtained from a dichlorocarbene addition reaction (65CI(L)184). Unfortunately the equilibrium appeared to favor the keto tautomer to the apparent exclusion of the enolic oxepin form (193). 

Trichloromethyl carbanions can dissociate into dichlorocarbenes also into the interfacial region where they hydrolyze, giving chloride and formate anions. However, in the system with and without the catalyst this hydrolysis is a slow process because the chloride anions produced, being less hydrophilic than hydroxide anions, occupy preferentially the interfacial region, thus shifting equilibrium (95) to the left ... 

The exact formulation of the reactive intermediate in a-elimination reactions using organolithium compounds as bases has been difficult. Apart from the free carbene, various carbenoids are possible, including the a-haloorganolithium formed on metalation, and carbene-lithium halide complexes of various degrees of association. In the case of the dichlorocarbene-trichloromethyllithium equilibrium, the... 

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