Couplings of two carbenes

Alvarez-Toledano has investigated149 the reactivity of tertiary amines toward vinylketene complexes of type 221 and found that a number of unusual transformations take place. The formation of 237 and 238 is not directly linked to the presence of the amine, but instead relies on a thermo-lytic coupling of two carbene complexes formed from the vinylketene ligand, with additional loss of carbon monoxide. It is thought that 239 is formed by the deprotonation of the labile C-4 proton, followed by subsequent coordination of a capping Fe(CO)3 group. 

Interestingly, Arduengo observed that Villa was stable with respect to dimerization in the absence of a Brpnsted or Lewis acid catalyst." Similarly, in the absence of an acid catalyst, dimerization of IXb is extremely slow and is first order in carbene. Therefore, the observed formal dimerization of Villa and IXb,c does not involve the coupling of two carbenes, but the nucleophilic attack of one carbene upon its conjugate acid, followed by proton elimination, as already suggested by Chen and Jordan (Scheme 8.13). It is important to keep in mind that even A,A-dialkylimidazolium ions have pXa values of 24 in DMSO, and based on the calculated proton acidity, Alder estimated the pXa values for acyclic diamino-carbenes to be from 2 to 6 pXa units higher than for imidazol-2-ylidenes. " ... 

The configurational (l,l)-ligogenic cases are exemplified by the hypothetical coupling of two carbenes to form a carbon-to-carbon double bond (Figure 14.7). 

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