Imidazolidene Carboxylate Bound MBPh4 Complexes (M = Li, Na) and Their Relevance in Transcarboxylation Reactions

Combination of 1,3-bis(2,6-diisopropylphenyl)imidazolum-2-carboxylate (IPrCO2) with the Lewis acids MEPh4, where M = Li or Na, provided two separate complexes. The crystal structures of these complexes revealed that coordination to NaBPh4 yielded a dimeric species, yet coordination of IPrCO2 with LiBPh4 yielded a monomeric species. Combination of 1,3-bis(2,4,6-trimethylphenyl)imidazolum-2-carboxylate (IMesCO(2)) with LiBPh4 also afforded a dimeric species that was similar in global structure to that of the IPrCO2+NaBPh4 dimer. In all three cases, the cation of the organic salt was coordinated to the oxyanion of the zwitterionic carboxylate. Thermogravimetric analysis of the crystals demonstrated that decarboxylation occurred at lower temperatures than the decarboxylation temperature of the parent NHC center dot CO2 (NHC = N-heterocyclic carbene). Kinetic analysis of the transcarboxylation of IPrCO2 to acetophenone with NaBPh4 to yield sodium benzoylacetate was performed. First-order dependences were observed for IPrCO2 and acetophenone, whereas zero-order dependence was observed for NaBPh4. Direct dicarboxylation was observed when (IBuCO2)-Bu-t was added to MeCN in the absence of added MBPh4.