Imidazolidene carboxylate bound MBPh_4 complexes (M = Li, Na) and their relevance in transcarboxylation reactions

摘要

Combination of 1,3-bis(2,6-diisopropylphenyl)imidazolum-2-carboxylate (IPrCO_2) with the Lewis acids MBPh_4, 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 IPrCO_2 with LiBPh_4 yielded a monomeric species. Combination of 1,3-bis(2,4,6-trimethylphenyl)imidazolum-2-carboxylate (IMesCO_2) with LiBPh_4 also afforded a dimeric species that was similar in global structure to that of the IPrCO_2+NaBPh _4 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·CO_2 (NHC = N-heterocyclic carbene). Kinetic analysis of the transcarboxylation of IPrCO_2 to acetophenone with NaBPh _4 to yield sodium benzoylacetate was performed. First-order dependences were observed for IPrCO_2 and acetophenone, whereas zero -order dependence was observed for NaBPh_4. Direct dicarboxylation was observed when ItBuCO_2 was added to MeCN in the absence of added MBPh4.