Journal of the American Chemical Society, Vol.127, No.41, 14223-14230, 2005
Catalysis of enantioselective [2+1]-cycloaddition reactions of ethyl diazoacetate and terminal acetylenes using mixed-ligand complexes of the series Rh-2(RCO2)(n) (L-4-n(star)). Stereochemical heuristics for ligand exchange and catalyst synthesis
This paper describes the synthesis of mixed Rh-2(11) complexes containing bridging acetate and R,R-diphenyl-N-triflylimidazolidinone (DIPTI) ligands (1, 2, and 9-19), and their function as enantioselective catalysts for the conversion of ethyl diazoacetate and terminal acetylenes to chiral cyclopropenes. Of these catalysts, 1 and 10 functioned with the highest enantioselectivity, in accord with a mechanistic model in which one of the ligand bridges is broken in the intermediate Rh-carbene complex. The synthetic results allow conclusions with regard to kinetically and thermodynamically favored pathways for the synthesis of mixed acetate-DPTI complexes. A new C-2-symmetric complex having only two anti-DTBTI bridges (23) is shown to be a highly effective chiral catalyst, as expected from the model.