Experimental and theoretical investigation of the cycloisomerization of N-propargylcarboxamide catalyzed by NHC-Au-X in green solvents

For the first time, NHC-Au-X [NHC = (1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene, X- = BF4-, OTf-, OTs- , TFA-] catalysts were tested in the cycloisomerization of N-(prop-2-yn-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline in a wide set of green solvents. We found that most of these solvents (cyclohexanone, isopropyl acetate, MIBK, ethyl lactate, furfuryl alcohol, ?-valerolactone, propylene carbonate and propionic acid) are comparable or better solvents with respect to traditional VOS. Furthermore, the catalyst activity is related to the basic strength of the anion and the performances of the catalysts decrease gradually with increasing basicity and hydrogen-bond acceptor power of X- . Kinetic experiments and DFT calculations seem to indicate that both the characteristic of the solvent and counterion should be taken into account.

Automated summary

The study showed that the activity of NHC-Au-X catalysts in the cycloisomerization of N-(prop-2-yn-yl)benzamide is related to the basic strength of the anion, with performance decreasing gradually with increasing basicity and hydrogen-bond acceptor power of X-. The characteristics of the solvent and counterion should both be taken into account in considering the reaction.