NHC-Catalyzed Synthesis of Coumarins

Significance

Reported is the NHC-catalyzed synthesis of substituted coumarins from the reaction of salicylaldehydes with α,β-unsaturated aldehydes. The present work is an evolution on previous results from the Bräse laboratories in which stoichiometric quantities of NHC precursors were used. The substrate scope has been modestly examined, with electron-rich but not strongly electron-withdrawing aromatic substitution being tolerated. Substitution ortho to the phenolic oxygen appears to be detrimental to the reaction, presumably on steric grounds. α,β-Unsaturated aldehydes where R⁵ and R⁶ are both substituted are not tolerated and, in fact, only poor to moderate yields are observed in the six cases examined where R⁵ ∂ Me. The reaction was optimized with respect to the catalyst, catalyst loading, solvent, temperature and base.