Transition-metal-catalyzed cyclization of alkynoic acids to alkylidene lactones

Alkynoic acids I-IV can be effectively cyclized to the corresponding exocyclic enol lactones with a new Rh(I) complex, [(cycphos)RhCl]₂ (4) (cycphos = 1,2-bis(dicyclohexylphosphino)ethane), that we have developed. The high-yield lactonizations can be performed at room temperature, and methylene chloride appears to be the best solvent for the cyclization. Exclusive (Chemical Equation Presented) I R=H, n=1 II =CH₃, n=1 II R=Ph, n=1 IV R=H, n=2 formation of Z-olefinic isomers was observed, i.e., trans addition of the carboxylate OH across the triple bond. There is also a strong preference for the formation of five- vs six-membered rings in the substituted pentynoic acid systems. Other electron-rich group VIII transition-metal complexes and mercuric salts were tested for catalytic activity under similar reaction conditions and were found to be less effective. The proposed mechanism involves initial OH activation by the metal center followed by nucleophilic attack of carboxylate anion on the metal-coordinated alkyne of the intermediate, and subsequent reductive elimination from the resulting hydrido-vinyl complex releases the lactone product and regenerates the catalyst.