Theory-Guided Design of Brønsted Acid-Assisted Phosphine Catalysis: Synthesis of Dihydropyrones from Aldehydes and Allenoates.

The phosphine-catalyzed addition of 2,3-butadienoates to aldehydes has been extended to the formation of disubstituted dihydro-2-pyrones. The requisite shift in equilibrium of the intermediate zwitterionic beta-phosphonium dienolates toward the s-cis intermediate was accomplished through the use of a Brønsted acid additive, which disrupts the favorable Coulombic interaction present in the s-trans intermediate. The detailed nature of the synergistic interactions involving the Brønsted acid additives and phosphine involved in the formation of s-cis beta-phosphonium dienolates was analyzed through a series of DFT calculations. Unlike previously reported annulations of aldehydes with allenoates, where trialkylphosphines are optimal catalysts, in this study triphenylphosphine was also found for the first time to be a suitable catalyst for the synthesis of dihydropyrones. This method provides a one-step route toward functionalized dihydropyrones from simple, stable starting materials. In addition, new reaction pathways of phosphine-catalyzed allene annulations are unveiled, with the formation of dihydropyrones being the first example of dual activation in this sphere.