The Phospha-Bora-Wittig Reaction

We report the phospha-bora-Wittig
reaction for the direct preparation
of phosphaalkenes from aldehydes, ketones, esters, or amides. The
transient phosphaborene Mes*PB–NR2 reacts
with carbonyl compounds to form 1,2,3-phosphaboraoxetanes, analogues
of oxaphosphetane intermediates in the classical Wittig reaction.
1,2,3-Phosphaboraoxetanes undergo thermal or Lewis acid-promoted cycloreversion,
yielding phosphaalkenes. Experimental and density functional theory
studies reveal far-reaching similarities between classical and phospha-bora-Wittig
reactions.