Highly Selective Markovnikov Addition of HypervalentH-Spirophosphoranes to Alkynes Mediated by Palladium Acetate: Generality and Mechanism

Abstract Palladium acetate efficiently catalyzes the addition of an H-spirophosphorane (pinacolato)2PH to alkynes to give Markovnikov addition products highly selectively. The addition products can be easily converted to the corresponding alkenylphosphonates and phosphonic acids via simple hydrolysis or thermal decomposition. This new reaction is a general method for the introduction of phosphorus functionality to the internal carbons of terminal alkynes, resolving the problem of the regioselectivity associated with hydrophosphorylation reactions so far reported. Mechanistic studies confirmed that (a) palladium acetate was reduced to metallic palladium by H-spirophosphorane, (b) the P–H bond of H-spirophosphorane could be activated by zero-valent platinum complexes to give the corresponding hydridoplatinum complexes, and (c) an alkenylpalladium species was identified from the reaction of palladium acetate with H-spirophosphorane and diphenylacetylene. These results support a reaction mechanism that palladium acetate was first reduced by H-spirophosphorane to give zero-valent palladium. This zero-valent palladium might insert into the P–H bond of the H-spirophosphorane to give a hydridopalladium species which then added to alkyne via the addition of H–Pd bond to form an alkenylpalladium species with the hydrogen atom added to the terminal carbon of alkynes. Reductive elimination of the alkenylpalladium affords the addition product.