Effects of Phosphorus Substituents on Reactions of α-Alkoxyphosphonium Salts with Nucleophiles

The effects of phosphorus substituents on the reactivity of α‐alkoxyphosphonium salts with nucleophiles has been explored. Reactions of α‐alkoxyphosphonium salts, prepared from various acetals and tris(o‐tolyl)phosphine, with a variety of nucleophiles proceeded efficiently. These processes represent the first examples of high‐yielding nucleophilic substitution reactions of α‐alkoxyphosphonium salts. The reactivity of these salts was determined by a balance between steric and electronic factors, respectively, represented by cone angles θ and CO stretching frequencies ν (steric and electronic parameters, respectively). In addition, a novel reaction of α‐alkoxyphosphonium salts derived from Ph3P with Grignard reagents was observed to take place in the presence of O2 to afford alcohols in good yields. A radical mechanism is proposed for this process that has gained support from isotope‐labeling and radical‐inhibition experiments. A dramatic change in the reactivity of an α‐alkoxyphosphonium salt toward nucleophiles is observed due to the steric and electronic nature of the phosphine substituents. By changing the type of phosphorus substituents, the reaction pathway can be controlled to proceed selectively by substitution or a new radical reaction (see scheme; OTf=trifluoromethansulfonate, TMS=trimethylsilyl, o‐tol=tolyl).