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A Fresh Twist on the Phospha-(Aza)-Wittig Reaction The reactivity of an unsupported phosphinidene oxide, BnArNP═O (Bn = benzyl; Ar = bulky aryl group), as the electrophilic partner in Wittig reactions with ylides is described. Reactions with methylene-triphenylphosphorane (H2C═PPh3) and ethylidene-triphenyl-phosphorane (HMeC═PPh3), proceed as expected, giving rise to the phosphaalkene metathesis products and triphenylphosphine oxide. This reaction can be extended to other ylides such as N-(triphenylphosphoranylidene)methanamine (MeN═PPh3), to afford an aminoiminophosphane BnArNP═NMe. In these reactions the phosphinidene oxide plays the role of an electrophile, which would typically be the remit of an organic carbonyl in classical Wittig reactions. Further mechanistic insight into such transformations can be gained by altering the nature of the phosphorus-ylide. Upon reacting BnArNP═O with H2C═PMe3 (which possesses a smaller, more Lewis basic, phosphine) an alternative product is formed. This transformation supports the formation of a betaine intermediate that subsequently undergoes hydrogen-migration to afford an oxidized phosphorus(V) compound related to phosphorus acid. https://pubs.acs.org/doi/full/10.1021/jacs.5c02865