On the reactivity of a masked N‐heterocyclic phosphinidene towards carbonyls and isonitriles

Abstract

AbstractReactions of a masked N‐heterocyclic phosphinidene (mNHP, 1), a four‐membered heterocycle with a divalent P atom, with various isonitriles (tBu−NC, Dmp−NC) and carbonyls (CO, Fe(CO)5, and benzalacetone) have been studied. The isonitriles (tBu−NC and Dmp−NC) insert into a P−N bond of the four‐membered ring of 1, leading to the formation of a five‐membered heterocycle (2tBu, 2Dmp), indicating carbene‐like reactivity of the isonitriles. In exactly this way, CO, which is isolobal to isonitriles, reacts to form a five‐membered ring (3) with an exocyclic C=O double bond. If, on the other hand, Fe(CO)5 is used as a CO source, a CO molecule is inserted analogously, however, the remaining Fe(CO)4 fragment is bound via a phosphorus‐iron dative bond to the five‐membered heterocycle. When mNHP 1 reacts with the α,β‐unsaturated ketone benzalacetone, phosphinidene reactivity is observed for 1. This means that the four‐membered ring opens and the P atom reacts with the benzalacetone in a cyclization reaction to form a 1,2‐oxaphospha‐cyclopentene. Theoretical studies of the five‐membered heterocycles 2 and 3 carrying eight π electrons show that zwitterionic Lewis representations dominate in the resonance scheme and only a very low biradical character (<5 %) is found.