A Computational Study on the Stability of Oxaphosphirane Rings towards Closed‐Shell Valence Isomerization

Abstract

The stability of both unligated oxaphosphiranes 1 and their P‐M(CO)5 complexes 2 (M = Cr, W) has been explored with respect to their closed‐shell isomers, formed from either heterolytic C–O or P–C ring bond cleavage. C–O bond cleavage of 1 leads to valence isomers 3 featuring trigonal‐planar hybridization at P, whereas pyramidalized isomers 3′ are obtained for push–pull systems featuring electron‐donating substituents at C and electron‐withdrawing groups at P. Complexes 2 undergo C–O ring cleavage, affording side‐on complexes 4, except for the unstable C,C‐diamino‐substituted complex 2d, which yielded the end‐on complex 4′d. Most of the C–O bond‐cleavage processes are exergonic, but have moderately high energy barriers; C,C‐difluoro‐substituted oxaphosphiranes were identified as thermodynamically stable. C–P bond‐cleavage pathways afford zwitterionic species 6 or 7 as a result of pericyclic 4π‐electron conrotatory processes and are always disfavoured, compared with C–O bond cleavages. The cases of 1n,u,v and 2e are noteworthy, as attempted C–P bond cleavage led to ring fragmentation to give carbonic acid difluoride and phosphinidene derivatives.