Exocyclic Bond Cleavage in Oxaphosphirane Complexes?

Abstract

A first computational insight into the intrinsic strength of exocyclic bonds to phosphorus in oxaphosphirane κP-pentacarbonylmetal(0) complexes 1a-f (M=Cr, Mo) is provided using a set of P-R derivatives (R=Me, tBu, CPh(3)). Whereas homolytic cleavage of the exocyclic P-R bond was found to be always unfavored (for neutral complexes), heterolytic cleavage leading to a carbocation R(+) moiety and the oxaphosphiranide complex 2(-) constitutes the lowest-energy process, especially if R is bulky and can stabilize the positive charge, that is, triphenylmethyl (trityl), efficiently. The energies required for P-M bond cleavage are about 30 kcal mol(-1), and decrease with the increasing bulk of the R substituent (from Me to trityl) and ongoing from Cr to Mo. The reactivities of complexes 1a-f towards oxidative and reductive single electron transfer (SET) reactions were analyzed using the facile variation of bond-strength-related descriptors (VBSD) methodology, thus enabling the design of synthetically useful strategies addressing decomplexation and P-functionalization. Reductive SET reactions with sodium naphthalenides enable selective P-M bond cleavage (i.e., decomplexation) for the case of P-Me and P-tBu substitution, whereas reductive P-R bond cleavage is favored in the case of the P-trityl complexes 1c,f, and results in the formation of the (anionic) oxaphosphiranide complex 2(-), which may be regarded as a potential key intermediate for further P-functionalization.