Thermodynamic, kinetic, and mechanistic study of oxygen atom transfer from mesityl nitrile oxide to phosphines and to a terminal metal phosphido complex

The enthalpies of oxygen atom transfer (OAT) from mesityl nitrile oxide (MesCNO) to Me ₃P, Cy ₃P, Ph ₃P, and the complex (Ar[ ^tBu]N) ₃MoP (Ar = 3,5-C ₆H ₃Me ₂) have been measured by solution calorimetry yielding the following P-O bond dissociation enthalpy estimates in toluene solution (±3 kcal mol ^-1): Me ₃PO [138.5], Cy ₃PO [137.6], Ph ₃PO [132.2], (Ar[ ^tBu]N) ₃MoPO [108.9]. The data for (Ar[ ^tBu]N) ₃MoPO yield an estimate of 60.2 kcal mol ^-1 for dissociation of PO from (Ar[ ^tBu]N) ₃MoPO. The mechanism of OAT from MesCNO to R ₃P and (Ar[ ^tBu]N) ₃MoP has been investigated by UV-vis and FTIR kinetic studies as well as computationally. Reactivity of R ₃P and (Ar[ ^tBu]N) ₃MoP with MesCNO is proposed to occur by nucleophilic attack by the lone pair of electrons on the phosphine or phosphide to the electrophilic C atom of MesCNO forming an adduct rather than direct attack at the terminal O. This mechanism is supported by computational studies. In addition, reaction of the N-heterocyclic carbene SIPr (SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene) with MesCNO results in formation of a stable adduct in which the lone pair of the carbene attacks the C atom of MesCNO. The crystal structure of the blue SIPr·MesCNO adduct is reported, and resembles one of the computed structures for attack of the lone pair of electrons of Me ₃P on the C atom of MesCNO. Furthermore, this adduct in which the electrophilic C atom of MesCNO is blocked by coordination to the NHC does not undergo OAT with R ₃P. However, it does undergo rapid OAT with coordinatively unsaturated metal complexes such as (Ar[ ^tBu]N) ₃V since these proceed by attack of the unblocked terminal O site of the SIPr·MesCNO adduct rather than at the blocked C site. OAT from MesCNO to pyridine, tetrahydrothiophene, and (Ar[ ^tBu]N) ₃MoN was found not to proceed in spite of thermochemical favorability.