5 years ago

Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes

Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes
Naoki Hayakawa, Tomonari Wakabayashi, Tsukasa Matsuo, Kazuya Sadamori, Shota Tsujimoto, Miho Hatanaka
The reaction of the bulky diphosphenes (Rind)P=P(Rind) (1; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHCP(Rind) (2), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2, thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol−1 for 2 b. P=P bond-breaking: Bulky R8-s-hydrindacen-4-yl (Rind)-substituted diphosphenes (R=Me, Et) react with two equivalents of N-heterocyclic carbene (NHC) to produce the NHC-coordinated phosphinidene adducts. The mechanism for P=P cleavage through formation and breaking of NHC-coordinated, highly polarized diphosphene intermediates is proposed.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201701201

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