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Photochemical C−H Activation and Ligand Exchange Reactions of CpRe(PPh3)2H2. Phosphine Dissociation Is Not Involved

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journal contribution
posted on 06.04.1999, 00:00 by William D. Jones, Glen P. Rosini, John A. Maguire
The dihydride CpRe(PPh3)2H2 (1) catalyzes H/D exchange between C6D6 and other arenes or alkanes. Compound 1 also undergoes photochemical phosphine substitution with PMe3 to give CpRe(PPh3)(PMe3)H2 and then CpRe(PMe3)2H2. Mechanistic studies of these reactions are inconsistent with [CpRe(PPh3)H2] as an intermediate. An alternative mechanism is presented proposing that the active species for H/D exchange is the 14-electron cyclic allyl intermediate [(η3-C5H7)Re(PPh3)2] (E), in which both hydrides have migrated from the rhenium to the cyclopentadienyl ligand. This intermediate accounts for the fact that (1) deuterium does not exchange into the hydride ligands of complex 1 during the H/D exchange catalysis and (2) phosphine substitution occurs by an associative pathway. The precursor to intermediate E, [(η4-C5H6)Re(PPh3)2H] (D), can undergo reversible orthometalation, allowing H/D exchange between the hydride ligands and the ortho phosphine positions. Evidence is presented to support this new mechanism as well as to rule out other feasible mechanisms.