Mechanistic Studies of the Transfer Dehydrogenation of Cyclooctane Catalyzed by Iridium Bis(phosphinite)p-XPCP Pincer Complexes

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Abstract
Reaction of bis(phosphinite) PCP iridium pincer complexes (p-XPCP)IrHCl (5a−f) [X = MeO (5a), Me (5b), H (5c), F (5d), C6F5 (5e), ArF(= 3,5-bis(trifluoromethyl)phenyl) (5f)] with NaOtBu in neat cyclooctane (COA) generates 1:1 mixtures of the respective (p-XPCP)IrH2 complexes 4a−f and the cyclooctene (COE) olefin complexes (p-XPCP)Ir(COE) (6a−f) at 23 °C. At higher temperatures, complexes 4 and 6 are equilibrated because of the degenerate transfer dehydrogenation of COA with free COE (6 + COA ⇌ 4 + 2COE), as was shown by temperature-dependent equilibrium constants and spin saturation transfer experiments at 80 °C. At this temperature, the COE complexes 6 exchange with free COE on the NMR time scale with the more electron-deficient complexes 6 exchanging COE faster. The exchange is dissociative and zero order in [COE]. Further analysis reveals that the stoichiometric hydrogenation of COE by complex 4f, and thus the separated back reaction 4f + 2COE → 6f + COA proceeds at temperatures as low as −100 °C with the intermediacy of two isomeric complexes (p-ArFPCP)Ir(H)2(COE) (8f, 8f‘). COE deuteration with the perdeuterated complex 4f-d38 at −100 °C results in hydrogen incorporation into the hydridic sites of complexes 8f,8f‘-d38 but not in the hydridic sites of complex 4f-d38, thus rendering COE migratory insertion in complexes 8f,8f‘ reversible and COE coordination by complex 4f rate-determining for the overall COE deuteration.

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