Selective Catalytic Dehydrogenation of Alkanes to Alkenes

Article abstract of DOI:10.1021/ja00260a013

Linear and cyclic alkanes can be selectively dehydrogenated to the corresponding alkenes both thermally and photochemically (254 nm) with iridium complexes as catalysts.In the photochemical case, a sacrificial hydrogen acceptor is not required and H2 is evolved directly.Preferential initial formation of the least stable alkene (e.g., methylenecyclohexane from methylcyclohexane) is explained by attack at unhindered C-H bonds by an oxidative addition to the metal.A subsequent β-elimination gives the alkene.A key feature of the catalyst, 2-O2CCF3)(PR3)2> (4), is that the chelating acetate group can open up to allow β-elimination to take place in the alkyl hydride intermediate 2-O2CCF3)(PR3)2>.Prolonged reaction times lead to progressive isomerization of the alkene to give the thermodynamic product (e.g., 1-methylcyclohexene from methylcyclohexane).Up to 32 turnovers of dehydrogenation are seen.Deactivation of the catalyst takes place by P-C hydrogenolysis of the PAr3 ligand in the thermal case; the rise in the amount of ArH formed parallels the fall-off in activity of the catalyst.P-C cleavage does not take place in the photochemical system (R = cyclohexyl).A reversiblecyclometalation of the catalyst 4a (R = p-FC6H4) is observed after removal of the hydride ligands with t-BuCH=CH2.In the presence of C6H6, a C-H oxidative addition product, 2-O2CCF3)(PR3)2>, is formed instead (k_H/k_D = 4.5) and can be isolated from the reaction mixtures; This supports the oxidative addition pathway proposed for the alkanes.Equilibrium constants for the reaction of 4a with alkenes to give 1-O2CCF3)(PR3)2> are reported.