Preparation and Degradation of Rhodium and Iridium Diolefin Catalysts for the Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols.

Rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols have been prepared, and their degradation has been investigated, during the study of the reactivity of the dimers [M(μ-Cl)(η 4 -C 8 H 12 )] 2 (M = Rh ( 1 ), Ir ( 2 )) and [M(μ-OH)(η 4 -C 8 H 12 )] 2 (M = Rh ( 3 ), Ir ( 4 )) with 1,3-bis(6'-methyl-2'-pyridylimino)isoindoline (HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane, to afford equilibrium mixtures of 1 , the mononuclear derivative RhCl(η 4 -C 8 H 12 ){κ 1 - N py -(HBMePHI)} ( 5 ), and the binuclear species [RhCl(η 4 -C 8 H 12 )] 2 {μ- N py , N py -(HBMePHI)} ( 6 ). Under the same conditions, complex 2 affords the iridium counterparts IrCl(η 4 -C 8 H 12 ){κ 1 - N py -(HBMePHI)} ( 7 ) and [IrCl(η 4 -C 8 H 12 )] 2 {μ- N py , N py -(HBMePHI)} ( 8 ). In contrast to chloride, one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(η 4 -C 8 H 12 )] 2 (μ-OH){μ- N py , N iso -(BMePHI)} (M = Rh ( 9 ), Ir ( 10 )), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. In the presence of KO t Bu, the [BMePHI] - ligand undergoes three different degradations: alcoholysis of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.