Gas-phase reduction of cyclic and acyclic α,β-unsaturated ketones by hydrogen transfer on MgO. Effect of the ketone structure

The gas-phase hydrogen transfer reduction (HTR) of cyclic and acyclic α,β-unsaturated ketones to the corresponding unsaturated alcohols (UOL) using 2-propanol as hydrogen donor was studied on MgO as an alternative to the less selective conventional hydrogenation using high pressure H2. The HTR of 2-cyclohexenone and mesityl oxide were used as model reactions. The MgO activity and selectivity toward the unsaturated alcohol depended on the ketone chemical structure. Cyclic 2-cyclohexenone was in fact less reactive but more selective to UOL formation than acyclic mesityl oxide, yielding about 85% UOL (91% selectivity) at 573 K. The rigid structure of 2-cyclohexenone enforces a s-trans conformation that favors selective reduction of the Cdouble bond; length as m-dashO bond and thereby enhances the UOL formation. In contrast, the less rigid structure of the acyclic ketone affords the simultaneous reduction of both unsaturated bonds, Cdouble bond; length as m-dashC and Cdouble bond; length as m-dashO, forming also the saturated alcohol; as a consequence, maximum UOL yields of about 45% (47% selectivity) were obtained at 573 K from HTR of mesityl oxide. The unsaturated ketone conversion pathways toward UOL and other compounds also depended on the ketone structure. UOL formed on MgO as a primary product from both reactants 2-cyclohexenone and mesityl oxide, via a cyclic six-membered intermediate according to the Meerwein–Ponndorf–Verley mechanism. However the saturated alcohol was produced by consecutive UOL reduction in 2-cyclohexenone reactions but directly from mesityl oxide reduction. Reduction of the Cdouble bond; length as m-dashC bond toward the saturated ketone was negligible regardless of the reactant structure whereas competing reactions such as the Cdouble bond; length as m-dashC bond shift were more likely to contribute during reduction of the acyclic reactant.