Synthesis and structure of Pd(II) pincer complexes: catalytic application in β-alkylation of secondary alcohols involving sequential dehydrogenation of alcohols via the borrowing hydrogen approach.

Herein, we report an efficient and sustainable approach for the selective synthesis of ketones by palladium pincer catalyzed β-alkylation of secondary alcohols with aromatic primary alcohols via the borrowing hydrogen (BH) approach for the first time. A set of new Pd(II) ONO pincer complexes was synthesized and characterised by elemental analysis and spectral techniques (FT-IR, NMR and HRMS). The solid-state molecular structure of one the complexes was corroborated by X-ray crystallography. A range of α-alkylated ketone derivatives (25 examples) was obtained in excellent yields up to 95% through sequential dehydrogenative coupling of secondary and primary alcohols with 0.5 mol% catalyst loading with a substoichiometric amount of the base. Control experiment studies were carried out for the coupling reactions which revealed that the reaction involves an aldehyde, a ketone and chalcone intermediates, and eventually established the borrowing hydrogen strategy. Gratifyingly, this protocol is simple and atom economical, with water/hydrogen as byproducts. In addition, large-scale synthesis also demonstrated the synthetic usefulness of the present protocol.