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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-07, Vol.52 (29), p.10038-10044
• Main Authors: Pranesh Kavin, Sekar, Ramesh, Rengan
• Format: Article
• Language: English
• Subjects: Alcohol; Alcohols; Aldehydes; Alkylation; Chemical analysis; Chemical reactions; Chemical synthesis; Crystallography; Dehydrogenation; Hydrogen; Ketones; Molecular structure; NMR; Nuclear magnetic resonance; Palladium; Spectral methods
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt01628e

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 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.