Mechanistic study on the conversion of primary alcohols and butadiene into branched ketones using a rhodium catalyst

Abstract

The direct formation of saturated branched ketones from primary alcohols and butadiene in the absence of premetalated intermediates or other hydrogen sources is a mechanistically intriguing transformation. Catalyzed by a rhodium(I) catalyst (Rh(COD)₂BAr^F₄/PPh₃) under basic conditions, this reaction proceeds with moderate to high experimental yields. Applying density functional theory, we investigate the mechanism of the conversion of 3-methoxybenzyl alcohol and butadiene into (branched) isobutyl ketone. This reaction involves four key steps: (i) oxidation of the alcohol to the corresponding aldehyde, with the generation of an Rh(I) hydride complex as an active catalyst, (ii) hydrogenation of butadiene to form an allyl–Rh(I) complex, (iii) carbonyl addition from the allylic carbon, forming an Rh(I) alkoxide intermediate, and (iv) an intramolecular hydrogen-transfer processes to generate the desired ketone product. The rate-determining states correspond to the carbonyl addition, involving both the intermediate and transition states, with an energy barrier of +30.4 kcal/mol. We found that two PPh₃ ligands are coordinated to the Rh center throughout the reaction. Therefore, carbonyl addition to the linear ketone is inhibited by steric hindrance around the metal center. Stereoisomerism among intermediates has a negligible effect on the reaction energetics and does not affect the configuration of the final product, which is released in an enolate form. Calculated positions of the transferred hydrogens align with the experimental deuterium labeling results, highlighting a hydrogen (auto)transfer mechanism in the transformation of primary alcohols to branched ketones.