Reduction of Carboxylic Acids to Alcohols via Manganese(I) Catalyzed Hydrosilylation

Emanuele Antico,Peter Schlichter,Christophe Werlé,Walter Leitner

doi:10.1021/jacsau.1c00140

Emanuele Antico, Peter Schlichter + Show 2 more

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https://doi.org/10.1021/jacsau.1c00140

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Journal: JACS Au	Publication Date: May 11, 2021
Citations: 28	License type: CC BY 4.0

Affiliation: Max Planck Institute for Chemical Energy Conversion

Abstract

The reduction of carboxylic acids to the respective alcohols, in mild conditions, was achieved using [MnBr(CO)5] as the catalyst and bench stable PhSiH3 as the reducing agent. It was shown that the reaction with the earth-abundant metal catalyst could be performed either with a catalyst loading as low as 0.5 mol %, rare with the use of [MnBr(CO)5], or on a gram scale employing only 1.5 equiv of PhSiH3, the lowest amount of silane reported to date for this transformation. Kinetic data and control experiments have provided initial insight into the mechanism of the catalytic process, suggesting that it proceeds via the formation of silyl ester intermediates and ligand dissociation to generate a coordinatively unsaturated Mn(I) complex as the active species.

Highlights

The direct reduction of carboxylic acids to alcohols is a more challenging transformation compared to the respective reduction of other carbonyl or carboxyl derivatives
There are many examples reported for the hydrosilylation of esters[17] and recently for amides.[18−21] only very few examples exist for free carboxylic acids: they require a large excess of the silane reagent; make use of noble metal catalysts like Ru,[22,23] Rh,24 and Ir;[25] are performed in halogenated solvents like chloroform;[26] or require a specific experimental apparatus.[27,28]
We chose to study the catalytic performances of several manganese complexes for this transformation (Scheme 1): the commercial carbonyl complex [MnBr(CO)5] Mn-1, previously shown to be active for other hydrosilylation and hydrogenation reactions;[37,36,41] the triazole PNN manganese complex Mn-2, recently reported in hydrosilylation reactions by our group;[32] the complex Mn-3, whose ligand framework has been recently reported by Kirchner and co-workers for the hydrosilylation of carbon dioxide;[42] and Mn-4, a manganesetriazine pincer complex active in hydrogenation reactions.[43]

Summary

Introduction

The direct reduction of carboxylic acids to alcohols is a more challenging transformation compared to the respective reduction of other carbonyl or carboxyl derivatives. It is traditionally performed using stoichiometric or even excess amounts of strong reducing reagents such as diborane, LiAlH4, or DIBAL-H. Since alcohols are widely employed in many chemical industries,[3] the development of catalytic methods to reduce carboxylic acids to alcohols continues to be a highly active research field. While the use of hydrogen offers a highly attractive reduction path, there are few reported examples in the literature for catalytic hydrogenation of carboxylic acids to alcohols,[4−11] usually requiring rather high H2 pressures. There are many examples reported for the hydrosilylation of esters[17] and recently for amides.[18−21] only very few examples exist for free carboxylic acids: they require a large excess of the silane reagent; make use of noble metal catalysts like Ru,[22,23] Rh, and Ir;[25] are performed in halogenated solvents like chloroform;[26] or require a specific experimental apparatus.[27,28] the development of a direct catalytic reduction from free carboxylic acids to their corresponding alcohols still seems highly desirable.[29,30]

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Conclusion