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Manganese-catalyzed hydroboration of carbon dioxide and other challenging carbonyl groups.

ABSTRACT: Reductive functionalization of the C=O unit in carboxylic acids, carbonic acid derivatives, and ultimately in carbon dioxide itself is a challenging task of key importance for the synthesis of value-added chemicals. In particular, it can open novel pathways for the valorization of non-fossil feedstocks. Catalysts based on earth-abundant, cheap, and benign metals would greatly contribute to the development of sustainable synthetic processes derived from this concept. Herein, a manganese pincer complex [Mn(Ph₂PCH₂SiMe₂)₂NH(CO)₂Br] (1) is reported to enable the reduction of a broad range of carboxylic acids, carbonates, and even CO₂ using pinacolborane as reducing agent. The complex is shown to operate under mild reaction conditions (80-120?°C), low catalyst loadings (0.1-0.2?mol%) and runs under solvent-less conditions. Mechanistic studies including crystallographic characterisation of a borane adduct of the pincer complex (1) imply that metal-ligand cooperation facilitates substrate activation.

SUBMITTER: Erken C

PROVIDER: S-EPMC6207666 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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Manganese-catalyzed hydroboration of carbon dioxide and other challenging carbonyl groups.

Erken Christina C Kaithal Akash A Sen Suman S Weyhermüller Thomas T Hölscher Markus M Werlé Christophe C Leitner Walter W

Nature communications 20181030 1

Reductive functionalization of the C=O unit in carboxylic acids, carbonic acid derivatives, and ultimately in carbon dioxide itself is a challenging task of key importance for the synthesis of value-added chemicals. In particular, it can open novel pathways for the valorization of non-fossil feedstocks. Catalysts based on earth-abundant, cheap, and benign metals would greatly contribute to the development of sustainable synthetic processes derived from this concept. Herein, a manganese pincer co ...[more]

PMID: 30375381

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Project description:A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic γ,δ-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic γ,δ-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)2S2]+, (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh-H (preferred pathway) or Rh-B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B-C or H-C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major γ-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh-H bond (pathways I, I-1, and I-2), (2) a potential pathway proceeding via migratory insertion into the Rh-B bond (pathway II), and two potential competing routes to a β-borylated byproduct (pathway III). The results find that the Rh-H migratory insertion pathway I-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing β-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.

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Manganese-catalyzed hydroboration of carbon dioxide and other challenging carbonyl groups.

Publications

Manganese-catalyzed hydroboration of carbon dioxide and other challenging carbonyl groups.

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