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On the role of pre- and post-electron-transfer steps in the SmI2 /amine/H(2)O-mediated reduction of esters: new mechanistic insights and kinetic studies.

ABSTRACT: The mechanism of the SmI2 -mediated reduction of unactivated esters has been studied using a combination of kinetic, radical clocks and reactivity experiments. The kinetic data indicate that all reaction components (SmI2 , amine, H2 O) are involved in the rate equation and that electron transfer is facilitated by Brønsted base assisted deprotonation of water in the transition state. The use of validated cyclopropyl-containing radical clocks demonstrates that the reaction occurs via fast, reversible first electron transfer, and that the electron transfer from simple Sm(II) complexes to aliphatic esters is rapid. Notably, the mechanistic details presented herein indicate that complexation between SmI2 , H2 O and amines affords a new class of structurally diverse, thermodynamically powerful reductants for efficient electron transfer to carboxylic acid derivatives as an attractive alternative to the classical hydride-mediated reductions and as a source of acyl-radical equivalents for C?C bond forming processes.

SUBMITTER: Szostak M

PROVIDER: S-EPMC4497321 | biostudies-literature | 2014 Apr

REPOSITORIES: biostudies-literature

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On the role of pre- and post-electron-transfer steps in the SmI2 /amine/H(2)O-mediated reduction of esters: new mechanistic insights and kinetic studies.

Szostak Michal M Spain Malcolm M Procter David J DJ

Chemistry (Weinheim an der Bergstrasse, Germany) 20140311 15

The mechanism of the SmI2 -mediated reduction of unactivated esters has been studied using a combination of kinetic, radical clocks and reactivity experiments. The kinetic data indicate that all reaction components (SmI2 , amine, H2 O) are involved in the rate equation and that electron transfer is facilitated by Brønsted base assisted deprotonation of water in the transition state. The use of validated cyclopropyl-containing radical clocks demonstrates that the reaction occurs via fast, reversi ...[more]

PMID: 24615806

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Project description:Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters including pK(a) and bond dissociation energy (BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes, [Ru(II)(Hdmp)(TPA)](ClO(4))(2) (1), [Ru(II)(Hdmdmp)(TPA)](ClO(4))(2) (2), [Ru(II)(dmp(-))(TPA)]ClO(4) (3), and [Ru(II)(dmdmp(-))(TPA)]ClO(4) (4) (Hdmp = 6,7-dimethylpterin, Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, TPA = tris(2-pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between 1 and one-electron oxidized species, [Ru(III)(dmp(-))(TPA)](2+), was determined to be 89 kcal mol(-1), which was large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from phenol derivatives to [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants (k) were determined to exhibit a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP; BDE = 79.15 kcal mol(-1)) to [Ru(III)(dmp(-))(TPA)](2+), the activation parameters were determined to be DeltaH(double dagger) = 1.6 +/- 0.2 kcal mol(-1) and DeltaS(double dagger) = -36 +/- 2 cal K(-1) mol(-1). This small activation enthalpy suggests a hydrogen-bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru(III)(dmp(-))(TPA)](2+), the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate, and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.

Dataset Information

On the role of pre- and post-electron-transfer steps in the SmI2 /amine/H(2)O-mediated reduction of esters: new mechanistic insights and kinetic studies.

Publications

On the role of pre- and post-electron-transfer steps in the SmI2 /amine/H(2)O-mediated reduction of esters: new mechanistic insights and kinetic studies.

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