Abstract:Catalytic reductions of carbonyl‐containing compounds are highly important for the safe, sustainable, and economical production of alcohols. Herein, we report on the efficient transfer hydrogenation of ketones catalyzed by a highly potent Mn(I)−NHC complex. Mn−NHC 1 is practical at metal concentrations as low as 75 ppm, thus approaching loadings more conventionally reserved for noble metal based systems. With these low Mn concentrations, catalyst deactivation is found to be highly temperature dependent and bec… Show more
“…in 31 P NMR compared to the initial cationic complex 3 ( δ = 37.7 p.p.m.). At the same time complex 4 , as well as its parent complex 3 exhibited restricted mesityl group rotation dynamics evidenced by the loss of equivalency between ortho -methyl substituents of the mesityl group on the NMR timescales—a typical feature of Mn(I)–NHC complexes 35 . Fig.…”
Section: Resultsmentioning
confidence: 97%
“…Importantly, catalyst 3 tolerates elevated reaction temperatures of 80 and 100 °C, that marks a significant improvement of thermal stability over the parent CN bidentate that rapidly degraded as the temperatures were elevated over 70 °C (ref. 35 ). Even at 100 °C hydrogenations with 3 led to quantitative yields requiring only 50 p.p.m.…”
Section: Resultsmentioning
confidence: 99%
“…The activity of Mn catalysts is generally lower than that of noble metal catalysts with majority of Mn-catalyzed hydrogenations requiring relatively high catalyst loadings of 0.1–5 mol%—a feature that strongly limits their practical utility. We recently demonstrated that reliance on such high metal loadings in Mn catalysis might stem from the limited stability of Mn pre-catalysts, most noticeable when Mn loadings are low 35 . Namely, we noted that Mn(I)–NHC complexes featuring aminocarbene “CN” bidentate ligands were highly competent hydride transfer catalysts at low reaction temperatures and high metal loadings, but a rapid catalyst degradation took place upon even marginal increase of reaction temperatures or reduction of catalyst loading <100 p.p.m.…”
Section: Introductionmentioning
confidence: 99%
“…Namely, we noted that Mn(I)–NHC complexes featuring aminocarbene “CN” bidentate ligands were highly competent hydride transfer catalysts at low reaction temperatures and high metal loadings, but a rapid catalyst degradation took place upon even marginal increase of reaction temperatures or reduction of catalyst loading <100 p.p.m. with respect to reduction substrate 35 . Addressing the catalyst stability in this work, we developed an active and highly stable Mn(I) catalyst that can promote hydrogenation reactions at catalyst loadings as low as 5 parts per million.…”
Any catalyst should be efficient and stable to be implemented in practice. This requirement is particularly valid for manganese hydrogenation catalysts. While representing a more sustainable alternative to conventional noble metal-based systems, manganese hydrogenation catalysts are prone to degrade under catalytic conditions once operation temperatures are high. Herein, we report a highly efficient Mn(I)-CNP pre-catalyst which gives rise to the excellent productivity (TOF° up to 41 000 h−1) and stability (TON up to 200 000) in hydrogenation catalysis. This system enables near-quantitative hydrogenation of ketones, imines, aldehydes and formate esters at the catalyst loadings as low as 5–200 p.p.m. Our analysis points to the crucial role of the catalyst activation step for the catalytic performance and stability of the system. While conventional activation employing alkoxide bases can ultimately provide catalytically competent species under hydrogen atmosphere, activation of Mn(I) pre-catalyst with hydride donor promoters, e.g. KHBEt3, dramatically improves catalytic performance of the system and eliminates induction times associated with slow catalyst activation.
“…in 31 P NMR compared to the initial cationic complex 3 ( δ = 37.7 p.p.m.). At the same time complex 4 , as well as its parent complex 3 exhibited restricted mesityl group rotation dynamics evidenced by the loss of equivalency between ortho -methyl substituents of the mesityl group on the NMR timescales—a typical feature of Mn(I)–NHC complexes 35 . Fig.…”
Section: Resultsmentioning
confidence: 97%
“…Importantly, catalyst 3 tolerates elevated reaction temperatures of 80 and 100 °C, that marks a significant improvement of thermal stability over the parent CN bidentate that rapidly degraded as the temperatures were elevated over 70 °C (ref. 35 ). Even at 100 °C hydrogenations with 3 led to quantitative yields requiring only 50 p.p.m.…”
Section: Resultsmentioning
confidence: 99%
“…The activity of Mn catalysts is generally lower than that of noble metal catalysts with majority of Mn-catalyzed hydrogenations requiring relatively high catalyst loadings of 0.1–5 mol%—a feature that strongly limits their practical utility. We recently demonstrated that reliance on such high metal loadings in Mn catalysis might stem from the limited stability of Mn pre-catalysts, most noticeable when Mn loadings are low 35 . Namely, we noted that Mn(I)–NHC complexes featuring aminocarbene “CN” bidentate ligands were highly competent hydride transfer catalysts at low reaction temperatures and high metal loadings, but a rapid catalyst degradation took place upon even marginal increase of reaction temperatures or reduction of catalyst loading <100 p.p.m.…”
Section: Introductionmentioning
confidence: 99%
“…Namely, we noted that Mn(I)–NHC complexes featuring aminocarbene “CN” bidentate ligands were highly competent hydride transfer catalysts at low reaction temperatures and high metal loadings, but a rapid catalyst degradation took place upon even marginal increase of reaction temperatures or reduction of catalyst loading <100 p.p.m. with respect to reduction substrate 35 . Addressing the catalyst stability in this work, we developed an active and highly stable Mn(I) catalyst that can promote hydrogenation reactions at catalyst loadings as low as 5 parts per million.…”
Any catalyst should be efficient and stable to be implemented in practice. This requirement is particularly valid for manganese hydrogenation catalysts. While representing a more sustainable alternative to conventional noble metal-based systems, manganese hydrogenation catalysts are prone to degrade under catalytic conditions once operation temperatures are high. Herein, we report a highly efficient Mn(I)-CNP pre-catalyst which gives rise to the excellent productivity (TOF° up to 41 000 h−1) and stability (TON up to 200 000) in hydrogenation catalysis. This system enables near-quantitative hydrogenation of ketones, imines, aldehydes and formate esters at the catalyst loadings as low as 5–200 p.p.m. Our analysis points to the crucial role of the catalyst activation step for the catalytic performance and stability of the system. While conventional activation employing alkoxide bases can ultimately provide catalytically competent species under hydrogen atmosphere, activation of Mn(I) pre-catalyst with hydride donor promoters, e.g. KHBEt3, dramatically improves catalytic performance of the system and eliminates induction times associated with slow catalyst activation.
“…15,16 Indeed, for Mn, the introduction of simple bi- or tridentate nitrogen-donor ligands was sufficient to promote hydrogenation of carbon dioxide to formate and formamide 17 and transfer hydrogenation of C=X bonds (X = O, N), e.g., ketones, imines, and aldimines. 18−25…”
The catalytic asymmetric
transfer hydrogenation (ATH) of ketones
is a powerful methodology for the practical and efficient installation
of chiral centers. Herein, we describe the synthesis, characterization,
and catalytic application of a series of manganese complexes bearing
simple chiral diamine ligands. We performed an extensive experimental
and computational mechanistic study and present the first detailed
experimental kinetic study of Mn-catalyzed ATH. We demonstrate that
conventional mechanistic approaches toward catalyst optimization fail
and how apparently different precatalysts lead to identical intermediates
and thus catalytic performance. Ultimately, the Mn–N,N complexes
under study enable quantitative ATH of acetophenones to the corresponding
chiral alcohols with 75–87% ee.
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