Alkali Metal Alkoxides in Noyori‐Type Hydrogenations

Highly Enantiomerically Enriched Secondary Alcohols via Epoxide Hydrogenolysis

In this article, we report the development of ruthenium-catalyzed hydrogenolysis of epoxides to selectively give the branched (Markovnikov) alcohol products. In contrast to previously reported catalysts, the use of Milstein's PNN-pincer-ruthenium complex at room temperature allows the conversion of enantiomerically enriched epoxides to secondary alcohols without racemization of the product. The catalyst is effective for a range of aryl epoxides, alkyl epoxides, and glycidyl ethers and is the first homogeneous system to selectively promote hydrogenolysis of glycidol to 1,2-propanediol, without loss of enantiomeric purity. A detailed mechanistic study was conducted, including experimental observations of catalyst speciation under catalytically relevant conditions, comprehensive kinetic characterization of the catalytic reaction, and computational analysis via density functional theory. Heterolytic hydrogen cleavage is mediated by the ruthenium center and exogenous alkoxide base. Epoxide ring opening occurs through an opposite-side attack of the ruthenium hydride on the less-hindered epoxide carbon, giving the branched alcohol product selectively.

HiREX: High-Throughput Reactivity Exploration for Extended Databases of Transition-Metal Catalysts

A method is introduced for the automated analysis of reactivity exploration for extended in silico databases of transition-metal catalysts. The proposed workflow is designed to tackle two key challenges for bias-free mechanistic explorations on large databases of catalysts: (1) automated exploration of the chemical space around each catalyst with unique structural and chemical features and (2) automated analysis of the resulting large chemical data sets. To address these challenges, we have extended the application of our previously developed ReNeGate method for bias-free reactivity exploration and implemented an automated analysis procedure to identify the classes of reactivity patterns within specific catalyst groups. Our procedure applied to an extended series of representative Mn(I) pincer complexes revealed correlations between structural and reactive features, pointing to new channels for catalyst transformation under the reaction conditions. Such an automated high-throughput virtual screening of systematically generated hypothetical catalyst data sets opens new opportunities for the design of high-performance catalysts as well as an accelerated method for expert bias-free high-throughput in silico reactivity exploration.

Hydrogenation of CO2 to MeOH Catalyzed by Highly Robust (PNNP)Ir Complexes Activated by Alkali Bases in Alcohol

Despite receiving significant attention, well-defined homogeneous complexes for hydrogenation of carbon dioxide (CO2) to methanol (MeOH) are scarce and suffer issues of low catalyst turnover numbers (TONs) at high catalyst concentrations and deactivation in the presence of CO and at elevated temperatures. Herein, we disclose a system deploying sterically demanded (PNNP)Ir complexes for a sustained activity for hydrogenation of CO2 to MeOH at temperatures ∼200 °C in an alcohol solvent. Through reaction optimization, we achieved a TON of ∼9000 for MeOH formation, which exceeds most active homogeneous systems reported to date, and robustness on par with or exceeding most reactive systems utilizing amine additives was demonstrated. The key to achieving sustained catalyst turnover for the system was utilizing a catalytic amount of an alkali base additive, which serves the dual purpose of facilitating more efficient outer-sphere reduction of CO2 and HCO2Et and enhancing the selectivity of MeOH over in situ formed CO.

On the mechanism of acceptorless dehydrogenation of N-heterocycles catalyzed by tBuOK: a computational study

The catalytic acceptorless dehydrogenation (ADH) of saturated N-heterocycles has recently gained considerable attention as a promising strategy for hydrogen release from liquid organic hydrogen carriers (LOHCs). Recently, a simple ^tBuOK base-promoted ADH of N-heterocycles was developed by Yu et al. (Adv. Synth. Catal. 2019, 361, 3958). However, it is still open as to how the ^tBuOK plays a catalytic role in the ADH process. Herein, our density functional study reveals that the ^tBuOK catalyzes the ADH of 1,2,3,4-tetrahydroquinoline (THQ) through a quasi-metal-ligand bifunctional catalytic channel or a base-catalyzed pathway with close energy barriers. The hydride transfer in the first dehydrogenation process is determined to be the rate determining step, and the second dehydrogenation can proceed directly from 34DHQ regulated by the ^tBuOK. In addition, the computational results show that the cooperation of a suitable alkali metal ion with the ^tBuO^- group is so critical that the ^tBuOLi and the isolated ^tBuO^- are both inferior to ^tBuOK as a dehydrogenation catalyst.

Pressurized Formic Acid Dehydrogenation: An Entropic Spring Replaces Hydrogen Compression Cost

Formic acid is unique among liquid organic hydrogen carriers (LOHCs), because its dehydrogenation is highly entropically driven. This enables the evolution of high-pressure hydrogen at mild temperatures that is difficult to achieve with other LOHCs, conceptually by releasing the "spring" of energy stored entropically in the liquid carrier. Applications calling for hydrogen-on-demand, such as vehicle filling, require pressurized H2. Hydrogen compression dominates the cost for such applications, yet there are very few reports of selective, catalytic dehydrogenation of formic acid at elevated pressure. Herein, we show that homogenous catalysts with various ligand frameworks, including Noyori-type tridentate (PNP, SNS, SNP, SNPO), bidentate chelates (pyridyl)NHC, (pyridyl)phosphine, (pyridyl)sulfonamide, and their metallic precursors, are suitable catalysts for the dehydrogenation of neat formic acid under self-pressurizing conditions. Quite surprisingly, we discovered that their structural differences can be related to performance differences in their respective structural families, with some tolerant or intolerant of pressure and others that are significantly advantaged by pressurized conditions. We further find important roles for H2 and CO in catalyst activation and speciation. In fact, for certain systems, CO behaves as a healing reagent when trapped in a pressurizing reactor system, enabling extended life from systems that would be otherwise deactivated.

Structure, reactivity and catalytic properties of manganese-hydride amidate complexes

The high efficiency of widely applied Noyori-type hydrogenation catalysts arises from the N-H moiety coordinated to a metal centre, which stabilizes rate-determining transition states through hydrogen-bonding interactions. It was proposed that a higher efficiency could be achieved by substituting an N-M' group (M' = alkali metals) for the N-H moiety using a large excess of metal alkoxides (M'OR); however, such a metal-hydride amidate intermediate has not yet been isolated. Here we present the synthesis, isolation and reactivity of a metal-hydride amidate complex (HMn-NLi). Kinetic studies show that the rate of hydride transfer from HMn-NLi to a ketone is 24-fold higher than that of the corresponding amino metal-hydride complex (HMn-NH). Moreover, the hydrogenation of N-alkyl-substituted aldimines was realized using HMn-NLi as the active catalyst, whereas HMn-NH is much less effective. These results highlight the superiority of M/NM' bifunctional catalysis over the classic M/NH bifunctional catalysis for hydrogenation reactions.

Ir/f-Ampha complex catalyzed asymmetric sequential hydrogenation of enones: a general access to chiral alcohols with two contiguous chiral centers

A general and highly efficient method for asymmetric sequential hydrogenation of α,β-unsaturated ketones has been developed by using an iridium/f-Ampha complex as the catalyst, furnishing corresponding chiral alcohols with two contiguous stereocenters in high yields with excellent diastereo- and enantioselectivities (up to 99% yield, >20 : 1 dr and >99% ee). Control experiments indicated that the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C and CO bonds of the enones were hydrogenated sequentially, and the final stereoselectivities were determined by the dynamic kinetic resolution of ketones. Moreover, DFT calculations revealed that an outer sphere pathway was involved in both reduction of CC and CO bonds of enones. The synthetic utility of this method was demonstrated by a gram-scale reaction with very low catalyst loading (S/C = 20 000) and a concise synthetic route to key chiral intermediates of the antiasthmatic drug CP-199,330.

A general and efficient method for asymmetric sequential hydrogenation of α,β-unsaturated ketones has been developed. A dynamic kinetic resolution and an outer sphere pathway were involved in this transformation.