Alkali Metal Alkoxides in Noyori‐Type Hydrogenations

Abstract

This minireview discusses the accelerative role of alkali metal alkoxides in two important catalytic processes: hydrogenation of ketones and esters with Noyori‐type molecular catalysts. Here I offer my perspective on the unique mechanistic aspects of these reactions – a subject that remains controversial and often misunderstood.

Selective Reduction of Carboxylic Acids to Alcohols in the Presence of Alcohols by a Dual Bulky Transition-Metal Complex/Lewis Acid Catalyst

“…The main role of the Lewis-acidic additive is to catalyze the esterification, which is followed by ES hydrogenation by the IrPCY2-derived catalyst. Considering the extensive experimental and theoretical studies by Dub, , Gusev, , and Schaub for the catalytic hydrogenation of esters, in addition to our previous results including ESI-MS studies, it seems more likely that H 2 is donated to the ES through the revised Noyori’s MLB mechanism [with the N–X (X = H or Na, Figure ) bond of X–N–M-type complexes (M = e.g., Ru) kept intact throughout the catalytic cycle], although the original Noyori’s MLB mechanism , that involves the simultaneous donation of a nitrogen-bound proton and an Ir-bound hydride from 5x (Figure , X = H) to RCO 2 R′ to generate RCHO, R’OH, and 3 (non-innocent N–H bond) could not be fully ruled out. Species 3 , which is off-cycle in the revised mechanistic scenario, could also follow one of two possible pathways.…”

“…Particularly, when conventionally less reactive alcohols for esterification such as t -BuOH were utilized, extremely sluggish esterification was observed; thus, 1-BuOH was ultimately chosen due to its low boiling point (118 °C at 1 atm), which is favorable for the separation of the desired ALs from 1-BuOH. Alkali metal effects , were also examined as follows: with LiH, esterification was completed but activity for hydrogenation was not obtained over 40 h of reaction time, whereas both KH and KO t -Bu gave similar results to give 2a in 90% yield with ES (∼5%) (Table S2). Other (PNNP)Ir derivatives, in which the Cy group, was replaced with Et, i -Pr, or Ph were also tested, but their effectiveness was comparable to or lower than that of IrPCY2 (Table S6), which highlights the importance of the steric demand of the Ir catalyst.…”

Pressurized formic acid dehydrogenation: an entropic spring replaces hydrogen compression cost

“…26 Often, complexes in this class require pre-activation via hydrodechlorination with KOH or KO t Bu to generate their active hydride forms. 9,26,[31][32][33] Nevertheless, under self-pressurizing conditions, there is an increase in conversion from 29% (entry 2a) to 84% (entry 6a) without such pre-activation. For example, self-pressurization enables complex 4 (entry 4b) to achieve 79% conversion, comparable to its activated dihydride derivative (entry 5, 79%).…”

“…This dramatic enhancement of reactivity upon pressurization suggests that one of the reaction products, like H 2 or CO, is necessary to enable or maintain catalytic activity. Hydrogenation is known to activate amine-containing Noyori-type complexes such as 1-6 in the presence of base, 9,[31][32][33] which is a possible explanation. Further, we observe that thermal decarbonylation of FA is possible at our operating temperature (vide infra).…”

Heterogeneous Enantioselective Hydrogenation of Ketones by 2-Amino-2′-hydroxy-1,1′-binaphthyl-Modified CeO2-Supported Ir Nanoclusters

“…The enantioselective hydrogenation of aromatic ketones without directing groups such as acetophenone is generally more difficult than that of ketones having some directing groups such α-keto esters for heterogeneous catalysts. , As for homogeneous catalysts, various effective Ru and Ir complexes combined with chiral ligands were reported to be effective homogeneous catalysts for the enantioselective hydrogenation of aromatic ketones without directing groups, achieving a high ee (up to >99%). ,,− In contrast, the reports on heterogeneous catalysts for the enantioselective hydrogenation of aromatic ketones without directing groups are limited, and the summary for heterogeneous catalyst systems is shown in Table S1. Most of the heterogeneous catalyst systems require the stabilization treatment of the metal species with organic (chiral) ligands and strong base additives for high activity and enantioselectivity, − and the requirement of strong bases is similar to the case of typical homogeneous catalysts. − ,− There are only two reports on heterogeneous catalysts without strong base additives, , and these catalysts are composed of metal nanoparticles and chiral modifiers without supports, chiral Ir nanoparticles, and cinchonidine-stabilized Rh nanoparticles immobilized on an ionic liquid (BMIMOH), although the ionic liquid in the latter case may act as a base. The former heterogeneous catalyst is chiral secondary phosphine oxide-stabilized Ir nanoparticles, where the chiral secondary phosphine oxide works as a chiral modifier and a stabilizer for Ir nanoparticles to provide a moderate ee (55%) in enantioselective hydrogenation of acetophenone.…”