Reductive Coupling of Carbon Dioxide and an Aldehyde Mediated by a Copper(I) Complex toward the Synthesis of α-Hydroxycarboxylic Acids

Abstract

Copper-mediated reductive coupling between CO 2 and an aldehyde to form α-hydroxycarboxylic acid was achieved using silylborane as a reductant. CO 2 cleanly inserted into a copper−carbon bond that was formed by the reaction between a silylcopper−NHC complex and an aldehyde. A series of reactions that regenerate the silylcopper complex were developed for the synthesis of an αhydroxycarboxylic acid. After repeating each step iteratively for two cycles, 0.62 equiv of α-hydroxycarboxylic acid based on the copper complex was obtained.

Activation of the Si–B interelement bond related to catalysis

“…Using the same strategy, the Nozaki group realised a coppercatalysed reductive coupling reaction of p-tolualdehyde and CO 2 with Et 3 Si-Bpin in 2020; the yield is low though (153b -184b, Scheme 57). 75 In 2018, a silylative pinacol-type reductive dimerization of aromatic aldehydes and acetophenones mediated by Au/TiO 2 nanoparticles was accomplished by Stratakis and co-workers 76 Aromatic aldehydes bearing substituents in the orthoposition and aliphatic aldehydes did not participate in this transformation. The same outcome was obtained with bulky acetophenones and aliphatic ketones.…”

Photocatalytic Carbinol Cation/Anion Umpolung: Direct Addition of Aromatic Aldehydes and Ketones to Carbon Dioxide

“…Here, we report a photocatalytic umpolung method of generating carbinol anions from aromatic aldehydes and ketones through a novel pathway in which the resulting ketyl radicals undergo a rapid one-electron reduction prior to radical–radical coupling (Scheme B, upper). Aromatic aldehydes and ketones reacted with CO 2 to form α-hydroxycarboxylic acids in the presence of a photocatalyst and a reductant and in the absence of any other additive. − …”

Visible-light photoredox-catalyzed umpolung carboxylation of carbonyl compounds with CO2

“…After demonstrating the viability of different kinds of ketones, we turned our attention to more challenging aryl aldehydes. Although umpolung carboxylations of aryl aldehydes with CO 2 are rare 43 , 68 , 69 , α-hydroxycarboxylic acids synthesis typically suffer from low yields, low selectivity, and limited scope. When aryl aldehydes were applied as substrates to the standard reaction conditions, we observed a complicated mixture of products due to intense side reactions, including pinacol coupling and disproportionation, which might arise from the lower steric hindrance of aldehydes and the strong base.…”

“…Compared to cyanide and carbon monoxide, CO 2 represents a better choice as a C1 source in carboxylation due to its abundance, nontoxicity, and recyclability 31 – 39 . Although significant progress has been achieved in recent decades to generate α-hydroxycarboxylic acids with CO 2 18 – 20 , 40 – 43 , most of these methods suffer from the utilization of stoichiometric amounts of metal reductants, low functional group tolerance, limited substrate scope, and/or competitive side reactions, especially the pinacol coupling and reduction, thus hampering industrial application. Therefore, a general and practical method is desirable.…”