Ir(NHC)‐Catalyzed Synthesis of β‐Alkylated Alcohols via Borrowing Hydrogen Strategy: Influence of Bimetallic Structure

Abstract

Multi N‐heterocyclic carbene(NHC)‐modified iridium catalysts were employed in the β‐alkylation of alcohols; dimerization of primary alcohols (Guerbet reaction), cross‐coupling of secondary and primary alcohols, and intramolecular cyclization of alcohols. Mechanistic studies of Guerbet reaction, including kinetic experiments, mass analysis, and density functional theory (DFT) calculation, were employed to explain the fast reaction promoted by bimetallic catalysts, and the dramatic reactivity increase of monometallic catalysts at the late stage of the reaction.

Synthesis of α-Hydroxy Acids via Dehydrogenative Cross-Coupling of a Sustainable C2 Chemical (Ethylene Glycol) with Alcohols

“…C 6 –C 9 and C 12 AHAs ( 6 – 10 ) were obtained with 70–76% yields under the indicated conditions. In addition to the formation of AHAs, β-alkylated alcohols via the Guerbet reaction were also observed as a side product . The reaction of ethylene glycol and benzyl alcohols forms acid 11 .…”

“…As illustrated in Figure , the consumption rate of ethylene glycol is slightly higher than the yield of AHAs; the conversion of ethylene glycol is 38%, and the yield of AHAs is 24% at 1 h, implying that most dehydrogenated ethylene glycols participate in the reaction forming AHA. The amount of consumed pentanol is 4.9 times more than that of consumed ethylene glycol, likely due to a competitive Guerbet reaction of pentanol . The conversion rate of pentanol appears low due to the use of excess (20 equiv) ethylene glycol, and the absolute amount of consumed pentanol is much higher than that of ethylene glycol.…”

“…N -Heterocyclic carbene (NHC)-modified Ir catalysts have been employed in dehydrogenative cross-coupling using bioalcohols, ,− wherein Ir catalysts induce the dehydrogenation of alcohols, and the resulting carbonyl intermediates undergo further C–C bond formation. Compared to the cross-dehydrogenative coupling involving the oxidation of C–H bonds, , Ir-catalyzed dehydrogenative cross-coupling reactions do not use oxidants, but hydrogen is evolved as a byproduct.…”

Recent advances in C/N-alkylation with alcohols through hydride transfer strategies

“…Recently, Jang et al described an improvement of the Guerbet type reaction for synthesizing heavily branched primary alcohols employing a bimetallic Ir(NHC) complex as the catalyst (Scheme 36). 47 In short reaction times (8 h), the bimetallic complex offered higher yields. However, as the reaction time increased (16 h), the monometallic catalyst exhibited better conversion yields, even higher than the bimetallic one in some cases, due to the slow transition of the monometallic complex to the bimetallic complex during the reaction.…”

Tuning the selectivity in iridium-catalyzed acceptorless dehydrogenative coupling of primary alcohols

“…However, there remains a particularly challenging selectivity problem for the ADC of primary alcohols, since this process can easily form three different products, namely, carboxylic acids/carboxylates in the presence of a high base concentration 2 or base-free conditions, 3 esters via the dehydrogenative coupling of alcohols, 3 b ,4 and Guerbet alcohols from the dimerization of primary alcohols via β-alkylation. 4 p ,5 Product selectivity is one of the most important issues for the success of a chemical reaction and, as such, it is be highly desirable to control the selectivity of the ADC of primary alcohols through a single catalyst for the selective formation of carboxylic acids, esters, and Guerbet alcohols.…”