Highly efficient NHC-iridium-catalyzed β-methylation of alcohols with methanol at low catalyst loadings

Abstract

Science China Chemistry4 scope and up to quantitative yields were achieved at low catalyst loadings with only 5 hydrogen and water as by-products. The protocol was readily extended to the 6 β-alkylation of alcohols with several primary alcohols. Control experiments, along 7 with DFT calculations and crystallographic studies revealed that ligand effect is 8 critical for their excellent catalytic performance, shedding light on more challenging 9 Guerbet reactions with simple alcohols.

Geometry-Constrained N,N,O-Nickel Catalyzed α-Alkylation of Unactivated Amides via a Borrowing Hydrogen Strategy

“…Recently, transition metal-catalyzed α-alkylation of carbonyl compounds with alcohols through a BH pathway has received considerable attractiveness . Among various carbonyl compounds, great progress has been mainly made in the α-alkylation of ketones and β-alkylation of alcohols . In contrast, the α-alkylation of amides is considered more challenging because amides usually exhibit slight Brønsted acidity, which is commonly ascribed to the resonance stabilization effect of the adjacent NR 2 group.…”

Modular Access to Quaternary α-Hydroxyl Acetates by Catalytic Cross-Coupling of Alcohols

“…Acceptorless dehydrogenation of alcohol is a green and atom-economic alternative approach, which provides aldehyde (or ketone) without the use of sacrificial acceptor molecules, and the side product is molecular hydrogen. − By using an acceptorless dehydrogenation strategy, transition-metal-catalyzed C-alkylation of primary or secondary alcohols with other primary alcohols has received considerable attention, in which the primary alcohols are considered as environmentally friendly alkylating agents, and hydrogen and water are produced as clean byproducts. − Furthermore, the cross-coupling of alcohols may also constitute one of the most efficient protocols to access value-added compounds, such as upgraded alcohols, carboxylic acids, and even hydroxyl acids. − However, when 1,2-diols are utilized and coupled with primary alcohols, besides possible alkylation, other thermodynamically favorable products after dehydration, dehydrogenation, oxidization, esterification and even polymerization may also be generated . Even in the case of the alkylation, dehydrogenative cross-coupling of 1,2-propylene glycol and methanol may produce not only ethylene glycol, ethanol, and n - and iso -propanol, but also butane-1,2- and 2,3-diols, as well as other byproducts formed by the methylation of lactate (Figure c), − in which α-hydroxyl acetates may be generated by dehydrogenative oxidization of 1,2-propylene glycol .…”

NHC-Iridium-Catalyzed Deoxygenative Coupling of Primary Alcohols Producing Alkanes Directly: Synergistic Hydrogenation with Sodium Formate Generated in Situ

“…In light of the possible formation of formates after decarbonylation or decarboxylation of the key α,β-unsaturated aldehyde intermediate produced by the coupling of primary alcohols, we posited that the resulting formate could function as a mild reductive reagent in the further hydrogenation of alkene to alkane in the presence of a suitable catalyst . Following our recent studies of NHC-Ir catalyzed cross-coupling of diverse alcohols and NHC-Ru-catalyzed reductive amination of levulinic acid using ammonium formate as a hydrogen source, we achieved the direct homo- and cross-coupling of various primary alcohols producing a variety of alkanes in good to quantitative yields with low (0.8 mol %) catalyst loading (Figure b). A plausible mechanism is proposed, in which sodium formate generated in situ is pivotal for improving the reaction selectivity for alkanes.…”

“…Initially, the homocoupling of 2-phenylethanol ( 1 ) to 1,3-diphenylpropane ( 3 ) was selected as a model reaction for catalyst screening (Figure ). In view of the excellent performance of NHC-Ir complexes in various coupling reactions involving alcohols, , several mono- and bis-NHC-Ir complexes were investigated. Good conversions but poor selectivity (the ratio between 2 and 3 ) for homocoupling reactions were achieved using the mono-NHC-Ir complexes ( 4a – 4c , Figure ), in which the best result was attained with complex 4c with a bulky N -phenyl substitute (78% for compound 3 ).…”

“…Good conversions but poor selectivity (the ratio between 2 and 3 ) for homocoupling reactions were achieved using the mono-NHC-Ir complexes ( 4a – 4c , Figure ), in which the best result was attained with complex 4c with a bulky N -phenyl substitute (78% for compound 3 ). Better selectivity was achieved with bis-NHC-Ir complexes ( 5a – 5c , Figure , 97–99% selectivity), which might be attributed to the lower ionization potential (IP) of the Ir center along with the increased number and steric bulkiness of NHC ligands . Remarkably, quantitative yields and selectivity of 1,3-diphenylpropane ( 3 ) were achieved in the presence of 0.8 mol % bis-NHC-Ir complex ( 5c ) with bulky N -phenyl substituents and 0.6 equiv of NaO t Bu in 0.6 mL t -amyl alcohol at 140 °C within 24 h without the addition of any extra hydrogen source.…”