Synthesis and Reactivity of Metal–Ligand Cooperative Bifunctional Ruthenium Hydride Complexes: Active Catalysts for β-Alkylation of Secondary Alcohols with Primary Alcohols

Recent advances in cross-coupling of alcohols via borrowing hydrogen catalysis

Application of the borrowing hydrogen strategy facilitates utilization of abundantly available alcohols for linear or branched long-chain alcohols. Selective synthesis of such alcohols is highly challenging and involves the utilization of transition metal catalysts towards the desired cross-coupled product. Herein, we have highlighted recent advances (from 2015 to 2023) towards the synthesis of higher alcohols. Major focus has been given to the development of ligands, including transition metal catalysts. Judicious catalyst design plays a key role in the alkylation process and is summarised in this review.

One pot tandem dehydrogenative cross-coupling of primary and secondary alcohols by ruthenium amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene complexes

One-pot tandem dehydrogenative cross-coupling of primary and secondary alcohols was catalyzed by three ruthenium complexes [1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene]Ru(p-cymene)Cl [R = Et (1b), i-Pr (2b), Bn (3b)], of amido-functionalized 1,2,4-triazole derived N-heterocyclic carbene (NHC) ligands. Density Functional Theory (DFT) calculations were employed for the ruthenium (1b) precatalyst to understand this reaction mechanism completely, and the mechanisms adapted are divided categorically into three steps (i) nucleophilic substitution of chloride ions by alcohols, (ii) dehydrogenation of primary and secondary alcohols, and (iii) olefin and ketone hydrogenation. Our mechanistic study reveals that the formation of a deprotonated Ru-alcoholate (A) or (E) intermediate is favorable compared to the protonated form (A') or (E') from (1b) by associative nucleophilic substitution. Though an ionic pathway that proceeds through (A') or (E'), has less barriers in the dehydrogenation and olefin/ketone hydrogenation steps than that of the neutral pathway, proceeding through (A) or (E), a steep energy barrier was observed in the first nucleophilic substitution step, prohibiting the reaction to proceed via the intermediate (A') or (E'). Thus, our thorough mechanistic study reveals that the reaction proceeds via deprotonated Ru-alcoholate (A) or (E) species. Furthermore, the 1,4 addition of an α,β-unsaturated carbonyl compound is kinetically and thermodynamically favorable over the 1,2 addition, and the experiments support these observations. As a testimony towards practical application in synthesizing bio-active flavonoid based natural products, five different flavan derivatives (16-20), were synthesized by the dehydrogenative coupling reaction using the neutral ruthenium (1-3)b complexes.

Ruthenium Complexes Bearing α-Diimine Ligands and Their Catalytic Applications in N-Alkylation of Amines, α-Alkylation of Ketones, and β-Alkylation of Secondary Alcohols

New Ru(II) complexes encompassing α-diimine ligands were synthesized by reacting ruthenium precursors with α-diimine hydrazones. The new ligands and Ru(II) complexes were analyzed by analytical and various spectroscopic methods. The molecular structures of L₁ and complexes 1, 3, and 4 were determined by single-crystal XRD studies. The results reveal a distorted octahedral geometry around the Ru(II) ion for all complexes. Moreover, the new ruthenium complexes show efficient catalytic activity toward the C-N and C-C coupling reaction involving alcohols. Particularly, complex 3 demonstrates effective conversion in N-alkylation of aromatic amines, α-alkylation of ketones, and β-alkylation of alcohols.

Guerbet-type β-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex

β-Alkylation of alcohols has been efficiently accomplished using readily available 3d metal Cr under microwave conditions in air. Well-defined molecular Cr is involved with a KIE of 7.33 and insertion of α-alkylated ketone into Cr–H bond as the RDS.

Ligand-free Guerbet-type reactions in air catalyzed by in situ formed complexes of base metal salt cobaltous chloride

Inexpensive, earth-abundant & environmentally benign CoCl2 efficiently catalyses the β-alkylation of alcohol in unprecedented yields (89%) & turnovers (8900). Mechanistic studies are indicative of in situ generated homogeneous molecular Co catalysts.

Dehydrogenative alcohol coupling and one-pot cross metathesis/dehydrogenative coupling reactions of alcohols using Hoveyda–Grubbs catalysts

Grubbs catalysts are shown to catalyze one-pot cross-metathesis/dehydrogenative alcohol coupling reactions in an efficient manner.

Recent Advances in Homogeneous Catalysis via Metal–Ligand Cooperation Involving Aromatization and Dearomatization

Recently, an increasing number of metal complex catalysts have been developed to achieve the activation or transformation of substrates based on cooperation between the metal atom and its ligands. In such “cooperative catalysis,” the ligand not only is bound to the metal, where it exerts steric and electronic effects, but also functionally varies its structure during the elementary processes of the catalytic reaction. In this review article, we focus on metal–ligand cooperation involving aromatization and dearomatization of the ligand, thus introducing the newest developments and examples of homogeneous catalytic reactions.

Phosphine-free pincer-ruthenium catalyzed biofuel production: high rates, yields and turnovers of solventless alcohol alkylation

High TONs and TOFs are observed for the β-alkylation of alcohols using phosphine-free pincer-ruthenium catalysts at a very low base loading. Kinetic studies and DFT calculations were complementary and provide a clear understanding on the mechanism.

Bidentate Ru(ii)-NC complexes as catalysts for the dehydrogenative reaction from primary alcohols to carboxylic acids

Complex 1 is active for alcohol dehydrogenative reactions, and two critical intermediates were isolated and characterized.

Unusual C–O bond cleavage of aromatic ethers in ruthenium complexes bearing a 2-alkoxypyridyl fragment

The S_N2 C–O cleavage mechanism with neutral H₂O switches to S_NAr when KOH is added.