Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

Abstract

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for CC bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and β-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH3 )(cod)(MeIm(2- methoxybenzyl))][BF4 ] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0 ) of 1283 h(-1) , and also in the β-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan-2-ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross-aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new CC bond that involves the reaction of an O-bound enolate generated in the basic medium with the electrophilic aldehyde.

Asymmetric Transfer Hydrogenation of α,β‐Unsaturated Carbonyl Compounds to Saturated Alcohols as Catalyzed by Iridium Complexes of Tricyclic Bioxazoline‐Fused Imidazole‐Derived N‐Heterocyclic Carbene Ligands

“…In order to gain insight on the catalyst mode of action, a detailed mass spectrometric studies were performed. In particular, a proposed mechanism consisting of two catalytic cycles working in tandem is outlined in Scheme , based on previously reported mechanism for the transfer hydrogenation of ketones, and α,β‐unsaturated ketones ,,. In particular, in the first catalytic cycle, the reaction of Ir‐NHC complex ( 1 e ) in presence of i ‐PrOH would give an i ‐propoxy bound Ir‐NHC species ( A ).…”

A Ferrocenyl‐Benzo‐Fused Imidazolylidene Complex of Ruthenium as Redox‐Switchable Catalyst for the Transfer Hydrogenation of Ketones and Imines

“…Some of the products were identified according to commercially available samples: 1‐phenylethanol, cyclohexanol, α‐methyl‐2‐naphtalenemethanol, 4‐bromo‐α‐methylbenzyl alcohol, 4‐methoxy‐α‐methylbenzyl alcohol and N ‐benzylaniline. The spectroscopic features of 1‐phenylhexan‐1‐ol were obtained from the literature …”

Recent Advances in Cascade Reactions Initiated by Alcohol Oxidation

“…Most of the traditional C−C coupling protocols suffer from the production of stoichiometric amounts of waste salts, whereas the catalytic one‐pot acceptorless dehydrogenation of alcohols to the corresponding carbonyl compounds followed by aldol condensation and hydrogenation in one pot (so‐named Guerbet reaction, Scheme ) represents a more elegant way to form new carbon–carbon bonds . During the last decade, it has been demonstrated that different functionalized alcohols can be used as green alkylating agents, mainly upon ruthenium, iridium, and rhenium catalysis …”