Electronic and steric factors for enhanced selective synthesis of 2-ethyl-1-hexanol in the Ir-complex-catalyzed Guerbet reaction of 1-butanol

1-Butanol is a potential bio-based fermentation product obtained from cellulosic biomass. As a value-added chemical, 2-ethyl-1-hexanol (2-EH) can be produced by Guerbet conversion from 1-butanol. This work reports the enhanced catalytic Guerbet reaction of 1-butanol to 2-EH by a series of Cp*Ir complexes (Cp*: 1,2,3,4,5-pentamethylcyclopenta-1,3-diene) coordinated to bipyridine-type ligands bearing an ortho-hydroxypyridine group with an electron-donating group and a Cl- anion. The catalytic activity of the Cp*Ir complex increased by increasing the electron density of the bipyridine ligand when functionalized with the para-NMe2 and ortho-hydroxypyridine groups. A record turnover number of 14047 was attained. A mechanistic study indicated that the steric effect of the ethyl group on the alpha-C of 2-ethylhexanal (2-EHA) and the conjugation effect of C=C-C=O in 2-ethylhex-2-enal (2-EEA) benefits the high selectivity of 2-EH from 1-butanol by inhibiting the cross-aldol reaction of 2-EHA and 2-EEA with butyraldehyde. Nuclear magnetic resonance study revealed the formation of a carbonyl group in the bipyridine-type ligand via the reaction of the Cp*Ir complex with KOH. (C) 2021, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Automated summary

This study reports an enhanced catalytic Guerbet reaction of 1-butanol to 2-ethyl-1-hexanol using a series of Cp*Ir complexes, achieving a record turnover number of 14047. The mechanistic study indicated that the steric and conjugation effects play a role in the high selectivity of 2-EH from 1-butanol.