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

Abstract

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 α-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.

Structural Evolution of MOF-Derived RuCo, A General Catalyst for the Guerbet Reaction

“…Our previous studies were aimed at the development of the optimized MOF-derived catalytic systems for the synthesis of isolated alcohol substrates, which are currently produced on large scales in a nonrenewable fashion. However, many Guerbet alcohols, with feature distinctive β-branching that engenders high liquidity and oxidative stability, find application in lubrication, paper processing and personal care products. ,, Yet, reports, especially of a recent nature, on self-condensation of short-chain alcohols other than ethanol to high-volume Guerbet alcohols have been scarce. ,,− Here, we show that MOF-derived alloys can function as general Guerbet catalysts with high catalytic activity for a suite of alcohol substrates, such as 1-propanol, 1-butanol and 1-pentanol. We have previously shown the superior catalytic performance of MOF-derived RuCo, formed in situ from bench stable precursors, over RuCo nanoparticles supported on a variety of traditional supports .…”

Scent of knowledge: The molecular fingerprint of volatiles in an emblematic historical library in Italy

“…The presence of alcohols such as heptanol and 2‐ethyl‐1‐hexanol among VOCs from naturally aged paper was already mentioned; 7,8,12,15 the former develops during autoxidation of fatty acids, 4,7,8,11–15 while the latter was considered a decomposition marker of cellulose 12 but its formation as a result of a degradation pathway was never clarified; it is here hypothesized for the first time that its synthesis may be the result of a Guerbet reaction that converts a primary alcohol into its β‐alkylated dimer with loss of one equivalent of water. This way, butanol, which can easily be provided by the acetone‐butanol‐ethanol biochemical conversion of sugars, might be converted into 2‐ethyl‐1‐hexanol 37,38 and the chirality of the found 2‐ethyl‐1‐hexanol lends support to its relationship with paper biodegradation pathways 12 . Alcohols such as heptanol and 2‐ethyl‐1‐hexanol contribute to a green and sweet odor even if their odor threshold is higher than their corresponding aldehyde 36 …”