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

doi:10.1016/S1872-2067(20)63772-X

Abstract

Abstract:

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 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.

Key words: 2-Ethyl-1-hexanol, Guerbet reaction, 1-Butanol, Iridium, Hydrogenation

Zhanwei Xu, Peifang Yan, Changhui Liang, Songyan Jia, Xiumei Liu, Z. Conrad Zhang. Electronic and steric factors for enhanced selective synthesis of 2-ethyl-1-hexanol in the Ir-complex-catalyzed Guerbet reaction of 1-butanol[J]. Chinese Journal of Catalysis, 2021, 42(9): 1586-1592.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(20)63772-X

https://www.cjcatal.com/EN/Y2021/V42/I9/1586

Figures/Tables 10

Scheme 1. Alternative route for the synthesis of 2-ethyl-1-hexanol.

Scheme 2. Chemical structures of Cp*Ir complexes in this work.

Table 1 Guerbet reaction of 1-butanol to 2-EH by using a Cp*Ir complex as catalyst.


Entry	Catalyst	T (°C )	TON	Selectivity (%)
Entry	Catalyst	T (°C )	TON	2-EHA	2-EEA	2-EHE	2-EH
1	[Cp*IrCl₂]₂	120	411	4.1	2.9	18.7	74.3
2	[Cp*IrClL1]Cl	120	629	3.8	7.6	26.6	62.0
3	[Cp*IrClL2]Cl	120	1124	3.5	6.9	27.6	62.0
4	[Cp*IrClL3]Cl	120	2900	1.1	6.1	15.0	77.8
5	[Cp*IrClL4]Cl	120	2192	1.3	5.4	17.7	75.6
6	[Cp*IrClL3]Cl	130	3492	0.7	4.5	8.4	86.4

Fig. 1. (a) Effect of the amount of KOH on TON by using the [Cp*IrClL3]Cl complex as the catalyst (conditions: 0.5 μmol of [Cp*IrClL3]Cl, and 2 g of 1-butanol under 10 bar of N2 atmosphere at 120 °C for 6 h); (b) The effect of the amount of KOH on the selectivity by using [Cp*IrClL3]Cl as the catalyst (conditions: 0.5 μmol of [Cp*IrClL3]Cl and 2 g of 1-butanol under 10 bar of N2 atmosphere at 120 °C for 6 h).

Scheme 3. Proposed pathways of the Guerbet reaction of 1-butanol to 2-EH.

Scheme 4. Steric and conjugation effects on product selectivity in the aldol reactions of butyraldehyde, 2-EHA, and 2-EEA. Reaction conditions: butyraldehyde (1 mmol), 2-EHA (0.5 mmol), 2-EEA (0.5 mmol), KOH (30 mg), 1-butanol (2 g), at 25 °C for 5 min.

Table 2 Control experiments for the hydrogenation of 2-EHA and 2-EHE.

Entry	Substrate	TOF (h^-1)	Selectivity (%)
1	—	—	—
2		1409	92.6
3		56	> 99

Scheme 5. The proposed reaction of [Cp*IrClL3]Cl complex with KOH.

Fig. 2. The 1H NMR and 13C NMR spectra of the [Cp*IrClL3]Cl complex with/without KOH. (a) The 1H NMR spectrum of [Cp*IrClL3]Cl without KOH; (b) The 1H NMR spectrum of [Cp*IrClL3]Cl with KOH; (c) The 13C NMR spectrum of [Cp*IrClL3]Cl without KOH; (d) The 13C NMR spectrum of [Cp*IrClL3]Cl with KOH.

Scheme 6. The proposed mechanism of the Guerbet reaction of 1-butanol to 2-EH.

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