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Issue 30, 2020
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Catalyst control of selectivity in the C–O bond alumination of biomass derived furans

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Abstract

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furans proceeded between 25 and 80 °C leading to dearomatised products due to the net transformation of a sp2 C–O bond into a sp2 C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH = +19.7 (±2.7) kcal mol−1, ΔS = −18.8 (±7.8) cal K−1 mol−1 and ΔG298 K = +25.3 (±0.5) kcal mol−1 and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4 + 1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an α-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp2 C–O bond through population of the σ*-orbital. Inclusion of [Pd(PCy3)2] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans and improves selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to the sp2C–H bond. The aluminium(III) dihydride reagent [{(MesNCMe)2CH}AlH2] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination occurs with a KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to the palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy3)2]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to important platform chemicals from biomass.

Graphical abstract: Catalyst control of selectivity in the C–O bond alumination of biomass derived furans

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Supplementary files

Article information


Submitted
03 Apr 2020
Accepted
03 Jul 2020
First published
08 Jul 2020

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2020,11, 7850-7857
Article type
Edge Article

Catalyst control of selectivity in the C–O bond alumination of biomass derived furans

T. N. Hooper, R. K. Brown, F. Rekhroukh, M. Garçon, A. J. P. White, P. J. Costa and M. R. Crimmin, Chem. Sci., 2020, 11, 7850 DOI: 10.1039/D0SC01918F

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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