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Ruthenium-catalysed decarboxylative unsymmetric dual ortho-/meta-C–H bond functionalization of arenecarboxylic acids† † Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc01226c

ABSTRACT: Here, we describe a ruthenium-catalysed decarboxylative unsymmetric ortho-C–H azaarylation/meta-C–H alkylation via a traceless directing group relay strategy. The installation of a 2-pyridyl functionality via carboxyl directed ortho-C–H activation is critical to promote decarboxylation and enable meta-C–H bond alkylation to streamline the synthesis of 4-azaaryl-benzo-fused five-membered heterocycles. This protocol is characterized by high regio- and chemoselectivity, broad substrate scopes, and good functional group tolerance under redox-neutral conditions. We report a Ru-catalysed decarboxylative unsymmetric ortho-C–H azaarylation/meta-C–H alkylation via a traceless directing group relay strategy, which shows high regio- and chemoselectivity with broad substrate scopes under redox-neutral conditions.

SUBMITTER: Li X

PROVIDER: S-EPMC10208063 | biostudies-literature | 2023 Apr

REPOSITORIES: biostudies-literature

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Project description:Lignin is the most abundant renewable feedstock to produce aromatic chemicals, however its depolymerisation involves the breaking of several C-O and C-C inter-unit linkages that connect smaller aromatic units that are present in lignin. Several strategies have been reported for the cleavage of the C-O inter-unit linkages in lignin. However, till today, only a few methodologies have been reported for the effective breaking or the conversion of the recalcitrant C-C inter unit linkages in lignin. Here we report the ruthenium ion catalysed oxidative methodology as an effective system to activate or convert the most recalcitrant inter unit linkages such as β-5 and 5-5' present in lignin. Initially, we used biphenyl as a model compound to study the effectiveness of the RICO methodology to activate the 5-5' C-C linkage. After 4 h reaction at 22 °C, we achieved a 30% conversion with 75% selectivity towards benzoic acid and phenyl glyoxal as the minor product. To the best of our knowledge this is the first ever oxidative activation of the C-C bond that connects the two phenyl rings in biphenyl. DFT calculation revealed that the RuO4 forms a [3 + 2] adduct with one of the aromatic C-C bonds resulting in the opening of the phenyl ring. Biphenyl conversion could be increased by increasing the amount of oxidant; however, this is accompanied by a reduction in the carbon balance because of the formation of CO2 and other unknown products. We extended this RICO methodology for the oxidative depolymerisation of lignin model hexamer containing β-5, 5-5' and β-O-4 linkages. Qualitative and quantitative analyses of the reaction mixture were done using 1H, 13C NMR spectroscopy methods along with GC-MS and Gel Permeation Chromatographic (GPC) methods. Advanced 2D NMR spectroscopic methods such as HSQC, HMBC and 31P NMR spectroscopy after phosphitylation of the mixture were employed to quantitatively analyse the conversion of the β-5, 5-5' and β-O-4 linkages and to identify the products. After 30 min, >90% of the 5-5' and linkages and >80% of the β-5' are converted with this methodology. This is the first report on the conversion of the 5-5' linkage in lignin model hexamer.

Dataset Information

Ruthenium-catalysed decarboxylative unsymmetric dual ortho-/meta-C–H bond functionalization of arenecarboxylic acids† † Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc01226c

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