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Combined experimental and computational investigations of rhodium-catalysed C - H functionalisation of pyrazoles with alkenes.

ABSTRACT: Detailed experimental and computational studies have been carried out on the oxidative coupling of the alkenes C2 H3 Y (Y=CO2 Me (a), Ph (b), C(O)Me (c)) with 3-aryl-5-R-pyrazoles (R=Me (1?a), Ph (1?b), CF3 (1?c)) using a [Rh(MeCN)3 Cp*][PF6 ]2 /Cu(OAc)2 ?H2 O catalyst system. In the reaction of methyl acrylate with 1?a, up to five products (2?aa-6?aa) were formed, including the trans monovinyl product, either complexed within a novel Cu(I) dimer (2?aa) or as the free species (3?aa), and a divinyl species (6?aa); both 3?aa and 6?aa underwent cyclisation by an aza-Michael reaction to give fused heterocycles 4?aa and 5?aa, respectively. With styrene, only trans mono- and divinylation products were observed, whereas with methyl vinyl ketone, a stronger Michael acceptor, only cyclised oxidative coupling products were formed. Density functional theory calculations were performed to characterise the different migratory insertion and ?-H transfer steps implicated in the reactions of 1?a with methyl acrylate and styrene. The calculations showed a clear kinetic preference for 2,1-insertion and the formation of trans vinyl products, consistent with the experimental results.

SUBMITTER: Algarra AG

PROVIDER: S-EPMC4517174 | biostudies-literature | 2015 Feb

REPOSITORIES: biostudies-literature

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Combined experimental and computational investigations of rhodium-catalysed C - H functionalisation of pyrazoles with alkenes.

Algarra Andrés G AG Davies David L DL Khamker Qudsia Q Macgregor Stuart A SA McMullin Claire L CL Singh Kuldip K Villa-Marcos Barbara B

Chemistry (Weinheim an der Bergstrasse, Germany) 20141217 7

Detailed experimental and computational studies have been carried out on the oxidative coupling of the alkenes C2 H3 Y (Y=CO2 Me (a), Ph (b), C(O)Me (c)) with 3-aryl-5-R-pyrazoles (R=Me (1 a), Ph (1 b), CF3 (1 c)) using a [Rh(MeCN)3 Cp*][PF6 ]2 /Cu(OAc)2 ⋅H2 O catalyst system. In the reaction of methyl acrylate with 1 a, up to five products (2 aa-6 aa) were formed, including the trans monovinyl product, either complexed within a novel Cu(I) dimer (2 aa) or as the free species (3 aa), and a divin ...[more]

PMID: 25521823

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Project description:Synthetic methods that utilise iron to facilitate C-H bond activation to yield new C-C and C-heteroatom bonds continue to attract significant interest. However, the development of these systems is still hampered by a limited molecular-level understanding of the key iron intermediates and reaction pathways that enable selective product formation. While recent studies have established the mechanism for iron-catalysed C-H arylation from aryl-nucleophiles, the underlying mechanistic pathway of iron-catalysed C-H activation/functionalisation systems which utilise electrophiles to establish C-C and C-heteroatom bonds has not been determined. The present study focuses on an iron-catalysed C-H allylation system, which utilises allyl chlorides as electrophiles to establish a C-allyl bond. Freeze-trapped inorganic spectroscopic methods (57Fe Mössbauer, EPR, and MCD) are combined with correlated reaction studies and kinetic analyses to reveal a unique and rapid reaction pathway by which the allyl electrophile reacts with a C-H activated iron intermediate. Supporting computational analysis defines this novel reaction coordinate as an inner-sphere radical process which features a partial iron-bisphosphine dissociation. Highlighting the role of the bisphosphine in this reaction pathway, a complementary study performed on the reaction of allyl electrophile with an analogous C-H activated intermediate bearing a more rigid bisphosphine ligand exhibits stifled yield and selectivity towards allylated product. An additional spectroscopic analysis of an iron-catalysed C-H amination system, which incorporates N-chloromorpholine as the C-N bond-forming electrophile, reveals a rapid reaction of electrophile with an analogous C-H activated iron intermediate consistent with the inner-sphere radical process defined for the C-H allylation system, demonstrating the prevalence of this novel reaction coordinate in this sub-class of iron-catalysed C-H functionalisation systems. Overall, these results provide a critical mechanistic foundation for the rational design and development of improved systems that are efficient, selective, and useful across a broad range of C-H functionalisations.

Dataset Information

Combined experimental and computational investigations of rhodium-catalysed C - H functionalisation of pyrazoles with alkenes.

Publications

Combined experimental and computational investigations of rhodium-catalysed C - H functionalisation of pyrazoles with alkenes.

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