title: 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 β-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.
authors.affiliation: Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS (UK).
authors.forename: Andrés G
authors.initials: AG
authors.lastname: Algarra
authors.forename: David L
authors.initials: DL
authors.lastname: Davies
authors.forename: Qudsia
authors.initials: Q
authors.lastname: Khamker
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Macgregor
authors.forename: Claire L
authors.initials: CL
authors.lastname: McMullin
authors.forename: Kuldip
authors.initials: K
authors.lastname: Singh
authors.forename: Barbara
authors.initials: B
authors.lastname: Villa-Marcos
keywords: CH activation; coupling reactions; density functional calculations; reaction mechanisms; rhodium