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

Detailed experimental and computational studies have been carried out on the oxidative coupling of the alkenes C₂H₃Y (Y = CO₂Me (a), Ph (b), C(O)Me (c)) with 3-aryl-5-R-pyrazoles (R = Me (1a), Ph (1b ), CF₃ (1c)) using a[Rh(MeCN)₃Cp∗][PF₆]₂/Cu(OAc)₂·H₂O catalyst system. In the reaction of methyl acrylate with 1a, up to five products (2aa-6aa) were formed, including the trans monovinyl product, either complexed within a novel CuI dimer (2aa) or as the free species (3aa), and a divinyl species (6aa); both 3aa and 6aa underwent cyclisation by an aza-Michael reaction to give fused heterocycles 4aa and 5aa, 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 1a 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.