Rhodium-catalyzed hydro(deuterio)formylation of vinylidenic olefins containing a phenyl and a pyridyl group: crucial role of the β-hydride elimination in determining regio- and chemoselectivity

Deuterioformylation of the vinylidenic substrates 1-phenyl-1-(n-pyridyl)ethenes, in the presence of phosphine modified Rh4(CO)12 as catalyst precursor, was carried out at 100 atm of CO and D2 (1:1), 80 °C and at partial substrate conversion. The direct 2H NMR analysis of the crude reaction mixture allowed to conclude that, under these conditions, the branched alkyl rhodium intermediate is almost exclusively formed. It can β-eliminate, undergo migratory insertion or oxidative addition of deuterium in a different degree depending on the position of the nitrogen atom with respect to the olefinic double bond, accounting for the observed different chemo- and regioselectivity.