Evidence for formation and different evolution of tertiary rhodium alkyl intermediates under rhodium-catalyzed deuterio-(hydro)formylation of 1-(n-pyridyl)-1-phenylethenes

Deuterioformylation of the vinylidenic substrates 1-(n-pyridyl)-1-phenylethenes, 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 and complete substrate conversion. The direct 2H NMR analysis of the crude reaction mixture led to the conclusion that, under these conditions, the branched alkyl rhodium intermediate is formed almost exclusively. It can: (i) β-eliminate, (ii) undergo migratory insertion on CO or (iii) oxidative addition of deuterium to various extents depending on the position of the nitrogen atom with respect to the olefinic double bond, thus accounting for the observed different chemo- and regioselectivities.