Rhodium-catalyzed hydroformylation of 1-allylpyrrole as an unexpected way to 5,6-dihydroindolizine synthesis

When 1-allylpyrrole was subjected to hydroformylation conditions with Rh4(CO)12 as the catalyst precursor, at 120 atm total pressure, at 20 and 100°C, 5,6-dihydroindolizine was found unexpectedly, together with the expected branched aldehyde, the linear isomer being obtained in traces amounts only. An annulation via a nucleophilic attack of the pyrrole C2 carbon atom on the carbonyl group of the linear aldehyde, followed by dehydration of the intermediate alcohol, possibly generates the indolizine structure.