A rhodium catalyst system conjured up by Zhijian Liu and John F. Hartwig of the University of Illinois, Urbana-Champaign, can catalyze intramolecular cyclizations of aminoalkenes under mild conditions (J. Am. Chem. Soc., DOI: 10.1021/ja710126x). The reaction, when optimized, could significantly expand the scope of hydroaminations of olefins, one of the simplest ways to prepare alkylamines.
The reactions proceed with unactivated aminoalkenes containing primary or secondary amines, terminal or internal alkenes, and linkers that possess or lack substituents that bias the substrate toward cyclization. In addition, the reactions occur at low temperature and aren't disrupted by spectator functional groups. That diverse combination of possibilities, which is not matched by existing catalyst systems, leads to a variety of five- and six-membered-ring products, the researchers note.
University of Michigan chemist Melanie S. Sanford, whose work has included rhodium-mediated cyclizations, comments that Liu and Hartwig's hydroamination results "address an important problem in organic synthesis that had not been previously solved."
A lot of progress had already been made in developing catalytic processes for hydroaminations, which involve the addition of an N-H bond across the multiple bond of an alkene, allene, or alkyne. Lanthanides, group 4 early-transition metals, and some late-transition metals have been reported to add amines to olefins, Hartwig says, but those catalysts have been limited in one way or another. In some cases, the metals are too sensitive to air or moisture. In other cases, the metals only facilitate additions involving secondary amines or terminal olefins. And in yet other cases, the metals can't tolerate the presence of polar functional groups.
Liu and Hartwig created their catalyst by combining the precursor [Rh(cyclooctadiene)2]BF4 with a variety of arylphosphine ligands that permitted them to control the selectivity of the reactions. They focused on an amino biarylphosphine ligand, which converted a range of aminoalkenes to pyrrolidines and piperidines and N-substituted pyrrolidines and piperidines.