Asymmetric Hydrogenation Approaches to Valuable, Acyclic 1,3-Hydroxymethyl Chirons

Research into directed hydrogenations for acyclic stereocontrol peaked in the 1980s.1 Much of that work involved 1,1-disubstituted alkenes that are sterically noncongested and easy to hydrogenate. Examples involving trisubstituted alkenes are rarer and tend to feature homoallylic alcohols.2 In these cases, high diastereoselec-tivities are possible because simultaneous coordination of the metal to the hydroxyl and alkene gives rigid, chairlike intermediates. However, there are relatively few examples of useful acyclic stereocontrol in hydrogenations of trisubstituted alkenes that are allylic alcohols, or where no alcohol at all is present. There is a good reason for this. Most of the work on directed hydrogenations has featured rhodium and iridium catalysts of the type MP2 where P2 is a chelating diphosphine ligand, and these do not hydrogenate trisubstituted alkenes at a significant rate if a directing group is not present. This is not so, however, for Crabtree’s catalyst3–6 an