A new paradigm in N-heterocyclic carbenoid ligands

We report a new class of very readily prepared chiral N-heterocyclic carbenoid ligand that also contains two different types of chirality: an asymmetric centre and an atropoisomeric unit, but contained in two separate N-substituents, such that both can easily be varied. In addition to its simplicity and flexibility, this approach has potential advantages over systems containing only atropoisomeric units, because the inclusion of an additional fixed asymmetric centre means that the atropoisomers are diastereoisomers and therefore chemically distinct entities. Complexation to a metal centre increases the barrier to rotation in the atropoisomeric unit so that the two diastercoisomers may be separable by simple methods such as standard chromatography. In addition, a novel cis-substituted palladium complex has been characterised by X-ray crystallography. (c) 2005 Elsevier B.V. All rights reserved.