A review of soluble transition-metal nanoclusters as arene hydrogenation catalysts

A critical review of the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds is presented. The review begins with a brief introduction to arene hydrogenation and to nanocluster science. The introductory material is followed by a detailed discussion of the approximately 20 papers in the literature that deal with the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds. Metal particle catalysts on solid supports are not reviewed herein, and are considered only as far as they serve to compare and contrast with soluble transition-metal nanoclusters. The major findings of this review are: (i) soluble nanocluster catalysts are implicated as the true catalysts in many putatively homogeneous arene hydrogenations; (ii) with few exceptions, nanocluster catalysts used for arene hydrogenation are poorly characterized; (iii) soluble nanocluster catalysts for arene hydrogenation have modest activity and lifetime; (iv) Rh and Ru are used almost exclusively as the active metals; (v) two catalyst systems, one developed by Roucoux and co-workers and the other by our own research group, stand out from the rest in terms of activity and lifetime; (vi) selective arene hydrogenationj especially for the synthesis of the all-cis diastereomer of substituted cyclohexanes, has received considerable attention and is a promising area for future study and, perhaps, fine chemical applications (selectivities >90% for the all-cis diastereomer have been achieved by several groups). (C) 2002 Elsevier Science B.V. All rights reserved.