Half-Sandwich Complexes with Aminocarboxylate Ligands and Their Use as Enantioselective Hydrogen Transfer Catalysts

Abstract

Chiral-at-metal half-sandwich complexes of rhodium, iridium, ruthenium, or osmium of the general formula [(ηn-ring)M(Aa)Cl] [(ηn-ring)M = (η5-C5Me5)Rh, (η5-C5Me5)Ir, (η6-p-MeC6H4iPr)Ru, (η6-p-MeC6H4iPr)Os; Aa = L-α-aminocarboxylate] can readily be prepared from the corresponding dimers [{(ηn-ring)MCl}2(μ-Cl)2]. The compounds have been prepared as diastereomeric mixtures of the two epimers at the metal center. In general, alkynyl aminocarboxylate derivatives [(ηn-ring)M(Aa)(C≡CR)] are obtained by treating the aforementioned chlorides with the corresponding alkynes in basic media. However, the reaction of the (alaninato)rhodium chloride [(η5-C5Me5)Rh(Ala)Cl] with the alkynes HC≡CR (R = Ph, p-tolyl) produced the alkynylcyclobutadiene complexes [(η5-C5Me5)Rh(η4-C4HR2C≡CR)] (R = Ph, p-tolyl). Treatment of the chlorides [(ηn-ring)M(Aa)Cl] with AgBF4 afforded the cationic trimers [{(ηn-ring)M(Aa)}3](BF4)3. Trimerization occurs with chiral self-recognition: only the (R,R,R) or (S,S,S) configuration at the metal trimers can be detected. The trimers [{(ηn-ring)M(Aa)}3](BF4)3 reacted with tertiary phosphanes, leading to the cationic mononuclear complexes [(ηn-ring)M(Aa)(PR3)](BF4). The assignment of the configuration at the metal center was accomplished by X-ray diffraction, circular dichroism and NMR spectroscopy. Most of the aminocarboxylate derivatives epimerized at the metal center. On the basis of kinetic and spectroscopic data, a general mechanism is proposed for the epimerization process. Neutral [(ηn-ring)M(Aa)Cl] and cationic [{(ηn-ring)M(Aa)}3](BF4)3 complexes are active catalysts for the hydrogen transfer reaction from 2-propanol to acetophenone. Conversions of up to 97% and enantioselectivities up to 75% were achieved. A proposal about the origins of the enantioselectivity is given. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)