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Half‐Sandwich Complexes with Aminocarboxylate Ligands and Their Use as Enantioselective Hydrogen Transfer Catalysts
Author(s) -
Carmona Daniel,
Lamata María Pilar,
Oro Luis A.
Publication year - 2002
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(200209)2002:9<2239::aid-ejic2239>3.0.co;2-1
Subject(s) - chemistry , rhodium , cationic polymerization , osmium , ruthenium , diastereomer , ring (chemistry) , stereochemistry , iridium , medicinal chemistry , metal , enantioselective synthesis , catalysis , crystallography , polymer chemistry , organic chemistry
Chiral‐at‐metal half‐sandwich complexes of rhodium, iridium, ruthenium, or osmium of the general formula [(η n ‐ring)M(Aa)Cl] [(η n ‐ring)M = (η 5 ‐C 5 Me 5 )Rh, (η 5 ‐C 5 Me 5 )Ir, (η 6 ‐ p ‐MeC 6 H 4 i Pr)Ru, (η 6 ‐ p ‐MeC 6 H 4 i Pr)Os; Aa = L ‐α‐aminocarboxylate] can readily be prepared from the corresponding dimers [{(η n ‐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 [(η n ‐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 [(η 5 ‐C 5 Me 5 )Rh(Ala)Cl] with the alkynes HC≡CR (R = Ph, p ‐tolyl) produced the alkynylcyclobutadiene complexes [(η 5 ‐C 5 Me 5 )Rh(η 4 ‐C 4 HR 2 C≡CR)] (R = Ph, p ‐tolyl). Treatment of the chlorides [(η n ‐ring)M(Aa)Cl] with AgBF 4 afforded the cationic trimers [{(η n ‐ring)M(Aa)} 3 ](BF 4 ) 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 [{(η n ‐ring)M(Aa)} 3 ](BF 4 ) 3 reacted with tertiary phosphanes, leading to the cationic mononuclear complexes [(η n ‐ring)M(Aa)(PR 3 )](BF 4 ). 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 [(η n ‐ring)M(Aa)Cl] and cationic [{(η n ‐ring)M(Aa)} 3 ](BF 4 ) 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)