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

Abstract

Keywords: Chiral-at-metal complexes / Asymmetric hydrogen transfer catalysis / Rhodium / Ruthenium / Iridium / Osmium / Aminocarboxylate ligands 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 -pMeC 6 H 4 iPr)Ru, (η 6 -p-MeC 6 H 4 iPr)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)(CϵCR)] 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 HCϵCR (R = Ph, p-tolyl) produced the alkynylcyclobutadiene complexes [(η 5 -C 5 Me 5 )Rh(η 4 -C 4 HR 2 CϵCR)] (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 [a] MICROREVIEWS: This feature introduces the readers to the authors' research through a concise overview of the selected topic. Reference to important work from others in the field is included. 2239 (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.

Octametallic Cluster of Cp*Ir(glycinato) Cations

“…The actual solution ratio of 92/8 is in good agreement given that there is a solvent effect on this ratio that is not accounted for in the DFT experiments. The diastereomeric ratio is also consistent with what has been reported by others …”

Octametallic Cluster of Cp*Ir(glycinato) Cations

“…The actual solution ratio of 92/8 is in good agreement given that there is a solvent effect on this ratio that is not accounted for in the DFT experiments. The diastereomeric ratio is also consistent with what has been reported by others …”

Interaction of Mono- and Dinuclear Metal Complexes with Mono- and Oligonucleotides for Analytical, Radio- and Chemotoxic Purposes

“…In the solid state, the complex forms a trinuclear structure with the carboxylato group of the amino acid binding to two subunits by one oxygen each. Similar structural features have also been observed with [(η narene)M(L) 3 ] with M = Ru, Os, Rh and L = amino acid [24]. In aqueous solution, the trinuclear structure is probably cleaved and mononuclear species of the composition [Re(OH 2 )(pro)(CO) 3 ] are formed.…”

Syntheses, Structures, and Reactivity of New Pentamethylcyclopentadienyl-Rhodium(III) and -iridium(III) 4-Acyl-5-Pyrazolonate Complexes

“…They are salt-like, both in solution and in the solid state. The latter assumption is confirmed from their IR spectra, which exhibit bands from noncoordinated nitrate (10) and perchlorate groups (11). In both cases, the halide exchange is accompanied by coordination of a solvent molecule, consistent with the low tendency of Rh to coordinate NO 3 and ClO 4 groups.…”

Octametallic Cluster of Cp*Ir(glycinato) Cations

“…The actual solution ratio of 92/8 is in good agreement given that there is a solvent effect on this ratio that is not accounted for in the DFT experiments. The diastereomeric ratio is also consistent with what has been reported by others …”

Interaction of Mono- and Dinuclear Metal Complexes with Mono- and Oligonucleotides for Analytical, Radio- and Chemotoxic Purposes

“…In the solid state, the complex forms a trinuclear structure with the carboxylato group of the amino acid binding to two subunits by one oxygen each. Similar structural features have also been observed with [(η narene)M(L) 3 ] with M = Ru, Os, Rh and L = amino acid [24]. In aqueous solution, the trinuclear structure is probably cleaved and mononuclear species of the composition [Re(OH 2 )(pro)(CO) 3 ] are formed.…”

Syntheses, Structures, and Reactivity of New Pentamethylcyclopentadienyl-Rhodium(III) and -iridium(III) 4-Acyl-5-Pyrazolonate Complexes

“…They are salt-like, both in solution and in the solid state. The latter assumption is confirmed from their IR spectra, which exhibit bands from noncoordinated nitrate (10) and perchlorate groups (11). In both cases, the halide exchange is accompanied by coordination of a solvent molecule, consistent with the low tendency of Rh to coordinate NO 3 and ClO 4 groups.…”

Octametallic Cluster of Cp*Ir(glycinato) Cations

“…The actual solution ratio of 92/8 is in good agreement given that there is a solvent effect on this ratio that is not accounted for in the DFT experiments. The diastereomeric ratio is also consistent with what has been reported by others …”

Interaction of Mono- and Dinuclear Metal Complexes with Mono- and Oligonucleotides for Analytical, Radio- and Chemotoxic Purposes

“…In the solid state, the complex forms a trinuclear structure with the carboxylato group of the amino acid binding to two subunits by one oxygen each. Similar structural features have also been observed with [(η narene)M(L) 3 ] with M = Ru, Os, Rh and L = amino acid [24]. In aqueous solution, the trinuclear structure is probably cleaved and mononuclear species of the composition [Re(OH 2 )(pro)(CO) 3 ] are formed.…”

Syntheses, Structures, and Reactivity of New Pentamethylcyclopentadienyl-Rhodium(III) and -iridium(III) 4-Acyl-5-Pyrazolonate Complexes

“…They are salt-like, both in solution and in the solid state. The latter assumption is confirmed from their IR spectra, which exhibit bands from noncoordinated nitrate (10) and perchlorate groups (11). In both cases, the halide exchange is accompanied by coordination of a solvent molecule, consistent with the low tendency of Rh to coordinate NO 3 and ClO 4 groups.…”