Daniel Carmona

Recent advances in homogeneous enantioselective Diels–Alder reactions catalyzed by chiral transition-metal complexes

Abstract The homogeneous enantioselective Diels–Alder reactions catalyzed by chiral transition-metal complexes are reviewed. Special attention has been paid to the mechanistic aspects of the process in order to account for the stereochemical outcome of the reactions.

Trimerisation of the Cationic Fragments [(η‐ring)M(Aa)]+ ((η‐ring) M=(η5‐C5Me5)Rh, (η5‐C5Me5)Ir, (η6‐p‐MeC6H4iPr)Ru; Aa=α‐amino acidate) with Chiral Self‐Recognition: Synthesis, Characterisation, Solution Studies and Catalytic Reactions of the Trimers [{(η‐ring)M(Aa)}3](BF4)3

The formation of [{(η-ring)M(Aa)}3](BF4)3trimers [(η-ring)M=(η5-C5Me5)Rh, (η5-C5Me5)Ir, (η6-p-MeC6H4iPr)Ru; Aa = α-amino acidate, one cation shown schematically] takes place by chiral self-recognition, the RMRMRM or SMSMSM trimers are equally configurated at the metal centres and are the only diastereomers detected. The equilibrium constant for the diastereomerisation process between both isomers depends on the solvent, amino acidate, and metal. The trimers catalyse the reduction of unsaturated aldehydes to unsaturated alcohols and the reduction of acetophenone to 2-phenylethanol with up to 75u2009% ee.

Enantioselective hydride transfer hydrogenation of ketones catalyzed by [(η6-p-cymene)Ru(amino acidato)Cl] and [(η6-p-cymene)Ru(amino acidato)]3(BF4)3 complexes

The new complexes (RRuSC, SRuSC)-[(η6-pCym)Ru(l-Aze)Cl] (6a, b), (RRuSC, SRuSC)-[(η6-pCym)Ru(l-Pip)Cl] (7a, b), (RRuRRuRRuSCSCSCSNSNSN, SRuSRuSRuSCSCSCSNSNSN)-[{(η6-pCym)Ru(l-Aze)}3](BF4)3 (8a, b) and (RRuRRuRRuSCSCSCSNSNSN, SRuSRuSRuSCSCSCSNSNSN)-[{(η6-pCym)Ru(l-Pip)}3](BF4)3 (9a, b) (l-Aze=l-2-azetidinecarboxylate, l-Pip=l-2-piperidinecarboxylate) were prepared, characterized and used, together with the known [{(η6-pCym)Ru(l-Pro)}3](BF4)3, 5 and [{(η6-pCym)Ru(l-Ala)}3](BF4)3, 10 (l-Pro=l-prolinate, l-Ala=l-alaninate), in hydride transfer reduction of acetophenone, a series of substituted acetophenones and several other ketones with moderate to high conversions and enantioselectivities up to 86% e.e.

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

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)

On the Sense of the Enantioselection in Hydrogen Transfer Reactions from 2-Propanol to Ketones

An explanation for the reversal in the sense of the enantioselectivity observed in hydrogen transfer reactions from 2-propanol to ketones catalyzed by the ruthenium or osmium amino acidates [(η6-p-MeC6H4-i-Pr)M(Aa)Cl] and [(η6-p-MeC6H4-i-Pr)M(Aa)]3[BF4]3 [Aa=piperidine-2-carboxylate (pip), N-methyl-L-phenylalaninate (MePhe)] is given; the molecular structures of [(η6-p-MeC6H4-i-Pr)Os(Pip)Cl] (1), [(η6-p-MeC6H4-i-Pr)Os(Pip)]3[BF4]3 (2), [(η6-p-MeC6H4-i-Pr)M(MePhe)Cl] [M=Ru (3), Os (4)] are also reported.

Optically active pseudoctahedral rhodium(III), iridium(III), and ruthenium(II) complexes with α-amino acidato ligands. Crystal structures of RIrSCSN- and SIrSCSN-[(C5Me5)Ir(pro)Cl] · 12H2O (Hpro = l-proline)

Abstract The synthesis and characterization of optically active amino acidato complexes of the types [(C 5 Me 5 )M(aa)Cl], [( p -cymene)Ru(aa)Cl], [(C 5 Me 5 )M(aa)(PPh 3 )]BF 4 , and [( p -cymene)Ru(aa)(PPh 3 )]BF 4 (M = Rh, Ir; Haa = l -alanine, l -proline), in which the metal is a chiral centre, are reported. The cationic species were prepared via the solvato-complexes [(C 5 Me 5 )M(aa)(MeOH)] + and [( p -cymene)Ru(aa)(MeOH)] + , which epimerize rapidly on the 1 H NMR time scale. The crystal structure of the complex [(C 5 Me 5 )Ir(pro)Cl] is reported; the asymmetric unit contains two independent molecules differing in the configuration at the metal.

Chiral rhodium complexes as catalysts in Diels–Alder reactions

The first rhodium enantioselective catalysts for the Diels–Alder reaction between methacrolein and cyclopentadiene are described; the molecular structure of the catalyst precursor [(η5-C5Me5)Rh(R-Prophos)(H2O)][SbF6]2 is also presented.

Bis(diphenylphosphino)amine and their dichalcogenide derivatives as ligands in rhodium(III), iridium(III), and ruthenium(II) complexes. Crystal structures of [(η5-C5Me5)MCl{η2-(SePPh2)2N}] (M=Rh, Ir)

Abstract Reaction of the dimers [{(η 5 -C 5 Me 5 )MCl} 2 (μ-Cl) 2 ] (M=Rh, Ir) or [{(η 6 -arene)RuCl} 2 (μ-Cl) 2 ] (arene= p -MeC 6 H 4 i Pr, C 6 Me 6 ) with NH(PPh 2 ) 2 in the presence of AgA (A=BF 4 , PF 6 ) leads to the mononuclear cationic complexes [(η 5 -C 5 Me 5 )MCl{η 2 -(PPh 2 ) 2 NH}]A (M=Rh ( 1 ), Ir ( 2 )) or [(η 6 -arene)RuCl{η 2 -(PPh 2 ) 2 NH}]A (arene= p -MeC 6 H 4 i Pr ( 3 ), C 6 Me 6 ( 4 )). Similar reactions using the chalcogenide derivatives NH(EPPh 2 ) 2 (E=S, Se) yield the neutral complexes [(η 5 -C 5 Me 5 )RhCl{η 2 -(EPPh 2 ) 2 N}] (E=S ( 5 ), Se ( 6 )), [(η 5 -C 5 Me 5 )IrCl{η 2 -(EPPh 2 ) 2 N}] (E=S ( 7 ), Se ( 8 )), [(η 6 -arene)RuCl{η 2 -(SPPh 2 ) 2 N}] (arene=C 6 H 6 ( 9 ), p -MeC 6 H 4 i Pr ( 10 )) and [(η 6 -arene)RuCl{η 2 -(SePPh 2 ) 2 N}] (arene=C 6 Me 6 ( 11 ), p -MeC 6 H 4 i Pr ( 12 )). Chloride abstraction from complexes 5 – 8 with AgPF 6 in the presence of PPh 3 gives the cationic complexes [(η 5 -C 5 Me 5 )Rh{η 2 -(EPPh 2 ) 2 N}(PPh 3 )]PF 6 (E=S ( 13 ), Se ( 14 )) and [(η 5 -C 5 Me 5 )Ir{η 2 -(EPPh 2 ) 2 N}(PPh 3 )]PF 6 (E=S ( 15 ), Se ( 16 )). Complexes 13 – 16 can also be synthesised from the starting dinuclear complexes, AgPF 6 , NH(EPPh 2 ) 2 and PPh 3 . Using this alternative synthetic route the related ruthenium complexes [(η 6 -C 6 Me 6 )Ru{η 2 -(EPPh 2 ) 2 N}(C 5 H 5 N)] BF 4 (E=S ( 17 ), Se ( 18 )) can be prepared. All described compounds have been characterised by microanalysis and NMR ( 1 H, 31 P) and IR spectroscopy. The crystal structures of the neutral complexes [(η 5 -C 5 Me 5 )MCl{η 2 -(SePPh 2 ) 2 N}] (M=Rh ( 6 ), Ir ( 8 )) have been determined by X-ray diffraction methods. Both complexes exhibit analogous pseudo-octahedral molecular structures with a C 5 Me 5 group occupying three coordination positions and a bidentate chelate Se,Se′-bonded ligand and a chloride atom completing the coordination sphere.

Pentamethylcyclopentadienyl-iridium(III) complexes with pyridylamino ligands: synthesis and applications as asymmetric catalysts for Diels–Alder reactions

Reaction of the dimer [(Cp*IrCl)2(P-Cl)2] with chiral pyridylamino ligands (pyam, L1-L5) in the presence of NaSbF6 gave complexes [Cp*IrCl(pyam)][SbF6] 1-5 as diastereomeric mixtures, which have been fully characterised, including the X-ray molecular structure determination of the complexes (S(Ir),R(N),R(C))-[Cp*IrClL1][SbF6] 1a and (R(Ir),S(N),S(C))-[Cp*IrClL5][SbF6] 5a. Treatment of these cations with AgSbF6 affords the corresponding aqua species [Cp*Ir(pyam)(H2O)][SbF6]2 6-10 which have been also fully characterised. The molecular structure of the complex (S(Ir),R(N),R(C))-[Cp*IrL,(H2O)][SbF6]2 6 has been determined by X-ray diffractometric methods. The aqua complexes [Cp*Ir(pyam)(H2O)][SbF6]2 (6, pyam = L2 (7), L3 (8)) evolve to the cyclometallated species [Cp*Ir{kappa3(N,N,C)-(R)-(C6H4)CH(CH3)NHCH2C5NH4}][SbF6] (11), [Cp*Ir{kappa3(N,N,C)-(R)-(C10H6)CH(CH3)-NHCH2C5NH4)}][SbF6] (12), and [Cp*Ir{kappa3(N,N,C)-(R)-(C10H6)CH(CH3)NHCH2C9NH6)}][SbF6] (13) respectively, via intramolecular activation of an ortho C-H aryl bond. Complexes 6-10 are enantioselective catalysts for the Diels-Alder reaction between methacrolein and cyclopentadiene. Reaction occurs rapidly at room temperature with good exo : endo selectivity (from 81 : 19 to 98 : 2) and moderate enantioselectivity (up to 72%). The involved intermediate Lewis acid-dienophile compounds [Cp*Ir(pyam)(methacrolein)][SbF]2 (pyam = L4 (14), L5 (15)) have been isolated and characterised.

Chiral iridium(III) α-amino acidato complexes (RIr,SN,SC)- and (SIr,SN,SC)-[(η5-C5Me5)Ir(L-prolinate)(CCCMe3)]

Abstract Diastereomers (S Ir ,S N ,S C )- and (R Ir ,S N ,S C )-[(η 5 -C 5 Me 5 )Ir(L-prolinate)(Cue5fcCue5f8CMe 3 )] have been prepared and the structure of the later determined by X-ray diffraction methods.