Asymmetric catalysis. Part 128: Diastereomeric rhodium(I) complexes in the enantioselective hydrogenation of ketopantolactone

In the hydrogenation of ketopantolactone, new rhodium complexes bearing (R,R)-diop and various bidentate chiral N,N′ co-ligands with (R)- or (S)-configuration were used. On the one hand, the N,N′ co-ligands consist of pyrroleimines, which derive from (R)- and (S)-1-phenylethylamine, (R)- and (S)-1-cyclohexylethylamine (and benzylamine), and on the other hand of pyrroleoxazolines and pyridineimines. Stereoselectivities of 31–33% ee for (R)-pantolactone were achieved using related compounds (RR-R) and (RR-S), respectively, with no double stereoselectivity being observed. It is assumed that during catalysis the pyrroleimines bind in a monodentate way at the sixth coordination site of the rhodium atom by the pyrrole nitrogen with the imine nitrogen carrying the different chiral substituents far away from the rhodium atom. Monodentate deltacyclane phosphanes, chloro ligands or solvent molecules, bound at the sixth coordination site of the catalyst, led to widely differing enantioselectivities in the ketopantolactone hydrogenation.     

Asymmetric cyclopropanation catalyzed by C2-symmetric bis-(ephedrine)-Cu(II) complexes

The asymmetric cyclopropanation of styrene with alkyl diazoacetate was performed with a series of Cu(II) complexes of novel chiral ligands, which were derived from substitution of 1,3-dibromopropane, 1,2-dibromoethane, α,α′-dibromo-m-xylene and 2,6–bis(bromomethyl)pyridine with 1R, 2S-(−)-ephedrine. These prepared catalysts shown to be highly active in the enantioselective cyclopropanation with ee higher than 89% under the optimal conditions.     

Two novel chiral rhodium complexes as catalysts for the enantioselective allylation of arylaldehydes

Two novel chiral rhodium complexes were successfully synthesized from the reaction of a new class of bidentate nitrogen ligands with RhCl₃·3H₂O in ethanol under reflux. The crystal structure of 4a was unambiguously established by X-ray analysis. Their corresponding cationic metal complexes prepared in situ from the reaction of 4a or 4b with AgBF₄ catalyze the enantioselective allylation of arylaldehydes with allylstannane in 5–50% ee.     

Cationic thiocarbonyl complexes of rhodium(I)

The syntheses are described of a range of cationic rhodium(I) thiocarbonyl complexes containing tertiary phosphine, phosphinite, phosphonite and phosphonite ligands.     

Chiral iridium(I) bis(NHC) complexes as catalysts for asymmetric transfer hydrogenation

The common use of NHC complexes in transition‐metal mediated C–C coupling and metathesis reactions in recent decades has established N‐heterocyclic carbenes as a new class of ligand for catalysis. The field of asymmetric catalysis with complexes bearing NHC‐containing chiral ligands is dominated by mixed carbene/oxazoline or carbene/phosphane chelating ligands. In contrast, applications of complexes with chiral, chelating bis(NHC) ligands are rare. In the present work new chiral iridium(I) bis(NHC) complexes and their application in the asymmetric transfer hydrogenation of ketones are described. A series of chiral bis(azolium) salts have been prepared following a synthetic pathway, starting from L ‐valinol and the modular buildup allows the structural variation of the ligand precursors. The iridium complexes were formed via a one‐pot transmetallation procedure. The prepared complexes were applied as catalysts in the asymmetric transfer hydrogenation of various prochiral ketones, affording the corresponding chiral alcohols in high yields and moderate to good enantioselectivities of up to 68%. The enantioselectivities of the catalysts were strongly affected by the various, terminal N‐substituents of the chelating bis(NHC) ligands. The results presented in this work indicate the potential of bis‐carbenes as stereodirecting ligands for asymmetric catalysis and are offering a base for further developments. Copyright © 2010 John Wiley & Sons, Ltd.     

Chiral and diastereoisomeric rhodium(I) coordination compounds with carvone as bidentate ligand

The chiral dienone p-mentha-6,8-dien-2-one (carvone) coordinates readily with rhodium(I) to form complexes of pronounced stability. Novel diastereoisomeric planar coordination compounds of rhodium(I) containing two chiral bidentate ligands have been prepared for the first time. The synthesis of the compounds is simple, and the yields are high.     

Catalyst structure and the enantioselective cyclopropanation of alkenes by copper complexes of biaryldiimines: the importance of ligand acceleration

The use of chiral non-racemic biaryl copper(I) complexes in the enantioselective cyclopropanation of a number of olefins with either ethyl or tert-butyl diazoacetate is described. Lack of ligand acceleration and the presence of equilibrium amounts of catalytically active uncomplexed Cu(I) ions account for lowered enantioselectivity when using certain ligands.     

Development of a new class of C1-symmetric bisphosphine ligands for rhodium-catalyzed asymmetric hydrogenation

A new class of C₁-symmetric bisphosphine ligands with three hindered quadrants have been obtained through facile synthesis from chiral BINOL derivatives. Their rhodium complexes have exhibited high enantioselectivities (up to 98% ee) in the asymmetric hydrogenation of various unsaturated prochiral olefins, providing an efficient catalytic system for the enantioselective synthesis of chiral amino acids and amines.     

Catalytic enantioselective organozinc addition toward optically active tertiary alcohol synthesis

A highly enantioselective organozinc (R₂Zn) addition to a series of aldehydes and ketones was developed based on conjugate Lewis acid–Lewis base catalysis. Optically active secondary and tertiary alcohols were obtained in high yields with high enantioselectivities without Ti(IV) compounds. Bifunctional chiral 3,3′‐diphosphoryl‐BINOL ligands were designed and prepared through a phospho‐Fries rearrangement as a key step. On the other hand, bifunctional chiral phosphoramide ligands were designed and prepared from L ‐valine. Mechanistic studies were performed by X‐ray analyses of Zn(II) cluster and chiral ligands, a ³¹P NMR experiment on Zn(II) complexes, and stoichiometric reactions with some chiral or achiral Zn(II) complexes to propose a transition state assembly that includes monomeric active intermediates. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 143–155; 2008: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20146     

Binuclear and polynuclear rhodium complexes containing chiral dithiolate ligands derived from lactic acid

Two precursors of the chiral dithiolato ligands, di‐[(2R)‐acetylmercaptopropyl] phthalate and isophthalate, 1 and 2 respectively, were synthesized from (S)‐lactic acid. Reactions of 1 and 2 with [Rh₂(μ)‐OMe)₂(cod)₂] (cod = 1,5‐cyclo‐octtadiene) yielded rhodium thiolato complexes of different nuclearities. The mixtures of complexes were analyzed by gel‐permeation chromatography (GPC). The reaction with ligand 1 produced a mixture of oligomeric complexes, where the binuclear species was the main component. Higher‐nuclearity complexes were the main products of the reaction with ligand 2. The rhodium complexes, in the presence of PPh₃, were tested as catalyst precursors for the asymmetric hydroformylation of styrene. Moderate activity and regioselectivity were achieved in most cases, but no enantioselective discrimination was observed. Copyright © 2000 John Wiley & Sons, Ltd.     

Rh(I) complexes with new C2‐symmetric chiral diphosphoramidite ligands: Catalytic activity for asymmetric hydrogenation of olefins

The design and synthesis of three new C ₂‐symmetric chiral diphosphoramidite ligands starting from simple and cheap building blocks have been developed. Rhodium(I) cationic complexes bearing these chelate ligands have been prepared and applied in asymmetric hydrogenation of model olefins. A rhodium complex with a diphosphoramidite containing a chiral diamine configurationally stable and two fluxional chiral biphenyl units gave higher enantioselectivity with increasing hydrogen pressure (87% ee) in the hydrogenation of dimethyl itaconate.     

Asymmetric synthesis of dipeptides by means of homogeneous hydrogenation catalyzed by chiral rhodium complexes

Asymmetric hydrogenation of dehydrodipeptides, α-acylaminocinnamoyl-(S)-amino esters, catalyzed by rhodium complexes with chiral diphosphines gave either (R)-N-acylphenylalanyl-(S)-amino esters or (S)-N-acylphenylalanyl-(S)-amino esters with high diastereomeric purity up to 98–99% on using proper chiral ligands.     

Catalytic asymmetric Mannich reactions of glycine derivatives with imines. A new approach to optically active alpha,beta-diamino acid derivatives

Imines of glycine alkyl esters react with imines in a diastereo- and highly enantioselective Mannich reaction in the presence of chiral copper(I) complexes as the catalyst to give optically active alpha,beta-diamino acid derivatives. A series of imines of glycine esters derived from glycine and aromatic carbonyl compounds has been screened as substrates for the Mannich reaction with different imines in the presence of various combinations of metal salts and chiral ligands. The benzophenone imine of glycine esters was found to react with N-protected imines in a diastereoselective fashion giving functionalized alpha,beta-diamino acid esters with excellent enantioselectivities. The most effective chiral catalysts are chiral copper(I) complexes having phosphino-oxazoline (P,N)-ligands, and among these ligands, those derived from (1R,2S)-dihydroxy-1,2,3,4-tetrahydronaphthalene gave the best results. The scope of this new catalytic asymmetric reaction of the benzophenone imine glycine esters is demonstrated for the reaction with different N-protected-C-aryl and C-alkyl imines giving the Mannich adducts with excellent optical purity. Furthermore, the synthetic aspects of the reaction are presented by converting the Mannich adducts into alpha,beta-diamino acid derivatives. The relative and absolute configuration of the Mannich adduct have been determined and based on the stereochemical outcome of the reaction a tetrahedral chiral-copper(I)-imino glycine alkyl ester intermediate is proposed. In this intermediate the Re-face of the benzophenone imine glycine ester is shielded by the chiral ligand leaving the Si-face available for approach of the Si-face of the imine. A series of semiempirical calculations has been performed to support the structure of the tetrahedral chiral-copper(I) complex and to account for the influence of the substituents in the chiral phosphino-oxazoline ligands.     

Study on catalytic properties of the macromolecules containing pendant pyridine groups-supported rhodium complexes for the carbonylation reactions

The experiments reveal that anchoring rhodium(I) macromolecules containing pendant pyridine groups exhibit remarkably different carbonylation catalytic behaviours, depending on the quaternary degree of polymer ligands. The catalysts with the quaternized ligands are more active than that with the non-quaternized ligands, but the best of the lots isn't the highestly quaternized. A new structural type of the compounds called the polydentate hetero-bound rhodium(I) ion complexes is recognized to be responsible for higher carbonylation reactivities.     

Phosphorus Heterocycles: From Laboratory Curiosities to Ligands in Highly Efficient Catalysts

Phosphabenzenes and phosphaferrocenes were among the first compounds with P−C multiple bonds. For nearly 30 years the chemistry of these molecules was essentially a domain left to basic researchers. Recently, however, it was reported that transition metal complexes with phosphabenzene and phosphaferrocene ligands exhibit remarkable potential as catalysts. Catalysts based on rhodium (I ) and various phosphabenzenes appear to be superior to classical systems in the hydroformylation of terminal and internal alkenes. In addition planar‐chiral phosphaferrocene species display an excellent performance as directing ligands in a series of enantioselective asymmetric syntheses.     

Synthesis and characterization of asymmetric NHC complexes

New chiral and non-chiral rhodium(I)–NHC complexes were synthesized. The first attempt by deprotonation of an imidazolinium salt with KOtBu and reaction with [Rh(COD)Cl]₂ leads to the corresponding rhodium(I) complex. Due to the basic conditions during the reaction a loss of chirality occurs. An alternative transmetallation reaction with a silver(I)–NHC complex yields the desired rhodium(I)–NHC complex under retention of chirality. Both Rh complexes were fully characterized by analytical methods.     

Enantiocatalytic activity of substituted 5-benzyl-2-(pyridine-2-yl)imidazolidine-4-one ligands

Currently, asymmetric synthesis represents one of the main streams of organic synthesis. Although an extensive research has been carried out in this area, the synthesis of chiral compounds with the required enantiomeric purity is still a challenging issue. Herein, we focus on the preparation of new enantioselective catalysts based on pyridine-imidazolidinones. The substituted 5-benzyl-2-(pyridine-2-yl)imidazolidine-4-ones 5–8 were prepared by condensation of chiral amino acid amides (α-methylDOPA and α-methylphenylalanine) with 2-acetylpyridine and pyridine-2-carbaldehyde. The individual isomers of the described ligands 5–8 were separated chromatographically. The copper(II) complexes of these chiral ligands were studied as enantioselective catalysts for the asymmetric Henry reaction of substituted aldehydes with nitromethane or nitroethane. The ligands containing a methyl group at the 2-position of the imidazolidinone ring 6a and 8a exhibit a high degree of enantioselectivity (up to 91% ee). The nitroaldols derived from nitroethane (2-nitropropan-1-ols) were obtained with a comparable enantiomeric purity to derivatives of 2-nitroethanol. This group of ligands represents a new and promising class of enantioselective catalysts, which deserve further attention.     

Development of homogeneous and heterogenized rhodium(I) and palladium(II) complexes with ligands based on a chiral proton sponge building block and their application as catalysts

Chiral compounds prepared from proton sponge building block 8-((2R,5R)-2,5-dimethylpyrrolidin-1-yl)naphthalen-1-amine were found to be effective chiral ligands for obtaining complexes of rhodium(I) and palladium(II) by reaction with [RhCl(cod)](2), PdCl(2)(cod) or Pd(OAc)(2). The complexes bearing triethoxysilane groups were immobilized on mesoporous MCM-41 in order to obtain new heterogeneous catalysts. Both materials are active in the hydrogenation of alkenes and could be recycled without loss of activity or enantioselectivity.     

Catalytic asymmetric hydroboration of heterofunctional allylic substrates: an efficient heterogenized version

The hydroboration of heterofunctional allylic systems with catecholborane (HBcat) using neutral and cationic rhodium complexes modified with P–P and P–N bidentate chiral ligands has been described in order to produce the secondary heteroorganoboronate ester as a major product with moderate enantioselectivity. The immobilization of cationic chiral rhodium complexes onto clays has beneficial effects on the recyclability and reuse of the catalytic system in particular for the hydroboration of allyl aryl sulfones.     

Preparation and catalytic activity of cationic rhodium tetrafluorobenzobarrelene complexes with nitrogen and phosphorus donor ligands

The preparation and properties of twenty five new cationic rhodium(I) complexes with tetrafluorobenzobarrelene and mono- or bidentate nitrogen or phosphorus donor ligands are described. The complexes with tertiary phosphines show high selectivities in the hydrogenation of 1-hexyne and several diolefins towards monoolefins. The dependence of the reduction rate upon the basicity of the phosphine has been studied.