Chiral Rh Complexes Research Articles

Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction.

The mechanism of Rh2 (S-NTTL)4 catalyzed carbene insertion into C(3)-H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal-associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water-assisted proton transfer is proposed and confirmed through deuterium-labeling experiments. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X-H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

Heterogeneous Chiral Diene‐Rh Complexes for Asymmetric Arylation of α,β‐Unsaturated Carbonyl Compounds, Nitroalkenes, and Imines

AbstractA chiral diene ligand with tertiary alkyl amine‐derived secondary amide moiety was immobilized on cross‐linked polystyrene (PS) by radical polymerization, which was combined with Rh to form heterogeneous chiral Rh complexes (PS‐diene Rh−Cl). PS‐diene Rh−Cl catalyzed asymmetric arylation reactions of α,β‐unsaturated carbonyl compounds (ketones, esters, and amides), nitroalkenes, and imines afforded the desired products in high yields with excellent enantioselectivities. PS‐diene Rh−Cl is stable in air, can be stored for several months, and can be reused more than 10 times without any reduction of either yield or enantioselectivity. We also developed a method of activation of PS‐diene Rh−Cl to generate more active species.magnified image

Enantioselective Access to Bicyclo[3.2.1]octadienone Skeleton: Total Syntheses of (+)-Engelharquinone and Its Epoxide.

The first enantioselective total syntheses of engelharquinone (2) and its epoxide 3 have been achieved. The key steps include (1) catalytic asymmetric 1,4-addition of a naphthylboronic acid derivative to a masked naphthoquinone derivative by using a chiral Rh-complex and (2) thiolate-promoted stereospecific construction of the bicyclo[3.2.1]octadienone scaffold.

Highly Enantioselective Ferrocenyl Palladacycle-Acetate Catalysed Arylation of Aldimines and Ketimines with Arylboroxines.

Benzylic N-substituted stereocenters constitute a frequent structural motif in drugs. Their highly enantioselective generation is hence of technical importance. An attractive strategy is the arylation of imines with organoboron reagents. Chiral Rh complexes have reached a high level of productivity for this reaction type. In this article we describe that an electron rich PdII catalyst also performs well in the arylation of aldimines, comparable to the best Rh catalysts. The ferrocenyl palladacycle-acetate catalyst allows for a broad substrate scope and very high enantioselectivities. Commonly observed side reactions like aryl-aryl homocouplings and imine hydrolysis could be blocked. Mechanistic studies implicate that a) the acetate ligand is crucial for transmetallation, b) the active catalyst is most likely a palladacycle-OAc monomer, c) the rate limiting step is probably the product release. By added KOAc the arylation could also be applied to ketimines.

ChemInform Abstract: A Rhodium Catalyst Superior to Iridium Congeners for Enantioselective Radical Amination Activated by Visible Light.

AbstractThe chiral Rh complex is highly efficient in the title reaction (detailed mechanism).

ChemInform Abstract: Rh‐Catalyzed Asymmetric 1,4‐Addition Reactions to α,β‐Unsaturated Carbonyl and Related Compounds: An Update

AbstractReview: [356 refs.

Enantioselective hydroformylation by a Rh-catalyst entrapped in a supramolecular metallocage.

Regio- and enantioselective hydroformylation of styrenes is attained upon embedding a chiral Rh complex in a nonchiral supramolecular cage formed from coordination-driven self-assembly of macrocyclic dipalladium complexes and tetracarboxylate zinc porphyrins. The resulting supramolecular catalyst converts styrene derivatives into aldehyde products with much higher chiral induction in comparison to the nonencapsulated Rh catalyst. Spectroscopic analysis shows that encapsulation does not change the electronic properties of the catalyst nor its first coordination sphere. Instead, enhanced enantioselectivity is rationalized by the modification of the second coordination sphere occurring upon catalyst inclusion inside the cage, being one of the few examples in achieving an enantioselective outcome via indirect through-space control of the chirality around the catalyst center. This effect resembles those taking place in enzymatic sites, where structural constraints imposed by the enzyme cavity can impart stereoselectivities that cannot be attained in bulk. These results are a showcase for the future development of asymmetric catalysis by using size-tunable supramolecular capsules.

Stereoselective Synthesis of Norephedrine and Norpseudoephedrine by Using Asymmetric Transfer Hydrogenation Accompanied by Dynamic Kinetic Resolution

Each of the enantiomers of both norephedrine and norpseudoephedrine were stereoselectively prepared from the common, prochiral cyclic sulfamidate imine of racemic 1-hydroxy-1-phenyl-propan-2-one by employing asymmetric transfer hydrogenation (ATH) catalyzed by the well-defined chiral Rh-complexes, (S,S)- or (R,R)-Cp*RhCl(TsDPEN), and HCO(2)H/Et(3)N as the hydrogen source. The ATH processes are carried out under mild conditions (rt, 15 min) and are accompanied by dynamic kinetic resolution.

ChemInform Abstract: Chiral Rh‐ and Ir‐Catalyzed Intramolecular Cycloaddition of Hexaynes for the Construction of New Chiral Skeletons.

AbstractThe chiral Rh complex catalyzes the consecutive [2+2+2] cycloadditions, whereas the chiral neutral Ir complex causes a single [2 + 2 + 2] cycloaddition.

ChemInform Abstract: Rhodium‐Catalyzed Asymmetric Arylation of Azomethine Imines.

AbstractThe reaction is catalyzed by the chiral Rh‐complex and leads to the best results when using tetraarylborates as arylating agents.

Catalyst Recycling via Adsorption on Carbon Nanotubes

In this paper, asymmetric hydro­genation of α-dehydroamino acid esters and a new method for recycling chiral catalysts were described. Hydrogenation of 2 was carried out with 1 mol% of a chiral Rh complex of (R,R)-1 to give the corresponding amino acid derivatives 3 in >99% yield with 92-96% ee (4 examples). When the hydrogenation was performed in the presence of carbon nanotubes (CNTs), the catalyst was easily recovered from the reaction mixture by simple filtration.

Asymmetric Transfer Hydrogenation of α‐Aminoalkyl α′‐Chloromethyl Ketones with Chiral Rh Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Asymmetric hydrogenation using chiral Rh complexes immobilised with a new ion-exchange strategy

Rh diphosphine complexes using DuPhos and JosiPhos as chiral ligands have been immobilised by ion exchange into the mesoporous material MCM-41. When used as catalysts for the enantioselective hydrogenation of dimethyl itaconate and methyl-2-acetamidoacrylate, these heterogeneous catalysts give catalytic performance in terms of yield and enantioselection that are comparable to the corresponding homogeneous catalysts. Furthermore, the heterogeneous catalysts can be readily recovered and reused without loss of catalyst performance. A second immobilisation strategy is described in which [Rh(COD)2]+BF4- is initially immobilised by ion exchange and subsequently modified by the chiral diphosphine and this give comparable catalyst performance. This immobilisation strategy opens up the possibility of easy ligand-screening for parallel synthesis and libraries.

A Practical Synthesis of Optically Active Aromatic Epoxides via Asymmetric Transfer Hydrogenation of α‐Chlorinated Ketones with Chiral Rhodium‐Diamine Catalyst.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Asymmetric transfer hydrogenation of alpha-aminoalkyl alpha'-chloromethyl ketones with chiral Rh complexes.

Asymmetric transfer hydrogenation of N-substituted (3S)-3-amino-1-chloro-4-phenyl-2-butanones in the presence of CpRhCl[(R,R)-Tsdpen] (S/C = 1000) with a mixture of formic acid/triethylamine gave N-substituted (2R,3S)-3-amino-1-chloro-2-hydroxy-4-phenylbutanes with up to 93% de in a quantitative yield, and reduction with the enantiomeric catalyst CpRhCl[(S,S)-Tsdpen] gave (2S,3S)-diastereomeric alcohol with up to 96% de.

A practical synthesis of optically active aromatic epoxides via asymmetric transfer hydrogenation of α-chlorinated ketones with chiral rhodium–diamine catalyst

A practical method for the synthesis of optically active aromatic epoxides has been developed via the formation of optically active α-chlorinated alcohols and intramolecular etherification. Optically active alcohols with up to 99% ee can be obtained from the asymmetric reduction of aromatic ketones with a substrate/catalyst ratio of 1000–5000 using a formic acid/triethylamine mixture containing a well-defined chiral Rh complex, Cp*RhCl[( R, R)-Tsdpen]. The asymmetric reduction of α-chlorinated aromatic ketones with a chiral Rh catalyst is characterized by a rapid and carbonyl group-selective transformation because of the coordinatively saturated nature of diamine-based Cp*Rh(III) hydride complexes. The outcome of the reduction is significantly influenced by the structures of the ketonic substrates as well as the hydrogen source such as formic acid or 2-propanol. Commercially available reagents and solvents can be used in this reaction without special purification. This epoxide synthetic process in either a one- or two-pot procedure is practical and particularly useful for the large-scale production of optically active styrene oxides from α-chlorinated ketones.

Practical Synthesis of Optically Active Styrene Oxides via Reductive Transformation of 2‐Chloroacetophenones with Chiral Rhodium Catalysts.

A practical method for the synthesis of optically active styrene oxides has been developed via formation of optically active 2-chloro-1-phenylethanols generated by reductive transformation of ring-substituted 2-chloroacetophenones. The optically active alcohols with up to 98% ee are obtainable from the asymmetric reduction of acetophenones with an S/C = 1000−5000 with a formic acid triethylamine mixture containing a well-defined chiral Rh complex, Cp*RhCl[(R,R)-Tsdpen].

Practical synthesis of optically active styrene oxides via reductive transformation of 2-chloroacetophenones with chiral rhodium catalysts.

[reaction: see text] A practical method for the synthesis of optically active styrene oxides has been developed via formation of optically active 2-chloro-1-phenylethanols generated by reductive transformation of ring-substituted 2-chloroacetophenones. The optically active alcohols with up to 98% ee are obtainable from the asymmetric reduction of acetophenones with an S/C = 1000-5000 with a formic acid triethylamine mixture containing a well-defined chiral Rh complex, CpRhCl[(R,R)-Tsdpen].

Homogeneous and heterogeneous catalytic asymmetric reactions IV. Hydrogenation of the Ni complexes of cinnamic acid salts over MRNi catalyst

Substituted cinnamic acids containing a prochiral CC bond, and their LiI, KI, NiII and CaII salts, were prepared and their surface complexes on Ni were studied over a Raney-Ni catalyst modified with tartaric acid. The hydrogenation product of sodium cinnamate is optically active. Of the alkali metal salts of (E)-α-phenylcinnamic acid, the hydrogenation of the Na salt gives the highest optical yield (17%). The solvent used significantly influences the selectivity. The published data for the chiral RH complexes are listed for comparison with the results of the homogeneous and heterogeneous hydrogenations described.The enantioselectivity in the presence of Raney-Ni is the result of a complex formed by the interaction of (R,R)-tartaric acid with the substrate.