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Relevant academic research and scientific papers

Dirhodium tetraprolinate-catalyzed asymmetric cyclopropanations with high turnover numbers

(Matrix presented) The bridged dirhodium tetraprolinate Rh 2(S-biTISP)2 (2) catalyzes the asymmetric cyclopropanation reaction between methyl phenyldiazoacetate and styrene at room temperature with high turnover number (92 000) and t

[Rh2(MEPY)4] and [BiRh(MEPY)4]: Convenient Syntheses and Computational Analysis of Strikingly Dissimilar Siblings

[Rh2(MEPY)4] is a versatile catalyst for asymmetric synthesis but its preparation requires purification by chromatography on surface-modified silica. A higher yielding procedure based on a more convenient work-up is presented herein. Moreover, a much improved method for the preparation of [BiRh(OTfa)4] is described, which makes this heterobimetallic complex readily available. Subsequent exchange of the trifluoroacetate ligands opens access to a so far underappreciated family of (chiral) paddlewheel complexes. While [BiRh] complexes comprising four carboxylate ligands are highly adequate for intermolecular asymmetric cyclopropanation reactions, [BiRh(MEPY)4] as the heterobimetallic cousin of [Rh2(MEPY)4] was found to be surprisingly unreactive; DFT calculations uncover the reasons for this inertia.

A Heteroleptic Dirhodium Catalyst for Asymmetric Cyclopropanation with α-Stannyl α-Diazoacetate. “Stereoretentive” Stille Coupling with Formation of Chiral Quarternary Carbon Centers

The heteroleptic dirhodium paddlewheel catalyst 7 with a chiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+1] cycloadditions with α-diazo-α-trimethylstannyl (silyl, germyl) acetate. Originally discovered as a trace impurity in a sample of the homoleptic parent complex [Rh2((R)-TPCP)4] (5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective. The -NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding. The resulting stannylated cyclopropanes undergo “stereoretentive” cross coupling, which shows for the first time that even chiral quarternary carbon centers can be made by the Stille–Migita reaction.

Chiral Heterobimetallic Bismuth–Rhodium Paddlewheel Catalysts: A Conceptually New Approach to Asymmetric Cyclopropanation

Cyclopropanation reactions of styrene derivatives with donor–acceptor carbenes formed in situ are significantly more enantioselective when catalyzed by the heterobimetallic bismuth–rhodium complex 5 a endowed with N-phthalimido tert-leucine paddlewheel ligands rather than by its homobimetallic dirhodium analogue 1 a. This virtue is likely the result of two synergizing factors: the conical shape of 5 a translates into a narrower calyx-like chiral binding site about the catalytically active Rh center; the Bi atom, although fully solvent exposed, does not decompose aryl diazoacetates and is hence incapable of promoting a racemic background reaction. Moreover, ligand variation proved that successful catalyst design mandates that the anisotropy of the conical heterobimetallic core be matched by proper directionality of the ligand sphere.

Design and Synthesis of Novel Chiral Dirhodium(II) Carboxylate Complexes for Asymmetric Cyclopropanation Reactions

A novel approach to the design of dirhodium(II) tetracarboxylates derived from (S)-amino acid ligands is reported. The approach is founded on tailoring the steric influences of the overall catalyst structure by reducing the local symmetry of the ligand's N-heterocyclic tether. The application of the new approach has led to the uncovering of [Rh2(S-tertPTTL)4] as a new member of the dirhodium(II) family with extraordinary selectivity in cyclopropanation reactions. The stereoselectivity of [Rh2(S-tertPTTL)4] was found to be comparable to that of [Rh2(S-PTAD)4] (up to >99 % ee), with the extra benefit of being more synthetically accessible. Correlations based on X-ray structures to justify the observed enantioinduction are also discussed.

Guide to enantioselective dirhodium(II)-catalyzed cyclopropanation with aryldiazoacetates Dedicated to Professor Melanie Sanford in recognition of her receipt of the Tetrahedron Young Investigator Award

Catalytic enantioselective methods for the generation of cyclopropanes have been of long standing pharmaceutical interest. Chiral dirhodium(II) catalysts prove to be an effective means for the generation of diverse cyclopropane libraries. Rh2(R-DOSP)4 is generally the most effective catalyst for asymmetric intermolecular cyclopropanation of methyl aryldiazoacetates with styrene. Rh2(S-PTAD)4 provides high levels of enantioinduction with ortho-substituted aryldiazoacetates. The less-established Rh2(R-BNP)4 plays a complementary role to Rh2(R-DOSP)4 and Rh2(S-PTAD)4 in catalyzing highly enantioselective cyclopropanation of 3-methoxy substituted aryldiazoacetates. Substitution on the styrene has only moderate influence on the asymmetric induction of the cyclopropanation.