Enantioselective reactions of donor/acceptor carbenoids derived from alpha-aryl-alpha-diazoketones

Stereoselective Synthesis of Polysubstituted Conjugated Dienes Enabled by Photo-Driven Sequential Sigmatropic Rearrangement

We here reported a highly stereoselective method for the synthesis of polysubstituted conjugated dienes from α-aryl α-diazo alkynyl ketones and pyrazole-substituted unsymmetric aminals under mild conditions, which was promoted by photo-irridation and involved with 1,6-dipolar intermediate and quadruple sigmatropic rearrangements, was successfully developed. In this transformation, the cleavage of four bonds and the recombination of five bonds were implemented in one operational step. This protocol provided a modular tool for constructing dienes from amines, pyrazoles and α-alkynyl-α-diazoketones in one-pot manner. The results of mechanistic investigation indicated that the plausible reaction path underwent the 1,6-sigmatropic rearrangement instead of the 1,5-sigmatropic rearrangement.

Diaryldiazoketones as Effective Carbene Sources for Highly Selective Rh(II)-Catalyzed Intermolecular C-H Functionalization

A novel donor/acceptor carbene intermediate has been developed using diaryldiazoketones as carbene precursors. In the presence of the chiral dirhodium catalyst, Rh2(S-TPPTTL)4, diaryldiazoketones undergo highly regio-, stereo-, and diastereoselective C-H functionalization of activated and unactivated secondary and tertiary C-H bonds. Computational studies revealed that the arylketo group behaves differently than the carboxylate acceptor group because the orientation of the arylketo group predetermines which face of the carbene will be attacked.

Ligand-Metal Cooperation Enables Net Ring-Opening C-C Activation / Difunctionalization of Cyclopropyl Ketones

Reactions that cleave C-C bonds and enable functionalization at both carbon sites are powerful strategic tools in synthetic chemistry. Stereodefined cyclopropyl ketones have become readily available and would be an ideal source of 3-carbon fragments, but general approaches to net C-C activation / difunctionalization are unknown. Herein we demonstrate the cross-coupling of cyclopropyl ketones with organozinc reagents and chlorotrimethylsilane to form 1,3-difunctionalized, ring-opened products. A combination of experimental and theoretical studies rule out more established mechanisms and shed light on how cooperation between the redox-active terpyridine (tpy) ligand and the nickel atom enables the C-C bond activation step. The reduced (tpy•-)NiI species activates the C-C bond via a concerted asynchronous ring-opening transition state. The resulting alkylnickel(II) intermediate can then be engaged by aryl-, alkenyl-, and alkylzinc reagents to furnish cross-coupled products. This allows quick access to products that are difficult to make by conjugate addition methods, such as β-allylated and β -benzylated enol ethers. The utility of this approach is demonstrated in the synthesis of a key (±)-taiwaniaquinol B intermediate and the total synthesis of prostaglandin D1.

Catalytic Enantioselective Sulfur Alkylation of Sulfenamides for the Asymmetric Synthesis of Sulfoximines

Sulfoximines are increasingly incorporated in agrochemicals and pharmaceuticals, with the two enantiomers of chiral sulfoximines often having profoundly different binding interactions with biomolecules. Therefore, their application to drug discovery and development requires the challenging preparation of single enantiomers rather than racemic mixtures. Here, we report a general and fundamentally new asymmetric synthesis of sulfoximines. The first S-alkylation of sulfenamides, which are readily accessible sulfur compounds with one carbon and one nitrogen substituent, represents the key step. A broad scope for S-alkylation was achieved by rhodium-catalyzed coupling with diazo compounds under mild conditions. When a chiral rhodium catalyst was utilized with loadings as low as 0.1 mol %, the S-alkylation products were obtained in high yields and with enantiomeric ratios up to 98:2 at the newly generated chiral sulfur center. The S-alkylation products were efficiently converted to a variety of sulfoximines with complete retention of stereochemistry. The utility of this approach was further demonstrated by the asymmetric synthesis of a complex sulfoximine agrochemical.

Iridium-Catalyzed Enantioselective C(sp3 )-H Borylation of Aminocyclopropanes

Transition-metal-catalyzed regio- and stereo-controllable C-H functionalization remains a formidable challenge in asymmetric catalysis. Herein, we disclose the first example of iridium-catalyzed C(sp³ )-H borylation of aminocyclopropanes by using simple imides as weakly coordinating directing groups under mild reaction conditions. The reaction proceeded via a six-membered iridacycle, affording a vast range of chiral aminocyclopropyl boronates. The current method features a broad spectrum of functional groups (36 examples) and high enantioselectivities (up to 99 %). We also demonstrated the synthetic utility by a preparative scale C-H borylation, C-B bond transformations, and conversion of the directing group.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Stereoselective Cyclopropanation of Enamides via C―C Bond Cleavage of Cyclopropenes

This work describes a straightforward protocol for the stereoselective synthesis of vinylcyclopropylamides in high E/Z and syn/anti ratios by cyclopropanation of N-tosyl substituted enamides with cyclopropenes in the presence of...

Elucidating mechanism and reactivity of reaction between donor-acceptor-acceptor 1,3-bisdiazo compound and cinnamyl alcohol catalyzed by Rh2(OAc)4: a DFT study

It is important for diazo compound to take part in organic synthesis. More theoretical and experimental studies focus on mono-diazo compound but fewer on bi-diazo compounds. Here, the mechanism and...

Bis-Rhodamines Bridged with a Diazoketone Linker: Synthesis, Structure, and Photolysis

Two fluorophores bound with a short photoreactive bridge are fascinating structures and remained unexplored. To investigate the synthesis and photolysis of such dyes, we linked two rhodamine dyes via a diazoketone bridge (−COCN₂−) attached to position 5′ or 6′ of the pendant phenyl rings. For that, the mixture of 5′- or 6′-bromo derivatives of the parent dye was prepared, transformed into 1,2-diarylacetylenes, hydrated to 1,2-diarylethanones, and converted to diazoketones Ar¹COCN₂Ar². The high performance liquid chromatography (HPLC) separation gave four individual regioisomers of Ar¹COCN₂Ar². Photolysis of the model compound—C₆H₅COCN₂C₆H₅—in aqueous acetonitrile at pH 7.3 and under irradiation with 365 nm light provided diphenylacetic acid amide (Wolff rearrangement). However, under the same conditions, Ar¹COCN₂Ar² gave mainly α-diketones Ar¹COCOAr². The migration ability of the very bulky dye residues was low, and the Wolff rearrangement did not occur. We observed only moderate fluorescence increase, which may be explained by the insufficient quenching ability of diazoketone bridge (−COCN₂−) and its transformation into another (weaker) quencher, 1,2-diarylethane-1,2-dione.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

We have synthesized the first isolable geminal bisenolates L2K2Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

We have synthesized the first isolable geminal bisenolates L2 K2 Ge[(CO)R]2 (R=2,4,6-trimethylphenyl (2 a,b), L=THF for (2 a) or [18]-crown-6 for (2 b)), a new synthon for the synthesis of organometallic reagents. The formation of these derivatives was confirmed by NMR spectroscopy and X-ray crystallographic analysis. The UV/Vis spectra of these anions show three distinct bands, which were assigned by DFT calculations. The efficiency of 2 a,b to serve as new building block in macromolecular chemistry is demonstrated by the reactions with two different types of electrophiles (acid chlorides and alkyl halides). In all cases the salt metathesis reaction gave rise to novel Ge-based photoinitiators in good yields.

Enantioselective Catalytic Cyclopropanation-Rearrangement Approach to Chiral Spiroketals

A highly enantioselective synthesis of chiral heterobicyclic spiroketals is reported via a "one-pot" cyclopropanation-rearrangement (CP-RA) cascade reaction that is sequentially catalyzed by a chiral Rh(II) catalyst and tetrabutylammonium fluoride (TBAF). Exocyclic vinyl substrates form spirocyclopropanes with tert-butyldimethylsilyl-protected enoldiazoacetates in excellent yields and with excellent enantioselectivities when catalyzed by chiral dirhodium(II) carboxylates, and following desilylation with simultaneous rearrangement in the presence of TBAF, they give (S)-spiroketals in high yields with excellent chirality retention (>95% ee).

A New Ligand Design Based on London Dispersion Empowers Chiral Bismuth-Rhodium Paddlewheel Catalysts

Heterobimetallic bismuth-rhodium paddlewheel complexes with phenylglycine ligands carrying TIPS-groups at the meta-positions of the aromatic ring exhibit outstanding levels of selectivity in reactions of donor/acceptor and donor/donor carbenes; at the same time, the reaction rates are much faster and the substrate scope is considerably wider than those of previous generations of chiral [BiRh] catalysts. As shown by a combined experimental, crystallographic, and computational study, the new catalysts draw their excellent application profile largely from the stabilization of the chiral ligand sphere by London dispersion (LD) interactions of the peripheral silyl substituents.

Dual-Target Compounds against Type 2 Diabetes Mellitus: Proof of Concept for Sodium Dependent Glucose Transporter (SGLT) and Glycogen Phosphorylase (GP) Inhibitors

A current trend in the quest for new therapies for complex, multifactorial diseases, such as diabetes mellitus (DM), is to find dual or even multi-target inhibitors. In DM, the sodium dependent glucose cotransporter 2 (SGLT2) in the kidneys and the glycogen phosphorylase (GP) in the liver are validated targets. Several (β-D-glucopyranosylaryl)methyl (het)arene type compounds, called gliflozins, are marketed drugs that target SGLT2. For GP, low nanomolar glucose analogue inhibitors exist. The purpose of this study was to identify dual acting compounds which inhibit both SGLTs and GP. To this end, we have extended the structure-activity relationships of SGLT2 and GP inhibitors to scarcely known (C-β-D-glucopyranosylhetaryl)methyl arene type compounds and studied several (C-β-D-glucopyranosylhetaryl)arene type GP inhibitors against SGLT. New compounds, such as 5-arylmethyl-3-(β-D-glucopyranosyl)-1,2,4-oxadiazoles, 5-arylmethyl-2-(β-D-glucopyranosyl)-1,3,4-oxadiazoles, 4-arylmethyl-2-(β-D-glucopyranosyl)pyrimidines and 4(5)-benzyl-2-(β-D-glucopyranosyl)imidazole were prepared by adapting our previous synthetic methods. None of the studied compounds exhibited cytotoxicity and all of them were assayed for their SGLT1 and 2 inhibitory potentials in a SGLT-overexpressing TSA201 cell system. GP inhibition was also determined by known methods. Several newly synthesized (C-β-D-glucopyranosylhetaryl)methyl arene derivatives had low micromolar SGLT2 inhibitory activity; however, none of these compounds inhibited GP. On the other hand, several (C-β-D-glucopyranosylhetaryl)arene type GP inhibitor compounds with low micromolar efficacy against SGLT2 were identified. The best dual inhibitor, 2-(β-D-glucopyranosyl)-4(5)-(2-naphthyl)-imidazole, had a K_i of 31 nM for GP and IC₅₀ of 3.5 μM for SGLT2. This first example of an SGLT-GP dual inhibitor can prospectively be developed into even more efficient dual-target compounds with potential applications in future antidiabetic therapy.

Assembly of polysubstituted chiral cyclopropylamines via highly enantioselective Cu-catalyzed three-component cyclopropene alkenylamination

Enabled by a commercial bisphosphine ligand, the Cu-catalyzed three-component cyclopropene alkenylamination with alkenyl organoboron reagent and hyroxyamine esters proceeds with exceptionally high enantioselectivity to deliver poly-substituted cis-1,2-alkenylcyclopropylamines that contain up to all three stereogenic centers on the cyclopropane.

Synthesis of chiral pyrimidine-substituted diester D-A cyclopropanes via asymmetric cyclopropanation of phenyliodonium ylides

A highly enantioselective cyclopropanation to synthesize pyrimidine-substituted diester D-A cyclopropanes is reported. Various N1-vinylpyrimidines react well with phenyliodonium ylides, delivering chiral cyclopropanes in up to 97% yield with up to 99% ee. Through simple [3+2] annulation with benzaldehyde or ethyl glyoxylate, different chiral pyrimidine nucleoside analogues with a sugar ring could be obtained.

Finding Opportunities from Surprises and Failures. Development of Rhodium-Stabilized Donor/Acceptor Carbenes and Their Application to Catalyst-Controlled C-H Functionalization

Catalyst-controlled C-H functionalization by means of the C-H insertion chemistry of rhodium carbenes has become a powerful synthetic method. The key requirements for the development of this chemistry are donor/acceptor carbenes and the chiral dirhodium tetracarboxylate catalysts. This perspective will describe the stages involved in developing this chemistry and illustrate the scope of the donor/acceptor carbene C-H functionalization.

Catalytic Enantio- and Diastereoselective Cyclopropanation of 2-Azadienes for the Synthesis of Aminocyclopropanes Bearing Quaternary Carbon Stereogenic Centers

We report the catalytic enantio- and diastereoselective preparation of aminocyclopropanes by the cyclopropanation of terminal and (Z)-internal 2-azadienes with donor/acceptor carbenes derived from α-diazoesters. The resulting cyclopropanes bear quaternary carbon stereogenic centers vicinal to the amino-substituted carbon and are formed as a single diastereomer in up to 99:1 er and 97% yield with 0.5 mol % of Rh2(DOSP)4 and only 1.5 equiv of the diazo reagent. Transformations with internal azadienes afford cyclopropanes with three contiguous stereogenic centers.

Catalyst-Dependent Chemoselectivity in the Dirhodium-Catalyzed Cyclization Reactions Between Enodiazoacetamide and Nitrosoarene: A Theoretical Study

N-O heterocycle compounds play an important role in various fields. Doyle et al. (Cheng et al., 2017) have recently reported an efficient catalyst-controlled selective cyclization reactions of tert-butyldimethylsilyl (TBS)-protected enoldiazoacetamides with nitrosoarenes in which the multifunctionalized products 5-isoxazolones and 1,3-oxazin-4-ones were formed through [3+2]- and [5+1]-cyclizations using Rh2(oct)4 and Rh2(cap)4 as catalysts, respectively. The present work studied the mechanism of the reactions in question and the origins of the catalyst-dependent chemoselectivity by the density functional theory (DFT) calculations at the M06-D3/SMD/6-311+G(d,p)//B3LYP-D3/6-31G(d,p) level of theory. The computed results illustrate the importance of the different dirhodium catalysts to gain diversified N-O heterocycle compounds and suggest the specific interaction between the reactants by the real catalyst model. Meanwhile, it is the steric hindrance and electronic effect of the ligands of dirhodium catalysts that control the reaction mechanism. Furthermore, the other auxiliary theoretical analysis, natural bond orbital calculation and distortion/interaction analysis, make the electronic effects, and steric hindrance more distinct.

Concise Synthesis of (-)-Cycloclavine and (-)-5- epi-Cycloclavine via Asymmetric C-C Activation

To illustrate the synthetic significance of C-C activation methods, here we describe an efficient strategy for the enantioselective total syntheses of (-)-cycloclavine and (-)-5- epi-cycloclavine, which is enabled by an asymmetric Rh-catalyzed "cut-and-sew" transformation between benzocyclobutenones and olefins. Despite the compact structure of cycloclavine with five-fused rings, the total synthesis was accomplished in 10 steps with a 30% overall yield. Key features of the synthesis include (1) a Pd-catalyzed tandem C-N bond coupling/allylic alkylation sequence to construct the nitrogen-tethered benzocyclobutenone, (2) a highly enantioselective Rh-catalyzed carboacylation of alkenes to forge the indoline-fused tricyclic structure, and (3) a diastereoselective cyclopropanation for preparing the tetrasubstituted cyclopropane ring. Notably, an improved catalytic condition has been developed for the nitrogen-tethered cut-and-sew transformation, which uses a low catalyst loading and allows for a broad substrate scope with high enantioselectivity (94-99% e.e.). The C-C activation-based strategy employed here is anticipated to have further implications for syntheses of other natural products that contain complex fused or bridged rings.

An unusual stereoretentive 1,3-quaternary carbon shift resulting in an enantioselective RhII-catalyzed formal [4+1]-cycloaddition between diazo compounds and vinyl ketenes

Enantioselective quaternary carbon construction in the assembly of cyclopentenones employing a RhII-catalyzed, formal [4+1]-cycloaddition is described. A Rh2(S-TCPTTL)4-catalyzed cyclopropanation of a vinyl ketene with a disubstituted diazo compound initiates a stereoretentive, accelerated ring expansion to provide the cycloadduct in good to excellent yields and enantioselectivity.

A Coupling Approach for the Generation of α,α-Bis(enolate) Equivalents: Regioselective Synthesis of gem-Difunctionalized Ketones

Regioselective α,α-difunctionalization adjacent to a ketone is a significant synthetic challenge. Here, we present a solution to this problem through the transition-metal-free coupling of esters with geminal bis(boron) compounds. This forms an α,α-bis(enolate) equivalent which can be trapped with electrophiles including alkyl halides and fluorinating agents. This presents an efficient, convergent synthetic strategy for the synthesis of unsymmetrical blocked ketones.

Streamlined asymmetric α-difunctionalization of ynones

Ynones are a unique class of structural motifs that show remarkable chemical versatility. Chiral ynones, particularly those possessing an α-stereogenic center, are highly attractive templates for structural diversification. So far, only very limited examples have been reported for asymmetric α-functionalization of ynones. Asymmetric double α-functionalization of ynones remains elusive. Here we describe a streamlined strategy for asymmetric α-difunctionalization of ynones. We developed a gold-catalyzed multicomponent condensation reaction from a simple ynone, an amine, and an electrophilic alkynylating reagent to generate a 1,2-dialkynyl enamine, a key stable and isolable intermediate. This intermediate can undergo asymmetric fluorination catalyzed by a chiral phosphoric acid derivative. Chiral ynones with an α-quaternary carbon and containing a fluorine and an alkyne can be synthesized in high yield and high ee. The synthetic utility of this method is demonstrated by the synthesis of enantioenriched tri(hetero)arylmethyl fluorides.

Chiral ynones with an α-quaternary carbon are attractive synthetic building blocks for natural and pharmaceutical products. Here, the authors report an asymmetric α-difunctionalization of simple ynones, involving a gold-catalyzed step and yielding enantioenriched fluorinated quaternary stereocentres.

Asymmetric Catalytic Preparation of Polysubstituted Cyclopropanol and Cyclopropylamine Derivatives

The catalytic asymmetric carbometalation of cyclopropenes followed by either an electrophilic oxidation or amination reaction provides a unique approach to the formation of diastereomerically pure and enantiomerically enriched cyclopropanol and cyclopropylamine derivatives, respectively.

Stereoselective cis-Vinylcyclopropanation via a Gold(I)-Catalyzed Retro-Buchner Reaction under Mild Conditions

A highly stereoselective gold(I)-catalyzed cis-vinylcyclopropanation of alkenes has been developed. Allylic gold carbenes, generated via a retro-Buchner reaction of 7-alkenyl-1,3,5-cycloheptatrienes, react with alkenes to form vinylcyclopropanes. The gold(I)-catalyzed retro-Buchner reaction of these substrates proceeds by simple heating at a temperature much lower than that required for the reaction of 7-aryl-1,3,5-cycloheptatrienes (75 °C vs 120 °C). A newly developed Julia–Kocienski reagent enables the synthesis of the required cycloheptatriene derivatives in one step from readily available aldehydes or ketones. On the basis of mechanistic investigations, a stereochemical model for the cis selectivity was proposed. An unprecedented gold-catalyzed isomerization of cis- to trans-cyclopropanes has also been discovered and studied by DFT calculations.

Beyond Iron: Iridium-Containing P450 Enzymes for Selective Cyclopropanations of Structurally Diverse Alkenes

Enzymes catalyze organic transformations with exquisite levels of selectivity, including chemoselectivity, stereoselectivity, and substrate selectivity, but the types of reactions catalyzed by enzymes are more limited than those of chemical catalysts. Thus, the convergence of chemical catalysis and biocatalysis can enable enzymatic systems to catalyze abiological reactions with high selectivity. Recently, we disclosed artificial enzymes constructed from the apo form of heme proteins and iridium porphyrins that catalyze the insertion of carbenes into a C-H bond. We postulated that the same type of Ir(Me)-PIX enzymes could catalyze the cyclopropanation of a broad range of alkenes with control of multiple modes of selectivity. Here, we report the evolution of artificial enzymes that are highly active and highly stereoselective for the addition of carbenes to a wide range of alkenes. These enzymes catalyze the cyclopropanation of terminal and internal, activated and unactivated, electron-rich and electron-deficient, conjugated and nonconjugated alkenes. In particular, Ir(Me)-PIX enzymes derived from CYP119 catalyze highly enantio- and diastereoselective cyclopropanations of styrene with ±98% ee, >70:1 dr, >75% yield, and ∼10,000 turnovers (TON), as well as 1,2-disubstituted styrenes with up to 99% ee, 35:1 dr, and 54% yield. Moreover, Ir(Me)-PIX enzymes catalyze cyclopropanation of internal, unactivated alkenes with up to 99% stereoselectivity, 76% yield, and 1300 TON. They also catalyze cyclopropanation of natural products with diastereoselectivities that are complementary to those attained with standard transition metal catalysts. Finally, Ir(Me)-PIX P450 variants react with substrate selectivity that is reminiscent of natural enzymes; they react preferentially with less reactive internal alkenes in the presence of more reactive terminal alkenes. Together, the studies reveal the suitability of Ir-containing P450s to combine the broad reactivity and substrate scope of transition metal catalysts with the exquisite selectivity of enzymes, generating catalysts that enable reactions to occur with levels and modes of activity and selectivity previously unattainable with natural enzymes or transition metal complexes alone.

Enantioselective Cyclopropanation of a Wide Variety of Olefins Catalyzed by Ru(II)-Pheox Complexes

The transition-metal-catalyzed asymmetric cyclopropanation of olefins with diazoacetates has become one of the most important methods for the synthesis of optically active cyclopropane derivatives, which are key pharmaceutical building blocks and present in a large number of natural products. To date, significant progress has been made in this area of research, and efficient stereocontrolled synthetic approaches to cyclopropane derivatives have been developed using rhodium, ruthenium, copper, and cobalt catalysts. However, the vast majority of these strategies are limited to electron-rich olefins, such as styrene derivatives, due to the electrophilicity of the metal-carbene intermediates generated from the reaction of the metal with the diazo compound. Recently, the D₂-symmetric Co(II)-phophyrin complexes developed by Zhang et al. were shown to be the most efficient catalysts for the asymmetric cyclopropanation of electron-deficient olefins. This catalytic system is mechanistically distinct from the previous rhodium and copper catalytic systems, proceeding via radical intermediates. However, the asymmetric cyclopropanation of vinyl carbamates, allenes, and α,β-unsaturated carbonyl compounds has rarely been reported. Therefore, the development of new powerful catalysts for the asymmetric cyclopropanation of a wide range of olefinic substrates is the next challenge in this field. In this Account, we summarize our recent studies on the Ru(II)-Pheox-catalyzed asymmetric cyclopropanation of various olefins, including vinyl carbamates, allenes, and α,β-unsaturated carbonyl compounds. We demonstrate that the developed catalytic system effectively promotes the asymmetric cyclopropanation of a wide variety of olefins to produce the desired cyclopropane products in high yields with excellent stereocontrol. The use of succinimidyl-, ketone-, and ester-functionalized diazoacetates as carbene sources was found to be crucial for the high stereoselectivity of the cyclopropanation reactions. In addition, we describe reusable chiral Ru(II)-Pheox catalysts, namely, water-soluble Ru(II)-hm-Pheox and polymer-supported PS-Ru(II)-Pheox, which can be reused at least five times in inter- and intramolecular cyclopropanation reactions without any significant loss of catalytic activity or enantioselectivity. These Ru(II)-Pheox-catalyzed asymmetric cyclopropanation reactions provide an elegant method to access a series of optically active cyclopropane derivatives, including cyclopropylamines, dicarbonyl cyclopropanes, alkylidenecyclopropanes, and cyclopropane-fused γ-lactones, which are intermediates in the syntheses of various biologically active compounds. The novel chiral Ru(II)-Pheox complexes are readily synthesized in high yield from inexpensive, commercially available benzoyl chloride and amino alcohols, then fully characterized using X-ray diffraction analysis, NMR, and elemental analysis. These catalysts are easy to handle and stable under ordinary temperatures and conditions and can be used after three months of storage without any loss of catalytic activity or stereoselectivity.

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-^tert PTTL)₄ ] as a new member of the dirhodium(II) family with extraordinary selectivity in cyclopropanation reactions. The stereoselectivity of [Rh₂ (S-^tert PTTL)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.