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

Rh-Catalyzed Enantioselective Aryl C-H Bond Cyclopropylation

Herein, we disclose the discovery and development of a site-, regio-, diastereo-, and enantioselective aryl C-H bond cyclopropylation using diazomethyl hypervalent iodine reagents, styrenes, and paddlewheel dirhodium carboxylate catalysts. A key aspect of this work was the catalytic generation of a chiral Rh(II) carbene through an electrophilic aromatic substitution with chiral Rh(II) carbynoids. The strategy allows the construction of cyclopropane rings using aryl C-H bonds from aromatic feedstocks and drug molecules and promises to reach an unexplored "cyclopropanated" chemical space highly difficult to reach by current strategies.

C 4-Symmetric Bowl-Shaped Diruthenium Tetracarboxylate Catalysts for Enantioselective C-H Functionalization Using Donor/Acceptor Carbenes

Cationic diruthenium (II,III) tetracarboxylate catalysts have been shown to catalyze selective intermolecular C-H functionalization reactions using donor/acceptor carbenes in high yield and with high levels of enantioselectivity. The diruthenium catalysts were compared to the analogous dirhodium (II,II) tetracarboxylate and showed similar levels of enantioselectivity for most reactions. A distinctive feature of the diruthenium catalysts is a greater preference for C-H functionalization over cyclopropanation compared to the corresponding dirhodium catalysts. Also, the diruthenium catalysts have a greater preference for sterically more accessible sites compared with their dirhodium counterparts. These studies show that the diruthenium catalysts are generally effective catalysts for enantioselective intermolecular C-H functionalization, but further optimization would be needed for them to match the dirhodium catalysts in terms of functional group compatibility, turnover frequency, and turnover numbers.

Diversity Synthesis Using Glutarimides as Rhodium Carbene Precursors in Enantioselective C-H Functionalization and Cyclopropanation

Cereblon E3 ligase modulatory drugs (CELMoDs) can be used to target proteins and mark them for proteasomal degradation by recruiting them to cereblon (CRBN), the substrate receptor of the CRL4CRBN E3 ubiquitin ligase complex. Modifications to the stereochemistry and regiochemistry of distal functionality on CELMoDs have been shown to have large effects on degradation activity and selectivity; however, methods allowing the rapid and selective introduction of enantioenriched moieties are rare. Herein, we report that classical CRBN-binding glutarimide cores can be successfully derivatized to aryl diazoacetates. These diazo derivatives, when in the presence of a dirhodium catalyst, successfully undergo high-yielding and highly enantioselective C-H functionalization of hydrocarbons and cyclopropanation of styrene. These products can be used to create not only molecular glue degrader-like compounds but also intermediates that can be elaborated into effective bifunctional ligand-directed degraders. Our findings highlight both the effectiveness of dirhodium catalysis in a drug discovery context and a new method for preparing diverse and stereoenriched glutarimide-containing compounds.

Dirhodium-Catalyzed Asymmetric Transformations of Alkynes via Carbene Intermediates

ConspectusFunctionalization of alkynes is an established cornerstone of organic synthesis. While numerous transition metals exhibit promising activities in the transformations of alkynes via π-insertion or oxidative cyclometalation, Lewis π-acids offer a different approach. By coordinating with alkynes through π-bonding, Lewis π-acids facilitate nucleophilic addition, leading to the formation of alkenyl metal species. These species can undergo electron rearrangement to generate metal carbenes, which are crucial intermediates for subsequent carbene transfer reactions. This reaction pathway provides a versatile route for alkyne functionalization, especially in an asymmetric manner. Although the Lewis π-acid, gold(I), pioneered this reaction mode, the development of asymmetric variants remains challenging due to the linear coordination of gold(I). Therefore, expanding the range of catalysts beyond gold(I) complexes to other metal catalysts would facilitate further advances in chiral molecule construction and the exploration of novel reaction modes.In this Account, we present a concise review of alkyne multifunctionalization via dirhodium-catalyzed asymmetric transformations, providing the development of the modulation strategies and substrates and plausible reaction mechanisms. In the aromatization-driven strategy, the furanyl dirhodium carbene is generated from an enynone, which is terminated by enantioselective intramolecular C-H insertion, cyclopropanation, aromatic substitution, or the Büchner reaction, giving chiral dihydroindoles, dihydrobenzofurans, cyclopropane-fused tetrahydroquinolines, fluorenes, or cyclohepta[b]benzofurans. The cap-tether modulation strategy was developed in a subsequent study to balance the reactivity and selectivity of an azo-enyne. This strategy gave the first catalytic asymmetric cycloisomerization of azo-enyne, affording centrally and axially chiral isoindazole derivatives. The synergistic activation strategy, i.e., EWG activation and C-H···O interaction, was introduced to achieve the first dirhodium-catalyzed asymmetric cycloisomerization of enynes, providing a range of chiral cyclopropane-annulated bicyclic systems from enynals. Benefiting from these successes, difluoromethyl-substituted enynes were designed and proven to be effective substrates. With the corresponding benzo-1,6-enynes as the substrates, the enantioselective biscyclopropanation and the cascaded cyclopropanation/cyclopropenation were achieved using alkynes as dicarbene equivalents. Additionally, benzo-1,5-enynal generated vinyl dirhodium carbene, which reacted with a variety of alkenes via [2 + 1] cycloaddition, [4 + 3] cycloaddition, or formal allylation, giving spiro and fused polycyclic heterocycles. Coupling the synergistic activation strategy with desymmetrization, we further successfully achieved the asymmetric cycloisomerization of diynals, constructing furan-fused dihydropiperidines with an alkyne-substituted aza-quaternary stereocenter. Notably, by analyzing X-ray structures of several dirhodium-alkyne π-complexes, together with the results of DFT calculations and control experiments, we obtained evidence supporting the synergistic activation mode, making the well-defined paddlewheel-like dirhodium(II) stand out among the other metal complexes. We anticipate that our research will significantly advance the fields of dirhodium, alkyne, and carbene chemistry.

Rh(II)/Pd(0) Dual Catalysis: Carbenoid N-H Insertion/Allylation Cascade Reaction to Construct Highly Functionalized and Polysubstituted Pyrrolidines

In the category of drugs approved by the U.S. FDA, pyrrolidine is the most frequently used core of five-membered nonaromatic heterocycles containing nitrogen. Herein, a Rh(II)/Pd(0) dual-catalyzed carbenoid N-H insertion/allylation cascade reaction has been developed. This protocol provide an efficient approach for the construction of diverse highly functionalized and polysubstituted pyrrolidines in high yields (up to 91%) with excellent chemoselectivities and high diastereoselectivities (>20:1) under mild reaction conditions.

Recent advances in metal carbene-induced semipinacol rearrangements

As has been well-recognized, the semipinacol rearrangements (SPRs) function as a powerful and versatile tool for the construction of all-carbon and heteroatom-containing quaternary stereocenters, which are present in various natural products and bioactive molecules. In recent years, considerable attention has been paid to exploring the metal carbene-induced semipinacol rearrangements, providing an attractive and powerful strategy for obtaining various important carbonyl compounds. However, to date, no review has been published that summarizes the significant advances in the preparation of functionalized carbonyl compounds using these migration rearrangement reactions. In this review article, we have summarised the recent advances in the field of metal carbene-induced SPR reactions according to different metal classifications. Mechanistic insights, synthetic applications, and their limitations are discussed. The challenges and opportunities in this field are also outlined.

Rh(II)-Catalyzed Enantioselective S-Alkylation of Sulfenamides with Acceptor-Acceptor Diazo Compounds Enables the Synthesis of Sulfoximines Displaying Diverse Functionality

The Rh(II)-catalyzed enantioselective S-alkylation of sulfenamides with α-amide diazoacetates at 1 mol % catalyst loading to obtain sulfilimines in high yields and enantiomeric ratios of up to 99:1 is reported. The enantioenriched sulfilimine products incorporate versatile amide functionality poised for further elaboration to diverse sulfoximines with multiple stereogenic centers, including by highly diastereoselective sulfilimine and sulfoximine α-alkylation with alkylating agents and epoxides and by interconversion of the amide to N-tert-butanesulfinyl aldimines, followed by diastereoselective additions.

D4-Symmetric Dirhodium Tetrakis(binaphthylphosphate) Catalysts for Enantioselective Functionalization of Unactivated C-H Bonds

Dirhodium tetrakis(2,2'-binaphthylphosphate) catalysts were successfully developed for asymmetric C-H functionalization with trichloroethyl aryldiazoacetates as the carbene precursors. The 2,2'-binaphthylphosphate (BNP) ligands were modified by introduction of aryl and/or chloro functionality at the 4,4',6,6' positions. As the BNP ligands are C2-symmetric, the resulting dirhodium tetrakis(2,2'-binaphthylphosphate) complexes were expected to be D4-symmetric, but X-ray crystallographic and computational studies revealed this is not always the case because of internal T-shaped CH-π and aryl-aryl interactions between the ligands. The optimum catalyst is Rh2(S-megaBNP)4, with 3,5-di(tert-butyl)phenyl substituents at the 4,4' positions and chloro substituents at the 6,6' positions. This catalyst adopts a D4-symmetric arrangement and is ideally suited for site-selective C-H functionalization at unactivated tertiary sites with high levels of enantioselectivity, outperforming the best dirhodium tetracarboxylate catalyst developed for this reaction. The standard reactions were conducted with a catalyst loading of 1 mol % but lower catalyst loadings can be used if desired, as illustrated in the C-H functionalization of cyclohexane in 91% ee with 0.0025 mol % catalyst loading (29,400 turnover numbers). These studies further illustrate the effectiveness of donor/acceptor carbenes in site-selective intermolecular C-H functionalization and expand the toolbox of catalysts available for catalyst-controlled C-H functionalization.

Trifluoroethylation and Pentafluoropropylation of C(sp3)-H Bonds

Polyfluorinated substituents often enhance effectiveness, improve the stability within metabolic processes, and boost the lipophilicity of biologically active compounds. However, methods for their introduction into aliphatic carbon chains remain very limited. A potentially general route to integrate the fluorinated scaffolds into organic molecules involves insertion of fluorine-containing carbenes into C(sp3)-H bonds. The electron-withdrawing characteristics of perfluoroalkyl groups enhances the reactivity of these carbenes which should enable the functionalization of unactivated C(sp3)-H bonds. Curiously, it appears that use of perfluoroalkyl-containing carbenes in alkane C-H functionalization is exceedingly rare. This concept describes photolysis, enzymatic catalysis, and transition metal catalysis as three primary approaches to C(sp3)-H functionalization by trifluoromethylcarbene and its homologues.

Doyle-Kirmse Reaction on Alkynyl Hydrazone Carboxylates: Synthesis of 1,4-Allenyne and 1,5-Enyne Thioaryl Carboxylates

The first Doyle-Kirmse reaction on alkynyl diazoacetates using allyl/propargyl sulfides is reported. The development provides diversified 1,5-enyne and 1,4-allenyne thioaryl carboxylates in good yields under ligand-/additive-free AuCl and Rh2(OAc)4 catalysis, respectively (48 examples, up to 96% yield). The study demonstrated the dual role of allyl sulfide as a ligand and substrate. Also, we have exemplified various synthetic modifications of the products to showcase the utility of different functional groups.

Catalytic, asymmetric carbon-nitrogen bond formation using metal nitrenoids: from metal-ligand complexes via metalloporphyrins to enzymes

The introduction of nitrogen atoms into small molecules is of fundamental importance and it is vital that ever more efficient and selective methods for achieving this are developed. With this aim, the potential of nitrene chemistry has long been appreciated but its application has been constrained by the extreme reactivity of these labile species. This liability however can be attenuated by complexation with a transition metal and the resulting metal nitrenoids have unique and highly versatile reactivity which includes the amination of certain types of aliphatic C-H bonds as well as reactions with alkenes to afford aziridines. At least one new chiral centre is typically formed in these processes and the development of catalysts to exert control over enantioselectivity in nitrenoid-mediated amination has become a growing area of research, particularly over the past two decades. Compared with some synthetic methods, metal nitrenoid chemistry is notable in that chemists can draw from a diverse array of metals and catalysts , ranging from metal-ligand complexes, bearing a variety of ligand types, via bio-inspired metalloporphyrins, all the way through to, very recently, engineered enzymes themselves. In the latter category in particular, rapid progress is being made, the rate of which suggests that this approach may be instrumental in addressing some of the outstanding challenges in the field. This review covers key developments and strategies that have shaped the field, in addition to the latest advances, up until September 2023.

A Carbene Relay Strategy for Cascade Insertion Reactions

Insertion reactions that involve stabilized electrophilic metallocarbenes are of great importance for installing α-heteroatoms to carbonyl compounds. Nevertheless, the limited availability of carbene precursors restricts the introduction of only a single heteroatom. In this report, we describe a new approach based on an I(III) /S(VI) reagent that promotes the cascade insertion of heteroatoms. This is achieved by sequentially generating two α-heteroatom-substituted metal carbenes in one reaction. We found that this mixed I(III) /S(VI) ylide reacts efficiently with a transition metal catalyst and an X-H bond (where X=O, N). This transformation leads to the sequential formation of a sulfoxonium- and an X-substituted Rh-carbenes, enabling further reactions with another Y-H bond. Remarkably, a wide range of symmetrical and unsymmetrical α,α-O,O-, α,α-O,N-, and α,α-N,N-subsituted ketones can be prepared under mild ambient conditions. In addition, we successfully demonstrated other cascades, such as CN/CN double amidation, C-H/C-S double insertion, and C-S/Y-H double insertion (where Y=S, N, O, C). Notably, the latter two cascades enabled the simultaneous installation of three functional groups to the α-carbon of carbonyl compounds in a single step. These reactions demonstrate the versatility of our approach, allowing for the synthesis of ketones and esters with multiple α-heteroatoms using a common precursor.

"Sandwich" Diimine-Copper Catalyzed Trifluoroethylation and Pentafluoropropylation of Unactivated C(sp3 )-H Bonds by Carbene Insertion

We report here "sandwich"-diimine copper complex-catalyzed trifluoroethylation and pentafluoropropylation of unactivated C(sp3 )-H bonds in alkyl esters, halides, and protected amines by employing CF3 CHN2 and CF3 CF2 CHN2 reagents. Reactions proceed in dichloromethane solvent at room temperature. Identical C-H functionalization conditions and stoichiometries are employed for generality and convenience. Selectivities for C-H insertions are higher for compounds possessing stronger electron-withdrawing substituents. Preliminary mechanistic studies point to a mechanism involving a pre-equilibrium forming a "sandwich"-diimine copper-CF3 CHN2 complex followed by rate-determining loss of nitrogen affording the reactive copper carbene. It reacts with trifluoromethyldiazomethane about 6.5 times faster than with 1-fluoroadamantane explaining the need for slow addition of the diazo compound.

Diruthenium Tetracarboxylate-Catalyzed Enantioselective Cyclopropanation with Aryldiazoacetates

A series of chiral bowl-shaped diruthenium(II,III) tetracarboxylate catalysts were prepared and evaluated in asymmetric cyclopropanations with donor/acceptor carbenes derived from aryldiazoacetates. The diruthenium catalysts self-assembled to generate C4-symmetric bowl-shaped structures in an analogous manner to their dirhodium counterparts. The optimum catalyst was found to be Ru2(S-TPPTTL)4·BArF [S-TPPTTL = (S)-2-(1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl)-3,3-dimethylbutanoate, BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate], which resulted in the cyclopropanation of a range of substrates in up to 94% ee. Synthesis and evaluation of first-row transition-metal congeners [Cu(II/II) and Co(II/II)] invariably resulted in catalysts that afforded little to no asymmetric induction. Computational studies indicate that the carbene complexes of these dicopper and dicobalt complexes, unlike the dirhodium and diruthenium systems, are prone to the loss of carboxylate ligands, which would destroy the bowl-shaped structure critical for asymmetric induction.

Catalytic generation of ortho-quinone dimethides via donor/donor rhodium carbenes

Substrates engineered to undergo a 1,4-C-H insertion to yield benzocyclobutenes resulted in a novel elimination reaction to yield ortho-quinone dimethide (o-QDM) intermediates that undergo Diels-Alder or hetero-Diels-Alder cycloadditions. The analogous benzylic acetals or ethers avoid the C-H insertion pathway completely and, after hydride transfer, undergo a de-aromatizing elimination reaction to o-QDM at ambient temperature. The resulting dienes undergo a variety of cycloaddition reactions with high diastereo- and regio-selectivity. This is one of the few examples of catalytic generation of o-QDM without the intermediacy of a benzocyclobutene and represents one of the mildest, ambient temperature processes to access these useful intermediates. This proposed mechanism is supported by DFT calculations. Moreover, the methodology was applied to the synthesis of (±)-isolariciresinol in 41% overall yield.

Rh(II)/Pd(0) Dual Catalysis: Interception of Ammonium Ylide with Allyl Palladium to Construct 2,2-Disubstituted Tetrahydroquinoline Derivatives

New synthetic methods to construct 2,2-disubstituted tetrahydroquinoline derivatives are of significant value in pharmaceutical chemistry. Herein, a Rh(II)/Pd(0) dual-catalyzed diazo α-aminoallylation reaction has been developed between allylpalladium(II) and ammonium ylides derived from the Rh₂(OAc)₄-mediated intramolecular N-H bond insertion reaction of diazo compounds, affording various 2,2-disubstituted tetrahydroquinoline derivatives in good yields up to 93% with high chemoselectivities under mild reaction conditions. A substrate scope investigation reveals broad ester substituent tolerance, and control experiments provide the basis for a proposed reaction mechanism.

N-Phthalimide as a Site-Protecting and Stereodirecting Group in Rhodium-Catalyzed C-H Functionalization with Donor/Acceptor Carbenes

The rhodium-catalyzed enantioselective C-H functionalization of unactivated C-H bonds by means of donor/acceptor carbene-induced C-H insertion was extended to substrates containing nitrogen functionality. The rhodium-stabilized donor/acceptor carbenes were generated by rhodium-catalyzed decomposition of aryldiazoacetates. The phthalimido group was the optimum nitrogen protecting group. C-H functionalization at the most sterically accessible methylene site was achieved using Rh₂(S-2-Cl-5-BrTPCP)₄ as catalyst, whereas Rh₂(S-TPPTTL)₄ was the most effective catalyst for C-H functionalization at tertiary C-H bonds and for the desymmetrization of N-phthalimidocyclohexane.

Synthesis of Alkynyl Hydrazones from Unprotected Hydrazine and Their Reactivity as Diazo Precursors

Alkynyl hydrazones are synthesized conveniently from 2-oxo-3-butynoates and hydrazine by suppressing the susceptible formation of pyrazoles. The resultant hydrazones are transformed into alkynyl diazoacetates under metal-free and mild oxidative conditions in excellent yields. Further, the alkynyl cyclopropane and propargyl silane carboxylates are synthesized in good yields by developing an unprecedented copper-catalyzed alkynyl carbene transfer reaction.

Dirhodium C-H Functionalization of Hole-Transport Materials

Hole-transport materials (HTMs) based on triarylamine derivatives play important roles in organic electronics applications including organic light-emitting diodes and perovskite solar cells. For some applications, triarylamine derivatives bearing appropriate binding groups have been used to functionalize surfaces, while others have been incorporated as side chains into polymers to manipulate the processibility of HTMs for device applications. However, only a few approaches have been used to incorporate a single surface-binding group or polymerizable group into triarylamine materials. Here, we report that Rh-carbenoid chemistry can be used to insert carboxylic esters and norbornene functional groups into sp² C-H bonds of a simple triarylamine and a 4,4'-bis(diarylamino)biphenyl, respectively. The norbenene-functionalized monomer was polymerized by ring-opening metathesis; the electrochemical, optical, and charge-transport properties of these materials were similar to those of related materials synthesized by conventional means. This method potentially offers straightforward access to a diverse range of HTMs with different functional groups.

Generating Fischer-Type Rh-Carbenes with Rh-Carbynoids

We describe the first catalytic generation of Fischer-type acyloxy Rh(II)-carbenes from carboxylic acids and Rh(II)-carbynoids. This novel class of transient donor/acceptor Rh(II)-carbenes evolved through a cyclopropanation process providing access to densely functionalized cyclopropyl-fused lactones with excellent diastereoselectivity. DFT calculations allowed the analysis of the properties of Rh(II)-carbynoids and acyloxy Rh(II)-carbenes as well as the characterization of the mechanism.

Rhodium-Catalyzed Allylic C-H Functionalization of Unactivated Alkenes with α-Diazocarbonyl Compounds

A redox-neutral mild methodology for the allylic C-H alkylation of unactivated alkenes with diazo compounds is demonstrated. The developed protocol is able to bypass the possibility of the cyclopropanation of an alkene upon its reaction with the acceptor-acceptor diazo compounds. The protocol is highly accomplished due to its compatibility with various unactivated alkenes functionalized with different sensitive functional groups. A rhodacycle π-allyl intermediate has been synthesized and proved to be the active intermediate. Additional mechanistic investigations aided the elucidation of the plausible reaction mechanism.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

Asymmetric Cyclopropanation with 4-Aryloxy-1-sulfonyl-1,2,3-triazoles: Expanding the Range of Rhodium-Stabilized Donor/Acceptor Carbenes to Systems with an Oxygen Donor Group

Rhodium-catalyzed enantioselective synthesis of 1-phenoxycyclopropane-1-carbaldehydes by intermolecular cyclopropanation of terminal alkenes followed by imine hydrolysis is described. This methodology utilizes 4-aryloxy-1-sulfonyl-1,2,3-triazoles as the carbene precursors and the chiral dirhodium(II) tetracarboxylates Rh2(S-NTTL)4 or Rh2(S-DPCP)4 as the catalysts. These reactions are considered to proceed via rhodium-stabilized donor/acceptor carbene intermediates, and these studies demonstrate that a heteroatom donor group is compatible with an enantioselective transformation.

In-situ Kinetic Studies of Rh(II)-Catalyzed C-H Functionalization to Achieve High Catalyst Turnover Numbers

Detailed kinetic studies on the functionalization of unactivated hydrocarbon sp3 C-H bonds by dirhodium-catalyzed reaction of aryldiazoacetates revealed that the C-H functionalization step is rate-determining. The efficiency of this step was increased by using the hydrocarbon as solvent and using donor/acceptor carbenes with an electron-withdrawing substituent on the aryl donor group. The optimum catalyst for these reactions is the tetraphenylphthalimido derivative Rh2(R-TPPTTL)4 and a further beneficial refinement was obtained by using N,N'-dicyclohexylcarbodiimide as an additive. Under the optimum conditions with a catalyst loading of 0.001 mol %, effective enantioselective C-H functionalization (66-97% yield, 83-97% ee) was achieved of cycloalkanes with a range of aryldiazoacetates as long as the aryldiazoacetate was not to sterically demanding. The reaction with cyclohexane using a catalyst loading of 0.0005 mol % could be recharged twice with additional aryldiazoacetate, resulting in an overall dirhodium catalyst turnover number of 580,000.

Heteroarylation of unactivated C-H bonds suitable for late-stage functionalization

The late-stage introduction of diverse heterocycles onto complex small molecules enables efficient access to new medicinally relevant compounds. An attractive approach to such a transformation would utilize the ubiquitous aliphatic C-H bonds of a complex substrate. Herein, we report a system that enables direct C-H heteroarylation using a stable, commercially available O-alkenylhydroxamate with heterocyclic sulfone partners. The C-H heteroarylation proceeds efficiently with a range of aliphatic substrates and common heterocycles, and is a rare example of heteroarylation of strong C-H bonds. Importantly, the present approach is amenable to late-stage functionalization as the substrate is the limiting reagent in all cases.

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.

Design, Synthesis, and Evaluation of Extended C4-Symmetric Dirhodium Tetracarboxylate Catalysts

The synthesis and evaluation of six C4-symmetric bowl-shaped dirhodium tetracarboxylate catalysts are described. These elaborate high symmetry catalysts are readily generated by means of the self-assembly of four C1-symmetric ligands around the dirhodium core. These catalysts are capable of highly site-selective, diastereoselective and enantioselective C-H functionalization reactions by means of donor/acceptor carbene-induced C-H insertions.

Leveraging Regio- and Stereoselective C(sp3)-H Functionalization of Silyl Ethers to Train a Logistic Regression Classification Model for Predicting Site-Selectivity Bias

The C-H functionalization of silyl ethers via carbene-induced C-H insertion represents an efficient synthetic disconnection strategy. In this work, site- and stereoselective C(sp3)-H functionalization at α, γ, δ, and even more distal positions to the siloxy group has been achieved using donor/acceptor carbene intermediates. By exploiting the predilections of Rh2(R-TCPTAD)4 and Rh2(S-2-Cl-5-BrTPCP)4 catalysts to target either more electronically activated or more spatially accessible C-H sites, respectively, divergent desired products can be formed with good diastereocontrol and enantiocontrol. Notably, the reaction can also be extended to enable desymmetrization of meso silyl ethers. Leveraging the broad substrate scope examined in this study, we have trained a machine learning classification model using logistic regression to predict the major C-H functionalization site based on intrinsic substrate reactivity and catalyst propensity for overriding it. This model enables prediction of the major product when applying these C-H functionalization methods to a new substrate of interest. Applying this model broadly, we have demonstrated its utility for guiding late-stage functionalization in complex settings and developed an intuitive visualization tool to assist synthetic chemists in such endeavors.

“Sandwich” Diimine‐Copper Catalysts for C−H Functionalization by Carbene Insertion

We report here "sandwich" diimine-copper(I) catalysts for C(sp³ )-H bond functionalization. Reactions of alkanes and ethers with trimethylsilyldiazomethane, ethyl diazoacetate, and trifluoromethyl-diazomethane have been demonstrated. We also report C(sp³ )-H bond methylation, benzylation, and diphenylmethylation by diazomethane, aryldiazomethanes, and diphenyldiazomethane. These reactions are rare examples of base-metal catalyzed, intermolecular C(sp³ )-H functionalizations by employing unactivated diazo compounds. Electrophilicity and unique steric environment of "sandwich"-copper catalysts are likely reasons for their catalytic efficiency.

Iridium(III)-Catalyzed Asymmetric Site-Selective Carbene C-H Insertion during Late-Stage Transformation

C-H functionalization has recently received considerable attention because C-H functionalization during the late-stage transformation is a strong and useful tool for the modification of the bioactive compounds and the creation of new active molecules. Although a carbene transfer reaction can directly convert a C-H bond to the desired C-C bond in a stereoselective manner, its application in late-stage material transformation is limited. Here, we observed that the iridium-salen complex 6 exhibited efficient catalysis in asymmetric carbene C-H insertion reactions. Under optimized conditions, benzylic, allylic, and propargylic C-H bonds were converted to desired C-C bonds in an excellent stereoselective manner. Excellent regioselectivity was demonstrated in the reaction using not only simple substrate but also natural products, bearing multiple reaction sites. Moreover, based on the mechanistic studies, the iridium-catalyzed unique C-H insertion reaction involved rate-determining asynchronous concerted processes.

Enantioselective Organocopper-Catalyzed Hetero Diels-Alder Reaction through in Situ Oxidation of Ethers into Enol Ethers

We disclose a catalytic method for the enantio- and diastereoselective union of alkyl ethers and heterodienes. We demonstrate that a chiral Cu-BOX complex catalyzes the efficient oxidation of ethers into enol ethers in the presence of trityl acetate. Then, the organocopper promotes stereoselective hetero Diels-Alder reaction between the in situ generated enol ethers and β,γ-unsaturated ketoesters, allowing for rapid access to an array of dihydropyran derivatives possessing three vicinal stereogenic centers.

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.

Diversification of aliphatic C-H bonds in small molecules and polyolefins through radical chain transfer

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and polymers. Herein, we report an approach to aliphatic carbon-hydrogen bond diversification using radical chain transfer featuring an easily prepared O-alkenylhydroxamate reagent, which upon mild heating facilitates a range of challenging or previously undeveloped aliphatic carbon-hydrogen bond functionalizations of small molecules and polyolefins. This broad reaction platform enabled the functionalization of postconsumer polyolefins in infrastructure used to process plastic waste. Furthermore, the chemoselective placement of ionic functionality onto a branched polyolefin using carbon-hydrogen bond functionalization upcycled the material from a thermoplastic into a tough elastomer with the tensile properties of high-value polyolefin ionomers.

Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp3 )-H Bonds

The selective functionalization of C(sp3 )-H bonds using transition-metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp3 )-H bonds, and the selection among very similar C-H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C-C and C-heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal-catalyzed C(sp3 )-H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.

Heteroleptic dirhodium(II,II) paddlewheel complexes as carbene transfer catalysts

This review highlights the applications of dirhodium(II,II) paddlewheel complexes with a heteroleptic scaffold. Dirhodium(II,II) paddlewheel complexes are well known as highly efficient and selective carbene transfer catalysts. While the majority of described complexes are homoleptic, comparatively fewer studies have concerned heteroleptic complexes. Here, we emphasise the use of heteroleptic complexes in order to highlight their benefits as carbene transfer catalysts and spur future research. Methods to synthesise heteroleptic dirhodium(II,II) paddlewheel complexes are discussed as well as a categorical review of their types of heteroleptic complexes and the carbene reactions in which they have been used.

Dirhodium(II)/Xantphos-Catalyzed Relay Carbene Insertion and Allylic Alkylation Process: Reaction Development and Mechanistic Insights

Although dirhodium-catalyzed multicomponent reactions of diazo compounds, nucleophiles and electrophiles have achieved great advance in organic synthesis, the introduction of allylic moiety as the third component via allylic metal intermediate remains a formidable challenge in this area. Herein, an attractive three-component reaction of readily accessible amines, diazo compounds, and allylic compounds enabled by a novel dirhodium(II)/Xantphos catalysis is disclosed, affording various architecturally complex and functionally diverse α-quaternary α-amino acid derivatives in good yields with high atom and step economy. Mechanistic studies indicate that the transformation is achieved through a relay dirhodium(II)-catalyzed carbene insertion and allylic alkylation process, in which the catalytic properties of dirhodium are effectively modified by the coordination with Xantphos, leading to good activity in the catalytic allylic alkylation process.

Formal Allylation and Enantioselective Cyclopropanation of Donor/Acceptor Rhodium(II) Azavinyl Carbenes

A highly efficient formal allylation of dihydronaphthotriazoles with alkenes under rhodium(II) catalysis is reported. Various allyl dihydronaphthalene derivatives were furnished via rhodium(II) azavinyl carbenes with moderate to good yields and excellent chemoselectivity. When monosubstituted alkenes are used, cyclopropanation occurs and good to excellent enantioselectivities have been achieved. Particularly noteworthy is the allylic C(sp²)-H activation instead of traditional C(sp³)-H activation in the formal allylation process.

Selective carbene transfer to amines and olefins catalyzed by ruthenium phthalocyanine complexes with donor substituents

Electron-rich ruthenium phthalocyanine complexes were evaluated in carbene transfer reactions from ethyl diazoacetate (EDA) to aromatic and aliphatic olefins as well as to a wide range of aromatic, heterocyclic and aliphatic amines for the first time. It was revealed that the ruthenium octabutoxyphthalocyanine carbonyl complex [(BuO)8Pc]Ru(CO) is the most efficient catalyst converting electron-rich and electron-poor aromatic olefins to cyclopropane derivatives with high yields (typically 80-100%) and high TON (up to 1000) under low catalyst loading and nearly equimolar substrate/EDA ratio. This catalyst shows a rare efficiency in the carbene insertion into amine N-H bonds. Using a 0.05 mol% catalyst loading, a high amine concentration (1 M) and 1.1 eq. of EDA, a number of structurally divergent amines were selectively converted to mono-substituted glycine derivatives with up to quantitative yields and turnover numbers reaching 2000. High selectivity, large substrate scope, low catalyst loading and practical reaction conditions place [(BuO)8Pc]Ru(CO) among the most efficient catalysts for the carbene insertion into amines.

Computational Mapping of Dirhodium(II) Catalysts

The chemistry of dirhodium(II) catalysts is highly diverse, and can enable the synthesis of many different molecular classes. A tool to aid in catalyst selection, independent of mechanism and reactivity, would therefore be highly desirable. Here, we describe the development of a database for dirhodium(II) catalysts that is based on the principal component analysis of DFT-calculated parameters capturing their steric and electronic properties. This database maps the relevant catalyst space, and may facilitate exploration of the reactivity landscape for any process catalysed by dirhodium(II) complexes. We have shown that one of the principal components of these catalysts correlates with the outcome (e.g. yield, selectivity) of a transformation used in a molecular discovery project. Furthermore, we envisage that this approach will assist the selection of more effective catalyst screening sets, and, hence, the data-led optimisation of a wide range of rhodium-catalysed transformations.