Intermolecular C–H functionalization versus cyclopropanation of electron rich 1,1-disubstituted and trisubstituted alkenes

Electrochemical Asymmetric Radical Functionalization of Aldehydes Enabled by a Redox Shuttle

Aminocatalysis is a well-established tool that enables the production of enantioenriched compounds under mild conditions. Its versatility is underscored by its seamless integration with various synthetic approaches. While the combination of aminocatalysis with metal catalysis, photochemistry, and stoichiometric oxidants has been extensively explored, its synergy with electrochemical activation remains largely unexplored. Herein, we present the successful merger of electrochemistry and aminocatalysis to perform SOMO-type transformations, expanding the toolkit for asymmetric electrochemical synthesis. The methodology harnesses electricity to drive the oxidation of catalytically generated enamines, which ultimately partake in enantioselective radical processes, leading to α-alkylated aldehydes. Crucially, mechanistic studies highlight how this electrochemical strategy is enabled by the use of a redox shuttle, 4,4'-dimethoxybiphenyl, to prevent catalyst degradation and furnishing the coveted compounds in good yield and high enantioselectivity.

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.

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.

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 selective mono- and difluoroalkylation using fluorinated silyl enol ethers

This feature article highlights our recent achievements in catalytic selective mono- and difluoroalkylation using fluorinated silyl enol ethers (FSEEs).

Rhodium-Catalyzed Intermolecular C-H Functionalization as a Key Step in the Synthesis of Complex Stereodefined β-Arylpyrrolidines

The synthesis of β-arylpyrrolidines via a catalytic enantioselective intermolecular allylic C(sp)3-H functionalization of trans-alkenes followed by immediate reduction, ozonolysis, and then in situ diversification of the resulting cyclic hemiaminal to furnish highly substituted, stereoenriched β-arylpyrrolidines is reported. This methodology utilizes 4-aryl-1-sulfonyl-1,2,3-triazoles as carbene precursors and the dirhodium tetracarboxylate catalyst Rh2( S-NTTL)4. A variety of β-arylpyrrolidines were prepared in good yields with high levels of diastereo- and enantioselectivity over four linear steps, requiring only a single purification procedure.

A computational mechanistic study of substrate-controlled competitive O–H and C–H insertion reactions catalyzed by dirhodium(ii) carbenoids: insight into the origin of chemoselectivity

DFT calculations disclosed the chemoselectivity of rhodium carbenoid and water co-catalyzed O–H and C–H insertion reactions with three 1,3-diketone substrates.

Highly Stereoselective Gold-Catalyzed Coupling of Diazo Reagents and Fluorinated Enol Silyl Ethers to Tetrasubstituted Alkenes

We report a highly stereoselective synthesis of all-carbon or fluorinated tetrasubstituted alkenes from diazo reagents and fluorinated enol silyl ethers, using C-F bond as a synthetic handle. Cationic Au^I catalysis plays a key role in this reaction. Remarkable fluorine effects on the reactivity and selectivity was also observed.

Using IR vibrations to quantitatively describe and predict site-selectivity in multivariate Rh-catalyzed C-H functionalization

Achieving selective C-H functionalization is a significant challenge that requires discrimination between many similar C-H bonds. Yet, reaction systems employing Rh2(DOSP)4 and Rh2(BPCP)4 were recently demonstrated to afford high levels of selectivity in the C-H insertion of carbenes into toluene-derived substrates. Herein, we explore the origin of this selectivity through a systematic analysis of substrate and reagent features that alter levels of selectivity from 20 : 1 to 1 : 610 for secondary (or tertiary)-to-primary benzylic C-H functionalization of toluene derivatives. Describing this variation using infrared vibrations and point charges, we have developed a mathematical model from which are identified features of the systems that determine levels of site-selectivity and are applied as predictive factors to describe the selectivity behavior of new substrate/reagent combinations.

Catalytic asymmetric synthesis of polysubstituted spirocyclopropyl oxindoles: organocatalysis versus transition metal catalysis

Recent progress in catalytic asymmetric synthesis of spirocyclopropyl oxindoles via organocatalysis and transition metal catalysis are summarized and discussed.

Role of sterically demanding chiral dirhodium catalysts in site-selective C-H functionalization of activated primary C-H bonds

The influence of sterically demanding dirhodium tetracarboxylate catalysts on the site selectivity of C-H functionalization by means of rhodium carbene-induced C-H insertion is described. The established dirhodium tetraprolinate-catalyzed reactions of aryldiazoacetates cause preferential C-H functionalization of secondary C-H bonds as a result of competing steric and electronic effects. The sterically more demanding dirhodium tetrakis(triarylcyclopropanecarboxylate) catalysts, exemplified by dirhodium tetrakis[(R)-(1-(biphenyl)-2,2-diphenylcyclopropanecarboxylate)] [Rh2(R-BPCP)4], favor C-H functionalization of activated primary C-H bonds. Highly site-selective and enantioselective C-H functionalization of a variety of simple substrates containing primary benzylic, allylic, and methoxy C-H bonds was achieved with this catalyst. The utility of this approach has been demonstrated by the late-stage primary C-H functionalization of (-)-α-cedrene and a steroid.

Guide to Enantioselective Dirhodium(II)-Catalyzed Cyclopropanation with Aryldiazoacetates

Catalytic enantioselective methods for the generation of cyclopropanes has been of longstanding pharmaceutical interest. Chiral dirhodium(II) catalysts prove to be an effective means for the generation of diverse cyclopropane libraries. Rh₂(R-DOSP)₄ is generaally 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.

Highly stereoselective olefin cyclopropanation of diazooxindoles catalyzed by a C2-symmetric spiroketal bisphosphine/Au(I) complex

A spiroketal bisphosphine (SKP) derived chiral digold complex is identified as a powerful catalyst for the highly diastereo- and enantioselective synthesis of spirocyclopropyloxindoles from diazooxindoles and a broad range of alkenes, including both cis and trans 1,2-disubstituted alkenes.

The combined C-H functionalization/Cope rearrangement: discovery and applications in organic synthesis

The development of methods for the stereoselective functionalization of sp(3) C-H bonds is a challenging undertaking. This Account describes the scope of the combined C-H functionalization/Cope rearrangement (CHCR), a reaction that occurs between rhodium-stabilized vinylcarbenoids and substrates containing allylic C-H bonds. Computational studies have shown that the CHCR reaction is initiated by a hydride transfer to the carbenoid from an allyl site on the substrate, which is then rapidly followed by C-C bond formation between the developing rhodium-bound allyl anion and the allyl cation. In principle, the reaction can proceed through four distinct orientations of the vinylcarbenoid and the approaching substrate. The early examples of the CHCR reaction were all highly diastereoselective, consistent with a reaction proceeding via a chair transition state with the vinylcarbenoid adopting an s-cis conformation. Recent computational studies have revealed that other transition state orientations are energetically accessible, and these results have guided the development of highly stereoselective CHCR reactions that proceed through a boat transition state with the vinylcarbenoid in an s-cis configuration. The CHCR reaction has broad applications in organic synthesis. In some new protocols, the CHCR reaction acts as a surrogate to some of the classic synthetic strategies in organic chemistry. The CHCR reaction has served as a synthetic equivalent of the Michael reaction, the vinylogous Mukaiyama aldol reaction, the tandem Claisen rearrangement/Cope rearrangement, and the tandem aldol reaction/siloxy-Cope rearrangement. In all of these cases, the products are generated with very high diastereocontrol. With a chiral dirhodium tetracarboxylate catalyst such as Rh(2)(S-DOSP)(4) or Rh(2)(S-PTAD)(4), researchers can achieve very high levels of asymmetric induction. Applications of the CHCR reaction include the effective enantiodifferentiation of racemic dihydronaphthalenes and the total synthesis of several natural products: (-)-colombiasin A, (-)-elisapterosin B, and (+)-erogorgiaene. By combining the CHCR reaction into a further cascade sequence, we and other researchers have achieved the asymmetric synthesis of 4-substituted indoles, a new class of monoamine reuptake inhibitors.

Thermally induced cycloadditions of donor/acceptor carbenes

The thermal decomposition of aryldiazoacetates and aryldiazoketones in the absence of a catalyst leads to synthetically useful transformations. The thermal reaction of aryldiazoacetates with alkenes generates cyclopropanes in 68-97% yield and with good diastereoselectivity (up to 19:1 dr) when the aryl substituent is electron-rich. The thermal reaction of aryldiazoketones with alkenes generated cyclobutanones in 71-94% yield and with good diastereocontrol (≥9:1 dr).

Controlling factors for C-H functionalization versus cyclopropanation of dihydronaphthalenes

Rhodium(II)-catalyzed reactions of vinyldiazoacetates with dihydronaphthalenes were systematically studied. These substrates underwent cyclopropanantion and/or the combined C-H activation/Cope rearrangement in good overall yield and with good diastereo- and enantiocontrol. The selectivity of these reactions was profoundly influenced by the nature of the chiral catalyst, the vinyldiazoacetate, and the dihydronaphthalene. The best combinations for achieving the highest selectivity in the cyclopropanation and the combined C-H activation/Cope rearrangement of 1,2-dihydronaphthalenes are methyl 2-diazopent-3-enoate (2a)/Rh(2)(S-DOSP)(4) and methyl 3-(tert-butyldimethylsilyloxy)-2-diazopent-3-enoate (2b)/Rh(2)(S-PTAD)(4). These combinations are very effective at enantiodivergent reactions of 1-methyl-1,2-dihydronaphthalenes.

Enantioselective C-C bond formation by rhodium-catalyzed tandem ylide formation/[2,3]-sigmatropic rearrangement between donor/acceptor carbenoids and allylic alcohols

The rhodium-catalyzed reaction of racemic allyl alcohols with methyl phenyldiazoacetate or methyl styryldiazoacetate results in a two-step process, an initial oxonium ylide formation followed by a [2,3]-sigmatropic rearrangement. This process competes favorably with the more conventional O-H insertion chemistry as long as donor/acceptor carbenoids and highly substituted allyl alcohols are used as substrates. When the reactions are catalyzed by Rh(2)(S-DOSP)(4), tertiary alpha-hydroxycarboxylate derivatives with two adjacent quaternary centers are produced with high enantioselectivity (85-98% ee).

Asymmetric synthesis of neolignans (-)-epi-conocarpan and (+)-conocarpan via Rh(II)-catalyzed C-H insertion process and revision of the absolute configuration of (-)-epi-conocarpan

Catalytic asymmetric synthesis of neolignan natural products (-)-epi-conocarpan and (+)-conocarpan has been achieved by exploiting an enantio- and diastereoselective intramolecular C-H insertion reaction to construct a cis-2-aryl-2,3-dihydrobenzofuran ring system as a key step. The C-H insertion reaction of 5-bromoaryldiazoacetate catalyzed by Rh(2)(S-PTTEA)(4), a new dirhodium(II) carboxylate complex that incorporates N-phthaloyl-(S)-triethylalaninate as chiral bridging ligands, provided 2-aryl-5-bromo-3-methoxycarbonyl-2,3-dihydrobenzofuran with exceptionally high diastereoselectivity (cis/trans = 97:3) and high enantioselectivity for the cis isomer (84% ee).