Achieving Site Selectivity in Metal-Catalyzed Electron-Rich Carbene Transfer Reactions from N-Tosylhydrazones

Rh(II)-Catalyzed Si-H Insertion with Nosyl-hydrazone-Protected Aryl Donor Diazo Compounds

Dirhodium(II,II) paddlewheel catalysts were evaluated in silyl-hydrogen insertion reactions of aryl diazo compounds generated from o-nosyl hydrazones. The high reactivity of aryl diazo compounds necessitates their in situ generation from sulfonyl-protected hydrazones. Herein, we describe our efforts to evaluate this transformation utilizing Rh(II) catalysts, including those with tethered, axially coordinating ligands. The heteroleptic catalyst, Rh2(OAc)3(2-OX), provided the highest yield of silanes when dioxane was the solvent.

De Novo Diastereoselective Synthesis of 1-Hydroxyl Allogibberic Methyl Ester en Route to Diverse Bioactive Molecules

The first de novo synthesis of 1-hydroxyl allogibberic methyl ester, en route to pharbinilic acid and other bioactive molecules, is accomplished in diastereoselective manner. Key reactions of the synthesis include a Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, a Lewis acid-catalyzed reductive Prins cyclization reaction, and a SmI₂-mediated transannular pinacol coupling reaction. The synthesis provides a new avenue to access diverse relevant bioactive molecules.

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.

Transition Metal Catalyzed Insertion Reactions with Donor/Donor Carbenes

Donor/donor carbenes are relatively new in the field of carbene chemistry; although applications in C-H and X-H insertion reactions are few in number, they demonstrate exquisite chemo- and stereo-selectivity. Recent reports have shown that C-H, N-H, B-H, O-H, S-H, Si-H, Ge-H, Sn-H and P-H insertion reactions are feasible with a variety of transition metal catalysts, both inter- and intramolecularly. Furthermore, high degrees of diastereo- and enantioselectivity have been observed in several cases. Methods typically involve the formation of a diazo-based carbene precursor, but procedures using diazo-free metal carbenes have been developed with significant success. This Minireview covers transition-metal catalyzed insertion reactions with donor/donor and donor carbenes, providing context for future developments in this emerging field.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Controlling the Selectivity Patterns of Au-Catalyzed Cyclization-Migration Reactions

As little as 2 mol % of (XPhos)AuNTf₂ catalyzes the transformation of a broad range of o-acetylene-substituted styrenes into 1,2-dihydronaphthalenes. Our data suggests that this transformation occurs via a gold-stabilized cyclopropyl carbinyl cation, which triggers either a [1,2] carboxylate shift or a less favorable [1,2] aryl shift. The relative rates of these migrations can be controlled by the identity of the ligand or by stabilizing the mesomeric cation.

Computational insights into different chemoselectivities in Rh2(ii)-catalyzed N-aryl nitrene and analogous Rh2(ii)/Cu(i)-catalyzed aryl-substituted carbene involving reactions

Computational studies were carried out to investigate the mechanisms and the origins of chemoselectivities among multiple reactive sites for Rh₂(ii)-catalyzed N-aryl nitrene and analogous Rh₂(ii)/Cu(i)-catalyzed aryl carbene involving reactions.

Rhodium-Catalyzed B-H Bond Insertion Reactions of Unstabilized Diazo Compounds Generated in Situ from Tosylhydrazones

Although transition-metal-catalyzed B-H bond insertion of carbenes into stable borane adducts has emerged as a promising method for organoborane synthesis, all the diazo compounds used to date as carbene precursors have had an electron-withdrawing group to stabilize them. Herein, we report a protocol for rhodium-catalyzed B-H bond insertion reactions of unstabilized diazo compounds generated in situ from tosylhydrazones. In addition, by using chiral dirhodium catalysts, we also achieved an asymmetric version of the reaction with good to excellent enantioselectivities (up to 98:2 e.r.). This is the first enantioselective heteroatom-hydrogen bond insertion reaction to use unstabilized diazo compounds as carbene precursors. The protocol exhibited good functional group tolerance and could be carried out on a gram scale. It also enabled one-pot transformation of a carbonyl group to a boryl group enantioselectively. The B-H bond insertion products could be easily transformed into chiral alcohols and other widely used organoboron reagents with enantiomeric fidelity.

Enantioselective Radical Cyclization for Construction of 5-Membered Ring Structures by Metalloradical C-H Alkylation

Radical cyclization represents a powerful strategy for construction of ring structures. Traditional radical cyclization, which is based on radical addition as the key step, necessitates the use of unsaturated substrates. Guided by the concept of metalloradical catalysis, a different mode of radical cyclization that can employ saturated C-H substrates is demonstrated through the development of a Co(II)-based system for catalytic activation of aliphatic diazo compounds for enantioselective radical alkylation of various C(sp3)-H bonds. It allows for efficient construction of chiral pyrrolidines and other valuable 5-membered cyclic compounds. This alternative strategy of radical cyclization provides a new retrosynthetic paradigm to prepare five-membered cyclic molecules from readily available open-chain aldehydes through the union of C-H and C=O elements for C-C bond formation.