Rhodium(III)-Catalyzed Transannulation of Cyclopropenes with N -Phenoxyacetamides through CH Activation

Insights into the Activation Mode of α-Carbonyl Sulfoxonium Ylides in Rhodium-Catalyzed C-H Activation: A Theoretical Study

A computational study has been performed to investigate the mechanism of RhIII -catalyzed C-H bond activation using sulfoxonium ylides as a carbene precursor. The stepwise and concerted activation modes for sulfoxonium ylides were investigated. Detailed theoretical results showed that the favored stepwise pathway involves C-H bond activation, carbonization, carbene insertion, and protonation. The free energy profiles for dialkylation of 2-phenylpyridine were also calculated to account for the low yield of this reaction. Furthermore, the substituent effect was elucidated by comparing the energy barriers for the protonation of meta- and para-substituted sulfoxonium ylides calculated by density functional theory.

Catalytic Synthesis of Trifluoromethyl Cyclopropenes and Oligo-Cyclopropenes

The synthesis of trifluoromethylated cyclopropenes is often associated with important applications in drug discovery and functional materials. In this report, we describe the use of readily available chiral rhodium(II) catalysts for a highly efficient asymmetric cyclopropenation reaction of fluorinated donor–acceptor diazoalkanes with a broad variety of aliphatic and aromatic alkynes. Further studies highlight the unique reactivity of fluorinated donor–acceptor diazoalkanes in the synthesis of oligo‐cyclopropenes. Subsequent C−H functionalization of trifluoromethyl cyclopropenes furnishes densely substituted cyclopropene frameworks and also allows the alternative synthesis of bis‐cyclopropenes.

Reaching Green: Heterocycle Synthesis by Transition Metal-Catalyzed C-H Functionalization in Sustainable Medium

Catalytic C-H functionalization has emerged as an efficient alternative to traditional coupling reactions. However, some of these reactions depend on environmentally harmful solvents, weakening the overall green nature of these methods. As organic processes consume large amount of solvents, the use of less harmful solvents enhance the sustainability of these reactions. Herein, we present an overview of transition metal-catalyzed C-H functionalization reactions for the synthesis of heterocycles in sustainable solvents based on CHEM21 solvent selection guide.

Pd-catalyzed Oxidative Cross-coupling of Alkyl Chromium(0) Fischer Carbene Complexes with Organoboronic Acids

Alkyl chromium(0) carbene complexes have been explored as the cross-coupling partners in the palladium-catalyzed reaction with aryl or alkenyl boronic acids. This coupling reaction displays the versatile reactivities of alkyl chromium(0) carbenes under palladium catalysis. Mechanistically, this transformation is proposed to involve deprotonation of the alkyl chromium carbene substrate to generate a vinyl chromium anion intermediate that undergoes transmetalation to organopalladium species and reductive elimination.

Cross-Coupling of α-Carbonyl Sulfoxonium Ylides with C-H Bonds

The functionalization of carbon-hydrogen bonds in non-nucleophilic substrates using α-carbonyl sulfoxonium ylides has not been so far investigated, despite the potential safety advantages that such reagents would provide over either diazo compounds or their in situ precursors. Described herein are the cross-coupling reactions of sulfoxonium ylides with C(sp² )-H bonds of arenes and heteroarenes in the presence of a rhodium catalyst. The reaction proceeds by a succession of C-H activation, migratory insertion of the ylide into the carbon-metal bond, and protodemetalation, the last step being turnover-limiting. The method is applied to the synthesis of benz[c]acridines when allied to an iridium-catalyzed dehydrative cyclization.

Fused Multifunctionalized Chromenes from Tetraynes and α,β-Unsaturated Aldehydes

The synthesis of fused, highly substituted chromenes in a one-pot-three-step cascade hexadehydro-Diels-Alder reaction of tetraynes followed by an intermolecular α,β-unsaturated aldehyde rearrangement is reported. The target compounds were prepared by the condensation of malonates with 3-bromo-1-propyne, and the resulting 2,2-di(1-propyn-3-yl)malonates underwent cross-coupling with phenylethynyl bromides to afford 2,2-di(5-phenyl-2,4-pentadiynyl)malonates, which underwent intramolecular cyclization to produce tricyclic chromenes. The overall transformation involves the formation of four new C-C bonds and one new C-aryl-O-C-aryl bond by both intramolecular cyclization and intermolecular rearrangement reactions. These constitute a key strategy for the construction of a highly substituted natural chromene core and a robust method for producing other chromenes.

Redox-Neutral Rhodium-Catalyzed [4+1] Annulation through Formal Dehydrogenative Vinylidene Insertion

A synthetic protocol for the expedient construction of 5-methylene-1H-pyrrol-2(5H)-one derivatives through rhodium-catalyzed [4+1] annulation with gem-difluoroacrylate as the C₁ component was reported. By taking advantage of the twofold C-F bond cleavage occurring during the annulation, this reaction not only allows the synthesis of these heterocyclic compounds under overall oxidant-free conditions but also renders the transformation stereospecific. The very mild reaction conditions employed ensure compatibility with a wide variety of synthetically useful functional groups.

Cationic Cobalt(III) Catalyzed Indole Synthesis: The Regioselective Intermolecular Cyclization of N-Nitrosoanilines and Alkynes

The unique regioselectivity and reactivity of cobalt(III) in the direct cyclization of N-nitrosoanilines with alkynes for the expedient synthesis of N-substituted indoles is demonstrated. In the presence of a cobalt(III) catalyst, high regioselectivity was observed when using unsymmetrical meta-substituted N-nitrosoanilines. Moreover, internal alkynes bearing electron-deficient groups, which are almost unreactive in the [Cp*Rh(III)]-catalyzed system, display good reactivity in this transformation.

Enantioselective Rhodium(I) Donor Carbenoid-Mediated Cascade Triggered by a Base-Free Decomposition of Arylsulfonyl Hydrazones

The reaction of diyne arylsulfonyl hydrazone substrates under rhodium(I)/BINAP catalysis gives access to sulfonated azacyclic frameworks in a highly enantioselective manner. This new cascade process considerably increases the molecular complexity by generating two C-C bonds, one C-S bond, and one C-H bond. Theoretical calculations, competitive experiments, and deuterium labeling have jointly been used to propose a mechanism that accounts for the reaction. The mechanism involves the formation of vinyl rhodium carbenoids, hydride migratory insertion, and intermolecular stereoselective nucleophilic attack. The last two steps are the key to the stereoselectivity of the process.