Rhodium-Catalyzed Tandem Cyclopropanation/Cope Rearrangement of 4-Alkenyl-1-sulfonyl-1,2,3-triazoles with Dienes

Site-Selective C-H Allylation of Alkanes: Facile Access to Allylic Quaternary sp3 -Carbon Centers

The construction of allylic quaternary sp³ -carbon centers has long been a formidable challenge in transition-metal-catalyzed alkyl-allyl coupling reactions due to the severe steric hindrance. Herein, we report an effective carbene strategy that employs well-defined vinyl-N-triftosylhydrazones as a versatile allylating reagent to enable direct assembly of these medicinally desirable structural elements from low-cost alkane feedstocks. The reaction exhibited excellent site selectivity for tertiary C-H bonds, broad scope (>60 examples and >20 : 1:0 r. r.) and good efficiency, even on a gram-scale, making it a convenient alternative to the well-known Trost-Tsuji allylation reaction for the formation of alkyl-allyl bonds. Combined experimental and computational studies were employed to unravel the mechanism and origin of site- and chemoselectivity of the reaction.

Synthesis of Chiral Endocyclic Allenes by Palladium-Catalyzed Asymmetric Annulation Followed by Cope Rearrangement

Catalytic asymmetric synthesis of chiral endocyclic allenes remains a challenge in allene chemistry owing to unfavored tension and complex chirality. Here, we present a new relay strategy merging Pd-catalyzed asymmetric [3+2] annulation with enyne-Cope rearrangement, providing a facile route to chiral 9-membered endocyclic allenes with high efficiency and enantioselectivity. Moreover, theoretical calculations and experimental studies were performed to illustrate the critical, but unusual Cope rearrangement that allows for the complete central-to-axial chirality transfer.

Catalytic Functionalization of Metallocarbenes Derived from α-Diazocarbonyl Compounds and Their Precursors

Short and efficient synthesis of heterocyclic compounds are highly desirable in synthetic organic chemistry. It is a dream approach to accomplish these syntheses from readily available starting materials in a single step. In this personal account, we discuss our contribution in the synthesis of heterocyclic compounds and beyond from N-sulfonyl-1,2,3-triazoles and α-diazocarbonyl compounds, which are the precursors for α-imino (carbonyl) metal carbenes in the presence of transition metal catalysts. Functionalization of α-imino(carbonyl) metal carbenes has been achieved through in-situ generated metal-stabilized ylides followed by either intramolecular trapping by non-polar bonds, rearrangement, cycloaddition, or 1,3-insertion fashion, which led to the efficient synthesis of various synthetically important intermediates and heterocyclic compounds.

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.

Gold(I)-Catalyzed Desymmetrization of 1,4-Dienes by an Enantioselective Tandem Alkoxylation/Claisen Rearrangement

An enantioselective alkoxylation/Claisen rearrangement reaction was achieved by a strategic desymmetrization of 1,4-dienes under the catalysis of (S)-DTBM-Segphos(AuCl)2/AgBF4. This reaction system was highly selective for the formation of 3,3-rearrangement products, providing cycloheptenes with various substitutions in good yield and good to excellent enantioselectivity. This transformation was further extended to bicyclic ring substrates, providing the opportunity to easily assemble 5,6- and 6,7-fused ring systems.

Rhodium-catalyzed NH insertion of pyridyl carbenes derived from pyridotriazoles: a general and efficient approach to 2-picolylamines and imidazo[1,5-a]pyridines

A general and efficient NH insertion reaction of rhodium pyridyl carbenes derived from pyridotriazoles was developed. Various NH-containing compounds, including amides, anilines, enamines, and aliphatic amines, smoothly underwent the NH insertion reaction to afford 2-picolylamine derivatives. The developed transformation was further utilized in a facile one-pot synthesis of imidazo[1,5-a]pyridines.

Expedient synthesis of fused azepine derivatives using a sequential rhodium(II)-catalyzed cyclopropanation/1-aza-Cope rearrangement of dienyltriazoles

A general method for the formation of fused dihydroazepine derivatives from 1-sulfonyl-1,2,3-triazoles bearing a tethered diene is reported. The process involves an intramolecular cyclopropanation of an α-imino rhodium(II) carbenoid, leading to a transient 1-imino-2-vinylcyclopropane intermediate which rapidly undergoes a 1-aza-Cope rearrangement to generate fused dihydroazepine derivatives in moderate to excellent yields. The reaction proceeds with similar efficiency on gram scale. The use of catalyst-free conditions leads to the formation of a novel [4.4.0] bicyclic heterocycle.