Intramolecular Redox-Mannich Reactions: Facile Access to the Tetrahydroprotoberberine Core

Cyclic amines such as pyrrolidine undergo redox-annulations with 2-formylaryl malonates. Concurrent oxidative amine α-CH bond functionalization and reductive N-alkylation render this transformation redox-neutral. This redox-Mannich process provides regioisomers of classic Reinhoudt reaction products as an entry to the tetrahydroprotoberberine core, enabling the synthesis of (±)-thalictricavine and its epimer. An unusually mild amine-promoted dealkoxycarbonylation was discovered in the course of these studies.

Metal-Free Oxidative C-C Bond Formation through C-H Bond Functionalization

The formation of C-C bonds embodies the core of organic chemistry because of its fundamental application in generation of molecular diversity and complexity. C-C bond-forming reactions are well-known challenges. To achieve this goal through direct functionalization of C-H bonds in both of the coupling partners represents the state-of-the-art in organic synthesis. Oxidative C-C bond formation obviates the need for prefunctionalization of both substrates. This Minireview is dedicated to the field of C-C bond-forming reactions through direct C-H bond functionalization under completely metal-free oxidative conditions. Selected important developments in this area have been summarized with representative examples and discussions on their reaction mechanisms.

C-H alkenylations with alkenyl acetates, phosphates, carbonates, and carbamates by cobalt catalysis at 23 °C

Inexpensive cobalt catalysts with N-heterocyclic carbene ligands enable direct arene alkenylations with easily accessible alkenyl acetates through regioselective C-H/C-O functionalizations in a stereoconvergent fashion. The versatile cobalt catalyst was broadly applicable and thus also allowed for the efficient conversion of alkenyl phosphates, carbonates, and carbamates at ambient temperature.

Ruthenium(II)-catalyzed C-H activation/alkyne annulation by weak coordination with O2 as the sole oxidant

Aerobic oxidative CH functionalizations of weakly coordinating benzoic acids have been accomplished with versatile ruthenium(II) biscarboxylates under ambient oxygen or air. Mechanistic studies identified the key factors controlling the elementary step of the oxidation of the ruthenium(0) complex.