Direct Access to Cobaltacycles via C–H Activation: N-Chloroamide-Enabled Room-Temperature Synthesis of Heterocycles

Spiro[indene-1,4'-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4-Phenyl-1,3-oxazol-2(3H)-ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C-H activation synthesis of heterocyclic spiro[4,4]nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing C═C bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro[indene-1,4'-oxa-zolidinones] synthesis via Rh(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity.

Stereoselective β-F Elimination Enabled Redox-Neutral [4 + 1] Annulation via Rh(III)-Catalyzed C-H Activation: Access to Z-Monofluoroalkenyl Dihydrobenzo[ d]isoxazole Framework

An efficient and practical Rh(III)-catalyzed redox-neutral [4 + 1] annulation of N-phenoxy amides with α, α-difluoromethylene alkynes has been realized to give direct access to the Z-configured monofluoroalkenyl dihydrobenzo[ d]isoxazole framework with broad substrate compatibility and good functional group tolerance, which was further enhanced by the late-stage C-H modification of complex bioactive compounds. Subsequent density functional theory calculations revealed that the stereoselective β-F elimination involving an allene species played a decisive role in determining the reaction outcome and such Z-selectivity.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Recent Advances on Mechanistic Studies on C–H Activation Catalyzed by Base Metals

During the last ten years, base metals have become very attractive to the organometallic and catalytic community on activation of C-H bonds for their catalytic functionalization. In contrast to the statement that base metals differ on their mode of action most of the manuscripts mistakenly rely on well-studied mechanisms for precious metals while proposing plausible mechanisms. Consequently, few literature examples are found where a thorough mechanistic investigation have been conducted with strong support either by theoretical calculations or experimentation. Therefore, we consider of highly scientific interest reviewing the last advances on mechanistic studies on Fe, Co and Mn on C-H functionalization in order to get a deep insight on how these systems could be handle to either enhance their catalytic activity or to study their own systems in a similar systematic fashion. Thus, in this review we try to cover the most insightful articles for mechanistic studies on C-H activation catalyzed by Fe, Co and Mn based on kinetic and competition experiments, stoichiometric reactions, isolation of intermediates and theoretical calculations.

RhIII-Catalyzed Synthesis of Isoquinolones and 2-Pyridones via Annulation of N-Methoxyamides and Nitroalkenes

The Rh(III)-catalyzed synthesis of 4-substituted isoquinolones and 2-pyridones by the annulation of N-methoxyamides and nitroalkenes has been developed. Both aliphatic and aromatic nitroalkenes were effective inputs. Annulations also proceeded for aromatic, alkenyl, and heteroaromatic C-H bond starting materials. Moreover, benzoic acid provided a novel nitrodihydroisocoumarin. The structure and relative stereochemistry of this compound, which is an oil at room temperature, was determined unambiguously by single crystal X-ray diffraction of its inclusion complex with a hydrogen-bonded host framework.

Mechanistic insights into the SN2-type reactivity of aryl-Co(iii) masked-carbenes for C-C bond forming transformations

Herein we describe the synthesis and characterization of a family of C-metalated aryl-Co(iii) enolates, which can be considered as masked-carbenes, using diazoacetates as coupling partners. These species have been proved to be necessary intermediates in the C(sp2)-C(sp3) bond forming event to obtain cyclic amides, taming the elusive Co(iii)-carbene species. The scope of diazoacetates has been exhaustively examined, varying the nature of the ester and the α-substitution, and a clear preference for electron-poor carbene precursors is observed. Exhaustive experimental and theoretical studies indicate that an unprecedented intramolecular SN2-type process governs the formation of the newly formed C-C bond. Furthermore, the key role of several Lewis acids as carboxylate-activating reagents is further explored by DFT calculations.

Impact of the Valence Charge of Transition Metals on the Cobalt- and Rhodium-Catalyzed Synthesis of Indenamines, Indenols, and Isoquinolinium Salts: A Catalytic Cycle Involving MIII/MV [M = Co, Rh] for [4 + 2] Annulation

Reaction mechanisms for the synthesis of indenamines, indenols, and isoquinolinium salts through cobalt- and rhodium-catalysis were investigated using density functional theory calculations. We found that the valence charge of transition metals dramatically influences the reaction pathways. Catalytic reactions involving lower-oxidation-state transition metals (M^I/M^III, M = Co and Rh) generally favor a [3 + 2] cyclization pathway, whereas those involving higher oxidation states (M^III/M^V) proceed through a [4 + 2] cyclization pathway. A catalytic cycle with novel M^III/M^V as a crucial species was successfully revealed for isoquinolinium salts synthesis, in which highly valent M^V was encountered not only in the [RhCp*]-catalysis but also in the [CoCp*]-catalysis.

Synthesis of 2,5-disubstituted oxazoles via cobalt(iii)-catalyzed cross-coupling of N-pivaloyloxyamides and alkynes

An efficient synthesis of 2,5-disubstituted oxazoles via cobalt(iii)-catalyzed cross-coupling of N-pivaloyloxyamides and alkynes is described herein.

Co(iii)-Catalyzed N-chloroamide-directed C–H activation for 3,4-dihydroisoquinolone synthesis

The first time synthesis of 3,4-dihydroisoquinolones via Co(iii)-catalyzed C–H activation at room temperature is described herein.