Cobalt(III)-Catalyzed Redox-Neutral Synthesis of Unprotected Indoles Featuring an N−N Bond Cleavage

Subba Reddy B. Ru(II)-Catalyzed Oxidative Functionalization of Arylhydrazine-1,2-dicarboxylates with Internal Alkynes for the Synthesis of Enecarbamates

A novel method has been developed for the synthesis of highly substituted enecarbamates and trisubstituted alkenes from arylhydrazine-1,2-dicarboxylates and 1,2-disubstituted alkynes through a sequential C-C and C-N bond formation. It is entirely a new strategy to produce enecarbamates in a highly regio- and stereoselective manner.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Rhodium-Catalyzed [4 + 1] Cyclization via C-H Activation for the Synthesis of Divergent Heterocycles Bearing a Quaternary Carbon

The development of an efficient approach to construct fused polycyclic systems bearing a quaternary carbon center represents a great challenge to synthetic chemistry. Herein, we report a Rh(III)-catalyzed [4 + 1] annulation of propargyl alcohols with various heterocyclic scaffolds under an air atmosphere. Diverse fused heterocycles containing a quaternary carbon center were obtained in moderate to good yields. Additionally, this method features a high atom-economy, metal oxidant free, simple operation, and compatibility with various functional groups.

Computational Study on N-N Homolytic Bond Dissociation Enthalpies of Hydrazine Derivatives

The hydrazine derivatives have been regarded as the important building blocks in organic chemistry for the synthesis of organic N-containing compounds. It is important to understand the structure-activity relationship of the thermodynamics of N-N bonds, in particular, their strength as measured by using the homolytic bond dissociation enthalpies (BDEs). We calculated the N-N BDEs of 13 organonitrogen compounds by eight composite high-level ab initio methods including G3, G3B3, G4, G4MP2, CBS-QB3, ROCBS-QB3, CBS-Q, and CBS-APNO. Then 25 density functional theory (DFT) methods were selected for calculating the N-N BDEs of 58 organonitrogen compounds. The M05-2X method can provide the most accurate results with the smallest root-mean-square error (RMSE) of 8.9 kJ/mol. Subsequently, the N-N BDE predictions of different hydrazine derivatives including cycloalkylhydrazines, N-heterocyclic hydrazines, arylhydrazines, and hydrazides as well as the substituent effects were investigated in detail by using the M05-2X method. In addition, the analysis including the natural bond orbital (NBO) as well as the energies of frontier orbitals were performed in order to further understand the essence of the N-N BDE change patterns.

C-H functionalization by high-valent Cp*Co(iii) catalysis

Significant progress has been accomplished in directed C-H functionalization through the use of earth-abundant and inexpensive first-row transition metals. Among these base metals, Co is especially attractive in view of its versatile applications in C-H functionalization, in both low- and high-valent states. In this vein, catalytic Co(iii) species can be generated from the dissociation of a Cp*Co(iii) catalyst or through the oxidation of a low-valent cobalt catalyst in the presence of an oxidant. In this feature article, we will discuss the breakthroughs in Cp*Co(iii)-promoted C-H functionalization. In this field, C(sp²)-H functionalization has been extensively studied and developed. In contrast, few C(sp³)-H functionalization reactions have been reported.

Oxidative ring-opening of 3-aminoindazoles for the synthesis of 2-aminobenzoates

A novel oxidative ring-opening of 3-aminoindazoles based on N–N bond cleavage was developed, in which various 2-aminobenzoates were obtained in good yields under mild conditions.

Cp*Co(iii)-Catalyzed oxidative [5+2] annulation: regioselective synthesis of 2-aminobenzoxepines via C–H/O–H functionalization of 2-vinylphenols with ynamides

A Cp*Co(iii)-catalyzed [5+2] C–H annulation reaction of 2-vinylphenols with ynamides toward the regioselective synthesis of 2-aminobenzoxepines has been reported.

Cobalt-catalyzed C–H activation: recent progress in heterocyclic chemistry

Cobalt-catalyzed C–H activation has gone through some major advancements in the past couple of decades.

Co(III)-Catalyzed C-H Activation: A Site-Selective Conjugate Addition of Maleimide to Indole at the C-2 Position

The cobalt(III)-catalyzed site-selective C-2 functionalization of indole has been developed using the pyrimidinyl group as a directing group. This reaction furnishes 3-arylated succinimide derivatives in excellent yields in a shorter duration using an inexpensive Co catalyst. Highly selective C-2 functionalization of indoles was achieved in the presence of the highly reactive C-3 position. This protocol is compatible with a variety of N-pyrimidinyl indole and maleimide derivatives, and it can be easily scaled-up. This method is also applicable for maleic ester and heteroarenes.

Cp*Co(III)-Catalyzed Annulation of Carboxylic Acids with Alkynes

A new procedure for oxidative coupling of aromatic and acrylic acids with alkynes has been developed using abundant, nontoxic, and air stable Cp*Co(III) catalyst. The coupling involves initial cyclometalation via weak chelation-assisted C-H bond activation followed by alkyne coordination, insertion, and reductive elimination leading to diverse isocoumarins (α-pyranones) in good yields under mild conditions.

Cobalt-catalysed C-H carbonylative cyclisation of aliphatic amides

A cobalt-catalysed C-H carbonylation of aliphatic carboxamide derivatives is described, employing commercially available Co(ii)-salts in the presence of a silver oxidant. This operationally simple process utilises an atmospheric pressure of CO and generates a range of substituted succinimide products bearing diverse functional groups that can be successfully accessed via this methodology.

Cobalt catalyzed carbonylation of unactivated C(sp3)-H bonds

A general efficient regioselective cobalt catalyzed carbonylation of unactivated C(sp3)-H bonds of aliphatic amides was demonstrated using atmospheric (1-2 atm) carbon monoxide as a C1 source. This straightforward approach provides access to α-spiral succinimide regioselectively in a good yield. Cobalt catalyzed sp3 C-H bond carbonylation is reported for the first time including the functionalization of (β)-C-H bonds of α-1°, 2°, 3° carbons and even internal (β)-C-H bonds. Our initial mechanistic investigation reveals that the C-H activation step is irreversible and will possibly be the rate determining step.

One-pot synthesis of 2,3-difunctionalized indoles via Rh(iii)-catalyzed carbenoid insertion C–H activation/cyclization

A new and versatile Rh(iii)-catalyzed carbenoid insertion C–H activation/cyclization for one-pot synthesis of 2,3-difunctionalized indoles has been developed.