Mechanistic Insights on Orthogonal Selectivity in Heterocycle Synthesis

Synthesis of unsymmetrical ketones via dual catalysed cross-coupling of α,β-unsaturated carboxylic acids with aryldiazonium salts

A visible light-enabled synthesis of unsymmetrical ketones has been accomplished by the cross-coupling of α,β-unsaturated carboxylic acids and aryldiazonium salts embracing a synergistic eosin Y and Co(OAc)2·4H2O catalysis. The reaction involves decarboxylative aerobic CC bond cleavage, and is endowed with the creation of new C-C and C-O bonds with good substrate scope.

Chemodivergent Photocatalyzed Heterocyclization of Hydrazones and Isothiocyanates for the Selectivity Synthesis of 2-Amino-1,3,4-thiadiazoles and 1,2,4-Triazole-3-thiones

A photocatalytic chemodivergent reaction for the selectivity formation of C-S and C-N bonds in a controlled manner was proposed. The reaction medium, either neutral or acidic, is critical to dictate the formation of 2-amino-1,3,4-thiadiazoles and 1,2,4-triazole-3-thiones from isothiocyanates and hydrazones. This is a practical protocol to achieve the chemoselectivity under mild and metal-free conditions.

Free Amine, Hydroxyl and Sulfhydryl Directed C-H Functionalization and Annulation: Application to Heterocycle Synthesis

Transition metal-catalyzed direct C-H bond functionalization is recognized as an efficient strategy to assemble heterocyclic frameworks. For this purpose, directing groups (DGs) installation on an organic molecule has remained a widely exploited strategy for the years. The installation of directing groups, especially for the amine, alcohol and thiol containing reactants and their removal after the reaction need additional steps. In this regard, the use of free amine, hydroxyl and sulfhydryl as directing groups in native form is advantageous and in recent times, these transformations have stirred undisputable advancements for applications to heterocycle synthesis. In this review, the aromatic sp² -C attached free amine, hydroxyl and sulfhydryl as native functionalities are shown to be useful for the construction of five to seven-membered N-, O- and S-heterocycles.

Prediction of NHC-catalyzed chemoselective functionalizations of carbonyl compounds: a general mechanistic map

Generally, N-heterocyclic carbene (NHC) complexed with carbonyl compounds would transform into several important active intermediates, i.e., enolates, Breslow intermediates, or acylazolium intermediates, which act as either a nucleophile (Nu) or an electrophile (E) to react with the other E/Nu partner. Hence, the key to predicting the origin of chemoselectivity is to compute the activity (i.e., electrophilic index ω for E and nucleophilic index N for Nu) and stability of the intermediates and products, which are suggested in a general mechanistic map of these reactions. To support this point, we selected and studied different cases of the NHC-catalyzed reactions of carbonyl compounds in the presence of a base and/or an oxidant, in which multiple possible pathways involving acylazolium, enolate, Breslow, and α,β-unsaturated acylazolium intermediates were proposed and a novel index ω + N of the E and Nu partners was employed to exactly predict the energy barrier of the chemoselective step in theory. This work provides a guide for determining the general principle behind organocatalytic reactions with various chemoselectivities, and suggests a general application of the reaction index in predicting the chemoselectivity of the nucleophilic and electrophilic reactions.

A novel index ω + N can be used to predict the chemoselectivity according to the general NHC-catalyzed reaction mechanism.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Catalytic α-Selective Deuteration of Styrene Derivatives

We report an operationally simple protocol for the catalytic α-deuteration of styrenes. This process proceeds via the base-catalyzed reversible addition of methanol to styrenes in DMSO -d₆ solvent. The concentration of methanol is shown to be critical for high yields and selectivities over multiple competing side reactions. The synthetic utility of α-deuterated styrenes for accessing deuterium-labeled chiral benzylic stereocenters is demonstrated.