Sequential One-Pot Carbene-Catalyzed Intramolecular Stetter Reaction and Acid-Mediated Condensation: Access to Heteroatom Analogues of π-Extended Polyaromatic Hydrocarbons

NHC-Catalyzed Umpolung Driven Intramolecular Cyclizations for the Generation of 3,4-Cycloheptanone Annulated Tetracyclic Indole Derivatives

In this Letter, we disclose the development and exploitation of umpolung reactivity of N-heterocyclic carbene (NHC) organocatalysts in generating tetracyclic indole derivatives with the introduction of a cycloheptane ring forming event. The NHC-catalyzed intramolecular vinylogous Stetter, Stetter, benzoin, and formal cross-dehydrogenative coupling transformations have been executed, enabling the construction of 3,4-cycloheptannulated indole derivatives in good to excellent yields. The developed protocols utilize an inexpensive catalyst and feature operational simplicity, atom economy, gram-scale syntheses, and postsynthetic availability.

Transition metal-catalyzed transformations of 2-formylarylboronic acids

2-Formylarylboronic acids are easily available precursors in organic chemistry. Different types of transition metal catalysts, such as Pd(0), Pd(II), Rh(I), Ir(I), Ni(II), Cu(I), Cu(II), and Co(II), can efficiently catalyze coupling reactions of 2-formylarylboronic acids with other organic reactants. In this review, we describe the synthesis of a diverse range of carbocyclic and heterocyclic compounds, as well as acyclic compounds, via transition metal-catalyzed reactions of 2-formylarylboronic acids over the past two decades.

Carbene-Catalyzed Tandem Imine Umpolung-Intramolecular Aza-Conjugate Addition-Oxidation to Access N-Substituted Isoindolinones

Herein, we have described a novel N-heterocyclic carbene (NHC)-catalyzed synthesis of N-substituted isoindolinone acetates. The presented transformation proceeds through NHC-catalyzed tandem imine umpolung-intramolecular aza-Michael addition followed by oxidation, while molecular oxygen in air acts as a sole oxidant. Atom efficiency, operational simplicity, large-scale syntheses, and mild reaction conditions are the salient features of this method. Mechanistic studies were indicative of the necessity of molecular oxygen in air as oxidant for the conversion of imine to amide.

Transition metal-free and temperature dependent one-pot access to phenanthrene-fused heterocycles via a 1,3-dipolar cycloaddition pathway

A versatile, operationally simple, temperature-dependent, and transition metal-free one-pot protocol has been devised for the preparation of novel phenanthrene-fused pyrazoles. Notably, the overall process involved an intermolecular condensation, an intramolecular 1,3-dipolar cycloaddition, and an aromatization sequence starting from biaryl-2,2'-aldehydes bearing enoate esters with various hydrazine hydrochlorides. Notably, the sequential one-pot three-component operation has also been achieved. Importantly, it was also shown that this protocol was amenable to hydroxylamine hydrochloride as the nitrogen source and furnished phenanthrene-fused isoxazoles. Notably, the temperature dependent nature of this protocol was also demonstrated, which led to the formation of dealkoxylcarbonylated phenanthrene-fused pyrazoles at slightly higher temperatures and longer reaction times. Remarkably, this metal-free protocol effectively constructed two C-N bonds and one C-C bond and exhibited a broad substrate scope.

Tandem Ene/[4 + 2] Cycloaddition Reaction for the Synthesis of 9-Benzylphenanthrenes from Arynes and α-(Bromomethyl)styrenes

A tandem reaction for the synthesis of phenanthrenes from arynes and α-(bromomethyl)styrenes is reported. The transformation proceeds via an ene reaction of α-(bromomethyl)styrenes with arynes, followed by a [4 + 2] cycloaddition reaction. The reaction generates 9-benzylphenanthrene derivatives in moderate to excellent yields.