Brønsted Acid-Promoted Intermolecular Dearomative Photocycloaddition of Bicyclic Azaarenes with Olefins under Aerobic Conditions

Photochemical dearomative skeletal modifications of heteroaromatics

Dearomatization has emerged as a powerful tool for rapid construction of 3D molecular architectures from simple, abundant, and planar (hetero)arenes. The field has evolved beyond simple dearomatization driven by new synthetic technology development. With the renaissance of photocatalysis and expansion of the activation mode, the last few years have witnessed impressive developments in innovative photochemical dearomatization methodologies, enabling skeletal modifications of dearomatized structures. They offer truly efficient and useful tools for facile construction of highly complex structures, which are viable for natural product synthesis and drug discovery. In this review, we aim to provide a mechanistically insightful overview on these innovations based on the degree of skeletal alteration, categorized into dearomative functionalization and skeletal editing, and to highlight their synthetic utilities.

Dearomative Construction of 2D/3D Frameworks from Quinolines via Nucleophilic Addition/Borate-Mediated Photocycloaddition

Dearomative construction of multiply-fused 2D/3D frameworks, composed of aromatic two-dimensional (2D) rings and saturated three-dimensional (3D) rings, from readily available quinolines has greatly contributed to drug discovery. However, dearomative cycloadditions of quinolines in the presence of photocatalysts usually afford 5,6,7,8-tetrahydroquinoline (THQ)-based polycycles, and dearomative access to 1,2,3,4-THQ-based structures remains limited. Herein, we present a chemo-, regio-, diastereo-, and enantioselective dearomative transformation of quinolines into 1,2,3,4-THQ-based 6-6-4-membered rings without any catalyst, through a combination of nucleophilic addition and borate-mediated [2+2] photocycloaddition. Detailed mechanistic studies revealed that the photoexcited borate complex, generated from quinoline, organolithium, and HB(pin), accelerates the cycloaddition and suppresses the rearomatization that usually occurs in conventional photocycloaddition. Based on our mechanistic analysis, we also developed further photoinduced cycloadditions affording other types of 2D/3D frameworks from isoquinoline and phenanthrene.

Photocatalytic Synthesis of β-Keto Primary Chlorides by Selective Chlorocarbonylation of Olefins

Functionalized primary alkyl chlorides are precursors to a plethora of scaffolds but their access from chemical feedstocks remains challenging. Herein, we report a concise dual Ni/photoredox catalytic protocol for regioselective chlorocarbonylation of unactivated alkenes that enables rapid access to β-keto primary chlorides. The catalytic process features an extensive substrate scope, scalability and functional group tolerance. The Ni/photocatalytic Cl⋅ generation and subsequent cross-coupling is implicated for the process based on the control experiments and DFT study. The synthetic utility of the protocol has been further corroborated through functionalization of complex substrates and modifications of the product.

Photo-induced intramolecular dearomative [5 + 4] cycloaddition of arenes for the construction of highly strained medium-sized-rings

Medium-sized-ring compounds have been recognized as challenging synthetic targets in organic chemistry. Especially, the difficulty of synthesis will be augmented if an E-olefin moiety is embedded. Recently, photo-induced dearomative cycloaddition reactions that proceed via energy transfer mechanism have witnessed significant developments and provided powerful methods for the organic transformations that are not easily realized under thermal conditions. Herein, we report an intramolecular dearomative [5 + 4] cycloaddition of naphthalene-derived vinylcyclopropanes under visible-light irradiation and a proper triplet photosensitizer. The reaction affords dearomatized polycyclic molecules possessing a nine-membered-ring with an E-olefin moiety in good yields (up to 86%) and stereoselectivity (up to 8.8/1 E/Z). Detailed computational studies reveal the origin behind the favorable formation of the thermodynamically less stable isomers. Diverse derivations of the dearomatized products have also been demonstrated.

Photoredox-Enabled Dearomative [2π + 2σ] Cycloaddition of Phenols

Dearomative photocycloaddition of monocyclic arenes is an appealing strategy for comprehending the concept of "escape from flatland". This brings the replacement of readily available planar aromatic hydrocarbon units with a 3D fused bicyclic core with sp3-enriched carbon units. Herein, we outline an intermolecular approach for the dearomative photocycloaddition of phenols. In order to circumvent the ground-state aromaticity and to construct conformationally restrained building blocks, bicyclo[1.1.0]butanes were chosen as coupling partners. This dearomative approach renders straightforward access to a bicyclo[2.1.1]hexane unit fused to a cyclic enone moiety, which further contributed as a synthetic linchpin for postmodifications. Mechanistic experiment advocates for a plausible onset from both the reactants, depending on the redox potential.

Rejuvenation of dearomative cycloaddition reactions via visible light energy transfer catalysis

Dearomative cycloaddition is a powerful technique to access sp3-rich three-dimensional structural motifs from simple flat, aromatic feedstock. The building-up of unprecedentedly diverse polycyclic scaffolds with increased saturation and stereochemical information having various applications ranging from pharmaceutical to material sciences, is an essential goal in organic chemistry. However, the requirement of large energy inputs to disrupt the aromaticity of an arene moiety necessitates harsh reaction conditions for ground state dearomative cycloaddition. The photochemical requirement encompasses use of ultraviolet (UV) light to enable the reaction on an excited potential energy surface. The microscopic reversibility under thermal conditions and the use of high energy harmful UV irradiation in photochemical manoeuvres, however, constrain their widespread use from a synthetic point of view. In this context, the recent renaissance of visible light energy transfer (EnT) catalysis has become a powerful tool to initiate dearomative cycloaddition as a greener and more sustainable approach. The excited triplet state population is achieved by triplet energy transfer from the appropriate photosensitizer to the substrate. While employing mild visible light energy as fuel, the process leverages an enormous potential of excited state reactivity. The discovery of an impressive portfolio of organic and inorganic photosensitizers with a range of triplet energies facilitates visible light photosensitized dearomative cycloaddition of various substrates to form sp3-rich fused polycyclic architectures with diverse applications. The tutorial review comprehensively surveys the reawakening of dearomative cycloadditions via visible light-mediated energy transfer catalysis in the past five years. The progress ranges from intra- and intermolecular [2π + 2π] to [4π + 2π], and ends at intermolecular [2π + 2σ] cycloadditions. Furthermore, the review provides potential possibilities for future growth in the growing field of visible light energy transfer catalysis.