Nickel-Catalyzed Regio- and Stereoselective Reductive Coupling of Oxa- and Azabicyclic Alkenes with Enones and Electron-Rich Alkynes

Nickel-catalysed highly regioselective synthesis of β-acyl naphthalenes under reductive conditions

Over the past decade, significant progress has been made in the direct C-H acylation of naphthalenes, occurring at the α or β-positions to yield valuable ketones through Friedel-Crafts acylation or transition-metal-catalysed carbonylative coupling reactions. Nevertheless, highly regioselective acylation of naphthalenes remains a formidable challenge. Herein, we developed a nickel-catalysed reductive ring-opening reaction of 7-oxabenzonorbornadienes with acyl chlorides as the electrophilic coupling partner, providing a new method for the exclusive preparation of β-acyl naphthalenes.

Enantioselective Palladium(II)-Catalyzed Desymmetrizative Coupling of 7-Azabenzonorbornadienes with Alkynylanilines

A PdII -catalyzed, domino enantioselective desymmetrizative coupling of 7-azabenzonorbornadienes with alkynylanilines is disclosed herein. This operationally simple transformation generates three covalent bonds and two contiguous stereocenters with excellent enantio- and diastereo-selectivity. The resulting functionalized indole-dihydronaphthalene-amine conjugates served as an appealing platform to streamline the diversity-oriented synthesis (DOS) of other valuable enantioenriched compounds. DFT calculations revealed that the two stabilizing non-covalent interactions contributed to the observed enantioselectivity.

Asymmetric alkene-alkene reductive cross-coupling reaction via visible-light photoredox/cobalt dual catalysis

The first example of asymmetric alkene-alkene reductive coupling is demonstrated via visible-light-fueled photoredox/cobalt dual catalysis. The desymmetrization reaction provided products (>20 examples) with up to five chiral centers in single-step operation in up to 95% yields with very high relative (>99 : 1 dr) and absolute stereochemistry (up to 98 : 2 er) control. The preliminary mechanistic investigations suggested that the critical mechanistic steps involved light-mediated controlled low-valent cobalt complex generation, oxidative ene-ene cyclization, and protonation.

Chemistry of Unsymmetrical C1-Substituted Oxabenzonorbornadienes

Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate, which can be readily activated by transition metal complexes with great face selectivity due to its dual-faced nature and intrinsic angle strain on the alkene. To date, the understanding of transition-metal catalyzed reactions of OBD itself has burgeoned; however, this has not been the case for unsymmetrical OBDs. Throughout the development of these reactions, the nature of C1-substituent has proven to have a profound effect on both the reactivity and selectivity of the outcome of the reaction. Upon substitution, different modes of reactivity arise, contributing to the possibility of multiple stereo-, regio-, and in extreme cases, constitutional isomers, which can provide unique means of constructing a variety of synthetically useful cyclic frameworks. To maximize selectivity, an understanding of bridgehead substituent effects is crucial. To that end, this review outlines hitherto reported examples of bridgehead substituent effects on the chemistry of unsymmetrical C1-substituted OBDs.

Asymmetric Synthesis of 1,2-Dihydronaphthalene-1-ols via Copper-Catalyzed Intramolecular Reductive Cyclization

We describe a copper-catalyzed intramolecular reductive cyclization of easily accessible benz-tethered 1,3-dienes containing a ketone moiety. This process provided biologically active 1,2-dihydronaphthalene-1-ol derivatives in good yields with excellent enantio- and diastereoselectivity. Mechanistic investigations using density functional theory revealed that (Z)- and (E)-allylcopper intermediates formed in situ from the diene and copper catalyst undergo isomerization and selective intramolecular allylation of the (E)-allylcopper form of the major product through a six-membered boatlike transition state. The resulting products were further transformed to fully saturated naphthalene-1-ols by reactions of the olefin moiety.

Palladium-Catalyzed syn-Stereocontrolled Ring Opening of Oxabicyclic Alkenes with Arylsulfonyl Hydrazides

A novel palladium-catalyzed ring-opening reaction of oxabicyclic alkenes with arylsulfonyl hydrazides was first developed. In this work, we provide an efficient one-pot reaction to afford the corresponding cis-2-aryl-1,2-dihydronaphthalen-1-ols and 2-aryl-naphthalenes in moderate to excellent yields (up to 95%) under an open-air condition. Various types of functional groups attached to the substrates were tolerated well in this method. Among them, the cis-1,2-configuration of product 3ag was confirmed by X-ray crystallographic analysis. In addition, a plausible mechanism for ring opening was also proposed.

Recent Advances in Transition Metal-Catalyzed Reactions of Oxabenzonorbornadiene

BACKGROUND

Oxabenzonorbornadiene (OBD) is a useful synthetic intermediate capable of undergoing multiple types of transformations due to three key structural features: a free alkene, a bridged oxygen atom, and a highly strained ring system. Most notably, ring-opening reactions of OBD using transition metal catalysts and nucleophiles produce multiple stereocenters in a single step. The resulting dihydronaphthalene framework is found in many natural products, which have been shown to be biologically active.

OBJECTIVE

This review will provide an overview of transition metal-catalyzed reactions from the past couple of years including cobalt, copper, iridium, nickel, palladium and rhodium- catalyzed reactions. In addition, the recent derivatization of OBD to cyclopropanated oxabenzonorbornadiene and its reactivity will be discussed.

CONCLUSION

It can be seen from the review, that the work done on this topic has employed the use of many different transition metal catalysts, with many different nucleophiles, to perform various transformations on the OBD molecule. Additionally, depending on the catalyst and ligand used, the stereo and regioselectivity of the product can be controlled, with proposed mechanisms to support the understanding of such reactions. The use of palladium has also generated a cyclopropanated OBD, with reactivity similar to that of OBD. An additional reactive site exists at the distal cyclopropane carbon, giving rise to three types of ring-opened products.

Platinum-catalyzed syn-stereocontrolled ring-opening of oxabicyclic alkenes with sodium arylsulfinates

A new efficient platinum-catalyzed ring-opening reaction of oxabicyclic alkenes with a wide range of sodium arylsulfinates was developed.

Iridium-Catalyzed Asymmetric Ring-Opening of Oxabenzonorbornadienes with N-Substituted Piperazine Nucleophiles

Iridium-catalyzed asymmetric ring-opening of oxabenzonorbornadienes with N-substituted piperazines was described. The reaction afforded the corresponding ring-opening products in high yields and moderate enantioselectivities in the presence of 2.5 mol % [Ir(COD)Cl]₂ and 5.0 mol % (S)-p-Tol-BINAP. The effects of various chiral bidentate ligands, catalyst loading, solvent, and temperature on the yield and enantioselectivity were also investigated. A plausible mechanism was proposed to account for the formation of the corresponding trans-ring opened products based on the X-ray structure of product 2i.