Palladium-Catalyzed Enantioselective Ring Opening of Oxabicyclic Alkenes with Organozinc Halides

Cu-Catalyzed cis-selective ring-opening cross-coupling of an oxabicyclic olefin with an organoboron reagent

The Cu(i)-catalyzed ring-opening cross-coupling of an oxabicyclic olefin with an organoboron reagent provides access to cis-2-aryl-1,2-dihydronaphthalen-1-ol derivatives, in contrast to the exclusive trans-selectivity in related Cu-catalyzed reactions with Grignard reagents. DFT calculations suggest that the reaction possibly proceeds via boronic ester by-product assisted ring-opening as an alternative pathway to the canonical β-oxygen elimination as the rate-determining step.

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.

Catalytic asymmetric transformations of oxa- and azabicyclic alkenes

This review provides an overview of the fundamental concepts and recent developments in a wide range of enantioselective transformations involving oxa- and azabicyclic alkenes.

Oxa- and Azabenzonorbornadienes as Electrophilic Partners under Photoredox/Nickel Dual Catalysis

Herein, the introduction of oxa- and azabenzonorbornadienes into photoredox/nickel dual catalysis in a regioselective and diastereoselective transformation is disclosed. The inherent advantages of this dual catalytic system allow the use of alkyl motifs forming exclusively cis-1,2-dihydro-1-naphthyl alcohol backbones using readily accessible 4-alkyl-1,4-dihydropyridines (DHPs). Whereas previous studies have emphasized the use of nucleophilic organometallic coupling partners, this protocol grants access to a rather unexplored core featuring alkyl residues, while avoiding the use of highly reactive organometallic species (i.e., M = Al, Mg, Li, Zn, Zr). DFT calculations support a oxidative addition/reductive elimination mechanism, followed by a Curtin-Hammett scenario that controls the regioselectivity of the process, unlike previously reported transformations that proceed via a carbometalation/ β-oxygen elimination mechanism.

Regioselective palladium-catalyzed ring-opening reactions of C1-substituted oxabicyclo[2,2,1]hepta-2,5-diene-2,3-dicarboxylates

Palladium-catalyzed ring-opening reactions of C1 substituted 7-oxanorbornadiene derivatives with aryl iodides were investigated. The optimal conditions for this reaction were found to be PdCl₂(PPh₃)₂, ZnCl₂, Et₃N and Zn in THF. Both steric and electronic factors played a role in the outcome of the reaction as increasing the steric bulk on the bridgehead carbon decreased the yield. These reactions were found to be highly regioselective, giving only one of the two possible regioisomers in all cases. A diverse collection of novel, highly substituted biphenyl derivatives were obtained.

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.

Regioselective palladium-catalyzed ring-opening reactions of C1-substituted oxabenzonorbornadienes

The effect of ethyl and methoxycarbonyl substitution on the C1 position of oxabenzonorbornadienes undergoing palladium-catalyzed nucleophilic ring opening was investigated with a variety of aryl iodide nucleophiles. Electron-withdrawing groups in the C1 position or on the aryl iodide afforded lower yields and led to aromatization of products. The presence of an electron-donating group in either position provided high yields in all cases. Despite variances in electronic nature, all trials produced a single regioisomeric product resulting from addition of the aryl group to the olefin carbon farthest from the C1 substituent. Based on these findings, a mechanism has also been proposed.

Nickel-catalyzed asymmetric ring opening of oxabenzonorbornadienes with arylboronic acids

A new, versatile, and highly efficient nickel-catalyzed asymmetric ring-opening (ARO) reaction of oxabenzonorbornadienes with a wide variety of arylboronic acids has been developed, yielding cis-2-aryl-1,2-dihydronaphthalen-1-ols in high yields (up to 99%) with good to excellent enantioselectivities (up to 99% ee) under very mild conditions. The effects of various nickel precursors, chiral bidentate ligands, catalyst loadings, bases, solvents, and temperatures on the yield and enantioselectivity of the reaction were also investigated. A plausible mechanism was proposed to account for the formation of the corresponding cis-ring-opened products based on the X-ray structure of product 4b.

Platinum-catalyzed anti-stereocontrolled ring-opening of oxabicyclic alkenes with Grignard reagents

A new platinum-catalyzed anti-stereocontrolled ring-opening of oxabicyclic alkenes with various Grignard reagents was reported, which afforded the corresponding anti-2-substituted-1,2-dihydronaphthalen-1-ol products with moderate to good yields in the presence of a catalytic amount of Pt(PPh3)4 (2.5 mol%) under mild conditions. The effects of catalyst loading, solvent and temperature on the yield were also investigated. Furthermore, the trans-configuration of the product 5i was confirmed by X-ray diffraction analysis.

Iridium-catalyzed asymmetric ring-opening reactions of oxabicyclic alkenes with secondary amine nucleophiles

Iridium-catalyzed asymmetric ring-opening reactions of oxabicyclic alkenes with various aliphatic and aromatic secondary amines are reported for the first time. The reaction gave the corresponding trans-1,2-dihydronaphthalenol derivatives in good yields with moderate enantioselectivities in the presence of 2.5 mol % [Ir(COD)Cl](2) and 5 mol % bisphosphine ligand (S)-p-Tol-BINAP. The trans-configuration of 3f was confirmed by X-ray crystallography.

Ruthenium(II)-Catalyzed Cyclization of Azabenzonorbornadienes with Alkynes

The ruthenium-catalyzed cyclization of azabenzonorbornadienes with alkynes leads to an unanticipated dihydrobenzoindole framework. Depending on the structure of the alkyne and the Ru catalyst, either a dihydrobenzoindole and/or a [2+2] cycloaddition product could be formed. Cp*Ru(COD)Cl was found to be an active catalyst for the cyclization of an azabenzonorbornadiene with a propargylic alcohol to produce the dihydrobenz[g]indole as a single regio and stereoisomer in good yield. For other alkynes, selective formation of the dihydrobenz[g]indole is possible by using a cationic Ru catalyst, [Cp*Ru(CH3CN)3]PF6.

Recent Applications of Chiral Ferrocene Ligands in Asymmetric Catalysis

Despite the impressive progress achieved in asymmetric catalysis during the last decade, an increasing number of new catalysts, ligands, and applications are reported every year to satisfy the need to embrace a wider range of reactions and to improve the efficiency of existing processes. Because of their availability, unique stereochemical aspects, and wide variety of coordination modes and possibilities for the fine-tuning of the steric and electronic properties, ferrocene-based ligands constitute one of the most versatile ligand architectures in the current scenario of asymmetric catalysis. Over the last few years ferrocene catalysts have been successfully applied in an amazing variety of enantioselective processes. This Review documents these recent advances, with special emphasis on the most innovative asymmetric processes and the development of novel, efficient types of ferrocene ligands.