Copper-catalyzed asymmetric ring opening of oxabicyclic alkenes with organolithium reagents

Recent synthetic strategies for the functionalization of fused bicyclic heteroaromatics using organo-Li, -Mg and -Zn reagents

This review highlights the use of functionalized organo-Li, -Mg and -Zn reagents for the construction and selective functionalization of 5- and 6-membered fused bicyclic heteroaromatics. Special attention is given to the discussion of advanced syntheses for the preparation of highly functionalized heteroaromatic scaffolds, including quinolines, naphthyridines, indoles, benzofurans, benzothiophenes, benzoxazoles, benzothiazoles, benzopyrimidines, anthranils, thienothiophenes, purine coumarins, chromones, quinolones and phthalazines and their fused heterocyclic derivatives. The organometallic reagents used for the desired functionalizations of these scaffolds are generally prepared in situ using the following methods: (i) through directed selective metalation reactions (DoM), (ii) by means of halogen/metal exchange reactions, (iii) through oxidative metal insertions (Li, Mg, Zn), and (iv) by transmetalation reactions (organo-Li and Mg transmetalations with ZnCl2 or ZnO(Piv)2). The resulting reactive organometallic reagents allow a wide range of C-C, C-N and C-X cross-coupling reactions with different electrophiles, employing in particular Kumada or Negishi protocols among other transition metal (Pd, Ni, Co, Cu, Cr, Fe, etc.)-catalyzed processes. In addition, key developments concerning selective metalation techniques will be presented, which rely on the use of RLi, LDA and TMP metal bases. These methods are now widely employed in organic synthetic chemistry and have proven to be particularly valuable for drug development programs in the pharmaceutical industry. New and improved protocols have resulted in many Li, Mg and Zn organyls now being compatible with functionalized aryl, heteroaryl, alkenyl, alkynyl and alkyl compounds even in the presence of labile functional groups, making these reagents well-suited for C(sp2)-C(sp2), C(sp2)-C(sp) and C(sp2)-C(sp3) cross-coupling reactions with fused heteroaryl halides. In addition, the use of some transition metal-catalyzed processes occasionally allows a reversed role of the reactants in cross-coupling reactions, providing alternative synthetic routes for the preparation of fused heteroaromatic-based bioactive drugs and natural products. In line with this, this article points to novel methods for the functionalization of bicyclic heteroaromatic scaffolds by organometallic reagents that have been published in the period 2010-2023.

Desymmetrization of Inert meso-Diethers through Copper-Catalyzed Asymmetric Allylic Alkylation with Grignard Reagents

We have developed a highly regio-, diastereo-, and enantioselective Cu-catalyzed desymmetrization of inert meso-diethers using Grignard reagents. Moreover, previous inaccessible sterically hindered organometallic reagents are realized in the reaction with broad secondary alkyl Grignard reagents. Finally, detailed control experiments and density functional theory calculations revealed the desymmetrization of meso-diethers exploits a direct anti-SN2' pathway, in the absence of an in situ-generated allyl bromine intermediate. The following oxidative addition is the crucial rate-determining and enantioselectivity-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.

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.

Copper-catalysed alkylation of heterocyclic acceptors with organometallic reagents

Copper-catalysed asymmetric C–C bond-forming reactions using organometallic reagents have developed into a powerful tool for the synthesis of complex molecules with single or multiple stereogenic centres over the past decades. Among the various acceptors employed in such reactions, those with a heterocyclic core are of particular importance because of the frequent occurrence of heterocyclic scaffolds in the structures of chiral natural products and bioactive molecules. Hence, this review focuses on the progress made over the past 20 years for heterocyclic acceptors.

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.

Rhodium-Catalyzed Enantioselective Synthesis of Oxazinones via an Asymmetric Ring Opening-Lactonization Cascade of Oxabicyclic Alkenes

The rhodium-catalyzed asymmetric ring opening reaction of oxabicyclic alkenes is shown to be an efficient method for synthesizing chiral heterocycles. We demonstrate that the pairwise combination of chiral catalyst with chiral amino-acid-derived pronucleophiles results in a stereodivergent synthesis of diastereomeric hydroxyesters. A favorable conformational preference induces the subsequent lactonization of one diastereomer leading to the highly enantioselective synthesis of oxazinones.

Three-Component Cycloaddition To Synthesize Aziridines and 1,2,3-Triazolines

An efficient three-component cycloaddition of oxa(aza)bicyclic alkenes/norbornene in the presence of NaN₃ and arylsulfonyl chlorides was developed, affording the corresponding aziridine products in good yields (up to 82%) with moderate to good endo/exo selectivities (up to >99:1 endo/exo). Further studies showed that the cycloaddition of oxa(aza)bicyclic alkenes in the presence of NaN₃ and chloroalkanes could afford the exo-cycloadduct 1,2,3-triazolines in good to excellent yields (up to 95%). Compared with the existing methodologies, the current protocol demands very simple and mild reaction conditions and is a metal-free catalyzed reaction. In addition, a plausible mechanism for the cycloaddition reaction was also proposed.

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.

Ir-Catalyzed ring-opening of oxa(aza)benzonorbornadienes with water or alcohols

A highly efficient Ir-catalyzed ring-opening reaction of oxa(aza)benzonorbornadienes with water was firstly developed, which afforded a mild access to substituted dihydronaphthalene products with excellent yields (up to 99%) within 30 min.

Study on the mechanism of platinum(ii)-catalyzed asymmetric ring-opening addition of oxabicyclic alkenes with arylboronic acids

The mechanism of an asymmetric ring-opening (ARO) addition of oxabicyclic alkenes catalyzed by a platinum(ii) catalyst was investigated by M06-2X/6-311G(d,p) using density functional theory (DFT). All the structures were optimized in the solvent model density (SMD) solvation model (solvation = the mixture of H2O/CH2Cl2 1 : 10, v/v) for consistence with experimental conditions. The overall mechanism is considered as a four-step reaction including transmetalation, carboplatinum, β-oxygen elimination, and hydrolysis. The transmetalation and carboplatinum steps are multi-step processes, and both the regioselectivity and the enantioselectivity lie in the carboplatinum process. Based on the natural population analysis (NPA) and the orbital composition analysis of oxabicyclic alkenes, the preferable coordination site with a platinum(ii) center is considered as the bridging oxygen atom by exo-coordination because of the less steric hindrance and the stronger electronic effect. This coordination is thought of as origin of the regioselectivity and the enantioselectivity, which is different from that proposed previously. The Gibbs free energy profiles show that the rate-determining step involves the migration of an aryl group from the platinum(ii) center to one of the closer enantiotopic carbon atoms in an alkene of the oxabicyclic alkenes. The theoretically predicted enantiomeric excess (ee) value of 82% for this reaction is very close to the experimental ee value of 80%. It was found that the hydrogen bonds between the oxabicyclic alkenes and water molecules promotes the platinum(ii) catalyst leaving the reaction system effortlessly and entering the next catalysis recycle. In the overall catalytic cycle, the highest free energy barrier is 30.1 kcal mol-1 and the process releases an energy of 26.3 kcal mol-1. The results confirm that the Pt(ii)-catalyzed ARO reactions take place at mild experimental conditions, which is consistent with the experiment observations. Thus, this study is important for understanding the catalytic behavior of the transition metal platinum(ii) in an asymmetric ring-opening reaction.

Cu-catalyzed enantioselective allylic alkylation with organolithium reagents

This protocol describes a method for the catalytic enantioselective synthesis of tertiary and quaternary carbon stereogenic centers, which are widely present in pharmaceutical and natural products. The method is based on the direct reaction between organolithium compounds, which are cheap, readily available and broadly used in chemical synthesis, and allylic electrophiles, using chiral copper catalysts. The methodology involves the asymmetric allylic alkylation (AAA) of allyl bromides, chlorides and ethers with organolithium compounds using catalyst systems based on Cu-Taniaphos and Cu-phosphoramidites. The protocol contains a complete description of the reaction setup, a method based on ¹H-NMR, gas chromatography-mass spectrometry (GC-MS) and chiral HPLC for assaying the regioselectivity and enantioselectivity of the product, and isolation, purification and characterization procedures. Six Cu-catalyzed AAA reactions between different organolithium reagents and allylic systems are detailed in the text as representative examples of these procedures. These reactions proceed within 1-10 h, depending on the nature of the allylic substrate (bromide, chloride, or ether and disubstituted or trisubstituted) or the chiral ligand used (Taniaphos or phosphoramidite). However, the entire protocol, including workup and purification, generally requires an additional 4-7 h to complete.

Mechanism, reactivity, and regioselectivity in rhodium-catalyzed asymmetric ring-opening reactions of oxabicyclic alkenes: a DFT Investigation

The origin of the enantio- and regioselectivity of ring-opening reaction of oxabicyclic alkenes catalyzed by rhodium/Josiphos has been examined using M06-2X density functional theory(DFT). DFT calculations predict a 98% ee for the enantioselectivity and only the 1,2-trans product as one regio- and diastereomer, in excellent agreement with experimental results. The solvent tetrahydrofuran(THF) plays a key role in assisting nucleophilic attack. Orbital composition analysis of the LUMO and the NPA atomic charge calculations were conducted to probe the origins of the regioselectivity. The orbital composition analysis reveals two potential electrophilic sites of the Rh-π-allyl intermediate M3 and the NPA atomic charges demonstrate that Cα carries more positive charges than Cγ, which suggests that Cα is the electrophilic site.

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/Copper Co-catalyzed Anti-Stereoselective Ring Opening of Oxabenzonorbornadienes with Grignard Reagents

Cooperative catalysis has been widely considered as one of the most powerful strategies to improve synthetic efficiency. A new iridium/copper cocatalyst was developed for the ring-opening reaction of oxabenzonorbornadienes with a wide variety of Grignard reagents, which afforded the corresponding anti-2-substituted 1,2-dihydronaphthalen-1-ols in high yields (up to 99% yield) under mild conditions. The effects of catalyst loading, Lewis acid, Grignard reagent loading, and reaction temperature on the yield were investigated. To the best of our knowledge, it represents the first example of ring-opening reactions of oxabicyclic alkenes with Grignard reagent nucleophiles in a trans-stereoselective manner.

Chiral Diarylmethanes via Copper-Catalyzed Asymmetric Allylic Arylation with Organolithium Compounds

A highly enantioselective copper/N-heterocyclic carbene catalyzed allylic arylation with organolithium compounds is presented. The use of commercial or readily prepared aryllithium reagents in the reaction with allyl bromides affords a variety of chiral diarylvinylmethanes, comprising a privileged structural motif in pharmaceuticals, in high yields with good to excellent regio- and enantioselectivities. The versatility of this new transformation is illustrated in the formal synthesis of the marketed drug tolterodine (Detrol).

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.

Iridium/copper-cocatalyzed asymmetric ring opening reaction of azabenzonorbornadienes with amines

A combination of iridium/copper associated with (R)-Difluorphos catalyst for the asymmetric ring opening reaction of azabenzonorbornadienes with amines was developed, which afforded chiral trans-vicinal diamines in 80-97% yields with 93-95% enantioselectivities.

Catalyst-controlled reverse selectivity in C–C bond formation: NHC-Cu-catalyzed α-selective allylic alkylation with organolithium reagents

An efficient and highly α-selective copper-catalyzed allylic alkylation of allylic halides with organolithium reagents is presented.

Copper-catalysed α-selective allylic alkylation of heteroaryllithium reagents

2-Allyl-substituted thiophenes and furans are synthesised efficiently in a direct procedure using 2-heteroaryllithium reagents and allyl bromides and chlorides catalysed by ligand-free copper(i). The reactions take place under mild conditions, with excellent α-selectivity, high functional group tolerance and good yields for the SN2 products.

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.