Stereoselective synthesis of morpholines via copper-promoted oxyamination of alkenes

Emerging Trends in Copper-Promoted Radical-Involved C-O Bond Formations

The C-O bond is ubiquitous in biologically active molecules, pharmaceutical agents, and functional materials, thereby making it an important functional group. Consequently, the development of C-O bond-forming reactions using catalytic strategies has become an increasingly important research topic in organic synthesis because more conventional methods involving strong base and acid have many limitations. In contrast to the ionic-pathway-based methods, copper-promoted radical-mediated C-O bond formation is experiencing a surge in research interest owing to a renaissance in free-radical chemistry and photoredox catalysis. This Perspective highlights and appraises state-of-the-art techniques in this burgeoning research field. The contents are organized according to the different reaction types and working models.

Rhodium-catalyzed diastereoselective synthesis of highly substituted morpholines from nitrogen-tethered allenols

Rhodium-catalyzed intramolecular cyclization of nitrogen-tethered allenols was investigated for the synthesis of functionalized morpholines. By using this strategy, various N-protected 2,5- and 2,6-disubstituted as well as 2,3,5- and 2,5,6-trisubstituted morpholines were obtained via an atom-economic pathway with high to excellent yields, diastereo- and enantioselectivities (up to 99% yield, up to >99 : 1 dr and up to >99.9 ee). The utilities of the synthesized morpholines in ozonolysis, hydration, metathesis and epoxidation reactions were also investigated.

Asymmetric hydrogenation for the synthesis of 2-substituted chiral morpholines

Asymmetric hydrogenation of unsaturated morpholines has been developed by using a bisphosphine-rhodium catalyst bearing a large bite angle. With this approach, a variety of 2-substituted chiral morpholines could be obtained in quantitative yields and with excellent enantioselectivities (up to 99% ee). The hydrogenated products could be transformed into key intermediates for bioactive compounds.

2-Substituted chiral morpholines were synthesized via a newly developed asymmetric hydrogenation of dehydromorpholines catalyzed by a bisphosphine–rhodium complex bearing a large bite angle.

Copper-Promoted Conjugate Addition of Carboxylic Acids to Ethenesulfonyl Fluoride (ESF) for Constructing Aliphatic Sulfonyl Fluorides

A CuO-promoted direct hydrocarboxylation of ethenesulfonyl fluoride (ESF) was developed using carboxylic acid as a nucleophile under mild conditions. A variety of molecules containing both ester group and aliphatic sulfonyl fluoride moiety exhibit great potential in medicinal chemistry and chemical biology. Furthermore, the modification of the known drugs Ibuprofen and Aspirin was also demonstrated.

Copper-catalyzed enantioselective alkene carboetherification for the synthesis of saturated six-membered cyclic ethers

The enantioselective copper-catalyzed oxidative coupling of alkenols with styrenes for the construction of dihydropyrans, isochromans, pyrans and morpholines is reported. A concise formal synthesis of a σ₁ receptor ligand using this alkene carboetherification methodology was demonstrated. Ligand, solvent and base all impact reaction efficiency. DFT transition state calculations are presented.

Enantio- and Regioselective Palladium(II)-Catalyzed Dioxygenation of (Aza-)Alkenols

An oxidative Pd-catalyzed intra-intermolecular dioxygenation of (aza-)alkenols has been reported, with total regioselectivity. To study the stereoselectivity, different chiral ligands as well as different hypervalent-iodine compounds have been compared. In particular, by using a C-6 modified pyridinyl-oxazoline (Pyox) ligand and hypervalent iodine bearing an aromatic ring, an excellent enantio- and diastereoselectivity has been achieved.

Facile synthesis of 1,4-oxazines by ruthenium-catalyzed tandem N-H insertion/cyclization of α-arylamino ketones and diazo pyruvates

Herein, we report an efficient strategy for the rapid construction of 1,4-oxazines starting from simple α-amino ketones and diazo pyruvates under mild reaction conditions. This transformation is efficiently catalyzed by RuCl3 through a tandem N-H insertion/cyclization sequence via an enol formation. This reaction shows broad functional group tolerance, and the resulting 1,4-oxazine products show promising anticancer activities toward HCT116.

Beyond osmium: progress in 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes

Olefin 1,2-difunctionalization has emerged as a popular strategy within modern synthetic chemistry for the synthesis of vicinal amino alcohols and derivatives. The advantage of this approach is the single-step simplicity for rapid diversification, feedstock nature of the olefin starting materials, and the possible modularity of the components. Although there is a vast number of possible iterations of 1,2-olefin difunctionalization, 1,2-amino oxygenation is of particular interest due to the prevalence of both oxygen and nitrogen within pharmaceuticals, natural products, agrochemicals, and synthetic ligands. The Sharpless amino hydroxylation provided seminal results in this field and displayed the value in achieving methods of this nature. However, a vast number of new and novel methods have emerged in recent decades. This review provides a comprehensive review of modern advances in accomplishing 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes that move beyond osmium to a range of other transition metals and more modern strategies such as electrochemical, photochemical, and biochemical reactivity.

Copper-catalyzed enantioselective synthesis of bridged bicyclic ketals from 1,1-disubstituted-4-methylene-1,6-hexanediols and related alkenols

Bridged bicyclic ketals display a range of bioactivities. Their catalytic enantioselective synthesis from acyclic 1,1-disubstituted alkene diols is disclosed. This reaction combines asymmetric catalysis with a distal radical migration. Alkynes and arenes undergo the group transfer.

Visible light photoredox alkylazidation of alkenes with sodium azide and heteroarenium salts: entry to azido-containing 1,4-dihydropyridines

A three-component alkene alkylazidation using sodium azide as the azido resource and heteroarenium salts as functionalized alkyl reagents for producing highly valuable 2-azido-1-(1,4-dihydropyridin-4-yl)-ethanes is described. This reaction allows the incorporation of both an azido group and a 1,4-dihydropyridin-4-yl group across C[double bond, length as m-dash]C bonds to construct two new bonds in a single reaction step, and represents a practical and mechanistically distinct alternative that harnesses an electrophilic heteroarenium ion to accomplish the alkene difunctionalization reaction.

Efficient Stereoselective Carbocyclization to cis-1,4-Disubstituted Heterocycles Enabled by Dual Pd/Electron Transfer Mediator (ETM) Catalysis

An efficient Pd/ETM (ETM = electron transfer mediator)-cocatalyzed stereoselective oxidative carbocyclization of dienallenes under aerobic oxidation conditions has been developed to afford six-membered heterocycles. The use of a bifunctional cobalt complex [Co(salophen)-HQ] as hybrid ETM gave a faster aerobic oxidation than the use of separated ETMs, indicating that intramolecular electron transfer between the hydroquinone unit and the oxidized metal macrocycle occurs. In this way, a class of important cis-1,4-disubstituted six-membered heterocycles, including dihydropyran and tetrahydropyridine derivatives were obtained in high diastereoselectivity with good functional group compatibility. The experimental and computational (DFT) studies reveal that the pendent olefin does not only act as an indispensable element for the initial allene attack involving allenic C(sp³)–H bond cleavage, but it also induces a face-selective reaction of the olefin of the allylic group, leading to a highly diastereoselective formation of the product. Finally, the deuterium kinetic isotope effects measured suggest that the initial allenic C(sp³)–H bond cleavage is the rate-limiting step, which was supported by DFT calculations.

Oxyamination of Unactivated Alkenes with Electron-Rich Amines and Acids via a Catalytic Triiodide Intermediate

An aerobic catalytic oxidation process is described for the olefin oxyamination using acids and primary amines as the sources of O and N. Our mechanistic findings point to the formation of triiodide as a critical catalytic intermediate to account for the tolerance of electron-rich nucleophiles. This dual iodide and copper catalytic system is suitable for a formal [5+1] annulation process to access valuable lactam structures and highlighted by the synthesis of the pharmaceutical Zamifenacin.

Selenophosphoramide-catalyzed diamination and oxyamination of alkenes

Scavenging fluoride from a selenophosphoramide-catalyzed alkene oxidation reaction suppresses the known syn-elimination pathway, enabling alkene diamination/oxyamination reactions via substitution.

A new selenophosphoramide-catalyzed diamination of terminal- and trans-1,2-disubstituted olefins is presented. Key to the success of this transformation was the introduction of a fluoride scavenger, trimethylsilyl trifluoromethanesulfonate (TMSOTf), to prevent a competitive syn-elimination pathway, as was the use of a phosphoramide ligand on selenium to promote the desired substitution reaction. A screen of catalysts revealed that more electron-rich phosphine ligands resulted in higher yields of the desired product, with selenophosphoramides giving the optimal results. A broad range of substrates and functional groups were tolerated and yields were generally good to excellent. For (E)-1,2-disubstituted olefins, diastereoselectivities were always high, giving exclusively anti products. The conditions were also applied to substrates bearing internal nucleophiles such as esters and carbonates, giving rise to 1,2-aminoesters and cyclic carbonates, respectively.

Ligand Controlled Ir-Catalyzed Regiodivergent Oxyamination of Unactivated Alkenes

An intramolecular Ir(III)-catalyzed regiodivergent oxyamination of unactivated alkenes provides valuable γ-lactams, γ-lactones and δ-lactams. The regioselectivity is controlled by the electronically tunable cyclopentadienyl Ir(III)-complexes enabling oxyamination via either 5-exo or 6-endo pathways. With respect to the mechanism, we propose a highly reactive [3.1.0] bicycle intermediate derived from Ir(V) nitrene-mediated aziridination to be a key intermediate toward the synthesis of γ-lactams.

Pd(ii)-Catalyzed aerobic 1,2-difunctionalization of conjugated dienes: efficient synthesis of morpholines and 2-morpholones

A novel and efficient methodology concerning the Pd(ii)-catalyzed intermolecular difunctionalization of conjugated dienes is reported to synthesize a series of functionalized morpholines and 2-morpholones. Widely distributed and easily obtained β-amino alcohols and α-amino acids, as starting nitrogen and oxygen sources, are successfully applied in the difunctionalization of conjugated dienes respectively. The majority of the desired products were obtained in moderate to excellent yields. Oxygen was successfully employed as a terminal oxidant. Further transformation of the generated products allowed for the expansion of structural diversity.

Rhodium-Catalyzed Cyclization of O,ω-Unsaturated Alkoxyamines: Formation of Oxygen-Containing Heterocycles

O,ω-Unsaturated N-tosyl alkoxyamines undergo unexpected Rh^III -catalyzed intramolecular cyclization by oxyamination to produce oxygen-containing heterocycles. Mechanistic studies show that an aziridine intermediate seems to be responsible for the formation of the heterocycles, possibly via a Rh^V species.

Stereoselective and Regioselective Synthesis of Heterocycles via Copper-Catalyzed Additions of Amine Derivatives and Alcohols to Alkenes

This Perspective describes the development of a family of copper(II)-catalyzed alkene difunctionalization reactions that enable stereoselective addition of amine derivatives and alcohols onto pendant unactivated alkenes to provide a range of valuable saturated nitrogen and oxygen heterocycles. 2-Vinylanilines and related substrates undergo alternative oxidative amination or allylic amination pathways, and these reactions will also be discussed. The involvement of both polar and radical steps in the reaction mechanisms have been implicated. Major product formation is a function of the lowest energy pathway, which in turn is a function of structural aspects of the various reaction components.

Practical Synthetic Procedures for the Iron-Catalyzed Intermolecular Olefin Aminohydroxylation Using Functionalized Hydroxylamines

A set of practical synthetic procedures for the iron-catalyzed intermolecular olefin aminohydroxylation reactions in gram scale is reported. In these transformations, a bench-stable functionalized hydroxylamine is applied as the amination reagent. This method is compatible with a broad range of synthetically valuable olefins including those that are incompatible with the existing aminohydroxylation methods. It also provides valuable amino alcohol building blocks with regio- and stereo-chemical arrays that are complementary to known methods.

Catalyst free annulative thioboration of unfunctionalized olefins

A direct and catalyst-free annulative thioboration of unfunctionalized olefins has been developed.

Copper-mediated annulation of 2-(1-arylvinyl) anilines and aryl nitrosos towards 2,3-diaryl-2H-indazoles

A copper-mediated annulation of 2-(1-substituted vinyl)anilines and aryl nitrosos is developed, giving 2,3-diaryl pyrazoles in moderate to good yields.

Catalytic Asymmetric Synthesis of Morpholines. Using Mechanistic Insights To Realize the Enantioselective Synthesis of Piperazines

An efficient and practical catalytic approach for the enantioselective synthesis of 3-substituted morpholines through a tandem sequential one-pot reaction employing both hydroamination and asymmetric transfer hydrogenation reactions is described. Starting from ether-containing aminoalkyne substrates, a commercially available bis(amidate)bis(amido)Ti catalyst is utilized to yield a cyclic imine that is subsequently reduced using the Noyori-Ikariya catalyst, RuCl [(S,S)-Ts-DPEN] (η⁶-p-cymene), to afford chiral 3-substituted morpholines in good yield and enantiomeric excesses of >95%. A wide range of functional groups is tolerated. Substrate scope investigations suggest that hydrogen-bonding interactions between the oxygen in the backbone of the ether-containing substrate and the [(S,S)-Ts-DPEN] ligand of the Ru catalyst are crucial for obtaining high ee's. This insight led to a mechanistic proposal that predicts the observed absolute stereochemistry. Most importantly, this mechanistic insight allowed for the extension of this strategy to include N as an alternative hydrogen bond acceptor that could be incorporated into the substrate. Thus, the catalytic, enantioselective synthesis of 3-substituted piperazines is also demonstrated.

Azidation in the Difunctionalization of Olefins

Organic azides are key motifs in compounds of relevance to chemical biology, medicinal chemistry and materials science. In addition, they also serve as useful building blocks due to their remarkable reactivity. Therefore, the development of efficient protocols to synthesize these compounds is of great significance. This paper reviews the major applications and development of azidation in difunctionalization of olefins using azide reagents.

Copper-catalyzed aminooxygenation of styrenes with N-fluorobenzenesulfonimide and N-hydroxyphthalimide derivatives

A copper-catalyzed aminooxygenation reaction of styrenes with N-fluorobenzenesulfonimide and N-hydroxyphthalimide derivatives has been developed. The aminooxygenation product could be converted into the corresponding alcohol or free amine through the cleavage of the N–O or C–N bond of the N-hydroxyphthalimide moiety.