Unusual Skeletal Reorganization of Oxetanes for the Synthesis of 1,2-Dihydroquinolines

Skeletal reorganization is a type of fascinating transformations owing to their intriguing mechanisms and utility in complex molecule synthesis. However, only a limited amount of examples are known for most functional groups. Herein, we describe such an unusual process of oxetanes. In the presence of In(OTf)₃ as catalyst, oxetane-tethered anilines reacted unexpectedly to form 1,2-dihydroquinolines. This process not only provides expedient access to dihydroquinolines, but also represents a new reaction of oxetane. Mechanistically, it is believed that the reaction proceeds through initial nitrogen attack rather than arene attack followed by a series of bond cleavage and formation events. Control experiments provided important insights into the mechanism.     

Mild and Convenient Synthesis of 1,2‐Dihydroquinolines from Anilines and Acetone Catalyzed by Ytterbium(III) Triflate in Ionic Liquids

A mild, convenient, and efficient process has been developed for the synthesis of 2,2,4‐trimethyl‐1,2‐dihydroquinolines by the reaction of anilines with acetone catalyzed by ytterbium(III) triflate [Yb(OTf)₃] in ionic liquids. The catalyst and ionic liquids can be easily recovered and reused, making this method friendly and environmentally acceptable.     

Unraveling the Bonding Nature Along the Photochemically Activated Paterno-Büchi Reaction Mechanism

The photochemically activated Paterno-Büchi reaction mechanism following the singlet excited-state reaction path was analyzed based on a bonding evolution framework. The electronic rearrangements, which describe the mechanism of oxetane formation via carbon-oxygen attack (C−O), comprises of the electronic activation of formaldehyde and accumulation of pairing density on the O once the reaction system is approaching the conical intersection point. Our theoretical evidence based on the ELF topology shows that the C−O bond is formed in the ground-state surface (via C−O attack) returning from the S₁ surface accompanied by 1,4-singlet diradical formation. Subsequently, the reaction center is fully activated near the transition state (TS), and the ring-closure (yielding oxetane) involves the C−C bond formation after the TS. For the carbon-carbon attack (C−C), both reactants (formaldehyde and ethylene) are activated, leading to C−C bond formation in the S₁ excited state before reaching the conical intersection region. Finally, the C−O formation occurs in the ground-state surface, resulting from the pair density flowing primarily from the C to O atom.     

Brønsted acid (HNO3)-catalyzed tandem reaction of α-ketoesters and arylamines: efficient synthesis of 1,2-dihydroquinoline derivatives

A simple and convenient Brønsted acid (HNO₃)-catalyzed tandem reaction of α-ketoesters with primary or secondary aromatic amines for the synthesis of polysubstituted 1,2-dihydroquinolines has been developed via a tandem process, which has the advantages of ready availability of catalyst, operation simplicity, atom efficiency as well as low toxicity. In particular, tricyclic dihydroquinolines, generally prepared with multi-processes, could also be constructed in this one-pot procedure.     

Synthesis of multihydroxyl branched polyethers by cationic copolymerization of 3,3‐bis(hydroxymethyl)oxetane and 3‐ethyl‐3‐(hydroxymethyl)oxetane

The cationic ring‐opening polymerization of 3,3‐bis(hydroxymethyl)oxetane (BHMO) and the copolymerization of BHMO with 3‐ethyl‐3‐(hydroxymethyl)oxetane (EOX) were studied. Medium molecular weight polymers (number‐average molecular weight ≈ 2 × 10³) were obtained in bulk polymerization. Poly[3,3‐bis(hydroxymethyl)oxetane], as highly insoluble, was only characterized by gel permeation chromatography and NMR methods in the esterified form. Copolymers of BHMO and EOX that were slightly soluble in organic solvents were characterized in more detail. In a copolymerization from a 1:1 mixture, the comonomers were consumed at equal rates. Matrix‐assisted laser desorption/ionization time‐of‐flight analysis confirmed that a random 1:1 copolymer was formed. ¹³C NMR analysis indicated that in contrast to previously described homopolymers of EOX in which the degree of branching was limited, the homopolymers of BHMO were highly branched. This pattern was preserved in the copolymers; EOX units were predominantly linear, whereas BHMO units were predominantly branched. The copolymerization of BHMO with EOX provides, therefore, a route to multihydroxyl branched‐polyethers with various degrees of branching containing ? OH groups exclusively as ≡C? CH₂? OH units. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1991–2002, 2002     

FeCl3‐Catalyzed Ring‐Closing Carbonyl–Olefin Metathesis

Exploiting catalytic carbonyl–olefin metathesis is an ongoing challenge in organic synthesis. Reported herein is an FeCl₃‐catalyzed ring‐closing carbonyl–olefin metathesis. The protocol allows access to a range of carbo‐/heterocyclic alkenes with good efficiency and excellent trans diastereoselectivity. The methodology presents one of the rare examples of catalytic ring‐closing carbonyl–olefin metathesis. This process is proposed to take place by FeCl₃‐catalyzed oxetane formation followed by retro‐ring‐opening to deliver metathesis products.     

Synthesis of multi‐functionalized quinolines and 1,2‐dihydroquinolines through FeCl3‐mediated reactions of carbonyl compounds with 2‐vinylanilines

A facile and efficient approach toward the synthesis of functionalized quinolines and 1,2‐dihydroquinolines from carbonyl compounds and 2‐vinylanilines is described. The protocol utilizes the nonhazardous and less expensive FeCl₃ as catalyst with wide functional group tolerance and avoiding heavy metal impurities in the products.     

Theoretical study on photooxidation mechanism of ruthenium complex [Ru(II)‐(bpy)2(TMBiimH2)]2+ with molecular oxygen

Photoinduced reactions of ruthenium complexes with molecular oxygen have attracted a lot of experimental attention; however, the reaction mechanism remains elusive. In this work, we have used the density functional theory method to scrutinize the visible‐light induced photooxidation mechanism of the ruthenium complex [Ru(II)‐(bpy)₂(TMBiimH₂)]²⁺ (bpy: 2, 2‐bipyridine and TMBiimH₂: 4, 5, 4, 5‐tetramethyl‐2, 2‐biimidazole) initiated by the attack of molecular oxygen. The present computational results not only explain very well recent experiments, also provide new mechanistic insights. We found that: (1) the triplet energy transfer process between the triplet molecular oxygen and the metal‐ligand charge transfer triplet state of the ruthenium complex, which leads to singlet molecular oxygen, is thermodynamically favorable; (2) the singlet oxygen addition process to the S₀ ruthenium complex is facile in energy; (3) the chemical transformation from endoperoxide to epidioxetane intermediates can be either two‐ or one‐step reaction (the latter is energetically favored). These findings contribute important mechanistic information to photooxidation reactions of ruthenium complexes with molecular oxygen. © 2016 Wiley Periodicals, Inc.     

Redox initiated cationic polymerization of oxetanes

3,3‐Disubstituted oxetane monomers were found to undergo rapid, exothermic redox initiated cationic ring‐opening polymerization in the presence of a diaryliodonium or triarylsulfonium salt oxidizing agent and a hydrosilane reducing agent. The redox reaction requires a noble metal complex as a catalyst and several potential catalysts were evaluated. The palladium complex, Cl₂(COD)Pd^II, was observed to provide good shelf life stability while, at the same time, affording high reactivity in the presence of a variety of hydrosilane reducing agents. A range of structurally diverse oxetane monomers undergo polymerization under redox cationic conditions. When a small amount of an alkylated epoxide was added as a “kick‐start” accelerator to these same oxetanes, the redox initiated cationic polymerizations were extraordinarily rapid owing to the marked reduction in the induction period. A mechanistic interpretation of these results is offered. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1854–1861     

Synthesis of polyethers with unsymmetrical structures by the polyaddition of 3‐ethyl‐3‐(glycidyloxymethyl)oxetane with diacyl chlorides

Polyethers with unsymmetrical structures in the main chains and pendant chloromethyl groups were synthesized by the polyaddition of 3‐ethyl‐3‐(glycidyloxymethyl)oxetane (EGMO) with certain diacyl chlorides with quaternary onium salts or pyridine as catalysts. The unsymmetrical polyaddition of EGMO containing two different cyclic ether moieties such as oxirane and oxetane groups with terephthaloyl chloride proceeded smoothly in toluene at 90 °C for 6 h to give polymer 1 with a number‐average molecular weight (M_n) of 51,700 in a 93% yield when tetrabutylammonium bromide (TBAB) was used as a catalyst. The polyaddition also proceeded smoothly under the same conditions when other quaternary onium salts, such as tetrabutylammonium chloride, tetrabutylammonium iodide, tetrabutylphosphonium chloride, and tetrabutylphosphonium bromide, and pyridine were used as catalysts. However, without a catalyst no reaction occurred under the same reaction conditions. Polyadditions of EGMO with isophthaloyl chloride and adipoyl chloride gave polymer 2 (M_n = 28,700) and polymer 3 (M_n = 25,400) in 99 and 65% yields, respectively, under the same conditions. The chemical modification of the resulting polymer, polymer 1 , which contained reactive pendant chloromethyl groups, was also attempted with potassium 3‐phenyl‐2,5‐norbornadiene‐2‐carboxylate with TBAB as a phase‐transfer catalyst, and a polymer with 65 mol % pendant norbornadiene moieties was obtained. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 368–375, 2001     

Synthesis of photoreactive calixarene derivatives containing pendant cyclic ether groups

New photoreactive calixarene derivatives containing cationically polymerizable pendant oxetane groups (calixarenes 1a , b , 2a , b , and 3a , b ) were synthesized in good yields by the substitution reaction of C‐methylcalix[4]resorcinarene (CRA), p‐methylcalix[6]arene (MCA), and p‐tert‐butylcalix[8]arene (BCA) with (3‐methyloxetan‐3‐yl)methyl 4‐toluenesulfonate and (3‐ethyloxetan‐3‐yl)methyl 4‐toluenesulfonate with potassium hydroxide as a base and tetrabutylammonium bromide as a phase‐transfer catalyst in N‐methyl‐2‐pyrrolidone, respectively. Calixarene derivatives containing cationically polymerizable pendant oxirane groups (calixarenes 4 , 5 , and 6 ) were also prepared in good yields by the substitution reaction of CRA, MCA, and BCA with epibromohydrin, respectively, with cesium carbonate as a base in N‐methyl‐2‐pyrrolidone. The thermal stability of the obtained calixarene derivatives containing pendant oxetane groups or oxirane groups was examined with thermogravimetric analysis, and it was found that these calixarene derivatives had thermal stability beyond 340 °C. The photochemical reaction of calixarenes 1 , 2 , and 3 containing pendant oxetane groups was examined with certain photoacid generators in the film state. In this reaction system, calixarene 1a , composed of a CRA structure and pendant (3‐methyloxetan‐3‐yl)methyl groups, showed the highest photochemical reactivity when bis‐[4‐(diphenylsulfonio)phenyl]sulfide bis(hexafluorophosphate) was used as the catalyst. The photochemical reaction of calixarenes 4 , 5 , and 6 containing pendant oxirane groups was also examined, and it was found that the photoinitiated cationic polymerization of calixarenes 4 , 5 , and 6 proceeded smoothly under the same conditions; however, the reaction rates were lower than those of the corresponding calixarenes 1 , 2 , and 3 containing pendant oxetane groups. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1169–1179, 2001     

Lactonization of C(sp2)—H Bonds in Enamides with CO2

Herein, we report a novel synthesis of 1,3‐oxazin‐6‐ones from enamides with CO₂ through C—H carboxylation and one‐pot cyclization. This transition‐metal‐free and redox‐neutral process features broad substrate scope, good functional group tolerance and facile product derivatization. The nucleophilic attack to CO₂ from the electron‐rich alkene is demonstrated for this reaction.     

Molybdatophosphoric Acid/NaNO2/Wet SiO2 as an Efficient System for the Aromatization of 1,2‐Dihydroquinolines under Mild and Heterogeneous Conditions

A combination of molybdatophosphoric acid and NaNO₂ in the presence of wet SiO₂ was used as an effective oxidizing agent for the oxidation of 1,2‐dihydroquinolines to their corresponding quinoline derivatives in dichlomethane at room temperature with excellent yields.     

Photoinitiated cationic polymerization of oxetane formulated with oxirane

Photoinitiated cationic polymerization of oxetane, oxirane (epoxide), and a formulation of both was carried out and their reactivity compared. To investigate a formulated system of oxetane and oxirane in photoinitiated cationic polymerization, computational and experimental methods were used. In the computational study, we employed a semiempirical molecular orbital method (AM1). On the other hand, the reactivities of each system were evaluated and compared experimentally by a real-time FT-IR method. The computational study reveals that oxetane seems to polymerize in S_N2 mechanism, but two possibilities, of S_N1 mechanism through the α-cleavage and of S_N2 mechanism through β-cleavage, are implied for oxirane. Using the real-time FT-IR method, the formulation of oxetane and oxirane was proved to possess rather high reactivities of oxetane toward photoinitiated cationic polymerization. The formulated system exhibited slightly lower number-average molecular weight than oxetane but higher than oxirane. These experimental observations are well explained in terms of the calculated reaction paths. © 1995 John Wiley & Sons, Inc.     

Synthesis and properties of oxetane‐based polysiloxanes used for cationic UV curing coatings

The polysiloxanes end‐capped with oxetane group (PSiO‐H and PSiO‐L) were synthesized via hydrosilylation reaction based on α,ω‐dihydrogen‐terminated polydimethylsiloxanes with a higher (0.23%, PDSi‐H) and lower (0.12%, PDSi‐L) hydrogen amount. The molecular structures were characterized by FT‐IR and ¹H NMR spectroscopy. The polysiloxanes were added into a commercial oxetane‐based resin, 3,3′‐(oxydi(methylene)) bis(3‐ethyloxetane) (DOX), as an additive to prepare a series of cationic UV curable formulations. The photo‐DSC results showed that the maximum photopolymerization rate decreased while the oxetane conversion increased with the polysiloxane content increasing. The surface hydrophobic property of cured films was improved having the water contact angles of 97° and 99° compared with 82° of the cured DOX film with only 1 wt% PSiOs, respectively. The dynamic mechanical thermal analysis results showed that both the storage modulus on the rubbery plateau region and the glass transition temperature decreased with increasing PSiO‐H loading. Moreover, the decrease became more obvious as PSiO‐L was added instead of PSiO‐H due to its lower concentration of oxetane group. The thermal stability of cured films was enhanced by the addition of PSiOs from the thermogravimetric analysis. And the DOX/PSiO‐H film possessed higher thermal degradation temperatures than DOX/PSiO‐L film. Copyright © 2011 John Wiley & Sons, Ltd.     

Rhodium‐Catalyzed Intramolecular Hydroarylation of 1‐Halo‐1‐alkynes: Regioselective Synthesis of Semihydrogenated Aromatic Heterocycles

The regioselective intramolecular hydroarylation of (3‐halo‐2‐propynyl)anilines, (3‐halo‐2‐propynyl) aryl ethers, or (4‐halo‐3‐butynyl) aryl ethers was efficiently catalyzed by Rh₂(OCOCF₃)₄ to give semihydrogenated aromatic heterocycles, such as 4‐halo‐1,2‐dihydroquinolines, 4‐halo‐3‐chromenes, or 4‐(halomethylene)chromans, in good to excellent yields. Some synthetic applications taking advantage of the halo‐substituents of the products are also illustrated.     

Dihydroquinolines,Dihydronaphthyridines and Quinolones by Domino Reactions of Morita-Baylis-Hillman Acetates

An efficient synthetic route to highly substituted dihydroquinolines and dihydronaphthyridines has been developed using a domino reaction of Morita-Baylis-Hillman (MBH) acetates with primary aliphatic and aromatic amines in DMF at 50–90 °C. The MBH substrates incorporate a side chain acetate positioned adjacent to an acrylate or acrylonitrile aza-Michael acceptor as well as an aromatic ring activated toward S_NAr ring closure. A control experiment established that the initial reaction was an S_N2′-type displacement of the side chain acetate by the amine to generate the alkene product with the added nitrogen nucleophile positioned trans to the S_NAr aromatic ring acceptor. Thus, equilibration of the initial alkene geometry is required prior to cyclization. A further double bond migration was observed for several reactions targeting dihydronaphthyridines from substrates with a side chain acrylonitrile moiety. MBH acetates incorporating a 2,5-difluorophenyl moiety were found to have dual reactivity in these annulations. In the absence of O₂, the expected dihydroquinolines were formed, while in the presence of O₂, quinolones were produced. All of the products were isolated in good to excellent yields (72–93%). Numerous cases (42) are reported, and mechanisms are discussed.     

Lewis and Brønsted Acid Cocatalyzed Reductive Deoxyallenylation of Propargylic Alcohols with 2‐Nitrobenzenesulfonylhydrazide

Reductive deoxyallenylation of sterically hindered tertiary propargylic alcohols was realized on reaction with 2‐nitrobenzenesulfonylhydrazide (NBSH) by the combined use of Lewis and Brønsted acid catalysts. This method features a broad substrate scope, mild reaction conditions, and good functional‐group tolerance, and affords various mono‐, di‐, and trisubstituted allenes in good‐to‐excellent yields. The synthetic utility of this method was demonstrated by the synthesis of 2H‐chromenes and 1,2‐dihydroquinolines.     

Enantioselective Rhodium‐Catalyzed Dearomative Arylation or Alkenylation of Quinolinium Salts

A highly enantioselective rhodium(I)‐catalyzed dearomative arylation or alkenylation of easily available N‐alkylquinolinium salts is reported, thus providing an effective and practical approach to the synthesis of dihydroquinolines in up to 99 % ee. This reaction tolerates a wide range of functional groups with respect to both the organic boronic acids and the quinoline starting materials. Moreover, the synthetic utility of this protocol is demonstrated in the formal asymmetric synthesis of bioactive tetrahydroquinoline and the total syntheses of (?)‐angustureine and (+)‐cuspareine.     

Synthesis of polyesters with pendant oxetane groups by the chemoselective alternating copolymerization of 3‐ethyl‐3‐(glycidyloxymethyl)oxetane with carboxylic anhydride and its photochemical reaction

The synthesis of polyesters with pendant oxetane groups by the chemoselective alternating copolymerization of 3‐ethyl‐3‐(glycidyloxymethyl)oxetane (EGMO) with carboxylic anhydride and the photochemical reaction of the resulting polymer was examined. The alternating copolymerization of EGMO with phthalic anhydride proceeded chemoselectively with quaternary onium salts under appropriate reaction conditions, and the corresponding soluble polymers with pendant oxetane groups with number‐average molecular weights of 4700–7200 were obtained in 72–87% yields. Furthermore, the photochemical reaction of the resulting polymers was examined with certain photoacid generators in the film state upon UV irradiation, and it was found that the photocrosslinking reaction of the pendant oxetane groups proceeded smoothly to give the insoluble polymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1952–1961, 2003