Highly Conjugated π-Systems Arising from Cannibalistic Hexadehydro-Diels–Alder Couplings: Cleavage of C−C Single and Triple Bonds

We have investigated the cannibalistic self-trapping reaction of an ortho-benzyne derivative generated from 1,11-bis(p-tolyl)undeca-1,3,8,10-tetrayne in an HDDA reaction. Without adding any specific trapping agent, the highly reactive benzyne is trapped by another bisdiyne molecule in at least three different modes. We have isolated and characterized the resulting products and performed high-level calculations concerning the reaction mechanism. During the cannibalistic self-trapping process, either a C≡C triple bond or an sp–sp³ C−C single bond is cleaved. Up to seven rings and nine C−C bonds are formed starting from two 1,11-bis(p-tolyl)undeca-1,3,8,10-tetrayne molecules. Our experiments and calculations provide considerable insight into the variety of reaction pathways which the ortho-benzyne derivative, generated from a bisdiyne, can take when reacting with another bisdiyne molecule.     

Mechanistic Insights into Rhenium-Catalyzed Regioselective C-Alkenylation of Phenols with Internal Alkynes

A (μ-aryloxo)rhenium complex was isolated and confirmed as a key precatalyst for rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes. The reaction exclusively provided ortho-alkenylphenols; the formation of para or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products was not observed. Several mechanistic experiments excluded a classical Friedel–Crafts-type mechanism, leading to the proposed phenolic hydroxyl group assisted electrophilic alkenylation as the most plausible reaction mechanism. For this purpose, the use of rhenium, a metal between the early and late transition metals in the periodic table, was key for the activation of both the soft carbon–carbon triple bond of the alkyne and the hard oxygen atom of the phenol, at the same time. ortho-Selective alkenylation with allenes also provided the corresponding adducts with a substitution pattern different from that obtained by the addition reaction with alkynes.     

不饱和半夹芯式16e化合物Cp*Ir(S2C2B10H10)与邻、间位取代苯基叠氮的反应性

考虑取代基的位置和电子效应对反应体系的影响, 本文系统地研究了16e化合物Cp^*Ir(S₂C₂B₁₀H₁₀) (1)与邻、间位取代苯基叠氮的反应。研究结果表明:与邻、间位取代苯基叠氮反应均生成苯环邻位碳发生C-H 活化形成C-S 键的金属配合物。这些配合物通过核磁(¹H、¹¹B、¹³C)、红外、质谱、元素分析和单晶结构解析进行了全面地表征。在光照反应结果的基础上, 提出了形成这类产物的自由基机理。     

HIGH TEMPERATURE REACTIONS OF HYDROGEN SULFIDE AND THIOLS WITH ORGANIC COMPOUNDS

Abstract

The high temperature reaction of hydrogen sulfide with chloro- and bromosubstituted aromatic and heteroaromatic compounds is a convenient method for synthesis of the corresponding thiols and sulfides. The reaction of hydrogen sulfide with ortho-dihalosubstituted aromatic compounds may be directed toward the formation of thianthrene or dibenzo thiophene and their derivatives. The high temperature reaction of thiophenol with chloro- and bromoderivatives of aromatic and heteroaromatic compounds affords the corresponding mixed sulfides. The reaction with ortho-substituted halogen derivatives leads to formation of the sulfur heterocycles of the thianthrene, thioxanthene and dibenzothiophenene series.

The high temperature reaction of hydrogen sulfide with vinyl chloride gives vinylthiol and thiophene. Hydrogen sulfide initiates pyrolytic transformations of thiophene, aniline and benzaldehyde into dithienyls, 5.10-dihydrophenazine and stilbene, respectively. The reaction mechanism has been discussed.     

Mesomorphic behaviour and TSDC measurements of ortho-metallated palladium(II) and platinum(II) complexes with S,O-donor co-ligands

A series of ortho-metallated Pd and Pt complexes containing an imine ligand carrying three alkoxy chains and N-benzoylthiourea derivatives as co-ligands were prepared and their liquid crystalline properties investigated. Their structures were assigned based on elemental analysis, IR and ¹H NMR spectroscopy, whereas thermal properties were investigated by differential scanning calotimetry and polarising optical microscopy. All the compounds exhibit monotropic transitions involving nematic and smectic A phases, with the mesomorphic behaviour strongly related to the type of N-benzoylthiourea as well as the metal centre used. The thermally stimulated depolarisation current technique was employed to determine the conduction mechanism, phase transition temperature and the activation energies for one of the ortho-metallated Pd complexes.     

Non conventional syntheses of heterocyclic compounds. 2. Synthesis of 1H-4,5-dihydro-1,3,4-benzotriazepine derivatives

Arylazoanils carrying aromatic methyl or ethyl groups in the position ortho to the imine function thermally cyclize to give 1H-4,5-dihydro-1,3,4-benzotriazepine derivatives in fairly good yields. A study of the effect of the substituents on both the aryl rings is reported. The reaction is interpreted as an internal oxo-reduction process.     

Gold(I)‐Catalyzed Generation of the Two Components of a Formal [4+2] Cycloaddition Reaction for the Synthesis of Tetracyclic Pyrano[2,3,4‐de]chromenes

Ortho‐Alkynylbenzaldehydes have been widely used to generate isochromenylium derivatives through gold‐catalyzed cycloisomerization. These isochromenylium derivatives have been exploited as formal diene derivatives for reactions with different dienophiles. Herein, we describe the behavior of ortho‐alkynylsalicylaldehydes, a particular case of ortho‐alkynylbenzaldehydes. The gold‐catalyzed cycloisomerization of ortho‐alkynylsalicylaldehydes delivers an unusual heterodiene derivative that reacts with electron‐rich alkenes through a formal [4+2] cycloaddition. In this reaction, both the diene and dienophile are generated in situ through gold‐catalyzed cycloisomerization of appropriate alkynamines or alkynols. This reaction was used to synthesize complex tetracyclic pyrano[2,3,4‐de]chromenes from two very simple starting materials (an ortho‐alkynylsalicylaldehyde and an alkynamine or alkynol) with complete atom economy and with selective formation of bonds, cycles, and stereocenters.     

Synthesis of Cyclic and Acyclic ortho-Aryloxy Diaryliodonium Salts for Chemoselective Functionalizations

Two regioselective, high-yielding one-pot routes to oxygen-bridged cyclic diaryliodonium salts and ortho-aryloxy-substituted acyclic diaryliodonium salts are presented. Starting from easily available ortho-iodo diaryl ethers, complete selectivity in formation of either the cyclic or acyclic product could be achieved by varying the reaction conditions. The complimentary reactivities of these novel ortho-oxygenated iodonium salts were demonstrated through a series of chemoselective arylations under metal-catalyzed and metal-free conditions, to deliver a range of novel, ortho-functionalized diaryl ether derivatives.     

Closing the Cycle as It Begins: Synthesis of ortho-Iodobiaryls via Catellani Reaction

Despite the advances in the field of carbon-halogen bond formation, the straightforward catalytic access to selectively functionalized iodoaryls remains a challenge. Here, we report a one-pot synthesis of ortho-iodobiaryls from aryl iodides and bromides by palladium/norbornene catalysis. This new example of Catellani reaction features the initial cleavage of a C(sp²)−I bond, followed by the key formation of a palladacycle through ortho C−H activation, the oxidative addition of an aryl bromide and the ultimate restoration of the C(sp²)−I bond. A large variety of valuable o-iodobiaryls has been synthesized in satisfactory to good yields and their derivatization have been described too. Beyond the synthetic utility of this transformation, a DFT study provides insights on the mechanism of the key reductive elimination step, which is driven by an original transmetallation between palladium(II)-halides complexes.     

Thermally Induced Twin Polymerization of 4H‐1,3,2‐Benzodioxasilines

The twin monomer 2,2′‐spirobi[4H‐1,3,2‐benzodioxasiline] ( 1 ) can be polymerized to nanostructured SiO₂/phenolic‐resin composite material by thermally induced twin polymerization. Thermally induced twin polymerization represents a way to produce nanocomposites simply by thermal induction of twin monomers. Besides 1 , the thermal reaction of several related salicylic (2‐oxybenzylic) silicon molecules has been investigated. The thermal cleavage of the molecules is studied by using several trapping reagents (e.g., vinyl compounds). A significant occurrence of quinone methide adducts indicates that the thermal mechanism proceeds not only by a ring opening at the oxymethylene position, but also with the ortho‐quinone methide as a central or alternative intermediate. This is supported by product analyses of thermally initialized reactions of 1 and its substituted analogues as well as by quantum chemical calculations.     

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus - Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C₄H₄ to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals. The o-C₆H₄+C₄H₄ reaction is highly exothermic, by ∼143 kcal/mol to form naphthalene and by 31–32 kcal mol⁻¹ to produce naphthyl radicals plus H, but features relatively high entrance barriers of 9–11 kcal mol⁻¹. Although the reaction is rather slow, much slower than the reaction of phenyl radical with vinylacetylene, it forms naphthalene and 1- and 2-naphthyl radicals directly, with their relative yields controlled by the temperature and pressure, and thus represents a viable source of the naphthalene core under conditions where ortho-benzyne and vinylacetylene are available.     

Chiral N,N′‐Dioxide–Scandium(III) Complex‐Catalyzed Asymmetric Friedel–Crafts Alkylation Reaction of ortho‐Hydroxybenzyl Alcohols with C3‐Substituted N‐Protected Indoles

The first Lewis acid catalyzed asymmetric Friedel–Crafts alkylation reaction of ortho‐hydroxybenzyl alcohols with C3‐substituted indoles is described. A chiral N,N′‐dioxide Sc(OTf)₃ complex served not only to promote formation of ortho‐quinone methides (o‐QMs) in situ but also induced the asymmetry of the reaction. This methodology enables a novel activation of ortho‐hydroxybenzyl alcohols, thus affording the desired chiral diarylindol‐2‐ylmethanes in up to 99 % yield and 99 % ee. A range of functional groups were also tolerated under the mild reaction conditions. Moreover, this strategy gives concise access to enantioenriched indole‐fused benzoxocines.     

ortho‐Directing Chromium Arene Complexes as Efficient Mediators for Enantiospecific C(sp2)–C(sp3) Cross‐Coupling Reactions

A new strategy for the coupling of a broad scope of electronically diverse aromatics to boronic esters is reported. The coupling sequence, which relies on the directed ortho‐lithiation of chromium arene complexes followed by boronate formation and oxidation, occurs with complete ortho‐selectivity and enantiospecificity to give the coupling products in excellent yields and with high functional group tolerance. An intermediate chromium arene boronate complex was characterized by X‐ray, NMR, and IR experiments to elucidate the reaction mechanism.     

1,3-Dipolar cycloaddition of azomethine imines to ethynyl hetarenes: A synthetic route to 2,3-dihydropyrazolo[1,2-a]pyrazol-1(5H)-one based heterobiaryls

π-Deficient ethynyl hetarenes were used as dipolarophiles in a 1,3-dipolar cycloaddition reaction with azomethine imines (2-arylidene-5-oxopyrazolidin-2-ium-1-ides). Both CuI-catalyzed and catalyst-free thermally induced reactions proceeded with high regioselectivity providing 6-hetaryl-5-aryl-2,3-dihydropyrazolo[1,2-a]pyrazol-1(5H)-ones in moderate to excellent yields. The ethynyl hetarenes (pyridines, pyrazines, quinoxalines, pteridines and pyrimido[4,5-c]pyridazines) with ortho-methyl, ortho-cyano and ortho-alkynyl substituents were applicable to this reaction. 1,3-Dipolar cycloaddition reactions of alkynyl hetarenes with azomethine imines or other 1,3-dipole reagents can be considered as an alternative synthetic approach to heterobiaryls.     

Iodane-Guided ortho C−H Allylation

A metal-free C−H allylation strategy is described to access diverse functionalized ortho-allyl-iodoarenes. The method employs hypervalent (diacetoxy)iodoarenes and proceeds through the iodane-guided “iodonio-Claisen” allyl transfer. The use of allylsilanes bearing electron-withdrawing functional groups unlocks the functionalization of a broad range of substrates, including electron-neutral and electron-poor rings. The resulting ortho-allylated iodoarenes are versatile building blocks, with examples of downstream transformation including a concise synthesis of the experimental antimitotic core of Dosabulin. DFT calculations shed additional light on the reaction mechanism, with notable aspects including the aromatic character of the transition-state structure for the [3,3] sigmatropic rearrangement, as well as the highly stereoconvergent nature of the trans-product formation.     

Pulse radiolysis studies on the polymerization of 1,6-hexanediol diacrylate in cyclohexane solvent

The mechanism of initiation and kinetics of radiation induced polymerization of 1,6-hexanediol diacrylate (HDDA) was studied by pulse radiolysis with optical detection in cyclohexane solution and in pure HDDA at room temperature. In both cases a transient light absorption decreasing with the wavelength in the 270–400 nm wavelength range, but possessing a shoulder at 310 nm was observed. The spectrum did not change with time after the pulse. The absorption is due to α-carboxyalkyl radicals: the assignment was supported by the results obtained from the reaction of electrons with methyl 2-chloropropionate leading to dechlorination. The radicals produced from HDDA or from the chloro compound decayed in a second order process with rate parameters of 5 · 10⁸ and of 4 · 10⁹ mol^-1 dm³ s^-1 in cyclohexane. In the presence of oxygen the corresponding peroxy radicals decaying in very slow process (several milliseconds) were observed. The results are interrupted in terms of radical polymerization mechanism.     

Rhodium(III)‐Catalyzed ortho Alkenylation of N‐Phenoxyacetamides with N‐Tosylhydrazones or Diazoesters through CH Activation

A coupling reaction of N‐phenoxyacetamides with N‐tosylhydrazones or diazoesters through Rh^III‐catalyzed C? H activation is reported. In this reaction, ortho‐alkenyl phenols were obtained in good yields and with excellent regio‐ and stereoselectivity. Rh–carbene migratory insertion is proposed as the key step in the reaction mechanism.     

Synthesis and properties of copoly(pyromellitimide–benzimidazopiperidone)

Some new polymers and model compounds were obtained from the reaction of N,N-protected glutamic anhydride with primary amines and the ring-opening reaction was proved to belong to γ-orientation. When the primary amine is aromatic ortho-diamine, it was proved that the reaction route of ring-closure dehydration of the ring-opening addition products was through imidazolecarboxylic acid rather than imide amine, and at the last step it becomes imidazopiperidone. The corresponding polymer containing pyromellitdiimide and benzimidazopiperidone in the main chain was synthesized by a three-step reaction of 2,2′-pyromellitdiimidodiglutaric anhydride with 3,3′-diaminobenzidine. In the first step, polyaminoamic acid having pyromellitdiimide ring was prepared in polar solvents. In the second step, the conversion of polyaminoamic acid to the copolyimidoimidazole acid was carried out thermally in solution or solid at 140–200°C. At last, the conversion of copolyimidoimidazole acid to the copolypyromellitimidobenzimidazopiperidone was performed either thermally (300°C) or by chemical means. Infrared spectra and NMR spectra were studied and compared with corresponding model compounds. Form solubility and TG analysis, the polymer belongs to a heat-resistant polymer.     

Proximity effects in mass spectrometry. Electron impact ionization-induced cyclization of 1,2-bis(1,4-dithiafulven-6-yl)benzenes

The electron impact ionization mass spectra of o-, m- and p-bis(1,4-dithiafulven-6-yl)benzenes were studied by means of accurate mass measurements, metastable analysis and collision-induced dissociation. Differences observed in the spectra of the ortho isomers are due to a cyclization reaction leading to molecular ions with the same structure as those generated from certain cyclic compounds, as confirmed by comparison of linked scans at constant B/E of metastable and collisionally activated molecular ions. Parallels of this cyclization of molecular ions with their electrochemical or acid-induced isomerization are also discussed.     

Direct observation of o-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

Reactive ortho-benzyne derivatives are believed to be the initial products of liquid-phase [4 + 2]-cycloadditions between a 1,3-diyne and an alkyne via what is known as a hexadehydro-Diels–Alder (HDDA) reaction. The UV/VIS spectroscopic observation of o-benzyne derivatives and their photochemical dynamics in solution, however, have not been reported previously. Herein, we report direct UV/VIS spectroscopic evidence for the existence of an o-benzyne in solution, and establish the dynamics of its formation in a photoinduced reaction. For this purpose, we investigated a bis-diyne compound using femtosecond transient absorption spectroscopy in the ultraviolet/visible region. In the first step, we observe excited-state isomerization on a sub-10 ps time scale. For identification of the o-benzyne species formed within 50–70 ps, and the corresponding photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions, we employed two intermolecular trapping strategies. In the first case, the o-benzyne was trapped by a second bis-diyne, i.e., self-trapping. The self-trapping products were then identified in the transient absorption experiments by comparing their spectral features to those of the isolated products. In the second case, we used perylene for trapping and reconstructed the spectrum of the trapping product by removing the contribution of irrelevant species from the experimentally observed spectra. Taken together, the UV/VIS spectroscopic data provide a consistent picture for o-benzyne derivatives in solution as the products of photo-initiated HDDA reactions, and we deduce the time scales for their formation.

We report the transient ultraviolet/visible absorption spectrum of an o-benzyne species in solution for the first time.