Synthesis and easy aromatisation of 5-substituted 6-(alkylthio)-2-methoxy-2,3-dihydropyridines. A new approach to the pyridine ring

Reaction of lithiated methoxyallene, 1-ethoxyethoxyallene, 1-(methylthio)propyne and 2-butyne with methoxymethyl isothiocyanate, MeOCH₂N=C=S followed by methylation affords the imidothioates H₂C=C=C(R)C(SMe)=NCH₂OMe [R=Me, OMe, OCH(Me)OEt, SMe]. Rearrangement to the fully conjugated systems H₂C=CH---C(R)=C(SMe)---N=CHOMe and subsequent electrocyclisation of these compounds leads to the 5-substituted 6-(methylthio)-2-methoxy-2,3-dihydropyridines with good to excellent yields. In the presence of acidic catalysts or by heating at elevated temperatures these dihydropyridines eliminate methanol to afford 3-substituted 2-(methylthio)pyridines. The aroma compound 2-(methylthio)-3-pyridinol was obtained by acid-catalysed treatment of 3-(1-ethoxyethoxy)-2-(methylthio)pyridine.     

He I and Ne I photoelectron spectra of dichloro- and dibromomalononitrile

He I and Ne I photoelectron spectra are reported for the densely populated valence regions of gaseous malononitrile, H₂C(CN)₂, dichloromalononitrile, Cl₂C(CN)₂, and dibromomalononitrile, Br₂C(CN)₂. A minor reassessment of the H₂C(CN)₂ assignments is extended to permit plausible assignments for the previously unreported dihalodicyano species. Semiempirical calculations, HAM/3 and MNDO for H₂C(CN)₂, and MNDO for the X₂C(CN)₂ molecules, are shown to be of limited value for the location of strongly localised nitrogen orbitals.     

Chemical fixation of carbon dioxide to cyclic carbonates under extremely mild conditions with highly active bifunctional catalysts

Chemical fixation of carbon dioxide to cyclic carbonates proceeds effectively under extremely mild temperature and pressure by using a bifunctional nucleophile–electrophile catalyst system of tetradentate Schiff-base aluminum complexes ((Salen)AlX) in conjunction with a quaternary ammonium salt (n-Bu₄NY) in the absence of any organic solvent. Electrophilicity of central Al³⁺ ion and the steric factor of substituent groups on the aromatic rings of (Salen)AlX (electrophile), and nucleophilicity and leaving ability of the anion Y⁻ of n-Bu₄NY (nucleophile) have a great effect on the catalytic activity of the bifunctional catalyst.     

Ergebnisse aus versuchen zur darstellung von bis(trifluormethylsulphanyl)thioketen (CF3S)2C=C=S

The attempted preparation of bis(trifluoromethylsulphanyl)thioketene is described. Mono-and di-(trifluoromethylsulphanyl)-substituted orthothioesters may be prepared fromCH₃C(SC₂H₅)₃ and CF₃SCl in the presence of anhydrous ZnCl₂. The unstable compoundshave been isolated and characterized. The corresponding CF₃Se and CF₃SO₂ derivativesare only formed as intermediates which decompose to ketene diethylmercaptal. Suchmono- and di-substituted products are obtained in good yield from H₂C=C(SC₂H₅)₂ andCF₃ECl (E=S, Se). The reaction of H₂C=C(SC₂H₅)₂ with CF₃SO₂F gave only poor yieldsof (CF₃SO₂)_nCH_2−n=C(SC₂H₅)₂ (n=1, 2) which were only capable of characterizationin etheral solution by spectral means. Attempts to prepare (CF₃S)₂C=C=S by refluxing(CF₃S)₂CHC(O)Cl, (CF₃S)₂CHC(O)OH or (CF₃S)₂C=C=O with P₄S₁₀ in toluene yieldedonly the cyclic dimer and the corresponding 1,3,4-trithiolan.     

Thiourea-catalyzed enantioselective cyanosilylation of ketones

The new chiral amino thiourea catalyst 3d promotes the highly enantioselective cyanosilylation of a wide variety of ketones. The hindered tertiary amine substituent plays a crucial role with regard to both stereoinduction and reactivity, suggesting a cooperative mechanism involving electrophile activation by thiourea and nucleophile activation by the amine.     

Generation and reactivity of simple chloro(aryl)carbenes within the cavities of nonacidic zeolites

A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus the sigma+ substituent parameter support a change in mechanism as the carbene center toggles between being electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is proposed upon reaction of the nucleophilic [Si-O-Al]- bridge with the carbene center, and for the electron-rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li+ cation is proposed.     

单相Co_2C的制备及其费-托合成催化性能研究

采用CO与金属钴在温度280℃,压力2 MPa的条件下反应48 h后制备得到单相Co_2C催化剂。通过XRD、H2-TPR、TEM和XAS对催化剂的结构和组成进行表征并考察了单相Co_2C催化剂在费-托合成反应中的稳定性与催化性能。结果表明,随着费-托合成反应的进行,Co_2C催化剂的活性缓慢上升,同时伴随着产物中甲烷的选择性逐渐降低,C5+的选择性逐渐升高。对比反应前后催化剂发现,反应后的催化剂为Co_2C和少量金属Co的混合相,表明在费-托合成反应条件下,单相Co_2C会发生部分分解,生成的金属Co会导致CO的转化率和产物的选择性发生变化。     

Probing the Electronic Demands of Transmetalation in the Palladium-Catalyzed Cross-Coupling of Arylsilanolates

The electronic characteristics of coupling partners in the transmetalation step for the cross-coupling reaction of arylsilanolates have been investigated. The ability to interrogate the transmetalation event by in situ preparation of the arylpalladium(II) silanolate intermediate has enabled a Hammett analysis for both the aryl electrophile and arylsilanolate to be conducted. These studies reveal that electron-donating groups on the silicon nucleophile and electron-withdrawing groups on the electrophile accelerate the transmetalation process.     

Ambiphilic boryl groups in a neutral Ni(ii) complex: a new activation mode of H2

The concept of metal–ligand cooperation opens new avenues for the design of catalytic systems that may offer alternative reactivity patterns to the existing ones. Investigations of this concept with ligands bearing a boron center in their skeleton established mechanistic pathways for the activation of small molecules in which the boron atom usually performs as an electrophile. Here, we show how this electrophilic behavior can be modified by the ligand trans to the boron center, evincing its ambiphilic nature. Treatment of diphosphinoboryl (PBP) nickel–methyl complex 1 with bis(catecholato)diboron (B₂Cat₂) allows for the synthesis of nickel(ii) bis-boryl complex 3 that promotes the clean and reversible heterolytic cleavage of dihydrogen leading to the formation of dihydroborate nickel complex 4. Density functional theory analysis of this reaction revealed that the heterolytic activation of H₂ is facilitated by the cooperation of both boryl moieties and the metal atom in a concerted mechanism that involves a Ni(ii)/Ni(0)/Ni(ii) process. Contrary to 1, the boron atom from the PBP ligand in 3 behaves as a nucleophile, accepting a formally protic hydrogen, whereas the catecholboryl moiety acts as an electrophile, receiving the attack from the hydride-like fragment. This manifests the dramatic change in the electronic properties of a ligand by tuning the substituent trans to it and constitutes an unprecedented cooperative mechanism that involves two boryl ligands in the same molecule operating differently, one as a Lewis acid and the other one as a Lewis base, in cooperation with the metal. In addition, reactivity towards different nucleophiles such as amines or ammonia confirmed the electrophilic nature of the Bcat moiety, allowing the formation of aminoboranes.

A bis(boryl)nickel complex promotes the facile and reversible activation of H₂ through a cooperative mechanism that involves the metal and both boryl moieties in a concerted five-center process.     

Regioselectivity in the ring opening of non-activated aziridines

In this critical review, the ring opening of non-activated 2-substituted aziridines via intermediate aziridinium salts will be dealt with. Emphasis will be put on the relationship between the observed regioselectivity and inherent structural features such as the nature of the C2 aziridine substituent and the nature of the electrophile and the nucleophile. This overview should allow chemists to gain insight into the factors governing the regioselectivity in aziridinium ring openings (81 references).     

The electronic structure of a diarsaallene –As=C=As– and a phosphaarsaallene –P=C=As–: UV photoelectron spectroscopy and theoretical studies

The electronic properties of a diarsaallene ArAs=C=AsAr and a phosphaarsaallene ArP=C=AsAr (Ar: 2,4,6-tri-tert-butylphenyl) have been investigated by using UV photoelectron spectroscopy and by density functional calculations on model compounds [(CH₃)₂C₆H₃Pn=C=AsC₆H₃(CH₃)₂, Pn: As, P]. Moreover, a comparison of the geometrical and electronic structures of the parent heteroallenes with those of the arsaethene H₂C=AsH and phosphaethene H₂C=PH has also been undertaken in order to determine the magnitude of the interaction between the π bond and the pnictogen lone pair n_Pn.     

四苯基卟啉及其衍生物的结构与性质研究(Ⅳ)——二质子四苯基卟啉及其衍生物的光谱、pKa与取代基常数间的线性关系

以HMO法剖析了二质子四苯基卟啉(H₂TPP²⁺)及其衍生物的紫外-可见光谱,确定了四苯基卟啉(TPP)和H₂TPP²⁺及其衍生物的λ_max、pK_a与取代基常数σ间的线性关系。     

Design of highly active binary catalyst systems for CO2/epoxide copolymerization: polymer selectivity, enantioselectivity, and stereochemistry control

Asymmetric, regio- and stereoselective alternating copolymerization of CO(2) and racemic aliphatic epoxides proceeds effectively under mild temperature and pressure by using a binary catalyst system of a chiral tetradentate Schiff base cobalt complex [SalenCo(III)X] as the electrophile in conjunction with an ionic organic ammonium salt or a sterically hindered strong organic base as the nucleophile. The substituent groups on the aromatic rings, chiral diamine backbone, and axial X group of the electrophile, as well as the nucleophilicity, leaving ability, and coordination ability of the nucleophile, all significantly affect the catalyst activity, polymer selectivity, enantioselectivity, and stereochemistry. A bulky chiral cyclohexenediimine backbone complex [SalcyCo(III)X] with an axial X group of poor leaving ability as the electrophile, combined with a bulky nuclephile with poor leaving ability and low coordination ability, is an ideal binary catalyst system for the copolymerization of CO(2) and a racemic aliphatic epoxide to selectively produce polycarbonates with relatively high enantioselectivity, >95% head-to-tail connectivity, and >99% carbonate linkages. A fast copolymerization of CO(2) and epoxides was observed when the concentration of the electrophile or/and the nucleophile was increased, and the number of polycarbonate chains was proportional to the concentration of the nucleophile. Electrospray ionization mass spectrometry, in combination with a kinetic study, showed that the copolymerization involved the coordination activation of the monomer by the electrophile and polymer chain growth predominately occurring in the nucleophile. Both the enantiomorphic site effect resulting from the chiral electrophile and the polymer chain end effect mainly from the bulky nucleophile cooperatively control the stereochemistry of the CO(2)/epoxide copolymerization.     

Group 14 chalcogenides featuring a bicyclo[3.3.0]octane skeleton

Reactions of mixtures of Cl₂MeSiSiMeCl₂ (1) and Me₂MCl₂ (M=Si, Ge, Sn) with either H₂S/NEt₃ or Li₂E (E=Se, Te) yielded the bicyclo[3.3.0]octanes Me₂M(E)₂Si₂Me₂(E)₂MMe₂. A carbon containing analog, (CH₂)₅C(S)₂Si₂Me₂(S)₂C(CH₂)₅, was prepared from 1 and (CH₂)₅C(SH)₂. Crystal structures of three of these compounds were determined and the observed conformations of the bicyclo[3.3.0]octane skeletons compared with results of density functional theory calculations. Another class of silchalcogenides featuring a bicyclo[3.3.0]octane skeleton, E(Me₂Si)₂Si₂Me₂(SiMe₂)₂E, was formed from the doubly branched hexasilane (ClMe₂Si)₂Si₂Me₂(SiMe₂Cl)₂ and H₂S/NEt₃ or Li₂E. All products were characterized by multinuclear NMR (¹H, ¹³C, ²⁹Si, ⁷⁷Se, ¹¹⁹Sn, and ¹²⁵Te).     

Conformational analysis of some acrylates using dipole moment calculations by CNDO/force method

CNDO MO calculations are carried out for four acrylates, H₂C=C (R₁)-COO (R₂) with R₁=H, Me and R₂=Me, Et for dipole moment determination using the Pople-Segal expression.⁷ Energy minimization for various conformers in each molecular system was achieved by the gradient method. Dipole moments for comparatively stable conformers of each of the molecules considered are reported. Theoretically calculated energy values are used to arrive at the equilibrium geometry and the corresponding dipole moments are compared with the experimental values in each system considered.     

Kinetic and thermodynamic stability of acenes: Theoretical study of nucleophilic and electrophilic addition

To understand the reactivity of acenes, particularly pentacene, the addition of HCl and water to acenes was studied for the benzene-nonacene series at the B3LYP/6-31G(d) level of theory. Surprisingly, the reactivity of the acenes increases along the series up to hexacene and remains constant from hexacene and above due to the biradical character of the ground state of higher acenes. While the exothermicity of HCl and water additions are very similar, the activation barriers for HCl and water additions differ by a constant factor of ca. 27 kcal/mol. The barrier for the addition of HCl varies from 44 kcal/mol for benzene to 16-18 kcal/mol for pentacene-nonacene, whereas the barrier for the addition of water varies from 71 kcal/mol for benzene to 43-46 kcal/mol for pentacene-nonacene. The transition states (TSs) for the addition of water to acenes are relatively "late" on the reaction coordinate, compared to the "earlier" TSs for the addition of HCl. There is a substantial substituent effect on the energy barriers for these reactions. HCl behaves as an electrophile, with rhoHCl (vs rho p) = -4.48 and -3.39 for anthracenes and pentacenes, respectively, while water behaves as a nucleophile, with rhoHCl (vs rho p) = 2.35 and 1.39 for anthracenes and pentacenes, respectively.     

Evidence of reversibility in azo-coupling reactions between 1,3,5-tris(N,N-dialkylamino)benzenes and arenediazonium salts

The reactions between strongly electron-rich aromatic substrates (1,3,5-tris(N,N-dialkylamino)benzenes, neutral carbon super nucleophiles) and diazonium salts produce moderately stable sigma complexes (Wheland complexes). The reactivity of Wheland complexes with electrophiles (other diazonium salts, or 4,7-dinitrobenzofuroxan) produces exchange reactions in the electrophilic part: the better electrophile replaces the less powerful electrophile. In the same way, in Wheland complexes with the 1,3,5-tris(morpholinyl)benzene, the 1,3,5-tris(piperidinyl)benzene replaces the less powerful nucleophile 1,3,5-tris(morpholinyl)benzene. Evidence is reported here indicating that for the title system the reaction of the attack of the electrophilic reagent producing Wheland complexes is a reversible process. The final products of the diazo-coupling reactions undergo a further attack of some diazonium salts. From the final products of the double diazo-coupling reactions (diazo compounds), we collected evidence that is a clear instance of complete reversibility of the diazo-coupling reaction.     

Synthesis and structural characterization of mixed ligand η-2-hydroxyacetophenone complexes of cobalt(III)

A tridentate Schiff base ligand [(CH₃)₂NCH₂CH₂N=C(CH₃)C₆H₄OH)] (LH) has been synthesized from 2-hydroxyacetophenone and 2-dimethylaminoethylamine. This ligand forms the neutral complexes [Co(L)(N₃){o-(CH₃C=O)C₆H₄O}] (1) and [Co(L)(SCN){o-(CH₃C=O)C₆H₄O}]·1/2H₂O (2) in presence of equivalent amount of Co(II) acetate, and sodium azide for 1 and sodium thiocyanate for 2. The complexes have been characterized by spectroscopic and crystallographic methods. The coordination geometry around Co(III) in both the complexes is distorted octahedral with one tridentate ligand L, one bidentate 2-hydroxyacetophenone and one monodentate azide for 1 and thiocyanate for 2. The azide and thiocyanate ligands in the two complexes occupy different positions relative to the coordination sites of L.     

Nucleophilicity and solvent effects on the kinetics of 4-(pyren-1-yl)thiazol-2-amine interaction with 4,6-dinitrobenzofuroxan

A multistep synthesis of novel pyrene-based thiazole moiety been has been realized following some synthetic challenges and complications. The chemical structure of the synthesized compound has been established on the basis of both spectroscopic and analytical tools. Its nucleophilic reactivity with 4,6-dinitrobenzofuroxan (DNBF) has been successfully studied in solution. A kinetic study of the covalent electrophile/nucleophile combination of dinitrobenzofuroxan (DNBF, electrophile) and 4-(pyren-1-yl)thiazol-2-amine (nucleophile) resulting in the formation of the corresponding σ-adduct in solution is reported. The rate constant (k₁) of the second-order relating to the CC bond forming step of this complexation process has been found to fit into the linear correlation log k = s_N (N + E), thereby permitting the evaluation of the nucleophilicity parameter (N) of the 4-(pyren-1-yl)thiazol-2-amine. 4-(Pyren-1-yl)thiazol-2-amine has been subsequently ranked according to its reactivity profile on the general nucleophilicity scale developed recently by Mayr et al., leading to an interesting and direct comparison over a large domain of π-, σ-, and n-nucleophiles.     

钒-铝催化丁二烯、丙烯交替共聚合的动力学研究(Ⅰ)——交替共聚合速度方程

本文研究了VO[OCH₂C(CH₃)₃]₂Cl-Ali-Bu₃催化丁二烯、丙烯交替共聚合的动力学特征,测定了单体和催化剂的反应级数,建立了聚合速度方程,并求得了-80℃-20℃温度范围内的一级速率常数K和总活化能E_T.