Stereochemie von [3.3]- und [5.5]-sigmatropischen umlagerungen

The knowledge of the stereochemical course of Cope and Claisen rearrangements is important both for mechanistic and synthetic reasons. In the first part an analysis of the possible transition state geometries (cf Table 1) for these reactions is given and methods which allow to distinguish between the various geometries are discussed (cf Schemes 1 and 2). In the second part known examples are analyzed. Generally, in Cope and Claisen rearrangements of acyclic systems a chair-like transition state (C) is favoured above a boat-like (B). Steric effects may alter this preference.     

Synthesis and structure of dilacunary decatungstogermanate, [gamma-GeW10O36]8-

The dilacunary decatungstogermanate [gamma-GeW10O36]8- (1) has been synthesized and structurally characterized in solution and in the solid state. Reaction of germanium dioxide with sodium tungstate in aqueous acidic medium results in the formation of [beta2-GeW11O39]8- (2), which is then used as a precursor for the synthesis of 1. The (183)W spectrum of 2 shows the expected 11 peaks of equal intensity, whereas that of 1 exhibits the expected three peaks with relative intensities 2:2:1. Polyanion 1 represents a novel lacunary polyoxometalate, giving rise to a multitude of derivatives by reaction with transition metals, lanthanides, and other electrophiles.     

Die Dienol-Benzol-Umlagerung von Allyldienolen: aromatische [1,2]-, [3,3]- und [3,4]-sigmatropische Umlagerungen

The dienol-benzene rearrangement of syn and anti-4-allyl-4-methylcyclohexa-2,5-dien-1-ol (syn and anti 15) occurs by formation of a benzonium ion intermediate in p-toluene-sulphonic acid in ether below 0° and leads to a mixture of 2-, 3- and 4-allyltoluenes in the ratio 54:10:36. By the introduction of ¹⁴C-, D- and methyl labelled dienols it is shown that only the allyl group migrates and that this rearrangement is an intramolecular, one-step process. The formation of 2-allyltoluene occurs with retention, whereas the 3- and 4-allyltoluenes are formed by inversion of the carbon skeleton of the migrating allyl group. These rearrangements can be therefore classified as suprafacial, aromatic sigmatropic reactions of the order [1,2], [3,3] and [3,4]. The transition state can be postulated as representing a positively charged complex consisting of interacting allyl and tolyl radicals. The interaction of the two parts is controlled by the symmetry of the highest occupied π-orbitals (ψ₃ for toluene and ψ₂ for the allyl group) in agreement with the Woodward-Hoffmann rules. The better “distribution” of the charge in the transition state of these reactions in comparison to the ground state is chiefly responsible for the CoPE-like [3,3] sigmatropic reaction occurring at low temperatures. In general, sigmatropic reactions in charged systems are faster. The rearrangement of syn and anti 2-allyl-2-methylcyclohexa-3,5-dien-1-ol (syn and anti 28) gives results similar to those obtained with the para-allyldienols. The thermal rearrangement of 15 and 28 gives 3-allyltoluene by a [3,3] sigmatropic Cope rearrangement followed by elimination of water.     

Homogeneous [mono-125I-Tyr10]- and [mono-125I-Tyr13] glucagon

The two monoiodinated forms of glucagon were prepared by lactoperoxidase-catalysed iodination followed by separation by reversed-phase high-performance liquid chromatography. The intramolecular distribution of 125I was analysed by tryptic and chymotryptic cleavage of the isolated isomers. The results show that [mono-125I-Tyr10]- and [mono-125I-Tyr13]glucagon can be separated from each other and from the respective unlabelled polypeptide and thus can be obtained in a pure state with the highest possible specific activity. We have studied the receptor binding ability of both tracer isomers to isolated intact rat hepatocytes. The resulting Kd values were 2.0 +/- 0.2 nM for the tyrosine-13-labelled glucagon and 4.2 +/- 0.3 nM for the tyrosine-10-labelled glucagon.     

Dicyclobuta[de,ij]naphthalene and dicyclopenta[cd,gh]pentalene: a theoretical study

The structures, energetics, and aromatic character of dicyclobuta[de,ij]naphthalene, 1, dicyclopenta[cd,gh]pentalene, 2, dihydrodicyclobuta[de,ij]naphthalene, 3, and dihydrocyclopenta[cd,gh]pentalene, 4, have been examined at the B3LYP/6-311++G//B3LYP/6-31G level of theory. All molecules are bowl-shaped, and the pentalene isomers, 2 and 4, are most stable. A comparison with other C(12)H(6) and C(12)H(8) isomers indicates that 2 is approximately 25 kcal/mol less stable than 1,5,9-tridehydro[12]annulene and 4 is approximately 100 kcal/mol higher in energy than acenaphthylene, both of which are synthetically accessible. The transition state structure for bowl-to-bowl inversion of 1 is planar (D(2)(h)()) and lies 30.9 kcal/mol higher in energy than the ground state; the transition state for inversion of 2 is C(2)(h)() and lies 46.6 kcal/mol higher in energy. Symmetry considerations, bond length alternations, and NICS values (a magnetic criterion) all indicate that the ground states of 1, 3, and 4 are very aromatic; however, HOMA values (a measure of bond delocalization) indicate that 3S and 4S are aromatic but that 1S is less so. NICS values for the ground state of 2 strongly indicate aromaticity; however, bond localization, symmetry, and HOMA values argue otherwise.     

2-苯基苯并噁唑的光解反应

聚对苯撑苯并二噁唑(PBO)纤维对光较为敏感,在紫外光照射下会发生降解.本文研究了该纤维的单体2-苯基苯并噁唑(PO)的初级光化学反应机理.当PO分子吸收一个光子而跃迁到第一激发态后,克服25.59kJ·mol-1能垒而越过过渡态,此时噁唑环打开,且两个苯环形成大约90°的二面角而得到产物,该产物可进一步与空气中的水发生次级反应.计算结果表明在第一激发态上噁唑开环反应很容易,但在基态势能面并没有发现噁唑的开环路径.分子中的原子(AIM)的计算结果与上述分析过程相吻合.     

Stereoselective [2,3]-sigmatropic rearrangements of unstabilized nitrogen ylides

The steric course of the [2,3]-rearrangement of several unstabilized nitrogen ylides has been investigated. The reactions proceed cleanly through an anti transition state, affording modest to good yields of a single diastereomer of the product. In two examples containing an N-cinnamyl group, a competing [1,2]-rearrangement affords a minor product.     

Unsubstituted bicyclo[1.1.0]but-2-ylcarbinyl cations

A synthesis for the unsubstituted bicyclo[1.1.0]but-2-ylmethanols (endo- and exo-9) from 1,3-butadiene has been developed. Solvolyses of their sulfonates 10 and 11 took entirely different courses, as the endo compound 10 gave rise exclusively to rearranged products such as cyclopent-3-en-1-ol (14), while the exo compound 11 underwent only the substitution of the tosylate group with complete retention of the exo-bicyclo[1.1.0]but-2-ylmethyl skeleton. Under solvolytic conditions, 10 reacted at very similar rates to the corresponding monocyclic substrate, that is, cyclopropylcarbinyl mesylate (19); in contrast, 11 reacted only three times as fast as n-butyl tosylate and about 1000-fold slower than 10. The nature of the bicyclo[1.1.0]but-2-ylcarbinyl cations has been probed by quantum chemical calculations. Whereas, the exo isomer (exo-18) corresponds to a local energy minimum, the endo isomer is only a transition state [endo-18(TS)] for an automerization of the nonclassical cyclopent-3-en-1-yl cation (13) and converts into 13 by a Wagner-Meerwein rearrangement. The most favorable isomerization of exo-18 also leads to 13 but via a transition state resembling the 2-vinylcycloprop-1-yl cation [25(TS)]. On the introduction of methyl groups at positions 1 and 3 of exo-18, the cation is no longer an energy minimum and it becomes a transition state [27(TS)] for an automerization of the nonclassical 1,3-dimethylcyclopent-3-en-1-yl cation (28). The large effect of the methyl substitution rationalizes the puzzling results of the previous product and rate studies, which utilized various substituted derivatives of bicyclo[1.1.0]but-2-ylcarbinyl sulfonates as substrates.     

(4-溴甲基双环[4.4.1]-1,3,5,7,9-十一碳五烯基-3-)甲醇环氧化反应机理的理论研究

A reaction mechanism of epoxidation reaction of the 4-(bromomethyl)bicyclo4.4.1undeca-1,3,5,7,9-pentaen-3-ylmethanol has been studied by using the density functional theory(DFT) method at B3LYP level with 6 31G* basis set. The geometric structures of reactant, product and transition state have been optimized. The transition state is found by the QST2 method and characterized by the vibration frequency analysis. The intrinsic reaction coordinate(IRC) for this reaction is traced and confirms the reaction mechanism. The changes and the nature of related chemical bonds along the IRC path have been analyzed by the theory of electronic charge density. The result shows that the elimination reaction and ring closing reaction are synergistic, and the hydrogen in the elimination reaction derives from hydroxyl. In addition, the activation energy of the reaction is 139.2 kJ/mol.     

AlCl歧化反应分解法制备金属铝过程中[AlCl]_n的形成机理

利用密度泛函理论(DFT)中的广义梯度近似(GGA)法,对氧化铝真空还原氯化歧化法制备金属铝的过程中[AlCl]n(n=1-10)团簇的稳定结构、能量和团簇形成过程的过渡态及其振动频率进行了理论研究,预测了其基态结构及成长趋势.结果表明:[AlCl]n(n=1-10)团簇可能存在的结构都是以[Al]n骨架外接n个Cl原子成型,且具有较好的几何对称性;从过渡态计算得到的活化能来看,正过程的活化能总是比逆过程的活化能要小,即[AlCl]n(n=1-10)团簇具有较好的成长趋势.以上研究结果有助于进一步了解用碳热还原氯化歧化法制备金属铝过程中液态金属铝的生成.     

A multistate switchable [3]rotacatenane

Rotacatenanes are exotic molecular compounds that can be visualized as a unique combination of a [2]catenane and a [2]rotaxane, thereby combining both the circumrotation of the ring component (rotary motion) and the shuttling of the dumbbell component (translational motion) in one structure. Herein, we describe a strategy for the synthesis of a new switchable [3]rotacatenane and the investigation of its switching properties, which rely on the formation of tetrathiafulvalene (TTF) radical π-dimer interactions-namely, the mixed-valence state (TTF(2) )(+.) and the radical-cation dimer state (TTF(+.) )(2) -under ambient conditions. A template-directed approach, based on donor-acceptor interactions, has been developed, resulting in an improved yield of the key precursor [2]catenane, prior to rotacatenation. The nature of the binding between the [2]catenane and selected π-electron-rich templates has been elucidated by using X-ray crystallography and UV/Vis spectroscopy as well as isothermal titration microcalorimetry. The multistate switching mechanism of the [3]rotacatenane has been demonstrated by cyclic voltammetry and EPR spectroscopy. Most notably, the radical-cation dimer state (TTF(+.) )(2) has been shown to enter into an equilibrium by forming the co-conformation in which the two 1,5-dioxynaphthalene (DNP) units co-occupy the cavity of tetracationic cyclophane, thus enforcing the separation of TTF radical-cation dimer (TTF(+.) )(2) . The population ratio of this equilibrium state was found to be 1:1. We believe that this research demonstrates the power of constructing complex molecular machines using template-directed protocols, enabling us to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.     

Proton as the simplest of all catalysts for [2 + 2] cycloadditions: DFT study of acid-catalyzed imine metathesis

The mechanism of imine metathesis was studied as a prototype reaction for the impact that heteroatom substitution has on thermally forbidden [2 + 2] addition reactions using high-level density functional theory in combination with a continuum solvation model. The intuitively expected high activation barriers were confirmed for N-alkyl- and N-aryl-substituted imine reactants with transition state free energies of 78.8 and 68.5 kcal/mol, respectively, in benzene. The computed reaction energy profiles were analyzed to discover possible strategies for lowering the transition state energy. Protonation of the imine nitrogen was proposed as a possible catalytic route and was explicitly modeled. The computed reaction energy profile shows that protonation of one of the imine reactants has an enormous effect on the overall rate of metathesis and lowers the activation barrier by as much as 37.3 and 30.6 kcal/mol for the N-alkyl and N-aryl reactants, respectively. These results suggest that acid-catalyzed imine metathesis should be amenable at elevated temperatures. Furthermore, the protonation of both reactants of the metathesis reaction is predicted to be not productive owing to electrostatic repulsion of the reactants, thus suggesting that there should be an optimum pH for the catalytic turnover. A detailed analysis of the catalytic mechanism is presented, and the primary driving force for the catalysis is identified. Upon protonation of the imine nitrogen, the key [2 + 2]-addition step becomes asynchronous and one of the two intermolecular N-C bonds is formed before traversing the transition state, resulting in a substantial net decrease of the overall energy requirement. The general applicability of this intuitively understandable mechanism for designing structural features for lowering the energy of transition state structures is explored.     

Electrochemistry and [60]fullerene displacement reactions of (dihapto-[60]fullerene) pentacarbonyl metal(0) (M = Cr, Mo, W)

Cyclic voltammetry (CV) measurements on (eta(2)-C(60))M(CO)(5) complexes (M = Cr, Mo, W) in dichloromethane show three [60]fullerene-centered and reversible reduction/oxidation waves. The E(1/2) values of these waves are shifted to positive values relative to the corresponding values of the uncoordinated [60]fullerene in the same solvent. A Jahn-Teller type distortion of the spherical surface of [60]fullerene promoted by [60]fullerene-metal pi-backbonding may explain the observed positive shifts. Lewis bases (L = piperidine and triphenyl phosphine) displace [60]fullerene from (eta(2)-C(60))M(CO)(5) complexes. Analysis of the activation parameters for the metal-[60]fullerene dissociation, the metal-[60]fullerene bond enthalpies (from DFT computations), and metal-solvent (benzene) bond enthalpies (from DFT computations) suggests appreciable solvent contribution to the transition state leading to formation of the intermediate species solvent-M(CO)(5). Appreciable transition state stabilization due to solvation of the intermediate species is inferred for M = Mo and W. For M = Cr, stabilization of the intermediate species due to solvation is not accompanied by the corresponding transition state stabilization.     

Thermal [3,3]-rearrangement of 1,1-disubstituted allyl carboxylates: lone pair participation and the geminal bond participation

The diastereoselectivity of the [3,3]-rearrangement of 1,1-disubsstituted allyl carboxylates was studied. In this heteroatom-containing system, the transition state has a boat-like transition structure (TS) because of the participation of the lone pairs and the secondary orbital interaction. Although the TS for the [1,3]-rearrangement has a far higher barrier, it does not proceed in the usual antarafacial manner due to the cyclic orbital interaction among two lone pairs of the carboxylate and the allylic lumo. In conjunction with the geminal bond participation, delocalization to the σ-bond at the Z-position shows a bonding character in the transition state of the [3,3]-rearrangement. Therefore, we predicted that a more electron-withdrawing σ-bond prefers the Z-position in the product. We designed the 1,1-disubstituted substrates with trimethylsilyl and pentyl groups, and found that the trimethylsilyl group prefers the Z-position despite its steric bulkiness. We confirmed our prediction by experimentation. This Z-selectivity was improved when a trimethylgermyl group was used.     

[2+2] Cycloaddition of chlorosulfonyl isocyanate to allenyl-sugar ethers

The direction and magnitude of asymmetric induction in the [2+2] cycloaddition of chlorosulfonyl isocyanate to 3-O-allenyl-α-d-xylofuranoses was investigated. It is shown that gem terminal dimethylallenes react more readily than methyl free congeners. The configuration of adducts was established by X-ray and CD-spectroscopy of alkylidene cephams. A stereochemical model of the transition state for the [2+2] cycloaddition of chlorosulfonyl isocyanate and allenyl ethers is proposed, based on the lowest energy conformation of the cumulene.     

[10b, 10c]Diazapyraceheptylene

The labile [10b, 10c]diazapyraceheptylene 1 was obtained by thermoflash dehydrogenation of a mixture of olefins 5a/b. The ¹H NMR spectrum indicates a strong influence of the 14π-electron periphery. A by-product of the synthesis of 1, i.e., pyrazino[2,1,6-cd:5,4,3-c′d′]dipyrrolizine 6, points to a rearrangement of the diazapyraceheptylene system, which may be frontier orbital controlled.     

Supramolecular spintronic devices: spin transitions and magnetostructural correlations in [Fe4IIL4]8+ [2x2]-grid-type complexes

The magnetism of a series of tetranuclear complexes of the [Fe4IIL4]8+ [2x2]-grid-type was investigated, revealing the occurrence of spin transition behavior within this class of compounds. The phenomenon depends directly on the nature of the substituent R(1) in the 2-position on the central pyrimidine group of the ligand L. All Fe(II) ions in compounds with R(1) substituents favoring strong ligand fields (R(1)=H; OH) remain completely in the diamagnetic low-spin state. Only complexes bearing R(1) substituents attenuating the ligand field by steric (and to a lesser extent electronic) effects (R(1)=Me; Ph) exhibit spin transition behavior triggered by temperature. In general, gradual and incomplete transitions without hysteresis were observed for magnetically active complexes. The systems described provide approaches to the development of (supra)molecular spintronics.     

二环丁烷[1.1.0]开环反应的量子化学研究

本文用MNDO方法研究了二环丁烷[1.1.0]开环反应的反应途径, 并优化了该反应的反应物、产物和过渡态的结构, 在理论上证明了此开环反应的反应机理。     

聚丙烯酸[2,5-双(对甲氧基苯甲酰氧基)苄酯]及聚甲基丙烯酸[2,5-双(对甲氧基苯甲酰氧基)苄酯]的合成

周其凤等曾报道聚丙烯酸［２,５－双（对甲氧基苯甲酰氧基）节酯］［１］和聚甲基丙烯酸［２,５－双（对甲氧基苯甲酰氧基）苄酯］［２］的合成．但后来的研究发现,在合成单体的条件下出现的一种未见报道的异常反应［３］使产物成分复杂化,因此当时报道的聚合物可能不是聚丙烯酸［２,５－双－（对甲氧基苯甲酰氧基）苄酯］或聚甲基丙烯酸［２,５－双（对甲氧基苯甲酰氧基）苄酯］,而可能是共聚物．针对这一问题,我们重新设计了合成路线以避免发生上述副反应,成功地合成了丙烯酸或ａ－甲基丙烯酸［２,５－双－（对甲氧基苯甲酰氧基…