Kinetics of thermal gas-phase isomerizations and fragmentations of cis- and trans-1-(E)-propenyl-2-methylcyclobutanes at 275 degrees C

Kinetic studies of the thermal isomerization and fragmentation reactions exhibited by cis- and trans-1-(E)-propenyl-2-methylcyclobutanes at 275 degrees C in the gas phase have provided first-order rate constants for cis,trans interconversions of the cyclobutanes, 1,3-carbon migrations leading to 3,4- and 3,6-dimethylcyclohexenes, isomerizations providing directly and indirectly four acyclic dienes, and fragmentations to ethylene, propene, and mixtures of pentadienes and hexadienes. Both cis and trans isomers of 1-(E)-propenyl-2-methylcyclobutane form trans-3,4-dimethylcyclohexene faster than they are converted to cis-3,4-dimethylcyclohexene; the trans reactant gives rise to cis-3,6-dimethylcyclohexene in preference to its trans isomer, while the cis starting material gives neither at measurable rates; both form the relatively minor product 1,6-(Z)-octadiene. The rate constants derived from 35 kinetic runs starting with four distinct 1-(E)-propenyl-2-methylcyclobutane samples are consistent to within narrow error limits. The stereomutations, isomerizations, and fragmentations of the 1-(E)-propenyl-2-methylcyclobutanes are interpreted in terms of competitive processes involving conformationally flexible short-lived 2-(E)-octene-4,7-diyl and 3-methyl-5-(E)-heptene-1,4-diyl diradicals.     

六配位磷质离子溶液的核磁共振研究

本文用含有两个不同双环五配位磷化物与甲氧基负离子反应, 通过加成和取代, 形成六配位磷负离子顺、反异构体. 反式体为动力学控制产物顺式体为热力学控制产物.     

α-三唑基查尔酮与苯肼的加成-关环反应研究

(Z)或(E)-1,3-二芳基-2-(1H-1,2,4-三唑-1-基)-2-丙烯-1-酮与苯肼在醋酸中反应均生成顺式和反式吡唑啉异构体混合物,但是在三乙胺催化下却只生成反式的异构体。制备了三对顺、反式的1-苯基-3,5-二芳基-4-(1H-1,2,4-三唑-1-基)-2-吡唑啉衍生物,其结构为^1HNMR和元素分析证实,初步论证了经α-氮原子的加成-环化反应机理。     

Harnessing the energy of molecular recognition in a nanomachine having a photochemical on/off switch

6A-Deoxy-6A-(N-methyl-3-phenylpropionamido)-beta-cyclodextrin operates as a molecular machine, where the amide group serves as a torsion bar to harness the work output resulting from extraction of 1-adamantanol and consequent complexation of the aryl substituent by the cyclodextrin, when the latter behave as the piston and cylinder, respectively, of a molecular pump. At 25 degrees C, the complexation changes the ratio of the amide (Z)- and (E)-isomers from 2.4:1 to 25:1, on which basis the work performed on the amide bond is calculated to be 1.4 kcal mol-1. trans-6A-Deoxy-6A-(N-methylcinnamido)-beta-cyclodextrin and the cis isomer function as a more advanced version of the machine, with the alkene moiety serving as a photochemical on/off switch. Irradiation at 300 nm converts the trans cinnamide to the cis isomer, while the reverse process occurs at 254 nm. With the cis isomer there is little interaction of the phenyl group with the cyclodextrin cavity, so in that mode the machine is turned off. By contrast, complexation of the aryl substituent by the cyclodextrin occurs with the trans cinnamide and changes the ratio of the amide (Z)- and (E)-isomers from 2.6:1 to 100:1. Consequently, in this mode the machine is turned on, and the work harnessed by the amide bond is 2.1 kcal mol-1.     

Oligometallic template strategy for ring-closing olefin metathesis: highly cis- and trans-selective synthesis of a 32-membered macrocyclic tetraoxime

An oligometallic template effect was observed on the cis/trans selectivity of a 32-membered macrocyclic tetraoxime in ring-closing olefin metathesis of an acyclic diallyl derivative H4L; the metathesis of heterotrinuclear complex LZn2M (M=Ca2+, La3+) afforded the cis isomer, whereas uncomplexed H4L gave the trans isomer.     

Ethyl (benzothiazol-2-ylsulfonyl)acetate: a new reagent for the stereoselective synthesis of alpha,beta-unsaturated esters from aldehydes

The title reagent engaged in the modified Julia olefination with aldehydes under mild reaction conditions (DBU, CH(2)Cl(2), rt or -78 degrees C) to yield alpha,beta-unsaturated esters; aryl aldehydes and aliphatic aldehydes possessing significant chain branching elements gave trans alkene products with high stereoselectivity (E : Z up to >98 : 2), while straight chain aliphatic aldehydes gave cis products preferentially (Z : E up to 92 : 8).     

Serine-cis-proline and serine-trans-proline isosteres: stereoselective synthesis of (Z)- and (E)-alkene mimics by Still-Wittig and Ireland-Claisen rearrangements

Two new amide isosteres of Ser-cis-Pro and Ser-trans-Pro dipeptides were designed and stereoselectively synthesized to be incorporated into potential inhibitors of the phosphorylation-dependent peptidylprolyl isomerase Pin1, an essential regulator of the cell cycle. The cis mimic, the (Z)-alkene isomer, was formed through the use of a Still-Wittig [2,3]-sigmatropic rearrangement, while the trans mimic, the (E)-alkene, was synthesized through the use of an Ireland-Claisen [3,3]-sigmatropic rearrangement. Starting from N-Boc-Ser(OBn)-N(OMe)Me, both mimics were synthesized in Boc-protected form suitable for peptide synthesis with an overall yield of 20% in 10 steps for the cis mimic and 13% in eight steps for the trans mimic.     

Photochemical E (trans)-->Z (cis) isomerization in substituted 1-naphthylacrylates.

Substituted naphthylacrylates, 1-3, not showing rotamerism have been synthesized with a view to study photochemical E (trans)-->Z (cis) isomerization. Photostationary state composition of the isomers upon direct excitation, triplet sensitized isomerization, quantum yield of isomerization, and steady state and time-resolved fluorescence behavior have been studied for these naphthylacrylates. The direct excitations of the compounds yield high Z (approximately 80%) isomer composition, whereas the triplet sensitization results in less Z (approximately 20%) isomer composition. This indicates that the singlet pathway is very efficient in converting the E isomer to the Z isomer. The naphthylacrylates 1 and 2 exhibit structured fluorescence at room temperature in hexane and upon changing the solvent to CH3CN; the structure of the fluorescence is lost, indicating that the singlet excited-state develops a polar character in a polar environment. The polar nature of the singlet excited state becomes more clear in the case of 3 from its fluorescence solvatochromism. The naphthylacrylates did not exhibit excitation wavelength-dependent fluorescence at room temperature suggesting that the ground state conformers (rotamers) are not involved. Fluorescence lifetimes measured for these compounds displayed biexponential behavior, which is explained using a two-state model.     

PHOTOACOUSTIC CALORIMETRY OF CYCLOHEPTENE DERIVATIVES. RELAXED TRIPLET ENERGIES OF 1-PHENYLCYCLOHEPTENE AND 1, 3-CYCLOHEPTADIENE AND THE ENERGETICS OF GROUND-STATE cis-trans ISOMERIZATION

Abstract— The heat of isomerization E _ct of cis -1-phenylcycloheptene ( c -PC7) to its highly strained trans isomer has been determined by a combination of a quantitative study of acid-catalyzed trapping of the trans isomer with time-resolved photoacoustic calorimetry. The relaxed triplet energy has been determined for PC7 and for cis, cis -1, 3-cycloheptadiene. The triplet energies are in line with previous studies of other alkenes but the E _ct value is significantly higher than that calculated by molecular mechanics calculations for cycloheptene. The significance of the results for cis-trans thermal isomerization is discussed.     

Stereoselective nitration of asymmetric hydrazones with nitric oxide

Nitration of asymmetric hydrazones with nitric oxide occurred stereoselectively at C1′-atom, giving mono-nitrated trans isomer as a major outcome. The ratio of trans to cis was up to >99. Higher isolated yields of nitrated products and higher trans/cis ratios of isomers suggest that this procedure offers advantages for synthesizing asymmetry nitro compounds.     

Synthesis of multisubstituted 2-(dihydrofuran-2(3H)-ylidene)acetates via intramolecular carboalkoxylation by platinum-olefin catalyst system

The cyclization of 6-(1-alkoxyethyl)hex-2-ynoates in the presence of a platinum-olefin catalyst system gave the corresponding multisubstituted 2-(dihydrofuran-2(3H)-ylidene)acetates in good to high yields. The Z/E selectivity is controlled by the electronic property of the ester group; the 2,2,2-trichloroethyl ester led to the Z isomer, while the phenyl ester gave the E isomer.     

Separation ability and stoichiometry of cyclodextrin complexes

Gas-liquid chromatography has been applied to search relations between selectivity towards isomers and stoichiometry of cyclodextrin complexes. The model tested compounds were: dimethylnaphthalenes and alpha- and beta-pinenes as constitutional isomers; cis/trans decalins, anetholes and isosafroles as diastereomers and as enantiomers (+/-)-alpha-pinenes and (+/-)-camphenes. Experimental retention data are used to confirm a simple theoretical model that allows distinguishing formation of G x CD complexes (1:1) and G x CD2 complexes (1:2). Based on the experimental data, stability constants K were evaluated. It has been found that remarkable selectivity factor alpha may appear both within the range of 1:1 stoichiometry (beta-CD complexes of decalins and of alpha- and beta-pinenes) and 1:2 stoichiometry (alpha-CD complexes with (+/-)-alpha-pinenes and (+/-)-camphenes). Occasionally selectivity arises from a different composition, when one isomer forms a 1:1 stoichiometry complex while another forms a 1:2 complex (dimethylnaphthalenes, cis/trans-anetholes and cis/trans-isosafroles).     

Darzens reaction of acyl phosphonates with alpha-bromo ketones: selective synthesis of cis- and trans-epoxyphosphonates

Acyl phosphonates with alpha-halo ketones in the presence of bases at room temperature afford cis- and trans-epoxyphosphonates in good chemical yields and high selectivities using different bases. The diastereoselectivity of this reaction is easily controlled by changing the base. Changing the base from Cs2CO3 to DBU changed the diastereomeric ratio (trans/cis) from 3/2 to 9/1. Moreover, the treatment of the trans isomer with DBU showed a complete conversion to the corresponding cis isomer.     

Selectivity in platinum-catalyzed cis-trans carbon-carbon double-bond isomerization

The isomerization of trans-2-butene to its cis conformer was found to be easier on Pt111 surfaces than the opposite cis to trans conversion. This kinetic trend, which is opposite to what would be expected on thermodynamic grounds, is explained by an increased stability of the cis isomer upon adsorption. A model where adsorption energies are affected by steric interactions between the side moieties of the olefin and the surface suggests that selectivity toward cis versus trans formation may be manipulated by controlling the structure of the surface of the catalyst.     

Oligomerization and hydroamination of terminal alkynes promoted by the cationic organoactinide compound [(Et2N)3U][BPh4

The three ancillary amido moieties in the cationic complex [(Et2N)3U][BPh4] are highly reactive and are easily replaced when the complex is treated with primary amines. The reaction of [(Et2N)3U][BPh4] with excess tBuNH2 allows the formation of the cationic complex [(tBuNH2)3(tBuNH)3U][BPh4]. X-ray diffraction studies on the complex indicate that three amido and three amine ligands are arranged around the cationic metal center in a slightly distorted octahedral mer geometry. The cationic complex reacts with primary alkynes in the presence of external primary amines to primarily afford the unexpected cis dimer and, in some cases, the hydroamination products are obtained concomitantly. The formation of the cis dimer is the result of an envelope isomerization through a metal-cyclopropyl cationic complex. In the reaction of the bulkier alkyne tBuC identical to CH with the cationic uranium complex in the presence of various primary amines, the cis dimer, one trimer, and one tetramer are obtained regioselectively, as confirmed by deuterium labeling experiments. The trimer and the tetramer correspond to consecutive insertions of an alkyne molecule into the vinylic CH bond trans to the bulky tert-butyl group. The reaction of (TMS) C identical to CH with the uranium catalyst in the presence of EtNH2 followed a different course and produced the gem dimer along with the hydroamination imine as the major product. However, when other bulkier amines were used (iPrNH2 or tBuNH2) both hydroamination isomeric imines Z and E were obtained. During the catalytic reaction, the E (kinetic) isomer is transformed into the most stable Z (thermodynamic) isomer. The unique reactivity of the alkyne (TMS) C identical to CH with the secondary amine Et2NH is remarkable because it afforded the trans dimer and the corresponding hydroamination enamine. The latter probably results from the insertion of the alkyne into a secondary metal-amide bond, followed by protonolysis.     

Stereochemistry of the [4 + 2] cycloadditions of trans,trans- and cis,trans-2,4-hexadiene to C(60)

The [4 + 2] cycloaddition of trans,trans-2,4-hexadiene with C(60) proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct 1a. However, when cis,trans-2,4-hexadiene reacts with C(60), isomerization of the cis,trans to the thermodynamically more stable trans,trans isomer occurs. Subsequently, the cis,trans diene isomerized to the trans,trans isomer and cycloadds to C(60), to form adduct 1a. When the reaction is carried out at higher temperatures, the formation of cycloadduct 1b is also obtained. This result is consistent with a concerted cycloaddition of cis,trans-2,4-hexadiene with C(60), which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.     

Influence of Michael acceptor stereochemistry on intramolecular Morita-Baylis-Hillman reactions

[reaction: see text] A study of the effect of Michael acceptor stereochemistry on the efficiency of intramolecular Morita-Baylis-Hillman (MBH) reactions has been performed. The reactions were catalyzed by a phosphine, and the reaction substrates studied were enones containing a pendant aldehyde moiety attached at the beta-position of the alkene group. In all cases examined with PPh3 as the catalyst, cyclization substrates possessing (Z)-alkene stereochemistry afforded a much higher yield of the desired product than did the E isomeric substrates under identical reaction conditions. This was also true when a polymer-supported phosphine catalyst was used. While both alkene isomers afforded the same product, in parallel reactions, the Z isomer afforded 2.5-8.5 times higher yield than did the corresponding E isomer. It is proposed that steric effects are a possible source of this dramatic difference in reactivity. Substrates where the beta-substituent is cis to the electron-withdrawing substituent are relatively more accessible to react with the nucleophile catalyst than are their trans counterparts. These findings are expected to be useful in the design of synthetic intermediates, as intramolecular MBH reactions are being increasingly used in the preparation of complex synthetic targets.     

Solvent-assisted conformational isomerization and the conformationally-pure REMPI spectrum of 3-aminophenol

3-Aminophenol (3AP) has two conformers, cis and trans, depending on the orientation of the OH group relative to the NH(2) group. While both conformers are found in the jet-cooled spectra of 3AP, only the trans isomer was found in the REMPI spectrum of the 3AP(NH(3))(1) cluster. It was suggested that the cis conformer of the cluster isomerizes to the more stable trans conformer in the ground state during supersonic expansion. Solvent-assisted conformational isomerization (SACI) is believed to drive the population into the more stable trans isomer. SACI also occurs for the 3AP monomer, reducing 50% of the cis/trans ratio when the ammonia concentration in the expansion is higher than 0.1%. Depending on the expansion condition, the cis conformer can be completely depleted. When other solvents were introduced in the expansion, SACI occurred with only certain solvents whose binding energy is higher than the isomerization barrier. SACI can be used as a means to prepare the most stable conformer of gas phase biomolecules.     

含绿色荧光蛋白发色团双自由基分子的非线性光学性质

采用密度泛函理论(DFT)方法研究了系列含绿色荧光蛋白发色团双自由基分子光学异构体的几何结构、极化率(αs)和第一超极化率(βtot).结果表明,引入电子给受体取代基使分子的极化率增大,而对第一超极化率有不同影响.对于光照前的反式结构,引入电子受体βtot值增加,且βtot值随取代基吸电子能力的增强而增大;引入电子给体βtot值降低,且βtot值随取代基给电子能力的增强而减小.当分子变成相应的顺式结构时,其βtot值变化趋势与反式结构的结果正好相反.光异构化前后分子的βtot值变化不同,引入电子受体使顺式结构的βtot值比反式结构的小,其中―NO2使顺式结构的βtot值减小为反式结构的1/6;引入电子给体使反式结构的βtot值比顺式结构的小,其中―NH2使反式结构的βtot值减小为顺式结构的1/6.从而,光异构化起到调节非线性光学(NLO)响应的作用.     

杂芳基乙烯光化学反应的研究 2: r-1, c-2, t-3, t-4-1, 3-二(4-甲基苯基)-2, 4-二(4-吡啶基)环丁烷的结构及光解反应

通过反-1-(4-甲基苯基)-2-(4-吡啶基)乙烯(E-MEP)在稀盐酸中的光二聚反应得到了接近定量的标题化合物(DMDPC)。用元素分析,IR, UV, 1H NMR和MS表征了其结构, 并用X射线衍射法测定了DMDPC的晶体结构。DMDPC属单斜晶系, 空间群为P21/c, 晶胞参数a=1.1376(6), b=1.7379(8), c=1.1590(5)nm, β=106.16(4)ⅲ, Z=4。由于四元环同侧苯环和吡啶环的相互排斥作用, DMDPC采取蝶式构象。研究发现, DMDPC在短波紫外光照射下易发生开环反应; 同时还发现,E-MEP与其顺式异构体(Z-MEP)间的反-顺异构化平衡受照射光波长控制。