The cycloaddition of ethylene to butene-2. I. Stereochemistry of the reaction

The rate of the thermal cycloaddition of ethylene to cis and trans butene-2 has been measured at 693°K and at pressures of about 12 atmospheres. The ratio of trans- to cis-1,2-dimethylcyclobutane from the reaction of trans-butene-2 with ethylene was 5.1, obtained from the initial rates of formation of the products. Similarly, the ratio of cis- to trans-1,2-dimethyl-cyclobutane from the reaction of cis-butene-2 with ethylene was 2.8. The results show that the cycloaddition reactions are the reverse of the decomposition reactions of the dimethyl-cyclobutanes and may be interpreted in terms of a biradical intermediate. Several ratios of rate constants have been measured as well as the rate constants for the reaction of the olefins to form the intermediate biradical.     

Investigations Concerning an Unexpected Reaction between the Dimsyl Anion ( = (Methylsulfinyl)methanide) and a γ,δ-Epoxy-ketone

The reaction of 2,2-dimethyl-5-(1,2-epoxypropyl)cyclohexanone ( 7 ) with t-BuOK in DMSO furnished a small amount of 5-(1-hydroxyprop-2-enyl)-2,2-dimethylcyclohexanone ( 12 ) and the 4 unexpected products 13–16 which contain one to three additional C-atoms (Scheme 2). The relative configuration of the major product 1-(4′,4′-dimethyl-2′,3′-dimethylidenecyclohexyl)propane-1,2-diol ( 15 ) was shown to be 1RS, 2RS,1′SR via NOE measurements performed on a derivative thereof. A crossover experiment in DMSO/[¹³C₂]DMSO 1:1 as solvent showed that the two additional C-atoms of this product originate from a single molecule of DMSO (Scheme 5). A tentative mechanistic scheme, consistent with all experimental observations, is proposed which involves a [2,3]-sigmatropic rearrangement of an (allylsulfinyl)methanide to a sulfenic acid as one of the key steps ( V → 24 , Scheme 8). We corroborated part of this hypothetic scheme by taking recourse to a model compound (7-(methylsulfinyl)-p-mentha-1,8-diene ( 32/33 ), readily prepared in two steps from perilla alcohol ( 30 )), which reacted as predicted by the proposed mechanism (Schemes 9 and 10).     

Kinetic Studies of Picolinic Acid Catalyzed Chromic Acid Oxidation of Cyclohexanone

The chromic acid oxidation of cyclohexanone catalyzed by picolinic acid in water undergoes a change from first-to zero-order dependence in both cyclohexanone and acidity. The mechanism proposed indicates the formation of an intermediate C₁ by picolinic acid and chromic acid. Then C₁ would react with enol form of cyclohexanone to give another intermediate C₂. C₂ finally cleaves into products.     

Kinetics and mechanisms of homogeneous catalytic reactions. Part 4. Hydrogenation of cyclohexanone and 2-cyclohexen-1-one catalysed by the complexes [MH(CO)(NCMe)2(PPh3)2]BF4 (M = Ru,Os)

Kinetic and mechanistic studies of the homogeneous hydrogenation of cyclohexanone were carried out using the cationic complexes [MH(CO)(NCMe)₂(PPh₃)₂]BF₄ (M = Ru, Os) as the catalyst precursors, which were very efficient under mild reaction conditions in 2-methoxyethanol solution. For both complexes, the catalytic hydrogenation of cyclohexanone proceeds according to the rate law r = k[M][H₂]. The activation parameters were also calculated, the activation energy for the osmium catalyst being higher than for the ruthenium(I). All experimental data are consistent with a mechanism involving the oxidative addition of hydrogen as the rate-determining step of the catalytic cycle. Finally, the [MH(CO)(NCMe)₂(PPh₃)₂]BF₄ complexes were efficient precatalysts for the selective reduction of 2-cyclohexen-1-one to cyclohexanone; the reduction of the CO group of cyclohexanone only begins to take place when the ,-unsaturated ketone has been consumed.     

Studies on the Biosynthesis of Tabtoxin (Wildfire Toxin). Origin of the Carbonyl C-Atom of the β-Lactam Moiety from the C1-Pool

Re-isolation of Pseudomonas tabaci strain NCPPB 2730 from its host, the tobacco plant, led to an activation of the bacteria in order to produce the β-lactam dipeptide tabtoxin (Wildfire toxin, 1 ). Incorporation of several ¹⁴C-labelled amino acids as well as L -[methyl-¹³C]methionine, L -[1,2-¹³C₂]- and L -[3,4-¹³C₂]aspartate, rac -[1,2-¹³C₂]glycerol, and [1,2-¹³C₂]acetate into isotabtoxion ( 2 ) demonstrated that the building blocks of tabtoxin ( 1 ) are L -threonine, L -aspartate, the Me group of L -methionine and a C₂-unit derived from the C₃-pool (Fig. 3). The Me group of L -methionine provides the carbonyl C-atom of the β-lactam moiety. These findings represent a novel pathway in β-lactam biosynthesis. Mechanistic aspects with respect to the β-lactam ring formation are discussed. A biradical 16 is proposed as an intermediate during the cyclization of a N-formyl-α-amino ketone 15 .     

Kinetics,mechanism, and stereochemistry of Diels-Alder reactions of CN-substituted ethenes with cyclohexa-1, 3-diene in the gas phase

The kinetics of the Diels-Alder additions of CH₂ = CHCN, CH₂ = C(CH₃) CN, and cis- and trans-CH₃CH = CHCN to cyclohexa-1, 3-diene have been studied in the gas phase. The stereochemistry of these reactions is discussed. In terms of a biradical mechanism, a minimum value of 4.1 ± 0.8 kcal mol^?1 for the stabilizing effect of a CN group vis-à-vis a methyl group is shown to fit the experimental activation energies.     

Kinetics and mechanism of unimolecular heterolysis of cage-like compounds: XXI. Solvent effect on the realtive rate of heterolysis of 2-methyland 2-phenyl-2-haloadamantanes. Role of activation parameters

The kinetics of heterolysis of 2-chloro-2-methyladamantane, 2-bromo-2-methyladamantane, 2-chloro-2-phenyladamantane, and 2-bromo-2-phenyladamantane in isopropyl alcohol, tert-butyl alcohol, acetonitrile, nitromethane, cyclohexanone, and γ-butyrolactone were studied using the verdazyl technique. The rate constant ratio k _Ph/k _Me decreases from three orders of magnitude to unity in the solvent series BuOH > i-PrOH > t-BuOH > MeCN > PhNO₂ > cyclohexanone > γ-butyrolactone > sulfolane, which results from weakening of conjugation between the phenyl group and emerging carbocationic center. The effect of solvent on the entropy and enthalpy of heterolysis in going from 2-methyl-substituted 2-haloadamantanes to their 2-phenyl analogs is discussed.     

Two Novel Thermal Biradical Cyclizations in Theory and Experiment: New Synthetic Routes to 6H-Indolo[2,3-b]quinolines and 2-Aminoquinolines from Enyne-Carbodiimides

The regioselectivity of the biradical cyclization of enyne-carbodiimides 1 can easily be controlled by variation of R¹ at the alkyne terminus. Attachment of a hydrogen atom (R¹=H) leads to C²–C⁷ cyclization and formation of biradical 2 , whereas C²–C⁶ cyclization to provide biradical 3 is observed with R¹=Me₃Si or Ph.     

两个氮氧双自由基桥连的稀土一维链的晶体结构、磁性和荧光性能

选用1,2-二苯氧基乙烷取代的氮氧双自由基(BNPhOEt)与稀土金属反应,得到了2例氮氧双自由基-稀土配合物[Ln(hfac)3(BNPhOEt)]·C6H14(Ln=Tb(1)、Ho(2);hfac=六氟乙酰丙酮),其均为2p-4f一维链状结构.磁性研究表明,在配合物1和2中分别存在铁磁和反铁磁耦合.此外,对2个配...     

Mass spectrometric studies of 6-chloro-6H-dibenz[c,e] [1,2]oxaphosphorine-6-sulphide and its derivatives

The electron-impact induced fragmentation of 6-chloro-6H-dibenz[c,e] [1,2]oxaphosphorine-6-sulphide and its 6-ethoxy and 6-hydroxy derivatives has been studied and it has been found that formation of the dibenz[c,e] [1,2]oxaphosphorin system and elimination of HCP are common features. β-Hydrogen rearrangement, in the case of the ethoxy derivative, is comparable to ethers with a P? O? CH₂? CH₃ linkage.     

Radical Reactivity of the Biradical [⋅P(μ-NTer)2P⋅] and Isolation of a Persistent Phosphorus-Cantered Monoradical [⋅P(μ-NTer)2P-Et]

The activation of C−Br bonds in various bromoalkanes by the biradical [⋅P(μ-NTer)₂P⋅] ( 1 ) (Ter=2,6-bis-(2,4,6-trimethylphenyl)-phenyl) is reported, yielding trans-addition products of the type [Br−P(μ-NTer)₂P−R] ( 2 ), so-called 1,3-substituted cyclo-1,3-diphospha-2,4-diazanes. This addition reaction, which represents a new easy approach to asymmetrically substituted cyclo-1,3-diphospha-2,4-diazanes, was investigated mechanistically by different spectroscopic methods (NMR, EPR, IR, Raman); the results suggested a stepwise radical reaction mechanism, as evidenced by the in-situ detection of the phosphorus-centered monoradical [⋅P(μ-NTer)₂P-R].< To provide further evidence for the radical mechanism, [⋅P(μ-NTer)₂P-Et] ( 3Et ⋅) was synthesized directly by reduction of the bromoethane addition product [Br-P(μ-NTer)₂P-Et] ( 2 a ) with magnesium, resulting in the formation of the persistent phosphorus-centered monoradical [⋅P(μ-NTer)₂P-Et], which could be isolated and fully characterized, including single-crystal X-ray diffraction. Comparison of the EPR spectrum of the radical intermediate in the addition reaction with that of the synthesized new [⋅P(μ-NTer)₂P-Et] radical clearly proves the existence of radicals over the course of the reaction of biradical [⋅P(μ-NTer)₂P⋅] ( 1 ) with bromoethane. Extensive DFT and coupled cluster calculations corroborate the experimental data for a radical mechanism in the reaction of biradical [⋅P(μ-NTer)₂P⋅] with EtBr. In the field of hetero-cyclobutane-1,3-diyls, the demonstration of a stepwise radical reaction represents a new aspect and closes the gap between P-centered biradicals and P-centered monoradicals in terms of radical reactivity.     

One-pot synthesis of new 6-(alkylamine)dibenzo[c,e][1,2]oxaphosphinine-6-oxides

Several new 6-(alkylamine)-6H-dibenz[c,e][1,2]oxaphosphinine-6-oxides were prepared through a one-pot reaction, starting with 2-phenylphenol, phosphorus trichloride, and a Zn catalyst, to form 6-chloro-6H-dibenz[c,e][1,2]oxaphosphine. The alkylamine derivatives were subsequently prepared through a nucleophilic substitution reaction involving aliphatic amines and H₂O₂ oxidation under soft conditions. This method has the advantages that it is a one-pot synthesis, does not require an inert atmosphere, and involves in situ catalyst formation.     

Vinyl polymerization initiated by chromic acid–reducing agent systems

A kinetic study of the thermal polymerization of acrylonitrile initiated by chromic acid–reducing agent (n-butanol, ethylene glycol, cyclohexanone, and acetaldehyde) systems was made. Chromic acid alone did not initiate polymerization under deaerated or undeaerated conditions. On the basis of the experimental determination of the dependencies of various variables on the rate of polymerization R_p, the rate of chromium (VI) disappearance ?R_M, the degree of polymerization DP, etc., a reasonable kinetic scheme was arrived at. The mechanism with the reducing agents, n-butanol, cyclohexanone, and ethylene glycol, was found to be similar but different from that with acetaldehyde. Evidence has been presented to prove the formation of radical intermediates formed by the oxidation of the reducing agent by Cr(IV). Rate parameters for oxidation of the reducing agent and polymerization of the monomer were evaluated.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XLII. Effect of Aprotic Solvent on the Rate of 3-Methyl-3-chloro-1-butene Dehydrochlorination. Correlation Analysis of Solvation Effects

The kinetics of 3-methyl-3-chloro-1-butene dehydrochlorination in propylene carbonate, γ-butyrolactone, sulfolane, acetone, MeCN, PhNO₂, PhCN, PhCOMe, MeCOEt, cyclohexanone, o-dichlorobenzene, PhCl, PhBr, 1,2-dichloroethane, dioxane, and AcOEt were studied; v = k[C₅H₉Cl], E1 mechanism. The reaction rate is satisfactorily described by the parameters of the polarity, electrophilicity, and cohesion of the solvent; the solvent nucleophilicity and polarizability exert no effect on the reaction rate.     

Photochemical Generation of Chiral N,B,X‐Heterocycles by Heteroaromatic C−X Bond Scission (X=S,O) and Boron Insertion

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X‐containing heterocycles, such as base‐stabilized 1,2‐thiaborinines and 1,2‐oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b‐dihydrobenzo[h][1,2]oxaborinino[4,3‐f]quinolines through a sequential conrotatory electrocyclization and a [1,5]‐H shift. The N,B,X‐containing heterocycles display strong blue‐green to orange‐red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.     

Synthesis and oxygen-atom transfer reactions of 3-hydroperoxy-3,4,4,5,5-pentasubstituted-1,2-dioxolanes

A series of 3-hydroperoxy-3,4,4,5,5-pentasubstituted-1,2-dioxolanes 2a-d were synthesized in good yield from the corresponding 3-hydroxy-1,2-dioxolanes by reaction with concentrated hydrogen peroxide in acetonitrile with p-toluenesulfonic acid as catalyst. The 3-hydroperoxy-1,2-dioxolanes were effective oxygen-atom transfer reagents for the oxidation of thioanisole, triethylamine and 2,3-dimethyl-2-butene to the sulfoxide, N-oxide and epoxide, respectively. The reactions occurred under mild conditions and were found to be of the second order overall. The second order rate constants (k₂) were determined for oxidation of thioanisole by 2a-d in deuteriochloroform. For 2a , k₂ values for N-oxidation and epoxidation were also measured. The 3-hydroperoxy-1,2-dioxolanes were found to be less reactive than the structurally similar cyclic α-azohydroperoxides but much more reactive than simple hydroperoxides. The mechanism of oxygen-atom transfer is postulated to occur via nucleophilic attack of the substrate on the terminal oxygen of the hydroperoxide. Intramolecular hydrogen bonding of the hydroperoxy proton to a dioxolane oxygen appears to account for the reaction order in aprotic media.     

Photochemical Generation of Chiral N,B,X‐Heterocycles by Heteroaromatic C−X Bond Scission (X=S,O) and Boron Insertion

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X‐containing heterocycles, such as base‐stabilized 1,2‐thiaborinines and 1,2‐oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b‐dihydrobenzo[h][1,2]oxaborinino[4,3‐f]quinolines through a sequential conrotatory electrocyclization and a [1,5]‐H shift. The N,B,X‐containing heterocycles display strong blue‐green to orange‐red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.     

Eine thermische,intramolekulare [2 + 2]-Cycloaddition eines Allenyl-benzols; Synthese von Allenylbenzolen durch säurekatalysierte Dienol-Benzol-Umlagerung

A thermal Intermolecular [2 + 2]-Cycloaddition of an Allenyl-Allyl-Benzene; Synthesis of Allenylbenzenes via Acid-Catalyzed Dienol-Benzene Rearrangement A few years ago, it has been shown that the acid-catalyzed dienol-benzene rearrangement of 2-propinyl-substituted cyclohexadienols is a convenient synthesis for allenyl-substituted benzene derivatives. The cyclohexadienols 20 and 21 were prepared via C-alkylation of the corresponding phenols with 2-propinylbromide (Scheme 3), followed by reduction of the cyclohexadienone 13 and 17 with LiAlH₄. Treatment of 20 and 21 with p-toluenesulfonic acid in ether at ?15°) yielded the desired allenyl benzenes 8 and 9 , respectively, via [3,4]-sigmatropic rearrangements (Scheme 4). The 2-propinylbenzenes 22–24 , formed via [1,2]-sigmatropic shift of the 2-propinylgroup, were found as by-products. Thermolysis of allenyl benzene 8 in decane yielded two bicyclic ( 25 and 26 ) and two tricyclic products ( 27 and 28 ; Scheme 5). For the formation of 25 and 26 , a pericyclic reaction mechanism (Scheme 6) as well as a mechanism via biradical intermediates (Scheme 7) is discussed. A [2 + 2]-cycloaddition of the α,β-allenic and the allylic C,C-double bound of 8 led to the tricyclic products 27 and 28 (Scheme 9). All attempts to realize a [1,7]-sigmatropic H-shift in the allene 9 failed so far, and the starting material underwent a rapid polymerisation.     

Kinetics of the thermal decomposition of 1,2-dioxaspiro[2,5]octane

The products and kinetics of the thermolysis of 1,2-dioxaspiro[2,5]octane in cyclohexanone and cyclohexanone-CCl₄ mixtures are studied. 1,2-Dioxaspiro[2,5]octane is consumed via two parallel routes: isomerization to oxepan-2-one and solvent (cyclohexanone) oxidation with the partial escape of radicals from the cage (17% at 25 °C). Under an inert atmosphere, the alkyl radicals formed by solvent oxidation initiate the chain radical decomposition of 1,2-dioxaspiro[2,5]octane. The mechanism of 1,2-dioxaspiro[2,5]octane thermolysis is discussed on the basis of the results obtained. The activation parameters of 1,2-dioxaspiro[2,5]octane isomerization to oxepan-2-one and reactions of dioxaspiro[2,5]octane with cyclohexanone are discussed.     

Ene reactions of olefins. I. The addition of ethylene to 2-butene and the decomposition of 3-methylpentene-1

The pyrolysis of ethylene–butene-2 mixtures has been studied in a static system over the temperature range of 689°-754°k and for initial pressures of each olefin of 20–200 torr. The two main addition products were cyclopentene and 3-methylpentene-1. Kinetic evidence indicated that cyclopentene was formed from radical processes while 3-methylpentene-1 was formed by the molecular “ene¨?” addition of ethylene to butene-2 through a six-center transition state. The following rate constants were obtained: The pyrolysis of 3-methylpentene-1 has been studied over the same temperature range and for initial pressures of 20–100 torr. Kinetic evidence showed that the products ethylene and butenes were formed in both radical and molecular processes. Estimates of the rate constant k_?1t and k_?1c were, however, in reasonable agreement with the measurements of k_1t and k_1c. The mechanism of the ene reaction is discussed, and it is concluded that the transition state does not involve the formation of a biradical.