Six-membered Cyclic Transition State Research Articles

A theoretical study of acetylene, ethylene, and cyclopropene additions to benzo[b]- and benzo[c]-fused heterocycles

The reactivity of benzo[b]- and benzo[c]-fused heterocycles as dienes for Diels-Alder reactions were evaluated on the basis of Frontier Molecular Orbital (FMO) energy gaps between reactants, necessary FMO changes for the transformation of reactants into transition state structures, uniformity of bond orders of heterocyclic rings, and uniformity of bond orders of the six-membered cyclic transition state structures All of these values were obtained with AM1 semiempirical methods. The selectivity of the cycloaddition reaction was also judged with Secondary Orbital Interactions (SOI) between two reactant moieties in isomeric transition state structures. For every possible reaction pathway, AM1 and B3LYP/AM1 energies were evaluated. The relative reactivity and selectivity obtained by computing activation barriers for these reactions were in full agreement with our qualitative studies as well as with experimental results. The usefulness of the FMO and bond order uniformity approach for the study of the Diels-Alder reaction was discussed.

Tandem beta-Fragmentation-Hydrogen Abstraction Reaction of Alkoxy Radicals in Steroidal Systems.

A number of tertiary or hemiacetalic steroidal alcohols, 2-hydroxy-3,4-dinor-2,3-secocholestan-5-one 2,5-hemiacetal (1), 2beta,3beta-dihydro-3'H-cyclopropa[2,3]-cholestan-5alpha-ol (7), 3beta-phenyl-5alpha-hydroxycholestan-2-one (10), 5alpha-hydroxycholestan-3-one (15), 3beta,5alpha-dihydroxycholestan-7-one 3-acetate (21), and 4,4-dimethyl-19-hydroxy-5alpha-cholestan-3-one 19,3-hemiacetal (27) have been prepared in order to test a new tandem beta-fragmentation-hydrogen abstraction reaction. The alkoxy radicals generated by irradiation of these alcohols with visible light in the presence of (diacetoxyiodo)benzene, iodine, and molecular oxygen undergo a beta-fragmentation reaction followed by peroxidation of the C-radical formed. The peroxy radical reacts further with iodine to give an alkoxy radical and iodoxyl radical (IO(*)). The new alkoxy radical can produce intramolecular functionalization of suitably positioned carbons by hydrogen abstraction through a six-membered cyclic transition state to give tetrahydrofurans or can be intramolecularly trapped by carbonyl groups suitably disposed so as to generate a new alkoxy radical that can afford gamma-lactones by beta-fragmentation.

C,C-coupling with sulfur-stabilized carbanions - 7. Diastereoselectivity in the addition of electrophiles to the carbanions of 2-(alkylthio)thiolane 1-oxides

Pure diastereoisomers of the 2-(alkylthio)thiolane 1-oxides 2 were prepared in a three-step synthesis. The carbanions of 2 were generated with different bases and treated with iodomethane and carbonyl compounds to form the methyl derivatives 6 and the carbinols 7, respectively. The conformational stability (α- diastereoselectivity) of the carbanions as well as the asymmetric induction during the reaction with prochiral electrophiles (β- diastereoselectivity) was studied. A mechanism for the reaction with carbonyl compounds is proposed, which involves a six-membered cyclic transition state formed from the sulfoxide moiety and the metal cation. - Steric effects of the substrate and the electrophile are discussed.

Theoretical study on the pyrolysis mechanism and kinetics of β‐hydroxyketones

The mechanism and kinetics for the decomposition of β-hydroxypropaldehyde, primary and secondary β-hydroxyketones, were studied by using ab initio RHF/6–31G and RHF/6–31G* methods. The activation barriers of these reactions were refined to be 39.57, 40.10, and 36.80 kcal mol−1 at the MP2/ /RHF/6–31G* level, respectively. The calculated results show that each decomposition is a concerted process with hydrogen transferring and bond breaking via a six-membered cyclic transition state. The thermal rate constants of the decomposition of primary and secondary β-hydroxyketones were obtained by calculating microcanonical probability fluxes through each transition state. It is theoretically confirmed that methyl substitution at the hydroxyl carbon of β-hydroxyketones causes a small enhancement in rates. The theoretical investigations of the mechanism and the rate constants are in agreement with the experimental results. © 1997 John Wiley & Sons, Inc.

Ozone-Mediated Nitration of Phenylalkyl Ethers, Phenylacetic Esters, and Related Compounds with Nitrogen Dioxide. The Highest Ortho Substitution Observed in the Electrophilic Nitration of Arenes

By the combined action of ozonized oxygen and nitrogen dioxide (the kyodai-nitration), the title compounds were smoothly nitrated in dichloromethane at subzero degrees with high ortho positional selectivity. Although the conventional nitration of phenylacetic acid and esters mainly produces m- and p-nitro derivatives, the present nitration offers a simple high-yield synthesis of o-nitro derivatives which are important as precursor in organic synthesis. The proportions of the ortho isomer in the nitration products from methyl 2-phenylethyl ether and methyl phenylacetate were 71 and 88%, respectively, the latter value being the highest ortho isomer proportion so far observed in the electrophilic aromatic nitration. The observed high ortho selectivity has been rationalized in terms of radical cation intermediate and six-membered cyclic transition state.

Homogeneous pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate in the gas phase

The pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate have been examined over the temperature range of 286–330°C and pressure range of 29–108 Torr. In a seasoned vessel and in the presence of the free radical inhibitor cyclohexene or toluene the reaction is homogeneous, unimolecular and obeys a first-order rate law. The elimination products are mainly acetone and ethyl acetate, and very small amounts of ethyl 3-butenoate, acetic acid, ethylene and H2O. The rate coefficient is expressed by the following equation: log k1(s−1)=(12.39±0.46)−(174.5±5.2) kJ mol−1 (2.303RT)−1. The mechanism appears to proceed via a six-membered cyclic transition state, where polarization of the (CH3)C(OH)δ+...δ-CH2COOCH2CH3 bond is rate determining.

Conjugate Addition of Alkenylsulfides with α,β-Unsaturated Carbonyl Compounds. Remarkable n-π* Orbital Interaction for Control of Regio- and Stereochemistry

A highly stereoselective conjugate addition reaction with α,β-unsaturated aldehydes was developed by using alkenylsulfide as a nucleophile. Under the influence of TiCl3(O i Pr), (1S*,2S*)-1-butyl-2-[(1Z)-1-(methylthio)propenyl]-1-(triethylsiloxy)cyclopentane reacted with enals to give 11,12-dimethyl-10-(methylthio)-14-(triethylsiloxy)-9,13-tetradecadien-5-one derivatives in >99% anti selectivity. The high regio- and stereoselection can be explained by assuming the chair-like six-membered cyclic transition state involving the orbital interaction between the sulfur and a carbonyl carbon.

Dualistic Reactivity of Lithium and Zinc Dienolates with Imines: Effects of Counterion, Temperature, and Substituents on .alpha.- and .gamma.-Coupling

Lithium and zinc dienolates of 3-butenoic, 3-methyl-2-butenoic, 3-pentenoic, and 4-phenyl-3-butenoic methyl esters are reacted with imines R1C(H)-NR2[3a: R1 = 2-pyridyl, R2 = Ph; 3b: R1 = MeaSiC=C; R2 = Ph; 3c: R1 = 2-pyridyl, R2 = (R)-PhCH(CHa); 3d: R1 = Me3Sic~C; R2 = (R)-PhCH(CH3)l. Depending on metal counterion, temperature, and substituents, the C-C coupling occurs either at the 2-position (a-coupling) or at the 4-position (y-coupling) of the dienolate, giving /?-amino esters and a,/?-unsaturated esters, respectively. The a-coupling occurs at -78 C and is reversible, whereas y-coupling products are formed upon warming to -20 C. The C-C coupled products may undergo irreversible ring closure to /?-lactams (from p-amino esters) or 5,6-dihydropyridin-2ones (from Z)-a,/?-unsaturated esters). Starting from enantiomerically pure imine 3c, high (7192%) asymmetric inductions were realized in both /?-lactams and 5,6-dihydropyridin-2-ones; y-coupling of enolate 2a and chiral imine 3d occurred with low diastereoselectivity (15%). Four factors favor the formation of /?-lactams: (i) the use of zinc dienolates, (ii) 4-substitution of the dienolates, (iii) nonaromatic imine N-substituents, and (iv) a low reaction temperature (-78 C). The product formation is discussed in terms of six-membered cyclic transition states, involving either zinc enolates (a-coupling) or isomeric allylzinc species (y-coupling).

Ab initio study on the thermal decarboxylation of but-3-enoic acid and its derivatives

The mechanism for thermal decarboxylation of but-3-enoic acid and its derivatives HXC=CYCH2COOH (X, Y=H, F, CH3, C2H5, and Cl) leading to carbon dioxide and olefins has been studied from the theoretical point of view by ab initio MO calculations. The transition states obtained by our ab initio calculations are completely consistent with the experimental data, and support the "synchronous" mechanism for thermal decarboxylation of but-3-enoic acid and its derivatives via a "twisted chair" six-membered cyclic transition state. The effects of β- and γ-substituents on the activation energy (Ea) can be explained in terms of electronic charge distribution. β-Substitution decreases the activation energy, while γ-substitution increases it. Changes in the activation energy are related to changes in the charges at Cγ(C1) and Cβ(C2) as the substituents are varied. The activation energy decreases with an increase of negative charge at Cγ, while it increases with an increase of negative charge at Cβ. The best estimate of 156.8 kJ/mol for the activation energy with MP2/6-31G*//HF/3-21G(*) is in reasonable agreement with the available experimental values of 164 ± 7 kJ/mol and 160 kJ/mol for decarboxylation of but-3-enoic acid. The calculated primary kH/kD, 2.86, and [Formula: see text] 1.03, for the decarboxylation of but-3-enoic acid, are also in excellent agreement with the available experimental values of 2.7 and 1.035, respectively, supporting the transition state structure obtained.

Quantum Chemical Study of the Mechanism of the Living Cationic Polymerization by Ester-Boron Trihaude Initiators

Abstract Semiempirical MNDO calculations of model systems simulating cationic initiating systems of the BCI3/ester type have been carried out. Interactions between BX3 (X = F, CI) and methyl acetate (MA) have been compared. Properties of six-membered cyclic transition states for the reaction between MA and propylene in the presence of BCI3, previously proposed as a mechanism of living cationic polymerization, have been investigated. An alternative mechanism of polymerization via zwitterionic complexes formed between BCI3 and monomer has been suggested. The cationic center of this zwitterionic complex has been found to be stabilized by the ester component of the initiating system.

Reaction Pathway and Transition State of the Zn-Promoted Barbier-Type Reactions of Benzaldehyde and Benzophenone with Allylic Iodides

Abstract The carbonyl-14C kinetic isotope effects and the substituent effects on the reactivity of the Zn-promoted Barbier-type reaction of allyl iodide (3-iodo-1-propene) with benzaldehyde and benzophenone were determined in THF at 25 °C. The observed normal carbon isotope effects as well as positive Hammett ρ values suggest that the reactions go through a direct nucleophilic addition mechanism. No indication of the occurrence of electron transfer was obtained by enone isomerization and dehalogenation probe experiments. Diastereoselectivity in the reaction of crotyl iodide (1-iodo-2-butene) with benzaldehyde was low and independent of the substituent on aldehyde, consistent with the six-membered cyclic transition state.

高配位ケイ素化合物の特性と有機合成への応用

Preparation, characterization, and synthetic application of hypercoordinate organosilicon compounds are described. Regiospecific and highly diastereoselective allylation of aldehydes, aldimines, α-hydroxy ketones, α-ketocarboxylicacids, β-hydroxy- and β-amino-α, β-enones, and 1, 2-diketones have been achieved by using pentacoordinate allylsilicates. Typically, reactions of stereohomogeneous (E) - and (Z) -crotyltrifluorosilanes with various aldehydes in the presence of CsF or a wide variety of hydroxy compounds and triethylamine in THE gave the corresponding anti- and syn-β-methylhomoallyl alcohols, respectively, in highly diastereoselective manner. The stereoselectivity and the results of ab initio MO calculations suggest that the present allylation proceeds via six-membered cyclic transition states including the coordination of carbonyl oxygen to the silicon of pregenerated pentacoordinate allylsilicates. Facile reduction of carbonyl compounds by using pentacoordinate hydridosilicates, structural characteristics of pentacoordinate silylsilicates, and oxidative Si-C bond cleavage of, alkyl- and aryltrifluorosilanes by trimethylamine-N-oxide are also described to present the unique feature of the organosilicates.

Thermal stability and degradation of aromatic polyamides. Part 2. Structure-reactivity relationships in the pyrolysis and hydrolysis of benzamides

An experimental study to determine the effect of ring and nitrogen substitution on the rate and product distribution of the pyrolytic and hydrolytic degradation of polyamides was carried out. Two families of benzamides were chosen as models of the amide functionality obtained from the reaction of an aromatic acid and an aliphatic amine. The first family included benzamide and other primary amides with ring substituents. The second family comprised secondary or tertiary amides. The benzamides were reacted in an inert argon atmosphere and in the presence of added water at 350°C. The influence of nitrogen substitution was dramatic; the rate of disappearance of the primary amides was an order of magnitude faster than that for secondary or tertiary amides. The major products formed from primary amides were the corresponding benzonitrile and benzoic acid, which was a secondary product of pyrolysis and a primary product of hydrolysis. Pyrolysis of secondary and tertiary benzamides led to the corresponding lesser-substituted benzamide. A combination of a free radical and a concerted mechanism was proposed that accounted for the conversion and product selectivities for both families of benzamides. A nonspecific free radical mechanism with short chain length combined with a [1,3] sigmatropic shift and subsequent fission described the reaction of primary benzamides and methyl substituted benzamides that lacked a β CH. The higher selectivity to benzamide and benzonitrile for benzamides witha β CH suggested β-carbon directed chemistry. Appliction of a free radical mechanism with nonzero chain length afforded benzamide and benzonitrile, but failed to account for the observed product selectivities. A concerted reaction mechanism with a six-membered cyclic transition state led to high selectivity of benzamide and benzonitrile.

Elimination kinetics of 2-methyl-2-pentanol catalyzed by HCl in the gas phase

The kinetics of the elimination of 2-methyl-2-pentanol in the gas phase were determined in a static system, seasoned with allyl bromide, and catalyzed by hydrogen chloride gas. The working temperature and pressure ranges were 385–445 °C and 38–291 Torr, respectively. Steric acceleration appears to be a reasonable explanation for rate enhancement in the dehydration process. This reaction is believed to proceed through a six-membered cyclic transition state type of mechanism.

Clay catalysed nitrodecarboxylation of aromatic acids

Nitration ofp-anisic acid in clay microenvironment yields 2,4-dinitroanisole in about 40–60% yield, in addition to the conventional nitration product, namely 3-nitro-4-methoxybenzoic acid. While clay directed nitration ofp-methoxyacetophenone gives only 3-nitro-4-methoxy-acetophenone, withm-nitrobenzoic acid, nitrodecarboxylation yieldingm-dinitrobenzene is the only reaction course. The observed greater ipso-nitration facilitated by a nitro group may be attributed to the greater ease with which the Wheland intermediate may decompose via a cyclic six-membered transition state.

Do metal perchlorates act as a conventional Lewis acid in the allylstannation of aldehydes?

Lithium, magnesium and calcium perchlorates are found to promote the addition of allylic tin compounds to aldehydes via a six-membered cyclic transition state, although not acting as a conventional Lewis acid like BF3.

Primary thermal degradation mechanisms of PET and PBT

The primary thermal degradation mechanisms of polyethyleneterephthalate (PET) and polybutyleneterephthalate (PBT) have been studied by direct pyrolysis mass spectrometry using negative chemical ionization (NCI). The NCI mass spectra of PET and PBT are dominated by series of ions which might be assigned either to cyclic or open-chain oligomers with olefin and carboxylic end-groups. The structures of the pyrolysis products have been identified by comparison of their daughter ion spectra with those of authentic samples of cyclic and open-chain oligomers with olefin and carboxyl end-groups. The authors' results provide strong evidence that cyclic oligomers are the primary pyrolysis products of PET and PBT, formed by intramolecular exchange (ionic) reactions. These further decompose by a β-CH hydrogen transfer reaction, which involves a six-membered cyclic transition state, and generates open chain oligomers with olefin and carboxylic end-groups. The pyrolysis of PET and PBT was also carried out in the presence of ammonium polyphosphate (APP) (an acid precursor). APP affects the pyrolysis by increasing the rate of decomposition and lowering the Polymer Decomposition Temperatures (PDT)s of both polymers. However, it does not change the nature of the compounds obtained in the pyrolysis of the pure polymers. This fact provides a firm argument in favour of the belief that the β-CH hydrogen transfer occurs by an ionic process.

Elimination kinetics of tertiary alcohols catalyzed by HBr in the gas phase

The homogeneous, molecular, gas phase elimination kinetics of several tertiary alcohols in seasoned, static reaction vessels, catalyzed by hydrogen bromide at temperatures of 320–392 °C and pressures of 40–174 torr are described. The steric factor appears to be responsible for rate enhancement in the dehydration process. A six-membered cyclic transition state appears to be a reasonable explanation for the mechanism of these reactions.

The maximally catalyzed pyrolysis kinetics of methyl 3-bromopropionate in the gas phase

The kinetics of the gas phase pyrolysis of methyl 3-bromopropionate, under maximum catalysis of HBr, were found to be of order 1.0. The reaction appears to undergo a molecular elimination of HBr, which follows first-order kinetics. The products are methyl acrylate and HBr. The pyrolysis, in s static system and seasoned vessel, was examined over the temperature range of 330.5–378.5°C and pressure range of 64–145 Torr. The rate coefficients, under maximum catalysis, are given by the Arrhenius equation log k1(s−1)=(11.19±0.64)−(171.6±7.7) kJ/mol/2.303 RT. The mechanism of the catalyzed pyrolysis of the bromoester appears to proceed through a six-membered cyclic transition state.

ChemInform Abstract: New Synthetic Reactions Using Alkoxy‐Substituted, Highly Coordinate Organosilicon Compounds

Syntheses and some reactions of highly coordinate organosilicon compounds as synthetic reagents, substituted by alkoxy groups, are described. Topics are (1) preparations of pentacoordinate allylsiliconates, isolated for the first time, as useful and storable reagents for regio-, chemo-, and stereoselective allylations of carbonyl compounds, (2) stereochemical outcomes of asymmetric allylation with optically active allylsiliconates which are interpreted by a six-membered cyclic transition state, (3) cross-coupling reactions of alkenylsiliconates with organic iodides and triflates catalyzed by a palladium complex in a stereoselective mode, and finally (4) chemo- and stereoselective reductions of carbonyl compounds using alkoxy-substituted hydridosiliconates derived from trialkoxysilane and alkali metal alkoxides or lithium salts of amino alcohols, and new catalytic asymmetric reductions of prochiral ketones using hydridosiliconates.