Conformation and rotational barriers of 2-substituted 1,3-diphenylallyl anions

The conformations and the rotational barriers of the 2-substituted 1,3-diphenylallylanions $\text{1b}$–$\text{g}$ (Tab. 2) have been determined. Increasing size of the substituents leads to more $\text{exo}$,$\text{endo}$- and $\text{endo}$,$\text{endo}$-conformers at the cost of the $\text{exo}$,$\text{exo}$-species. This trend is connected with decreasing ΔG³-value sof the rotational barriers; the barriers are essentially not affected by ion pair effects, which is in contrast to the parent “allyl anion”.     

A theoretical investigation of the rotational barriers of peroxyformic acid

The internal rotational barriers in peroxyformic acid have been studied employing ab initio MO calculations. The C-O and O-O rotational barriers were calculated to be 7.68 and 1.04 $\text{kcal}\text{mol}$, respectively. The relatively low O-O rotational barrier is attributed to a balance between electron repulsion and hydrogen bonding in the syn chelated conformer.     

Concerted and two-step conformational ring-flipping in [2.2] (3,3′,4,4′) biphenylophanes1

The [2.2] (3,3′,4,4′)biphenylophanes ($\text{4}$) and ($\text{7}$) undergo conformational ring-flipping. The activation energy barriers have been determined and the possible conformational pathways are presented.     

Stereochemistry of organic sulphur compounds : Part. 17 conformational analysis of some erythro- and threo-thio-derivatives of β-fluorinated and β-oxygenated ethanes

The synthesis and conformational analysis of 2-fluoro, 2-hydroxy and 2-methoxy thioderivatives (thioethers, sulphoxides, sulphones and sulphonium salts) of 1,2-dimethyl and 1,2-diphenylethanes ($\text{erythro}$ and $\text{threo}$) are reported. Steric effects in thioethers and electrostatic interactions in sulphonium salts are the main factors determining the stability rotamers. A balance of these factors controls the conformational equilibria in sulphoxides and sulphones. Electronic interactions between the $\text{π}$-aromatic electrons or the unshared β-heteroatomic electrons and the sulphur atom are suggested to explain large differences in rotamer populations induced by changes in the carbon sketelon or in the relative configuration of the sulphinylic sulphur.     

Reaction of dicarbonyl compounds with dimethyl 3-ketoglutarate. Influence of steric effects on success of the condensation

The influence of steric factors on the synthesis of substituted bicyclo[3.3.0]octane-3,7 diones $\text{via}$ the condensation of 1,2-dicarbonyl compounds with dimethyl 3-ketoglutarate $\text{2}$ is described.     

Photochemistry of thioanhydrides. Photofragmentation of cis-1,2-dihydrophthalic thioanhydrides

$\text{Cis}$-1,2-Dihydrophthalic thioanhydride ($\text{2}$) and some substituted derivatives were prepared and found to undergo irradiation induced, high quantum-yields CO+COS extrusion from a siglet and a (high) triplet excited state.     

Conformational populations and rotational barriers in esters

The $\text{trans}$ conformations of methyl, ethyl and isopropyl formate were shown to be present in equilibrium in a polar solvent with the $\text{cis}$ conformations to significant (> 1% 230 K) but much lower extents than for t-butyl formate; rotational barriers for the former compounds are greaters.     

Addition of boron enolates to isoquinolinium salts: A facile synthesis of 1-β-keto substituted 2-ethoxycarbonyl-1,2-dihydroisoquinolines and their cyclization

An efficient method to synthesize 1-β-keto substituted 2-ethoxycarbonyl (or 2-acetyl)-1,2-dihydroisoquinolines ($\text{3}$) is described, utilizing boron enolates and isoquinolinium salts. The products were treated with sodium ethoxide to afford the corresponding cyclic compounds ($\text{6}$).     

Stereochemistry of organic sulphur compounds. part 13. Configurational assignment of diastereoisomers of 2-methylsulphinyl-1,2-diphenylethanol and of their O-methyl and O-acetyl derivatives

The synthesis, isolation and conformational analysis of the diastereomeric 2-methylsulphinyl-1,2-diphenylethanol and of its $\text{O}$-methyl and $\text{O}$-acetyl derivatives are reported. Chemical correlations and the study of the influence of solvent polarity changes on the coupling constants have permitted the configurational assignment. Lanthanide shift reagents have been used also. to this effect. The role of hydrogen bonding in the hydroxysulphoxides has been evaluated in diluted solutions by IR and NMR spectroscopy. A donor-acceptor interaction between oxygen and sulphur has been invoked to explain the differences in conformational behaviour between epimeric sulphoxides at sulphur atom.     

Photoaddition reactions of trans dichloroethylene with benzenoid compounds: facile formation of tetracyclo[3.3.0.02,804,6]octanes and semibullvalenes

$\text{trans}$ 1,2-Dichloroethylene undergoes a stereospecific photoreaction with benzonitrile, the three tolunitriles, α,α,α-trifluorotoluene, fluorobenzene, and chlorobenzene to give substituted 6-$\text{exo}$ 7-$\text{endo}$ dichlorotricyclo[3,3,0,0_2,8]oct-3-enes which on treatment with base yield cyclised products or semibullvalenes: phenol yields dichlorobicyclo[3.2.1]oct-2-en-8-one with this ethylene photochemically.     

Nucleophilic reactions of fluoroolefins. II. Regioselectivity and elimination-addition competition in the reaction of 1-phenylpentafluoropropenes with sodium ethoxide

1-Phenylpentafluoropropene and its para-substituted analogs $\text{1}$ are susceptible to nucleophilic attack at both vinylic carbon atoms C-1 and C-2. They react with ethanolio sodium ethoxide to give predominantly substitution products, 1-ethoxy-1-phenyltetrafluoropropenes $\text{2}$ and 2-ethoxy-1-phenyltetrafluoropropenes $\text{3}$, with only little formation of adducts, $\text{viz}$. 2-ethoxy-1H-1-phenylpentafluoropropanes $\text{4}$. Alkenes $\text{1}$, where the para-substituent X  H,Cl,and CF₃ give additionally 1,2-diethoxy-1-phenyltrifluoropropenes $\text{5}$ and, where X = CF₃ also 2,2-diethoxy-1H-1-phenyltetrafluoropropane $\text{6}$. Overall regioselectivity of nucleophilic attack of the ethoxide ion on alkenes $\text{1}$ exhibits the Hammett type correlation with Ó_p values of substituents X: CH₃O and CH₃ groups favour the attack on the vinylic carbon C-1, while CF₃ and Cl substituents direct the attack on the C-2 carbon of alkenes $\text{1}$. The E and Z isomers of 1-ethoxy and 1,2-diethoxy substituted alkenes $\text{2}$ and $\text{5}$ were formed in comparable amounts, while the E isomers of 2-ethoxy substituted alkenes $\text{3}$ were always formed with a 93 – 97 % selectivity.     

Photochemical studies. Irradiation induced transformations of 1,2-dihydrophthalides

The photochemical behavior of $\text{cis}$-1,2-dihydrophthalide ($\text{1}$) and its 1,2-trimethylene derivative ($\text{4}$) was studied. Besides bicyclo[2.2.0]hexene formation in both cases, ($\text{1}$) transforms into its $\text{trans}$ isomer ($\text{3}$) whereas ($\text{4}$) undergoes a 1,2-shift to ($\text{7}$).     

Conformational analysis of 3-buten-2-ol: A model asymmetric olefin

The conformational energy of 3-buten-2-ol has been investigated using $\text{ab}$$\text{initio}$ molecular orbital theory. Six energy minima have been located, the lowest energy two of which are the same as those assigned in a recent microwave study of the molecule. Rationale for the observed preferences are advanced.     

Analysis of cycloheptanone conformation from temperature dependent circular dichroism measurements of (+)-(3R)-methylcycloheptanone and (−)-(4R)-methylcycloheptanone

Conformational analysis of methylcycloheptanones was achieved by relating CD rotational strengths to conformational free energies. Twist conformers with C₁ on the symmetry axis (TC₁) were found to be more stable than those with C₂ or C₇ on the symmetry axis (TC₂ or TC₇). The free energy differences were found to range from 0.2 to 1.2$\text{kcal}\text{mole}$. A similar analysis of (+)(3R)-methylcyclohexanone showed the equatorial chair conformer to be most stable, by 0.50–0.75 $\text{kcal}\text{mole}$.     

Stereospecificity in thermal isomerization of preisocalamendiol,a ten-membered ring sesquiterpene: An application of molecular mechanics calculations

Molecular mechanics calculations were carried out to explain stereospecificity in the cyclization reaction of prisocalamendiol ($\text{1}$), a ten-membered ring sesquiterpene, suggesting that this stereospecificity does not depend on relative stabilities of the conformational isomers of $\text{1}$, but is due to those of the corresponding conformers in each transition state. Particularly, molecular mechanics calculations, which were made on the basis of such a transition state model as $\text{3}$, can explain the experimental results much well.     

Zur deprotonierung von chloromethyleniminiumchloriden

Deprotonation of chloromethyleniminium chlorides ($\text{1}$, $\text{13}$) leads to cis-trans-isomeres 1,2-diamino-1,2-dichloro-ethenes ($\text{3a}$, $\text{3b}$ resp. $\text{14a}$, $\text{14b}$); their reactivity is described.     

1,4-Addition of dichlorocarbene to 1,2-bismethylenecycloheptane

The reaction of dichlorocarbene with 1,2-bismethylenecycloheptane $\text{3}$ yields, besides the 1,2-addition product $\text{4b}$, the 1,4-addition product $\text{6b}$ in a ration 99:1.     

Stereoselective syntheses of substituted methyl (Z,E,E)-deca-2,7.9-trienoates and substituted methyl (Z,E,E)-undeca-2,8,10-trienoates

Lindlar hydrogenation of substituted methyl ($\text{E}$,$\text{E}$)-deca-7,9-dien-2-ynoates and substituted methyl ($\text{E}$,$\text{E}$)-undeca-8,10-dien-2-ynoates affords selectively the corresponding ($\text{Z}$,$\text{E}$,$\text{E}$)-trienes.     

Peri-bridged naphthalenes. 4. chalcogen-bridged acenaphthylenes

Three new electron donors, acenaphtho[5,6-$\text{cd}$]-1,2-dithiole ($\text{1}$), acenaphtho[5,6-$\text{cd}$]-1,2-diselenole ($\text{2}$), and acenaphtho[5,6-$\text{cd}$]-1,2-ditellurole ($\text{3}$), can be prepared in 28%, 22%, and 14% yields respectively by reaction of the elemental chalcogens (S, Se, Te) with 5,6-dilithioacenaphthylene ($\text{4}$). Compound $\text{4}$ is generated by treatment of 5,6-dibromoacenaphthylen ($\text{5}$), for which a convenient preparation is described, with $\text{n}$-butyllithium (2 equiv.) in THF at ?78°C.     

Cross-conjugated biradicals: Kinetics and kinetic isotope effects in the thermal isomerizations of cis- and trans-2,3-dimethylemethylnecyclopropane,cis- and trans-3,4-dimethyl-1,2-dimethylenecyclobutane,and of 1,2,8,9-decatetraene

cis- and trans - 2,3 - Dimethylenemethylenecyclopropane ($\text{C}$ and $\text{T}$) interconvert at 160.0° with a small normal kinetic isotope effect (KIE) when the exo-methylene is deuterated, but the 1,3-shift products, 2-methylethylidenecyclopropane, show a large normal KIE, 1.35 and 1.31, when formed from $\text{C}$ and $\text{T}$, respectively. This data can be interpreted in terms of either parallel reactions or a common trimethylenemethane diradical intermediate formed with a normal KIE of 1.11 and closing to 1,3-shift product with a normal KIE of 1.29 due to the effect of deuterium in the required 90° rotation of the exo-methylene carbon.The kinetics of the thermal 1,3- and 3,3-shifts of cis- and rans-3,4-dimethyl-1,2-dimethylenecyclobutane ($\text{CB}$ and $\text{TB}$) were determined in a flow reactor. The first order rate constants are log k_CB (sec^?1) = 13.7 ? 42,200/2.3 RT and log k_TB (sec^?1) = 13.6 ? 41,900/2.3 RT (E_a in kcal/m) which compare favorably to that from the parent hydrocarbon. 1,2-dimethylenecyclobutane, after reasonable correction for dimethyl substitution.Rearrangement of $\text{TB}$ and its bis(dideuteriomethylene) derivative at 230.0° revealed a normal KIE of 1.08. This KIE could be interpreted in terms of either a methylene rotational isotope effect in a concerted reaction or formation of a bisallyl diradical with the expected normal rotational IE on closure to the 1,3-shift product of 1.12 with no IE in the ring opening when the result is corrected for return of the biradical to starting material.The kinetics of intramolecular 2 + 2 cycloaddition of 1,2,8,9-decatetraene were determined in a flow reactor. The first order rate constant is log k(sec^?1) = 9.4 ? 30,800/2.3 RT (E_a in kcal/m). These energetics are compared with those of other 2 + 2 cycloadditions. The major product is 3,4-dimethylenecyclooctene ($\text{DC}$) which is also found from the minor product, cis-7,8-dimethylenebicvyclo[4.2.0]octane ($\text{CO}$), at higher temperatures. The trans isomer, $\text{TO}$, also gives $\text{DC}$ at about the same rate as $\text{CO}$.