Kinetic and Quantum-Chemical Study of Oxirane Ring Cleavage with Acids

Kinetic and thermodynamic parameters of the reactions of phenyl glycidyl ether and epichlorohydrin with bis(alkylpolyethylene glycol) ether of orthophosphorus acid (oxyphos KD-6) are established. It is shown that the difference in the reactivity of the oxiranes is caused by the electronic effects of substituents and the protonation by the phenolic oxygen atom of phenyl glycidyl ether. Basic solvents decrease the reactivity of the systems. Based on AM1 semiempirical quantum-chemical calculations, a hydroxycarbocation mechanism of the oxirane ring opening was proposed, involving initial formation of unstable cis- and trans-oxonium structures.     

Substituent effect on the 1H NMR Spectra of trans aryl methyl oxiranes and arylpropenes

The ¹H NMR spectra of 1-aryl-2-methyl oxiranes substituted in the phenyl ring and of the corresponding vinyl derivatives have been analysed. The substituent effect on oxiran protons seems mostly polar in character, as shown by correlations with substituent constants, solvent effects and the fact that perturbation decreases with increasing distance from the substituent. The comparison with the corresponding vinyl derivatives, in which conjugation effects are present, confirms this point. The results also seem to exclude the possibility of substituents causing significant changes on the preferred conformation of the phenyl ring. Ring current contributions on oxirane protons, evaluated by SCF procedure, show that their change with substituents is very small and does not represent a significant part of the change of proton chemical shift with substituents.     

1H NMR spectra and electronic structure of 3-arylmethyl-idenephthalide and 3-arylthiomethylidenephthalide derivatives

The ¹H NMR spectra of trans-3-phenylmethylidenephthalide and trans-3-phenylthiomethylidenephthalide derivatives were investigated. After applying a correction for the anisotropy of substituents and/or for changes of ring current in the substituted aromatic ring, linear correlations were obtained between the chemical shifts of protons of the substituted phenyl group and the methine group and s? constants of substituents. The influence of the bridge heteroatom on the transfer of electronic effects of substituents through the molecules under study is discussed.     

17O NMR chemical shifts versus structure relationships in oxiranes

Natural-abundance ¹⁷O NMR spectra have been measured for twenty one oxiranes. Their chemical shifts covering a 100 ppm range were interpreted in terms of the paramagnetic β- and diamagnetic γ-effects. In addition, through-space orbital interaction between the ethylenic π and the Walsh orbitals of the oxirane ring was suggested by a lowfield shift in norbornadiene $\text{exo}$-oxide.     

Polymerization of oxiranes by polymeric organoantimony oxides

Polymeric arylantimony(V) oxides [poly(ArSbO₂), Ar = phenyl, p-chlorophenyl (CPh), and p-methylphenyl (Tol)] were employed as catalysts for the polymerization of oxirane [ethylene oxide (EO)] and also substituted oxiranes [propylene oxide (PO), 1,2-butylene oxide (BO), and epichlorohydrin (ECH)]. The polymerization of EO by ArSbO₂s proceeded 3–60 times faster than that by the other organoantimony and -tin compounds such as triphenylstibine oxide (Ph₃SbO) and arenestannoic acids (ArSnO₂H), respectively. Apparent activation energy for the polymerization of EO was estimated as 13.7, 13.3, and 13.6 kcal/mol for PhSbO₂, TolSbO₂, and CPhSbO₂, respectively. The results of the polymerization as well as ¹H-, ¹³C-, and ¹⁷O-NMR spectroscopy suggested that the polymerization was initiated by ArSbO₂ or Ar₂Sb₂O₄ fragments, which was derived from a nucleophilc solvation of the polymeric ArSbO₂ by oxiranes in situ.     

Conformational composition of the 2,4,6-trimethyl-1,3,2-dioxaborinane stereoisomers

A study of conformational transformations of 2,4,6-trimethyl-1,3,2-dioxaborinane cis- and trans-isomers in non-empirical quantum-chemical HF/6-31G (d) and PBE/3z approximations showed that the transformation routes include the sofa conformers with different orientations of the substituents at the ring atoms C⁴ and C⁶ through the transition states corresponding to diequatorial, axial-equatorial, and dixial conformations of the 2,5-twist form. The calculation and ¹H NMR data show that the conformational equilibrium of cis-isomer is almost entirely shifted to diequatorial sofa, and the trans-form is characterized by the interconversion between the two axial-equatorial sofa conformers.     

13C NMR shifts and conformations of substituted indans

¹³C NMR spectra of indan derivatives bearing substituents in the 1, 2, 5 and 6 positions are reported and assigned by LIS measurements and other techniques. Epimeric indanes bearing vicinal oxygen and phenyl or benzyl substituents show ring carbon shielding in the cis relative to the trans isomers, which is compared with corresponding cyclopentane shifts, and indicates the predominance of envelope conformations with pseudoaxial oxygen substituents for the cis isomers. Acetylation shifts show consistently larger shielding at C-β for the trans compounds. Introduction of oxygen at C-5 leads to asymmetric shielding effects at the ortho carbon atoms as soon as there is a substituent in the para position which can participate in mesomeric forms.     

Steric as well as n→π* Interaction Controls the Conformational Preferences of Phenyl Acetate: Gas‐phase Spectroscopy and Quantum Chemical Calculations

Herein, we report that the conformational preference of phenyl acetate is governed by steric effect and n→π* interaction. Conformation‐specific electronic and IR spectroscopy combined with quantum chemistry calculations confirm the presence of only the cis conformer of phenyl acetate in the experiment. The cis conformer of phenyl acetate has n→π* interaction between the lone‐pair electrons on the carbonyl oxygen atom and the π* orbitals of the phenyl group. The n→π* interaction is absent in the trans conformer which has additional steric repulsion between the methyl group and phenyl ring. The trans conformer is higher in energy than the cis conformer by ≈3 kcal mol^?1. We have found the effect of methyl substitution on the strength of the n→π* interaction, steric repulsion, and hyperconjugation in phenyl acetate. The red‐shift observed in the cis conformer of phenyl acetate with respect to the trans conformer is affected due to the influence of the methyl substituent on the strength of the n→π* interaction as well as hyperconjugation. The present result demonstrates that the introduction of a bulkier substituent can induce steric as well as electronic control to reduce conformational heterogeneity of a molecular system. Understanding the effect of bulkier substituents to promote defined conformations having specific non‐covalent interactions may have implication in better perception of the optimum structure and function of biomolecules as well as recognition of drugs by biomolecules.     

Critical examination of the 1H NMR spectra of aryl epoxides

The proton chemical shift of several derivatives of styrene oxides have been obtained in different solvents in order to check the relative importance of different contributions to the shielding of oxirane protons. It seems likely that ‘ring current’ and electrostatic effects can explain differences within the several compounds examined. An investigation of the conformational requirements of the phenyl group in these systems shows that the preferred conformation present in styrene oxide is also likely to be maintained in other compounds, where groups cis and trans to the phenyl ring are present. A discussion of the possibility of employing ‘ring currents,’ obtained both in the Johnson and Bovey scheme and with an SCF treatment for conformational analysis is given and it seems that this contribution, when considered alone, can only give very approximate information regarding the geometrical pattern of the molecule.     

Etude Stéréochimique par RMN 1H de Tetrahydropyrannes β-Chlorés α-Substitués

The ¹H NMR study of 2-alkyl-3-chlorotetrahydropyrans, obtained by reaction of Grignard reagents with a mixture of cis/trans-2,3-dichlorotetrahydropyrans, shows cis/trans configuration of two isomers in which the alkyl substituents are exclusively in the equatorial position. 3-Chloro-2-phenyltetrahydropyran exists in trans (eq-eq) configuration only. The ¹H NMR study of cis/trans 2-alkoxy (or aryloxy)-3-chlorotetrahydropyrans, obtained by reaction of alcohols or phenol with 2,3-dichlorotetrahydropyrans, shows the axial position of the alkoxy (or aryloxy) substituent.     

Protonenresonanz- und Röntgenstrukturuntersuchungen an unsymmetrisch substituierten Aminoboranen. III—Substituentenabhängigkeit der Struktur

A number of unsymmetrically substituted N-methylaminoboranes, each with a phenyl ring at nitrogen and boron, were synthesized. The ratio of the cis-trans isomers has been investigated by means of ¹H NMR spectroscopy and its dependence on the size of the aromatic moieties and the second substituent on boron is discussed. The structures of the compounds were established from the position of the N-methyl signal and were based on X-ray structure determinations of (4-bromophenylmethylamino) chlorophenylborane and (4-bromo-2-methylphenylmethylamino)chloro(2-methylphenyl)borane. In the case of (methylphenylamino)chlorophenylborane, the isomer with the phenyl group in cis position is highly favoured (90%) in the thermal equilibrium. Substitution of one of the phenyl groups by a 2-methyl- or 2,6-dimethylphenyl group decreases the fraction of the cis isomer. The same occurs when the chlorine substituent at boron is replaced by bromine or the methyl group. In absolute terms, the trans isomers are energetically more stable than the cis isomers only if one of the substituents at boron is a methyl or a 2,6-dimethylphenyl group or if there is a 2-methylphenyl substituent both at the nitrogen and the boron atom. Steric hindrance and electronic repulsion are probably responsible for the observed substitution effects. In addition, these experiments show that the isomer favoured in the crystal also predominates in solution.     

1H- and 13C-NMR. Studies of the molecular conformations of cyclo-tetraglycyl.

The ¹H- and ¹³C-NMR. spectra of cyclo-tetraglycyl show that the four peptide groups are magnetically equivalent, and different from either a standard trans or a standard cis peptide group. It is suggested that the observed NMR. features correspond to a non-planar form of the peptide groups. On the one hand these data confirm the earlier conclusions from theoretical investigations of the molecular geometry, that cyclic tetrapeptides could not contain four standard trans peptide groups. On the other hand they are not consistent with a previously suggested alternative molecular conformation according to which cyclo-tetraglycyl would adopt a conformation similar to cyclo-tetrasarcosyl, with two cis and two trans peptide bonds. The different behaviour of glycine and sarcosine under the steric strains of tetrapeptide ring closure would appear to suggest that with the exception of the X-Pro bonds, transoid peptide groups in polypeptide chains of the common amino acids should be more likely to occur than the cis form, which has as yet apparently not been observed for N-unsubstituted peptide groups in natural peptides or proteins.     

Photochromism in the arylaroylaziridine system

Photochromism of a number of arylaroylaziridines has been observed in the solid state or in the rigid matrix. The photochromic behavior is dependent on the intensity of the incident radiation, the reaction medium, the wavelength of light used and on the relationship of the substituents on the three-membered ring. From the visible absorption spectra it is clear that cis-arylaroylaziridines give different colored species than do the trans isomers. These results suggest that the coloration is due to an extensive electrical interaction between the bent bonds of the aziridine ring and the π orbitals of the benzoyl and phenyl groups. Attempts to trap the colored species by co-irradiation with substrates containing a multiple bond failed. A reaction has been found to occur upon heating various arylaroylaziridines with dimethylacetylene dicarboxylate in inert solvents.     

Synthesis and nuclear magnetic resonance spectroscopic studies of 1‐arylpyrroles

A series of m‐ and p‐substituted 1‐phenyl, 1‐benzyl, 1‐benzoyl, and 1‐(2‐phenylethyl)pyrroles was prepared and their ¹H and ¹³C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shift values of the β? H and the β? C of pyrroles [except 1‐(2‐phenylethyl)pyrroles] and the Hammettt σ. The observation may be explained in terms of the electronic effects of the substituents which are transmitted through bonds and through space by interaction of the p orbitals between β? Cs of the pyrrole ring and m‐ and p? Cs of the phenyl ring. Substituent constants of 1‐pyrrolyl, 1‐pyrrolylmethyl, and 1‐pyrroloyl groups for the ¹H and ¹³C chemical shifts of phenyl ring are also presented.     

Stereochemical characterization of some mono- and diaryl substituted ethylene oxides by NMR spectroscopy

NMR spectra of several styrene, stilbene and stilbazole oxides have been determined, and chemical shifts and coupling constants have been correlated with cis-and trans-configurations. Assignments have been made for all protons, and double resonance technique and ¹³C? H coupling constants have been used in some particular cases. An explanation is proposed for the observation that chemical shifts of oxirane protons are higher for cis than for trans isomers.     

Ring distortion effect on δ13C values of bicyclo[4.n.0] alkanes

The ¹³C NMR spectra of trans- and cis-fused bicyclo[4.n.0]alkanes, where n=4, 3 and 2, were determined. The distortion of the six membered ring, which is due to the strain arising from the 1,2 fusion of cyclohexane with a smaller ring, is apparent mainly for the chemical shifts of the ring junction carbons. Both for the trans-and for the cis-fused hydrocarbons the results support a cyclohexane in a chair conformation, distorted to a different degree according to the size of the other ring. The geometries of the different compounds in the series were calculated.     

Ring-opening polymerization of α,β-disubstituted oxiranes by α-methoxyphenylmethylium hexachloroantimonate

α-Methoxyphenylmethylium hexachloroantimonate was used as a novel initiator for the polymerization of α,β-disubstituted oxiranes such as cyclohexene oxide (CHO) and 2-butene oxide (trans and cis) (2-BO) at ?78°C with dichloromethane or dichloromethane-toluene mixtures as solvents. The CHO polymerization mixture became turbid and the polymer precipitated in dichloromethane. The CHO polymerization proceed quantitatively in dichloromethane–toluene mixtures. The molecular weight distribution of polyCHO obtained was bimodal regardless of the solvent used. The polymerization of trans-2-BO was heterogeneous in both dichloromethane and dichloromethane–toluene mixture. The polymerization mixtures of cis-2-BO were transparent but reached a limit yield which was less than the polymer yield of trans-2-BO. Furthermore, the microstructure of the poly2-BOs were analyzed by Vandenberg's method and the results confirmed Vandenberg's finding that inversion of configuration occurs in the propagation step.     

Regio- and Stereoselective Formation of 1,3-Oxathiolanes by Reactions of Thiocarbonyl Compounds with Oxiranes

Abstract

Lewis acid-catalyzed reactions of oxiranes with a variety of C═S compounds yield 1,3-oxathiolanes. The ring enlargement of monosubstituted oxiranes occurs regioselectively via cleavage of the O,C(3) bond of alkyl substituted oxiranes and the O,C(2) bond of phenyl oxirane. Furthermore, the reaction proceeds with inversion of the configuration at the center of the nucleophilic attack by the S-atom. The formation of thiocarbonylium ions as intermediates is supported by Wagner–Meerwein-type rearrangements. Enolized thioketones react with oxiranes to give enesulfanyl alcohols, which undergo an acid-catalyzed cyclization to yield 1,3-oxathiolanes.     

Effect of allylic strain on the conformations of diphenyl-substituted tetrahydro-1,3-oxazin-2-ones and hexahydropyrimidin-2-ones

The conformations of the cis and trans isomers of 4,6-diphenyl-, 4,5-diphenyl- and 5,6-diphenyltetrahydro-1,3-oxazin-2-one and 4,5-diphenylhexahydropyrimidin-2-one, and of some of their N-substituted derivatives, have been studied by ¹H NMR. Conformers with 4a, 6e-, 4a, 5e- and 5a, 6e-phenyl groups are preferred in the respective isomers of the N-H oxazinones, confirming a half-chair conformation of the ring. Allylic strain caused by N-substituents shifts strongly the a,e?e, a equilibria in trans-4,6-diphenyl- and cis-4,5-diphenyl-oxazinones, but only moderately the e,e?a,a equilibria in the compounds with trans-vicinal phenyl groups. In the latter, the diaxial conformation is preferred only in the case of bulky N-substituents. The diaxial conformation is more favoured in the trans-4,5-diphenylpyrimidones.     

Copper-catalyzed rearrangement of vinyl oxiranes

A novel copper-catalyzed vinyl oxirane ring expansion protocol has been developed. A wide range of vinyl oxiranes can be rearranged to 2,5-dihydrofurans in excellent yields in the presence of electrophilic copper(II) acetylacetonate catalysts. Regioisomeric vinyl oxiranes can be converted to a single dihydrofuran product using these conditions. This method uses low catalyst loadings (0.5-5 mol %), has good tolerance of substitution patterns, and can be done in the absence of solvent.