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.     

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.     

Synthese von threo-cis/threo-trans- und erythro-cis/erythro-trans-Dihydropalustrin. 17. Mitteilung über Schachtelhalmalkaloide

Synthesis of threo-cis/threo-trans- and erythro-cis/erythro-trans-dihydropalustrin The first synthesis of a threefold protected spermidine, namely ³-benzyloxycarbonyl-N¹-phthaloyl-N²-tosylspermidin ( 9 ) is presented. Each of the protecting groups can be removed selectively. After hydrazinolysis the resulting N³-benzyloxy-carbonyl-N²-tosylspermidine ( 10 ) has been condensed with methyl (2 E)-cis-7,8-epoxy-2-decenoate to the threo-cis/trans piperidines 17 , and with methyl (2 E)-trans-7,8-epoxy-2-decenoate to the erythro-cis/trans piperidines 17 , respectively. After catalytic removal of the Z group, the resulting aminoesters 13 and 18 , in a melt with imidazole, underwent ring closure to the 13-membered lactames 14 and 19 , respectively. reductive deprotection of the N-tosyl group with sodium/ammonia led to the stereoisomeric palustrines 15 and 20 , respectively.     

Two optically active iso­quinoline derivatives

In the two title optically active tetra­hydro­iso­quinoline derivatives, namely 3‐hydroxy­methyl‐4‐phenyl‐1,2,3,4‐tetra­hydro­isoquinolin‐2‐ium bromide methanol hemisolvate, C₁₆H₁₈NO⁺·Br^?·0.5CH₃OH, (IIb), and 2‐formyl‐3‐hydroxy­methyl‐4‐phenyl‐1,2,3,4‐tetra­hydro­iso­quinoline, C₁₇H₁₇NO₂, (III), the absolute configurations have been confirmed as 3R,4R by structure refinement using Bijvoet‐pair reflections. The hydroxy­methyl and phenyl groups in (IIb) are oriented in equatorial and pseudo‐equatorial positions, respectively, whereas in (III), the corresponding groups are in axial and pseudo‐axial positions, respectively; the hydroxy­methyl and phenyl groups are trans with respect to one another in both structures. The heterocyclic rings in (IIb) and (III) adopt envelope conformations inverted with respect to each other. In both structures, the mol­ecules are linked through hydrogen bonds.     

Structural and Conformational Aspects of Equatorial and Axial Trifluoromethyl,Difluoromethyl, and Monofluoromethyl Groups

The X‐ray crystal structures of cis‐ and trans‐1‐(indol‐3‐yl)‐4‐methyl cyclohexane and its congeners with stepwise fluorination of the methyl group are reported. The trans‐configured compounds adopted diequatorial conformations, whereas the cis analogues adopted regular cyclohexane chair conformations with the methyl group preferentially assuming the axial position, even in the case of the CF₃ group. Surprisingly, although the axial CF₃ derivative displayed distinct valence deformations in the cyclohexane moiety, the observed structural changes were relatively modest. The cis derivatives with axial mono‐ and difluorinated methyl groups exhibited conformational disorder in the crystals with significant population levels for the staggered conformations that had one fluorine atom in the endo position; their respective trans counterparts adopted unique conformations, but again with one fluorine atom in the endo position. Theoretical calculations for a series of cis‐ and trans‐1,4‐dimethyl cyclohexane model compounds with stepwise fluorination of one equatorial or axial methyl group reproduced the experimentally observed structural response patterns very well, reproduced the experimentally determined nonlinear correlation of the axial–equatorial energy difference with the degree of methyl fluorination in a satisfactory manner, and provided further insights into important conformational aspects of partially fluorinated methyl groups.     

Design and Theoretical Study of Nickel Catalysts for Syndiotactic Polyolefins

A nickel catalyst was nodeled with ligand L^2,[NH=CH-CH=CH-0]^-,which should have potential use as a syndiotactic plyolefin catalyst,and the reaction mechanism was studied by theoretical calculations using the density functinal method at the B3LYP/LANL2MB level.The mechanism involves the formation of the intermediate [NiL^2Me]^ ,in which the metal occupies a T-shaped geometry.This intermediate has two possible structures with the methyl group trans either to the oxygen or to the nitrogen atom of L^2.The results show that both structures can lead to the desired product via similar reaction paths,A and B.Thus,the polymerization could be considered as taking place either with the alkyl group occupying the position trans to the Ni-0 or trans to the Ni-N bond in the catalyst.The polymerization process thus favors the catalysis of syndiotactic polyolefins.The syndiotactic synthesis effects could also be enhanced by varations in the ligand substituents.From energy considerations,we can conclude that it is more favorable for the methyl ttrans-O position to form a complex than to occupy the trans-N position.From bond length considerations,it is also more favoured for ethene to occupy the trans-O position than to occupy the trans-N position.     

Stereochemical aspects of proton chemical shifts. VII. The γ effects of hydroxyl,methoxyl and methyl substituents

The ¹H NMR chemical shifts of some hydroxy, methoxy or methyl substituted trans-decalins, trans-1, 3-dioxadecalins and cyclohexanes are reported. It is concluded that the replacement in a g⁺g⁺ H? C? C? C? H fragment of one hydrogen by hydroxy, methoxy or methyl results in a modest (0.1 ppm) upfield shift of the other hydrogen atom. Experimental limitations to the transferability of shift increments from one molecular environment to another are demonstrated. The syntheses of 1α,5β-dimethoxy- and 1β,5α-dimethoxy-trans-decalin are given.     

Conformational Analysis,RIS Models and Single‐Chain Properties of Structurally Modified Polycarbonates, 1. Effect of Cyclohexyl and Phenyl Ring Substitutions

Conformational properties of segments and chains of structurally different polycarbonates are investigated in detail. Conformational analysis and rotational isomeric state (RIS) models for some of the polycarbonates and single‐chain properties of all the polycarbonates are reported here for the first time. Substitution of the methyl group on the bisphenol phenyl rings results in increased energy barriers to rotations as well as changes in positions of local minima, compared to the case without substitutions. Conformational structure about the isopropylidene linkage C_α atom is not altered by ortho methyl substitutions on the rings. Substitution by a cyclohexyl ring rigidly attached to the C_α atom restricts conformational mobility within the bisphenol unit. Rotational flexibility of the phenyl–oxygen bond is hindered by additional substitutions on the cyclohexyl ring. The carbonate group prefers the trans–trans conformation in all the polycarbonates. The energy difference between the cis–trans and trans–trans states of the carbonate group is lowered by the ortho methyl substituent on the phenyl rings. There is a reduction in 〈R²〉, 〈S²〉, and C_n accompanying the substitutions. The introduction of other substituents on a cyclohexyl polycarbonate results in an increase in all chain dimensions including the persistence length. Also, the cyclohexyl or trimethylcyclohexyl substituents do not significantly alter the overall average shape of the chains. Substitutions both on the phenyl rings and at the isopropylidene linkage lead to a compaction of the polymer chain, but the effect is more pronounced when due to substituents on phenyl rings.     

Thermische Lactonisierung von Estern γ-Brom-α, β-ungesättigter Carbonsäuren zu Δα-Butenoliden. Direkte γ-Bromierung von α, β-ungesättigten Säuren

By heating with iron powder at 120–150° some γ-bromo-α, β-unsaturated carboxylic methyl esters, and, less smothly, the corresponding acids, were lactonized to Δ^7alpha;-butenolides with elimination of methyl bromide. The following conversions have thus been made: methyl γ-bromocrotonate ( 1c ) and the corresponding acid ( 1d ) to Δ^α-butenolide ( 8a ), methyl γ-bromotiglate ( 3c ) and the corresponding acid ( 3d ) to α-methyl-Δ^α-butenolide ( 8b ), a mixture of methyl trans- and cis-γ-bromosenecioate ( 7c and 7e ) and a mixture of the corresponding acids ( 7d and 7f ) to β-methyl-Δ^α-butenolide ( 8c ). The procedure did not work with methyl trans-γ-bromo-Δ^α-pentenoate ( 5c ) nor with its acid ( 5d ). Most of the γ-bromo-α, β-unsaturated carboxylic esters ( 1c, 7c, 7e and 5c ) are available by direct N-bromosuccinimide bromination of the α, β-unsaturated esters 1a, 7a and 5a ; methyl γ-bromotiglate ( 3c ) is obtained from both methyl tiglate ( 3a ) and methyl angelate ( 4a ), but has to be separated from a structural isomer. The γ-bromo-α, β-unsaturated esters are shown by NMR. to have the indicated configurations which are independent of the configuration of the α, β-unsaturated esters used; the bromination always leads to the more stable configuration, usually the one with the bromine-carrying carbon anti to the carboxylic ester group; an exception is methyl γ-bromo-senecioate, for which the two isomers (cis, 7e , and trans, 7d ) have about the same stability. The N-bromosuccinimide bromination of the α,β-unsaturated carboxylic acids 1b , 3b , 4b , 5b and 7b is shown to give results entirely analogous to those with the corresponding esters. In this way γ-bromocrotonic acid ( 1 d ), γ-bromotiglic acid ( 3 d ), trans- and cis-γ-bromosenecioic acid ( 7d and 7f ) as well as trans-γ-bromo-Δ^α-pentenoic acid ( 5d ) have been prepared. Iron powder seems to catalyze the lactonization by facilitating both the elimination of methyl bromide (or, less smoothly, hydrogen bromide) and the rotation about the double bond. α-Methyl-Δ^α-butenolide ( 8b ) was converted to 1-benzyl-( 9a ), 1-cyclohexyl-( 9b ), and 1-(4′-picoly1)-3-methyl-Δ^α-pyrrolin-2-one ( 9 c ) by heating at 180° with benzylamine, cyclohexylamine, and 4-picolylamine. The butenolide 8b showed cytostatic and even cytocidal activity; in preliminary tests, no carcinogenicity was observed. Both 8b and 9c exhibited little toxicity.     

Stereochemistry of alkyl 3-substituted 4-halotetrahydro-2-oxo-3-furancarboxylates: 2—1H and 13C NMR spectra

The ¹H NMR parameters of methyl 3-substituted cis-4-halotetrahydro-2-oxo-3-furancarboxylates are reported, with assignments of the ring protons based on solvent-induced changes in the vicinal trans coupling constants, ³J(H-4, H-5). Preferred conformations, ce with a pseudo-equatorial halogen for the cis isomers and ta with a pseudo-axial halogen for the trans isomers, have been suggested on comparison of the magnitudes of J(trans) and J(gem) in both series. The ³J(¹³CH₃, H-4) values measured for methyl cis-4-bromotetrahydro-3-methyl-3-furancarboxylate, methyl trans-4-bromotetrahydro-3-methyl-3-furancarboxylate and trans-3,4-dibromodihydro-3-methyl-2(3H)-furanone have confirmed the stereochemical assignments.     

1H NMR spectra. Part 29§: proton chemical shifts and couplings in esters—the conformational analysis of methyl γ‐butyrolactones

The ¹H NMR spectra of 35 cyclic and acyclic esters are analysed to give the ¹H chemical shifts and couplings. The substituent chemical shifts of the ester group were analysed using three‐bond (γ) effects for near protons and the electric field, magnetic anisotropy and steric effect of the ester group for more distant protons. The electric field is calculated from the partial atomic charges on the O?C = O atoms, and the asymmetric magnetic anisotropy of the carbonyl group acts at the midpoint of the C = O bond. The values of the anisotropies Δχ_parl and Δχ_perp were for the aliphatic esters 10.35 and ?18.84 and for the conjugated esters 7.33 and ?15.75 (×10^?6 ų/molecule). The oxygen steric coefficients found were 104.4 (aliphatic C = O), 45.5 (aromatic C = O) and 16.0 (C–O) (×10^?6 Å⁶/molecule). After parameterisation, the overall RMS error for the data set of 280 entries was 0.079 ppm. The strongly coupled ¹H NMR spectra of the 2‐methyl, 3‐methyl and 4‐methyl γ‐butyrolactones were analysed and the methyl conformational equilibrium obtained from the observed couplings. The observed versus calculated density functional theory (DFT) ΔG(ax‐eq) was 1.0 (1.01), 0.34 (0.54) and 0.65 (0.71) kcal/mol res. The shielding effect of a methyl cis to a proton in the five‐membered lactone rings is ?0.40 ±0.05 ppm and deshielding trans effect 0.12 ±0.05 ppm, which is common to both five and six membered rings. The cis/trans isomerism in the vinyl esters methyl acrylate, crotonate and methacrylate and methyl furoate was examined using the ¹H chemical shifts. The calculated shifts of both the cis and trans isomers were in good agreement with the observed shifts. Copyright © 2012 John Wiley & Sons, Ltd.     

Stereochemical effects in the mass spectra of some cyclopropylsilanes explained by the MNDO molecular orbital method. I. Silicon-oxygen interactions

Large differences have been found between the mass spectra of cis- and trans-1-carbomethoxy-2-trimethylsilylcyclopropane. To help in explaining these differences, quantum chemical calculations were performed by the MNDO method on the neutral molecules, on the vertical and adiabatic molecular ions and also the [M – methyl]⁺ ions. To evaluate the results, heats of formation, bond orders, molecular geometry, spin and charge densities and orbital coefficients were used. These indicate the formation of a very strong bonding interaction between silicon and oxygen in the molecular ion of the cis but not the trans isomer, leading to spontaneous methyl loss and a stabilization energy of ～ 2.6 eV.     

Synthesis of difunctional 1,4-dimethyl-1,4-disilacyclohexanes

Transformations of HVinSiCl₂, HVinSi(Me)Cl, HVinSi(Me)Ph, and HVinSi(Me)NEt₂ in the presence of Pt catalyst were studied. In dilute solutions, the reaction gave a mixture of structural and stereoisomers of five- and six-membered disilacyclanes, resulting from intramolecular cyclization of the initially formed linear dimer. In the case of methyl(phenyl)disilacyclane, the structural isomers were separated andtrans-1,4-dimethyl-1,4-diphenyl-1,4-disilacyclohexane was isolated. The reaction of this product with HCl in the presence of AlCl₃ followed by hydrolysis resulted in the synthesis oftrans-1,4-dichloro- andtrans-1,4-dihydroxy-1,4-dimethyl-1,4-disilacyclohexanes. The structures of the structural and stereoisomers synthesized were confirmed by¹H,¹³C, and²⁹Si NMR and IR spectroscopies and mass spectrometry. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1734–1738, September, 1999.     

1H NMR spectra of oxiranes. cis-Arylmethyl oxiranes substituted in the phenyl ring

The ¹H NMR parameters for a number of cis-arylmethyl oxiranes are reported and discussed in comparison with those of trans derivatives and styrene oxides substituted in the phenyl ring. While the macroscopic behaviour of the results is in agreement with a perturbative mechanism through the oxirane ring that is mainly electrostatic in character, a small contribution due to conjugative interaction is not excluded. It seems likely that the effect of substituents in the phenyl ring is mainly transmitted through bonds rather than through space, and that the different polarisation of the C? H bonds in the oxirane ring is mainly responsible for the different behaviour of the protons in cis- and trans- arylmethyl oxiranes towards the effect of substituents.     

Donor-acceptor complexes in copolymerization. L. Preparation and microstructure of chlorine-containing alternating conjugated diene–acceptor monomer copolymers

Neighboring monomer units cause significant shifts in the infrared absorption peaks attributed to cis- and trans-1,4 units in conjugated diene-acceptor monomer copolymers. Conjugated diene-maleic anhydride alternating copolymers apparently have a predominantly cis-1,4-structure, while alternating diene-SO₂ copolymers have a predominantly trans-1,4 structure. Alternating copolymers of butadiene, isoprene, and pentadiene-1,3 with α-chloroacrylonitrile and methyl α-chloroacrylate, prepared in the presence of Et_1.5AlCl_1.5(EASC), have trans-1,4 unsaturation. Alternating copolymers of chloroprene with acrylonitrile, methyl acrylate, methyl methacrylate, α-chloroacrylonitrile, and methyl α-chloroacrylate prepared in the presence of EASC-VOCl₃ have trans-1,4 configuration. The reaction between chloroprene and acrylonitrile in the presence of AlCl₃ yields the cyclic Diel-Alder adduct in the dark and the alternating copolymer under ultraviolet irradiation. The equimolar, presumably alternating, copolymers of chloroprene with methyl acrylate and methyl methacrylate undergo cyclization at 205°C to a far lesser extent than theoretically calculated, to yield five and seven-membered lactones. The polymerization of chloroprene in the presence of EASC and acetonitrile yields a radical homopolymer with trans-1,4 unsaturation.     

Angular dependence of the vicinal interproton spin–spin coupling in silacyclohexanes. The conformational energy term of the methyl group in 1-methyl-1-silacyclohexane

An approximate Karplus-Conroy relationship between the vicinal coupling constants J(H SiCH ) in a silaethane fragment and the corresponding torsional angles τ in silacyclohexanes is proposed. The relationship ³J(H SiCH ) VS τ is asymmetric. The ¹H NMR parameters and conformational features of cis- and trans-3,5-dimethyl-1-silacyclonexane and 3-silabicyclo[3.2.1]octane are discussed, and the conformational energy term for the methyl group in 1-methyl-1-silacyclohexane is found to be approximately 1.45 kJ mole^?1 in favour of the axial position for the methyl group, in good agreement with previously calculated values.     

trans‐Di­chloro­tetrakis(4‐methylpyrimidine‐N1)­ruthenium(II)

The title compound, trans‐[Ru^IICl₂(N¹‐mepym)₄] (mepym is 4‐methylpyrimidine, C₅H₆N₂), obtained from the reaction of trans,cis,cis‐[Ru^IICl₂(N¹‐mepym)₂(SbPh₃)₂] (Ph is phenyl) with excess mepym in ethanol, has fourfold crystallographic symmetry and has the four pyrimidine bases coordinated through N¹ and arranged in a propeller‐like orientation. The Ru—N and Ru—Cl bond distances are 2.082 (2) and 2.400 (4) Å, respectively. The methyl group, and the N³ and Cl atoms are involved in intermolecular C—H?N and C—­H?Cl hydrogen‐bond interactions.     

Use of sulfur derivatives as an ortho directing group for the metalation of diazines. Metalation of diazines. XVIII

The metalation of thioethers, methyl and phenyl sulfoxides and sulfones of pyrazine and pyridazine has been performed. Methyl sulfoxides and sulfones were first metalated on the methyl group. The ortho directing effect of thioethers, sulfoxides and sulfones have been compared with the methoxy group. The sulfoxides were shown to be very good ortho directing groups.     

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.     

Organoborazines. III. Homo- and copolymerization of p-vinylphenylcyclotriborazines

The polymerization and copolymerization with styrene of a series of unsymmetrically B-p-vinylphenyl-N-methyl and N-phenyl borazines [R³(R²)₂B₃N₃(R¹)₃; R¹ = methyl, phenyl, R² = methyl phenyl, R³ = p-vinylphenyl] has been studied. The polymerization of these monomers yielded both tractable and crosslinked materials. The polymers obtained were characterized by ¹H- and ¹³C-NMR spectroscopy, elemental analysis, gel permeation chromatography, and thermogravimeteric analysis. The reactivity ratios for the copolymerization reaction were calculated by the Mortimer-Tidwell method.