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Isomerization conformer stabilization

In summary, we expect that 2,3-substitution will not greatly alter the conformational preference of the diene system, L e. conformational isomerism of 2,3-difluoro-butadiene will be subject to the same electronic factors as that of the unsubstituted diene and these factors operate in the same direction in both cases. It is expected that, by analogy to 1,3-butadiene, the order of conformer stability of 2,3-difluoro-butadiene will be dictated by steric effects, L e. it will be tram > gauche > cis. [Pg.107]

In general, conjugation efficiency is highest with chains composed entirely of sp C-C bonds. However, these chains are flexible and thus conformation-ally inhomogeneous. In addition, they are unstable with respect to other photochemical (like cis-trans isomerization, [2 + 2]cyclization) or thermal processes (radical initiated or electrocyclizations). The chemical and conformational stability may be increased by rigidifying the carbon backbone. [Pg.177]

The double interflavanyl linkage in A-type proanthocyanidins introduces a high degree of conformational stability which culminates in high-quality and unequivocal NMR spectra conspicuously free of the effects of dynamic rotational isomerism at the dimeric level. Compounds of this class are readily recognizable from the characteristic AB-doublet ( J3 4 = 3-4 Hz) of the C-ring protons in the heterocyclic region of their... [Pg.47]

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... [Pg.524]

The same mixture of H and I was obtained starting with either of the geometrically isomeric radical precursors E or F. A possible explanation is based on the assumption of a common radical conformer G, stabilized in the geometry shown by electron delocalization involving the radicaloid p-orbital, the p-peroxy oxygen and Jt of the diene unit. The structure of the compounds H and I were determined by H NMR spectra and the conversion of H to diol J, a known intermediate for the synthesis of prostaglandins. [Pg.298]

Reactants AB+ + CD are considered to associate to form a weakly bonded intermediate complex, AB+ CD, the ground vibrational state of which has a barrier to the formation of the more strongly bound form, ABCD+. The reactants, of course, have access to both of these isomeric forms, although the presence of the barrier will affect the rate of unimolecular isomerization between them. Note that the minimum energy barrier may not be accessed in a particular interaction of AB+ with CD since the dynamics, i.e. initial trajectories and the detailed nature of the potential surface, control the reaction coordinate followed. Even in the absence (left hand dashed line in Figure 1) of a formal barrier (i.e. of a local potential maximum), the intermediate will resonate between the conformations having AB+ CD or ABCD+ character. These complexes only have the possibilities of unimolecular decomposition back to AB+ + CD or collisional stabilization. In the stabilization process,... [Pg.82]

The foregoing stabilizing 1,3-diaxial interaction was shown to have potentially useful applications for stereochemical control of addition reactions56. The /l-trimethylstannyl cyclohexenone ketal 65 affords a nearly 1 1 mixture of isomeric c/s-diols 66 and 67 when hydroxylated with OsC>4 (equation 25). However, the chlorostannane 68 upon hydroxylation with OSO4, then Sn methylation, yields a 94 6 mixture favoring the a,a,-diol 66. Evidently, the conformational change induced by the 1,3-diaxial donor-acceptor... [Pg.226]

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... [Pg.339]

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. [Pg.612]

We first consider the case of conformational isomerism of 1,3-butadiene, the simplest 4 N pi electron molecule. We will discuss the relative stabilities of three important points on the rotational surface the cis isomer (6 = 0°),thegauche isomer (6 = 45°) and the trans isomer (6 = 180°). We shall focus our attention entirely on the effect of pi interactions on conformational preference. [Pg.24]

An alternative approach for determining the relative pi electronic stabilization of two torsional isomers utilizes a molecular dissection into two open shell radical fragments. This approach is illustrated by examining torsional isomerism in butadiene and 1,3,5-hexatriene. The ir MO s of the conformational isomers of 1,3,5-hexatriene can be constructed from the union of the n MO s of two formal allyl radicals. The two regiochemical modes of union of interest will be designated cis and tram ... [Pg.35]

Additional evidence for the greater stability of the cis conformation of allylic anions is provided by other base catalyzed isomerization studies of 1-butene and 1-pentene. It was found that the thermodynamically less stable cis isomers of 2-butene and 2-pentene were the major products of the reaction182-186). Furthermore, m-2-butene isomerizes, under the same conditions, faster than the tram isomer to give 1-butene. [Pg.99]

We now focus our attention on conformational isomerism in 1,2-disubstituted cyclohexanes. Our model system will be 1,2-difluorocyclohexane and we first examine the relative stabilities of the two trans 1,2-difluorocyclohexanes, i.e. axial-axial (aa) and equatorial-equatorial (ee). These two molecules are shown below along with a listing of the dominant stabilizing a—a interactions. [Pg.195]


See other pages where Isomerization conformer stabilization is mentioned: [Pg.586]    [Pg.544]    [Pg.20]    [Pg.49]    [Pg.544]    [Pg.20]    [Pg.22]    [Pg.28]    [Pg.272]    [Pg.40]    [Pg.169]    [Pg.32]    [Pg.353]    [Pg.647]    [Pg.80]    [Pg.264]    [Pg.168]    [Pg.26]    [Pg.184]    [Pg.610]    [Pg.151]    [Pg.6]    [Pg.552]    [Pg.369]    [Pg.183]    [Pg.228]    [Pg.258]    [Pg.9]    [Pg.146]    [Pg.399]    [Pg.860]    [Pg.28]    [Pg.74]    [Pg.9]   
See also in sourсe #XX -- [ Pg.399 , Pg.400 ]




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Conformation stabilization

Conformational stability

Conformational stabilizer

Conformations stability

Conformer stability

Isomerism conformational

Isomerization, conformational

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