Population of trans conformation

The conformation of the iV,iV-dimethyIaminopropyl side chain in CPZ HCl was found to be solvent dependent. The percent trans conformer calculated from the parameter N= J+f for each CH2CH2 fragment following the approach of Abraham ef are summarized in Table 4. The population of trans conformer for the C -C fragment was found to decrease drastically in water and in chloroform solutions, although the Cp-Cy fragment was predominantly trans in all three solvents. The side... 

Another interesting question concerns the rate at which each tosylate undergoes elimination. A tosylate sample contains molecules with several different conformations. The size of each conformer population depends on conformer energy, and the more reactive tosylate will probably be the one with the largest population of reactive conformers, i.e., molecules whose geometries allow anti elimination. Which tosylate, cis or trans, will have a larger population of reactive conformers Explain how you reached this conclusion. 

Relationship to Electronic Properties. As a result of the close connection between bond conformation and electronic properties (4), the analysis of chain conformation in the polysilylenes has been of interest to researchers in this field, both from the experimental and theoretical viewpoints. As reported by Trefonas et al. (5), most asymmetrically substituted alkyl polysilylenes in solution at room temperature display an electronic absorption with ranging from 303 to 309 nm. The variable-temperature absorption spectrum of PMHS is shown in Figure 4 (4). At room temperature, max is 308 nm, and as the solution is cooled, there is a continuous red shift with the X x reaching 328 nm at -95 °C. Some workers 4, 6) suggest that this observation is a reflection of an increasing population of trans rotational states in the silicon backbone as the temperature is lowered. This suggestion is supported by the finding that these spectra can be adequately modeled by a rotational isomeric-state treatment (4). 

Because of the leverage due to the presence of an axial methyl group, one might expect some evidence of an effect of boat conformations on the equilibria in Table XXVII. Since the equilibria were determined at two temperatures, assuming for the trans isomers that the log K vb I jT plots are linear, one can calculate the intercept and subtract the entropy changes due to creation of axes of symmetry or dl pairs. The residual entropies did indeed have a negative bias, rms A/S = — 0.4 0.5, but the scatter was too large for any reliable estimate of the populations of boat conformations. 

The i"C chemical shift thus reveals the relative configuration of substituents in molecules with a definite conformation, e.g. the axial position of the OH group in trans-3-methylcyclohexanol, cis-4-methylcyclohexanol, -D-arabinofuranose and a-D-xylopy-ranose (Table 2.12). It turns out, in addition, that these compounds also take on the conformations shown in Table 2.12 (arabinopyranose, C the others, CJ if they occurred as the other conformers, then the OH groups on C-l in these molecules would be equatorial with the result that larger shifts for C-l, C-3 and C-5 would be recorded. A ring inversion (50 50 population of both conformers) would result in an average C shift. 

The s-trans conformation is favored when X = Me. When X = H however the diene can adopt the s-cis conformation. The population of this conformation may be responsible for the change in stereoselectivity in these dienes... 

Although there is no spectroscopic evidence for an appreciable population of any conformer other than the more stable s-trans one 1 a, below 200 C2 it is apparently the. v-riy-like conformer lb that leads to reaction. The degenerate product lb is formed by transfer of hydrogen from lb via transition state 2 and subsequently rapidly equilibrates to its more stable conformer 1 a. Evidence for a concerted reaction has been presented using deuterium labeled pentadienes17. 

Flory, Mark, and Abe [158] used a 3-state RIS model to study the impact of placing small and/or flexible ( articulated ) substituents on alternating carbons of PE. They noted that steric interactions between such substituents could be minimized if one or both of the backbone bonds adjacent to the methine carbon had a gauche conformation. This leads to adoption of a higher relative population of gauche conformers versus trans conformers than that observed for PE. Since gauche conformers are more compact than trans conformers, smaller... 

Micelles not only can alter rate constants of reactions but they can alter the conformation of molecules and thus affect the outcome of a reaction [8]. Selfassociation of alkyl tyrosines enhances the population of one conformer (Fig. 11.1) (a) over conformer (b) and (c) (Table 11.1). The special aspect of conformer (a) is that it is the only one which has a trans carboxylate co-planar with the hydrocarbon and could expose its ionic group to the water phase while the alkyl group points to the centre of the aggregate, thus it will be preferred. Self-associating solutes are a special class of compounds (so called functional micelles") which are dealt with separately. It is more common to experience solutes which are more simply... 

These experimental findings were rationahzed by density functional theory calculations, and were attributed to differences in populations of reactive conformers [3]. Specifically, the lowest energy conformations of the dendralenes are comprised of unreactive orthogonal s-trans 1,3-butadienyl units (Figure 12.2). As a result of their odd number of alkenes, [3], [5], and [7] dendralenes contain at least one quasi s-cis unit, which is predisposed to react in a DA reaction. In both series, the first DA reaction occurs preferentially at the less sterically encumbered terminal diene sites hence, an even dendralene generates a more reactive odd dendra-lene, and vice versa. 

---