Conformation measurement trans isomer

The preference for the axial position in unhindered thiane-1-oxides has been known for some time. The spectra of the cis and trans isomers of the 2-, 3- and 4-methyl thiane-1-oxides, 169-171, were also measured. It was concluded from the 13C chemical shifts that the methyl groups preferred the equatorial positions. A comparison of the 170 chemical shifts obtained for sulfoxides 169-174 with those obtained for the cis and tram sulfoxide isomers of trans- 1-thiadecalin, 175 and 176, was consistent with this proposal. Sulfoxide 175 with the S=0 axial gave a shift about 17 ppm upfield from that of its equatorial isomer 176. For sulfoxides 169-174, the conformers proposed to have the S=0 axial gave shifts that were upfield from those of the supposed equatorial conformers. For tram-3, (rans-5-dimethylthiane-1 -oxide (177) with the oxygen axial, the 170 signal was 21 ppm upfield from the signal observed for the equatorial oxygen in cis-3, cis-5-dimethylthiane-l-oxide (178). 

Hogeveen138 measured apparent acidity constants of substituted /i-phenylthio-, /J-phenylsulfinyl- and / -phenylsulfonyl-acrylic acids (cis and trans) in 50% v/v ethanol. The p values for transmission through SCH=CH, SOCH=CH and S02CH=CH were 0.531, 0.389 and 0.320 respectively for the cis acids and 0.652, 0.282 and 0.331 for the trans acids. These results were discussed in considerable detail and compared with those of pertinent related systems. Little importance was attached to the small differences between p values for cis/trans isomers, and the relative transmissions were taken as the mean p values 0.59,0.34 and 0.33. The superior transmission of SCH=CH was attributed to greater polarizability. The values for pKJtrans) — pKJcis) of isomeric acids were also discussed. For the sulfonyl acids this is almost constant at 0.1 unit for the sulfinyl adds there is some variation about a mean value of 0.26 unit, and for the thio adds there is also some variation about a mean value of — 0.15 unit. The differing behavior of the three systems in this respect was explained in terms of hypothetical conformations and electrostatic interactions therein. 

The first experimental evidence that acetal hydrolysis is controlled by stereoelectronic factors was provided by Kirby and Martin (94). They have measured the rates of the acid catalyzed hydrolysis (0.1 N HC1) of the tricyclic acetal 130 which has no electron pair antiperiplanar to the leaving group, and the corresponding cis isomer 131 which does have an electron pair properly oriented to eject the leaving group. They found that the cis isomer 131 was rapidly hydrolyzed whereas the trans isomer 130 in which the conformation of the acetal centre is locked, was still not completely hydrolyzed after several weeks, indicating a difference in rate of at least 3000. 

A carbonyl group adjacent to a ring junction bearing a hydrogen atom allows conformational equilibration to take place at the ring junction through enolisation (p. 154). For the 4-oxocholestane system (7), optical rotatory dispersion measurements [47] have indicated the presence of 99% A/B trans-isomer (5a) at equilibrium. In contrast, similar data [47] for... 

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

Data for the 2,3-dimethyldecalins are in Table XXXV. The isomers of this set were all identified by the one-to-one correspondence of the measured and computed equilibrium yields. A point of interest is that the trans isomers were not eluted in the expected order. Isomer 2, despite its higher conformational energy, preceded isomer 3. 

Dimethyl oxalate (in dioxan) and diethyl oxalate in CT behave as mixtures of cis- and trans-isomers, none of which is planar measurements on diethyl malonate, succinate, adipate, etc. can be interpreted only if multicomponent mixtures of several (specified) conformations are present... 

Id usually appears as a thin film on a glass surface, and until now we have not succeeded in obtaining a single crystal for the X-ray analysis. However, there are some reasons to speculate about its structure. For disilenes of the type RR Si=SiR R, photochemical cis-tram isomerization in solution was shown to occur [11], the trans isomer being predominant under equilibrium conditions. Of course, symmetrically substituted 1 crumot have real cis—trans isomers, but, by analogy, a similar equilibrium with predominance of a conformer, close to the tram one, seems likely in solution. This assumption is eonfirmed by Raman polarization measurements for a solution of 1 in hexane, because the selection mles observed for the conformer predominant in solution are consistent with C2h symmetry, that is, with a quasi-/ra s structure of this conformer [12]. As both the Raman and UV-Vis absorption and fluorescence spectra of solid Id are similar to those of 1 in hexane solution [1,12], we can suggest for Id also a quasi-/ra/is structure as shown in Fig. I. 

From the arguments presented by the authors (based on NMR measurements and molecular mechanics calculations), one could conclude that, for cis-(14), only the half-chair with the t-butyl equatorial and the S=0 axial is populated. For the trans isomer, the most represented conformer has both the f-butyl and sulfinyl groups equatorial [46]. 

Independently, Hua and Flood synthesized a light-active aryl-triazole foldamer (42, Fig. 22) which has two azobenzene units on each end [54]. When the azobenzene units are photoisomerized from trans to cis with UV (365 nm) irradiation, the foldamer loses two of its three aromatic n-n interactions that were being employed to stabilize the hehcally folded form. Therefore, UV irradiation destabilizes the folded helix in favor of the unfolded random-coil conformation. The folded conformation could be reinstated by irradiation with visible light (436 nm). In solution, the foldamer binds with chloride in its trans isomer (/sTa = 3,000 MeCN) and releases chloride in the cis isomer (K = 380 M ). On account of the fact that free chloride ions conduct electricity through solution better than receptor-bound chloride, the anion binding-release cycle was observed by conductivity measurements. Consistently, a 1 1 solution of 42 and TBACl (MeCN) showed increased conductivity under UV light and decreased conductivity under Vis light. 

Note Refer to the text for definitions of conformations. AC is the relative energy for conformations of a given isomer, AE is the energy measured relative to the Bda conformation of the adduct formed by trans addition of the syn isomer. The reaction coordinates R, a, 3, Ys 5, and e are defined in Figure 3. 

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