Five-ring model compounds

A comparison of the absorption and emission spectra of Ooct-OPV5 with those of the fully conjugated, similarly substituted polymer Ooct-PPV shows that the absorption and luminescence maxima of the five-ring model compound are only slightly blue-shifted relative to those of the polymer (see Fig. 16-11). Hence, the... 

From these results it was concluded that the catalytic activity was caused by the vic-cis-diols of ribose rings but not by the nucleic acid base (e.g., cytosine), which might play a role in the formation of active centers. To elucidate the action mechanism of the catalysis, five polymeric model compounds without nuclei acid bases were synthesized by copolymerizing sugar derivatives with maleic anhydride, and subsequent hydrolysis (Scheme 14) [99, 100]. Starting from polymer 33, the carboxyl groups and I -OH groups were blocked by methylation and acetylation to result in 34 and 35, respectively. 

C-H and N-H bond dissociation energies (BDEs) of various five- and six-membered ring aromatic compounds (including 1,2,5-oxadiazole) were calculated using composite ab initio CBS-Q, G3, and G3B3 methods. It was found that all these composite ab initio methods provided very similar BDEs, despite the fact that different geometries and different procedures in the extrapolation to complete incorporation of electron correlation and complete basis set limit were used. A good quantitive structure-activity relationship (QSAR) model for the C-H BDEs of aromatic compounds... 

Scheme 3. New five-membered ring heterocyclic compounds predicted from the acetoin-ammonium sulfide model system.

M N(Bu )CH=CHNBu R] [R=TEMPO, OC6H2Bu 2-2,6-Me-4, OC6Me4-2,3,5,6-OH-4, Bu SiMe3, WCp(CO)3, Re(CO)5] together with EPR spectral data, DPT calculations on model compounds showed that spin delocalisation to the five-membered ring decreased in the sequence M = Ge > Si > C, implicating the importance of the zwitterionic structure 64. 

A simple model for this compound with both the five-rings planar gives an O — O distance oi about bid A, while BbSlEGOBBS [3] value for the covalent radius of oxygen is 1 23 A. Consequently there must be... 

Chapters 7 to 12 deal with factors affecting conformational equilibria and complex formation. Angyal explains the effect of calcium chloride on the anomeric equilibria of certain sugars by formation of especially stable complexes. These involve an axial-equatorial-axial sequence of three hydroxyl groups on a six-membered ring or a cis-cis sequence on a five-membered ring. Lemieux and Brewer use model compounds to study solvation effects on the orientation of the hydroxymethyl... 

Several aspects of aromaticity have been studied <2002JOC1333> using statistical analyses of quantitative definitions of aromaticity. ASEs, REs, magnetic susceptibility exaltation (A), nucleus-independent chemical shift (NIGS), the harmonic oscillator model of aromaticity (HOMA), (/j) and (Aj), evaluated for a set of 75 five-membered 7t-electron systems and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations between the various aromaticity criteria, provided the whole set of compounds is used. The data in Table 9 have been found for arsole (AsH) 1 (E = As, R = H), its anion (As ), and protonated species (AsH2 ). 

T 5.27) of equal intensity, respectively ascribed to the a-D and )8-D anomers of 277. A study of some model compounds has shown that the thioseptanoid ring is less stable than the corresponding six-membered ring. This conclusion is also reached from the results of iodine titration of 277 80% of the theoretical amount of iodine is consumed in one minute, and 100% in five minutes. A small proportion of the readily oxidizable aldehydo form having a free thiol group is present in equilibrium this compound can readily be re-formed from 277, when it is removed by oxidation. This behavior is similar to that shown by the thiofuranoid ring of 4-thio-D-ribofuranose (see Section IV,3 p. 222). 

Table 1 displays rate data for alkoxyamine-termi-nated polymers and low molecular model compounds and shows some important trends. At about the same temperature, the dissociation rate constants Ad of alkoxyamines (Schemes 12 and 30) with the same leaving radical (polystyryl, 1-phenylethyl) increase in the order 3 (TEMPO) < 6 < 8 (DEPN) < 1 (DBNO) by a factor of about 30. Acrylate radicals dissociate markedly slower than styryl radicals from 1 (DBNO), but there is no appreciable difference for 8 (DEPN). The dependence of Ad on the nitroxide structure has been addressed by Moad et al.104 They found the order five membered ring < six membered ring < open chain nitroxides and pointed out additional steric (compare 3 and 6) and polar effects. 

Dimethyltetrahydrofuran-3,4-dicarboxylic acid was prepared, as shown in Scheme 3, as the five-membered ring model three isomers were isolated, but the stereochemistry could not be assigned unequivocally. Inspection of the NMR spectra of the model compounds, plus that of commercially available... 

Another approach to the study of the cyclic units formed in these cyclopolymerizations is through the study of the radical cyclization reaction of selected model compounds. Thus, extensive studies have shown that the five-membered ring is predominant, while new evidence indicates that radical stability exerts a marked influence on the ring size. These conflicting aspects of cyclopolymerization have been discussed in detail by Butler(L7) he has pointed out that considerably more investigations will be necessary before definite conclusions can be drawn with respect to the ratio of five- to six-membered rings in the many cyclopolymers already synthesized, and a satisfactory explanation for these extensive variations from one system to another is available. 

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