Small ring compounds, table

TABLE 4.5 Some Strained Small-Ring Compounds... 

Table 28. organosilane reduction of small ring compounds... 

TABLE 28. ORGANOSILANE REDUCTION OF SMALL RING COMPOUNDS (Continued)... 

Comparisons have been made for some groups of small-ring compounds." Some examples are shown in Table 5.100 It can be seen that cyclopropane has larger C-H stretching force constants than propane, and that they are similar to those of ethene. This is in accord with the %s character in the C-H bonds. 

Small ring compounds such as cubane and norbomane arc strained for obvious reasons. Larger rings tend to be less strained, as perusal of Table 6 will show. 

Any of these sets of computed energies can then be used for calculation of reaction enthalpies by comparing reactants and products. Table 4.4 gives some data for hydrogenation, hydrogenolysis, and isomerization reactions at several levels of theory. The table includes some data for small-ring compounds, which represent a particularly challenging test of the accuracy of the computationally based methods. 

Table 30 contains transition data on some compounds with terminal small rings (compounds 2 and 3). Compared with the simple alkyloxy group (compound 1), the small rings have an effect comparable to branched alkyls, but unlike that of five- or six-mem-bered rings. In order to obtain chiral materials, the introduction of an oxirane or thi-irane ring in the terminal chains is useful [143]. 

Characteristic group frequencies (near 1000cm" have been suggested for cyclobutane and cyclohexane derivatives, and these are so listed in Table 5-IX. Vibrational analysis of small ring compounds is discussed in Reference (84). 

Initially, it was found that introducing a methyl group at the 1-position and a methyl or phenyl substituent at the 2-position of the indole ring in this series produced only small changes in activity, despite a marked increase in lipophilicity of the phenyl-substituted compounds Table 7.1). 

The diazirine precursors of carbenes (entry 3 in Table 10.3) are cyclic isomers of diazo compounds. The strain of the small ring and the potential for formation of nitrogen make them highly reactive on photoexcitation. They are, in general, somewhat less easily available than diazo compounds or arenesulfonylhydrazones. However, there are several useful synthetic routes.86 87 88... 

Conformational analysis of rings larger than cyclohexane is more complicated. These rings are also less common than cyclohexane, so we discuss their conformations only briefly. As can be seen from Table 6.1, the seven-membered ring compound cycloheptane has only a small amount of strain. Obviously, it is nonplanar to avoid angle strain. It does have some torsional strain, but the overall strain is comparable to that of cyclopentane. It is a fairly common ring system. 

Comparison between the reactivity of the N-oxides and the parent compounds (Table 11.1) shows that formation of the N-oxides increases the reactivity by 10-fold, except that at the 8-position of quinoline, where the reactivity remains the same. The difference between the result for the 8-position and the others probably again reflects the poor 1,5-conjugative interaction. The relatively small incremental activation for the other positions reflects the generally poor transmission of conjugative effects between one ring and the other in naphthalene-like molecules [68JCS-(B) 1112],... 

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