Systems profiler, structure rings

An interesting stereochemical profile of this cyclization is that in the five-membered product structures, substituents PhS and OH groups are placed cis to each other, whereas in six-membered products the placement is trans (Table 6, entry 7). The cis selectivity in the five-membered ring systems is not affected by a and fi-substituents of the alkoxides (entries 2, 3 and 4), indicating that the steric effect is not the dominant factor. Instead, interaction between the oxido and carbene center composes a five- (or seven-) membered transition state 28, which allows the carbene to abstract the nearest quasi axial hydrogen as a hydride to produce a carbonyl intermediate 29, leading to the cyclization products 27 and 30 (Scheme 13, Eq. 1). Similarly, the stereoselective stepwise cyclization of cis- and rra s-2-(3,3-dithiopropyl)cyclohexanol to 2-phenylthio-... 

Figure 9. B3LYP/6-3IG Energy profile (LE) for 6 ring (- - ) vs. 5 ring (—) dehydrocyclization in the 2-heptyl cation system. The arbitrary zero reference point is the cis 1-5-p-H-bridged cation 16 (see footnote (a) in Table II). The transition-state structures have been shown in Figure 6. The product ions are the I-2-dimethylcyclopentyl cation 20 and the I-methylcyclohexyl cation 21, respectively, and these have almost the same energy at B3LYP/6-3IG level.

Fig. 3.2. Profiles of the charge density along the C2 symmetry axis for the dissociation of the water molecule. In (a) p decreases monotonically from a maximum value at the oxygen nucleus. As Ro is increased, an abrupt change occurs in the form of the curve when a point of inflexion appears (b). This unstable critical point exists only for this single configuration X, of the nuclei and a further increase in Rq results in Its bifurcation to yield a maximum, corresponding to the formation of a bond critical point and a minimum, corresponding to the formation of a ring critical point a.s typified by (c). The system has entered the region of the ring structure.

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