Orbital control, subjacent

The rate of success falls whenever these conditions are not met. For example, condition (1) is not always satisfied in aromatic substitution reactions. The FOs of polycyclic aromatic hydrocarbons are not well separated from the other MOs,66 so subjacent orbital control may intervene. Particular care should be taken with non-alternant hydrocarbons because they tend to react under charge control. This is even truer in heteroaromatic systems (cf. Exercise 15, p. 119, and Exercise 16, p. 121), where Hiickel... 

Two transition structures with a retention (192, TSret) and an inversion (193, TSinv) configuration (Figure 2) were optimized for 1,3-silyl migration in allylsilane at HF/6-31G, MP2/6-31G and DFT/6-31G levels. The TSjnV 193 was found to be a distorted trigonal bipyramid (TBP) around the silicon with the two allylic carbons at the equatorial positions different from the TS illustrated by Kwart and Slutsky297,302, while 192 has a distorted square pyramid (SP) structure around silicon. Analysis of the orbital interaction in the transition states showed that the major stabilization of 193 was caused by the MO interaction as predicted by the Woodward-Hoffmann rules, while the major stabilization in 192 was ascribed to the subjacent orbital control. 192 was more stable than 193 at... 

The degenerate thermal rearrangement of methylenecyclobutanes has been examined using the MINDO/2 semi-empirical MO method. The retention of configuration of the migrating centre can be attributed to subjacent orbital control (of the kind described by Berson and Salem) and to antarafacial participation of the allyl unit because of Jahn-Teller instabilities in the biradical transition state. The course of the reaction should be very sensitive to substituent effects. 

Indole and benzofuran combine two problems they are nonalternant and they contain heteroatoms. The indole frontier orbital coefficients in the 8- and 9-positions78 are very similar, 0.491 and 0.493, respectively. The subjacent orbital lies only 0.256/1 below the HOMO, but it has very small coefficients in these positions. The overriding factor seems to be the net charge (—0.12 at position 9, essentially neutral at position 8), which strongly favors attack at position 9. The case of benzofuran is still more complicated. There is a difference between the frontier orbital coefficients at position 8 (0.54) and 9 (0.47), but charge control still prefers the 9-position (—0.10 versus —0.03 units for the 8-position). The experimental results show that the frontier orbital terms dominate. 

Electrocyclic reaction controlled by subjacent orbital interaction... 

After it was recognized that the interaction of the LUMO with such a subjacent orbital, as it was named by Berson and Salem [46], can be strong enough to allow an otherwise forbidden pathway, and thus to control the stereochemistry of a reaction, the frontier electrons lost much of their uniqueness. Instances were then cited in which the superjacent orbital, the one lying immediately above the LUMO, seems to have a dominant influence on the course of the reaction. [47]... 

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