Steric strain, release

The steric strain released or incurred when going from the reactants to the products is especially important for endothermic or thermoneutral reactions. 

For C70 and higher fullerenes addition also occurs only at the [6,6] fusions. However, these sites are no longer all equivalent, and a variety of regioisomers can result. For C70, of the four possible products, as illustrated in Fig. 10, addition occurs exclusively at the polar fusion, A (,14). This is the sterically most accessible site, and energy calculations have shown there is the greatest release of steric strain on com-plexation (91). The equatorial [6,6] fusion, D, has lowest bond order, and correspondingly, no complexations to it have been observed. For C84, complexation with Vaska s compound occurs at the fusion with the highest bond order, as shown by Htickel calculations (28). 

In (IV) the group R migrates to an electron-deficient centre, and it might, therefore, be expected that electron release by the group R would aid reaction. The sequence in (rel.) of the Me < Bu" < Bus < Bu could thus be due to the inductive ( + 1) effect of the alkyl groups. If there were any steric strain in the four-coordinate... 

It has been suggested that interaction of the dmbim pulls the Co into the corrin plane, sterically inhibiting nucleophilic displacement at the axial methyl [166]. A bound imidazole would play the same role. Therefore, release of this steric strain in base-off Cbls and in MeCbi+ should facilitate dealkylation base-off species dealkylate more rapidly [166], but different products are observed. It is essential to assess the relative importance of steric and electronic effects on... 

Photochemical E —> Z isomerization produces a sterically strained state in which the methyl substituents are positioned in close proximity. The strain is released when the methyl groups and the naphthyl moieties switch their positions. This second step is followed by back-isomerization from the Z to the E form of the double bond and again steric hindrance provides an energetically unfavorable state which can reach an energetically more favorable state by a second flip of the sub-... 

A similar behavior is observed in the cycloaddition reaction of isoquino-linium iodide with ethyl vinyl ether (Scheme 46). Steric strain of adjacent substituents is released in the TS resulting in an additional rate increase for R = t-Bu, as evidenced by an upward deviation in the Hammett plot (77JA2588). 

Angle bending is the next step in energy minimization. It occurs when strain release by rotation of substituents is not enough to accommodate all the steric strain or when rotation is not possible, as with spherical substituents. Quantitative results are scarce an interesting example is the methylation of a series of 3,5-dialkylimidazo(l,2-a)pyridines (81T83 81T91) (Scheme 114). 

When R is primary alkyl, the second-order rate constant k2 is obtained by taking the slope of kobs vs. concentration of the nucleophile. The plot passes through the origin, indicating a pure SN2 mechanism without SN1 participation. The reference pyridinium ion is the 2,4,6-triphenyl derivative (because pyrylium precursors with phenyl substituents are more easily prepared) (82AHC(Suppl 2)1) but numerous other substituents have been introduced into the ring. Rate constant values reported in Table XIX, where release of steric strain has a major influence, are in agreement with the role of structural factors discussed in Section IV,A. 

Rotating the totally eclipsed conformation 60° to a gauche conformation releases most, but not all, of this steric strain. The gauche conformation is still 3.8 kJ (0.9 kcal) higher in energy than the most stable anti conformation. 

Consequently, for the haloform case (Table 4.10), since repulsion for chlorine is less than that for fluorine, chlorine as a substituent facilitates carbanion formation much more than fluorine. The enhanced acidities of bromoform and iodoform have been attributed to the release of steric strain on deprotonation, while the increased availability of [Pg.110]

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