Electrophilic reactions, strain effects

The rates of hydration of the Bredt-olefins 69 and 176 (Table 27) have been used to estimate strain effects in electrophilic reactions of these strained olefins. It was estimated that the 105-fold rate increase of 69 over 2-methyl-but-2-ene (257a) is due to about 50% strain release in the transition state of protonation. 

Ion-radical organic reactions of the Sj j l type are less sterically restricted than classical Sj reactions. Generally, the nucleophilic (not Sj j ) reactivity varies with the steric demand at the reaction center. The electron-transfer reactivity does not depend on steric effects. To illustrate this, one can compare electron transfer and nucleophilic reactivity between ketene silyl acetals and cationic electrophiles (Fukuzumi et al. 2001). Nevertheless, space strains may determine the overall results of these reactions if either intermediate radicals or forming products are sterically hindered. 

The choice of mercury salt for this reaction appears to be less critical than intermolecular variants and many salts (X = Cl, OAc, NO3, SO4, CIO4, CF3CO2) have been successfully utilized. The electrophilicity of the salt can effect the regio- and stereo-chemistry of addition.434-438 The regioselectivity is also determined by the substitution pattern of the unsaturated alcohol. For cycloalkenes, steric factors, ring strain, and other reaction variables become important (equation 264). High regio- and stereo-selectivity are often encountered in these intramolecular processes. 

Subsequently, certain authors (126-128) have supported this interpretation, but alternative explanations have also been proposed (129-132). Another early discussion of the stereochemistry of the enolization process was that of Vails and Toromanoff (133). They proposed that if stereoelectronic effects are an important parameter, the cyclohexanone enolate should react by two different pathways, one involving a chair-like transition state (443 444) and the other a boat-like transition state (443 445 446) Thus, both of these reactions proceed by perpendicular attack (134) of the electrophile. Their energy difference results from the difference in strain between the chair (444) and the twist-boat (445) forms. 

The cyclopropyl effect has controlled the regioselectivity of the cross-coupling reactions of propargylic/allenylic metallic species with electrophiles afford alkynic cyclopropanes (Scheme 9).70 Cyclopropyl ring strain, which makes the formation of vinylidenecyclopropanes unfavourable, is believed to control the regioselectivity. 

There is rich experimental evidence showing that aryl positions adjacent to strained annelated rings exhibit reduced reactivity towards electrophilic reagents [2-7,98-101], These data are in harmony with the theoretical interpretation in terms of the bond fixation model presented above. It is possible that the ground state charge distribution in some particular fused molecular systems affects propensities of a and f3 atoms to undergo the electrophilic substitution reactions, but interpretation based solely on the rehybridization effect at carbon junction atoms offered by Siegel et al.[9] is obviously unjustified. 

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