Transitions terminal substituent effects

These substituent effects are due to the stabilization of the carbocation resulting from protonation at the center carbon. Even if allylic conjugation is not available in the transition state, the aryl and alkyl substituents make the terminal carbocation more stable than the alternative, a secondary vinyl cation. 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

As indicated already, the optical transition energy are an extremely sensitive probe for the electronic and steric properties of the three-electron-bonded species and their respective relative contributions. However, the effect of substituents on the optical transitions becomes of much lesser importance in intramolecular radical cations derived from open-chain dithianes (type 7-9). Changing the terminal substituents in R-S-(CH2)3-S-R from methyl to isopropyl results in a just 15 nm change (440 vs. 455 nm), i.e., structure clearly appears to be the dominating parameter. This is fully corroborated by the pulse radiolysis results on 2-substituted-l,3-dithiacyclopentanes.l23 As mentioned already, the radical cation (11), derived from 1,3-dithiacyclopentane (12), is very unstable if formed at all ( niax > 650 nm). The analogous radical cation generated upon oxidation of l,3-dithia-2,2-dimethylcyclopentane (13), on the other hand, exhibits a pronounced and blue-shifted absorption at 610 nm as well as a considerable kinetic and thermodynamic stability. 

TABLE 1.3. Effect of terminal substituents in (l.xxiv) compounds on the temperature of the transition to the isotropic phase. 

When this stereoelectronic requirement is combined with a calculation of the steric and angle strain imposed on the transition state, as determined by MM-type calculations, preferences for the exo versus endo modes of cyclization are predicted to be as summarized in Table 12.3. The observed results show the expected qualitative trend. The observed preferences for ring formation are 5 > 6, 6 > 7, and 8 > 7, in agreement with the calculated preferences. The relationship only holds for terminal double bonds. An additional alkyl substituent at either end of the double bond reduces the relative reactivity as a result of a steric effect. 

Evidence of variables that influence the relative rates of reaction of olefins and alcohols was obtained from experiments with compounds that have both olefinic and alcoholic functions and by the competitive oxidation of mixtures of olefins and alcohols. The data of Table VI show that when the double bond has no substituents, as in allyl alcohol, but-3-en-l-ol, or 2-methylbut-3-en-l-ol, only the epoxide is formed but when the double bond has substituents, the epoxida-tion rate is decreased and ketone and aldehyde products from the oxidation of the OH group are formed. This effect is more pronounced with a greater degree of substitution. Since the double bond and the OH group are part of the same molecule, the difference must arise from the different abilities of the reactants to coordinate and react at the titanium center restricted transition-state shape selectivity is a possibility. The terminal double bond, sterically less hindered, interacts strongly with titanium, preventing coordination of the competing OH... 

There exist two simple rationales to explain the observed direction of the dia-stereoselective bond activation in 7/Fe+, i.e., more pronounced loss of H2 from 7a/Fe+ in comparison to 7b/Fe+. At first, one can safely assume that the reaction proceeds via insertion of the docked Fe+ in a terminal C-H bond to form a six-membered ring. Depending on the relative stereochemistry at C(3) and C(4), the eliminations of H2 and HD, respectively, therefore involve quasi-axial or quasi-equatorial orientations of the methyl substituents in the intermediates eq- and ax-lOa of course, similar considerations apply to the associated transition structures (TSs). By analogy to conventional arguments of conformational analysis, an equatorial position of the methyl group is assumed to be preferred, thereby accounting for the experimentally observed H2/HD ratios. Thus, for the stereoisomer shown in Scheme 8, both the KIE and the equatorial position of the methyl substituent favor loss of H2, whereas the SE favors loss of HD from 7b/Fe+. However, for the latter this path is impeded by the operation of a kinetic isotope effect that slows down activation of a C-D bond. Secondly, one arrives at... 

Asymmetric Hydrogenations. Catalytic asymmetric hydrogenations of p-disubstituted-a-phenylacrylic acids have been achieved using the Rh complex of (4) (eq 9). Asymmetric hydrogenation of unsymmetrically substituted trisubstituted acrylic acids leads to the formation of two stereocenters in high ee. The variation of the terminal dialkylamino substituents has little effect on enantioselectivity. A study of a Ru° complex of (1) was reported as a model for understanding the stereoselective transition state of asymmetric hydrogenations. ... 

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