Rotational barriers substituent effects

It is possible that the effect of jS-substituents arises from small rotational barriers which impede hydrogen transfer in these short-lived biradicals. 

The apparent lowering of the rotational barrier in triafulvenes is open to interpretation either by substituent or solvent stabilization of ground-state polarity leading to a decrease of C3/C4 double bond character or by stabilization of a more polar - probably perpendicularly orientated184 — transition state by substituent or solvent effects. 

The effects of the substituents on nitrogen on rotational barriers were discussed by Yoder and Gardner (34) for formamides and acetamides. The pertinent data, given in Table 5, suggest that the barriers to rotation of formamides are not affected by the bulkiness of the alkyl group on nitrogen, but such a conclusion... 

Mannschreck et al. (44) examined the effect of substituents on the barriers to rotation in 2,4,6-trisubstituted benzamides. In /V-benzyl-A(-methyl-2,4,6-tri-bromobenzamide, the rotational barrier (AG ) is 23.8 kcal/mol at 35.8 to 40.6°C for the Z -> E process in quinoline (44). This should be compared with AG of 23.4 kcal/mol for the same process with the trimethyl compound (5). It is seen that steric effects are of primary importance, inasmuch as the van der Waals radii of the methyl and bromo groups are almost the same. 

Effect of Substituents on the Rotational Barriers and Equilibrium Constants of A/-(2,6-Disubstituted phenyl)-Af-methyl-haloacetamides (12)... 

Establishing that a smaller substituent in the peri position can raise the barrier, Oki and co-workers (149) were interested in finding the peri substituent effect on the barrier to rotation, and prepared a series of 9-(l,l-dimethyl-2-phenyl-ethyl)triptycenes (104). Data in Table 22 indicate that the barrier to rotation... 

Most of the data in Table 12 come from the work of Shvo et al. (78). Careful band-shape analysis and solvent-effect studies permitted evaluation of the rate constants and AG values at 298 K, which renders the discussion of substituent effects more meaningful than usual. The authors obtained reasonably linear Hammett plots when correlating log km with Or (79) for X and Y, holding one of these substituents constant. They also found that the dihydropyridine system may act as an unusually efficient donor, giving a AG of 17.6 kcal/mol with X, Y = H, CN, the only barrier below 25 kcal/mol reported for any donor-substituted cyanoethylene. However, with other acceptor combinations the dihydropyridine moiety is not so outstanding, and this illustrates the difficulty of measuring donor and/or acceptor effects by rotational barriers alone (vide infra). 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

The study of substituted allyl radicals (Sustmann and Brandes, 1976 Sustmann and Trill, 1974 Sustmann et al., 1972, 1977), where pronounced substituent effects were found as compared to the barrier in the parent system (Korth et al., 1981), initiated a study of the rotational barrier in a captodative-substituted allyl radical [32]/[33] (Korth et al., 1984). The concept behind these studies is derived from the stabilization of free radicals by delocalization of the unpaired spin (see, for instance, Walton, 1984). The... 

The experimental result seems to support this model. Table 11 lists values for rotational barriers in some allyl radicals (Sustmann, 1986). It includes the rotational barrier in the isomeric 1-cyano-l-methoxyallyl radicals [32]/ [33] (Korth et al., 1984). In order to see whether the magnitude of the rotational barriers discloses a special captodative effect it is necessary to compare the monocaptor and donor-substituted radicals with disubstituted analogues. As is expected on the basis of the general influence of substituents on radical centres, both captor and donor substituents lower the rotational barrier, the captor substituent to a greater extent. Disubstitution by the same substituent, i.e. dicaptor- and didonor-substituted systems, do not even show additivity in the reduction of the rotational barrier. This phenomenon appears to be a general one and has led to the conclusion that additivity of substituent effects is already a manifestation of a special behaviour, viz., of a captodative effect. The barrier in the 1-cyano-l-methoxyallyl radicals [32]/... 

An error-propagation analysis allows the conclusion that the captodative substituent effect on the rotational barrier in this allyl radical is at least additive and perhaps slightly greater. 

Benzylic radicals offer themselves to a similar analysis. Some barriers to rotation have been determined (Conradi et ai, 1979). The barrier to rotation of 9.8 + 0.8 kcal mol for the a-cyano-a-methoxybenzyl radical [21] (Korth et al., 1985) could not be interpreted rigorously in terms of a captodative effect because estimates had to be made for the effect of a single captor or donor substituent on the rotational barrier. Within these limitations the barrier does not reflect more than an additive substituent effect. 

Furthermore, the estimation of the rotational barriers of ortho-substituted 6-aryl-1,1,5-trimethylindanes (Fig. 1a) [24] by NMR as well as conformational NMR-studies of (9-anthryl)carbinol derivatives (Fig. 1b) [25] have shown the CF3 substituent to have steric effects that are comparable to those of an isopropyl group (Fig. 1). 

The 1 -(para-methoxyphenyl )-2-(triisopropylsilyl)vinyl cation 131 has also been generated, characterized, and studied323,333 to elucidate the importance of a-7t aryl and (3-0 hyperconjugative stabilization. A comparison was made with the para-methoxyphenylvinyl cation 124, which has a higher demand for a-aryl 7t-stabilization. The para carbon of cation 131 is 7 ppm more shielded compared to that of cation 124, indicating that the (3-silyl stabilization effect is operative. The experimentally determined rotation barrier of the para-methoxy substituent for 131 (<8 kcal mol-1) and 124 (9.0 kcal mol-1) shows a less significant double-bond character of the MeO-C(4)... 

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