Allylic substitution substituent effects

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

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]/... 

Another feature that is crucial in considering rearrangements in monosubstituted allyls is the effect on the chirahty and stereochemistry. In crotyl complexes, formation of a a-bond at the unsubstituted terminus provides a path for racemization for the stereogenic center at the substituted terminus (equation 21). Formation of the a-bond at the monosubstituted terminus, however, results in conversion to a different isomer (equation 22). The most stable isomer is the syn isomer (72) and, in the absence of a substituent on the central carbon, the anti isomer (74) will only occur to the extent of f 5Vo. Thus if one considers complexes hke (acac)Pd(allyl), some racemize, whereas others only isomerize because there is no path for racemization (equation 23). These concepts have been used effectively by Bosnich in the design of systems for asymmetric allylic alkylation. These concepts also allow the rationalization of why certain substrates give low enantiomeric yields. It should be noted here that the planar rotation found in some of the molybdenum complexes retains the chirahty in the allyl moiety. 

Substituent effects in the allyl ester rearrangements are very similar to those observed in the ester reverse ene-type eliminations. This is apparent from the relative rate comparisons of Table 8. At the a- and y-carbons, reaction rates are observed to increase in the order CF3 < H < CH3. The rate accelerations by methyl substitution for hydrogen at the a-carbons are factors of 40 and 23, and at the y-carbon are factors of 55 and 23. These effects should be compared with the rate accelerations by methyl for hydrogen substitution at the a-carbon in the ester ene reactions, i.e., from Table 2, i-PrOAc/EtOAc = 18.7 and t-BuOAc/i-PrOAc = 53. One may conclude that the positive formal charge densities at the a- and... 

An extensive study has been made of the effect of substituents on the rate of rearrangement of allyl-substituted phenyl ethers. These data are given in Table 28. The data in both diphenyl ether and carbitol were found to be best correlated by the ff parameter. The equation for the correlation in diphenyl ether is given by... 

A similar chiral substituent effect is also observed in kinetic resolution of racemic primary allylic alcohols (Scheme 8). For example, both the enantiomers of racemic E-allylic alcohol are epoxidized at almost the same rate, while efficient kinetic resolution occurs in the epoxidation of racemic Z- and 2-substitut-ed allylic alcohols [49]. 

In addition to enantiocontrol, the problem of regiocontrol arises in these reactions. There are various factors that influence the regioselectivity of allylic substitutions [3,4,13, 36, 37, 38, 39]. Electronic effects exerted by the catalyst and the allylic substituents, steric interactions between the nucleophile, the allyl system and the catalyst, and the relative stabilities of the Ti-olefin complexes formed after nucleophilic addition, can all play a role. The relative importance of these factors varies with the catalyst, the substrate, the nucleophile, the solvent and other reaction parameters and is difficult to predict. 

The second class of substrate of importance bears a CH2 group at one of the termini of the 7t-allyl unit. Until recently this family of substrates presented the biggest challenge because the vast majority of palladium-catalyzed reactions deliver the nucleophile to the less substituted carbon unless there are some special additional factors (i.e., ring size in an intramolecular reaction, substituent effects, etc.). 

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