Isopropyl group, 753 table

Whereas the van der Waals radius of the fluorine atom is the smallest one after that of hydrogen, its volume is actually closer to that of oxygen (Table 1.16). Note that if the volume is an intrinsic property, steric effects are dependent on the observed phenomena. They frequently appear in dynamic processes. This allows comparison of steric parameters of various groups, fluorinated or not. These parameters show that the CF3 group is at least as bulky as an isopropyl or isobutyl group (Table 1.17). These data are confirmed by the values of the rotation, or of inversion barriers, of fluorinated diphenyl-type compounds (Figure 1.6). 

From the relative roles of the primary and tertiary radicals which are statistically formed on p-cymene, the termination constant of p-cymene should lie between that of cumene and that of toluene or xylene. Experimentally, this interpretation can be easily checked (12) by comparing the oxidation of p-cymene diluted by benzene with aromatic hydrocarbon mixtures having equal concentrations of methyl and isopropyl groups in comparable mixtures (Table V). 

Although in this volume only the Re/Si system is applied, transformation from one system into the other is frequently necessary. Fortunately, for the most common case of heterotopic hydrogen atoms (see Table 3). always last in ligand rank, the equivalence pro-R or HR = 1IR( holds. This is also true when the heterotopic ligands possess the highest rank. However, for heterotopic groups of intermediate rank, as is typical for the methyls of isopropyl groups, e.g.. in isopropyl alcohol and valine, there is no equivalence. 

Despite the success in the 1,3-diphenylallyl system, use of many of these ligands in the alkylation of 1,3-dialkylallyl system as Equation 8E.4 has produced mixed results, as summarized in Table 8E.5. With the phosphinooxazoline-type ligands, good selectivities (>90% ee) are still obtained from the reactions of substrates possessing bulky allylic substituents such as isopropyl groups (entries 8-10), but smaller substrates such as 1,3-dimethylallyl derivatives give only a modest level of enantioselectivities (entries 1 -7). The disparity between these results appears to be sterically derived as the enhanced preference of syn versus anti orientation in the 7t-allyl structure by the bulky phenyl or isopropyl groups may not be present with the smaller substrates. 

Another remarkable feature of these results (Hirota and Weissman, 1960) is that hyperfine coupling to the /3-proton in the isopropyl group was only about 2-4 G. This is very small compared with the results for /3-protons in comparable alcohol radicals (Table 2), and is thought to arise because of restricted rotation about the C—C bond (Symons, 1962). Indeed, molecular models show that only the conformation of this ketyl in which the /3-proton lies in the radical plane is not severely strained. Under these conditions hyper conjugation is at a minimum, and the splitting detected may be a measure of residual coupling in the absence of hyperconjugation (Symons, 1962). 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

If the alkyl substituent is in position 3, the 2-position is the most favored from the electronic point of view, being ortho to the alkyl group and alpha to the heteroatom. The other a position (5) is, however, free from steric effects. Consequently, reactions of 3-alkyl thiophenes, -furans, and -pyrroles yield a mixture of 2- and 5-substituted isomers, the composition of which depends on the nature of the heteroatom, the electrophile, and the size of the alkyl group. Table XXI reports the isomer distribution in the acylation of 3-methyl-, 3-isopropyl-, and 3-i-butylthiophenes and shows the importance of the steric factor in determining the orientation. 

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