Steric acceleration

Table 15 shows that peroxyester stabiUty decreases for the alkyl groups in the following order tert — butyl > tert — amyl > tert — octyl > tert — cumyl > 3 — hydroxy — 1,1 dimethylbutyl. The order of activity of the R group in peroxyesters is also observed in other alkyl peroxides. Peroxyesters derived from benzoic acids and non-abranched carboxyUc acids are more stable than those derived from mono-a-branched acids which are more stable than those derived from di-a-branched acids (19,21,168). The size of the a-branch also is important, since steric acceleration of homolysis occurs with increasing branch size (236). Suitably substituted peroxyesters show rate enhancements because of anchimeric assistance (168,213,237). 

The most satisfactory method of dehydrating 12a-alcohols appears to be through the sulfonate esters Engel and coworkers have shown (ref. 236 and ref. cited therein) that treatment of such sulfonates with alumina gives A -compounds. The reaction appears to be subject to steric acceleration in that bulky IToc-substituents and cw-fused A-rings aid elimination, and that yields increase with increasing size of the sulfonate employed. 

The ionization of alkyl (E)-arylazo ethers is subject to general acid catalysis when the reaction is carried out in the presence of carboxylic acid buffers (see Scheme 6-3), and the ionization is also subject to steric acceleration in the presence of bulky substituents ortho to the azo ether group (Broxton and Stray, 1980 Broxton and McLeish, 1983 a, and earlier work of Broxton s group). 

The results for the polymethyl compounds in Table 259, show departure from additivity for the 2,6 and 3,4-compounds (where steric acceleration is obviously responsible) and for the 3,5 compound, the result of which is inexplicable. Some additional relative rates were obtained by Benkeser et al.133 as follows 3-Me, 2.51 3-i-Pr, 3.96 3-f-Bu, 5.50 4-Me, 10.6 4-Et, 11.5 4-/-Pr, 12.0 4-f-Bu, 14.0. It is not clear which of the conditions used by Benkeser et al.136 were employed for this study, especially since the relative rates quoted for 3-Me and 4-Me do not agree with either of the sets of data in Table 259. 

Table 3. Steric acceleration of thermolysis of rrans-azoalkanes 20 (180 0, ethylbenzene) and m-azoalkanes 24 (-28 °C, ethanol)...

As previously pointed out in the discussion of ring size effects on bond homolyses the largest steric acceleration by bulky substituents is expected for the thermal cleavage of C-C-bonds in tetra- or hexasubstituted ethanes 26. Im comparison to azoal-... 

Steric acceleration of the isomerization can apparently occur it is almost certainly Cla of 65 which is removed electrolytically, yet the major electrolysis product is 66(66 67 = 3.2 1) 27>, probably because steric compression between Clb and the nearby aromatic ring in 65 can be relieved through isomerization (inversion). 

The study of S -1 reaction in cyclohexane becomes difficult because of the ease with which the elimination reactions occur at the same time. However it has been shown that SN reaction will be sterically accelerated for an axial substituent, because the formation of carbocation will relieve the steric strain due to 1, 3 interactions which is absent with equatorial substituent. 

Further kinetic experiments with some stericaliy hindered phenylhydroxylamines gave results47 which suggest that under certain circumstances steric acceleration occurs, attributable to the buttressing effect of neighbouring 3-substituents. Thus the rate constants for the reactions of 41 and 42 are respectively greater than are those for 43 and 44. 

Nieuwstad, Klapwijk, and van Bekkum (105) have added to the knowledge of aromatic hydrogenation by their study of the influence of alkyl substituents in the 1 and 2 positions of naphthalene on the rate. Tetrahydro-naphthalenes were the products of hydrogenation over palladium at 80°C. The selectivity of the reaction was also followed and expressed as the ratio of the rate constants for the saturation of the unsubstituted and substituted rings, respectively. Steric effects play an important role, and, beside steric hindrance by the bulky substituents, steric acceleration also has been observed, the latter being caused by a release of the strain between the 1-alkyl group and hydrogen in position 8. 

A subtle example of steric acceleration associated with a dissociative mechanism is shown in the reaction... 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

With the first two entries the exchange is first-order (rate independent of [L]) and AH, AS and A V are markedly larger or more positive than with the third and fourth entries for which the rate law is second-order. Consideration of the magnitudes of the values, particularly AV, lead the authors to assign a D mechanism to the first two exchanges and an 4 (or possibly A) mechanism to the other two. Even steric acceleration and retardation for the two groups is observed. 

There is strong evidence for a dissociative type of mechanism for base hydrolysis. There is an =10 -fold rate enhancement (steric acceleration) for base hydrolysis of Co(iso-BuNH2)5CP+ compared to Co(NH3)jCF (mainly residing in while the corresponding factor for aquation is only =<10, emphasizing the different degrees of dissociation D vs /j). There is, incidentally, a LFER for log Atqh vs log slope 1.0, for reactions of a series of Co(III) complexes. Finally, on the basis of a mechanism, the estimated pro-... 

Table 4. Steric acceleration of thermolysis of peroxyesters 25 in ethylbenzene at 60...

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