R-Butyl peresters

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Table 99 lists kinetic data for the decomposition of various r-butyl peresters of unsaturated cyclic acids. Differences in delocalization energies between the perester and corresponding radical (R-) and ion (R-I-) are also given as ADE and ADHio . For further comparison the t-butyl peresters, where R is benzyl and benz-hydryl, are included. By the simple hmo calculation, that was employed for the benzyl and benzhydryl groups, there is no difference in the ADE values between the ions and radicals. However, a difference of one ]8-unit is calculated between the radical and ion of the cyclopropenyl group. The AH values for decomposition of peresters with R = benzhydryl and 2,3-di- -propylcyclopropenyl are approximately the same. Yet, the ADE value increases in energy from 1.55 / to 1.00 p. 

Using such an initiator to produce a polymer, followed by conversion of the carboxylic acid end group to the acid chloride and then reaction of this with t-butyl hydroperoxide results in polymer molecules containing r-butyl perester end groups that can be used to initiate polymerization of a second monomer. 

For r-butyl peresters there is also a variation in efficiency in the series where R is primary secondary>tertiary. The efficiency of /-butyl peroxypentanoate in initiating high pressure ethylene polymerization is >90%, that of /-butyl peroxy-2-ethylhexanoate ca 60% and that of/-butyl peroxypivalate ca 40%.Inefficiency is due to cage reaction and the main cage process in the case where R is secondary or tertiary is disproportionation with /-butoxy radicals to form /-butanol and an olefin. 

Predictive equations for the rates of decomposition of four families of free radical initiators are established in this research. The four initiator families, each treated separately, are irons-symmetric bisalkyl diazenes (reaction 1), trans-phenyl, alkyl diazenes (reaction 2), tert-butyl peresters (reaction 3) and hydrocarbons (reaction 4). The probable rate determining steps of these reactions are given below. For the decomposition of peresters, R is chosen so that the concerted mechanism of decomposition operates for all the members of the family (see below)... 

The three saturated long-chain rm-butyl peresters are members of a homologous series, and as such, the weighted least-squares regression analysis of the enthalpies of formation r.v. number of carbons yields a methylene increment of —26.7 kJmol-1, a typical value for liquids. The methylene increment for the tert-butyl esters of the Cr, Cio, C12 and C14 acids is —28.0 kJmol-1. The closeness of these two values ensures that the enthalpies of formal reaction 16 will be nearly constant. For the three pairs from Table 3, the value is —70.3 8.1 kJmol-1. The standard deviation from the mean is quite large because the arithmetic difference for the C12 ester and perester, —79.5 kJmol-1, is quite a bit more negative than the differences for the Cio and C14 pairs, —64.4 and —66.9 kJ mol-1, respectively. Unfortunately, the acids and esters are in different phases and so we are reluctant to attempt any comparison between them, such as a formal hydrolysis reaction or disproportionation with hydrogen peroxide. 

This mechanism, involving a free radical R , is compatible with the allylic rearrangements found.250 The finding that t-butyl peresters labeled with ibO in the carbonyl oxygen gave ester with 50% of the label in each oxygen251 is in accord with a combination of R with the intermediate 19, in which the copper is ionically bound, so that the oxygens are essentially equivalent. Other evidence is that f-butoxy radicals have been trapped with dienes.252 Much less is known about the mechanisms of the reactions with metal acetates.253... 

Kinetic data for some chloro-substituted peresters are given in Table 97. For comparison the kinetic data for t-butyl peracetate are included. The rate of decomposition of r-butyl peroxy chloroformate is about 3 x 10 times greater than... 

The secondary isotope effect is consistent with a two-bond homolysis for the peresters listed in Table 105. However, the isotope effect for the perester where R = /-butyl is significantly smaller than for reactions where a /-butyl cation is generated . Although previous data indicate the importance of the ionic structure (II) in the transition state, it appears that a significant contribution from the radical structure (I) occurs as well in the case of R = /-butyl. Carbon isotope effects (ki s/A ), originating from the carboxyl carbon atom, were found to be 0,964 and 0.945 for /-butyl triphenylperacetate and a,a-diphenylperacetate, respectively (ref. 411). The data are consistent with two-bond homolysis. 

Several investigations have been concerned with the effect of solvent upon the rate of perester decomposition. These data also shed some light on the importance of the ionic structure (II) in the transition state. Some data on the effect of solvent may be obtained from previous tables. For the decomposition of peresters where R is a primary alkyl group in RCO3C4H9-/ and one-bond homolysis is the mechanism of choice, changes in solvent polarity have little effect on the rate of decomposition. The rate coefficients for the decomposition of r-butyl percaprate at 110 °C in chlorobenzene, nitrobenzene and diphenyl ether are 8.30 x 10 , 6.58 x 10" and 6.39 X 10" sec"S respectively" . The rates are also independent of initial concentration of the peroxide this may indicate that induced decomposition is unimportant cf. sub-section 13.4.1). 

Martin and his co-workers have provided a firm experimental foundation for anchimeric assistance in the homolytic decomposition of per-benzoate derivatives (Table 6). A single o-phenylthio group was found to increase the rate of decomposition of f-butyl perbenzoate by a factor of ca. 45 thousand. The effect is not steric since an o-r-butyl group hardly affects the rate of perester decomposition [compare (95) versus (99), Table 6]. A noticeable but weak effect is observed with the homologous sulfide (96). In addition to sulfur, anchimeric assistance by iodine and vinyl groups in (97) and (98), respectively, was observed. The bridged canonical species... 

In the case of reaction 3, entries 1 and 2, that is, iert-butyl peracetate and (ert-butyl perpropionate, almost certainly decompose by a stepwise mechanism, rather than the concerted mechanism assumed for reaction 3. Entry 3, tert-butyl perisobutyrate, probably forms the least stable R radical by the perester decomposition mechanism which is still mostly concerted in nature (36). 

Figure 6.7-15 Infrared absorbance spectra, recorded during the thermal decomposition of tert-butyl peroxypivalate in //-heptane at 80 °C and 2000 bar r reaction time. The initial perester concentration is 0.01 mol L". ...

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