Carbon 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). 

Peresters are also sources of radicals. The acyloxy portion normally loses carbon dioxide, so peresters yield an alkyl (or aryl) and an alkoxy radical ... 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

The Cj - and 54-symmetric tetraesters of tricyclo[3.3.0.0 ]octane (430 and 431) have been prepared by oxidation of diene 429 To access the parent hydrocarbon (435), acid chloride 432 was transformed to the derived ketene which undergoes intramolecular [2+2] cycloaddition The resulting cyclobutanone (433) serves as precursor to perester 434 whose thermal decomposition proceeds with chain transfer in competition with cleavage The unique arrangement of the carbon atoms in 435 is such that the smallest rings are all five-membered. The highly symmetric structure may be viewed as a constrained cisoid bicyclo[3.3.0]octane (as well as the symbol of NATO). 

The three saturated long-chain tert-butyl peresters are members of a homologons series, and as such, the weighted least-squares regression analysis of the enthalpies of formation V5. number of carbons yields a methylene increment of —26.7 kJmol , a typical valne for liquids. The methylene increment for the terf-butyl esters of the Cg, Cjo, Cn and C14 acids is —28.0 kJmol. 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. The standard deviation from the mean is quite large because the arithmetic difference for the C12 ester and perester, —79.5 kJmol, is quite a bit more negative than the differences for the Cio and C14 pairs, —64.4 and —66.9 kJmol, 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. 

Suitable inert solvents include methyl ethyl ketone, benzene, ethylbenzene and toluene. Suitable initiators include peresters and peroxycarbonates such as ferf-butyl perbenzoate, ferf-butyl peroxy isopropyl carbonate, fcrf-butyl peroctoate, tert-butyl peroxy isonon-... 

Compound 403 is readily reduced with sodium borohydride at -78°C and yields the monoalcohol 405 (115). It also reacts with potassium t -butyl hydroperoxide at -20°C and gives the cis-enone-perester carbonate 406 in high yield (116). This last transformation can be explained by retro-Claisen fragmentation of intermediate 407 followed by the elimination of methoxide ion from 408. It is also possible that 407 undergoes a direct stereoelectronically controlled Grob type fragmentation to compound 406. 

Problem 5.15 a. The perester could serve as a free radical initiator. At first glance, it appears that two tetrahydrofuran molecules form the two rings of the product, but the product contains one additional carbon atom. Therefore, the perester must be the source of the atoms of one ring, as well as a source of radicals. Because the perester contains a carbonyl group, the simplest explanation is that the lactone ring is derived from the perester. These considerations suggest the following chain mechanism ... 

The tetrahydrofuranyl radical, 5-53, adds to the carbon-carbon double bond of the perester. 

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. 

Table 1 Organic peroxides consist mainly of eight different basic substance classes. Peroxodi-sulfates are also important radical generators on a smaller scale azo-initiators and carbon-carbon initiators are used Dialkylperoxides Hydroperoxides Perester Perketales Ketonperoxides Diacylperoxides Peroxidicarbonates Peracids Peroxodisulfates Azo-initiators CC-initiators...

The peresters (758 X = H, Ph, or Me) showed first-order kinetics on thermal decomposition in cyclohexene. Rate and activation data were indicative of a radical mechanism. Similarly, in the thermolysis of the peroxide (759) in carbon tetrachloride, the scrambling of oxygen in the products when 0-labelled peroxide was used indicated a radical cage mechanism. The products obtained on irradiation of the azaspriranes (760 n = 1 or 2) may be rationalized by homoallyl rearrangements of the intermediate spiro-radicals. ... 

However, detailed analysis for this oxidation reaction showed that the reaction mechanism did not involve radical intermediates hut rather a henzoyl perester intermediate (15). It was reported that the thermal decomposition of henzoyl 2,6-dimethylphenyl carbonate, which should generate the phenoxy radical, produced Poly-2,6-Me2P and DPQ in 35-38% and 10% yield, respectively (16). From these studies, it seems that some complicated experimental results and different imder-standings were involved in the above assumption of the C—C coupling selectivity in the free-radical coupling, which was pointed out before (17). 

In order to avoid the large extrapolation to common temperature, required for comparison of the thermolysis of both -butyl perbenzoate and the corresponding hyponitrite, we decided to study the photolysis of the perester under the same conditions of solvent and temperature as convenient for the thermal hypo-nitrite reaction (27°, hexane) (r7). Product studies showed hexyl benzoate was formed from a chain Induced reaction. The direct photolysis gave phenyl t-butyl ether and high relative yields of carbon dioxide. Both... 

Peroxides cure by decomposing on heating into oxy radicals which abstract a hydrogen from the elastomer to generate a polymer radical. The polymer radicals then react to form carbon-carbon crosslinks. With imsaturated elastomers, this occurs preferentially at the site of allylic hydrogens. The rate of crosslinking is directly proportional to the rate of decomposition of the peroxide. Cure rates and curing temperatures therefore depend on the stability of the peroxide, which decreases in the order dialkyl > perketal > perester or diaryl. The most commonly used of these crosslinkers is dicumyl peroxide. 

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