Perester

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). 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

Photopolymerizable compositions based on monomeric acryflc or other ethylenicaHy unsaturated acid derivatives are becoming increasingly popular. When multiftmctional derivatives are employed, three-dimensional networks having high strength and abrasion resistance are possible on exposure to light. A typical composition may contain an ethoxylated trimethylolpropane triacrylate monomer, a perester phenacjhdene initiator (69), and an acryflc acid—alkyl methacrylate copolymer as binder. 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

Polymers ndResins. / fZ-Butyl peroxyneopentanoate and other peroxyesters of neopentanoic acid can be used as free-radical initiators for the polymeri2ation of vinyl chloride [75-01-4] (38) or of ethylene [74-85-1]. These peresters have also been used in the preparation of ethylene—vinyl acetate copolymers [24937-78-8] (39), modified polyester granules (40), graft polymers of arninoalkyl acrylates with vinyl chloride resins (41), and copolymers of A/-vinyl-pyrrohdinone [88-12-0] and vinyl acetate [108-05-4] (42). They can also be used as curing agents for unsaturated polyesters (43). 

The corresponding aziridinyl radical has been generated by thermolysis of the appropriate perester and was also found to undergo ring opening to give (256) and (257) (75CB1527). 

KHARASH - SOSNOVSKY AHyDcOxidation AllyBc orpropargylic oxidation with t-butyl peresters. 

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

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I). " The initial step is the reductive cleavage of the perester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. The net effect is an allylic oxidation. 

Decomposition of the /rtin -decalyl perester A gives a 9 1 ratio of trans cis hydroperoxide product at all oxygen pressures studied. The product ratio from the cis isomer is dependent on the oxygen pressure. At 1 atm O2, it is 9 1 trans cis, as with the trans substrate, but this ratio decreases and eventually inverts with increasing O2 pressure. It is 7 3 cis trans at 545 atm oxygen pressure. What deduction about the stereochemistry of the decalyl free radical can be made from these data ... 

Since the perester may decompose explosively on excessive heating, an infrared spectrum of the residue should be run prior to distillation to check for complete reaction. For /-butyl perbenzoate, vc-o is 1775 cm- (5.63 fj.). 

Azoperoxydic initiators are particularly important due to their capacity to decompose sequentially into free radicals and to initiate the polymerization of vinylic monomers. The azo group is thermally decomposed first to initiate a vinyl monomer and to synthesize the polymeric initiator with perester groups at the ends of polymer chain (active polymer) [31,32]. 

For /-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%.196 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.196... 

Peroxyacetals 58106 and peresters such as 61107 are also effective transfer agents, however, at typical polymerization temperatures ( 60 CC) they are thermally unstable and also act as initiators. Compounds such as 62 which may give addition and 1,5-intramolecular substitution with fragmentation have also been examined for their potential as chain transfer agents (l,5-SHi mechanism).108... 

The first step of the procedure illustrates a general way of preparing aryl iert-butyl ethers.416 The second step is the best way to prepare 2-hydroxythiophene, inasmuch as the yield is good and ter/-butyl perbenzoate is a readily available perester that is relatively stable. The same procedure has been used to convert several other haloaromatic compounds to hydroxyaromatic compounds in good yield4 and is probably quite general. 

Oxidation, of Grignard reagents with peresters, 41, 91 43, 55 of 2-hydroxy-3-methylbenzoic acid to 2-hydroxyisophthalic acid by-lead dioxide, 40, 48 of indene, 41, S3... 

Decarboxylation reactions performed on activated or aromatic carboxylic acids, e.g., /1-keto acids, is a well-known synthetic transformation. However, the reaction has also been applied on other systems, e.g., N-carboxythiopyri-dones, N-acyloxyphthalimides and by thermolysis of peresters [104-106]. 

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. 

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)... 

In Table I are listed the radical products (R )(column 2), AE(x) values (column 3), EA values (column 4) and the experimental temperatures for the one- and ten hour half life rates for the decomposition of trona-symmetric bisalkyl diazenes (columns 5 and 6), (rona-phenyl,alkyl diazenes (columns 7 and 8), peresters (columns 9 and 10) and hydrocarbons (columns 11 and 12). 

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). 

The quality of fit to the linear equation 7 is excellent for the radical forming decompositions of Irons-symmetric bisalkyl diazenes (reaction 1 - Table II) and Irons-phenyl, alkyl diazenes (reaction 2 - Table II). The quality of fit to equation 7 is not as high for the radical forming decompositions of lerl-butyl peresters (reaction 3 - Table II) and hydrocarbons (reaction 4 - Table II). This suggests that transition state arguments may be used to rationalize the rates of reactivity very well for reactions 1 and 2, and fairly well for reactions 3 and 4. 

From Table IV the relative magnitudes of the reactant state "sensitivity factor" (N) are 4>1>2=3= zero. From this analysis the decomposition rates of traiw-phenyl, alkyl diazenes (2) and iert-butyl peresters (3) can be predicted by assuming a dependence only on transition state effects, with no need to incorporate the back strain of the reactants into the equation. 

Back strain effects are most important for the homolysis of hydrocarbons (4), a highly endothermic reaction, which does not produce a stable molecule byproduct, as do diazenes (N2) and peresters (CO2). Destabilization of the reactants in reaction 4 back strain is essential in lowering the energy of activation of reaction. The results of this study suggest that only reaction 4 requires the use of A values to obtain a good correlation between reaction temperatures and calculated product radical stabilities. 

Irons-phenyl, alkyl diazenes (2), peresters (3) and hydrocarbons (4). These equations are intended to be used for their predictive value for applications especially in the area of free radical polymerization chemistry. They are not intended for imparting deep understanding of the mechanisms of radical forming reactions or the properties of the free radical "products". Some interesting hypotheses can be made about the contributions of transition state versus reactant state effects for the structure activity relationships of the reactions of this study, as long as the mechanisms are assumed to be constant throughout each family of free radical initiator. 

CH Nj reactions, 382 COClj reactions, 383 free radical addition of hexafluoroacetone, 257 identification of oxidation products CHjNj to measure peracids as peresters, 385 extinction coefficients, 388-389/ iodometry to measure -OOH, 385 NO to measure alcohols and hydroperoxides, 386 residual, simplified carbonyl envelope which results from SF exposure, 386... 

It is somewhat contradictory and not yet fully understood why the back strain effect on the rate of perester decompositions is so large. We had reasoned before from the discussion of conformational effects that the Ca-CO-bond of 25 is only stretched to a small extent at transition state. From an analysis of bond energies5 18 it becomes questionable if the homolysis of C-N-bonds (as in 20 ) and C-C-bonds (as in 25) is likely to be directly comparable5,12a 18 In addition the extent of Ca-CO-cleavage at the transition state of fragmentation of 25 may well be itself dependent on the... 

CopperQ bromide, Limonene Wilson, C. W. et al., J. Agric. Food Chem., 1975, 23, 636 Addition of all the perester in one portion to limonene and catalytic amounts of copperQ bromide before oxidation had begun, as shown by development of a blue-green colour, led to an explosion. 

Fluorination of caesium heptafluoropropoxide at —40°C with nitrogen-diluted fluorine exploded violently after 10 h. This may have been caused by ingress of moisture, formation of some pentafluoropropionyl derivative and conversion of this to pentafluoropropionyl hypofluorite, known to be explosive if suitably initiated. Other possible explosive intermediates are peroxides or peresters. 

Such decay is known as concerted fragmentation. Peroxides have the weak O—O bond and usually decompose with dissociation of this bond. The rate constants of such decomposition of ROOR into RO radicals demonstrate a low sensitivity of the BDE of the O—O bond to the structure of the R fragment [4], Bartlett and Hiat [8] studied the decay of many peresters and found that the rate constants of their decomposition covered a range over 105 s-1. The following mechanism of decomposition was proposed in parallel with a simple dissociation of one O—O bond [3,4] ... 

The activation energy is equal to the dissociation energy of the weakest bond (/. w D 140-160 kJ moP1 for peresters). 

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