Kinetics 1,2-dioxetanes

In addition to ready thermal decomposition, 1,2-dioxetanes are also rapidly decomposed by transition metals (39), amines, and electron-donor olefins (10). However, these catalytic reactions are not chemiluminescent as determined by the temperature drop kinetic method. 

The first detailed investigation of the reaction kinetics was reported in 1984 (68). The reaction of bis(pentachlorophenyl) oxalate [1173-75-7] (PCPO) and hydrogen peroxide cataly2ed by sodium saUcylate in chlorobenzene produced chemiluminescence from diphenylamine (DPA) as a simple time—intensity profile from which a chemiluminescence decay rate constant could be determined. These studies demonstrated a first-order dependence for both PCPO and hydrogen peroxide and a zero-order dependence on the fluorescer in accord with an earher study (9). Furthermore, the chemiluminescence quantum efficiencies Qc) are dependent on the ease of oxidation of the fluorescer, an unstable, short-hved intermediate (r = 0.5 /is) serves as the chemical activator, and such a short-hved species "is not consistent with attempts to identify a relatively stable dioxetane as the intermediate" (68). 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

This group of highly strained cyclic peroxides, though thermodynamically unstable, contains some compounds of sufficient kinetic stability to exist as solids at ambient temperature [1], Interest in these compounds is increasing, most are very unstable, several have proved explosive when isolated [2,3], Not only are lower 1,2-dioxetanes dangerously unstable but so, above 0°C, are the precursor 1,2-bromohydroperoxides [4],... 

For isolated HEI such as dioxetanes and other cyclic and linear peroxides that act directly as reagents in the excitation step, kinetic studies lead to rate constants and activation parameters for this excitation step and conclusions with respect to the mechanism of chemiexcitation can be obtained from the structural and conditional dependence of these parameters. For complex CL systems, in which the HEI is formed in rate-limiting steps prior to the excitation step, the kinetic parameters of this essential reaction step can only be obtained indirectly (see below). 

This biradical-like concerted mechanism, in which the kinetic features reflect the biradical character and the formation of excited-state products can best be rationalized by the concerted namre of the complex reaction coordinate, was proposed to optimally reconcile the experimentally determined activation and excitation parameters of most 1,2-dioxetanes studied and has been called the merged mechanism . Specifically, bofh fhermal sfabil-ity and singlel and friplef quanfum yields in fhe series of mefhyl-subsliluled 1,2-dioxelanes, including fhe parenf 1,2-dioxefane" , could be readily rationalized on the basis of the merged mechanism and qualitative quanmm mechanics considerations . 

Catherall and coworkers studied the reaction kinetics of bis(pentachlorophenyl) oxalate and H2O2 catalyzed by salicylate in the presence of DBA and verified that the delayed addition of DPA did not alter the reaction kinetics. The authors also conducted several kinetic smdies on the peroxyoxalate system and proposed the participation of 4-hydroxy-4-(pentachlorophenoxy)-l,2-dioxetan-3-one (48, R=Cl5C60) as the HEI in the reaction ... 

Kinetically stabilized azetes also show a high tendency for cycloaddition with a variety of other reagents. Cycloaddition of (47) with triplet oxygen produced a fully characterized dioxetan adduct (48), which decomposed at 25°C into t-butyl cyanide and the a-dione fragment (88AG(E)272). 

Benzaldehyde is formed in the liquid-phase oxidation of f-butyl phenylacetate via a hydroperoxide and also by a non-radical pathway, probably via a dioxetane intermediate both reactions are catalysed by benzoic acid. The kinetic parameters have been calculated by solving an inverse kinetic equation.258... 

I, 2-dioxetane. An investigation of the chemiluminescence and decomposition kinetics of an unusually stable 1,2-dioxetane. Journal of the American Chemical Society, 97 (24), 7110-7118. 

A sensitive probe of the mechanism of dioxetane decomposition is the effect of deuterium substitution on the rate of reaction. Koo and Schuster (1977a) investigated the reaction of dioxetanes [9a] and [9b] and found no kinetic or... 

Despite the clear implication of the involvement of intramolecular electron transfer in the chemiluminescence of certain dioxetanes, there have been no clear examples of intermolecular electron exchange luminescence processes with dioxetanes. In a recent note, however, Wilson (1979) reports the observation of catalysis of the chemiluminescence of tetramethoxy-1,2-dioxetane by rubrene and, most surprisingly, by 9,10-dicyanoanthracene. While catalysis by the added fluorescers was not kinetically discernible, a lowering of the activation energy for chemiluminescence was observed. These results were interpreted not in terms of an actual electron transfer with the formation of radical ions, but rather in terms of charge transfer interactions between fluorescer and dioxetane in the collision complex. In any event, these results certainly emphasize the need for caution in considering the fluorescer as a passive energy acceptor in dioxetane chemiluminescence. 

A number of theoretical studies have appeared on the mechanism of the oxirane-forming reaction of olefins and 0( P). A kinetic investigation of the reaction of oxirane and the 0( P) atom has shown that H-abstraction occurs rather than insertion to form a dioxetane intermediate, The thermal and photochemical epoxidation of propylene in the presence of sulfur dioxide and acetonitrile have been reported. ... 

The kinetics of the thermal decomposition of 1,2-dioxetanes and a-peroxylactones are first order and are usually unimolecular. A variety of experimental methods can be used to monitor the rates. These include direct chemiluminescence of the excited carbonyl product,energy-transfer chemiluminescence of the chemienergized excited carbonyl product to an efficient fluorescer, dioxetane consumption or carbonyl product formation by nmr spectroscopy iodometry of the cyclic... 

Besides the isothermal kinetic methods mentioned above, by which activation parameters are determined by measuring the rate of dioxetane disappearance at several constant temperatures, a number of nonisothermal techniques have been developed. These include the temperature jump method, in which the kinetic run is initiated at a particular constant initial temperature (r,-), the temperature is suddenly raised or dropped by about 15°C, and is then held constant at the final temperature (7y), under conditions at which dioxetane consumption is negligible. Of course, for such nonisothermal kinetics only the chemiluminescence techniques are sufficiently sensitive to determine the rates. Since the intensities /, at 7 ,- and If at Tf correspond to the instantaneous rates at constant dioxetane concentration, the rate constants A ,- and kf are known directly. From the temperature dependence (Eq. 32), the activation energies are readily calculated. This convenient method has been modified to allow a step-function analysis at various temperatures and a continuous temperature variation.Finally, differential thermal analysis has been employed to assess the activation parameters in contrast to the above nonisothermal kinetic methods, in the latter the dioxetane is completely consumed and, thus, instead of initial rates, one measures total rates. 

In Table 3 the activation energies of tetramethyl-1,2-dioxetane by a variety of isothermal and nonisothermal kinetic methods are compared. The values range... 

TABLES. ACTIVATION ENERGIES OF TETRAMETHYL-1,2-DIOXETANE BY A VARIETY OF KINETIC METHODS... 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

The experimental procedure to determine 4>wa is quite analogous to that discussed for The experimental definition is given by Eq. 38, in which all the terms have been already defined. Again the dioxetane decomposition rate constant kj) is determined by following the first-order kinetics of the DPA-enhanced chemiluminescence decay. The initial or total DPA fluorescence intensity is standardized with a suitable light standard, usually with luminol or the scintillation cocktail. The photomultiplier tube should be corrected for wavelength response. ... 

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