1.2- Dioxetanes activation parameters

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). 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

Two extreme mechanisms have been proposed for the unimolecular dioxetane decomposition the concerted mechanism , whereby cleavage of the peroxide and the ring C—C bond occurs simultaneously, and the biradical mechanism whereby the initial cleavage of the 0—0 bond leads to the formation of a 1,4-dioxy biradical whose subsequent C—C bond cleavage leads to the formation of the two carbonyl fragments (Scheme 8). Although the biradical mechanism adequately explains the activation parameters obtained for most of the dioxetanes smdied, it appears not to be the appropriate mechanistic model for the rationalization of singlet and triplet quanmm yields. Therefore, an intermediate mechanism has been proposed, whereby the C—C and 0—0 bonds cleave in a concerted, but not simultaneous, manner (Scheme 8) . 

Although the activation parameters obtained from the thermal decomposition of a great number of diverse dioxetane derivatives have been interpreted on the basis of the biradical mechanism, no general interpretation of the excitation efficiencies has been given on fhe basis of fhis mechanism Furfhermore, most theoretical... 

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. 

TABLE 4. ACTIVATION PARAMETERS AND EXCITATION PARAMETERS OF SOME SELECTED DIOXETANES... 

Most of the experimental evidence also points to the diradical mechanism as the preferred decomposition mode. Thus, the very earliest experimental evidence in support of the diradical mechanism rests on the fact that alkyl and phenyl substitution does not significantly alter the activation parameters for dioxetane decompositon. It was argued that if C-C bond cleavage occurs simultaneously with 0-0 bond cleavage, the incipient carbonyl group in the activated complex (23) should be stabilized in the relative order phenyl > alkyl > hydrogen. Thus, the activation energies should obey the relative order ii a(Ph)< fl(R)< a(H), that is, lowest for phenyl-substituted dioxetanes. Since this expectation was not borne out by the experimental data, the diradical (24) was proposed as an intermediate. 

As additional support for the diradical mechanism, it was shown that the 3,4-diethoxy-l,2-dioxetane (8) and the p-dioxene-l,2-dioxetane had identical activation energies, implying that the C-C bond is not significantly stretched in the activated complex. That these notions on substituent effects in dioxetane decomposition are grossly oversimplified has come clearly into focus in recent years (Table 4). The fact that little yet is understood about the correspondence between activation parameters and dioxetane structure has already been amply expounded in Section V.l.B. Nevertheless, a few additional comments seem appropriate on this subject in... 

R5. Richardson, W. H and O Neal, H. E. O., Thermochemistry and estimated activation parameters for the thermal decomposition of 1,2-dioxetanedione, 4,4-dimethyl-l,2-dioxetan-3-one. 

The activation parameters are collected in Table III for the 1,2-dioxetanes (1) and a-peroxylactones (2). Clearly, variation of substituent structure has a minor effect on the activation parameters. A significant exception is the diadamantylidene system (lz), which is unusually thermally stable.38 Assuming a diradical mechanism for the thermal decomposition of 1,2-dioxetanes, O Neal and Richardson97 were able to reproduce the experimental activation parameters with good precision, employing thermokinetic calculations developed by Benson.9741... 

A kinetics study has been performed on the aqueous iodine oxidation of the thiolactam (78). The rate of oxidation of (78) was found to be approximately 100 times that of a simple cyclic thioether, and this enhancement was ascribed to transannular anchimeric assistance provided by the nitrogen of the amide group <87JOC258i>. The thermal decomposition of the fused dioxetane (79) has been monitored by chemiluminescence decay, and this has been shown to follow first-order kinetics. The activation parameters have been calculated <83CL431>. Second-order rate constants for the quatemization of octahydro-lfl-azonine (9) and other azacycloalkanes with iodomethane in acetonitrile and methanol have been measured <68CCC1429>. The kinetics of the hydrogen-deuterium... 

Kinetic data for the thermal decomposition of 1,2-dioxetans (209) and (210) indicate a first-order reaction, and the activation parameters suggest a two-step decomposition mechanism via the biradical (211). The analogous... 

Adam, W, Griesbeck, A.G., GoUnick, K., and Knutzen-Mies, K., 1,2-Dioxetanes derived from 4,5-dimethyl-2,3-dihydrofuran synthesis via photooxygenation, activation parameters and excitation properties, /. Org. Chem., 53,1492, 1988. 

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 . 

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