Thermolysis of 1,2-dioxetanes

In parallel with the ab initio calculations, also semiempirical smdies on the thermolysis of 1,2-dioxetane were performed. Most computations have been conducted by the PM3 method because it is the best semiempirical method for describing lone electron pairs on adjacent atoms . As an illustration, only the PM3 method reveals that in the dioxetane molecule the 0-0 bond is longer and weaker compared with the C—C one, as manifested by the computed values of bond lengths [rf(0—O) = 1.600 > d(C—C) = 1.522 A] and bond orders [n(0—O) = 0.973 < w(C—C) = 0.989] . In contrast, the AMI and MNDO semiempirical methods exhibit the opposite trends, i.e. AMI gives d 0—0) = 1.334 A, d(C-C) = 1.539 A, n(O-O) = 0.995 and n(C-C) = 0.976, whereas MNDO furnishes d(0-0) = 1.316 A, d(C-C) = 1.558 A, n(O-O) = 0.996 and n(C-C) = 0.9622 f-8. Nevertheless, despite the quantitative differences in the computed bond lengths, bond orders and bond angles, both the AMI and PM3 methods disclosed qualitatively similar reaction trajectories . 

The study of the production of excited states by the thermolysis of 1,2-dioxetanes is quite extensive (e.g. Nakamura and Goto, 1979a,b). It has been found that the addition of electron donors, e.g. amines and aromatic hydrocarbons (in the ground state), accelerate the decomposition. The process has... 

It is now our task to amalgamate the experimental data on the thermolysis of 1,2-dioxetanes into a consistent mechanism. This has not been at all easy, as witnessed by the mechanistic turmoil precipitated during the last couple of years. Two mechanisms have been argued. 

Studies of lanthanide-sensitized chemiluminescence on systems other than the thermolysis of 1,2-dioxetanes, include the thermolysis of diphenyldiazomethane in the presence of oxygen (Nazarov, 2000), the system H202-Na0H (Kaczmarek et al., 2003) and the oxidation of hydrazine by hypochlorite (Tsaplev, 1997). Given the fact that up to date the sensitized chemiluminescence of rare-earth /3-diketonate complexes has been explored for only a limited number of chemiluminescent reactions, it can be anticipated that a wealth of original research will be conducted in the near future. 

Chemiluminescence.—The triplet potential-energy surface of dioxetan has been found to intersect the singlet surface between dioxetan and the transition state for dioxetan decomposition to formaldehyde. This provides support for the argument that the reason for efficient triplet ketone formation on thermolysis of dioxetans is because intersystem crossing is an integral part of the reaction.293 The decomposition mechanism of 1,2-dioxetans has been the subject of another theoretical investigation.294 ... 

As to the nature of the electronically excited state, the investigation of the thermolysis of tetramethyl-1.2-dioxetane revealed a high yield (about 50%) of excited triplet acetone 34> ... 

In summary, although the computed structural details of the reaction profile depend on the method used for calculations, the general salient mechanistic conclusion is that the dioxetane thermolysis starts with the 0—0 bond rupture to generate the 0C(H2)—C(H2)0 triplet diradical, which is followed by C—C bond cleavage to afford the final ketone products one of them is formed preferentially in its triplet excited state. Since even simple 1,2-dioxetanes still present a computational challenge to resolve the controversial thermolysis mechanism, the theoretical elucidation of complex dioxetanes constitutes to date a formidable task. 

Exciplexes have also been detected in the thermolysis of some 1,2-dioxetanes (Zaklika et al., 1978). 

A dramatic solvent effect in the thermolysis of tetramethyldioxetane, which followed the isokinetic relationship A/7 = /3A5 for a variety of solvents, formed the basis for the postulation of the concerted mechanism. However, it was shortly thereafter reported that the dramatic solvent effect in methanol was the result of catalysis by transition-metal ion impurities. In the presence of metal-ion complexing agents such as EDTA or Chelex 100, the menacing catalysis could be suppressed. That utmost care must be taken in measuring reliable kinetic parameters in 1,2-dioxetane decomposition cannot be overemphasized. 

Oxidation of luminol (see Example 7.22) and thermolysis of endoperox-ides or 1,2-dioxetanes provide important examples of chemiluminescent reactions. Tetramethyl-1,2-dioxetane (181) has been studied in great detail the thermolysis is clearly first order and the activation enthalpy in butyl phthal-ate is A// 27 kcal/mol. The enthalpy difference between the reactants and ground-state products is A// = -63 kcal/mol. 

Lechtken P, Hoehne G. Thermolysis of tetramethyl-1,2-dioxetane. Angew Chem Inti Ed. 1973 12 772-3. 

Acetyl-protected 1,2,3,4-tetrahydropyrazines 105, which are prepared by treatment of 2,3-dihydropyrazine with acetic anhydride and zinc (Scheme 27), undergo photooxidation to produce new dioxetanes 106 <1995JA9690>. Upon thermolysis, the dioxetanes 106 decompose quantitatively to tetraacyl ethylenediamines 107. Dimethyldioxirane oxidation of tetrahydropyrazine 105 affords novel epoxide 108, which is also generated by deoxygenation of dioxetane 106 with dimethyl sulfide. In 2,3,4,5-tetrahydropyrazine 1-oxide 109, which is prepared... 

The chemistry of some ring systems having two heteroatoms, i.e. dioxetanes, dithietanes, oxathietanes and thiazetidines are described. Next, the review considers compounds having either silicon or boron in a four-membered ring. Some thermolysis processes are interesting in the silicon series and the first thermally stable 1,2-dihydro-1,2-diborete is described. 

Thermolysis of 2,1,3-silaphosphaoxetane 38 furnishes the transient silanone, which immediately dimerizes to the l,3-disila-2,4-dioxetane 65 (Scheme 4) <19960M1845>. 

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