Thermal decomposition 1,2-dioxetanes

Excitation appears to be general for this reaction but yields of excited products vary substantially with the substituent R. The highest yield reported is from tetramethyl-l,2-dioxetane [35856-82-7] (TMD) where the yield of triplet acetone is 50% of total acetone formed (18,19). Probably only one carbonyl of the two produced can be excited by the thermal decomposition, and TMD provides 100% of the possible yield of triplet acetone. Singlet excited acetone is also formed, but at the low yield of 0.1—0.3% (17—21). Other tetraaLkyldioxetanes behave similarly to TMD (22). 

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. 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

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... 

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... 

Lucigenin (10,10 -dimethyl-9,9 -biacridinium or bis-Af-methylacridinium (38)), in the presence of hydrogen peroxide in alkaline media, exhibits chemiluminescence with a maximum emission wavelength at 445 nm. Lucigenin chemiluminescence was first reported in 1935 by Glen and Petsch, and the 1,2-dioxetane 39 was postulated as a key intermediate. Nevertheless, the mechanism of lucigenin chemiluminescence was only elucidated by McCapra and Richardson, who also proposed the thermal decomposition... 

An alternative thermal decomposition of 1,2-dioxetanes bearing an aromatic electron donor involves a CIEEL mechanism and is described in Section 2.16.6.1. 

Figure 11. Molecules that are chiral in the excited state only, (a) (lS,3R,7R,9S)-tricyclo-[7.3.0.03 7]dodecane-5,11 -dione in the electronic ground state, (RS), and in locally excited n ->Jt states, (.RS ) and (RS).5S (b) 3-(1nJt )-(lS,6R)-bicyclo[4.4.0.]decane-3,8-dione prepared by thermal decomposition of enantiopure 1,2-dioxetanes. 56...

Undoubtedly, the most characteristic property of 1,2-dioxetanes and a-peroxylactones is the fact that they emit light on thermal decomposition. Since in liquid media in the presence of molecular oxygen triplet excited states are efficiently quenched, the observed direct chemiluminescence is ascribed to the fluorescence of the carbonyl product. This fluorescence occurs usually at 420 10nm and corresponds to n n excitation.The shortest wavelength emission has probably been observed for the indole-1,2-dioxetane (17) that occurs at 320 nm. ... 

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... 

Probably the strongest support in favor of the diradical mechanism is the lack of a deuterium isotope effect in the thermal decomposition of franx-3,4-diphenyl-1,2-dioxetane. In the concerted mechanism, the ring carbon of the dioxetane changes its hybridization state from sp to sp in the activated complex (23) and an inverse secondary isotope effect k lkp) would be expected. Consequently, a diradical mechanism was argued to accommodate these results. Similarly, in the thermal decarboxylation of the dimethyl a-peroxylactone, a negligible (A ///A ) = 1.06 0.04) secondary isotope effect was observed. Presumably, in the a-peroxylactone decomposition, a diradical mechanism similar to that of dioxetanes (Eq. 66) upholds. 

The fact that hyperenergetic molecules such as the 1,2-dioxetanes should be prone by catalytic decomposition is not surprising. Early examples include the protecting effect of molecular oxygen on the thermal decomposition of 3,4-diethoxydioxetane, the efficient catalytic decomposition of this dioxetane by amines, and of alkyl-substituted dioxetanes by transition-metal ion impurities. However, all of these catalytic decompositions are competing dark reactions that greatly diminish the chemiluminescence efficiency of the dioxetanes. 

Besides their thermal decompositions into carbonyl fragments, the chemistry of 1,2-dioxetanes is quite limited. Obviously one of the reasons for this is the great lability of the dioxetane ring system. However, a number of reactions with nucleophiles and electrophiles have been performed and will be briefly reviewed here. 

The specificity of this reaction has been used to chemically titrate both the excited-singlet acetone and the triplet acetone produced through thermal decomposition of tetramethyl-I,2-dioxetane. (Cf. Section 7.6.4.) For this purpose the thermolysis was carried out in the presence of /ra/ii-l,2-dicy-anoethylene, and the quantities of singlet and triplet acetone formed were obtained from the yields of dioxetane and c/s-I,2-dicyanoethylene, respectively (Turro and Lechtken, 1972). 

Further detailed information on the reactive excited states of p. -unsaturated ketones has been acquired with (3a-c). Inefficient intersystem crossing from the singlet to the triplet state precedes population of Tz(.n,Tt ) from which the 1,3-acyl shift, induced by Norrish type I cleavage, occurs more readily than internal conversion to Ti Ti has been estimated to lie within 8 W mol of the acetone triplet (335 kJ mol ). That the 1,3-acyl shift is coupled not only with singlet-state but also Tz-state reactivity has been unequivocally shown by two methods. Thermal decomposition of a dioxetane, a derivative of (3a), generates predominantly the T2 state from which the 1,3-acyl shift product is formed concurrently with the ODPM products (4a) and (5a) via A photo-CIDNP study has independently demon-... 

Dioxetanes such as tetramethyl-1,2-dioxetane (525) are known to undergo thermal decomposition to form two carbonyl compounds via a concerted or stepwise (radical) mechanism, accompanied by chemiluminescence (Section 5.6) (Scheme 6.254).135,511,1440 The degradation of 525 results principally in acetone phosphorescence (2max = 430 nm) and the reaction is very sensitive to quenching by oxygen. 

High energy dioxetane molecules give an electronically excited carbonyl fragment by thermal decomposition or by intramolecular charge-transfer induced decomposition. Thus, dioxetane lb is presumed to decompose similarly into the corresponding excited... 

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