Activated oxalates

Me Capra in particular proposed n> that the chemiluminescence reactions of a large number of organic compounds had this concerted dioxetane decomposition step as key reaction in the production of electronically excited products, namely acridinium salts 25,26,27) indolylperoxides 28>, activated oxalic esters 29>, diphenyl carbene 30>, tetrakis-dimethylamino-ethylene 31 32>, lucigenin 33>, and substituted imidazoles 23>. 

As mentioned in Section II. C., the concerted bond cleavage of 1.2-dioxetane derivatives has been proposed to be of general importance in respect of the excitation step of a large number of chemiluminescence reactions. The first experimental results concerning simple dioxetanes were obtained by M. M. Rauhut and coworkers in their work on activated oxalic ester chemiluminescence 24>. From experimental data on the reaction of e.g. bis (2.4-dinitrophenyl)oxalate with hydrogen peroxide in the presence of rubrene, they concluded that 1.2-dioxetanedione... 

It was found that the action of OH radicals on nitrobenzene or benzoic acid produced all the three isomeric phenolic compounds. OH radicals produced, for example, by reaction (1) can be used to generate other free radicals or atoms from inorganic or organic compounds. These reactions have been studied more recently by Kolthoff and Medalia (13), Merz and Waters (46), Stein and Weiss (45), and others. Reaction (1) also proved useful in the study of the so-called active oxalic acid (47). The free radical mechanism of hydrogen peroxide has been discussed also in connection with the mechanism of the action of the enzymes catalase and peroxydase, the prosthetic groups of which are iron porphyrin complexes which presumably also undergo oxido-reduction processes in the course of their catalytic activity (48). 

Weiss [240] suggested that the continuous production of active oxalic acid ion radical (C2O4 ) in this system is governed by the reaction... 

At room temperature, this active oxalic acid ion radical has a life of about hr. Therefore, the system behaves in such a manner that the aqueous polymerization caused by the reaction of monomer with carboxyl radicals tends toward its completion within hr or so after initiation. 

More control over the reaction conditions results in isolable peroxides, or at the very least, allows the exact structure of the peroxide to be inferred with some certainty. The reaction mechanisms which form the intellectual foundations of the phenomenon of organic chemiluminescence (and of bioluminescence) are all to be discoved here. In Chapter IV cyclic peroxides display a variety of mechanism, culminating in V with the very important dioxetans. Practical applications of these ideas must not be forgotten, and the chemistry of the active oxalates in Chapter VI brings together previous mechanistic concepts with the most well developed of all the chemiluminescent systems, the active oxalates. 

It is of crucial importance that the transfer of the electron back to the radical cation is facilitated by the structure of the peroxide reduction products. This idea was first indicated by McCapra [2] in an explanation of the very efficient active oxalate chemiluminescence, and serves as a good example of the above mechanism. 

The cage nature of the reaction is confirmed by the use of diphenylamine as activator. Normally diphenylamine emits at 360 nm, but in the presence of benzocoumarin (BC), an additional peak at 450 nm, ascribed to an exciplex is seen. This 450 nm peak is only seen in chemiluminescence. As in the case of the suggestion [2] for the mechanism of active oxalate chemiluminescence, the loss of CO2 generates a powerful reducing agent (Eq BC / BC = —1.92 V vs SCE)... 

This resulted in the synthesis of activated oxalic esters. One of the earliest and most efficient (0 = 0.23 einstein/mol) is 2,4-dinitrophenyl oxalate (3) ... 

Another group [14] has also investigated the gaseous products from the reaction of activated oxalates with hydrogen peroxide, and compared them with those from the CO2 afterglow [15-17], excited by microwave irradiation of CO2. Bis- (2.4-dinitrophenyl) oxalate (3) (p. 71) and bis (pentafluoro phenyl) oxalate (9) were used for the purpose. 

Since the minimum description of an excited state involves the separation of electrons into bonding and anti-bonding orbitals, radical processes have often featured in the explanations. However it is important to distinguish the various mechanisms since some are very much better understood than others. The most satisfactory mechanism to date is that named CIEEL by Schuster and anticipated by McCapra in his explanation for the chemiluminescence of the active oxalates. The essence of the chemically initiated electron exchange luminescence process is that there are two electron transfer steps. These steps take place within the first encounter complex formed from the two reacting molecules. The first transfer is strongly dependent on the difference in redox potential between the oxidant and reductant. This dependency is seen in the need for fluorescent molecules of low... 

There has been enormous progress in the development of more efficient non -enzymatic chemiluminescent systems - the most outstanding being active oxalic esters and other derivatives (p. 69). 

The last can be assayed either in the luminol reaction [74, 75], or by an appropriate activated oxalate, e.g. 2,4,6-trichlorophenyl oxalate, with perylene as fluorescer [76]. The detection limit of glucose in the latter system is 7 X 10 M. 

The peroxyoxalate chemiluminescence is used here, too warning capsules against infiltrating troops were developed containing an active oxalate, a fluorescer, and - as a hydrogen peroxide source, producing H2O2 on contact with air - an anthrahydroquinone derivative (258). 

Although these compounds react with quantum yields greater than luminol and less than that of the active oxalate esters, they are unsurpassed in producing very high intensities, albeit for a short time. 

Gruselle, M., B. Malezieux, S. Benard, C. Train, C. Guyard-Duhayon, P. Gredin, K. Tonsuaadu, and R. Clement. 2004. Chiral matrix effect of optically active oxalate-based networks Controlled helical conformation of an organic chromophore. Tetrahedron Asymm. 15 3103-3109. 

---