Organic superbase-induced chemiluminescent decomposition of a hydroxyaryl-substituted dioxetane: Unique effect of a bifunctional guanidine base on the chemiluminescence profile of a bicyclic dioxetane bearing a 4-(benzoxazol-2-yl)-3,5-dihydroxyphenyl moiety

Organic superbases represented by TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) effectively induced the decomposition of hydroxyaryl-substituted dioxetanes in acetonitrile to give bright light. The color of the chemiluminescence from a dioxetane bearing a 4-(benzoxazol-2-yl)-3,5-dihydroxyphenyl moiety varied depending on the base used. In addition to this change in the color of emission, TBD increased the chemiluminescence efficiency 2- to 5-fold compared to the results with other base systems and accelerated decomposition of the dioxetane. These unique effects of TBD may be due to its “bifunctional” character, which is different from those of other organic superbases. Chemiluminescent decomposition of the dioxetane was effectively induced by superbases even in apolar p-xylene.     

Color modulation for intramolecular charge-transfer-induced chemiluminescence of bicyclic dioxetanes bearing a 3-hydroxy-5-naphthylphenyl moiety in the coordination sphere

Bicyclic dioxetanes 2a, 2b, and 3, bearing a 3-hydroxy-5-naphthylphenyl moiety underwent charge-transfer-induced decomposition with accompanying emission of light, the color of which changed from red to blue responding to a complex of crown ether with potassium t-butoxide used as a base. Furthermore, they afforded unusual chemiluminescence, the spectra of which displayed two peaks in some cases. It was observed for chemiluminescences in the coordination sphere with crown ether that their spectra did not coincide with the spectra of authentic emitters.     

On mechanistic aspects of 5-deazaflavin dependent dehydrogenation of alcohol

The reaction of 5-deazaflavins with alcoholates was investigated and the direct hydride equivalent transfer from C₁ of alcoholates to C₅ of 5-deazaflavins was confirmed by chemical methods. 5-Alkoxy-10-butyl-3-methyl-5-deazaflavins were synthesized by treatment of 10-butyl-5-chloro-3-methyl-5-deazaflavin with the corresponding alcoholates. The 5-alkoxy-5-deazaflavins were reduced by sodium borodeuteride or sodium hydro-sulfite in deuterium oxide or monodeuteriomethanol to give 10-butyl-3-methyl-1,5-dihydro-5-deazaflavin-5,5-D₂ exclusively. 3,10-Dimethyl-5-deazaflavin radical anion was detected by esr technique on treatment of 3,10-dimethyl-5-deazaflavin with potassium in DMF. From the above reactions, a mechanism of 5-deazaflavin dependent dehydrogenation of alcoholate was proposed.     

Marked difference in singlet-chemiexcitation efficiency between syn-anti isomers of spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] for intramolecular charge-transfer-induced decomposition

Spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] syn-3 bearing a hydroxy group at the 6-position (as a model syn-rotamer of parent dioxetane 4 bearing a 3-hydroxyphenyl group) and its isomer anti-3 (as a model anti-rotamer of 4) were synthesized. When these spiro-dioxetanes were treated with tetrabutylammonium fluoride (TBAF) in DMSO, anti-3 emitted light with high efficiency (Φ^CL = 0.41), while the respective value for syn-3 was only 1/10 for anti-3. This significant difference in Φ^CL between syn-3 and anti-3 was attributed to the difference in their singlet-chemiexcitation efficiencies.     

Electron-transfer-induced decomposition of 1,2-dioxetanes in negative-mode matrix- assisted laser desorption/ionization time-of-flight mass spectrometry

1,2-Dioxetanes bearing an aromatic electron donor undergo intramolecular charge-transfer-induced chemiluminescence (CTICL). Although there has been some controversy regarding the mechanisms involved, there is little experimental evidence to strongly support any of the proposed mechanisms. In the course of our investigations, to clarify these mechanisms, we tried to effectively ionize dioxetanes bearing a phenolic group and found that poly(3-octylthiophene-2,5-diyl) was a promising matrix for negative-mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS). Electron-transfer ionization was found to take place for dioxetanes bearing a hydroxyphenyl moiety that had been further substituted with an aromatic group, which acted as an antenna to catch an electron from the matrix. Furthermore, the characteristic fragmentation of dioxetanes 3c-3d was thought to occur by the elimination of 2-methyl-1-propene (56 u) and pivalaldehyde (86 u) from deprotonated ion [M - H](-) of dioxetanes, based on the results of muliple mass spectrometry measurements of dioxetanes using MALDI quadrupole ion trap ToF-MS. Based on a comparison of fragmentation in dioxetanes and the corresponding keto esters, dioxetanes were presumed to initially generate excited keto esters from which fragmentation took place.     

Crucial dependence of chemiluminescence efficiency on the syn/anti conformation for intramolecular charge-transfer-induced decomposition of bicyclic dioxetanes bearing an oxidoaryl group

Thermally stable rotamers of bicyclic dioxetanes bearing 6-hydroxynaphthalen-1-yl (anti-5a and syn-5a), 3-hydroxynaphthalen-1-yl (anti-5b and syn-5b), and 5-hydroxy-2-methylphenyl groups (anti-5c and syn-5c) were synthesized. These dioxetanes underwent TBAF (tetrabutylammonium fluoride)-induced decomposition accompanied by the emission of light in DMSO and in acetonitrile at 25 °C. For all three pairs of rotamers, the chemiluminescence efficiency Φ(CL) for anti-5 was 8-19 times higher than that for syn-5, and the rate of CTID (charge-transfer-induced decomposition) for anti-5 was faster than that for syn-5. The chemiluminescence spectra of the rotamers for 5a and 5c, respectively, were different. This discrepancy in the chemiluminescence spectra between rotamers can presumably be attributed to the difference in the structures of de novo keto imide anti-14 and syn-14 in an excited state, which inherit the structures of the corresponding intermediary anionic dioxetanes anti-13 and syn-13. The important difference in chemiluminescence efficiency between anti-5 and syn-5 is discussed from the viewpoint of a chemiexcitation mechanism for CTID of oxidophenyl-substituted dioxetane.     

Alkaline metal ion-enhanced chemiluminescence of bicyclic dioxetanes bearing a hydroxyaryl group with an ‘even’ substitution pattern

Bicyclic dioxetane 5 bearing a 3-hydroxynaphthalen-2-yl group and its analogs 14 and 15 decomposed to give light with efficiencies of only 0.002-0.005% in a tetrabutylammonium fluoride (TBAF)/THF system, which was as expected for dioxetanes with a so-called ‘even’ substitution pattern. However, the chemiluminescence efficiencies (Φ^CL) markedly increased when these dioxetanes were decomposed with alkaline metal t-butoxide in THF. This enhancement of Φ^CL by alkaline metal ion was most likely due to the highly ordered conformation of an aromatic ring by chelate formation of the metal ion with both an oxido anion and oxygen atom of a tetrahydrofuran ring in an intermediary dioxetane like 12. Alkaline metal ion-enhanced chemiluminescence was similarly observed for dioxetane 6 bearing a 2-hydroxyphenyl group.     

Solvent-promoted chemiluminescent decomposition of a bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety

An aprotic polar solvent such as N-methylpyrrolidone (NMP) promoted the thermal decomposition of bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety 1 without the addition of any base. This solvent-promoted decomposition (SPD) gave light as effectively as the base-induced decomposition (BID) in an aprotic polar solvent. SPD caused intramolecular CT-induced chemiluminescence similar to BID, but, in contrast to BID, SPD proceeded through a pathway with a large negative entropy of activation. Dioxetane 1 was also shown to give light due to excited state intramolecular proton transfer (ESIPT) upon heating in p-xylene.     

Base-induced chemiluminescent decomposition of bicyclic dioxetanes bearing a (benzothiazol-2-yl)-3-hydroxyphenyl group: a radiationless pathway leading to marked decline of chemiluminescence efficiency

Charge-transfer-induced decomposition (CTID) of bicyclic dioxetanes 1b-d bearing a 3-hydroxylphenyl moiety substituted with a benzothiazol-2-yl group at the 2-, 6-, or 5-position was investigated, and their chemiluminescence properties were compared to each other, based on those for a 4-benzothiazolyl analogue 1a. Dioxetanes 1c and 1d underwent CTID to give the corresponding oxido anions of keto esters 8c or 8d in the singlet excited state with high efficiencies similarly to the case of 1a. On the other hand, 1b showed chemiluminescence with quite low efficiency, though it gave exclusively keto ester 2b. The marked decline of chemiluminescence efficiency for 1b was attributed to 1b mainly being decomposed to 8b through a radiationless pathway, in which intramolecular nucleophilic attack of nitrogen in the benzothiazolyl group to dioxetane O-O took place to give cyclic intermediate cis-11.