Thermodynamic aspects of thermal decomposition and charge-transfer-induced chemiluminescent decomposition for bicyclic dioxetanes bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety.

Uncatalyzed thermal decomposition (TD) and charge-transfer-induced decomposition (CTID) of dioxetanes were investigated to determine their thermodynamic characteristics. The dioxetanes examined were a series of 1-[4-(benzothiazol-2-yl)-3-hydroxyphenyl]-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptanes, 1a-d, bearing an alkyl substituent R, e.g., methyl, ethyl, isopropyl, or tert-butyl, at the 5-position and a parent dioxetane 1e with R = H. X-ray single crystallographic analysis was achieved for 1a-d. Both free energies of activation, DeltaG(double dagger)(TD) and DeltaG(double dagger)(CTID), increased in the order 1a < or = 1b < 1c < 1d. The free energy difference DeltaDeltaG(double dagger) = DeltaG(double dagger)(TD) - DeltaG(double dagger)(CTID) was ca. 27 kJ mol(-1) regardless of the substituent R. However, the use of Taft's dual-substituent parameter suggested that CTID was more sensitive to the polarity of the substituent R than TD. The entropy term for CTID, DeltaS(double dagger)(CTID), decreased from zero to a large negative value in the order of tert-butyl, isopropyl, ethyl, and methyl, whereas DeltaS(double dagger)(TD) did not show a similar tendency.