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, ΔG^⧧_TD and ΔG^⧧_CTID, increased in the order 1a ≤ 1b < 1c < 1d. The free energy difference ΔΔG^⧧ = ΔG^⧧_TD − ΔG^⧧_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, ΔS^⧧_CTID, decreased from zero to a large negative value in the order of tert-butyl, isopropyl, ethyl, and methyl, whereas ΔS^⧧_TD did not show a similar tendency.