The thermal isomerization of cyclobutenes. Part XV. Rates of isomerization of exo- and endo-tricyclo[4,2,1,02,5]nona-3,7-diene and exo- and endo-tricyclo[4,2,1,02,5]non-3-ene

Abstract

The thermal isomerization of exo-tricyclo[4,2,1,0^2,5]nona-3,7-diene (I) and its endo-isomer (II) have been studied in the temperature ranges 256–290 and 202–235 °C respectively. Similarly exo-tricyclo[4,2,1,0^2,5] non-3-ene (III) and its endo-isomer (IV) were thermolysed in the temperature ranges 285–325 and 244–283 °C respectively. In all cases the only product from (I) and (II) was bicyclo[4,2,1]nona-2,4,7-triene (V) and from (III) and (IV) bicyclo[4,2,1]nona-2,4-diene (VI) formed by thermolytic ring-opening. Reactions were accurately first-order and insensitive to increase of the surface area of the vessel. Arrhenius parameters were calculated on the basis of five rate constants obtained for each compound: log k_I(sec.^–1)= 14·203 ± 0·044 –(44,227 ± 108/2·303RT)(1), log k_II(sec.^–1)= 14·678 ± 0·109 –(40,549 ± 247/2·303RT)(2), log k_III(sec.^–1)= 14·284 ± 0·106 –(46,713 ± 280/2·303RT)(3), log k_IV(sec.^–1)= 14·594 ± 0·120 –(44,194 ± 294/2·303RT)(4)(where R= 1·987 cal. deg.^–1 mole^–1).

Factors influencing these relative rates are discussed and compared with available data. It is concluded that the presence of the remote double bond in (I) and (II) has little electronic influence on the structure of the transition state.