Table I. Relative Rates of Photooxygenation of 2-Siloxynorbornenes 8 1 7 17 K,CI 17 8.4 4.2 1 a R = SiMe3.quently, the peroxide ion has the choice of removing the easily detachable silyl group to give 8 or closing to the dioxetane 10. The same process occurs to a lesser degree on the endo face of 6. The endo zwitterionic peroxide gives mainly the product of silatropic rearrangement, 9. The corresponding endo-dioxetane is simply not detected, presumably owing to its even lower concentration than that of 9.The action of the gem-dimethyl grouping at C-7 is to force singlet oxygen to approach the endo face of the norbornadiene skeleton. This time the resulting zwitterionic peroxide 16 has three avenues of reaction available. Silatropic shift, as before, is favored over dioxetane formation. However, an alternativeis provided, as charge is also conveniently annihilated by closure to the endo-peroxynortricyclane derivative 12 which, incidentally, could be regarded as the consequence of a nonconcerted homo-Diels-Alder addition. The isolation and stability of l l and 12 at room temperature rule out concerted formation of 12 followed by equilibration to 11. One last, but important point, concerns reaction rates. Both 6 and 7 reacted rapidly with singlet oxygen; solutions (0.5 M) in deuteriochloroform containing meso-tetraphenylporphin M) were quantitatively oxidized in 5 min. Moreover, the usual tests confirmed the intermediacy of singlet oxygen as reagent.I2 Rates of 6 and 7 were also compared with their monoolefinic counterparts (Table I). It is significant that dienes react faster than monoenes and that the presence of methyl groups at C-7 is rate retarding.I3 This behavior parallels exactly that of 2-methylnorbornadiene and its 7,7-dimethyl derivative, where singlet oxygen reacts to give the allylically rearranged hydr~peroxide.'~ Consequently, in the present case it can be similarly concluded that singlet oxygen is behaving as an electrophile and that the transition state is controlled essentially by the HOMO of the d o x y olefins, modified by a steric effect.

Aus 4‐Chlor‐pyrazolonen‐(5) (I) entstehen unter der Einwirkung von Basen wie K2CO3 bei Raumtemp. syn‐ (II) und anti‐Isomere (III) der Titelverbindungen; unsymm. Verbindungen dieses Typs erhält man z.B. ausgehend von äquimolaren (Ia)‐(Ib)‐Gemischen.