Neighboring Carbon and Hydrogen. 33.1 Reactivities of 3, 5-Cyclocholestan-6-yl Derivatives. Strain and Reactivity in Homoallylic Systems2

The kinetics of solvolysis of the relatively reactive 3, 5-cyclocholestan-6-yl trichloroacetates and chlorides have been studied. in addition, the rates of acid-catalyzed isomerization of the 3, 5-cyclocholestan-6-ols and the corresponding acetates to cholesterol or cholesteryl acetate have been measured. A striking feature of the present results is the very high reactivity of the 3, 5-cyclocholestan-6β-yl derivatives in solvolysis. in acetic acid or 90% dioxane at 25°, 3, 5-cyclocholestan-6β-yl chloride is more reactive than cholesteryl chloride by a factor of 107-4. The ratio of 3, 5-cyclocholestan-6β-yl chloride to cholesteryl chloride at equilibrium may be estimated from the kinetic data, since the equilibrium constant, Ke, is the product of a reactivity ratio and a partition factor for reaction of the intermediate cation at the two reaction centers. on this basis, Ke is ca. 10-6, 4, the difference in molar standard ground state free energies of the two isomers being ca. 000 calories. The important role of ground state free energies in determining the relative reactivity pattern in homoallylic systems is discussed, the 5-norbornenyl system being contrasted with cholesteryl. as we go from one system to the other, ground state free energy factors are responsible for a 108 change in relative reactivity of the two isomers. A connection is drawn between the ground state free energy pattern and the magnitude of strain and electron delocalization effects on reactivity of homoallylic isomers. 3, 5-Cholestan-6β-yl derivatives are more reactive than the 6α-isomers in the various reactions by factors of 101-102. The bearing of these results on the question of the nature of the non-classical cation first produced by ionization is discussed. Further consideration is given the general question of plurality of homoallylic cations and the relation between homoallylic and bicyclobutonium cations.