THE ADDITION OF HYDROGEN-CHLORIDE TO CYCLOPROPYL BENZENE IN ACETIC ACID
The kinetics of cleavage of arylcyclopropanes with HCl in acetic acid have been investigated to probe the transition state and product forming steps in the protolytic cleavage of the cyclopropane ring. The method of initial rates was used to obtain the kinetic parameters. The addition of HCl to cyclopropylbenzene(1) was found to follow the rate law: Rate = k(,2) 1 HCl +k(,3) 1 HCl Cl('-) where k(,2) = 3.0 x 10('-6) M('-1)s('-1), k(,3) = 0.73 M('-2)s('-1), HCl is from undissociated HCl, and Cl('-) is from dissociate HCl. The kinetic reaction products are 1-chloro-1-phenylpropane and 1-acetoxy-1-phenylpropane. The isotope effect for reaction, k(,HCl)/k(,DCl), was found to be 2.4. The activation parameters for reaction were determined to be (DELTA)H('(DBLDAG)) = 16.6 Kcal/mole and (DELTA)S('(DBLDAG)) = -29 cu from rate measurements at 50(DEGREES), 60(DEGREES) and 70(DEGREES)C. The rates of cleavage for a series of aryl substituted phenylcyclopropanes gave a linear correlation with (sigma) and (rho) = -1.4.
The effects of substituents on the cyclopropyl ring were measured by the method of competitive rates. Alkyl substitution on the cyclopropyl ring produced a non-additive increase in the rate of ring cleavage. Phenyl substitution caused a decrease in the rate of ring cleavage. A mechanism is postulated which involves rate determining corner protonation of the cyclopropane ring giving a transition state with unsymmetrical charge delocalization over the cyclopropyl ring. The transition state is either a bimolecular complex which collapses to form the chloride or acetate products, or the transition state is a termolecular complex which collapses to form the chloride product.
A series of rigid bicyclic phenylcyclopropanes, which had the rings oriented between 0(DEGREES) and 90(DEGREES) with respect to optimal conjugation, were investigated to determine the conjugative stabilization of the cyclopropyl ring cleavage. The rate differences for ring cleavage within this series was only a factor of three. The phenyl group has only a minor effect in conjugative stabilization.
The spin-lattice relaxation time (T(,1)) of three phenylcyclopropanes were investigated in chloroform solutions. An unusual temperature dependence of the correlation times was found with all carbons reaching a minimum correlation time at 40(DEGREES)C and increasing on either side. This was attributed to a change in dominance between reorientational and angular correlation times.