engleski

A Case of Concerted Formation of the Five Membered Ring. The Structure of the Partially Cyclized Transition State

A number of cases in the chemical literature favour the kc over the k(c) process in solvolysis of the substrate in which the double bond is placed at C-4 relative to the reaction centre, i.e. the lack of the pi-participation in the transition state is assumed.In order to find an evidence for possible pi- and extended pi-participation for tertiary chlorides with the 4-alkenyl side chain, the solvolytic behaviour of the chlorides 1 and 2 were studied. The magnitude of the beta-deuterium kinetic isotope effect (KIE) for 1 is considerable reduced compared to its saturated analogue (kH/kD=1.80) suggesting concerted closure of the five membered ring. Nevertheless, the beta -KIE obtained with 2 is even more reduced, which can be rationalised in terms of the distribution of the positive charge in the reaction transition state. In the case of 2, the charge on the reaction centre in the transition state is much more reduced the with 1, suggesting the assistance of the second double bond in the rate determining step, i.e. extended pi participation.In order to evaluate the experimental results, theoretical calculation were carried out using the standard Gaussian 94 package. The transition states in which one (3) and two double bonds take part were located (at the B3LYP/6-31G(d) level of theory) on the energy surface. Also the theoretical values of the isotope effects were obtained using the program QUIVER

deuterium kinetic isotope effect ; transition state ; theoretical calculation

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