Effect of deuterium substitution on the solvolysis rates of (methylcyclopropyl)-carbinyl derivatives
The ionization hypothesis, and the introduction of a general mechanism known as the SNl reaction, was without doubt one of the most important developments in theoretical organic chemistry. The pioneering work of Ingold and (the late) Hughes stimulated an extensive investigation of so:.volytic displacement reactions and the development of more and more re:ined methods for the study of reaction mechanisms. One of the latest addenda to the set of tools used for this purpose was the application ofisotopes. Although 14C was, at the beginning, one ofthe most widely used isotopes in organic chemistry, more recently the light isotopes, particularly deuterium and tritium, have gained in importance. In connection with kinetic studies isotopic substitution proved to be of great help in the investigation of the rate-determining step in unimolecular di:;placeJnent reactions, and for the elucidation of the structure of short-lived ionic intermediates. In the limiting case of displacement reactions the influence of an internal nccleophile-the neighbouring group-has been the subject of extensive studies. Systematic investigation of unusual reactivities in systems with neighbouring group participation produced the hypothesis of resonancestHhilized transition states, and led to the idea of "non-classical" ionsl. Such ionic species have been proposedas relatively stable intermediates in a number of cases where rate enhancements and/or skeletal rearrangements W(!re observed (Figure 1). In all these structures the transition state stabilization is explained by a eh arge delocalization due to partial overlapping of the empty p-orbital at the reaction centre with the neighbouring carbon-carbon bond. The relative importance of non-classical intermediates recently became the subject of considerable controversy2. The nurober of non-classical structures that has been proposed has multiplied fantastically in recent years, and a critical re-examination of the basic concept is certainly justified. Brown expressed the opinion2 that essentially in every case where nonclassical structures have been suggested, strained initial states were involved. This raises the question as t:o what extent the observed rate accelerations are due to non-classical resonance, and how much to raised initial state free energy. The cyclopropylcarbinyl system is one of the numerous examples where high rea.ctivities have been attributed to non-classical stabilization of the tra.nsition state. The cationic reactions of this system have been thoroughly