THE FACILITATED TRANSITION HYPOTHESIS AS AN EXPLANATION FOR THE GEM-DIALKYL EFFECT

A new technique of searching the conformational space of transition st ates was used to explore the cause of the rate acceleration in the gem -dialkyl effect in intramolecular cyclization reactions. Several previ ous hypotheses were discarded and a new hypothesis was advanced based on this new data. This hypothesis, the ''Facilitated Transition'' hypo thesis, states that increased steric hindrance reduces the overall act ivation energy by facilitating rotation through the transition state. The older ''Reactive Rotamer'' hypothesis was eliminated by generating all conformations of the starting materials using the WIZARD and MM2 computer programs and demonstrating that no relationship is found betw een rate increase and a change in the concentration of ''reactive rota mers''.