| dc.description.abstract | Statistical and nonstatistical rate theories help organic chemists understand reactions. The most widely used statistical rate model is transition state theory (TST). TST is an extremely useful model for describing organic reactions; however, it has some well-known shortcomings. Despite this, reactions are often shoehorned into TST for the explanation of experimental rates and selectivities even when it is clear that TST could not account for experimental observations. Here, we present three mechanistic studies to test the validity of TST and other rate theories, including hydroboration of alkenes, thioboration of alkynes, and 1,5-hydrogen atom transfers (1,5-HAT). These were studied experimentally by the measurement of product ratios and 13C kinetic isotope effects (KIE), and computationally using dynamic trajectories, energy calculations, and tunneling approximations.
The hydroboration of alkenes is a reaction that has long been described by TST; however, it has no enthalpic barrier for reaction. This leads to a nonstatistical energy distribution in the molecule. Other rate theories have been proposed, but these also fail to predict the selectivity. We propose a new nonstatistical model, which does account for the experimental selectivities.
The mechanism of the formal thioboration of alkynes was studied. Through the experimental studies of kinetics and isotope effects, and computational studies, the mechanism was found to occur by simultaneous attacks to both carbons of the alkyne in an AdE3 mechanism. This reaction was determined to be governed by TST.
A study of a 1,5-HAT found an extremely large H/D KIE at room temperature of 150. A KIE this large cannot be explained with TST alone, and could only be accounted for by quantum mechanical tunneling. In addition, this 1,5-HAT had a linear Eyring plot across a large range of temperatures. The normal small curvature tunneling (SCT) algorithm was insufficient to describe all of the experimental observations, but by using the large curvature tunneling (LCT) algorithm, the experimental observations were reproduced with striking accuracy. | en |
