The Dynamic Effect in the Hydroboration of Alkenes
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The hydroboration of simple alkenes with BH3 preferentially occurs in an anti- Markovnikov fashion. The standard explanation for this preference, reproduced in all general organic chemistry textbooks, is that the selectivity arises from a greater stability for the anti-Markovnikov transition state. This explanation presupposes the applicability of the transition-state theory model for reactivity and selectivity. This dissertation explores the applicability of transition state theory to selectivity in hydroborations and finds that in some cases transition state theory fails to accurately account for observations. Experimental results for the hydroboration of propene-d6 and styrene-d8 with excess BH3 was analyzed by 2H-NMR to determine the percentage of the Markovnikov product for the BH3-mediated reaction. The experimental selectivities were then compared with predictions based on very high-level calculations using transition state theory. It was observed that the regioselectivity of the hydroboration of these alkenes is lower than can be accounted for by transition state theory. The regioselectivity discrepancy was explored through dynamic trajectory analysis. It is proposed here that the observed regioselectivity is that of a ?hot? reaction, resulting from an exothermic association of alkene with borane to form an intermediate complex. This complex then overcomes low-energy barriers to form anti-Markovnikov and Markovnikov products faster than excess energy is lost to solvent. This hypothesis was explored for the hydroboration of internal disubstituted and trisubstituted alkenes. The applicability of transition state theory and the role of dynamics in determining the selectivity was gauged by determining product ratios in the presence of large excesses of borane and by considering the energetics of the calculated hydroboration reaction path. In all cases the enthalpic barriers for the rate-limiting association step and the formation of products from the intermediate ? -complex were small. Isotope effects were determined experimentally and were found to be too small for the conventional mechanism to be the predominate pathway. When the hydroboration reaction of propene with BH2Cl or BHCl2 was explored through a series of experimental and theoretical studies, we observed that the regioselectivity was lower than that predicted from transition state theory. However, the calculated pathways indicated that energy barriers for product formation were too large for this reaction to be considered a ?hot? reaction. The regioselectivity discrepancy was attributed to the chloroboranes undergoing equilibration with selective reaction of the most highly reactive forms of the borane.