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 π -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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2010-12-8813 |
| Date | 2010 December 1900 |
| Creators | Oyola, Yatsandra |
| Contributors | Singleton, Daniel A. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | thesis, text |
| Format | application/pdf |
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