Chemistry and Biochemistry
Bayer School of Natural and Environmental Sciences
Jeffrey D. Evanseck
Jeffry D. Madura
Douglas J. Fox
Diels-Alder reaction, dative bond, Frontier Molecular Orbital Theory, Density Functional Theory, boron, Stereoelectronic effects
The focus of this dissertation is to correct misconceptions about Lewis acidity, uncover the physical nature of the coordinate covalent bond, and discusses how Lewis acid catalysts influence the rate enhancement of the Diels-Alder reaction. Large-scale quantum computations have been employed to explore many of Lewis' original ideas concerning valency and acid/base behavior. An efficient and practical level of theory able to model Lewis acid adducts accurately was determined by systematic comparison of computed coordinate covalent bond lengths and binding enthalpies of ammonia borane and methyl substituted ammonia trimethylboranes with high-resolution gas-phase experimental work. Of all the levels of theory explored, M06-2X/6-311++G(3df,2p) provided molecular accuracy consistent with more resource intensive QCISD(T)/6 311++G(3df,2p) computations.
Coordinate covalent bond strength has traditionally been used to judge the strength of Lewis acidity; however, inconsistencies between predictions from theory and computation, and observations from experiment have arisen, which has resulted in consternation within the scientific community. Consequently, the electronic origin of Lewis acidity was investigated. It has been determined that the coordinate covalent bond dissociation energy is an inadequate index of intrinsic Lewis acid strength, because the strength of the bond is governed not only by the strength of the acid, but also by unique orbital interactions dependent upon the substituents of the acid and base. Boron Lewis acidity is found to depend upon both substituent electronegativity and atomic size. Originally deduced from Pauling's electronegativities, boron's substituents determine acidity by influencing the population of its valence by withdrawing electron density. However, size effects manifest differently than previously considered, where greater σ-bond orbital overlap, rather than π-bond orbital overlap, between boron and larger substituents increase the electron density available to boron, thereby decreasing Lewis acidity. The computed electronegativity and size effects of substituents establish unique periodic trends that provide a novel and clearer understanding of boron Lewis acidity, consistent with first principle predictions.
Lastly, it is discovered that the energetics associated with the transition structure converge much slower than what was observed for coordinate covalent bonded ground states. Consequently, it is harder to model activation barriers, as compared to binding energies, to within experimental accuracy, because larger basis sets must be employed. Hyperconjugation within dienophile ground states, initiated by geminal Lewis acid interactions, is found to govern the strength of the coordinate covalent bond between the Lewis acid and the dienophile. A novel interpretation is presented where the strength of the coordinate covalent bond within the Lewis acid activated dienophile is governed by donor-acceptor orbital interactions between the π-density present on the carbonyl group to the σ* orbitals on the Lewis acid, rather than the main donor-acceptor motif between the oxygen lone pair and the empty 2p orbital on the Lewis acid. Moreover, the same hyperconjugation within the dienophile controls the rate enhancement of the Lewis acid catalyzed Diels-Alder reaction, by modulating the energy of the dienophile's lowest unoccupied molecular orbital. A new understanding of Lewis acidity and coordinate covalent bonding has been achieved to better describe and predict the structure and electronic mechanism of organic reactions.
Plumley, J. (2009). New Conceptual Understanding of Lewis Acidity, Coordinate Covalent Bonding, and Catalysis (Doctoral dissertation, Duquesne University). Retrieved from https://dsc.duq.edu/etd/1052