DFT studies on the mechanisms of reactions of boroles and ruthenium-catalyzed C-H bond functionalizations of arenes

Abstract

DFT calculations on the reaction mechanisms of stoichiometric organic reactions and catalytic organometallic reactions have been reported in this thesis. With the aid of DFT calculations, we have elucidated reaction mechanisms based on experimental results. In addition, interesting predictions have been made. In Chapter 2, we studied systematically the dimerization reactions of boroles in which Diels-Alder (DA) dimers, bridged-bicyclic (BB) dimers or spiro dimers (SD) are generated depending on the substituents on the borole. We investigated how different substituents at the carbon atoms of the butadiene backbone as well as at the boron atom influence the dimerization reaction pathways. The DFT results show that, in general, both the DA and BB dimers are easily accessible kinetically, and the DA dimers are thermodynamically more stable than the BB dimers. When the substituent-substituent repulsive steric interactions are alleviated to a certain extent, the BB dimers are more stable than the DA dimer, and become accessible. The SD dimers are generally kinetically difficult to obtain. However, we found that aryl substituents promote the formation of the SD dimers. In Chapter 3, we studied the reactions of borole with CO, in which Lewis acid-base adduct 1AD, tricyclic boracycle 1TB or ketene derivative 1KD are reported depending on the substituents on the borole. DFT calculations were performed to study systematically the influence of borole substituents on these reactions. It was found that the Lewis acid-base adduct 1AD is a kinetic product, and can further isomerize to ketene derivative 1KD. The Lewis acid-base adduct 1AD can also undergo ring expansion, followed-by dimerization and migrations to afford the tricyclic boracycle 1TB. The computational results show that strong electron-withdrawing perfluorophenyl substituents significantly stabilize the Lewis acid-base adduct 1AD, allowing its successful isolation. In most cases, the tricyclic boracycle 1TB is both kinetically and thermodynamically more favourable than the ketene derivative 1KD. However, a –B(C₆F₅)₂ substituent at the 3-position and a silyl substituent at the 5-position together are able to lower the barrier leading to the formation of the ketene derivative 1KD. In Chapter 4, DFT calculations were performed to investigate the reactions of borole with alkynes, in which bicyclic diene, borepin and/or borirene can be formed depending on the substituents on the borole and alkyne. Our computational results indicate that bicyclic diene and borepin are generally kinetically favoured, interconvert into each other easily, and isomerize to other form(s) of bicyclic diene and borepin. Their relative thermodynamic stability determines the final form(s) of bicyclic diene or borepin that were observed in experiments. Formation of borirene is kinetically unfavourable in most reactions. However, certain substituents can promote the formation of borirene by stabilizing an energetically high-lying zwitterionic intermediate that leads to the formation of borirene. Borafluorene reacted with most alkynes to give borepin exclusively due to its propensity to maintain its aromaticity. However, when it reacted with silyl-substituted alkyne(s), a novel product different from bicyclic diene, borepin and borirene was generated, which can be attributed to the well-known ability of silyl groups to migrate. In Chapter 5, DFT calculations have been performed to investigate how different nucleophiles and directing groups affect the preference of C-H versus C-O bond functionalization in the ruthenium-catalyzed coupling reactions of aryl ethers with organoboronates. Our results indicate that the preference depends on the relative stability of the transition state structures for the C-O bond activation in the C-O bond functionalization pathway and the transmetalation with boronate to form a Ru-C bond in the C-H functionalization pathway. When the transition state structure for the transmetalation with boronate lies lower in energy than that for the C-O bond activation, C-H bond functionalization is preferred, and vice versa. How different nucleophiles and directing groups affect the relative stability of the transition structures has been discussed in detail.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology