Portland State University. Department of Chemistry
Term of Graduation
Date of Publication
Doctor of Philosophy (Ph.D.) in Chemistry
Arynes, Boron iodane exchange, Dehydroboration, Diarylhalonium compounds, Hypervalency, Three-center four-electron bond
1 online resource (l, 591 pages)
Organic compounds containing functionalized benzenoid rings have found tremendous applications in pharmaceuticals, agrochemicals, commodity chemicals, and beyond. As a result, development of methods for the synthesis of functionalized benzenoid rings has gained much attention from the synthetic community. Although many transition-metal mediated methods to functionalize benzenoid rings are known, there has been an upsurge of activity in developing protocols for decoration of benzenoid cores promoted by p-block elements. This excitement stems from the sustainability of earth-abundant elements and unique reactivity patterns observed within the p-block. Among these, transformations promoted by hypervalent halogen compounds, and especially, the diarylhalonium salts are very popular. As a result, a fundamental understanding of bonding in diarylhalonium compounds and their reactivity warrants further investigations. Towards that end, this body of work describes fundamental studies on bonding, syntheses, and applications of diarylhalonium compounds in the context of benzenoid ring functionalization.
First, a theoretical and experimental study on the fundamental nature of bonding in diarylhalonium compounds is presented (Chapter 2). A combination of Density Functional Theory (DFT), Natural Bond Orbital (NBO) Theory, and X-ray structure data is used to correlate bonding and structure for a series of diarylhalonium salts and a refined bonding model for these compounds based on theory of fractional orbital contribution is proposed. The revised bonding model is used to account for both kinetic (aryl transfer, and aryne formation) and thermodynamic reactivity (Lewis acidity) for both acyclic phenyl(Mes)halonium and cyclic dibenzohalolium salts.
The unsymmetrical aryl(Aux)iodonium salts (Aux = 2,4,6 trimethylphenyl or 2,4,6 trimethoxyphenyl) are widely used aryl transfer reagents due to their high aryl-transfer selectivity in nucleophilic substitution reactions. Aryliodides have been the traditional synthetic starting materials for these reagents, and syntheses from more commercially available arylboron compounds is challenging for electron poor aryl(Mes)iodonium salts and unreported for aryl(TMP)iodonium salts. In this context, an improved approach for the syntheses of aryl(Mes)iodonium salts from readily available arylboron compounds is presented (Chapter 3). This problem is solved using two distinct approaches. 1) The use of potassium aryltrifluoroborate salts as aryl source overcomes the limitations of low nucleophilicity of previously used arylboronic acids; and 2) the use of “F+” oxidants to generate highly electrophilic fluoro(Mes)iodonium species overcomes the limitation of low electrophilicity of Mes-I(OAc)2. Combination of these two approaches improves the yields of boron-iodane exchange reaction for the synthesis of aryl(Mes)iodonium salts. Mechanistic studies suggest that the reaction proceeds by a σ-bond metathesis-like pathway. Second, a new method for synthesis of aryl(TMP)iodonium salts is reported which proceeds under mild conditions via H-bond activation of TMPI(OAc)2 with fluoroalcohols.
In a subsequent section (Chapter 4), the utility of arylboron compounds as umpolung reagents for aryne formation is demonstrated. The refined boron-iodane exchange reaction conditions developed in chapter 3 are utilized to facilitate a polarity reversal for arylboron compounds via conversion to aryl(Mes)iodonium salts which are subjected to downstream aryne forming reaction conditions in a one-pot protocol. Lastly, the complementarity of this one pot dehydroboration reaction to Chan-Lam reaction is demonstrated.
©2023 Shubhendu Karandikar
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Karandikar, Shubhendu, "Studies on Bonding, Syntheses, and Applications of Diarylhalonium Compounds" (2023). Dissertations and Theses. Paper 6407.
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