Neutral and Cationic Organoantimony Compounds as Lewis Acid Catalysts

Abstract

Organoantimony(V) compounds are potent Lewis acids which have been used for the complexation of anions or for the catalysis of organic reactions. Inspired by the broad range of applications developed for the stable Group 13 Lewis acid B(Cv6Fv5)v3, as well as the fact that SbFv5 is more acidic than BFv3, we set out to investigate organoantimony compounds in the context of Lewis acid catalysis. To this end, we synthesized electrophilic Sb(III) and Sb(V) complexes featuring electron-withdrawing halogenated ligands, cationic charges, and/or ancillary donor ligands, and tested these compounds in a number of organic transformations. Computational studies of their electronic structures provided us with insights into their unusual properties. We compared a series of triarylstibines with their tetrachlorocatecholate stiborane analogs, and demonstrated that the Lewis acidity of organoantimony(III) species can be readily enhanced by oxidation to the +V state, a phenomenon that is rationalized by the lowering of the antimony-based accepting Ó* orbital and a “deepening” of the associated Ó-hole upon oxidation. We also reported the synthesis of triarylfluoro- and triarylchloro- stibonium cations, among which the trimesitylchlorostibonium hexachloroantimonate is free from direct cation-anion interactions due to the steric shielding provided by the mesityl substituents. However, this cation is not as reactive as its phenyl derivative in the catalytic polymerization of THF and the Friedel-Craft dimerization of 1,1-diphenlyethylene. Additionally, we evaluated a series of tetraarylstibonium cations as catalysts for the cycloaddition of isocyanates to oxiranes. While all stibonium cations favor the 3,4-oxazolidinone products, the bulkier cations are found to be the most selective. Furthermore, we compared a series of ortho-phenylene based pnictogen cations and dications with their monofunctional derivatives as catalysts for the transfer hydrogenation of quinone derivatives with Hantzsch ester, and found that their catalytic reactivity follows the Lewis acidity trend in the order Sb dication > Sb monocation > P monocation. Lastly, this dissertation also investigates the electronic structures of selected organoantimony(V) compounds with the view to understand how coordination events at the antimony center affect the photophysical properties of these compounds.