Studies and Synthetic Methodology: 1. Cycloaddition of Allylic Azides and Alkynes, 2. Hexafluoro-2-propanol-promoted Friedel–Crafts Acylation Reactions

Abstract

Cycloaddition of allylic azides and alkynes. The 1,3-dipolar Huisgen azide-alkyne cycloaddition is a significant area of interest in modern chemistry. The use of allylic azides as dynamic reaction partners represent a novel variant of this chemistry as they undergo facile 1,3-allylic azide rearrangement, which is also known as the Winstein rearrangement. We combined such an allylic azide rearrangement with an intramolecular Huisgen cycloaddition to afford substituted triazoles in a diastereoselective fashion. Although modest diastereoselectivity was observed in most cases, the majority of diastereomeric pairs were separable. Also, depending on the conditions, a difference in the reactivity of various allylic azides was noticed. Under thermal conditions, vinyl-substituted triazoloxazines were formed, in contrast to copper(I)-catalyzed conditions which afforded dimerized macrocyclic products. Hexafluoro-2-propanol promoted Friedel–Crafts acylation reactions. The Friedel–Crafts acylation is one of the most important reactions in both academia and industry for the synthesis of aromatic ketones. The reaction is typically promoted by stoichiometric or greater amounts of acids, such as AlCl3, FeCl3, or H2SO4, which activate the carbonyl for attack by an aromatic group. A drawback of this extremely versatile acylation reaction is the generation of large amounts of corrosive aqueous waste following post-synthesis workup. We have shown that hexafluoro-2-propanol (HFIP) promotes both intramolecular and intermolecular FC acylation without additional catalysts or reagents. This solvent-promoted acylation is practically simple and accommodates a broad substrates scope. Our preliminary kinetic studies reflects involvement of 3 molecules of HFIP in rate determing step.