Superacid-induced ring-opening reactions of fluorinated heterocycles

Abstract

This paper is dedicated to Professor Ronald J . Gillespie on the occasion of his 65th birthday STEFAN P. KOTUN and DARRYL D. DESMARTEAU. Can. J. Chem. 67, 1724Chem. 67, (1989. HF/AsFS and HF/SbFS superacid mixtures react with fluorinated small-ring heterocycles such as 1,2-oxazetidines and oxetanes with consequent addition of HF to give alcohol and amine ring-opened products in high yield. Excess HF serves as solvent and reactant. Ring compounds containing both nitrogen and oxygen as heteroatoms protonate predominantly on nitrogen; a case with competing 0-protonation arises in the case of a chloro-substituted oxazetidine. In general, rings containing two heteroatoms retain the heteroatom-heteroatom bond and it is a carbon-heteroatom bond that opens. The resulting OH and NH functional groups are not protonated and lost in the superacid medium due to the instability of the highly fluorinated cations that would be left behind. Most of the heterocycles react at or below room temperature, although the very weakly basic 2,2-bis(trifluoromethyl)-3,3,4,4-tetrafluorooxetane requires the stronger HF/SbFS superacid system and more severe conditions. Key words: superacids, hydrogen fluoride, ring opening, fluorinated oxazetidines, fluorinated oxetanes.

The Chemistry of Highly Fluorinated Carbocations

“…A few years ago the preparation of alcohol 197 was reported, on the basis of ring opening of F -2,2-dimethyloxetane ( 45 ) by a mixture of HF and SbF 5 …”

“…In contrast to the result with oxetane, AsF 5 gave better yields of the products than SbF 5 , and even BF 3 showed some catalytic activity. The ratio of heterocyclic compound to Lewis acid is also said to be important, and the best result was achieved when a 2-fold molar excess of AsF 5 to 198 was used …”

“…The ratio of heterocyclic compound to Lewis acid is also said to be important, and the best result was achieved when a 2-fold molar excess of AsF 5 to 198 was used. 162 Reaction of monohydrooxazetidine 199 with HF/ AsF 5 is straightforward, giving only 200, isolated in 64% yield. However, the ring-opening reaction of chlorooxazetidine 201 under similar conditions leads to formation of two isomers 202a,b.…”