HIGHLY SELECTIVE FLUORINATING AGENTS - A COUNTERANION-BOUND N-FLUOROPYRIDINIUM SALT SYSTEM

A series of alkyl- or (trifluoromethyl)-substituted N-fluoropyridinium-2-sulfonates 2a-h, differing in fluorinating power, were synthesized, and assessment was made of the effectiveness of each selective fluorinating agent. N-Fluoropyridinium-3- and -4-sulfonates 3 and 4 were also synthesized. Power-variables 2a-h were found to be highly selective fluorinating agents for a wide range of nucleophilic substrates such as activated aromatics, enol trialkylsilyl and alkyl ethers, active methylene compounds, activated olefins, and sulfides. Thus, phenol, naphthol, phenylurethane, and the trimethylsilyl ether of phenol were exclusively or highly selectively fluorinated at the o-position with 2f-h. Conjugated enol trialkylsilyl ethers of a steroid were regioselectively fluorinated at the 6-position with moderately powerful 2b-e. This regioselectivity increased with the bulkiness of the silyl part, and with the most bulky triisopropylsilyl group exclusive B-fluorination was achieved. Preferential beta-stereoselective fluorination at the 6-position was observed. N-Fluoropyridinium-2-sulfonates were activated with an acid. This acid-catalyzed fluorination led to the preferential p-fluorination of anisole. The present results can be explained based on the capacity of the 2-sulfonate anion to interact with the hydroxy group of phenol or naphthol, NH group of phenylurethane, silicon atoms of silyl ethers, or protons of acids.