An Efficient Pyrrole Synthesis via Silaphenylmercuric Triflate Catalyzed Cyclization of Homopropargyl Azides

Abstract

A mixture of phenylmercuric acetate and trifluoromethanesulfonic acid or silica gel supported phenylmercuric tri­fluoromethanesulfonate (silaphenyl mercuric triflate) efficiently catalyzed the formation of pyrroles from homopropargyl azide derivatives. The reactions proceed using 20 mol% of the heterogeneous catalyst with yields of isolated pyrroles ranging from 74% to 99%.

Key words

cyclization - heterocycles - heterogeneous catalysis - azides - alkynes

References and Notes

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13

When the cyclization reaction of 1 with Hg(OTf)₂ was quenched using Et₃N and NaCl at -20 ˚C, the formation of pyrrolic mercury chloride corresponds to 7 was confirmed by ^¹H NMR spectroscopy. However, pyrrolic mercury chloride was unstable under acidic conditions. During column chromatography on silica gel pyrrolic mercury chloride decomposed into 2.

16

Preparation of Silaphenylmercuric Triflate (10) and a Typical Experimental Procedure for a Silaphenyl-mercuric Triflate Catalyzed Cyclization
To a suspension of dried silaphenylmercuric acetate 9 (0.2 mmol/g, 500 mg, 0.1 mmol) in MeNO₂ (5 mL) was added TfOH (17.4 µL, 0.2 mmol), and the mixture was stirred for 10 min at r.t. The filtered residue was washed with MeNO₂ (10 mL) and dried to give silaphenylmercuric triflate 10. Next, MeNO₂ (4 mL) and prepared 10 were added to a dried two-neck flask. To the stirred suspension of 10 was added a solution of 1 (86 mg, 0.5 mmol) in MeNO₂ (1 mL) at r.t. under argon. The mixture was stirred at r.t. for 5 min, and the catalyst was then removed by filtration and washed with MeNO₂ (10 mL). The combined filtrates were concentrated under reduced pressure. Purification by column chromatog-raphy on silica gel using hexane and EtOAc (10:1) gave pyrrole 2 (70 mg, 97%).

17

In all cases (Table  [³] ), recovery of 10 was between 99.29% and 99.89%.