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Synlett 2011(19): 2815-2818  
DOI: 10.1055/s-0031-1289566
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

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

Hirofumi Yamamoto*, Ikuo Sasaki, Mizuho Mitsutake, Ayumi Karasudani, Hiroshi Imagawa, Mugio Nishizawa
Faculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514, Japan
Fax: +81(88)6553051; e-Mail: hirofumi@ph.bunri-u.ac.jp;
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Publication History

Received 3 August 2011
Publication Date:
09 November 2011 (online)

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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

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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.

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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%).

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