2-Pyridineselenenyl Bromide

[91491-61-1]  · C5H4BrNSe  · 2-Pyridineselenenyl Bromide  · (MW 236.96)

(selenoxide elimination to afford alkenes;1-4 episelenonium ions3,4)

Physical Data: mp 115-116 °C (dec).

Solubility: sparingly sol THF, dichloromethane, methanol.

Preparative Method: by reaction of 2,2-Dipyridyl Diselenide with Bromine in an appropriate solvent. An analytically pure sample is obtained by the filtration of the precipitate thus formed and washing. This preparation should not be carried out in Acetonitrile since 2-pyridineselenenyl bromide adds to acetonitrile.

Handling, Storage, and Precautions: use in a fume hood.

Selenoxide Elimination.1-4

For the dehydrogenation of ketones to enones, the introduction of a 2-pyridineselenenyl group on the a-carbon of the ketone and subsequent oxidation of the selenide to induce the selenoxide elimination is one of the most convenient procedures.1,2 2-Pyridineselenenyl bromide (or chloride) is a useful reagent for this sequence and three kinds of reaction have been reported for the introduction of a 2-pyridineselenenyl group onto the a-carbon of the ketones. They are: (1) the reaction of ketones with 2-pyridineselenenyl bromide (PySeBr) in the presence of Hydrochloric Acid (eq 1); (2) the reaction of the lithium enolate of the ketones with PySeBr (eq 2); and (3) the reaction of the enol silyl ether of the ketones (or aldehydes) with PySeBr (eq 3). The 2-pyridineselenenyl group has been reported to be a better leaving group than the benzeneselenenyl group. The 2-pyridineselenenyl group has also been used in selenoxide eliminations to afford vinylic amides4 and vinylic ethers.3

Vinylic Selenides.

2-Pyridineselenenyl bromide is an important starting material for the preparation of 2-pyridyl vinylic selenides from alkenes via b-methoxyalkyl 2-pyridyl selenides (eq 4).3 Deprotonation and subsequent alkylation of 2-pyridyl vinylic selenides proceed with retention of the stereochemistry, which has not yet been realized by the use of other vinylic selenides.3 It should also be noted that the reaction conditions for the deprotonation of 2-pyridyl vinylic selenides are much milder than those required for the corresponding phenyl vinylic selenides. This may be due to the chelation of the nitrogen atom of the pyridine with a lithium atom, as depicted in eq 5.

Episelenonium Ions.

b-Methoxyalkyl 2-pyridyl selenides have also been used as intermediates for the introduction of 2-pyridineselenenyl and acylamino groups into trisubstituted alkenes (eq 6).4 This reaction represents the preparation of tertiary amine derivatives. The use of the 2-pyridineselenenyl group is essential as phenyseleno-substituted tertiary carbocations are too stable and do not react with nitriles.

Related Reagents.

Benzeneselenenyl Bromide; Benzeneselenenyl Chloride; 2,2-Dipyridyl Diselenide; 2,2-Dipyridyl Disulfide.

1. Toshimitsu, A.; Owada, H.; Terao, K.; Uemura, S.; Okano, M. JOC 1984, 49, 3796.
2. Takagaki, H.; Abe, M.; Watanabe, M.; Takeuchi, K.; Nakanishi, S.; Aihara, H.; Yamazaki, K. Jpn. Patent 63 277 694 (CA 1988, 111, 23 792j).
3. Toshimitsu, A.; Owada, H.; Terao, K.; Uemura, S.; Okano, M. JCS(P1) 1985, 373.
4. Toshimitsu, A.; Hayashi, G.; Terao, K.; Uemura, S. JCS(P1) 1988, 2113.

Akio Toshimitsu

Kyoto University, Japan

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