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TEMPO is used as a catalyst in organic synthesis and for the oxidation of primary alcohols to aldehydes. It finds use in the chemical industry for conversion of bisnoralcohol (a steroid) to bisnoraldehyde. It acts as a free radical scavenger, as a mediator in controlled radical polymerization and as a structural probe in electron spin resonance spectroscopy. Further, it is involved in the preparation of (S)-(+)-2-methylbutanal from (S)-(-)-2-methyl-1-butanol.
For brief features on TEMPO and related reagents, see: Synlett, 563 (2001); 1757 (2003); 657 (2006). For reviews on nitroxide radicals, see: Synthesis, 190, 401 (1971); Chem. Rev., 78, 37 (1978); J. Sci. Ind. Res., 54, 623 (1995).
In the presence of a catalytic amount of KBr, catalyzes the selective oxidation of primary and secondary alcohols to aldehydes and ketones by buffered NaOCl: J. Org. Chem., 50, 4888 (1985); 52, 2559 (1987); Org. Synth. Coll., 8, 367 (1993). High yields of aldehydes can also be obtained under mild, phase-transfer conditions with Oxone®: Org. Lett., 2, 1173 (2000). In the presence of CuCl, aerobic oxidation of alcohols to aldehydes and ketones has been accomplished in the ionic liquid 1-n-Butyl-3-methylimidazolium hexafluorophosphate, L19086: Org. Lett., 4, 1507 (2002). With NaOCl, ɑ-amino or ɑ-alkoxy alcohols have also been oxidized to the aldehydes: Tetrahedron Lett., 33, 5029 (1992). The use of I2 as cooxidant is useful for sensitive substrates: Org. Lett., 5, 235 (2003).
The addition of quaternary salts to the reaction mixture permits further oxidation of aldehydes to acids. Selective oxidation of a primary OH to an aldehyde can be achieved in the presence of a secondary OH: J. Org. Chem., 54, 2970 (1989); Tetrahedron Lett., 31, 2177 (1990). The oxidation can also be performed using NCS under phase-transfer conditions: J. Org. Chem., 61, 7452 (1996), with Iodosobenzene diacetate, B24531: J. Org. Chem., 62, 6974 (1997), or Trichloroisocyanuric acid, B23906: Org. Lett., 3, 3041 (2001).
For a review of the use of stable nitroxyl radicals for the oxidation of primary and secondary alcohols, see: Synthesis, 1153 (1996).
Chernick, E. T.; Casillas, R.; Zirzlmeier, J.; Gardner, D. M.; Gruber, M.; Kropp, H.; Meyer, K.; Wasielewski, M. R.; Guldi, D. M.; Tykwinski, R. R. Pentacene Appended to a TEMPO Stable Free Radical: The Effect of Magnetic Exchange Coupling on Photoexcited Pentacene. J. Am. Chem. Soc. 2015, 137 (2), 857-863.
Zhu, X.; Chiba, S. TEMPO-mediated allylic C-H amination with hydrazones. Org. Biomol. Chem. 2014, 12 (26), 4567-4570.
Gefahrenhinweise (EU): H314-H318
Causes severe skin burns and eye damage. Causes serious eye damage.
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