(R = H)
· Benzeneseleninic Acid
· (MW 189.07)
(R = Se(O)Ph) (anhydride)
· Benzeneseleninic Anhydride
· (MW 360.13)
(oxidizing agents of many functional groups1)
Physical Data: acid mp 121 °C; anhydride mp 164-165 °C.
Solubility: both compounds sol chloroform, dichloromethane, THF; the acid is amphoteric and sol in water at low and high pH.
Form Supplied in: both compounds are commercially available white solids.
Purification: the anhydride is readily hydrolyzed and may contain substantial amounts of the acid. It can be regenerated from the latter by heating at 120-130 °C under vacuum (ca. 0.1 Torr) for a few hours.
Handling, Storage, and Precautions: the acid is stable to prolonged storage on the shelf; the anhydride should be kept in a desiccator. Both compounds are toxic and can react vigorously with reducing agents; the acid forms an explosive complex with hydrogen peroxide. Use in a fume hood.
Benzeneseleninic anhydride (BSA) is the more commonly used reagent and benzeneseleninic acid probably functions in at least some cases by generating the anhydride in situ. Excess reagent can be removed at the end of a reaction by washing with aqueous K2CO3 or KOH. Diphenyl Diselenide is the usual byproduct and often requires chromatographic separation.
Oxidation of Alcohols and Phenols.
Primary benzylic alcohols are oxidized to the corresponding aldehydes with BSA in refluxing benzene, chlorobenzene, or THF, while various secondary alcohols afford ketones under similar conditions (eq 1).2 In principle, it is possible to oxidize 3 mol of the alcohol with 1 mol of the anhydride, but in practice an excess of the latter is generally more efficacious. The reagent can also be generated in situ from a catalytic amount of diphenyl diselenide and t-Butyl Hydroperoxide.2c The method is relatively mild and tolerates other easily oxidized groups such as selenide and sulfide. Under more forcing conditions, the product sometimes undergoes dehydrogenation (see below).
Phenols are typically oxidized to o-quinones with the anhydride and to p-quinones with the acid, often with high regioselectivity and in good yield (eq 2).3 However, the formation of hydroxydienones, or of coupled products, dominates in some cases (eq 3).4 When the reaction is performed with BSA in the presence of Hexamethyldisilazane, the corresponding N-phenylselenoimine is generally obtained, again with high ortho selectivity (eq 4). This provides a means for aromatic amination when used in conjunction with reduction of the imine.5
Dehydrogenation of Carbonyl Compounds.
Ketones, particularly of the steroid or terpenoid variety, produce enones or dienones6 when heated with the anhydride in chlorobenzene or diglyme at 95-132 °C The method may be used in conjunction with eq 1 to transform alcohols directly to dienones in one step. A further refinement permits the use of catalytic amounts of the anhydride in the presence of a co-oxidant such as iodoxybenzene or m-iodoxybenzoic acid (eq 5), which recycles the byproduct diphenyl diselenide.7 Lactams2b and lactones8 undergo similar dehydrogenations (eq 6), although the former compounds sometimes produce imides2b instead.
Oxidation of Amines, Hydrazines, and Hydrazones.
Primary amines afford ketones when oxidized with the anhydride under mild conditions (eq 7).9 Synthetically useful yields are generally limited to non-enolizable ketones, however. Enamides are oxidized to carbinolamides and other hydroxy or keto derivatives,10 and indolines produce indoles under similar conditions.11 Benzeneseleninic acid or anhydride react with Hydrazine to generate Diimide,12 whereas various types of 1,2- and 1,1-disubstituted derivatives produce azo compounds12,13 and tetrazenes,14 respectively (eqs 8 and 9), at or below rt. This procedure can also be employed to generate the reactive dienophile N-phenyl-1,2,4-triazoline-3,5-dione
from N-phenylurazole,15 and to oxidize hydroxylamines to nitroso compounds.13 N-Acylhydrazines (hydrazides) and N-alkyl- or N-arylsulfonylhydrazines (sulfonhydrazides) are converted into selenoesters12 and selenosulfonates,16 respectively, when treated with the seleninic acid under mild conditions (eqs 10 and 11).
BSA is a useful reagent for the regeneration of ketones from their hydrazones, oximes, or semicarbazones (eq 12).13 Phenyl-, p-nitrophenyl- and tosylhydrazones react rapidly in warm THF, whereas the 2,4-dinitrophenyl and N,N-dimethyl derivatives are considerably more resistant. Aldehydes are similarly obtained from their tosylhydrazones or oximes, but not from phenyl- or p-nitrophenylhydrazones, which afford N-acylazo products instead.
Oxidation of 1,3-Dithiolanes and Thiocarbonyl Compounds.
BSA is also an effective reagent for the deprotection of aldehydes and ketones from their 1,3-dithiolane derivatives (eq 13).17 The reaction proceeds at or near rt, is applicable to sterically hindered systems, and often gives cleaner results than conventional hydrolytic methods. Moreover, it can be employed under anhydrous conditions, an advantage if other moisture-sensitive groups are present. The similar deprotection of 1,3-dithianes and 1,3-oxathiolanes is, however, generally less efficacious, while selenoacetals afford the parent aldehydes and ketones in typically high yields.18 The conversion of thiones, xanthates, thionoesters, trithiocarbonates, thioureas,
and other thio- and selenocarbonyls into the corresponding carbonyl derivatives can also be accomplished with BSA at rt (eq 14),19 although enolizable thiones afford more complex products.
Benzylic hydrocarbons are oxidized to aldehydes in modest to good yields by benzeneseleninic anhydride at elevated temperatures (eq 15).20 Overoxidation to carboxylic acids and the competing formation of phenylselenenylated derivatives often limits the utility of the method. The comproportionation of benzeneseleninic anhydride with diphenyl diselenide generates electrophilic SeII species in situ. The mixture can thus be employed in the transformation of alkenes to a-phenylseleno ketones or aldehydes.21 Terminal alkenes afford chiefly the 1-phenylseleno-2-ketone regioisomers (eq 16), whereas the opposite regiochemistry is favored with ethers or esters of allylic alcohols (eq 17). Similarly, the comproportionation
of diphenyl diselenide and benzeneseleninic acid,22 or the reduction of the latter with Hypophosphorous Acid,23 in the presence of alkenes results in the addition of
PhSeOH to produce b-hydroxy selenides. a-Hydroxylations of polycyclic ketones occur at an adjacent ring junction position when these compounds are heated in refluxing toluene or chlorobenzene with BSA (eq 18).24 These reactions are, however, sluggish and often produce only modest yields.
Benzeneseleninic acid reacts with Hydrogen Peroxide to produce the corresponding perseleninic acid in situ. The latter is in turn a convenient reagent for the oxidation of sulfides to sulfones, aldehydes to carboxylic acids, alkenes to epoxides, and in Baeyer-Villiger reactions (see Benzeneperoxyseleninic Acid).
- 1. Ley, S. V. In Organoselenium Chemistry; Liotta, D., Ed.; Wiley: New York, 1987; Chapter 3.
- 2. (a) Barton, D. H. R.; Brewster, A. G.; Hui, R. A. H. F.; Lester, D. J.; Ley, S. V.; Back, T. G. CC 1978, 952. (b) Back, T. G. JOC 1981, 46, 1442. (c) Shimizu, M.; Kuwajima, I. TL 1979, 2801.
- 3. (a) Barton, D. H. R.; Brewster, A. G.; Ley, S. V.; Read, C. M.; Rosenfeld, M. N. JCS(P1) 1981, 1473. (b) Barton, D. H. R.; Finet, J.-P.; Thomas, M. T 1988, 44, 6397.
- 4. Barton, D. H. R.; Ley, S. V.; Magnus, P. D.; Rosenfeld, M. N. JCS(P1) 1977, 567.
- 5. (a) Barton, D. H. R.; Brewster, A. G.; Ley, S. V.; Rosenfeld, M. N. CC 1977, 147. (b) Holker, J. S. E.; O'Brien, E.; Park, B. K. JCS(P1) 1982, 1915.
- 6. Barton, D. H. R.; Lester, D. J.; Ley, S. V. JCS(P1) 1980, 2209.
- 7. Barton, D. H. R.; Godfrey, C. R. A.; Morzycki, J. W.; Motherwell, W. B.; Ley, S. V. JCS(P1) 1982, 1947.
- 8. Barton, D. H. R.; Hui, R. A. H. F.; Ley, S. V.; Williams, D. J. JCS(P1) 1982, 1919.
- 9. (a) Czarny, M. R. CC 1976, 81. (b) Barton, D. H. R.; Lusinchi, X.; Milliet, P. T 1985, 41, 4727.
- 10. (a) Back, T. G.; Ibrahim, N.; McPhee, D. J. JOC 1982, 47, 3283. (b) Magnus, P; Ladlow, M.; Cairns, P. M. TL 1987, 28, 3307.
- 11. Ninomiya, I.; Hashimoto, C.; Kiguchi, T.; Naito, T.; Barton, D. H. R.; Lusinchi, X.; Milliet, P. JCS(P1) 1990, 707.
- 12. Back, T. G.; Collins, S.; Kerr, R. G. JOC 1981, 46, 1564.
- 13. Barton, D. H. R.; Lester, D. J.; Ley, S. V. JCS(P1) 1980, 1212.
- 14. Back, T. G.; Kerr, R. G. CJC 1982, 60, 2711.
- 15. Barton, D. H. R.; Lusinchi, X.; Ramírez, J. S. TL 1983, 24, 2995.
- 16. Back, T. G.; Collins, S.; Krishna, M. V. CJC 1987, 65, 38.
- 17. (a) Cussans, N. J.; Ley, S. V.; Barton, D. H. R. JCS(P1) 1980, 1654. (b) Barton, D. H. R.; Bielska, M. T.; Cardoso, J. M.; Cussans, N. J.; Ley, S. V. JCS(P1) 1981, 1840.
- 18. Burton, A.; Hevesi, L.; Dumont, W.; Cravador, A.; Krief, A. S 1979, 877.
- 19. Cussans, N. J.; Ley, S. V.; Barton, D. H. R. JCS(P1) 1980, 1650.
- 20. Barton, D. H. R.; Hui, R. A. H. F.; Ley, S. V. JCS(P1) 1982, 2179.
- 21. (a) Shimizu, M.; Kuwajima, I. BCJ 1981, 54, 3100. (b) Shimizu, M.; Takeda, R.; Kuwajima, I. BCJ 1981, 54, 3510.
- 22. (a) Reich, H. J.; Wollowitz, S.; Trend, J. E.; Chow, F.; Wendelborn, D. F. JOC 1978, 43, 1697. (b) Hori, T.; Sharpless, K. B. JOC 1978, 43, 1689.
- 23. Labar, D.; Krief, A.; Hevesi, L. TL 1978, 3967.
- 24. Yamakawa, K.; Satoh, T.; Ohba, N.; Sakaguchi, R.; Takita, S.; Tamura, N. T 1981, 37, 473.
Thomas G. Back
University of Calgary, Canada
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