Bromine-Silver(I) Oxide


[7726-95-6]  · Br2  · Bromine-Silver(I) Oxide  · (MW 159.81) (Ag2O)

[20667-12-3]  · Ag2O  · Bromine-Silver(I) Oxide  · (MW 231.74)

(oxidant; used to form tetrahydrofurans from alcohols via insertion into unactivated C-H bonds)

Physical Data: none available; reagent generated in situ.

Solubility: used as suspension in pentane, CHCl3, CCl4, or THF; marginally sol aqueous media.

Form Supplied in: generated in situ from Bromine and Silver(I) Oxide.

Handling, Storage, and Precautions: reagent is generated in situ. Carrying out reaction in the dark is recommended by some authors, but is apparently unnecessary. Freshly prepared Ag2O6 (from Ag2NO3 and aq. NaOH followed by drying at 40-45 °C) is preferred to commercial samples.

Formation of Cyclic Ethers.

Remote functionalization of unactivated carbons using alcohol derivatives such as nitrites or hypohalites is well precedented.1 A related process can be effected directly using the free alcohols in the presence of silver(I) oxide and bromine at room temperature. For example, 1,3,3-trimethylcyclohexanol underwent conversion to the bicyclic ether in 75% yield (eq 1).2 The original report suggested that cyclization occurred via silver-assisted heterolytic cleavage of an intermediate hypobromite, with concomitant insertion of electrophilic oxygen into a d C-H bond. A homolytic mechanism was considered less likely because the reaction proceeded readily in the dark and was unaffected by radical inhibitors. However, subsequent studies provided evidence for the intermediacy of alkoxyl radicals in the form of characteristic decomposition products,3 racemization at the site of insertion,4 and stereochemical dependence on hydrogen vs. deuterium abstraction.5

Primary and secondary alcohols led to mixtures of tetrahydrofurans and ketones or aldehydes (eq 2).6 In some cases, yields were improved by carrying out the reaction in indirect daylight rather than in the dark. Cyclization of a benzylic alcohol to an ortho methyl group was found to be restricted to tertiary alcohols.7 In an examination of the reaction of two secondary alcohols with bromine and other silver(I) salts, it was noted that the ratio of THF to ketone was sensitive to the acidity of the medium, with the addition of 1 equiv of trifluoro- or trichloroacetic acid leading to nearly complete selectivity for the ketone.8 A more recent competition study has shown that the reaction is highly selective for methine carbons over methylenes or methyls (eq 3).9

The observation of an induction period and a sigmoid curve for appearance of cyclic ether led to the suggestion of a possible autocatalytic mechanism.10 In support of this, it was found that addition of 2 equiv of dimethyltetrahydrofuran eliminated the induction period and improved the yield of cyclization product. However, others have attributed this effect to the presence of peroxide impurities which could serve as initiators for a radical decomposition of the hypobromite intermediates.11

Overall, the fate of aliphatic alcohols in the presence of this reagent is analogous to that seen with Lead(IV) Acetate, although the yields of cyclic ethers typically appear to be higher. The preponderance of evidence seems to favor a homolytic mechanism. The likely role of the silver salts is to facilitate the cyclization of the intermediate d-bromo alcohols.11,12

1. (a) Breslow, R. COS 1991, 7, Chapter 1.3. (b) Rubottom, G. M. In Oxidation in Organic Chemistry, Part D, Trahanovsky, W. S., Ed.; Academic: New York, 1982, Chapter 1.
2. (a) Sneen, R. A.; Matheny, N. P. JACS 1964, 86, 3905. (b) Sneen, R. A.; Matheny, N. P. JACS 1964, 86, 5503.
3. Akhtar, M.; Hunt, P.; Dewhurst, P. B. JACS 1965, 87, 1807.
4. Smolinsky, G.; Feuer, B. I. JOC 1965, 30, 3216.
5. Green, M. M.; Moldowan, J. M.; McGrew, J. G., CC 1973, 451.
6. Mihailovic, M. Lj.; Cekovic, Z.; Stankovic, J. CC 1969, 981.
7. Baker, K. M.; Davis, B. R. T 1969, 25, 2487.
8. Roscher, N. M.; Jedziniak, E. J. TL 1973, 1049.
9. Roscher, N. M.; Shaffer, D. K. T 1984, 40, 2643.
10. (a) Deluzarche, A.; Maillard, A.; Rimmelin, P.; Schue, F.; Sommer, J. M. CC 1970, 976. (b) Deluzarche, A.; Rimmelin, P.; Sommer, J.-M. BSF 1973, 1810.
11. Boido, V.; Edwards, O. E. CJC 1971, 49, 2664.
12. Mihailovic, M. Lj.; Gojkovic, S.; Konstantinovic, S. T 1973, 29, 3675.

F. G. West & John A. Bender

University of Utah, Salt Lake City, UT, USA

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