· t-Butyl Hypoiodite
· (MW 200.02)
(reagent for radical and ionic iodinations)
Physical Data: orange-red solution, l
max 515 nm.
Solubility: sol CCl4 and benzene.
Form Supplied in: not commercially available.
Preparative Methods: the reagent has not been isolated in pure form and its precise nature is uncertain. Reagents considered to be
t-butyl hypoiodite have been commonly prepared by the reaction of t-Butyl Hypochlorite with either Iodine or metal iodide salts, or by the reaction of Potassium t-Butoxide with iodine. Tanner and co-workers have shown that the reagent generated by the first of these methods is quite different to that formed by the other two methods.1 Although the reagent formed by the latter two methods can be thought of as t-BuOI, the actual structure of the reagent is uncertain and there may be slight differences in the composition of different batches. This situation is amply illustrated for the reaction of t-BuOCl with AgI (eqs 1-4) The scheme adequately explains
the spectroscopic and molecular weight measurements of the product formed in this way, and also the liberation of varying amounts of iodine (eq 4).
Handling, Storage, and Precautions: reported to be very sensitive to moisture and is best generated in situ as required.
Barton and co-workers employed t-BuOI reagents, made by two of the above described methods, for a range of novel photochemical applications (eqs 5-7).2-4
Irradiation of a benzene solution of a carboxylic acid and the reagent derived from t-BuOK and iodine results in efficient decarboxylation to form the one-carbon shortened alkyl iodide (eq 5).2 This photochemical Hunsdiecker-type reaction presumably involves transformation of the starting carboxylic acid into the corresponding acyl hypoiodite, which then undergoes homolytic fission. This latter process is too facile to allow hydrogen atom abstraction by the carboxyl radical to compete. However, when amides are subjected to these reaction conditions, hydrogen atom abstraction can occur, resulting in a novel synthesis of lactones (eq 6).3 Similar remote functionalization is possible in steroid systems via alkoxyl radicals, generated using either the t-BuOCl-I2 or t-BuOK-I2 combinations (eq 7).4
Reaction of carboxylic acids with excess alkyl iodide, in the presence of iodine and t-BuOCl, in the dark can give good yields of esters (eq 8).5 Evidence points to this reaction proceeding via heterolytic SN1 or SN2 substitution, depending on the substrate structure.
Reagents derived from t-BuOCl and a metal iodide can be used to effect iodination of hydrocarbons and phenols (eqs 9 and 10).6,7 The selectivities displayed by the radical hydrocarbon iodination led Tanner and Gidley to propose a chain reaction involving an iodonyl radical as the hydrogen atom abstracting species.6
Addition reactions of t-BuOI (prepared from t-BuOCl and HgI2) to alkenes can be carried out under radical or ionic conditions (eqs 11 and 12) with regiocomplementary results.8 The additions catalyzed by BF3 etherate were examined in detail and found to be stereospecific, probably involving anti addition.
- 1. Tanner, D. D.; Gidley, G. C.; Das, N.; Rowe, J. E.; Potter, A. JACS 1984, 106, 5261.
- 2. Barton, D. H. R.; Faro, H. P.; Serebryakov, E. P.; Woolsey, N. F. JCS 1965, 2438.
- 3. Barton, D. H. R.; Beckwith, A. L. J.; Goosen, A. JCS 1965, 181.
- 4. Akhtar, M.; Barton, D. H. R. JACS 1964, 86, 1528.
- 5. Britten-Kelly, M. R.; Goosen, A.; Lovelock, J.; Scheffer, A. JCS(P1) 1977, 945.
- 6. Tanner, D. D.; Gidley, G. C. JACS 1968, 90, 808.
- 7. Kometani, T.; Watt, D. S.; Ji, T.; Fitz, T. JOC 1985, 50, 5384.
- 8. Heasley, V. L.; Berry, B. R.; Holmes, S. L.; Holstein III, L. S.; Milhoan, K. A.; Sauerbrey, A. M.; Teegarden, B. R.; Shellhamer, D. F.; Heasley, G. E. JOC 1988, 53, 198.
Nigel S. Simpkins
University of Nottingham, UK
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