· (MW 254.05)
(reagent for the reduction of tosylhydrazones of aldehydes, ketones, and enones to the corresponding methylene derivatives1,2)
Physical Data: mp 166-168 °C
Solubility: sol chloroform, THF, and pentane; reacts readily with water and other protic solvents.
Form Supplied in: white crystalline solid
Analysis of Reagent Purity: the technical bulletin Quantitative Analysis of Active Boron Hydrides, available upon request from the Aldrich Chemical Company, Milwaukee, WI, USA, describes methodology to determine reagent purity by hydrogen gas evolution.
Preparative Methods: available via the controlled addition of borane-THF complex to benzoic acid at 0 °C.1a,b Preparation using borane-dimethyl sulfide complex involves much milder reaction conditions (25 °C).2
Handling, Storage, and Precautions: shows good stability that results from adduct formation with solvent (THF or DMS). Solutions of the reagent in THF have shown little decomposition after storage at 0 °C for 10 days.1b
Reduction of Tosylhydrazones of Aldehydes, Ketones, and a,b-Unsaturated Aldehydes and Ketones.
Bis(benzoyloxy)borane has been used for the reduction of tosylhydrazones to the corresponding methylene derivatives. This method offers a mild alternative to the classical Wolff-Kishner and Clemmensen reduction procedures which preclude the presence of sensitive functional groups.3 This reagent is comparable in reactivity to Catecholborane, which had previously been demonstrated to be a very efficient reagent for the reduction of tosylhydrazones.4 Regiospecific alkene migration occurs during the reduction of a,b-unsaturated derivatives,5 and allenes result from the reduction of alkynic substrates.6 Given the commercial availability of catecholborane, it is probably the reagent of choice for tosylhydrazone reduction. Representative tosylhydrazone reductions employing bis(benzoyloxy)borane
are given in eqs 1-4. The reactions are typically complete in 0.5 to 2 h. As with catecholborane a regiospecific and stereospecific rearrangement occurs in the reduction of a,b-unsaturated tosylhydrazones, permitting the preparation of thermodynamically unstable alkenes.5,6 The related Bis(trifluoroacetoxy)borane has been used in the reduction of indoles to indolines.7
- 1. (a) Kabalka, G. W.; Summers, S. T. JOC 1981, 46, 1217. (b) Brown, H. C.; Stocky, T. P. JACS 1977, 99, 8218. (c) Pelter, A.; Hutchings, M. G.; Levitt, T. E.; Smith, K.; JCS(D) 1970, 347.
- 2. For chemistry of additional acyloxyborane derivatives, see: (a) Brown, H. C.; Heim, P.; Yoon, N. M.; JACS 1970, 92, 1637. (c) Lane, C. F.; Myatt, H. L.; Daniels, J.; Hopps, H. B.; JOC 1974, 39, 3052. (c) Lane, C. F. CRV 1976, 76, 773. (d) Brown, H. C.; Subba Rao, B. C. JACS 1960, 82, 681.
- 3. (a) Reusch, W. In Reduction; Augustine, R. L., Ed.; Dekker: New York, 1968; pp 171-211. (b) House, H. O. Modern Synthetic Reactions, 2nd ed.; Benjamin: Menlo Park, CA, 1972; Chapter 4.
- 4. Kabalka, G. W.; Baker, J. D. JOC 1975, 40, 1834.
- 5. (a) Kabalka, G. W.; Yang, D. T. C.; Baker, J. D., Jr. JOC 1976, 41, 574. (b) Kabalka, G. W.; Hutchins, R.; Natale, N. R.; Yang, D. T. C.; Broach, V. OS 1979, 59, 42.
- 6. Kabalka, G. W.; Newton, R. J., Jr.; Chandler, J. H.; Yang, D. T. C. CC 1978, 726.
- 7. Maryanoff, B. E.; McComsey, D. F. JOC 1978, 43, 2733.
Michael S. VanNieuwenhze
The Scripps Research Institute, La Jolla, CA, USA
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