F6lix Serratosa
University of Barcelona Barcelona, Spain
Basic Alumina: Source of Dihalocarbenes For organic laboratory experiments
N o training a t all and as little equipment as a chromatographic column is needed to prepare dihalocarbenes, as cyclopropane adducts, by means of basic alumina. Credit for basic alumina must be given to J. Castells and G . A. Fletcher (1) who described it for the first time in 1956. Since all its applications have come from a more or less direct personal contact with the authors, a brief survey on this subject will be given before describing the experiment. Although "chromatographic secondary reactions" have been observed by a great number of authors, (2) little advantage has been taken of them. However, since basic alumina appeared in the chemical literature, a few dramatic examples of its possibilities have been reported. The isolation of the very sensitive intermediate spiro[2,5] octa-1,4-dien-3-one (I) (9), and the synthesis of a fully conjugated decayne (11) (4), demonstrate the great selectivity of basic alumina (&03KOH) in dehydrohalogenation reactions, and its usefulness in either mechanistic or synthetic work.
We have found that chloroform and bromoform on basic alumina do give dihalocarbenes (111, X = C1 or Br), which have been trapped with cyclohexene. The technique is very simple: a 3 :1molar mixture of cyclohexene and haloform is adsorbed on a column of basic alumina and kept a t room temperature for 4 days. The column is then eluted with ether, the solvent and excess of reagents are removed in vacuo and the residue distilled to give 7,7-dihalobicyclo[4.1.0]heptane (IV, X = C1 or Br) (9) in moderate yields (1%150/, calculated on the base present in the alumina). However, the results are excellent when they are compared with those reported in aqueous solution of potassium hydroxide (less than 1%yield) (9), and give further evidence that dihalocarbenes react exclusively, or almost exclusively, with water and not with hydroxide ions, as pointed out by Robinson (10). The water used in the preparation of basic alumina is so strongly tied that solvolytic reactions are almost excluded, as shown in the above reported examples. Moreover, the reaction reported provides a unique direct confirmation of Hine's a-elimination mechanism (11) without having to resort to such an "exotic" base as potassium t-butoxide (9), Hine's original base (KOH) being used instead. Lileralura Cited
Acetylene compounds bearing alkali sensitive groupings (5) and perchlorocompounds (6) have also been successfully prepared by this method. Further examples are the double bond formation from tosylates (7). The increasing interest in carbenes (a), and the feelmg that rational use of basic alumina could become a powerful tool in isolating transient intermediates, prompted us to test the suitability of basic alumina as a source of dihalocarbenes.
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Journal of Chemical Educafion
(1) CASTELLS, J., AND FLETCHER, G. A,, J. C k m . Soe., 1956, 3245. E.,AND LEDERER, M., "Chromatographj," 2nd (2) LEDERER, D. 61. ed.. Eleevier Poblieation Co.. Amsterdam. 1957.. . (3) WINSTEIN, S., AND BNRD,R., J. Am. Chem. Soc., 79, 4238 (1957): 85.567 (1963). M. C., J . Chem. (4) JONES, E. R. H., LEE,H. H., AND WRITIN% SOC.,1960, 3483. (5) BEI,II,, C., CAST ELL^, J., CASTELLS,J., MESTRES,R., PASCUAL, J., AND SERRATOSA, F., Anales real sac. espaE. fis. y pub. (Madrid),57B, 619 (1961). (6) BALLESTER, M., AND CASTA~~ER, J., private communication. G.H., ELLINGTON, P. S., MEAKINS, G. D., AND (7) DOUGLAB, SWINDELLS, R., J. Chem. Sac. 1959, 1720; DJERASSI, C., R. D. H., AND VILLOTI, R., Pmc. Chem. Soe. MURRAY, D. H., J. Chem. 1961,450; DJERASSI, C., AND WILLIAMS, Soc., 1963,4046. (8) KIRMSE,W.,Angew. Chem., 73, 161 (1961); DE SELMS, R. C., "Organic Chemical Bulletin 34," No. 1, Research Laboratories of Eastman Kodak Company, . . 1962. (9) DOERING, W. VON E., AND HOFFMANN, A. I(.,J. Am. Chem. Sac., 76, 6162 (1964). E.A., J. Chem. Soe., 1961,1663. (10) ROBINSON, (11) HINE,J., J.Am. Chem. Soe., 72,2438 (1950).
