Bromo olefin elimination from

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

This procedure illustrates a recently published, simple, general method for the synthesis of conjugated dienes from olefins. The scope of the reaction is shown in Table I.5 In most of these examples hydrogen bromide. .elimination can be effected by stirring a solution of the olefin-bromo-methanesulfonyl bromide adduct in methylene chloride with one equivalent of triethylamine at room temperature. Only two equivalents of the more costly potassium tert-butoxide are then needed in the second elimination step the yields using the two-base procedure are generally superior to that obtained using only potassium tert-butoxide. 

The elimination of a molecule of halogen acid from halo acetals of acetaldehyde with powdered potassium hydroxide gives ketene acetals. However, the ti-bromo acetals of the homologs of acetaldehyde on similar treatment with potassium hydroxide or potassium 1-butoxide are converted into ctyjS-olefinic acetals. ... 

Relative reactivity of olefins with bromophenylcarbene generated either (1) by irradiation of 3-bromo-3-phenyldiazirine or (2) from base-induced a-elimination of a,a-dibromotoluene in the presence and in the absence of 18-crown-6 showed that addition of 18-crown-6 (via the complex of carbene with KCl/ KO-t-butanolate) as well as photolysis of diazirine afforded the free carbene. a-Ehmination normally proceeds via carbenoids (Scheme 3). 

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