1.2- Dibromo-ethane elimination reactions

Unfortunately yields are low when this reaction is extended to 1,2-dibromo-ethane, possibly because of steric problems and/or elimination of HBr. 

Ynamines are obtained by thiol elimination from ketene 5, A -acetals (1-aIkylthio-l-dialkylaminoalkenes) in 40-50% yields on treatment with LiNEt, at 20 °C or with NaNHo in boiling piperidine, or by leading them over solid NaNH.j at 150-165 °C. In the first two procedures the formed ynamines are fractionally distilled from the reaction mixture (equation 83) . When elimination is effected with KNHj in HMPT, aqueous work-up leads to the hydration of the ynamine. Therefore, 1,2-dibromo-ethane is added to the reaction mixture. It functions as a proton donor for the... 

Thus a support-bound benzylpiperazine (282) was allowed to undergo a Man-nich reaction with formaldehyde and a-keto-aldehyde hydrazones. Upon addition of 1,2-dibromo-ethane (388) the support-bound piperazine tether is N, N -dialky-lated to give a quinuclidinium species (389). Subsequent Hoffmann elimination released a diazaalkadiene (390) from the polymeric support, allowing it to react with isonitriles to give pyrazoles (395). With electron-rich dienophiles, HDA reactions were also reported to have been performed to yield compound (393) (Scheme 80). 

In this, as in many other cases in aqueous solution, OH" plays the role of the base. Note that for compounds such as 1,1,2,2-tetrachloroethane and pentachloroethane, the base catalyzed reaction is important at quite low pH values (/NB = 4.5, i.e., pH at which the neutral and base catalyzed reaction are equally important, see Table 13.7 and Section 13.3). In fact, for polyhalogenated alkanes a small7NB value (e.g., <7) is indicative of an E2 reaction, or, in special cases, of an E1CB reaction see below. Some other examples of compounds reacting by an E2-mechanism include 1,1,2-trichloro-ethane, 1,1,2-tribromoethane, and l,2-dibromo-3-chloroethane (see Table 13.7). A high /NB value (e.g., >10) does not, however, necessarily exclude ( elimination, because this reaction may also occur with water as base, or by an alternative to the SN1 mechanism (i.e., an El mechanism, see below). 

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