Rearrangement dehydrohalogenation

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

Iminoboianes have been suggested as intermediates in the formation of compounds derived from the pyrolysis of azidoboranes (77). The intermediate is presumed to be a boryl-substituted nitrene, RR BN, which then rearranges to the amino iminoborane, neither of which has been isolated (78). Another approach to the synthesis of amino iminoboranes involves the dehydrohalogenation of mono- and bis(amino)halobotanes as shown in equation 21. Bulky alkah-metal amides, MNR, have been utilized successfully as the strong base,, in such a reaction scheme. Use of hthium-/i /f-butyl(ttimethylsilyl)amide yields an amine, DH, which is relatively volatile (76,79). 

Pyrolysis of carboxylic acids Dehydrohalogenation of acyl halides Dehalogenation of a-halo acyl halides Rearrangement of diazo ketones (Wolff)... 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

Instead of 13C labeling, 12C-enriched precursors with a 13C abundance of less than 1.1 % can be used for mechanistic studies. In this case, the spectra are more easily analyzed as no homonuclear carbon-carbon coupling occurs. An illustrative example is the base-catalyzed dehydrohalogenation of 7,7-dichloronorcarene to benzocyclopropene [407]. For this reaction, two pathways A and B have been proposed, mechanism B involving a skeletal rearrangement ... 

The intermediate adduct is generally stable enough so that spontaneous dehydrohalogenation does not take place. For example, the 3-(arylthio)-2-halonitriles which are obtained from thiols and a-chloro-acrylonitrile, form rearranged products only in the presence of dehydro-halogenating base (Birum and Heininger, 1957 Heininger and Birum, 1965). 

Many examples of double bond migration appear in the reactions of dihalocyclo-propanes with alkoxide ion, in which the halocyclopropene is a presumed intermediate. Thus dehydrohalogenation of 2,3-dialkyl-l,l-dichlorocyclopropanes provides a very simple route to methylenecyclopropanes, which often rearrange on heating,... 

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