Apparent alpha-eliminations

There is ample scope for further investigations of the mechanism of beta-eliminations yielding imines and nitriles. Some of the base-catalysed reactions may be examples of the carbanion mechanism and hydrogen isotope studies and the application of the Bronsted catalysis law to rates of elimination under various reaction conditions could prove informative. 

Earlier in this chapter, (Section 2.1.4) the possibility that some beta-eliminations might proceed by a base-catalysed alpha-elimination followed by hydrogen migration was considered. In this section, evidence is presented to demonstrate that a number of apparent alpha-eliminations do not actually involve carbenoid intermediates. Earlier reviews are available on this aspect of elimination - . 

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The initial product of elimination is a highly electron-deficient species, usually a carbene or a nitrene, which subsequently reacts rapidly to give products. The base-catalysed hydrolysis of chloroform is an alpha-elimination . The properties of the electron-deficient species represent a rapidly expanding field of organic chemistry and this subject is discussed in the following chapter. In this chapter, only apparent alpha-eliminations, (e.g. reaction (5)), are included. The nature of the products indicates the intermediacy of electron-deficient species, but kinetic studies show the processes occur via reactions of the precursor carbanions, viz. 

Possibly the measurement of the chlorine isotope effect for a series of suitably substituted substrates may help to decide whether or not greater stereospecificity is associated with decreased carbanion character of the transition state for elimination from alkenes. Of course, such studies must be accompanied by labelling experiments to exclude the possibility that alkynes arise by an apparent alpha-elimination (Section 5.1) involving formation of a carbanion at the alpha carbon, which subsequently eliminates alpha halogen simultaneously with migration of a substituent from the beta carbon. Results with alcoholic media, however, tend to indicate this alternative mechanism is unimportant if a normal beta-elimination is possible. 

An apparent beta-elimination to give an olefin can occur by an alpha-elimination followed by hydrogen migration, viz. 

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