Allenes, from 1,1-dihalocyclopropane

Since the synthesis of allenes from dihalocyclopropanes has been reviewed,130"132 only representative examples highlighting the process, along with the more recent results, are presented in this section. The formation of diallenes (equation 48) illustrates the usefulness of this process.133,134 Two other interesting examples are given in equations (49) and (50).133-137... 

Interestingly, the alkene to allene conversion can be carried out directly without isolation of the intermediate dihalocyclopropane. This process involves the treatment of the alkene with 1 equiv. of carbon tetrabromide and 2 equiv. of methyllithium in ether at -65 °C.163 Ultrasonic irradiation facilitates the formation of cyclopropylidenes, and therefore the allenes, from dihalocyclopropanes under the influence of Li, Na or Mg.1 The reactions are usually complete in 5-15 min. A report165 on the use of n-butyllithium complexed with the chiral tertiary amine (-)-sparteine, leading to optically active allenes, seems to be of questionable value. 

Moore, W. R., Ward, H. R. Formation of allenes from gem-dihalocyclopropanes by reaction with alkyllithium reagents. J. Org. Chem. 1962, 27,4179 181. 

When the carbene or carbenoid resulting from a dihalocyclopropane is unable to rearrange to the al-lene due to steric or other factors, insertion or addition reactions characteristic of carbenes take place. Thus dibromonorcarane on reaction with methyllithium gives a bicyclobutane derivative by insertion of the carbene into a 0-C—H bond (equation 57).178 Allene formation is sterically unfavorable in this case. Similarly, dibromotetramethylcyclopropane gives l,2,2-trimethylbicyclo[1.1.0]butane instead of tetra-methylallene (equation 58).179 181 An example involving a tricyclic dibromocyclopropane is given in equation (59).182... 

In general, tetrasubstituted dibromocyclopropanes lead to bicyclobutanes predominantly, whereas mixtures of bicyclobutanes and allenes result from trisubstituted dihalocyclopropanes and those bearing bulky substituents (see also Section 4.7.3.3). 

In general, allenes are prepared by the same types of reactions which lead to alkenes elimination of hydrogen halides, halogens and water from adjacent carbon atoms. However, some methods are characteristic of allene synthesis, e.g. the dehalogenation of gem-dihalocyclopropanes, rearrangement of alkynes and the 1,4-addition to vinylalkynes. 

DOERING-LA FLAMME Allene Synthesis Allene synthesis from olefins via gem-dihalocyclopropanes (see 1st edition) Br Br... 

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