Synthesis via dihalocyclopropanes

Dehalogenation of dihalocyclopropanes with sodium or magnesium gives allenes (equation 47).147 Since the original observation, a number of reagents have been introduced for the allene synthesis via de-... 

It is abundantly clear from the preceding discussion that dihalocyclopropanes are versatile intermediates in organic synthesis. Although a wealth of chemistry has already been uncovered, prospects remain bright for interesting developments in the future. Areas such as the application of dihalocyclopropanes in heterocyclic synthesis via carbene insertion into C—H bonds adjacent to heteroatoms, reactions of dihalocyclopropanes with organometallics and the synthetic applications of metallated derivatives deserve further exploration. The chemistry of difluoro-, diiodo- and mixed dihalo-cyclopropanes can be expected to attract some attention. Finally, other heteroatom-substituted cyclopropanes derived ftom dihalocyclopropanes will also invoke further investigation. 

The dihalocyclopropane route is the method of choice for the synthesis of benzocyclopropenes and linearly fused cyclopropanaphthalenes, but is unsuccessful for angular cyclopropa[u]naph-thalene. Treatment of l,l-dichloro-la,2,3,7b-tetrahydro-l/f-cyclopropa[a]naphthalene(5) with potassium /ert-butoxide gave a mixture of 1-chloromethylnaphthalene, 1-terr-butoxymethyl-naphthalene and 2-terf-butoxy-l-chloromethylene-l,2,3,4-tetrahydronaphthalene, but none of the expected cyclopropa[n]naphthalene. The failure of the reaction has been attributed to difficulties in the aromatization. Isomerization of the initially produced cyclopropenyl double bond into the cyclohexane ring would require disruption of the aromatic character of the adjacent benzene nngT t-r>> 62 Similarly, attempted aromatization of l-chloro-l,2,3,7b-tetra-hydro-l//-cyclopropa[a]naphthalene (6) with 2,3-dichloro-5,6-dicyanobenzoquinone or via N-bromosuccinimide bromination followed by reaction with base afforded 4,5-benzotropone instead of cyclopropa[fl]naphthalenc. ... 

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

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