Dihalocyclopropanes, synthesis

Shortly after discovery of a convenient dihalocyclopropane synthesis their conversion to allenes was effected This area has been reviewed very recently ", and therefore only some very new examples will be mentioned Employing a repetitive cyclopropanation/allene formation sequence, 1,2,3-cyclononatriene—presumably the smallest isolable cyclic butatriene—has become available (equation 120). ... 

Treatment of geminal dihalocyclopropyl compounds with a strong base such as butyl lithium has been for several years the most versatile method for cumulenes. The dihalo compounds are easily obtained by addition of dihalocarbenes to double--bond systems If the dihalocyclopropanes are reacted at low temperatures with alkyllithium, a cyclopropane carbenoid is formed, which in general decomposes above -40 to -50°C to afford the cumulene. Although at present a number of alternative methods are available , the above-mentioned synthesis is the only suitable one for cyclic cumulenes [e.g. 1,2-cyclononadiene and 1,2,3-cyclodecatriene] and substituted non-cyclic cumulenes [e.g. (CH3)2C=C=C=C(CH3)2]. 

It is not unreasonable to expect preparatively useful photoreactions of appropriately functionalized dihalocyclopropanes also. Although the present trend in gew-dihalocyclopropane chemistry clearly favors the use of more highly functionalized substrates, the traditional role — mentioned in the introduction — of employing these strained molecules in hydrocarbon synthesis, is by no means an approach of the past. This point is stressed by the recent synthesis of octavalene... 

Dihalocyclopropanes are generally prepared by the addition of dihalocarbenes to alkenic substrates. As indicated in the introduction, the first synthesis of a dihalocyclopropane was accomplished by Doering and Hoffmann by the addition of dichlorocarbene, generated from chloroform and potassium r-butoxide (Bu OK), to cyclohexene giving dichloronorcarane (1), as shown in equation (l).s... 

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-... 

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... 

The conversion of dihalocyclopropane to allene can be extended to the synthesis of cumulenes by repeating the sequence, as illustrated by the synthesis of 1,2,3-cyclononatriene (Scheme 7).161... 

It should be pointed out that while the dihalocyclopropane-allene conversion discussed here represents the best general synthesis of allenes, occasional problems due to the formation of side products have been reported. An example is the formation of the bicyclobutane (16) along with the expected allene (equation 53).166 The method is especially unsuitable for the synthesis of tetrasubstituted allenes. Steri-cally demanding substituents on the dihalocyclopropanes are believed to be responsible for the change of course of the reaction. 

The allyl cation generated by the electrocyclic cleavage of dibromocyclopropanes (cf. Section 4.7.3.7.1) has been trapped by a carboxyl group in a highly efficient synthesis of furanones (equation 111) and pyranones.236 Acid-catalyzed openings of dihalocyclopropanes also give similar results.237... 

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. 

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