Cyclopropylidene to allene

Occasionally the cyclopropylidene to allene isomerization cannot take place for structural reasons. If, for example, the expected allene would be very highly strained, as is the case for certain cyclic allenes, then the reaction is forced to follow an alternative path. A case in point is provided by 1-alkyl-7,7-dibromonorcaranes which undergo a carbene insertion reaction when treated with methyl lithium to provide bicyclobutanes rather than allene derivatives. 

Interestingly, no allenic products are formed during this reaction. The two oxygen atoms seem to prevent a competing cyclopropylidene to allene isomerization, at least at the reported reaction temperature of —75 °C [126],... 

The generation and chemistry of cyclopropylidene has been extensively reviewed Of the unimolecular processes, the conversion of cyclopropylidene to allene represents a synthetically important type of reaction. However, this transformation is accompanied by competitive intra-and intermolecular processes such as C-H insertions and spiro[2.2]pentane formation from insertion to alkenes. Furthermore, vinylcyclopropylidenes undergo a concerted ring expansion to cyclopentenylidene (Skattebol rearrangement). Although cyclopropylidene to allene rearrangements can occur at temperatures below — 50 C, activation enthalpies of competing processes may be lower and therefore temperature effects can play an important role in the maximization of yields. 

While ab initio calculations of reaction paths for polyvalent atoms are still scarce, the usefulness of semiempirical calculations has recendy been demonstrated. Dewar employed his MINDO/2 molecular orbital method to examine the addition of carbon atoms to ethylene and trans-2-butene (75). An unrealistically large activation energy was foimd for rearrangement of the intermediate cyclopropylidene to allene. 

Cyclopropylidenes undergo ring opening to give allenes. Reactions that would be expected to generate a cyclopropylidene therefore lead to allene, often in preparatively useful yields. 

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. 

Cyclopropylidene has apparently never been isolated [225], so its halflife is likely to be short even well below room temperature. Employing a variety of methods, Bettinger et al. obtained a barrier for its rearrangement to allene of about 4 kcal mol-1, i. e ca. 17 kJ mol-1 [226], not far from our values of 18-24 kJ mol-1. 

Not all electrocyclic reactions are stereoselective. It turns out that none of the three of the possible interconversions between triplet cyclopropylidene and allene should show SS, according to an analysis given by Borden (1967). 

De Meijere, A., Faber, D., Heinecke, U., Walsh, R., Muller, T., Apeloig, Y. On the question of cyclopropylidene intermediates in cyclopropene-to-allene rearrangements - tetrakis(trimethylsilyl)cyclopropene, 3-alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes, and related model compounds. Eur. J. Org. Chem. 2001,663-680. 

Extended conjugated cyclopropylidenes, e.g. 123 and 126, rearrange to conjugated allenes with competing rearrangement to vinylcyclopentadienes. An additional competing process is the intramolecular insertion of cyclopropylidene to a double bond with formation of spiro[2.2]pentanes, e.g. formation of 131. ° ... 

Cyclopropylidene complexes have been reported to undergo ring opening to allene or allyl complexes. ... 

On the other hand, the MINDO/2 method had predicted that cyclopro-pylidene forms cyclopropene much faster than allene, but this is in disagreement with the experimental observation that cyclopropylidenes do open very efficiently to allenes. ... 

Certain types of carbenes have special modes of stabilization available to them. Cyclopropylidenes open to allenes ... 

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