Cyclopropylidene-allene rearrangement

The Doering-Moore-Skattebol method including a cyclopropylidene-allene rearrangement is often used for the synthesis of allenes. However, the reaction conditions applied are often not compatible with acceptor substituents. One of the rare exceptions is the transformation 76 —> 77 (Scheme 7.11) [122]. The oximes 77 are not accessible by the classical route starting from allenyl ketone and hydroxylamine (see Section 7.3.2). 

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. 

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. 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

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. 

Cyclopropylidene reaction Neither the barrier nor the equilibrium constant for the cyclopro-pylidene/allene reaction have been measured. The only direct experimental information of these species come from the failure to observe cyclopropylidene at 77 K (Chapman OL (1974) Pure and Applied Chemistry 40 511). This and other experiments (references in Bettinger HF, Schleyer PvR, Schreiner PR, Schaefer HF (1997) J Org Chem 62 9267 and in Bettinger HF, Schreiner PR, Schleyer PvR, Schaefer HF (1996) J Phys Chem 100 16147) show that the carbene is much higher in energy than allene and rearranges very rapidly to the... 

The temperature and solvent dependence for allene production is seen in the buta-1,3-dienyl)cyclopropylidene rearrangement carried out under three different sets of reaction conditions, e.g. reaction with 16, 17 and 18 as substrates. ... 

The diazotization route is frequently accompanied by products derived from solvolysis of the initially formed cyclopropylidene or the rearranged cyclopentenylidene (Skattebol rearrangement) in the case of vinylcyclopropylidene with alcohol solvent, although allenes still account for the major products in the case of vrc-disubstituted cyclopropylidenes. It is noteworthy that the stereochemistry of the ring substituents (Table 2, entries 2 and 3) is an important factor in affecting yields of allenes. gcw-Disubstitution of vinylcyclopropanes diminishes formation of allenes in favor of products from the Skattebol rearrangement (entry 6). 

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. ° ... 

The reaction of a 1,1-dibromocyclopropane with an alkyllithium generally leads to 1,1-elimination producing a cyclopropylidene, which normally rearranges to an allene, but may be trapped by other carbenic reactions. When there is a silyl substituent on C2, the carbene is apparently trapped by a 1,2-silicon shift giving a silyl-substituted cyclopropene. ... 

The photolytic conversion of 3,3-dimethyl-l-methylsulfanyl-2-trimethy]silylcyclopropene into 3-methyl-l-methylsulfanyl-l-trimethylsilylbuta-1,2-diene (2c) has been suggested to occur either through a vinylcarbene or a cyclopropylidene. However, a computational study of the rearrangement of tetrakis(trimethylsilyl)cyclopropene (3) to the corresponding allene 2j indicates that tetrakis(trimethylsilyl)cyclopropylidene (4) is not an intermediate. ... 

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. 

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