Alkylidenecycloproparenes synthesis

The CH2 group of cycloproparenes is relatively acidic. Extended HUckel calculations predict that the benzocyclopropenyl anion 294 should be a resonance-stabilized species, contrary to the cyclopropenyl anion 295 or cyclohepta-trienide (296), which are at least potentially antiaromatic. This prediction has been experimentally verified benzocyclopropene (1) may be deprotonated with BuLi, and the intermediate anion 294 has been trapped with trimethylsilane to afford 236. From the rate of hydrolysis of 236, the pX of 1 has been estimated to 36, i.e., some 5 units below that of toluene (pXj = 41). Theoretical calculations (STO-3G) give a pK of 33 for 1. Metallation at the CHj group of cycloproparenes is the key step for the synthesis of alkylidenecycloproparenes (see above) however, it should be noted that so far, no benzocyclopropenyl anions have... 

The third method for alkylidenecycloproparene synthesis provides triafulvene derivatives by way of cycloproparenylidene dimerization166. The reaction is limited in scope by the availability of appropriate carbene (carbenoid) precursors and thus far has provided 98a,b only (equation 22). Attempts to extend the method into the naphthalene series167 by using the known89 naphthalene analogue of 97a provides a very sensitive (air/light) deep red material that is fluorescent and, while uncharacterized, is likely the analogue of 98a. 

The lithiation of 1 has many synthetic applications beyond alkylidenecycloproparene synthesis. Thus Szeimies and Wimmer232 have employed 6 for the synthesis of a range of derivatives in which the cyclopropabenzenyl framework is maintained (Scheme 14). 

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