Benzocyclopropenyl anion

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... 

Electron photodetachment spectra for indenyl and fluorenyl anions have been measured and the electron affinities of the corresponding radical and bond dissociation energies of the corresponding neutrals determined. Comparison has been made with solution-phase data in an attempt to determine the dependence of gas and solution properties on ionic size and extent of charge distribution.3 Benzocyclopropenyl anion... 

The influence of the lone pair(s) on the bond fixation is studied in a series of heteroanalogs of cyclopropabenzene including benzocyclopropenyl anion (Fig. 14). Relevant... 

Figure 14. Schematic representation of heteroanalogs of cyclopropabenzene involving also benzocyclopropenyl anion.

Comparison of the C-H coupling constants for benzocyclopropene (24) (178 + 2 Hz ) and 1-methylcyclopropene (172 Hz ) provides little insight into the enhanced acidity of 24. On the other hand, STO-3G calculations of the energy difference between parent hydrocarbons and their respective anions led to predicted pXa values of 33,41 and 60 for 24, toluene, and cyclopropene, respectively. The source of the remarkable stability for the benzocyclopropenyl anion appeared to be associated with polarization of the n-electron system as represented in structure 28 together with concomitant a-induction of negative charge to C(l) and C(6) from the carbanion center (C(7)). Therefore, charge... 

Deprotonation of benzocyclopropene (17) gave the benzocyclopropenyl anion (17a) in the gas phase (eq 10), and the measured acidity A/7°acid of 386 3 kcal/mol was 34.5 kcal/mol more favorable than for the allylic position of cyclopropene. The greater... 

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