Cycloproparenyl cations

Insofar as electron impact mass spectrometry is concerned, the cycloproparenes almost always display a molecular ion. The primary source of fragmentation is by loss of a C(1) substituent radical to provide a cycloproparenyl cation. However, labelling studies have shown that loss of H from 1 and 11 (at least) occurs only after complete scrambling of the carbon atoms216217. The use of appearance energy measurements218 for the loss of Br from ionized 2-bromocyclopropabenzene (57a), when coupled with thermochemical data, have led to the prediction that cation 110 is more stable than the phenyl cation by at least 27.6 kcal mol"1 AHf of 110 is estimated at 311 kcalmol"1. 

The parent ion 5 and its naphtho[6] analogue have been generated by treatment of 1 and 11, respectively, with trityl tetrafluoroborate. The cations so formed are captured by water and the final product of reaction is the ring opened aryl aldehyde (equation 24)230 231. Kinetic measurements have shown230 that abstraction of hydride ion from 1 is first order and displays a deuterium isotope effect of 6.5. To the best of the author s knowledge, the isolation and spectroscopic characterization of an unsubstituted cycloproparenyl cation has yet to be achieved. 

The interaction of ge/w-dihalocycloproparenes at C(ll with nucleophiles is consistent with ionization to a cycloproparenyl cation, cf. 5, and subsequent capture of the nucleophile to give products of SN1 substitution at the sp3 centre as illustrated by equation 23 (Section IV. A). There are no known examples of nucleophilic aromatic substitution among the cycloproparenes because of an absence of appropriately functionalized compounds. 

Although a C(i) hydride can be removed from 1 and 11 by the trityl cation and the ensuing cycloproparenyl cation captured with a suitable nucleophile (equation 24, Section IV. A), electrophilic aromatic substitution of the cycloproparenes is frustrated by reactions which result in opening of the three-membered ring (Section V.B. 1). It is only by sterically blocking this part of the molecule that three-membered ring opening is prevented and reaction with the arene component observed. 

The C-resonance lines of the cycloproparenyl cations are shifted by ca. 10 ppm downfield in comparison to those of the corresponding dichloro derivatives, except for the resonance line of C2, C5 (benzocyclopropenes) and C2, C7 (cyclopropa[Z ]naphthalenes), which are insensitive to the ionization process and appear only ca. 2 ppm downfield. C-Chemical shift/ charge density correlations for the 1-fluorobenzocyclopropenylium ion have been attempted. A poor agreement was obtained when the simple Spiesecke and Schneider " relationship was applied. The quality of the correlation improved, however, when paramagnetic shift corrections were included. 

The generation of the parent benzocyclopropenylium and cyclopropa[6]naphthalenylium ions via hydride abstraction from benzocyclopropene (3a) and 1 //-cyclopropa[(j]naphthalene (3b)," respectively, has been attempted with triphenylmethyl fluoroborate, but only small quantities of the corresponding aromatic aldehydes 4 were isolated after quenching the mixture with water. Aldehyde formation is ascribed to attack of water on the intermediate cycloproparenyl cation. Benzocyclopropene is ca. 5 times less reactive towards triphenylmethyl fluoroborate than cycloheptatriene. 

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