Carbocations fluorenyl

Figure 1.3. Rate-equilibrium correlation for hydration of carbocations triarylmethyl and 9-aryl-9-fluorenyl ( and , respectively, slope =—0.60) diarylmethyl (A, —0.54) aryltropylium (O, —0.68) 9-xanthylium and cyclic phenyldialkoxycarbocations ( and A, respectively, slope = —0.63).

The transient zwitterion (15), obtained from the /(-hydroxy acid precursor by laser flash photolysis, has been characterized 46 it reacts with nucleophiles more slowly than does the 9-fluorenyl cation itself. The parent acid was also characterized.46 Evidence has been presented that (16) undergoes substantial El elimination via a primary carbocation.47 An analysis of solvolysis results for (17) is indicative of extensive charge delocalization throughout the fluorenyl ring at the transition state apparently... 

The photoionization of benzylic alcohols leading to carbocations has recently been reported197. Taking advantage of the stabilization provided by the /J-silicon effect, flash photolysis of 380 in trifluoroethanol gave rise to the fluorenyl cation 381 which was characterized spectroscopically (equation 46). 

The photochemical generation of arylmethyl cations from arylmethanols has been the topic of several papers this year. For mono-aryImethanols in which the aromatic group is phenyl, naphthyl, pyrenyl or phenanthryl, Wan has reported that ultra-violet light irradiation in aqueous alcoholic sulphuric acid solution yields the corresponding ether,presumably by way of an intermediate arylmethyl carbocation. Photolysis of (414) in aqueous alcohol to give (415) is found to proceed even in the absence of acid, while the non-cyclic analogue (416) reacts much less efficiently, even at low values of pH. This reactivity difference has been observed previously for fluorenol and benzhydrol and is attributed to extra stabilisation of the intermediate carbocation in the fluorenyl system. 

Two transients have been observed during flash photolysis of 9-fluorenol and both have been assigned the fluorenyl cation structure.A new study has concluded that while one of these transients is indeed the cation, the other corresponds to the fluorenyl radical cation.A transient assigned as the carbocation obtained by loss of hydroxide has also been observed by flash photolysis of arylxanthenol (417) in aqueous ethanol. 

Carbenes were the latecomers to the company of stable examples of organic reaction intermediates. The 100 anniversary of Gomberg s discovery of the stable triphenylmethyl radical was celebrated in 2000, and Gomberg and others were familiar with the triphenylmethyl carbocation by 1910. Stable anions derived from hydrocarbons like the fluorenyl anion were being investigated by Conant and McEwan in the decade 1930-40, but the first isolable species which might be considered to be carbenes were not reported until 1988 by Bertrand... 

Both the 9-methyl and 9-trifluoromethyl-9-fluorenyl cations (7 and 8) have been generated by laser flash photolysis of corresponding alcohol precursors. The product ions were then studied by time-resolved spectroscopy. Consistent with previous studies related to carbocation-bearing electron-withdrawing groups, ion (8) exhibits a significant bathochromic shift in the UV absorption compared to (7). The ions were quenched with methanol and reaction rates showed cation (8) to be a more reactive species. 

The cyclopentadienyl cation (5) and its mono- and dibenzannelated analogues the indenyl (27) and fluorenyl (28) carbocations have been the subject of frequent study. In 1925, Ziegler and SchnelP re-... 

The solvolytic reactivity of the 9-CFs substituted fluorenyl tosylate 46 was strongly depressed compared to R = H, and a rate factor of 10 due to antiaromatic destabilization was estimated. Solvolytic studies with formation of 9-fluorenyl carbocations with COzR, - CONMez, and CR=NOCH3 substituents have also been reported. On the basis of the calculated geometry it was suggested by Creaiy et al. that such cations avoid antiaromatic structures with cyclopentadienyl cation moieties and resemble bis(dienyl) cations 28a. 

In summary, antiaromatic character of the fluorenyl carbocation is expected to be strongly attenuated. However, in a number of examples cited there is evidence for the effects of residual antiaromaticity, either in the molecule as whole or just for the cyclopentadienyl fragment. 

It is suggested that the fluorenyl rings in 49 are orthogonal, and that a—n conjugation occurs from the C-C bond of one ring to the empty p orbital of the other (Figure 8). - This effect would explain the upfield shift at the carbocation center in 49. 

Intramolecular hydride ion migration in a substituted 9-fluorenyl carbocation 287... 

Intramolecular Hydride Ion Migration in a Substituted 9-Fluorenyl Carbocation... 

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