Fluorenyl cations acidity

The 9-fluorenyl cation also reacts at the diffusion limit with substituted styrenes, as indicated by the observation that the rate constants are 3 x 10 M s independent of the styrene substituent. The reaction of styrene with the phenethyl cation (71), the first step in the polymerization of styrene (Sty) catalyzed by Brpnsted acids, has also been directly observed (Scheme 1.8). Photoprotonation of styrene... 

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... 

Photoheterolytic cleavage of benzyl alcohol in neutral media occurs only when it gives rise to highly stabilized cations, such as triphenylmethyl cations (compare Sec. 15.1 the reaction may be at least in part adiabatic in this case and yield the excited cation) [79-81], xanthyl [82] or fluorenyl cations [83]. However, acid catalysis is effective and methyl ethers are by far the main products from benzyl alcohols in acidified aqueous methanol [84-88]. Electron donating substituents in the ortho and, to a lesser extent, in the meta position enhance the quantum yield (acid catalysis may be... 

The 9-phenyl-9-fluorenyl (PhFl, 118, Scheme 56) amino-protecting group has been proposed by Christie and RapoportF as it is significantly more stable to solvolysis in acid conditions than the trityl group because of the antiaromatic character of the fluorenyl cation. In fact, solvolysis of Trt-Cl is 6000 times faster than PhFl-Cl.f The AI -PhFl amino acid derivatives are prepared with PhFl-Br (commercially available or prepared from fluorenone and phen-yUithium and subsequent substitution with HBr) and lead nitrate, which is required to trap the bromide (Scheme 56).F f2i] lead-free alternative using nitromethane as solvent has also been reported. ... 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured." (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate," and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-... 

The high acidity of 56 in the photoexcited state as well as the efficient photodecarboxylative formation of 58 are paralleled by the ease of photoionization of fluorenol forming the fluorenyl cation 28. These discoveries by Wan et al. - provide an additional useful criterion of antiaromaticity. 

This group was reported to be 6000 times more stable to acid than the trityl group because of destabilization of the cation by the fluorenyl group. 

The appeaience of a long wavelength band (600—700 nm) in these systems is neither due to the formation of acid-olefin ir-complexes nor to cation radicals derived from the diarylethylene but rather to minor side reactions producing fluorenyl-type carbenium ions, as diown by Sauvet ... 

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. 

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