Xanthyl cation

Generation of xanthyl cation from 2-(9-xanthyl)ethanol [87] is an extended Grob fragmentation. The intervening chain of separating the terminal donor groups may also incorporate heteroatoms, as shown in the deconvolution of a decalindione monoxime tosylate [88]. 

The 9-Cyclopropyl-9-xanthyl cation 33 is so stable that the positive charge is hardly delocalized into the cyclopropyl group. The alpha and beta hydrogens of the cyclopropyl group show absorptions at <5 H 2.96,1.38 and 1.96, comparable to that of cyclopropylam-monium ion (<5 H 2.93 and 0.93), rather than a typical cyclopropylcarbinyl cation, such as dimethylcyclopropylcarbinylcation (<5 H 3.4, 4.0)64. 

The only known examples of this case are the xanthylketones, 26, which can be used to generate xanthyl cations, 27, Eq. (9), by LFP [67]. However, this reaction occurs by P cleavage of the n,7t triplet of the ketone and the xanthyl radical formed is oxidized to the cation by oxygen. In the absence (nitrogen purging) of oxygen, the xanthyl radical persists. 

Thioxanthyl cations 6-9 exhibit adiabatic cation formation upon irradiation of the corresponding thioxanthenols in neutral aqueous acetonitrile [25,27]. Shukla and Wan noted an enhanced adiabatic fluorescence intensity for the thioxanthyl cations compared to the xanthyl cations [27]. It was suggested that the thioxanthyl cations are less susceptible to nucleophilic attack by water and thus are not deactivated as readily as the xanthyl systems [27],... 

Cozens and co-workers found that the xanthyl cation was spontaneously generated by adsorption of 9-xanthenoI within several acidic zeolites [33]. The cation was characterized spectroscopically and was found to be stable over a long period of time. Diffuse reflectance spectra of the zeolite composites exhibited absorption bands similar to those for the xanthyl cation in solution [10]. Fluorescence spectra also corresponded to cation solution spectra. The 9-phenylxanthyl, 9-phenylfluorenyl, and triphenylmethyl carbocations are similarly formed and readily detectable on dry montmorillonite clay minerals [34]. 

Cations with the xanthyl backbone have been the subject of considerable photochemical study and have been generated by each of the techniques described in the previous section. The xanthyl cations studied include those with 9-alkyl (R = Me, c-Pr, i-Pr, and r-Bu) and 9-aryl (aryl substituent = H, p-F, m-F, p-Me, m-Me, /n-OMe, and p-OMe) substituents, as well as the parent xanthyl cation. The xanthyl cation exhibits a characteristic absorption spectrum, with maxima at 260, 370, and 450 nm, invariant with the different alkyl or aryl substituents, and with the various techniques and media used for cation generation [6-8,10-15,24,28,32,33]. Only the p-OMe-substituted cation exhibits a different absorption spectrum, with the long-wavelength band less well resolved and shifted to 500 nm [13]. 

Fluorescence lifetimes and quantum yields measured for several 9-substi-tuted xanthyl cations show some variation according to the media in which the cations were generated (Tables 1 and 2). Cation lifetimes measured in TFA-TFE exhibit significantly longer lifetimes than those in other solvent systems. 

Fluorescence lifetimes have been determined for several 9-arylxanthyl cations generated Irom their corresponding alcohol in strongly acidic media, or as their tetrafluoroborate salt in acetonitrile (Table 2) [11,28,30]. Excellent agreement between the two methods was found for the parent 9-phenyl-, p-Me-, and m-Me-substituted cations, with poorer agreement in the case of the tn-OMe cation. The error associated with the very weak fluorescence of the m-OMe-substituted cation may be responsible for the difference in the lifetime estimates. The two values reported for the fluorescence lifetime of the p-F xanthyl cation show the widest variation, with a report of 18.8 ns for the cation prepared as its tetrafluoroborate salt in acetonitrile versus 47 ns in TFA-TFE [13,28,30]. 

Aryl substitution has a dramatic effect on the lifetimes and quantum yields for the xanthyl cations. Lifetimes vary from the subnanosecond range for the m-OMe-substituted cation to the tens of nanoseconds for the 9-phenyl- and p-F-substituted cations. A similar trend is seen with the fluorescence quantum yields, those cations that exhibit short lifetimes are also more weakly fluorescent [11,30]. Implications of the substituent effects on cation photophysical parameters will be discussed in Section IV. 

Figure 6 Transient reflectance spectra recorded 1 ps after 355-nm excitation of the xanthyl cation in three N2-purged zeolites. (A) HY, (B) HZSM5, (C) HMor. (From Ref. 33.)...

Bimolecular excited state rate constants for water quenching of several 9-alkylxanthyl cations and the parent xanthyl cation were determined by Stem-... 

With both the substituent effect and relative quenching order different from the ground state reactivities, the possibility that excited state cations may be quenched by water, alcohols, and ethers through an electron transfer process was considered [28,29], Application of the Rehm-Weller equation [43,44] shows that for an electron transfer mechanism to be operating, cation reduction potentials must account for the observed substituent effect on reactivity [28,29], In qualitative terms, an ease in reduction of the cations should parallel an increase in the quenching rate constants. Reduction potentials for the xanthyl cations [45] demonstrate that the trend for ease of reduction of the cations decreases with a concomitant increase in the magnitude of the quench-... 

The 9-xanthylium radical was observed following irradiation of the xanthyl cation generated via adsorption of the corresponding alcohol in zeolite cavities [33]. This suggests that the singlet-excited cation has undergone electron transfer from a donor. It was speculated that zeolite environment may act as an electron donor in this case. 

Triplet-excited xanthyl cations have also been shown to undergo electron transfer processes with aromatic donors [12,13]. The reactivity of the triplet state 9-phenylxanthyl cation, its p-fluoro analog, and the 9-phenylthioxanthyl cation was examined using transient absorption techniques. Irradiation of the 9-phenylxanthyl cation 1 in the presence of biphenyl resulted in an enhanced rate constant for decay of triplet 1, with concomitant production of transients corresponding to the 9-phenylxanthyl radical at 340 nm and the biphenyl radical cation at 670 nm (Fig. 10). Rate constants for triplet decay or radical growth were measured as a function of added quencher concentration for a variety of aromatic donors. Plots of the observed first-order rate constant versus the... 

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