Photolysis of carbocationic species. Triphenylcarbonium ion

Abstract

Triphenylcarbonium ion was photolyzed with visible light in various acidic media. In 72% aqueous sulfuric acid, bis-µ,p'-diphenylmethylbiphenyl (3), p-diphenylcarbinol-p'-diphenylmethylbiphenyl (5), and bis-µdiphenylcarbinolbiphenyl (4) were formed, each in about 20% yield. Mechanistic investigations indicate that this dimerization involves interaction of an excited, triplet tritylcarbonium ion with a ground-state tritylcarbonium ion. Dimerization was suppressed by increasing the acidity of the medium, and in 96% sulfuric acid the only product obtained was 9-phenylfluoren-9-ol (formed in 60 % yield). In the presence of acetic acid and lower concentrations of sulfuric acid, the excited triplet tritylcarbonium again plays a role, cyclizing to the 9-phenylfluorenyl system or oxidizing the solvent forming trityl radical, which then reacts further. In 3 % sulfuric acidacetic acid the products were 21 % triphenylmethane, 22% 9-phenylfluorene, 17% bis-9-phenylfluorenyl peroxide, and 10% benzophenone. In 3 % sulfuric acid-16 % tolueneacetic acid the products were 20% of a mixture of 9phenylfluorene and triphenylmethane, 45% 1,1,1,2-tetraphenylethane, and 11 % 9-benzyl-9-phenylfluorene. In the presence of oxygen, photolysis of the tritylcarbonium ion in 3% sulfuric acid-acetic acid yielded 37% benzophenone, 28% 2,2-diphenylmethylenedioxybenzene, and 11% of the novel peroxide acetate 17. Under still more weakly acidic conditions the aldehyde acetate 18 was formed. In 30 % sulfuric acid-acetic acid, diphenyl-ptritylphenylcarbinol was formed in 30% yield, along with 12% 5, while in 60% sulfuric acid-acetic acid the only product isolated was 4.

Electron transfer reactions of triplet 9-arylxanthenium and 9-arylthioxanthenium cations

“…During the last two years detailed studies of the fluorescence behavior and intermolecular reactivity of di-and triarylmethyl cationsI8 and of various 9-substituted xanthenium cations' [8][9][10][11][12][13][14][15][16][17][18][19][20] have been undertaken. The reactivity of the first excited singlet state of the 9-phenylxanthenium cation toward a variety of substituted aromatic donors has been measured and shown to involve electron transfer to give 9-phenylxanthenyl radical plus the radical cation of the aromatic donor.18-20 Rapid back electron transfer provides the major decay pathway for thesinglet geminate radical/radical ion pair.…”

Characterization of the triplet excited state of the phenylxanthenium carbocation

“…Despite the large volume of data on the reactivities of photogenerated ground state carbocations (I), there have been relatively few investigations of the excited state behavior of these intermediates (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Most of the available information deals with intramolecular rearrangements such as the photo-cyclization of the triphenylmethyl cation (2) and valence isomerizations of protonated unsaturated carbonyl compounds (3,5,6) and a variety of aromatics (7).…”

“…In this regard it should also be noted that a triplet state has been postulated as an intermediate in some of the photochemistry of the triphenylmethyl cation, although this was not based on any direct spectroscopic evidence (2,7). Work aimed at generating other diarylmethyl cations by either direct excitation or sensitization and examining their reactivity is currently in progress.…”

Photoinduced electron transfer of carbanions and carbocations

“…Although some researchers have recently employed carbocations as electron acceptors in PET reactions, much of this preliminary work has concentrated on the investigation of mechanism, rather than synthetic utility. In the early I970s, van Tamelen and co-workers reported on the steady state photolysis of several stabilized carbocations [69][70][71][72]. The reactions were often complex, and very dependent on the solvent conditions.…”