Xanthenes, formation

It was concluded at the formation of a xanthene is a key step in the graphitization of 3,5-dimethylphenol resins. Xanthene formation is an efficient way of removing heteroatoms. This step would not be possible if the 3,5-dimethylphenol resin was not significantly ortho-ortho linked. However, the ortho-ortho methylene orientation, though essential, is not the only influencing factor. The methyl groups in the 3- and 5-positions also influence the xanthene formation process, as this reaction was not detectable in bis(2-hydroxyphenyl)methane under comparable conditions. 

Crystalline derivatives may be prepared with xanthhydrol (9-hydroxy-xanthen), but the reagent is comparatively, expensive. Xanthhydrol reacts with primary amide,s with the formation of crystalline xanthyl-amides or 9-acylamidoxanthens ... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

In a similar fashion, 9-(a-hydroxybenzyl)xanthene is converted to 10-phenyldibenz[b,/]oxepin (6) by treatment with p-toluenesulfonic acid in formic acid.196 Substituents in one of the benzene rings can give rise to the formation of two different products. In this study,196 the location... 

Treatment of fluorene, xanthene, thioxanthene, phenyl-4-pyridyl-methane, phenyl-2-pyridylmethane, 4,5-methylenephenanthrene, tetra-phenylcyclopentadiene, or phenalene with base and a trace of oxygen in DMSO (80% )-tert-buty alcohol (20%) solution gave significant amounts of the ketyls—i.e., Figure 6. These reactions may involve the initial formation of the ketone followed by reduction by the carbanion to yield the ketyl. 

The thioxanthone ring system comprises the nucleus for two of the more efficacious orally active antischitosomal drugs. One synthesis for these compounds starts with the chlorsulfonation of pora-chlorotoluene (21-1) to give the sulfonyl chloride (21-2). Treatment of the product with zinc in aqueous sulfuric acid leads to a reduction of the sulfonyl chloride to form the thiophenol (21-3). The key to the formation of this and related thioxanthones and xanthenes consists of the UUman reaction, which... 

A similar method, which involves the formation of the epoxide through reaction of diazomethane on l-formyl-2-hydroxyxanthone, leads directly to ll -furo[2,3-a]xanthen-ll-one.321... 

TABLE 10 Quantum Yield of Fluorescence, Triplet Formation, and Singlet Oxygen Formation for Simple Xanthenes°... 

The literature of the xanthenes provides many examples of compounds with various substituted aromatic nuclei at C-9, and other examples with different substituents on the xanthene ring. The basic skeleton, however, has not been modified in recent years. It is apparent that the crowded aromatic subunit will slow reactions involving the formation of intermediates involving reaction at C-9 and may retard the subsequent reactions of the intermediates as well. This observation predicts slower rates of electron transfer, for example, and subsequent photoreduction than would be expected for unsubstituted xanthenes. Our interest was to synthesize a new series of dyes without substituents at that position and to investigate the effect of C-9 substitution on photochemical and photophysical properties. 

The anion derived from 2,3-dimethyl-l,4-naphthoquinone behaves as a quinone methide and undergoes a [l,4]-cycloaddition with the benzoquinone (193). The product is the xanthene derivative (194) (70JCS(C)722). There is no indication of the formation of the isomeric xanthene. A [l,3]-cycloaddition occurs simultaneously which leads to the fluorene derivative (195). 

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

Aryliodonium salts have been found to be coinitiators for photooxidizable dye sensitization (105). Smith polymerized aerylamide-bis(acrylamide) mixtures using acridine, xanthene, or cyanine dyes and, for example, diphenyllodonium chloride as an electron acceptor. Reduction of the salt results in the formation of phenyl radicals. 

The reactions of 9-(p-tolyl)- 10-telluroniaanthracene perchlorate with ammonium halogenides, NaOH, MeONa (89KGS691), or NaN3 in tetrahy-drofuran (87KGS279) led to the respective 9-substituted 9-(p-tolyl)tellura-xanthenes 107 in high yields (64-96%). In some cases, for instance, in the reaction with ammonium chloride, the formation of 106b was detected by means of ESR spectroscopy. 

A series of rearrangements is proposed to account for the formation of symmetrically substituted xanthenes when hydroquinones react with 2-methylprop-2-en-l-ol (95S693). In an equally simple reaction, dibenzofajjxanthenes are produced when aliphatic aldehydes react with an excess of 2-naphthol under acidic conditions (95JCR(S)502). 

Under similar conditions, even benzylic C-H bonds of some hydrocarbons (xanthene and fluorene) are converted to the ketones (e.g., fluoren-9-one). Notwithstanding the parallel activities of the two catalysts, different mechanisms were tentatively proposed (277-279). Thus, for R.UC0AI-LDH-CO3, it was postulated that the presence of Co in the structure facilitated the formation of high-valent Ru(V)=0 species. In contrast, for the Ru-HAP catalyst, it was proposed that after coordination of the reactant to Ru as an alcoholate, the carbonyl compound was eliminated, leaving a Ru-H compound, which in a next step is reoxidized by O2. The basic nature of the HAP (or LDH) support may actually favor the latter route, with formation of an alcoholate. Filtration tests and elemental analyses confirm the stability of the supported species in both catalysts. 

In this family of the xanthene dyes, excitation of oxygen free aqueous solution of the dye alone is sufficient to induce the formation of the semioxidized and semireduced form of the dianionic starting material and the electron transfer steps always concern the triplet dye X 3 [167-173]. Electron exchange may occur between triplet and ground state or between two triplet excited states (steps 23 and 24). 

Similar processes have been observed with other aldehydes, such as glyoxylic acid. However, the carboxymethine-linked oligomers resulting from reaction with glyoxylic acid proceeded to xanthylium salts rather than to larger polymers (Es-Safi et al. 1999b). The postulated pathway involves dehydration and cyclisation of the carboxymethine dimer (Eig. 9B.6(4)) followed by oxidation of the resulting xanthene (Eig. 9B.6(5)) that was also detected in the medium. Formation of xanthylium salts was also shown in the case of furfural and hydroxymethylfurfural (Es-Safi et al. 2000). 

Intramolecular cyclisation following halogen-metal exchange in the benzonitrile derivatives 25 provides a route to xanthones and thioxanthones. Incorporation of a second aryl halide function into the benzonitrile substrate allows an anionic cascade ring-closing sequence and the formation of pentacyclic xanthene derivatives 26 <03JOC4091>. 

---