Xanthylium cations

It is difficult to reach a firm conclusion about the structure of the transient detected optically at low temperature. However, the epr results rule out its being 3XA. Other reasonable candidates are JXA, the xanthyl radical (XAH ), or the xanthylium cation (XAH+). The last two possibilities can be excluded by examination of the chemical behavior of the transient. 

The reduction of xanthylium cations by BNAH apparently involves rate-determining electron transfer followed by fast hydrogen atom abstraction. Conformational studies... 

Mass spectra of xanthylium salts have not been reported. However, elimination of a hydrogen radical from the molecular ion of 9//-xanthene gives a xanthylium cation as base peak which fragments further by loss of a -CHO radical (see Section 2.22.6.1). 

The extent of this failure is evident from comparisons of experimental measurements of rate and equilibrium constants. One comparison in the literature is provided by Ritchie and coworkers study of the relatively stable cation, pyronin (the 3,6-bis(dimethylamino)xanthylium cation 71) with a series of nucleophiles.252 Another example is McClelland s measurements of rate and equilibrium constants for the reactions of halide and acetate ions with the trityl cation.19 As already mentioned fluoride and acetate are less reactive than bromide and chloride despite their equilibrium affinities being much greater. This is reflected indeed in the much lower rates of solvolysis of the fluoride and acetate than bromide or chloride as leaving groups... 

Electrochemical oxidation of thiopyrans 74 and 75 to the corresponding thiopyrylium ions proceeds by successive losses of an electron, a proton, and another electron (84KGS318). The same behavior has been shown by 1,2,3,4,5,6,7,8-octahydrothio- and seleno-xanthenes (91KGS47). Electrochemical reduction of 1,2,3,4,5,6,7,8-octahydrothio- and seleno-xanthylium cations has been also investigated (9IKGS47). 

The task of finding systems of electrophiles and nucleophiles for which rates and equilibria of the simple combination reactions can be measured is not an easy one, and data have accumulated slowly. The efforts in my laboratory have focused primarily on carbocations, with pKR values in the range measurable in dilute aqueous solution, reacting with common anionic and neutral nucleophiles. The pyronin cation [3,6-bis(dimeth-ylamino)xanthylium cation] is particularly well suited for such studies (1). This cation has an unusually high pKR of 11.5 and gives measurable rates and equilibria with a wide range of nucleophiles. Pyronin is also stable enough in dimethyl sulfoxide [(CH3)2SO] solution to allow studies in that solvent (2) for... 

The initiation mechanism, as suggested by Ledwith, follows the path of reaction (A) and is a result of a direct addition of the cation to the olefin. This is based on observations of the reaction of the xanthylium cation with 1,1-diphenylethylene. It is also based on the reaction of the tropillium cation with AT-vinyl caibazole. The last may, perhaps, be influenced by steric factors. The high initiating efficiency of the tropillium ion, that has a stable six r-electron system, may be a result of formation of a charge transfer complex ... 

Xanthosine, 959 Xanthoxylin, 1549 Xanthoxylum spp., 1549 Xanthylium cation, 1784 Xenobiotics, 2450, 2472 Xeno-estrogens, 2386 Xenohormetic compounds, 2572... 

Historically, most of the PS tested for their antimicrobial properties were already knovm to be effective for cancer treatment. They include the following classes of organic dyes porphyrin-related structures, phthalo-cyanines, phenothiazinium dyes, xanthylium dyes, and cationic fuller-enes (Dai et ah, 2009) (Table 3.3). 

Chromeno[43,2-g/i]phenaiithridine 55 is formed when the anion derived from the addition product from the reaction of benzotriazole and the xanthylium ion is oxidised with copper iodide (Scheme 36). Thioxanthylium cations behave in a similar manner <99JHC927>. 

Ritchie was the first to directly measure the absolute reactivity of cations toward solvent and added nucleophiles. The cations were highly stabilized examples, triarylmethyl cations bearing stabilizing substituents such as 30 and 31, xanthylium ions (e.g., 32) and tropylium ions (e.g., 33). The feature (and requirement) of these cations was that they had a lifetime in water such that kinetics could be followed by conventional or stopped-flow spectroscopy whereby one solution containing the pre-formed cation was added to a second solution. The time required to mix these solutions was the important factor and limited measurements to cations with lifetimes longer than several milliseconds. The lifetimes in water for 30-33 are provided below. Lifetime is defined as the reciprocal of the first-order rate constant for the decay of the cation in solvent. 

Pyrylium cations form pyridines with ammonia and pyridinium salts with primary amines (B-82MI 505-02). For example, 2,4,6-triphenylpyrylium cation (261 Z=0) yields 2,4,6-triphenylpyridine with ammonia, the corresponding 1-methylpyridinium salt with methylamine, and pyridine 1-arylimines with phenylhydrazine. Xanthylium ions (210), where ring opening cannot readily occur, form adducts (262) with ammonia, amines, amides, ureas, sulfonamides and imides. Similar adducts (e.g. 263) are formed by benzo[( ]pyrylium ions. 

All of the compounds discussed are based on three molecules 2/f-pyran (1), 4//-pyran (2) and the pyrylium cation (3). Names which have been used for the benzologue (4) of 2//-pyran include 2H- 1-benzopyran, benzo-a-pyran, chrom-3-ene and 2//-chromene. A similar situation exists for the corresponding derivative (5) of 4/f-pyran. The unambiguous and simplest name chromene is used in the present work. The benzologue (6) of pyrylium is known both as benzopyrylium and chromylium the former name is preferred here. Higher benzologues are referred to as naphthopyrans, such as 2H-naphtho[ 1,2-6 jpyran (7), but the names xanthene and xanthylium are used for (8) and (9). 

Charge densities have been calculated by CNDO/2 and EHMO for xanthene (8) and dibenzoxanthene and the derived cations (75JA5472). The 13C chemical shifts correlate well with the CNDO/2 charge densities and indicate extensive delocalization of the charge in the xanthylium species. 

Disubstituted pyrylium salts, such as (41), are attacked at C-4 only (74JOU2015) and in the presence of perchloric acid a new pyrylium salt (42) is formed. Flavylium (43) and xanthylium (44) salts also react at the y-position, for instance, with CH-acidic reagents such as pentane-2,4-dione, malonic acid derivatives or aromatic electron-rich compounds like IV V-dimethylaniline, 1,3-dimethoxybenzene or IV-methylindole (59CB46, 74CHE1019) some examples are shown in Scheme 1. 2,4-Disubstituted pyrylium cations, e.g. (45), react at C-6 (80JOC5160). 

Fig. 10 Plot of E parameters against open circles, benzhydryl cations fdled circles, trityl cations squares, organometallic cations fdled triangles, tropylium ions open triangles, flavylium, xanthylium and other O-, S-, or N-conjugated cations.

Pyrylium salts with a free - or -position react in a similar way without ring fission, flavylium adds dimethylaniline and the product aromatizes to give cation 312 xanthylium ions form adducts at the 9-position with -diketones, -keto esters, and malonic esters (e.g., 313). 

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