Xanthylium ions

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. 

Some pseudo-bases do not ring open the xanthylium ion (210) gives xanthydrol (211) which can be isolated or oxidized with dilute nitric acid to xanthone (212). 

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. 

Pyrylium salts with a free a- or -position react in a similar way without ring fission, e.g. flavylium ions (220) add dimethylaniline and the product aromatizes to give (297) xanthylium ions (210) form adducts at the 9-position with (3-diketones, 3-keto esters and malonic esters e.g. 298). 

Six-membered unsaturated oxygen heterocyclic compounds are based on three molecules 2/7-pyran 1,4/7-pyran 2, and the pyrylium ion 3. Based on this, the benzo analog of 2/7-pyran is named 277-1-benzopyran (commonly 2H-chromene) and the benzo analog of 477-pyran is called 4/7-1-benzopyran (commonly 477-chromene). The benzo analog of 3 is known as 1-benzopyrylium 6 (sometimes chromylium). Related naphthyl analogs are exemplified by 2/7-naphtho[l,2,A]pyran 7, the xanthylium ion 8, and xanthene 9. 

C data for pyrylium and xanthylium ions are summarized in Figure 6. 

This effect is even more pronounced with two benzannulations thus, the dibenzo[b,e]pyrylium ion (xanthylium ion) shows shifts at 5 = 10.18 (9-H) = 165.1 (C-9) (CF3COOD). 

Xanthene 340 Xanthine 478 Xanthommatin 446 Xanthone 340 Xanthopterin 489 Xanthylium ion 327... 

Secondary kinetic isotope effects for hydride transfer reactions between alcohols and xanthylium ions show that the hydride transfer occurs by tunneling. The rehybridizations at both the donor and acceptor carbons lag behind the H-tunelling. 

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