Pyrilium salt

This system gave results that were further improved upon by the addition of a photosensitizer (e.g., a thiopy-rilium, pyrilium, and selena-pyrilium salt) to bring the absorption of light into the visible region. 

A. Preparation.—The preparation of phosphorins from pyrilium salts and tris(hydroxymethyl)phosphine has been extended to include phosphorins with indolyl and pyrrolyl groups as substituents. ... 

Since unsaturated ketones themselves are highly reactive compounds, they may react with excess of acid chloride in an overall diacylation process. The unsaturated diketones readily undergo cyclization to form pyrilium salts. In fact, this is often the best way to synthesize pyrilium salts.116... 

Cyclohexane can be acylated with acetyl chloride and AICI3 to yield l-acetyl-2-methylcyclopentene in 37% yield.127 Alkanes, such as 2-methylbutane, cyclohexane, methylcyclopentane, and methylcyclohexane, are easily acylated with a,p-unsaturated acyl chlorides in the presence of A1C13 and a hydride acceptor to afford mono- or bicyclic products after secondary transformations.128 Isoalkanes (isopentane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane) undergo diacylation and eventually form pyrilium salts under Friedel-Crafts acylation conditions with acetyl chloride or acetic anhydride.129... 

A related matrix effect of considerable analytical interest is the enhancement in absolute ion yield sometimes observed under conditions of high dilution in a solid matrix (22). Comparison of the SIMS spectrum of a neat pyrilium salt with that of the same salt diluted 1000-fold in NH Cl shows that the intact cation signal is observed in about three times greater abundance for the NH Cl-diluted sample. The threefold increase is observed even when the absolute amount of salt analyzed in the dilute sample is one thousand times less than that in the neat sample. An additional aspect of this experiment is the persistence of the enhanced signal. Ion bombardment yields products for one day in the NH Cl matrix, but for only about one hour in the neat sample under identical conditions. Effective desorption of ammonium chloride, which entrains analyte, is one way of accounting for these observations. 

A typical pyrilium salt synthesis is illustrated by the preparation of salt 9.12. The precursor to 9.12 is pyran 9.11, available by dehydration of 1,5-diketone 9.10. Note the similarity of this sequence to the Hantzch pyridine synthesis, Chapter 5. Also, the dehydrative cyclisation of a diketone to oxygen heterocycle 9.11 is reminiscent of furan synthesis, Chapter 2. 

The reaction of pyrilium salts with nucleophiles may involve electrocyclic ring-opening of the intermediate dienes, as in the formation of ketone 9.27. 

Likewise, isothiochroman derivatives can be prepared via a (thia)pyrilium salts-promoted photocycloaddition between thiobenzophenone and arylalkenes (Scheme 9.32) [53]. 

The pyrilium salt 30a was obtained from benzo-9-crown-3 in 29% yield in two steps by formylation with hexamine in the presence of CF3CO2H, followed by reaction with 2 equiv of acetophenone in the presence of POCI3 <2002JOC2065>. In the same manner, the Vilsmeier formylation of the /V-phenyl dithiazonine and the subsequent condensation reaction with 2-aminobenzenethiol resulted in substituted benzothiazole 43 in 38% yield < 1999J(P2)1273>. Benzo-9-crown-3 ether trimerizes in the presence of fed and aqueous sulfuric acid to produce tris-(9-crown-3)-triphenylene 28 in 25% yield <2001CJC195>. 

Primary amines via pyrilium salts react with 2-pyrid ie N-oxides and on decomposition give aldehydes in fair yields (equation 36). ... 

The pyrilium salts are stable aromatic cations and are responsible as metal complexes for some flower colours. 

Reaction of nitromethane with pyhlium salts. Dimroth 62) in a number of papers since 1956 63] has described an interesting reaction in the formation of aromatic nitrocompounds by reacting nitromethane with pyrilium salts (e.g. perchlorate) substituted in positions 2.4,6 in basic medium (7) with a yield varying from 25 to 607o ... 

Pyrans, pyrones, pyrilium salts, mutual transformations of 85UK1971. 2-Pyrones, 6-substituted, synthesis and reactivity of 83MI6. Tetrahydropyrans, 2.6-disubstituted, synthesis of 83CRV379. 47/-Tetrahydropyrans, 4-functionalized, chemistry of 81AKZ728. Tetrahydropyrans, synthesis via intramolecular cyclization of unsaturated... 

The reactivity described above for borates in the presence of DCN has also been observed for tetra-alkylsilanes, tetra-alkylgermanes and tetra-alkylstannanes in the presence of dicyano-benzene and tetracyanobenzene, and in the presence of pyrilium salts.Thus ultra-violet irradiation of 1,4-dicyanobenzene in acetonitrile with one of tetrabutylsilane, tetrabutylstannane or tetrabutylgermane yielded 4-butylcyanobenzene, and excitation of 2,4,6-triphenylpyrilium tetrafluoroborate under the same conditions yielded the addition products (147) and (148)Both reactions are thought to proceed by electron transfer from the tetra-alkyl metal to the excited state of the salt the radical cation produced then dissociates to the trialkyl metal cation and an alkyl radical which couples with the cyanoarene or the pyrilium ring. 

Indolo[2,3-c]pyrilium salt 487 was obtained by acid catalyzed acylation of 2-(indol-3-yl)dimedone 486 with trichlorogermylpropionyl chloride (98CHE592). Treatment of 487 with ammonia and triethanol amine gave the indoloquinoline 488 (98CHE592) (Scheme 103). 

The O + /P exchange from pyrilium salt using a source of PH3 is the classical procedure that allowed the synthesis of the first phosphinine, the ubiquitous 2,4,6-triphenylderivative 1 reported by Markl in 1966. This straightforward approach which relies on the use of easily available and inexpensive compounds... 

PROBLEM 19.95 A synthesis of pyridines involves the condensation of an a,P-unsaturated ketone (1) with a methyl ketone (2). This reaction gives a pyrilium salt (3), which then reacts with ammonia to produce a 2,4>6-trisubstituted pyridine. 

Propose mechanisms for the formation of pyrilium salt 3 from the condensation of 1 and 2, and for the formation of the pyridine 4 from 3 and ammonia. Hints The formation of 3 occurs best when 1.5—2.0 equivalents of 1 are used. The ketone 5 is a by-product of the condensation how can it be formed Think redox. ... 

In a mild alternative to the Hofmann elimination, acrydinium salts (20), prepared from the corresponding pentacyclic pyrilium salt and primary amines, are converted into terminal olefins (21) on heating with the non-nucleo-philic base triphenylpyridine. Small quantities of 2-alkenes are formed as by-products. 

Preparation.—A quaternary ammonium anion-exchange resin, in its carbonate form, is the reagent in a new and convenient procedure for the hydrolysis of primary, allyl, and benzyl halides to the corresponding alcohols. A shortened (two-stage) sequence for the conversion of primary alkylamines to alcohols mediated by pyrilium salts (c/. 2,113 4,138) is outlined in Scheme l treatment of the intermediate pyridinium salt (1) with sodium 2-hydroxymethylbenzoate (or thiobenzoate) leads directly to isolation of the alcohol, rather than the esters obtained when sodium acetate or sodium trifluoroacetate are employed. 

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