Pyrilium cation

Other workers have used pyrilium cations [137] as sensors for anions such as ATP or sulphate. Charge-transfer complexes between calixpyrroles and chlo-... 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

The enantioselective reduction of pyrilium cations has been achieved. For example, the calcone (196) is treated with catalytic chiral acid (BINOL-derived phos-phonic acid or triflimide) and subjected to photolysis in the presence of the Hantzsch ester. A mechanism is proposed involving formation of the pyrilium cation (197). The chiral counterion provides an asymmetric environment around the ion (197) and the hydride is delivered stereoselectively from the Hantzsch ester. Reduction of the pyrilium ion gives product (198). 

Preparation.—From Amines or Alcohols. Katritzky has reviewed mechanistic and preparative aspects of the conversion of primary amino to other functional groups mediated by pyrilium cations according to the general Scheme 34, where nucleophiles include the halide anions (cf. 4,157) and carboxylates (c/. 4,138), Other recent publications in this area have included full details of the preparation of primary alkyl and benzyl fluorides by this route using 2,4,6-triphenylpyrilium salts (4,157). The conversion of primary amines to the corresponding bromides has now been improved, either by a modified pyrolysis procedure but still... 

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. 

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

A number of reviews and texts whose topics include photochemical reactions of aromatic compounds have been published during the year and will be mentioned here. A new volume of the Organic Photochemistry series has appeared and contains a chapter detailing the photochemical reactions of aromatic and heteroaromatic cations (e.g. cyclopropenlum ions, tropylium ions, pyrilium ions and... 

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. 

From the experimental point of view, A-benzylideneaniline has been found to add to electron-rich alkenes upon FT photosensitization by pyrilium sals. The mechanism of this imino Diels-Alder reaction has been studied in detail through time-resolved measurements and DFT calculations. It resulted that the radical cation of the olefin combines with the imine and this intermediate undergoes cyclization, 1,3-H shift and back electron transfer to give the final products (see Scheme 21). ... 

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