Pyrilium ion

Since many furans are not particularly sensitive to light they often form as products of photochemical reactions. A high pressure mercury arc can be used to eject nitrogen from pyridazine Ai-oxides and form cyclopropenyl ketones and their furan isomers.29 High yields of 2-acetylfuran can be achieved by irradiating the pyrilium ion in Scheme 5 (i.e., the requisite pyrone dissolved in sulfuric acid) although the conditions, mainly the acidity, have to be carefully controlled.30 Surprisingly, the intermediate cyclopen-tenone diol can be isolated with 50% sulfuric acid it is converted to the furan in a separate step. 

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 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). 

Nucleophilic attack by water does not work well under the conditions of the reaction of Scheme 19.35. Performing the photolysis in 10% aqueous sulfuric acid is, however, successful (Scheme 19.36). The pyrilium ion derived from protonation of 137 undergoes electrocychzation to a 3 1 mixture of cyclopentenones 138 and 139. Under the... 

Figure 1.36 Structural differences between 2H-pyran and the pyrilium ion.

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. 

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