Catalytic hydrogenation of pyrylium salts

Abstract

On catalytic reduction of 2,6-diphenylpyrylium salts, hydrogenolysis occurs at the C-O bond with the formation of 1,5-diphenylpentanes. The principal direction of the hydrogenation of 9-phenyl-symoctahydroxanthylium tetrafluoroborate is the formation of a mixture of products of partial and complete reduction of the heterocycle. The structure of the substances obtained was established by IR and 13C NMR spectroscopy.Catalytic hydrogenation is a practically uninvestigated aspect of the well-studied chemistry of pyrylium salts. Up to the present time, only individual examples of similar reactions are known [1, 2]. We have studied the catalytic hydrogenation of pyrylium tetrafluoroborates (I)-(IV), differing in the number and nature of substituents, and their condensed analogs, the hydrochromylium (V) and (VI) and octahydroxanthylium (VII) tetrafluoroborates.Reaction was carried out in an autoclave in the temperature range 40-100~ and pressure 7-12 MPa on various catalysts. It was established that pyrylium salts (I)-(IV) react with rupture of the heterocycle and the formation in high yield (80-92%) of 1,5-diphenylpentanes (VIII)-(XI). The degree of substitution and the nature of the substituents (alkyl, aryl) have practically no influence on the yield of final product. A study has been made of the effect on the yield of hydrocarbon (X) of temperature, hydrogen pressure, and catalyst using 2,4,6-triphenylpyrylium tetrafluoroborate (III) as an example. At 40~ the yield of 1,3,5-triphenylpentane (X) was 68%, at 50~ it had increased to 80%, and remained practically unchanged on increasing the temperature to 100~ A further increase in temperature leads to hydrogenation of the benzene rings and the formation of a mixture of hydrocarbons separable with difficulty. On increasing the pressure from 7 to 10 MPa, the yield of product was increased by 20%. Of the catalysts investigated, Raney Ni, Pal/C, and Ni/Ru, the latter proved to be the most active (Table 1).The catalytic hydrogenation of 2-phenyl-5,6,7,8-tetrahydrochromylium (V) and 2,4-diphenyl-5,6,7,8-tetrahydrochromylium (VI) tetrafluoroborates leads to a complex mixture of products, which we failed to separate.On hydrogenating 9-phenyl-sym-octahydroxanthylium tetrafluoroborate (VII), the hydrocarbon (XII) was isolated in a yield of only 18% and the main direction of the reaction was the formation of the oxygen-containing heterocycles 9-phenyldecahydroxanthene (XIII) and 9-phenyl-dodecahydroxanthene (XIV), the combined yield of which was 51%.The reason for the different reaction direction seemingly follows from consideration of the various stabilities towards hydrogenolysis of the C-O bond of the saturated oxygen-containing heterocycles formed. The acid released on reduction of N.

meso-Tetrahydropyranylperoxides: Molecular Structures in Solution, in the Crystal, and by DFT Calculations and Their Isomerization to the Racemate

“…We have previously shown that by the catalytic (Pd/C) hydrogenation of pyrylium salts bearing 2,6-dialkyl-substituents ( 1 ) one may conviently obtain, in preparatively useful yields, the correspondingly substituted all -cis -tetrahydropyrans 2 . , If the initial pyrylium salt has 2,6-diaryl substituents , or is polycyclic, the hydrogenation is less chemoselective leading to complicated mixtures.…”