Electron-Induced (EI) Mass Fragmentation is Directed by Intra- molecular H-Bonding in Two Isomeric Benzodipyran Systems
2004
Cornelis J. Van der Schyf | Stéphane Mabic
The striking differences observed in the electron-induced (EI) mass fragmentationpathways of two isomeric benzodipyrans are attributable to hydrogen bonding in thesemolecules. In the "angular" isomer, 6-butyryl-5-hydroxy-2,2,8,8-tetramethyl-3,4,9,10-tetra-hydro-2H,8H-benzo[1,2-b:3,4-b1]dipyran (2), H-bonding occurs between the aromatic OHgroup and the alpha carbonyl moiety contained in the ortho-phenone group, whereas in the"linear" isomer, 10-butyryl-5-hydroxy-2,2,8,8-tetramethyl-3,4,6,7-tetrahydro-2H,8H-benzo-[1,2-b:5,4-b1]dipyran (3), the aromatic OH group is para to the phenone moiety, effectivelyprecluding any H-bonding. Semi-empirical molecular orbital calculations (AM1) were used tocompare predicted sites of ionization with associated fragmentation patterns. In bothmolecules, the highest occupied molecular orbital (HOMO) was located predominantly on thearomatic moiety. Similarly, in the radical cation species of both compounds, maximum spindensity was located over the aromatic rings. Neither the HOMO nor the spin density mapsprovided a rational explanation for the differences in fragmentation patterns of the twobenzodipyran isomers. The H-bonding favors EI alpha aromatic ring C-O bond cleavage in the"angular" benzodipyran and in 5,7-dihydroxy-2,2-dimethyl-8-butyryl chroman (1), a relatedmonochroman also containing a hydrogen proximal to the aromatic ring C-O bond. In contrast,fragmentation of the "linear" benzodipyran followed a different route, which was exhibited byits base peak resulting from the loss of a propyl group from the butyryl side-chain.
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