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Bromination of Non‐α‐Tocopherols: A Comparative Synthetic, Kinetic and Computational Study
Author(s) -
Patel Anjan,
Böhmdorfer Stefan,
Hofinger Andreas,
Netscher Thomas,
Rosenau Thomas
Publication year - 2009
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200900495
Subject(s) - halogenation , chemistry , tocopherol , substituent , cationic polymerization , quinone methide , organic chemistry , quinone , medicinal chemistry , computational chemistry , stereochemistry , antioxidant , vitamin e
The bromination chemistry of the three non‐α‐tocopherols and of their truncated model compounds in apolar solvents was extensively studied and compared to that of the α‐congener. Bromination occurs at free aromatic positions for all non‐α‐tocopherols. In the case of δ‐tocopherol, there is a preference for C‐5 over C‐7. In the presence of a 5‐methyl substituent, as in β‐tocopherol, formation of the 5a‐bromo derivative by an oxidation/addition mechanism via the corresponding o ‐quinone methide becomes a competitive process. By measurement of product ratios at different temperatures, the relative and absolute activation energies of the reaction systems with parallel reactions going on (β‐ and δ‐tocopherol) were established. The kinetic data were in very good agreement with DFT results that showed the product ratio for δ‐tocopherol bromination to correlate with the stabilities of the cationic bromination intermediates. All products were comprehensively characterized, providing reliable analytical standards and reference compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)