Bromination of Non-a-Tocopherols: A Comparative Synthetic, Kinetic andComputational Study

摘要

The bromination chemistry of the three non-a-tocopherolsand of their truncated model compounds in apolar solventswas extensively studied and compared to that of the a-conge-ner. Bromination occurs at free aromatic positions for all non-a-tocopherols. In the case of 6-tocopherol, there is a prefer-ence for C-5 over C-7. In the presence of a 5-methyl substitu-ent, as in 13-tocopherol, formation of the 5a-bromo derivativeby an oxidation/addition mechanism via the correspondingo-quinone methide becomes a competitive process. By mea-surement of product ratios at different temperatures, the rela- tive and absolute activation energies of the reaction systemswith parallel reactions going on (I3- and 5-tocopherol) wereestablished. The kinetic data were in very good agreementwith DFT results that showed the product ratio for 8-tocoph-erol bromination to correlate with the stabilities of the cat-ionic bromination intermediates. All products were compre-hensively characterized, providing reliable analytical stan-dards and reference compounds.