Extensive Halogen Scrambling and Buttressing Effects Encountered upon Treatment of Oligobromoarenes with Bases

Abstract

As a rule, tri-, tetra- and pentahaloarenes readily undergo ortho-lithiation when treated with amide-type bases. However, halogen migration occurs whenever the substrate contains three or more contiguous halogen atoms, provided that at least one of them is bromine or iodine. Dismutation and reduction processes often take place concomitantly. In this manner, a variety of organometallic intermediates may be formed, the driving force always being a decrease in basicity. When no such energy gain can be achieved, a sterically crowded substrate may just turn out to be inert; this was found to be the case with 1,5-dibromo-3-fluoro-2-(trimethylsilyl)benzene, 1,5-dibromo-3-fluoro-2,4-bis(trimethylsilyl)benzene, and 1,5-dibromo-3-fluoro-2,4-diiodobenzene. Buttressing effects are apparently strong enough to prevent expedient deprotonation of those substrates.