Birch Reduction - Experimental Testing and Computational Verification

Experimental Testing and Computational Verification

Then in 1990 and 1993 a method was finally devised to experimentally assess whether the anisole and toluene radical anion protonated ortho or meta. The esoteric method began with the premise that the isotope selectivity in protonation in a protium–deuterium medium would be greater for the radical anion, of the first protonation step, than for the carbanion of the penultimate step. The reasoning was that carbanions are much more basic than the corresponding radical anions and thus will react more exothermically and less selectively in protonation. Experimentally it was determined that less deuterium at the ortho site than meta resulted (1:7) for a variety of methoxylated aromatics. This is a consequence of the greater selectivity of the radical anion protonation. Computations (e.g. ROHF/6-31g) of the electron densities concurred with the experimental observations. Also, it was ascertained that frontier orbital densities did not, and these had been used in some previous reports.

Subsequently, in 1992 and 1996 Birch published twice still suggesting that meta protonation was preferred. This was a reversal of his earlier views as published with Leo Radom.

However, textbooks, publishing on the mechanism of the Birch Reduction, have noted that ortho protonation of the initial radical anion is preferred.