Electrostatic repulsion by the charged tail of a radical controls the stereochemistry of coupling with anthracenide. Reversibility of benzylic fragmentation1,2

Abstract

Reaction of anthracenide A˙^– with 1-pivaloyl-2,3-diphenylaziridines cis-4a and trans-4a yield the same products, namely PhCH₂CHPhNHCOCMe₃ (11) and AH–CHPhCHPhNHCOCMe₃ (6) (AH = dihydroanthryl). The steric differentiation is lost when the two ketyls 12 formed from 4a undergo homolytic ring cleavage forming the same anionic radical 13. Extremely short reactions (⩽10 s) give 6 as the erythro isomer exclusively or nearly so. Coupling of 13 with A˙^– does not form the precursor (threo-8) of threo-6 owing to electrostatic repulsion between A˙^– and the anionic tail of 13 in the preferred conformation of the latter. Radical coupling is not completed within this short time so 11 can be formed directly from 13 via 10, the amide anion of 11. Reduction of 13 to carbanion 9 by outer-sphere electron transfer or via 8 and its benzylic fragmentation (BFR) is the other path to 11. Extending the time to 1 or 2 min has the following effects. Coupling of 13 with A˙^– is completed at the expense of 11. Second, more than a trace of threo-6 is detected indicating that BFR 8→9 + A (A = anthracene) is reversible and that addition of dianion 9 to A proceeds without pronounced stereochemical preference. With even more time the erythro∶threo ratio changes in favour of threo-6 and finally can even reach a value slightly less than 1. Simultaneously the amount of 11 increases slowly at the expense of the total 6 indicating that part of BFR which becomes irreversible by carbanion protonation 9 + THF→10. With much longer reaction times imidate ion 10 eliminates (E)-stilbene. Both isomers of 6 have been independently synthesized from the two isomers of 4a and anthracene hydride AH^–.