How Alkali Metal Alkoxides Initiate Organic Radical Reactions

Alkali metal alkoxides have long been known to cause hydrodehalogenation of aryl halides; this conversion of aryl halides to arenes happens when the reactions are conducted in appropriate solvents (with weak C-H bonds). More recently, when aryl halides are heated with alkoxides in arene solvents, coupling to arenes occurs. Both of these reaction types are known to involve aryl radical intermediates. The consensus has been that alkali metal alkoxides undergo electron transfer to aryl halides to form radicals, but crucial evidence has been missing. We now refute this proposal and show through deuterium isotope studies that the deprotonation of the substrates leads to benzynes that initiate radical chemistry. Surprisingly, o-, m-, p- and, in appropriate cases, r- (remote) benzynes are simultaneously formed. During reactions with potassium tert-butoxide, we observed for the first time low-level methylation of arenes, resulting from methyl radicals derived from tert-butoxide. Although methyl radicals could, in principle, arise by electron transfer from tert-butoxide ions, followed by known radical fragmentation, we show that a different, previously unreported mechanism applies.