Reductive metalation of unsaturated compounds: the effect of allylic protons

Reductive metalation of alkali metals of hydrocarbons containing labile protons can be accompanied by proton transfer from the hydrocarbon to the derived radical anion and/or dianion. The extent of this reaction as a function of reaction conditions has been studied with trans-1,2-diphenylpropene (I). Three alkali metals, Li, Na and K, and two solvents, diethyl ether (DEE) and tetrahydrofuran (THF) were examined.No significant proton transfer was observed with the Li/DEE, Li/THF or Na/DEE system: the product (III) being the vicinal dianion of (I). In the case of Li/DEE, the limited solubility of the dianion prevented complete conversion of (I) to (III) except in dilute solutions.With Na/THF, substantial amounts of proton transfer occurred to form the substituted allylic anion (V) from (I). The extent of this reaction could be controlled by varying the temperature and was largely suppressed at 0°.Extensive proton transfer resulted with K/THF. However, in this case the allylic anion (V) so formed was further reduced with dimerization to the tetra-anion of 1,2,5,6-tetraphenylhexane, (IX).