HOMOLYTIC BASE-PROMOTED AROMATIC ALKYLATIONS BY ALKYLMERCURY HALIDES

Electron transfer chain reactions leading to substitution in electrone gatively substituted benzene derivatives can be observed with alkylmer cury halides in the presence of proton accepters such as DABCO. Promot ion by base involves the abstraction of a proton from the substituted cyclohexadienyl adduct radical to form a radical anion which readily t ransfers an electron to RHgX with the regeneration of R-.. Aromatic su bstitutions involving t-Bu-. are highly regioselective and yield produ cts of only para attack for PhCHO, PhCOCH3, PhCOCMe3, PhCOPh, PhCN, ph thalimides, or 1,2-dicyanobenzene. The ortho/para substitution product s are observed for isophthaldehyde or 1,3-dicyanobenzene, while 1,4-di cyanobenzene yields the ortho substitution product. At 25-35 degrees C substitution by t-Bu-. ortho to an ester group is not observed and m- or p-cyanobenzoate esters yield only products of substitution ortho to the cyano group. With the isopropyl radical substitution ortho to the ester function is observed with:diethyl isophthalate. Intramolecular radical cyclizations of the radical adducts of 1-aryl-4-penten-1-ones leading to alpha-tetralones. is also promoted by the presence of DABCO . When the aryl group contains a para eater function, spirocyclization occurs leading to a rearrangement acyl radical which,can be oxidized by t-BuHgCl to the acyl cation and the carboxylic acid.