Regrioselectivity of Palladium(II)-Induced Intramolecular Cyclization of 2-(3-Methyl-2-butenyl)phenol

Reaction of 2-(3-methyl-2-butenyl)phenol (1) with PdCl2 gives 2,2-dimethylchromene (3) and 2,2-dimethyl-4-methoxychroman (4) (six-membered products) predominantly, together with trace amounts of 2-isopropylbenzofuran (5) and 2-isopropenyl-2,3-dihydrobenzofuran (6) (five-membered products). The reaction involves attack of the oxygen atom of the phenoxyl group at 2- or 3-position of the allylic side chain. Addition of various sodium carboxylates to the reaction mixture changes the regioselectivity. In the presence of sodium salts of carboxylic acids bearing electron-releasing substituents, nearly equal amounts of five and six-membered products are formed. The presence of sodium salts of carboxylic acids bearing electron-withdrawing substituents results in the predominant formation of the six-membered products. These results are discussed en the basis of alternation of reactive Pd(II) species. Results obtained from 2-(2-butenyl)phenol (2) are also reported.