Neighboring methoxyl participation in solvolytic nucleophilic substitution

Abstract

In the present paper are summarized some of the results of the investigation of MeO-3,4,5,6 and 7 participation in solvolytic substitution. Among the ω-methoxy-1-alkyl bromobenzene-sulfonates, solvolysis of the 4-methoxy-1-butyl and 5-methoxy-1-pentyl esters is rapid and dominated by anchimerically assisted ionization in the common solvents. While MeO-5 and 6 participation is important, MeO-3,4 and 7 is not, the corresponding esters solvolyzing with a rate constant essentially equal to the k s value estimated from a ϱ*ν* correlation of solvolysis rates for primary bromobenzenesulfonates solvolyzing without anchimeric assistance. On the basis of the calculated k s values, ratios of anchimerically assisted and unassisted solvolysis rates ( kΔ/ k s ) are derived for the MeO-5 and 6 cases. In the case of MeO-5 assisted ionization of 4-methoxy-1-butyl bromobenzenesulfonate in acetic acid at 25°, an α- or a δ-methyl group is rate-enhancing by a factor of 6. Both the 5-methoxy-2-pentyl and 4-methoxy-1-pentyl esters give rise to an identical mixture of 5-methoxy-2-pentyl and 4-methoxy-1-pentyl acetates from methylene-O cleavage of the common cyclic oxonium ion intermediate. The participating methoxyl group may be an o-methoxyl substituent in a phenyl group, as in o-methoxyneophyl, 3- o-anisyl-1-propyl, 3-methyl-3- o-anisyl-1-butyl and trans-2- o-anisyl cyclopentyl arenesulfonates. With the compounds investigated, o-MeO-5, o-MeO-6, but not o-MeO-7 participation has proved to be important. Me-O cleavage of the intermediate oxonium ions formed by o-MeO-5 and 6 participation is important, so that benzodihydrofurans or benzodihydropyrans are produced, sometimes as the major product. The behavior of the cyclic oxonium ion intermediates as ion pairs determines some interesting kinetic features of the acetolysis of the methoxyl-substituted alkyl arenesulfonates solvolyzing with methoxyl participation.