1985 年 33 巻 12 号 p. 5207-5213
A simple method for the evaluation of the chemoselectivit of alkylating agents toward pyridine in aqueous media is proposed. The analytical method involves the evaluation of the relative rate of quaternization of pyridine with respect to the rate of hydrolysis in aqueous buffer (pH 6.0), i.e., the evaluation of chemoselectivity toward pyridine in competitive nucleophilic attacks of a given alkylating agent on the nitrogen of pyridine versus the oxygen functions in phosphate buffer containing a large excess of pyridine. The term Spy is tentatively defined as log {(N/(1-N)) × [H2O]/[py]}, where N is the molar fraction consumed for pyridine quaternization after the complete consumption of the given alkylating agent, and [H2O] and [py] are the concentrations of water and pyridine, respectively. Spy correlated linearly with the substrate constant (s) defined by Swain and Scott : s=0.163Spy+0.209 (r=0.995, n=8) at 37°C in 4% acetone-containing 1/15 M phosphate buffer (pH 6.0). The dependences of the chemoselectivity on the reaction temperature and the acetone content of the medium are briefly discussed.
