Kinetics and Mechanium of Pyridinolysis of Ethyl Y-Phenyl Chlorophosphates in Acetonitrile

초록

The kinetics and mechanism of the pyridinlysis of enthyl Y-Phenyl Chlorophosphates, (C2H5O)(Y-C6H4O)P(=O)CI) are invesingated at 35.0℃ in acetonitrile. We reported series of phosphoryl transfer resctions.1 The Hammett plots for substituent (Y) variations in the ubstrate (log k2 vw _Y) exhibit a convex downward and concave upward biphasic ype with reak at Y = m-CH3. For electron-donating Y groups the Hammett coefficients, _Y, change from positive to negative and cross-interaction constant2, _XY is positive. On the othr hand for electron withdrawing Y groups _Y value changes from negative to positive with a negative _XY value. These results are interpreted ad a stepwise mechanistic pathway with a trigonal bipyramidal penta-coordinate (TBP-5C) intermediate via rate determining step change at the break-point (_Y = -0.07). Due to convex downward and concve upward Hammett plots (Figure 1) with a break at Y = m-CH3 (δㅛ = -0.07), cross-interaction constants, XY = +0.96 (Y = p-OCH3-m-CH3) and XY = -0.78 (Y = m-CH3-m-OCH3) obtained. This indicates the change in rate determining step at Y = m-CH3, from rate limiting bond formation (Y = p-OCH3-m-CH3) to rate limiting expulsion of leavig group, i.e., bond breaking, (Y = m-CH3-m-OCH3) in a stepwise mechanism with a TBP-5C intermediate.