The nucleophilic substitution reactions of (2R,4R,5S)-(+)-2-chloro-3,4-dimethyl-5-phenyl-1,3,2-oxazaphospholidine 2-sulfide with X-pyridines are investigated kinetically in acetonitrile at $5.0^{circ}C$. The free energy relationships for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Ac. Unusual positive $ ho_X$ (= +4.73) and negative ${eta}_X$ (= -0.75) values are obtained with the weakly basic pyridines, and rationalized by the isokinetic relationship with isokinetic temperature at $t_{ISOKINETIC}=39.3^{circ}C$. A concerted mechanism involving a change of nucleophilic attacking direction from a frontside attack with the strongly basic pyridines to a backside attack with the weakly basic pyridines is proposed on the basis of greater magnitudes of selectivity parameters ($ ho_X$ = -6.15 and ${eta}_X$ = 1.11) with the strongly basic pyridines compared to those ($ ho_X$ = 4.73 and ${eta}_X$ = -0.75) with the weakly basic pyridines.