Kinetic Study on Nucleophilic Substitution Reactions of O-Phenyl O-Y-substituted-Phenyl Thionocarbonates with 1,8-Diazabicyclo[5.4.0]undec-7-ene in Acetonitrile

Abstract

Second-order rate constants (k(N)) for nucleophilic substitution reactions of O-phenyl O-Y-substituted-phenyl thionocarbonates (4a-4k) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in MeCN at 25.0 +/- 0.1 degrees C are reported. The reactivity of 4a-4k decreases as basicity of the leaving group increases except O-2,4-dinitrophenyl O-phenyl thionocarbonate (4a), which is less reactive than O-3,4-dinitrophenyl O-phenyl thionocarbonate (4b) although the former possesses 2 pK(a) units less basic nucleofuge than the latter. The Bronsted-type plot for the reactions of 4b-4k is linear with beta(lg)=-0.50, a typical beta(lg) value for reactions reported to proceed through a concerted mechanism. The Hammett plot correlated with sigma(-)(Y) constants for the reactions of 4b-4k results in a better linear correlation than that correlated with sigma(o)(Y) constants. Besides, the Yukawa-Tsuno plot exhibits an excellent linear correlation with rho(Y)=2.12, r=0.68 and R-2=0.990, indicating that a negative charge develops partially on the O atom of the leaving group in the rate-determining step (RDS). Thus, the reactions have been concluded to proceed through a forced concerted mechanism. Effects of steric hindrance on reactivity and reaction mechanism are also discussed in detail.