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Pyridinolysis of O-Aryl Phenylphosphonochloridothioates in Acetonitrile
Lumbiny, Bilkis Jahan,Adhikary, Keshab Kumar,Lee, Bon-Su,Lee, Hai-Whang Korean Chemical Society 2008 Bulletin of the Korean Chemical Society Vol.29 No.9
fThe kinetics and mechanism of the reactions of Y-O-aryl phenylphosphonochloridothioates with X-pyridines are investigated in acetonitrile at 35.0 ${^{\circ}C}$. The negative value of the cross-interaction constant, $\rho$XY = −0.46, indicates that the reaction proceeds by concerted $S_N2$ mechanism. The observed $k_H/k_D$ values involving d-5 pyridine ($C_5D_5N$) nucleophiles are greater than unity (1.05-1.11). The net primary deuterium kinetic isotope effects, $(k_H/k_D)_{net}$ = 1.28-1.35, excluding the increased $pK_a$ effect of d-5 pyridine are obtained. The transition state with a hydrogen bond between the leaving group Cl and the hydrogen (deuterium) atom in the C-H(D) is suggested for the studied reaction system.
Pyridinolysis of O,O-Diphenyl S-Phenyl Phosphorothiolates in Acetonitrile
Adhikary, Keshab Kumar,Lumbiny, Bilkis Jahan,Kim, Chan-Kyung,Lee, Hai Whang Korean Chemical Society 2008 Bulletin of the Korean Chemical Society Vol.29 No.4
The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at $35.0{^{\circ}C}$. The Hammett plots for substituent (Z) variations in the leaving group (log $k_2$ vs. $\sigma$ Z) are biphasic concave downwards with breaks at Z = H. The large magnitudes of ${\rho}X(\rho_{nuc})$, ${\beta}X(\rho_{nuc})$, and the cross-interaction constant, $\rho$XZ, suggest frontside nucleophilic attack toward the leaving group. The sign reversal of $\rho$Z from positive in $\sigma$ Z $\leq$ 0 to negative in $\sigma$ Z $\geq$ 0 is interpreted as the change in mechanism from concerted to stepwise with rate-limiting expulsion of the leaving group. The anomalous negative sign of $\rho$ Z for leaving groups with electron-withdrawing substituents is interpreted as the intramolecular ligand exchange process of the leaving group from the equatorial position in the intermediate to the apical position in the TS.
Adhikary, Keshab Kumar,Lumbiny, Bilkis Jahan,Dey, Shuchismita,Lee, Hai-Whang Korean Chemical Society 2011 Bulletin of the Korean Chemical Society Vol.32 No.8
The nucleophilic substitution reactions of Y-O-aryl phenyl phosphonochloridothioates with substituted anilines ($XC_6H_4NH_2$) and deuterated anilines ($XC_6H_4ND_2$) are kinetically investigated in acetonitrile at $55.0^{\circ}C$. The deuterium kinetic isotope effects (DKIEs) invariably increase from an extremely large secondary inverse ($k_H/k_D$ = 0.439; min) to a primary normal ($k_H/k_D$ = 1.34; max) as both substituents of nucleophile (X) and substrate (Y) change from electron-donating to electron-withdrawing. These results are opposite to the DKIEs on Y-O-aryl methyl phosphonochloridothioates, and can be rationalized by the gradual transition state (TS) variation from backside to frontside attack. The trigonal bipyramidal pentacoordinate TS is proposed for a backside attack, while the hydrogen-bonded, four-center-type TS is proposed for a frontside attack. The negative values of the cross-interaction constants (${\rho}_{XY(H)}$ = -0.38 for $XC_6H_4NH_2$ and ${\rho}_{XY(D)}$ = -0.29 for $XC_6H_4ND_2$) indicate that the reactions proceed by a concerted $S_N2$ mechanism.
Adhikary, Keshab Kumar,Lumbiny, Bilkis Jahan,Lee, Hai-Whang Korean Chemical Society 2011 Bulletin of the Korean Chemical Society Vol.32 No.10
Kinetic studies on the reactions of Y-S-aryl phenyl phosphonochloridothioates with X-pyridines have been carried out in MeCN at $55.0^{\circ}C$. The Hammett and Bronsted plots for substituent X variations in the nucleophiles are biphasic concave upwards with a break point at X = H. The Hammett plots for substituent Y variations in the substrates are biphasic concave upwards with a break point at Y = H, and the sign of ${\rho}_Y$ is changed from unusual negative (${\rho}_Y$ < 0) with the weaker electrophiles to positive (${\rho}_Y$ > 0) with the stronger electrophiles. The stepwise mechanism is proposed on the basis of the ${\rho}_X$, ${\beta}_X$, and ${\rho}_{XY}$ values as follows: a ratelimiting leaving group departure from the intermediate involving a frontside attack and product-like TS for the stronger nucleophiles and weaker electrophiles; a rate-limiting leaving group departure from the intermediate involving a backside attack and product-like TS for the weaker nucleophiles and electrophiles; a rate-limiting bond formation involving a frontside attack for the stronger nucleophiles and electrophiles; a rate-limiting bond formation involving a backside attack for the weaker nucleophiles and stronger electrophiles. The substituent effects of X and Y on the pyridinolysis mechanisms of $R_1R_2P$(=S)Cl-type substrates are discussed.
Pyridinolysis of O-Aryl Phenylphosphonochloridothioates in Acetonitrile
이본수,이해황,Bilkis Jahan Lumbiny,어디까리 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.9
The kinetics and mechanism of the reactions of Y-O-aryl phenylphosphonochloridothioates with X-pyridines are investigated in acetonitrile at 35.0 oC. The negative value of the cross-interaction constant, ρXY = −0.46, indicates that the reaction proceeds by concerted SN2 mechanism. The observed kH/kD values involving d-5 pyridine (C5D5N) nucleophiles are greater than unity (1.05-1.11). The net primary deuterium kinetic isotope effects, (kH/kD)net = 1.28-1.35, excluding the increased pKa effect of d-5 pyridine are obtained. The transition state with a hydrogen bond between the leaving group Cl and the hydrogen (deuterium) atom in the C-H(D) is suggested for the studied reaction system.
Pyridinolysis of O,O-Diphenyl S-Phenyl Phosphorothiolates in Acetonitrile
Keshab Kumar Adhikary,Bilkis Jahan Lumbiny,이해황,김찬경 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.4
The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at 35.0 oC. The Hammett plots for substituent (Z) variations in the leaving group (log k2 vs. σ Z) are biphasic concave downwards with breaks at Z = H. The large magnitudes of ρ X (ρ nuc), β X (ρ nuc), and the cross-interaction constant, ρ XZ, suggest frontside nucleophilic attack toward the leaving group. The sign reversal of ρ Z from positive in σ Z ≤ 0 to negative in σ Z ≥ 0 is interpreted as the change in mechanism from concerted to stepwise with rate-limiting expulsion of the leaving group. The anomalous negative sign of ρ Z for leaving groups with electron-withdrawing substituents is interpreted as the intramolecular ligand exchange process of the leaving group from the equatorial position in the intermediate to the apical position in the TS.