Concurrent primary and secondary deuterium kinetic isotope effects in anilinolysis of O-aryl methyl phosphonochloridothioates

Title
Concurrent primary and secondary deuterium kinetic isotope effects in anilinolysis of O-aryl methyl phosphonochloridothioates
Authors
Ul Hoque, M.E.; Guha, A.K.; Kim, C.K.; Lee, B.S.; Lee, H.W.
Keywords
NUCLEOPHILIC-SUBSTITUTION REACTIONS, METAL-ION CATALYSIS, Y-O-aryl methyl phosphonochloridothioates
Issue Date
2009
Publisher
ROYAL SOC CHEMISTRY
Abstract
The nucleophilic substitution reactions of Y-O-aryl methyl phosphonochloridothioates with substituted anilines (XC6H4NH2) and deuterated anilines (XC6H4ND2) are investigated kinetically in acetonitrile at 55.0 degrees C. The Hammett and Bronsted plots for substituent (X) variations in the nucleophiles are biphasic concave downwards with a break region between X = H and 4-Cl. The deuterium kinetic isotope effects (DKIEs) are primary normal (k(H)/k(D) = 1.03-1.30) for stronger nucleophiles (X = 4-MeO, 4-Me and H), and extremely large secondary inverse (k(H)/k(D) = 0.367-0.567) for weaker nucleophiles (X = 4-Cl, 3-Cl and 3-NO2). The cross-interaction constants are negative (rho(XY(H)) = -0.95 and rho(XY(D)) = -1.11) for stronger nucleophiles, while positive (rho(XY(H)) = +0.77 and rho(XY(D)) = +0.21) for weaker nucleophiles. These kinetic results indicate that the mechanism changes from a concerted process involving frontside nucleophilic attack for stronger nucleophiles to a stepwise process with a rate-limiting leaving group expulsion from the intermediate involving backside attack for weaker nucleophiles. A hydrogen-bonded, four-center-type transition state (TS) is suggested for a frontside attack, while a trigonal bipyramidal pentacoordinate TS is suggested for a backside attack. The unusually small DKIEs, as small as or equal to 0.4, for weaker nucleophiles seem to be ascribed to severe steric congestion in the TS.
URI
http://dspace.inha.ac.kr/handle/10505/1897
ISSN
1477-0520
Appears in Collections:
College of Natural Science(자연과학대학) > Chemistry (화학) > Journal Papers, Reports(화학 논문, 보고서)
Files in This Item:
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