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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 33, Number 3
BKCSDE 33(3)
March 20, 2012 

Kinetics and Mechanism of the Pyridinolysis of (2R,4R,5S)-(+)-2-Chloro-3,4-dimethyl- 5-phenyl-1,3,2-oxazaphospholidine 2-Sulfide in Acetonitrile
Hasi Rani Barai, Hai Whang Lee*
Thiophosphoryl transfer reaction, Pyridinolysis, (2R,4R,5S)-(+)-2-Chloro-3,4-dimethyl-5-phenyl- 1,3,2-oxazaphospholidine 2-sulfide, Biphasic concave upward free energy relationship, Isokinetic relationship
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 oC. 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 ρX (= +4.73) and negative βX (= –0.75) values are obtained with the weakly basic pyridines, and rationalized by the isokinetic relationship with isokinetic temperature at tISOKINETIC = 39.3 oC. 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 (ρX = –6.15 and βX = 1.11) with the strongly basic pyridines compared to those (ρX = 4.73 and βX = –0.75) with the weakly basic pyridines.
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