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
作者:Hasi Rani Barai、Hai-Whang Lee
DOI:10.5012/bkcs.2012.33.3.1047
日期:2012.3.20
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 $\rho_X$ (= +4.73) and negative $\beta}_X$ (= -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 ($\rho_X$ = -6.15 and $\beta}_X$ = 1.11) with the strongly basic pyridines compared to those ($\rho_X$ = 4.73 and $\beta}_X$ = -0.75) with the weakly basic pyridines.
在乙腈中于$5.0^\circ}C$条件下,研究了(2R,4R,5S)-(+)-2-氯-3,4-二甲基-5-苯基-1,3,2-氧氮磷杂环己烷-2-硫酮与X-吡啶衍生物的亲核取代反应动力学。核试剂中取代基X的变化的自由能关系表现出双相向上凹,并在X=3-乙酰基处有断裂点。对于弱碱性吡啶,得到了异常的正值$\rho_X$(=+4.73)和负值$\beta}_X$(=-0.75),并通过等温关系和等温温度$t_ISOKINETIC}=39.3^\circ}C$进行了合理化解释。基于与强碱性吡啶($\rho_X$=-6.15和$\beta}_X$=1.11)相比,弱碱性吡啶的选择性参数($\rho_X$=4.73和$\beta}_X$=-0.75)更大的数值,提出了一种协同机理,涉及从强碱性吡啶的前侧攻击到弱碱性吡啶的后侧攻击的亲核攻击方向的改变。