噁唑烷环-Schiff碱互变异构中取代基效应和溶剂效应的NMR研究

NMR Study of Substituent and Solvent Effects on the Ring-Schiff Base Tautomerism of Oxazolidines

合成了十个对位和间位取代的2-苯基-4,4-二甲基噁唑烷,并用NMR研究了它们在氯仿、丙酮和二甲基亚砜中与相应的Schiff碱之间的互变异构平衡,发现在平衡常数K与取代基常数σ^+之间存在着良好的线性自由能关系,说明平衡可能经历了带有正电荷的中间体,在2-位质子的化学位移δ与取代基常数σ间也存在着线性关系,logK/K_0-σ^+和δ-σ直线的斜率都与溶剂的极性有关。

Ten para-and meta-substituted 2-phenyl-4, 4-dimethyloxazolidinos have been synthesized and their tautomeric equilibria with the corresponding Schiff bases studied by NMR in chloroform, acetone and DMSO. Fairly good linear free energy relationships were found to be present between the equilibrium constant, K, and the substituent constant. σ^+, of the substituent, indicating a positively charged intermediate may be involved in the equilibrium. The chemical shift, δ, of tho 2-proton of the oxazolidines is also correlated linearly with the Hammett σ of the substituent. The slopes of both the log K/K_o-σ^+ and the δ-σ-lines are dependent on the solvent polarity.