Department of Chemistry, University of Technology and Life Sciences, Seminaryjna 3, PL-85-326 Bydgoszcz, Poland.
Int J Mol Sci. 2013 Feb 25;14(3):4444-60. doi: 10.3390/ijms14034444.
1H and 13C NMR spectra of eleven 2-phenacylbenzoxazoles (ketimine form) show that their CDCl(3)-solutions contains also (Z)-2-(benzo[d]oxazol-2-yl)-1-phenylethenols (enolimine form). Intramolecular hydrogen bonding in the latter tautomer was found to be significantly weaker than that one in respective (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines. Integrals of the 1H NMR signals were used to evaluate the molar ratio of the tautomers. Strong electron-donating substituents were found to stabilize the ketimine tautomer. pK(T) (negative logarithm of the equilibrium constant, K(T) = [ketimine]/[enolimine]) was found to be linearly dependent on the Hammett substituent constant σ. The results of the MP2 ab initio calculations reveal enolimine including an intramolecular OH…N hydrogen bond to be the most stable form both with electron-donor and electron-acceptor substituents. The stability of ketimines is an intermediate of those found for enolimines and enaminones i.e., (E)-2-(benzo[d]oxazol-2(3H)-ylidene)-1-phenylethanones. 13C CPMAS NMR spectral data reveal that in the crystalline state the ketimine tautomer is predominant in p-NMe(2) substituted congener. On the other hand, enolimine forms were detected there when the substituent has less electron-donating character or when it is an electron-acceptor by character.
11 种 2-苯甲酰基苯并恶唑（亚胺酮形式）的 1H 和 13C NMR 谱表明，其 CDCl3 溶液中还含有（Z）-2-（苯并[d]恶唑-2-基）-1-苯乙烯醇（烯醇亚胺形式）。在后者互变异构体中发现分子内氢键明显弱于相应的（Z）-2-（2-羟基-2-苯乙烯基）吡啶。1H NMR 信号的积分用于评估互变异构体的摩尔比。发现强给电子取代基稳定亚胺酮互变异构体。pK（T）（平衡常数 K（T）= [亚胺酮] / [烯醇亚胺]的负对数）被发现与哈米特取代常数 σ 呈线性相关。MP2 从头算计算的结果表明，烯醇亚胺包括一个分子内 OH…N 氢键，对于给电子和吸电子取代基都是最稳定的形式。亚胺酮的稳定性是烯醇亚胺和烯胺酮（即（E）-2-（苯并[d]恶唑-2（3H）-亚基）-1-苯乙酮）发现的稳定性的中间状态。13C CPMAS NMR 光谱数据表明，在 p-NMe2 取代同系物中，结晶状态下主要存在亚胺酮互变异构体。另一方面，当取代基具有较小的给电子性质或具有吸电子性质时，在那里检测到烯醇亚胺形式。
