Abstract
1H and 13C NMR spectra of eleven 2-phenacylbenzoxazoles (ketimine form) show that their CDCl3-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. pKT (negative logarithm of the equilibrium constant, KT = [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-NMe2 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.
Highlights
Acylation of the anion obtained by subtracting one of the methyl protons in 2-methyl(benzo)pyridines affords 2-phenacyl(benzo)pyridines [1,2,3]
If the pyridine ring is not benzo annulated or if the annulation locates at 4,5-position, their chloroform solutions always contain (Z)-2-(2-hydroxy-2phenylvinyl)pyridines [2,3] in addition to the ketimine form [1,2,3]
On the other hand, when the pyridine ring is benzo annulated at 3,4- or 5,6-positions or at both of them, the ketimine tautomer is always in equilibrium with (Z)-1,2-dihydro-2-benzoyl-methylenepyridines [1,3]
Summary
Acylation of the anion obtained by subtracting one of the methyl protons in 2-methyl(benzo)pyridines affords 2-phenacyl(benzo)pyridines [1,2,3]. The methyl group in 2-methylbenzoxazole is susceptible to loss of one proton [4,5] If this process is followed by acylation of the obtained anion, 2-phenacylbenzoxazoles are the final reaction products [6,7,8]. The ketimine form, K, of 2-phenacylbenzoxazole (Scheme 1) is expected to tautomerize into enolimine, O, i.e., (Z)-2-(2-hydroxy-2-phenylvinyl)benzoxazole [(Z)-2-(benzo[d]oxazol-2-yl)-1-phenylethenol)] and enaminone, E, i.e., (E)-1,2-dihydro-2-benzoylmethylenebenzoxazoles [(E)-2-(benzo[d]oxazol-2(3H)-ylidene)-1-phenylethanone]. Both O and E tautomers shown in Scheme 1 are stabilized by intramolecular hydrogen bonds. From the 1H and 13C NMR studies it is known, that except the ketimine form (2-phenacylbenzoxazoles), tautomeric enaminones (denoted as E in Scheme 1) are present in CDCl3 as well as in CCl4 and DMSO-d6 solutions [10]. 2-phenacyl-benzoxazole solutions will be discussed and compared with that of 2-phenacylquinolines from which 2-phenacylbenz-oxazoles differ by an additional heteroatom and the size of the heterocyclic ring
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