Reactions of Carbonyl Compounds in Basic Solutions. Part 35¹: The Alkaline Hydrolysis and Reactivity – Structure – Spectra Correlations of (Z)-4-(Substituted Benzylidene)-2-Phenyl-4H-Oxazol-5-Ones

Abstract

The kinetics of the alkaline hydrolysis and the configuration, conformation and electronic structure were studied using IR, NMR spectroscopy, X-ray analysis and AM1 theoretical calculations for a series of (Z)-4-(substituted benzylidene)-2-phenyl-4H-oxazol-5-ones (1) and compared with analogous results reported for (E)-4-benzylidene-2-phenyl-4H-furan-5-ones (5) and related compounds.

Spectral characterisation of oxazol-5(4H)-ones containing a furan and/or benzene ring in conjugation with the azlactone system

“…Each of the compounds I-III may exist in one of the two configurations E or Z with respect to the exocyclic C=C double bond (Scheme 3). As confirmed previously by several authors, 2,9 the Z-configuration is more stable. Calculations of the Frank-Condon triplet of the azlactone molecule show the existence of a considerable excess of spin density on the methylenic C=C double bond at position 4 of the oxazolone ring -the so called "twisted triplet" state typical of most ethylenic compounds.…”

“…In order to assign all vibrational levels involved in the Fermi resonance, we measured the IR spectras in the region of 940-810 cm -1 where the deformation vibration ω of the C-H group is present 9 . Spectral patterns are summarised in Table 9.…”

“…The stretching vibration of the endocyclic C=N group is present in the region of 1710-1620 cm -1 . 9 There is a single peak for each of the compounds but its frequency is strongly influenced by substituent effects (Table 9). The wavenumber of the C=N stretching vibration of (Z)-4-benzylidene-2-phenyloxazol-5(4H)-one is present at 1654.8 cm -1 .…”

“…8 Alkaline hydrolysis of (Z)-4-benzylidene-2-phenyloxazol-5(4H)-ones leads in the first step to a ring opening according to Scheme 1. The reaction rate is controlled by substituent effects 9,10 and it is very sensitive to the structural changes on the aromatic system in the position 2-of the γ-lactone ring. The reaction is enhanced by electron acceptor substituents, yielding the reaction constant ρ=0.74 (at 304.4 K).…”

“…Studies of IR absorption bands of substituted (Z)-4-benzylidene-2phenyl-oxazol-5(4H)-ones measured in CHCl 3 revealed a clear splitting due to the strong two-level Fermi resonance interaction between the C=O stretching mode and the first overtone of an out-of-plane deformation vibration mode of C-H group. 9,10 Oxazol-5(4H)-ones with furan-2-yl substituents, prepared recently, 11 are intermediates to furan containing aminoacids and peptides.…”