The Infrared Spectra of Complex Molecules

Abstract

Notes 1655 pattern centering at r 7.37 (range 6.9-7.7), while the benzylic proton appears as a triplet (two overlapping doublets, J-7 cps) centering at r 4.33 (range 4.1-4.5). The key difference between the spectra of lib and lb is the position of this benzylic proton which in lb is shifted downfield (compared with lib) by approximately 2 ppm by the lactonic oxygen. The aromatic protons appear as two peaks, the one at 1.86 representing the two protons ortho to the nitro group and that at 2.33 corresponding to the protons meto to it. Condensation of Sodiomalonic Ester and p-Nitrostyrene Oxide.-The condensation was carried out as described earlier4 and was sampled at the a-ethoxycarbonyl-d-nitrophenyl-7-butyrolactone stage without crystallization. The pmr spectrum of this crude material had no absorption in the 4^5 region where the resonance of the benzylic proton of the-nitrophenyl isomer might be expected to appear. Hydrolysis and decarboxylation of this unpurified material as described earlier4 gave a chloroform extract , which, after treatment with 25 mg of activated charcoal, was evaporated to dryness. A pmr spectrum of the residue in deuterated chloroform was identical with that of pure lib, and, in particular, no peak at 4.33 ascribable to lb could be detected. The sample of lib rapidly crystallized and melted at 107-110° without purification. A mixture melting point with pure lib, mp 112-113°, was 108-110°. a strong band at 2400 and 2300 cm-1, respectively. The yellow substance, stable only below room temperature , would thus appear to be a nitrilium salt. A closely related observation has been described recently by Grab, et al.4 They have isolated nitrilium salts from a Beckmann rearrangement of N-chloro-ketimines (V), induced by antimony pentachloride or ferric chloride. These salts show a characteristic intense infrared absorption at 2310 cm-1. R \ SbCli + C=N-Cl-)-R-C=N-R' / SbClr R' V The possibility that the nitrilium ion produced is II can be confirmed in that o-methylthioacetanilide (III) should be produced on hydrolysis (Scheme I). On the basis of a large rate enhancement and the absence of an appreciable kinetic isotope effect it was suggested that the conversion of sz/n-o-methylthio-benzaldoxime o-iodobenzoate to o-methylthiobenzo-nitrile involved the participation of sulfur.1 Proton loss from the isothiazolium intermediate results in nitrile formation. It was thus of interest to examine the reaction of la wherein proton loss is blocked. The corresponding acetophenone oxime derivatives were synthesized and their reactions were investigated. Treatment of the sodium salt of syn-methyl o-methylthiophenyl ketoxime with p-toluenesulfonyl chloride at 0° gave a yellow crystalline substance which underwent exothermic decomposition at 20°. The infrared spectrum of this compound at 0° displayed a very intense band at 2330 cm-1 in the region characteristic of triple-bond stretching .2 The infrared spectra of several types of carbon-nitrogen triple bond compounds were examined. The spectrum of benzonitrile in chloroform exhibits a strong band at 2230 cm-1, and that of phenyl iso-cyanide at 2130 cm-. p-nitrobenzoate Nj/ s-ch3 h2o ch3 N + CH3Z CH3-Cx / N-H sch3 III x\ // Upon working up the solution of la, three components were isolated. These were subsequently identified as 3-methylbenzisothiazole (32%, IV), the amide III (23%), and o-methylthioacetophenone, the latter produced by the hydrolysis of unrearranged oxime. Attempts to prepare a stable crystalline oxime ester were successful upon treating the sodium salt of the ketoxime with p-nitrobenzoyl chloride. Rearrangement of lb in tetrachloroethane produced 3-methyl-benzisothiazole (IV, 60% yield) and methyl p-nitro-benzoate (68%). The 3-methylbenzisothiazole was identified by micro-analysis, nmr, and conversion to its previously reported hydrochloride.5 The isolation of IV from the reaction of si/n-methyl o-methylthiophenyl ketoxime p-nitrobenzoate provides unequivocal evidence for ortho sulfur participation. The ketoxime ester reacts with participation to form a (4) C.