On the Oxidation of Different Iminic Bonds by Excess of 3-Chloroperbenzoic Acid

Abstract

Abstract:In the present work the behavior of different substituted iminic bonds toward the oxidative action of 3-chloroperbenzoic acid is reported. The C=N bond was or was not oxidized to oxaziridines, amides, oximes, nitroso-, nitro-, and azodioxy compounds depending on the substituents at the iminic group and on the imine/MCPBA stoichiometric ratio. Key words: imines, oxidation, oxaziridines, C-nitroso compound, oximesAlthough the reduction 1 and hydrolysis 2 of imines has been largely studied, only a few publications report its behavior toward oxidizing agents. It has been reported that benzylidene alkylamines lead to the corresponding oxaziridines by stoichiometric oxidation with peracids, 3 urea hydrogen peroxide, 4 and cobalt-mediated molecular oxygen 5 (Scheme 1). Scheme 1 Synthetic methodology for oxaziridine generationA number of thermally stable oxaziridines, obtained by oxidation of benzylidene alkylamines, 6 have been employed both as oxygenating and/or aminating agents of nucleophilic species 7 and as reagents in cycloaddition reactions with heterocumulenes, 8 alkenes, 9 alkynes, 10 and nitriles. 11 Reports of reactions of imines with excess MCPBA are scarce. Previously, we have reported that the oxidation of benzylidene alkylamines 1-3 by 1.1 mmol of MCPBA in CH 2 Cl 2 solution led to oxaziridines 1a-3a in good yields (>90%), 12 while nitroso compounds 1b-3b rapidly dimerized to azodioxy compounds 1c-3c and were obtained employing 2.2 mmol of MCPBA (Scheme 2). Furthermore compounds 2b, 3b, 2c, and 3c, having a hydrogen at the α position of R 1 , undergo isomerization into oximes 2d and 3d by heating in toluene solution.Moreover, the azodioxy dimer 3c was obtained in quantitative yield by reaction of 1.1 mmol MCPBA with the isolated oxaziridine 3a.The same result was obtained on oxidizing the cyclic imine 3,4-dihydro-2H-pyrrole 4 with 1.1 mmol of MCPBA; the condensed oxaziridine 4a (yield 98%) was obtained in this case. Product 4a was subsequently oxidized into nitroso compound 4b that rapidly dimerized to azoxydimer 4c when a further 1.1 mmol of MCPBA were added. Furthermore, on heating 4c in toluene (80°C), 4d was obtained (yield 80%, Scheme 3). 12 Continuing our studies on the oxidation of imines with MCPBA we have discovered outcomes strongly dependent on the C=N bond substituents. Due to the lower basicity of the nitrogen in 5-7 with respect to compounds 1-4, the second oxygen transfer on oxaziridines 5a-7a, formed on initial oxidation, did not take place. Instead, N,N-diarylamides 8-10 were obtained both with 1.1 mmol or 2.2 mmol of peracid, after a carbon-nitrogen migration of the aryl group (Scheme 4). Amides were also obtained in reactions of imines with sodium perborate 13 or with MCPBA and BF 3 ·OEt 2 . 14 A further decrease of basicity of the imine nitrogen as in oximes 11, isoxazolines 12, benzothiadiazines 13, and osazones 14 (Figure 1), due to the presence of a heteroatom on the nitrogen atom, diminished the reactivity towards C=N oxidation, and starting materials were recovered even ...

From seaweeds to biopolymeric electrolytes for third generation solar cells: An intriguing approach

“…This analysis was conducted to observe changes of the functional group before and after modification [31].…”