Halogenation of N-substituted p-quinone monoimines and p-quinone monooxime esters: X. Halogenation of N-aroyl-2,5(2,3)-dialkyl-1,4-benzoquinone monoimines and their reduction products

Abstract

Introduction of a strong electron-withdrawing substituent to the nitrogen atom of 2,5(2,3)-dialkyl-1,4-benzoquinone imines makes their halogenation products, the corresponding cyclohexene derivatives, very unstable and favors halogenation of methyl groups in the quinoid ring. Bromination of 4-amino-N-aroyl-2,5-dialkyl-6-bromophenols gave 2,5-dialkyl-6-benzoyloxy-3,5-dibromocyclohex-2-ene-1,4-diones. * For communication IX, see [1].We previously examined halogenation of N-arylsulfonyl-2,5-dialkyl-1,4-benzoquinone imines [2] and found that the main factors determining the direction of addition of halogen molecule are the size of substituents in the quinoid ring and the size of the halogen atom. The presence of a strong electron-withdrawing substituent on the nitrogen atom, in particular aroyl group, also considerably affects the halogenation process [1, 3,4]. Halogenation of N-aroyl-2,5(2,3)-dialkylsubstituted 1,4-benzoquinone imines and their reduction products, N-aroyl-2,5(2,3)-dialkyl-4-aminophenols was not reported previously.The goal of the present work was to study halogenation of N-aroyl-2,5(2,3)-dialkyl-1,4-benzoquinone imines, elucidate specific features of the halogenation process, and compare these reactions with halogenation of the corresponding N-arylsulfonyl derivatives. As substrates we used N-aroyl-1,4-benzoquinone imines which, like N-arylsulfonyl derivatives studied previously [2], contained different alkyl substituents (i-Pr, Me) in positions 2 and 5 of the quinoid ring: N-aroyl-2,5-dimethyl-, -2-isopropyl-5-methyl-, and -5-isopropyl-2-methyl-1,4-benzoquinone imines Ia-Ic, IIa-IIc, and IIIa-IIIc, and their reduction products, 4-amino-N-aroyl-2,5-dimethyl-, 2-isopropyl-5-methyl-, and -5-isopropyl-2-methylphenols IVa-IVc, Va-Vc, and VIa-VIc.Due to the presence of a methyl or isopropyl group in the ortho position with respect to the imino nitrogen atom, quinone imines I-III in solution exist as a single isomer, which is confirmed by the 1 H NMR data. In the 1 H NMR spectra of N-aroyl-2,5-dialkyl-1,4-benzoquinone imines, differences in the chemical shifts of protons in the quinoid ring were considerably smaller than those observed for analogous N-arylsulfonyl derivatives; therefore, assignment of signals in the spectra of compounds I-III was more difficult. The 3-H proton in I-III resonated in the 1 H NMR spectra in the region δ 6.58-6.66 ppm, while the 6-H signal was located at δ 6.44-6.59 ppm; the corresponding signals of N-arylsulfonyl derivatives appeared in the regions δ 7.89-7.96 and 6.50-6.64 ppm, respectively [2]. N-Aroyl-2,5-dialkyl-1,4-benzoquinone imines I-IIIwere synthesized by oxidation of the corresponding aminophenols IV-VI with lead tetraacetate in acetic acid. The chlorination of quinone imines I-III and aminophenols IV-VI was performed using gaseous chlorine, and the bromination was carried out with molecular bromine in CHCl 3 , AcOH, DMF, or DMFAcOH (1 : 5) at different substrate-to-reagent ratios under different temperature conditions. The results are given in Scheme 1. Unlike N-a...

Halogenation of N-substituted p-quinone monoimines and p-quinone monooxime ethers: XIII. Specificity of bromination of N-Acetyl(aroyl)-1,4-benzoquinone monoimines

“…Initial quinone monoimines Ia, Ib, IIa, IIb, IIIa, IIIb, IVb, IVc [1], IIf, IVf [20], Vg, VIg, VIIg [12], Va, Vb, Vd, VIa-VIe, VIIa-VIIe [3], XIVa, and XIVb [5] were synthesized according to known methods; their properties were consistent with published data.…”

“…Various halogen-containing derivatives were obtained and specificity of halogenation related to the effect of strong electron-withdrawing substituent on the nitrogen atom was revealed. In particular, low stability of intermediate cyclohexene structures [1,2], halogenation of methyl groups in the quinoid ring, and formation of 5-aroyloxycyclohex-2-ene-1,4-diones [2,3] were observed.…”

“…As shown previously, halogenation of N-substituted 1,4-benzoquinone monoimines follows different paths including halogen addition at the double C=C bond in the quinoid ring, dehydrohalogenation, hydrohalogenation, prototropic rearrangement, and electrophilic substitution, so that the reaction mixtures are complex multicomponent systems [1][2][3][4][5][6][15][16][17]. As the number of halogen atoms in the quinoid ring increases, further addition of halogen or hydrogen halide becomes more difficult, and more severe conditions are necessary [1-3, 12, 16, 17].…”

“…In the preceding communications of this series we described in detail halogenation of N-aroyl-1,4-benzoquinone monoimines and their reduction products [1][2][3][4][5], as well as of 4-acetylaminophenols having no other substituents in the benzene ring [6]. Various halogen-containing derivatives were obtained and specificity of halogenation related to the effect of strong electron-withdrawing substituent on the nitrogen atom was revealed.…”

“…In our previous studies, halogenation of various N-acetyl(aroyl)-1,4-benzoquinone monoimines gave only halogenated quinone monoimines, aminophenols, cyclohexene structures, and 5-aroyloxycyclohex-2-ene-1,4-diones [1][2][3][4][5][6], whereas hydrohalogenation of the same substrates afforded the corresponding 1,4-addition products [1][2][3][4][5][6]12]. All possible steps in the hydrohalogenation process were examined, while halogenation of N-acetyl-(aroyl)-1,4-benzoquinone monoimines having halogen atoms in the quinoid ring was not studied specially.…”