Halogenation of N-substituted p-quinone monoimines and p-quinonemonooxime ethers: XII. Halogenation of N-aroyl-2(3)-methyl-1,4-benzoquinone monoimines and their reduced forms

Abstract

A strong acceptor substituent at the nitrogen atom of the N-substituted p-quinone monoimine decreases the stability of the halogen-containing cyclohexene structures formed at the addition of a halogen molecule to the C=C bond of the quinoid ring. As a result of the bromination of N-benzoyl-2-methyl-1,4-benzoquinone monoimine alongside the usual products of addition and substitution the 5-benzoyloxy-2,3,6-tribromo-6-methylcyclohex-2-ene-1,4-dione was isolated.This study is a continuation of the research on the halogenation processes of N-substituted 1,4-benzoquinone monoimines. We formerly investigated in detail the halogenation of various N-arylsulfonyl-1,4-benzoquinone monoimines and their reduced forms [2-4], 4-amino-N-aroylphenol without substituents in the ring of the aminophenol [5], and N-aroyl-1,4-benzoquinone monoimines with two alkyl substituents in the ring [6,7]. The N-aroyl-1,4-benzoquinone monoimines unsubstituted in the quinoid ring were not previously examined due to their instability [5,8].In this study N-aroyl-2(3)-methyl-1,4-benzoquinone monoimines I, II and 4-amino-N-aroyl-2(3)-methyl-phenols III, IV were chosen as the objects of the investigation. In contrast to the corresponding N-arylsulfonyl-2-methyl-1,4-benzoquinone monoimines [3] the 1 H NMR spectra of N-aroyl-2-methyl-1,4-benzoquinone monoimines Ia, Ib contain a single set of signals. It was established formerly that the N-aroyl-1,4-benzoquinone monoimines had a lower barrier to the Z,E-isomerization [9] (ΔG ≠ 298K 44-46 kJ mol -1 ), than N-arylsulfonyl derivatives [10] (ΔG ≠ 298Q 65-80 kJ mol -1 ). Therefore in the solutions of compounds Ia, Ib already at the room temperature the fast Z,E-isomerization occurs, and notwithstanding the existence of two isomeric forms the 1 H NMR spectrum contains a single set of signals, namely, the degenerate Z,E-isomerization is observed. It was earlier established that the presence of the Z,E-isomerization signifi cantly affected the halogenation course of the N-arylsulfonyl-1,4-benzoquinone monoimines [3].The difference in the chemical shifts of protons H 5 and H 6 (H 2 and H 3 ) of the quinoid ring is not so pronounced in the 1 H NMR spectra of compounds I, II as in the case of N-arylsulfonyl derivatives. This is due to the fact that the chemical shifts of the protons of the quinoid ring of N-aroyl derivatives A because of the fast Z,E-isomerization depend mainly on the position of this proton in the ring and on the character of the substituents in the neighboring positions. In N-arylsulfonyl derivatives B the chemical shifts of the protons in the quinoid ring are strongly affected by the position of the substituent at the nitrogen atom (cis-or trans-): the signal of the proton in the position 5 of the quinoid ring is shifted downfi eld and appears in the region 7. 92-8.18 [3], of the proton H 3 , in the region 6.81-6.93 ppm. In aroyl derivatives A the signal of the atom H 5 of quinone imines Ia, Ib is shifted upfi eld compared with the sulfonyl derivatives and appears in the region 7.02-7.08, and th...

Reaction of some N-substituted 1,4-benzoquinone imines with sodium arenesulfinates

“…The properties of previously described compounds Ia [24], IIa, IIIa [25], IV [26], Va [27], VIa, VIb, VIIa, VIIb [28], IX, XIa, and XIb [9] were consistent with published data.…”