Raney Niを用いる芳香複素環N-オキシドの接触還元 (第5報) 含硫黄置換基を有するN-オキシドの還元

Abstract

Catalytic reduction at ordinary temperature and pressure over Raney nickel was carried out on N-oxide compounds with sulfur-containing substituent, 4-phenylthiopyridine 1-oxide (I), phenyl 4-pyridyl sulfone 1-oxide (II), 4-ethylthiopyridine 1-oxide (III), and sodium salt (IV) of 4-mercaptoquinoline 1-oxide. (I) and (II) underwent rapid reduction, as in the case of previously reported N-oxides, and respectively afforded 4-phenylthiopyridine (V) and phenyl 4-pyridyl sulfone (VI). In the case of (III), absorption of hydrogen stopped after about one-half the calculated amount of hydrogen had been absorbed and a part of the starting material was recovered together with 4-ethylpyridine (VII). This fact suggests that the sulfone group might work as a catalyst poison but thioether in aryl thioethers has not proved to be of great hazard. In the case of alkyl thioethers, the thioether group does act as a catalyst poison but the action of Raney nickel on the N-oxide group is so great that the reduction proceeds with considerable speed before the catalyst loses its activity. In the case of (IV), absorption of hydrogen also stops after about one-half the theoretical amount had been absorbed, and quinoline (VIII) and 4, 4′-diquinolyl sulfide 1, 1′-dioxide (IX) are obtained. This shows that the reduction of N-oxide group is followed by desulfurization by Raney nickel, by which the catalytic activity of Raney nickel is lost, and the unreacted material (IV) changes into (IX) during aftertreatment. 4-Mercaptoquinoline 1-oxide (X) itself does not undergo reduction of the N-oxide group and the starting compound is recovered almost completely.