A series of rhodium pyrazolylborate complexes [TpRRh(PPh3)2] {TpR = HBR‘3, R‘ = 3,5-dimethylpyrazolyl (1), pyrazolyl (3), 3-phenylpyrazolyl (4), or 3-phenyl-5-methylpyrazolyl (5)} and [BpRRh(PPh3)2] {BpR = H2BR‘2, R‘ = 3,5-dimethylpyrazolyl (2)} were prepared from [ClRh(PPh3)3] and KX
(X = pyrazolylborate anion). Complexes 2−5 were characterized crystallographically. The solution
structures of new complexes 4 and 5 were also examined by variable temperature NMR and IR
spectroscopy. The catalytic activity of complexes 1−5 in alkyne hydrothiolation was then examined. It
was found that disubstituted tris(pyrazolyl)borate complexes (1 and 5) give the best selectivity and yield
in alkyne hydrothiolation with aliphatic thiols.
Oximes and semicarbarones are oxidized by ceric ammonium nitrate to the parent carbonyl cornpound in good yield. Suitable solvent systcms includc aqucous alcohols, acetonitrile, and acetic acid. The reaction is rapid at 0" or below. This procedure for regeneration of aldehydes and ketones from these derivatives may be preferable to the well-known hydrolytic procedures in some cases. The stoichiometry of the reaction was studied by isothermal calorimetry; two equivalents of oxidant are consumed by one mole of oxime evolving approximately 40 kcal/mole. A smaller consunlption of oxidant by hexane-2,s-dione dioxinie was attributed to stabilization of the monoxime as a nitroso chelate. Gaseous products were studied by infrared spectroscopy and nitrous oxide was identified as the sole product absorbing between 3 and 15 p. A reaction mechanism is proposed involving one-electron oxidation to the iminoxy radical, oxidized by a second one-electron step to the cc-hydroxy nitroso compound. Dimerization and elimination of hyponitrous acid gives the carbonyl compound. Nitrocyclohexane was absent from the oxidation product from cycIohexanone oxime. Carvone oxime was oxidized to carvone without racemiration or significant attack of the olefin groups.
Oxidation of 1,3:2,5-di-0-methylene-L-rhamnitol yielded 2,5:4,6-di-0-methylene-1-deoxy-Larabo-3-hexulose, which was characterized as its 2,5-dichlorophenylhydrazone and by reduction to derivatives of L-rhamnitol and of 6-deoxy-L-talitol. Oxidation of 2,s-0-methylene-L-rhamnitol I-benzoate yielded a syrupy 3,4-diketose which was characterized as the crystalline quinoxaline derivative. Attempts to convert 1,2:4,5-di-0-isopropylidene-D-mannitol 6-benzoate to a ketose were unsuccessful.
INTRODUCTIOX
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