REDUCTION OF HETEROCUMULENES PROMOTED BY LOW-VALENT LANTHANOIDS

Heterocumulenes react with lanthanoid reductants such as Yb metal, YbC l3/Zn, and SmI2. In tetrahydrofuran (THF) or THF-hexamethylphosphoric triamide (HMPA), diphenylketene (1) is reduced with Yb or YbCl3/Zn to give dimeric compounds (2-5), whereas monomeric products (6-8) are obt ained with SmI2, one-electron reductant. Isocyanates (11 a-g) are redu ced with SmI2, to produce oxamides (12 a-g) in moderate to good yields . This reaction proceeds via one electron transfer from SmI2, followed by homocoupling of radical intermediates [H]. At room temperature 1-n aphthyl isocyanate (11 e) is treated with SmI2 to give 1-naphthylamine as a main product, while at lower temperature, corresponding oxamide (12 e) is obtained as a main product. Higher temperature and longer re action time are necessary to complete the reaction of aliphatic isocya nates. Furthermore, the SmI2/THF/HMPA system desulfurizes isothiocyana tes, R-NCS (13 a-c), under the mild conditions, to give isonitriles (1 4 a-c) in good yields, irrespective of the substituents (R). Thioketon e (15) also undergoes desulfurization by Yb metal in THF-HMPA to give olefin (16) at room temperature.