REACTIONS OF 1,2-OXAPHOSPHOLENE-2-OXIDES .5. CONSEQUENCES OF REDUCTION

The reactions of 5, 5-dimethyl-2-phenyl-l, 2-oxaphosphol-3-ene 2-oxide (1a) with a variety of reducing agents have been studied. Catalytic hydrogenation of la (Pd/C) resulted in reduction of the C=C bond, giving 5, 5-dimethyl-2-phenyl-l, 2-oxaphospholane 2-oxide (6). By contrast, hydroboration with excess BH3THF left the double bond intact, and reduced the P=O group. The product, 5, 5-dimethyl-2-phenyl-1, 2-oxaphosphol-3-ene (7), is itself unstable, but it can be isolated as the novel borano complex 8. The P=0 group in la can also be reduced by trichlorosilane. In this case the unstable 7 was isolated as the corresponding tungsten pentacarbonyl complex 10. Conversion of 1a to the corresponding P-sulfide 9 was readily accomplished, but 9 could not be selectively reduced with metallic sodium. Lithium aluminum hydride reduced both the C=C bond and the P=0 group, but the presumed product 5, 5-dimethyl-2-phenyl-l, 2-oxaphospholane (5) was unstable and underwent ring opening to a compound tentatively identified as (3-hydroxy-3-methylbutyl) phenylphosphinic acid (3), which is in equilibrium with its phosphine oxide tautomer 4. © 1990, Taylor & Francis Group, LLC. All rights reserved.