Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part XII. Conversion of aryl 2-nitroaryl ethers into novel 3-aryl-2,3-dihydro-1,3,2-benzoxazaphosph(V)oles (oxazaphosphoranes) and their 2-oxo-derivatives

Abstract

Reactions in cumene of aryl 2-nitroaryl ethers (2-NO₂·C₆H₄·O·C₆H₂R³₂R⁴: R³= Me, R⁴= H; R³= R⁴= Me; R³= OMe, R⁴= H; R³= H, R⁴= OMe; R³= R⁴= H; R³= H, R⁴= Me; R³= Me, R⁴= CO₂Me) with tervalent phosphorus reagents (4 mol. equiv.)(R²R¹₂P: R¹= R²= MeO, EtO, or PrⁱO; R¹= EtO, R²= Me; R¹= MeO, R²= Ph; R¹= Ph, R²= MeO; R¹₂=–O·CH₂·CH₂·O–, R²= Ph) give novel pentaco-ordinate phosphorus compounds, the 3-aryl-2,3-dihydro-1,3,2-benzoxazaphosph(V)oles (8)(12–95%). These are very rapidly hydrolysed in water to give tetraco-ordinate phosphorus compounds, 2-alkoxy-3-aryl-, 2-hydroxy-3-aryl-, 2-alkyl-3-aryl-, or 2,3-diaryl-2,3-dihydro-2-oxo-1,3,2-benzoxazaphospholes (15)–(17), according to conditions and the phosphorus ligands in the original oxazaphosphole (8). Hydrolysis in acid leads to 2-hydroxydiarylamines (13). The reaction of 4-methoxyphenyl 2-nitrophenyl ether with diethyl methylphosphonite gave as a major by-product (19.5%) a spiro-oxazaphosphole (32), probably via ligand exchange of the first-formed oxazaphosphole (8) with its hydrolysis product, 2-hydroxy-4′-methoxydiphenylamine. The reaction of ethane-1,2-diol with 2,3-dihydro-2,2-dimethoxy-2-phenyl-3-(2,4,6-trimethylphenyl)-1,3,2-benzoxazaphosphole (34) similarly gave the spiro-oxazaphosphole (35).

Small quantities, only, of non-phosphorus-containing heterocycles are formed as by-products in these reactions: 2,6-dimethylphenyl-, 2,4,6-trimethylphenyl-, and 2,6-dimethoxyphenyl 2-nitrophenyl ethers gave, respectively, 5,11-dihydro-4-methyldibenz[b,e][1,4]oxazepine, its 2,4-dimethyl analogue, and a mixture of 1,2-dimethoxy-phenoxazine and 1- or 4-methoxy-phenoxazines in 1–5% yields.

The mechanisms of these reactions, which are markedly different from those of corresponding reactions of aryl 2-nitroaryl sulphides, are briefly discussed.