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.