REDUCTION OF NITRO-COMPOUNDS AND NITROSO-COMPOUNDS BY TERVALENT PHOSPHORUS REAGENTS .16. FORMATION AND REACTIONS OF 2,3-DIHYDRO-1,3,2-BENZOTHIAZAPHOSPH(V)OLES [AMINO(THIYL)PHOSPHORANES]

In contrast to the corresponding ethers, which give 3-aryl-2,3-dihydro-1,3, 2-benzoxazaphosph(v)oles [amino-(oxy)phosphoranes] (1; X = O) on reaction with phosphorus(III) esters, aryl 2-nitroaryl sulphides give N-aryl-N-(2- alkylthiophenyl)phosphoramidates (4), in addition to N-S heterocycles. Yields of (4) are high (70%) when o-blocking methyl groups are present. Using 31P Fourier-transform n.m.r. spectroscopy we have shown that compounds (4) are probably derived via the intermediacy and isomerisation of amino(thiyl)phosphoranes (1; X = S). This has been confirmed, using 2-phenyl-1,3,2-dioxaphospholan, by the isolation of the novel 2,3-dihydro-2-phenyl-3-(2,4,6-trimethylphenyl)-1,3,2-benzothiazaphosph(v) ole-2-spiro-2′-1′,3′,2′-dioxaphospholan (6; Q=Y = Me) and its 2,6-dimethylphenyl- and 2,6-dimethoxyphenyl-analogues (6; Q = Me, Y = H and 6; Q = MeO, Y = H) in 32-76% yield. These do not isomerise to amidates corresponding to (4). The use of methyl diphenylphosphinite (Ph2POMe) in boiling toluene led to the first monocyclic amino(thiyl)phosphoranes, 2,3-dihydro-2,2-diphenyl-3-(2,6-dimethylphenyl)-2-methoxy-1,3, 2-benzothiazaphosph(v)ole (7; Ar = 2,6-Me2C6H3; 51%) and its 2,4,6-trimethylphenyl analogue (7; Ar = 2,4.6-Me3C 6H2; 48%). In boiling cumene (153°C) these isomerised into N-(2,6-dimethylphenyl)-N-(2-methylthiophenyl)-PP-diphenylphosphinamidate (8) and its trimethylphenyl analogue. A kinetic study, using a programmed n.m.r. technique, showed that this reaction is first order (105k 1 at 451 K = 2.4 ± 1.2 s-1), the high activation energy (Eact, 220 ± 60 kJ mol-1) pointing to a rate-determining fission of the phosphorane ring to give a quasi-phosphonium betaine (10), followed by fast alkyl transfer. By-products in the formation of (7) and (8) from methyl diphenylphosphinite and aryl 2-nitrophenyl ethers include N-(2,6-dimethylphenyl)-N-(2-thiophenyl)-PP-diphenylphosphinamidate (11), formed by acidic hydrolysis of (7); 2-(2,6-dimethylphenylthio)phenylimino-PP- diphenyl-P-methoxyphosphorane (14) and its thermally derived isomer, N-methyl-N-2-(2,6-dimethylphenylthio)phenyl-PP-diphenylphosphinamidate (17), and products of the hydrolysis of (14), N-2-(2,6-dimethylphenylthio)phenyl-PP- diphenylphosphinamidate (13) and the derived amine (12). The formation of 2-methylthio-2′,6′-dimethyldiphenylamine (15) is attributed to methylation of the thioquinone imine (16) postulated as an intermediate in the reaction. Analogous products were obtained from the 2,4,6-trimethylphenyl homologue. The spirophosphoranes (6) exhibit classical temperature-dependent ligand reorganisation observable by n.m.r. with Tc = 110-126°C and ΔG‡ = 80.4-89.8 kJ mol-1. Compounds (7) exhibit solvent chemical-shift changes from -37.8 δ(CDCl3) to +72.4 [(CF3)2CHOH-CDCl3] indicating ring fission to the isomeric quasi-phosphonium betaines.