Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part 16. Formation and reactions of 2,3-dihydro-1,3,2-benzothiazaphosph(V)oles [amino(thiyl)phosphoranes]

Abstract

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-Me₃C₆H₃; 51%) and its 2,4.6-trimethylphenyl analogue (7; Ar = 2,4,6-Me₂C₆H₂; 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 (10⁵k₁ at 451 K = 2.4 ± 1.2 s^–1), the high activation energy (E_act. 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 T_c= 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.