Trimethylsilyl- and trimethylstannyldimethylphosphane--convenient and versatile reagents for the synthesis of polyfluoroaryldimethylphosphanes.
Trimethylsilyldimethylphosphane (Me3SiPMe2) and the corresponding tin compound (Me3SnPMe2) were used as reagents for the substitution of fluorine by the Me2P group in polyfluoroarenes C6F5X (X = F, H, Cl, CF3) and C5NF5. The reactions occur even under mild conditions (T = 0-20 C), either in benzene or without solvent, to give as a rule 4-X-1-(dimethylphosphano)tetrafluorobenzenes (XC6F4PMe2, 1-4) and 4-(dimethylphosphano)tetrafluoropyridine (C5NF4PMe2, 5), respectively, in yields between 75 and 95%. In the case of C6F6, double substitution is also observed, which affords 1,4-bis(dimethylphosphano)tetrafluorobenzene (6). A very efficient route to the compounds XC6F4PMe2 (X = F, H, Cl, CF3) and C5NF4PMe2 was developed as a one-pot reaction of the corresponding fluoroarenes with tetramethyldiphosphane (P2Me4) and trimethyltin hydride (Me3SnH) at moderate temperatures. This process was tested for C6F6 and perfluorobiphenyl which gave C6F5PMe2 (1) and 4,4'-bis(dimethylphosphano)octafluorobiphenyl (7), respectively. The results, which included kinetic measurements that used the intensities of the 31P signals, revealed the influence of the substrate type on the rate of reaction in the sequence: C5NF5>C6F5CF3> C6F5Cl, C6F5PMe2>C6F5H>C6F6>> C6H5F. Ab initio calculations were carried out on the model reactions of pentafluoropyridine with silylphosphane, phosphane or phosphide to discriminate between possible reaction mechanisms. The novel phosphanes were characterised by spectroscopic investigations (NMR, MS), by preparation of the related thiophosphanes ArFP(=S)Me2 (8-14), their spectroscopic and analytic data and single crystal X-ray diffraction studies on five of these derivatives.
The preparation and properties of polyfluoro aromatic and heteroaromatic compounds
Abstract This review is concerned with ring systems formally recognised as “aromatic” and possessing a large proportion of fluorine atoms directly attached to the carbon skeleton. These include derivatives of the following: benzene and polynuclear benzenoid systems; pyridine, diaza- and triazabenzenes and some of their fused six-membered ring compounds; and a wide variety of fused ring carbocycles and heterocycles. Some highly fluorinated non-benzenoid aromatic compounds obeying the Huckel 4n + 2 π electron rule, as well as antiaromatic 4n π electron systems, are included in the survey. The wide variety of methods used for synthesising these highly fluorinated compounds, which enabled them to be regarded no longer as mere chemical curiosities, is presented. The chemical behaviour of polyfluoro aromatic and heteroaromatic compounds, at the fluorinated ring, forms the major proportion of this review: reactions with nucleophiles and a rationalisation of orientation reactions; reactions with electrophiles, free radicals, carbenes and nitrenes; the formation of stable salts of radical cations; photochemical reactions and the formation of valence isomers; and rearrangement reactions. Organometallic derivatives of polyfluorinated compounds have been extensively used in organic synthesis.
Studies in azide chemistry. Part V. Synthesis of 4-azido-2,3,5,6-tetrafluoro-, 4-azido-3-chloro-2,5,6-trifluoro-, and 4-azido-3,5-dichloro-2,6-difluoro-pyridine, and some thermal reactions of the tetrafluoro-compound
4-Azido-2,3,5,6-tetrafluoropyridine can be prepared by nitrosation of 2,3,5,6-tetrafluoro-4-hydrazinopyridine or, preferably, treatment of pentafluoropyridine with sodium azide in acetonitrile, a reagent that converts 3-chloro-2,4,5,6-tetrafluoro- and 3,5-dichloro-2,4,6-trifluoro-pyridine into 4-azido-3-chloro-2,5,6-trifluoro- and 4-azido-3,5-dichloro-2,6-difluoro-pyridine, respectively. 4-Azido-2,3,5,6-tetrafluoropyridine decomposes smoothly into nitrogen and an intractable solid at temperatures above ca. 130 °C, undergoes the Staudinger reaction with tri-phenylphosphine at room temperature, reacts with dimethyl sulphoxide at elevated temperatures to give a sulphoximide, partakes in 1,3-dipolar cycloaddition reactions with benzyne, diphenylacetylene, phenylacetylene, cyclopentadiene dimer, and norbornene, yields 4-anilino-2,3,5,6-tetrafluoropyridine with benzene at 175 °C, and reacts with cyclohexane to give the C–H ‘insertion’ product 4-(cyclohexylamino)-2,3,5,6-tetrafluoropyridine and 4-amino-2,3,5,6-tetrafluoropyridine. Thermal decomposition of the azide in a mixture of cyclo-hexane, -heptane, and -octane reveals that C–H ‘insertion’ is encouraged by increase in ring size, and thermolysis experiments with cis- and trans-1,2-dimethylcyclohexane show that this type of reaction lacks stereospecificity. The results of reactions between the azide and trans-4-methylpent-2-ene at elevated temperatures indicate that 1,3-dipolar cycloaddition occurs before nitrene generation can be achieved. Authentic samples of the new amino-pyridines [PhNHpyF, cyclo-CnH2n–1·NHpyF(n= 6, 7, or 8), cis- and trans-cyclo-1,2-Me2C6H9·NHpyF, and cis- and trans-Pri[graphic omitted]pyF(pyF= 2,3,5,6-tetrafluoro-4-pyridyl)] encountered in this work can be obtained via nucleophilic attack by the appropriate amines on pentafluoropyridine.