Azo-containing tertiary phosphines: synthesis, reactivity and structural characterisation

Abstract

6-Bromo-2-methoxynaphthalene 1 was converted into 6-diphenylphosphanyl-2-methoxynaphthalene 2a by preparing its Grignard and quenching with PPh₂Cl. Compound 2a was demethylated on refluxing in HBr yielding 6-(diphenylphosphanyl)naphthalen-2-ol 2b in good yield. Oxidation of 2b with either H₂O₂ or S₈ afforded the corresponding phosphine oxide 3a or sulfide 3b respectively. Treatment of 2b with a stoichiometric amount of NaH and quenching of the anion with [4-R-C₆H₄N₂][BF₄] (R = H, Me, Et, ⁱPr, ^tBu, NO₂ or NMe₂) yielded the C–N coupled azo-containing phosphines in good yield; similar coupling reactions of 3a, 3b afforded analogous compounds, again in good yield. Evidence is presented that shows the coupling reaction does not proceed through a P–N coupled intermediate, which would subsequently need to rearrange to the observed C–N coupled products. The latter all exist as tautomeric mixtures of the azo and hydrazone forms. The tautomerisation in some cases was suppressed on conversion into their acetic acid esters by reaction with NaH followed by acetyl chloride. All of the new compounds have been characterised by elemental analysis (C, H, N), FAB mass spectrometry, ¹H, ¹³C-{¹H}, ³¹P-{¹H} NMR and in selected cases by Uv-visible spectroscopy. The position of the C(2) resonance in the ¹³C-{¹H} NMR spectra has been used to calculate the position of the azo/hydrazone equilibrium and hence the mole fraction of each tautomer present in solution. These data were used to interpret the Uv-visible data. In addition, three compounds have been further characterised by single crystal X-ray diffraction studies.

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