Sandmeyer reactions. Part 4.1 An investigation into the cyclisation modes of Pschorr phenanthrene synthesis

Comparison of the cyclisation regiochemistries in the heterolysis and the copper-catalysed homolysis of methyl (E)-3-(2-diazoniophenyl)-2-(3-halophenyl)propenoate tetrafluoroborates indicates that the homolytic pathway involves direct closure of the six-membered ring and not a five-membered ring closure followed by ring expansion. From competition experiments in which homolytic cyclisation of the corresponding non-halogenated compound was run against hydrogen abstraction from hypophosphorous acid, a cyclisation rate constant kC = (3.0 ± 0.5) × 109 s–1 at ambient temperature was estimated which, when used in conjunction with a literature value for the homolytic phenylation of benzene, allows evaluation of a statistically corrected effective molarity of 2 × 104 mol dm–3 for homolytic Pschorr phenanthrene closure. Regioselectivity considerations imply that, by contrast, heterolytic Pschorr phenanthrene closure exhibits unit effective molarity. A mechanistic rationale is presented to explain these patterns of behaviour.