A Retro Diels–Alder Route to Diphosphorus Chemistry: Molecular Precursor Synthesis, Kinetics of P₂ Transfer to 1,3-Dienes, and Detection of P₂ by Molecular Beam Mass Spectrometry

Abstract

The transannular diphosphorus bisanthracene adduct P₂<b>A</b>₂ (<b>A</b> = anthracene or C₁₄H₁₀) was synthesized from the 7-phospha­dibenzo­norborna­diene Me₂NP<b>A</b> through a synthetic sequence involving chloro­phosphine ClP<b>A</b> (28–35%) and the tetracyclic salt [P₂<b>A</b>₂Cl]­[AlCl₄] (65%) as isolated intermediates. P₂<b>A</b>₂ was found to transfer P₂ efficiently to 1,3-cyclohexadiene (CHD), 1,3-butadiene (BD), and (C₂H₄)­Pt­(PPh₃)₂ to form P₂(CHD)₂ (>90%), P₂(BD)₂ (69%), and (P₂)­[Pt­(PPh₃)₂]₂ (47%), respectively, and was characterized by X-ray diffraction as the complex [CpMo­(CO)₃(P₂<b>A</b>₂)]­[BF₄]. Experimental and computational thermodynamic activation parameters for the thermolysis of P₂<b>A</b>₂ in a solution containing different amounts of CHD (0, 4.75, and 182 equiv) have been obtained and suggest that P₂<b>A</b>₂ thermally transfers P₂ to CHD through two competitive routes: (<i>i</i>) an associative pathway in which reactive intermediate [P₂<b>A</b>] adds the first molecule of CHD before departure of the second anthracene, and (<i>ii</i>) a dissociative pathway in which [P₂<b>A</b>] fragments to P₂ and <b>A</b> prior to addition of CHD. Additionally, a molecular beam mass spectrometry study on the thermolysis of solid P₂<b>A</b>₂ reveals the direct detection of molecular fragments of only P₂ and anthracene, thus establishing a link between solution-phase P₂-transfer chemistry and production of gas-phase P₂ by mild thermal activation of a molecular precursor