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

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