Synthesis and Reactivity of Five-Membered P(V)-Phosphapalladacycles

Reactions of Pd(PPh3)4 with P(E)(NiPr2)(naph) (6, E = S; 7, E = O) (naph = 1,8-naphthylene) having a strained four-membered P(V)-phosphacycle gave dimeric complexes [Pd{κ2P,C-μ2-PE- P(E)(NiPr2)(naph)}(PPh3)]2 (9, E = S; 10, E = O), in which a Pd metal has been inserted into a P−C bond of the phosphacycle to form a phosphapalladacycle having a P(V) donor, and the two phosphapalladacycle units have been mutually bridged with EP groups, as confirmed by X-ray structure analysis for thermodynamically more stable racemic isomers 9a and 10a. The meso-to-racemic isomerization observed for the simultaneously formed meso isomer 9b, and probably also for the corresponding meso isomer 10b, indicated partial dissociation of 9 and 10 taking place to their monomer units in solution, which were actually trapped as [Pd{κ2P,C-P(E)(NiPr2)(naph)}(dppe)] by treatment with bidentate dppe (dppe = 1,2-bis(diphenylphosphino)ethane). On the other hand, a similar treatment of 9a with monodentate PMe3 and PEt2Ph resulted in a trivial substitution of both PPh3 ligands to give dimeric PMe3 and PEt2Ph analogues of 9a, respectively. 10a was found to react with O2 to form an unprecedented oxidation product, [Pd{κ2P,O-μ2-PO-PO(O)(NiPr2)(naph)}(PPh3)]2, 17, in which an oxygen atom has been inserted into each P(V)−Pd bond. 9a and 10a exhibited moderate catalytic activities for a Heck reaction between PhI and styrene. The mercury test indicated that metallic Pd nanoparticles released from 9a and 10a would be practical catalysts.