Synthesis, crystal structure, dynamic behavior and reactivity of dinaphtho[2,1-b:1?,2?-d]phospholes and related atropisomeric phosphacyclic derivatives

7-Phenyldinaphtho[2,l-b:1',2'-d]phosphole (lb) has been prepared by reaction of dichlorophenylphosphine either with 2,2'-dilithio-l,l'-binaphthalene or with bis-dialin 7. In the latter case the relevant tetrahydro derivative 8 is formed at the same time. Displacement of the phenyl substituent in lb by alkyl groups can be accomplished through a dephenylation-alkylation protocol involving a lithium-promoted phosphole anion formation. By this way the unsubstituted phosphole la and a variety of alkyl-substituted derivatives have been prepared. The X-ray crystal structure of lb shows an intracyclic C-P-C angle of 89.3" indicating that phosphole ring is strained. Alkylphospholium iodides 4 undergo ready ring opening by reaction with LiAlH4 or other nucleophiles under mild conditions affording with fair diastereoselectivities (2-(1,l'-binaphthylyl))- substituted phosphines 6 or phosphines oxides 6, respectively. Dinaphthophospholes 1 are fluxional at ambient temperature because of the rapid interconversion of the atropisomeric conformers. Line shape analysis of the variable temperature NMR spectra lead to estimation of an energy barrier of 55-60 kJ mol-' for this process. Fluxionality is maintained both when the phosphorus center is tetrasubstituted, like in the relevant oxides 2, and when it is coordinated to a transition metal, like in the Pd-complexes 10. On the contrary, P-substituted dinaphthophosphepines 9 do not undergo atropisomerization even well above room temperature and can be successfully resolved under ambient conditions. The crystal structures of the P-phenyl-substituted derivatives lb, 8 and 9a, as determined by X-ray diffraction, show remarkable differences in the relative disposition of the naphthalene rings.