Intramolecular [4 + 2] Diels-Alder cycloaddition reactions of phospholes with vinylphosphines promoted by nickel

Article abstract of DOI:10.1021/ic00317a012

The reaction of divinylphenylphosphine with 1-phenyl-3,4-dimethylphosphole in the presence of anhydrous nickel(II) bromide leads to the formation of a single diastereomer of bromo[phenylbis(5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1]hept-5-en-2-yl) phosphine]nickel(II) bromide, [LNiBr]⁺Br^-, as the result of two sequential stereoselective intramolecular [4 + 2] Diels-Alder cycloadditions. This reaction provides a facile route to a new type of conformationally rigid tridentate phosphine. This new complex has been characterized by elemental analysis, physical properties, ¹H, ¹H{³¹P}, ¹³C{¹H}, and ³¹P{¹H} NMR spectroscopy, electronic and infrared spectroscopy, and cyclic voltammetry. The ligand has been isolated after liberation from nickel by cyanide displacement in a two-phase solvent system (CH₂Cl₂/H₂O) and converted to its trisulfide. It has also been transferred to palladium, platinum, and ruthenium to form the pentacoordinate LPdCl₂, the four-coordinate square-planar [LPtCl]⁺Cl^-, and mer-L Ru(PPh₃)Cl₂ and trans-L₂RuCl₂, respectively. The last complex contains two dangling-arm phosphines. These complexes have been characterized by the above techniques, including X-ray crystallography for mer-LRu(PPh₃)Cl₂. This molecule crystallizes in the monoclinic space group P2₁/n in a unit cell of dimensions a = 11.916 (3) ?, b = 22.074 (7) ?, c = 18.309 (4) ?, β = 104.43 (2)°, and V = 4664 (2) ?³, with Z = 4. Refinement converged to R = 0.038 for 5033 independent observed (I/σ(I) ≥ 3.0) reflections. The two Ru-P(bridging) distances are slightly different (2.350 (1), 2.320 (1) ?) and are longer than Ru-P(exocyclic) (2.308 (1) ?). The C-P-C angles for the bridging phosphorus atoms are small (79.5 (2), 79.1 (2)°). Bromobis[2-(vinylphenylpnosphino)-5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1] hept-5-ene]nickel(II) bromide was a minor product of the reaction of divinylphenylphosphine, 1-phenyl-3,4-dimethylphosphole, and nickel(II) bromide. The reaction of vinyldiphenylphosphine, 1-phenyl-3,4-dimethylphosphole, and nickel(II) bromide resulted in the formation of mono- and bis(2-(diphenylphosphino)-7-phenylphosphanorbornene) nickel(II) complexes. These complexes were also characterized by the above-mentioned techniques, including X-ray crystallography for dibromo[2-(diphenylphosphino)-5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1]hept- 5-ene]nickel(II). This molecule crystallizes in the triclinic space group P1 in a unit cell of dimensions a = 12.177 (3) ?, b = 12.448 (3) ?, c = 8.664 (2) ?, α = 100.83 (2)°, β = 99.11 (2)°, γ = 81.11 (2)°, and V= 1263.0 ?³, with Z = 2. Refinement converged to R = 0.038 for 2692 independent observed (I/σ(I) > 3) reflections. The donor abilities of the two phosphorus donors are different: Ni-P distances are 2.135 (1) and 2.169 (1) ?, the C-P-C angle for the bridging phosphorus is 81.4 (2)°, and the Ni-Br (2.3433 (8), 2.3230 (8) ?) distances are significantly different. The chelate ring is conformationally rigid, and the molecule contains four asymmetric centers.