X-ray structures of adducts of 1,2,5-triphenylphosphole with dimethyl acetylenedicarboxylate: A confirmation, a revision, and a new structure

Article abstract of DOI:10.1139/v93-235

Dimethyl acetylenedicarboxylate was reacted with 1,2,5-triphenylphosphole under mild conditions, and four products isolated. Regardless of conditions used, every preparation gave low yields of dimethyl 3,6-diphenylphthalate coincident with loss of the phosphorus bridge. NMR data and X-ray crystal structures are provided for a yellow, and a red, 2:1 adduct. The yellow adduct, tetramethyl 1,6,7-triphenyl-6-phospha(V)-tricyclo[7.3.1.0^1,9]nona-3,5,7-triene- 2,3,4,5-tetracarboxylate 6, crystallized in the triclinic space group P1? (No. 2) with two molecules in the unit cell. Refinement (a = 11.259(1) ?, b = 12.947(3) ?, c = 13.784(3) ?, α = 112.25(2)°, β = 103.44(1)°, and γ = 101.80(1)°) converged at R = 0.0453 (R(w)= 0.0453) for 537 parameters using 2880 reflections with I > 2σ(I). The red adduct, trimethyl 1,2,5-triphenylphosphoranylidene-4-methoxycyclopent-2-ene-5-one-2,3,4- tricarboxylate, an exocyclic phosphorane 8, crystallized in the monoclinic space group P2₁/c (No. 14) with four molecules in the unit cell. Refinement of this structure (a = 13.233(5) ?, b = 15.712(5) ?, c = 17.191(7) ?, α = 90°, β = 110.35(4)°, and γ = 90°) converged at R = 0.0916 (R(w) = 0.0937) for 400 parameters using 2052 reflections with I > 2.5σ(I). Small amounts of a white or buff amorphous material, which could represent a polymeric product from the lost phosphorus bridge, were also obtained. Thermal rearrangement of the yellow adduct gave a colourless 6,5,3-tricyclic isomer 11, possibly via the isomeric cyclopropyl derivative 15 or the nine-membered phosphonin 7. The structure of 11 was confirmed by mild oxidation of the colourless rearrangement product to its P-oxide. This product, tetramethyl 2,3,6-triphenyl-2-phosphatricyclo[6.1.0^1,8.0^5,9]-2-oxonona-3,6-d iene-1,7,8,9-tetracarboxylate 12, was also colourless. It crystallized in the orthorhombic space group Pbc2₁ (No. 29) with four pairs of molecules in the unit cell. Refinement of this structure (a = 8.918(1) ?, b = 22.605(4) ?, and c = 30.169(6) ?) converged at R = 0.0747 (R(w) = 0.0757) for 321 parameters using 3255 reflections with I ≥ 3σ(I). Unequivocal structures for these adducts and derivatives finally confirms, and establishes further understanding of, the complex reactions of the triphenylphosphole with dimethyl acetylenedicarboxylate.