- Intramolecular [4 + 2] Diels-Alder cycloaddition reactions of phospholes with vinylphosphines promoted by palladium and platinum
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Reactions of (vinylphosphine)2MX2 complexes with (1-R-3,4-dimethylphosphole)2MX2 complexes (where vinylphosphine = Ph2VyP, PhVy2P, Et2VyP; R = Ph, Bzl, t-Bu; X = Cl, Br, I; M = Pd, Pt) produce dihalo[2-(R′,R″-phosphino)-5,6-dimethyl-7-R-7-phosphabicyclo[2.2.1] hept-5-ene]metal(II) complexes via an intramolecular Diels-Alder [4 + 2] cycloaddition. These reactions occur at ambient temperature with very high diastereoselectivity, providing a facile route to a new class of rigid chiral bidentate phosphines. These substituted 7-phosphanorbornene palladium and platinum complexes have been characterized by elemental analysis, physical properties, infrared spectroscopy, mass spectrometry, thermal analysis, and 1H, 1H{31P}, 13C{1H}, 31P{1H}, 1H/13C HETCOR, COSY, and 2-D J nuclear magnetic resonance spectroscopy. The structures of two palladium complexes have been confirmed by X-ray crystallography. Dichloro[2-(diphenylphosphino)-5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1] hept-5-ene]palladium(II) crystallizes in the monoclinic space group P21/c in a unit cell of dimensions a = 11.184 (4) ?, b = 10.972 (5) ?, c = 23.351 (8) ?, β = 103.17 (3)°, and ρ(calcd) = 1.58 g cm-3, with Z = 4. Refinement converged to R = 0.060 with 1886 independent observed (I/σ(I) ≥ 3.0) reflections. The donor abilities of the two phosphorus donors are not significantly different; the PdP distances are 2.213 (4) and 2.251 (4) ?, the CPC angle for the bridging phosphorus is small (82.5 (6)°), and the PdCl distances (2.344 (4), 2.351 (4) ?) are not significantly different. The chelate ring is rigid, and the molecule contains four asymmetric centers. Dichloro[2-(phenylvinylphosphino)-5,6-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1] hept-5-ene]palladium(II) crystallizes in the orthorhombic space group Fdd2 in a unit cell of dimensions a = 28.737 (5) ?, b = 32.979 (8) ?, c = 10.253 (2) ?, and ρ(calcd) = 1.44 g cm-3, with Z = 16. Refinement converged to R = 0.036 with 2011 independent observed reflections. The donor abilities of the two phosphorus donors are not significantly different; the PdP distances are 2.224 (2) and 2.236 (2) ?, the CPC angle for the bridging phosphorus is small (82.3 (4)°), and the PdCl distances (2.356 (2), 2.366 (2) ?) are not significantly different. The chelate ring is rigid, and the molecule contains five asymmetric centers. Two of the ligands have been liberated from the complexes and converted to their respective disulfides with retention of stereochemical configuration. Thermal analyses (TG and DSC) show that the complexes and disulfides polymerize upon melting, probably by extrusion of phosphinidine. A mechanism for the [4 + 2] Diels-Alder cycloaddition is proposed.
- Rahn, Jeffrey A.,Holt, Mark S.,Gray, George A.,Alcock, Nathaniel W.,Nelson, John H.
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p. 217 - 226
(2008/10/08)
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