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Novel Stereochemistry, Reactivity, and Stability of an Arsenic Heterocycle in a Metal-Promoted Asymmetric Cycloaddition Reaction

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posted on 29.10.2007, 00:00 by Mengtao Ma, Sumod A. Pullarkat, Yongxin Li, Pak-Hing Leung
The organopalladium complex containing ortho-metalated (S)-[1-(dimethylamino)ethyl]phenylene as the chiral auxiliary has been used as the chiral template to promote the asymmetric cycloaddition reaction between diphenylvinylphosphine and 3,4-dimethyl-1-phenylarsole. A diphenylphosphino-substituted asymmetrical heterobidentate arsanorbornene (As−P) ligand was obtained stereoselectively on the chiral palladium template in moderate yield. The chiral benzylamine auxiliary could be removed chemoselectively from the template by treatment with HCl to produce the neutral complex [(As−P)PdCl2]. In contrast to their reported P−P analogue, the arsenic donor in the dichloro complex could be eliminated stereospecifically under mild reaction conditions to generate the corresponding 1-(diphenylphosphino)-3,4-dimethyl-2,4-cyclohexadiene, which remained as a bidentate ligand at the PdCl2 unit via phosphorus and the η2-C4−C5 double bond. The arsenic-elimination process was found to be influenced by the halo ligand in [(As−P)PdX2]. A similar process was observed with the analogous dibromo complex, but the corresponding diiodo species did not show similar reactivity. All of the novel As−Pd complexes have been characterized by X-ray crystallography.