6. Synthesis and Coordination Chemistry of the First C₂-Chiral Bisoxazine Ligand
   Clément Mazet, Lutz H. Gade*
   Inorg. Chem. 2003, 42, 210-215

The ligand 2,5-bis(oxazinyl)-3,4-diethylpyrrole (4) was obtained in three reaction steps from the known pyrrole derivative 3,4-diethylpyrrole-2,5-dicarboxylic acid (1) which was first coupled with 2 molar equiv of (S)-1-benzoxy-3-butylamine to give the corresponding diamide 2 using dicyclohexylcarbodiimide and 1-hydroxybenzitriazole as coupling reagents. Subsequent hydrogenolysis of the benzyl ether functions yielded the dialcohol 3 which was cyclized in high yield after methylsulfonation and treatment with an excess of NaOH giving the target compound 2,5-bis[2-{(S)-5-methyloxazinyl}]-3,4-diethylpyrrole (4). Lithiation of 4 by reaction with 1 molar equivalent of nBuLi at −78°C and addition of [PdCl₂(COD)] to the lithium pyrrolide cleanly gave the palladium complex 5 which was fully characterized. Complex 5 is unstable in solution and dimerizes to give a mixture of two diastereomeric helical dinuclear complexes, 6a and 6b, which cocrystallized in a 1:1 ratio to give X-ray quality single crystals. Both isomers possess virtual molecular 2-fold symmetry (though no crystallographic rotational symmetry), the molecular C₂-axis being orthogonal to the Pd···Pd vector.

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DOI de la version éditeur : 10.1021/ic025947b