Inorganic Chemistry, Vol.42, No.1, 210-215, 2003
Synthesis and coordination chemistry of the first C-2-chiral bisoxazine ligand
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 -78degreesC 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 C-2-axis being orthogonal to the Pd...Pd vector.