Chiral metal complexes. 27. Stereoselectivity in the synthesis and reaction of coordinated aminomalonic acid derivatives

Ewan F. Birse, Michael A. Cox, Peter A. Williams*, Frederick S. Stephens, Robert S. Vagg

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

21 Citations (Scopus)

Abstract

The synthesis is reported of a series of ternary cationic complexes of general form [Co(R,Rpicchxn)(ARMA)+ (where picchxn is the N4 tetradentate N,N′-di(2-picolyl)-1,2-diaminocyclohexane and ARMA is a bidentate α-substituted-α-aminomalonate dianion). The aminomalonic acid (NH2· C(COOH)2·R) derivatives investigated have R = -CH3 (AMMAH2), -CH2·CH3 (AEMAH2), -CH2· CH2·CH3 (APMAH2), -CH2·(CH2)2·CH3 (ABuTMAH2, -CH2·C6H5 (ABMAH2), -CH2·(p-C6H4)· C(CH3)3 (ABuBMAH2) and -CH2·C10H7 (ANMAH2). The isomeric species in the complex products have been separated using cation exchange chromatography and each isomer has been characterized using NMR and circular dichroism techniques. In each synthesis the major isomeric product obtained has a Λ-β1 topology. However, where ARMAH2 possesses a lengthy alkyl sidechain trace amounts of Δ-α-[Co(R,R-picchxn)(ARMA)]+ isomers have been observed during the synthetic reactions. This unusual isomeric form readily undergoes inversion of its absolute configuration in DMSO solution to yield the more thermodynamically stable Λ-β1-[Co(R,R-picchxn()R-ARMA)]+ species stereospecifically. In the case of Λ-β1-[Co(R,R,-pichxn)(S-APMA)]ClO 4·2NaClO4·5H2O the crystal structure has been determined. The compound crystallises in the orthorhombic space group P212121, with a = 10.056(3), b = 16.475(7), c = 23.370(7) Å and Z = 4. The structure was refined to R = 0.079 for 4460 non-zero reflexions, and confirms the absolute configuration of each chiral centre to be consistent with the NMR and circular dichroism interpretations. The decarboxylation of these chelate ARMAH2 derivatives under acid conditions leads to corresponding complexes containing mixtures of coordinated R-and S-α-aminoacids in various ratios. This ratio has been determined in each case, and factors which may influence the degree of chiral induction observed are discussed.

Original languageEnglish
Pages (from-to)45-56
Number of pages12
JournalInorganica Chimica Acta
Volume148
Issue number1
DOIs
Publication statusPublished - 1 Aug 1988

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