Differential reactivity pattern for the condensation reactions of copper(II) complexes of DL- and L-amino acids

Abstract

The condensation reactions of copper(II) complexes of DL and L forms of alanine, serine and threonine with aliphatic aldehydes reveal differential reactivity for the bis(amino acidato)copper(II) complexes. The reaction of [Cu(DL-ala)₂](where ala = alaninato anion) with formaldehyde and ammonia, at an acidic pH of 5, yields a pentamethylenediaza group containing product, [(αR,α′S)-N³N⁷-(1,3,5,7-tetraazabicyclo[3.3.1]nonane)di(2-methylacetato)]copper(II)1, but at higher pH conditions (pH > 8.0), yields the known oxazolidine group containing product, diaquabis(4-methyloxazolidine-4-carboxylato)-copper(II)2. However, the reaction of [Cu(L-ala)₂] with formaldehyde and ammonia at pH 8.5 is known to yield an optically active isomer of 1, viz. [(αS,α′S)-N³,N⁷-(1,3,5,7-tetraazabicyclo-[3.3.1]nonane)di(2-methylacetato)]copper(II)1a. Though the latter reaction also yields the oxazolidine group containing product 2 at very basic pH (> 10), the formation of 2 reveals racemization during the reaction process. Similarly, whereas the copper(II) complexes of DL-serine and DL-threonine readily undergo condensation reactions with aldehydes such as formaldehyde, propionaldehyde, butyraldehyde and valeraldehyde, yielding oxazolidine group containing products, the corresponding L-amino acidatocopper(II) complexes do not undergo condensation under the same experimental conditions except in the case of formaldehyde, demonstrating differential reactivity of DL- and L-amino acidatocopper(II) complexes. The crystal structures of the products obtained from the reactions of [Cu(DL-ser)₂](ser = serinato anion) and [Cu(DL-thr)₂](thr = threoninato anion) with formaldehyde, aqua[N,N′-(2-oxapropanediyl)bis(oxazolidine-4-carboxylato)]copper(II) Sesquihydrate 3 and [N,N′-(2-oxapropanediyl)bis(5-methyloxazolidine-4-carboxylato)]copper(II) monohydrate 4, respectively, have been determined. Complex 3 crystallizes in the triclinic space group P. a= 10.802(2), b= 11.039(2), c= 13.460(3)Å, α= 75.14(3), β= 69.57(3), γ= 87.53(3)° and Z= 4. Complex 4 also crystallizes in the triclinic space group P, a= 7.388(2), b= 10.017(2), c= 11.485(2)Å, α= 115.27(3), β= 101.89(3), γ= 91.79(3)° and Z= 2.