Synthesis and electrospray mass spectrometry study of Pd(II) complexes of low-rim amino acid substituted calix[4]arenes

Abstract

The reactions between Pd(II) and low-rim amino acid substituted calix[4]arenes {L^a  = 5,11,17,23-tetra-4-tert-butyl-25,27-di{[(2′-amino-4′-methylthio)-L-butyryl]aminoethoxy}-26,28-dihydroxycalix[4]arene, L^b  = 5,11,17,23-tetra-4- tert-butyl-25,27-di({[(N-tert-butoxycarbonyl)-2′-amino-4′-methylthio]-L-butyryl}aminoethoxy)-26,28-dihydroxycalix[4]arene (diN-BOC-L^a) and L^c  = 5,11,17,23-tetra-4-tert-butyl-25,27-di{[(N-tert-butoxycarbonyl)-2′-aminoacetyl]aminoethoxy}-26,28-dihydroxycalix[4]arene} were studied by electrospray mass spectrometry. When L^a was refluxed with 2.2 equiv. of trans-[Pd(Py)₂Cl₂] or 1.1 equiv. of PdCl₂, the mononuclear complex PdL^aCl₂ was formed. When refluxed with 2.2 equiv. of PdCl₂, the binuclear complex Pd₂L^aCl₄ was obtained. The same binuclear complex Pd₂L^aCl₄ was also obtained as the final adduct when L^b was refluxed with 3.5 equiv. of PdCl₂, during which two BOC groups are removed one-by-one after 6 and 16 h, respectively. However, when L^b was refluxed with trans-[Pd(Py)₂Cl₂] in methanol, no reaction was observed. In order to understand whether the coordination to Pd(II) of the S atom in methionine is necessary for the removal of BOC groups, the reaction of L^c containing diN-BOC-glycine with PdCl₂ was also conducted. The result showed that the BOC groups can also be removed and the Pd(II) complex is not formed until both BOC groups are detached from L^c. These data reveal that the detachment of BOC groups is likely induced by PdCl₂, which acts as a Lewis acid during the reaction. This method can be potentially applied to the preparation of analogs of Pd(II) and other metal complexes of calixarenes substituted at the lower rim by BOC-protected amino acids or peptides without prior treatment of the BOC groups.