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

The reactions between Pd(II) and low-rim amino acid substituted calix[4]are nes {L-a = 5,11,17,23-tetra-4-tert-butyl- 25,27-di{[(2'-amino-4 -methylthio )-L-butyryl]aminoethoxyl-26,28-dihydroxycalix[4]arene, L-b = 5,11,17,23-tet ra-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'-amino acetyl]aminoethoxyl-26,28-dihydroxycalix[4]arene} were studied by electrosp ray mass spectrometry. When L-a was refluxed with 2.2 equiv. of trans-[Pd(P y)(2)Cl-2] or 1.1 equiv. of PdCl2, the mononuclear complex (PdLCl2)-Cl-a wa s formed. When refluxed with 2.2 equiv. of PdCl2, the binuclear complex (Pd 2LCl4)-Cl-a was obtained. The same binuclear complex (Pd2LCl4)-Cl-a was als o obtained as the final adduct when L-b was refluxed with 3.5 equiv. of PdC l2, during which two BOC groups are removed one-by-one after 6 and 16 h, re spectively. However, when L-b was refluxed with trans-[Pd(Py)(2)Cl-2] in me thanol, no reaction was observed. In order to understand whether the coordi nation to Pd(II) of the S atom in methionine is necessary for the removal o f 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 fro m L-c. These data reveal that the detachment of BOC groups is likely induce d 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 m etal complexes of calixarenes substituted at the lower rim by BOC-protected amino acids or peptides without prior treatment of the BOC groups.