Transesterification and amide cis–trans isomerization in Zn and Cd complexes of the chelating amino acid ligand Boc–Asp(Dpa)–OBzl

摘要: The amino acid derivative Boc–Asp–OBzl (Boc = N-butyloxycarbonyl; Asp aspartic acid; Bzl benzyl) was functionalized by coupling its carboxylate side chain to dipicolylamine. This yielded the tridentate nitrogen donor ligand Boc–Asp(Dpa)–OBzl (1-OBzl). compound 1-OBzlcontains three different carbonyl groups: a tertiary amide linkage between and Dpa, C-terminal benzyl ester function, an N-terminal urethane protecting group. NMR spectra were used compare reactivity of these moieties. Boc group gives rise two isomers, (E, 9%) (Z, 91%). Coordination Cd(NO3)2 Zn(NO3)2 complexes 3 4. These compounds have significantly reduced barriers rotation about C–N bond compared with free (1-OBzl: 18.5 kcal mol–1 in CDBr3; 3: 12.9 (CD3)2CO; 4: 13.8 (CD3)2CO). Both readily undergo transesterification methanol or CD3OD. Experimental pseudo-first order rate constants determined CD3OD (3) (CD3)2CO : (3 1; 4). It found that zinc complex 4 (k (2.28 ± 0.02) × 10–4 s–1) is more reactive than cadmium (1.41 0.03) 10–6 s–1). In study their cis–trans isomerization, [(1-OCH3)Cd(NO3)2] (5) synthesized, [(1-OCD3)Zn(NO3)2] (6) generated situ 1). (5: 14.1 CD3OD; 6: 13.4 1)). Our results show stronger Lewis-acid zinc(II) active cadmium(II) acceleration transesterification. marked contrast which comparably fast both metal ions.