Model for the Reduced Schiff Base Intermediate between Amino Acids and Pyridoxal:  Copper(II) Complexes of N-(2-Hydroxybenzyl)amino Acids with Nonpolar Side Chains and the Crystal Structures of [Cu(N-(2-hydroxybenzyl)-d,l-alanine)(phen)]·H₂O and [Cu(N-(2-hydroxybenzyl)-d,l-alanine)(imidazole)]

Abstract

Copper(II) complexes with reduced Schiff base ligands of amino acids possessing nonpolar side chains with salicylaldehyde have been synthesized. Ternary complexes with imidazole, 1,10-phenanthroline, and pyridine have been prepared and characterized for N-(2-hydroxybenzyl)-D,L-alanine. The crystal structures of [(N-(2-hydroxybenzyl)-D,L-alanine)(1,10-phenanthroline)Cu(II)] monohydrate ([Cu(SAla)phen].H(2)O) and [(N-(2-hydroxybenzyl)-D,L-alanine)(imidazole)Cu(II)] ([Cu(SAla)Him]), have been determined. [Cu(SAla)phen].H(2)O crystallized in space group P&onemacr;, with a = 8.718(2) Å, b = 10.886(3) Å, c = 11.693(2) Å, alpha = 71.32(2) degrees, beta = 85.27(2) degrees, gamma = 70.21(2) degrees, and Z = 2. The copper atom is five coordinate, with SAla acting as a tridentate ONO chelator through the carboxylato and phenolato oxygens and the amine nitrogen. The remaining donors are provided by the phen nitrogens. [Cu(SAla)Him] crystallized in space group P2(1)/n, with a = 10.353(1) Å, b = 6.714(1) Å, c = 18.769(2) Å, beta = 91.71(1) degrees, and Z = 4. The copper atom is four coordinate, with SAla acting as a tridentate ONO chelator with the neutral imidazole moiety coordinated through nitrogen. In both complexes the ligand has two chiral centers due to the coordination of the N. Molecular mechanics calculations show that unfavorable steric interactions would occur in the nonobserved R,R and S,S diastereomers. Compounds prepared have been characterized by a range of physicochemical techniques. The complexes may serve as stable models for the intermediates in enzymatic amino acid transformations.

A structural and catalytic model for zinc phosphoesterases

“…The ethyl ester H 2 EtL1 was produced in a reaction sequence (Scheme 1) involving a statistical reaction between N-(2-pyridylmethyl)glycine ethyl ester, 36 …”

Copper(II) Complexes of Schiff‐Base and Reduced Schiff‐Base Ligands: Influence of Weakly Coordinating Sulfonate Groups on the Structure and Oxidation of 3,5‐DTBC

“…intermolecular N-H···O hydrogen bond between the NH 2 and sulfonate groups in the solid state and in solution has been utilized as a crystal engineering tool. [12] The properties and the functions of the N-H···O hydrogen bonds involving coordinated -SO 3 -groups in a dinuclear calcium complex have been reported. [24] As in 1, medium strong C-H···O hydrogen bonds have also been observed in the solid state.…”

“…Several copper complexes of reduced Schiff-base ligands formed between salicylaldehyde and amino acids were explored to serve as models for the intermediate species in biological racemization and transamination reactions. [3] Making use of these advantages, we employed such reduced Schiff-base ligands to synthesize dinuclear Cu II and Zn II complexes that exhibit interesting structural transformations in the solid state. [4] Further, in our recent report, we showed that incorporation of an additional carboxylate group in the side arm of the ligand N-(2-hydroxybenzyl)--glutamic acid formed between salicylaldehyde and -glutamic acid resulted in a novel structural display of a spiral staircase conformation.…”

“…[6] Various dicopper active sites found in copper-containing metalloenzymes have similar structural aspects with three histidine donors for each of the two Cu centers, which are separated by a distance of about 3.5 Å, and catechol oxidase as one of the prominent members of the type III copper proteins that catalyze the two-electron oxidation of ortho-diphenols to the corresponding quinones. [7] Conse-quently, quite a number of dinuclear copper complexes have been investigated as biomimetic catalysts for catechol oxidation by employing the most common and convenient model substrate, 3,5-di-tert-butylcatechol (3,5-DTBC), which can be oxidized to 3,5-di-tert-butylquinone (3,. [8] With regard to this, we have recently reported a series of dinuclear copper() complexes of the tridentate and binucleating reduced Schiff-base ligands and their functional relationship with catecholase activity.…”

A Novel Copper Chelate Catalyzed Ring Closure Reaction of 1,2-Bisketenes with Alcohols To Give 5-Alkoxy-2,3-dihydrofuran-2-ones

“…Since pseudo-square-planar monoligand chelates of tridentate Schiff bases derived from salicylic aldehyde and 1,2-amino alcohols 10 [11] or a-amino acids 11 [12] can be used as catalysts, we concluded that the metal center either has two free coordination sites in cis position or that theseÐas in the case of the tetrahedral complexesÐcan be readily created. Thus we assume the crucial step in the reaction to be the nucleophilic attack of the hydroxylic component at a copper bisketene complex that has a structure analogous to that described for a cobalt 1,2-bisketene chelate.…”