Metal complex amino acid synthons: syntheses, structures and stereoselective reactions of (iminoacetato)cobalt(iii) complexes

Abstract

Amino acid anions (AAO Ϫ ) chelated to cobalt() in [(en) 2 Co(AAO)](O 3 SCF 3 ) 2 (AA = Gly, Sar, Ala and Glu) were selectively oxidized to their imine derivatives by a new general procedure utilizing PBr 3 and N-bromosuccinimide in dmf. The new iminoacetato complexes, Λand ∆-[(en) 2 Co(O 2 CCH᎐ ᎐ NH)](O 3 SCF 3 ) 2 , constitute chiral glycine equivalents which can serve as synthons for stereoselective α-amino acid synthesis. In alkaline EtOH, quantitative addition of CH 2 (COMe) 2 , CH 2 (CO 2 Et) 2 or MeCOCH 2 CO 2 Et to the imine of the iminoacetate ligand initially produced both diastereomers of the product α-amino acid cobalt() complexes. However, subsequent crystallizationinduced asymmetric transformations in the heterogeneous reaction mixtures led to better than 90% excess of a single diastereomer after five days, and the diastereopure product triflate salts were obtained after recrystallization. Both enantiomers of isotopically substituted (3-13 C, 98%)aspartic acid were produced by facile synthesis from ∆-[(en) 2 Co(O 2 CCH᎐ ᎐ NH)](O 3 SCF 3 ) 2 and diethyl (2-13 C)malonate. The new N-methyliminoacetato complex, rac-[(en) 2 Co(O 2 CCH᎐ ᎐ NMe)](O 3 SCF 3 ) 2 , also yielded to imine addition reactions providing a route to the α-N-methylamino acid subclass. The molecular structures of the new imine complexes, Λ-(ϩ) 578 -[(en) 2 Co(O 2 CCH᎐ ᎐ NH)]Br 2 ؒH 2 O and rac-[(en) 2 Co(O 2 CCH᎐ ᎐ NMe)]S 2 O 6 ؒ1.5H 2 O, and the diethyl carboxy-aspartate addition product, (ΛS,∆R)-[(en) 2 Co{O 2 CCH(CH(CO 2 Et) 2 )NH 2 }](ClO 4 ) 2 , were determined by X-ray crystallography.

XAS examination of glutathione–cobalt complexes in solution

“…Co(III)-S distance average in tris(dithiolato) Co(III) complexes is 2.27 Å [67]. Co(III)-N amide bond lengths in mononuclear octahedral peptides complexes are ca 1.87 Å [68][69][70][71][72][73][74][75] and Co(III)-O carboxylate in these compounds as well as in monomeric carboxylate Co(III) complexes containing amino and/or thiolato ligands range between 1.88 and 1.99 Å [76][77][78][79][80][81]. One can reasonably conclude that the Co(III)-N/O and Co(III)-S distances found in Co:GSH pH12 (EXAFS data: 2.02 and 2.27 Å respectively) are in the range of classical distances reported for Co(III) peptides and polyaminothiols complexes listed above and in our previous article [36] in which mononuclear tris cysteinato Co(III) complex in solution exhibited Co(III)-N amine and Co(III)-S distances of 2.02 and 2.26 Å respectively.…”

Metal Complexes of Biologically Important Ligands, CLXXII [1]. Metal Ions and Metal Complexes as Protective Groups of Amino Acids and Peptides – Reactions at Coordinated Amino Acids

“…Oxidation of Co(III)-chelated hydroxyl-and chloroprolines leads to pyrrole-2-carboxylate complexes [198]. Instead of SOCl 2 also a mixture of PBr 3 and N-bromosuccinimide was used for oxidation of chelated Gly, Sar, Ala and Glu to give the corresponding imine compounds [(en) 2 Co (HN=CH(R)CO 2 )] 2+ [199]. Yamaguchi et al [200] Brought to you by | University of [196,200,201].…”

“…In α-imino acids an "Umpolung" at the α-C atom is achieved, nucleophiles; e. g. the hydride anion (as BH 4 − ) [202] or the nitromethanide anion, acetylacetone or dialkylmalonate [199,219], are added to the α-C atom to give α-amino acids. A recent example [220a] is the synthesis of 2-(nitromethyl) ornithine: by reaction of [(tren)Co(ornithinate)] 2+ with SOCl 2 /DMF the corresponding imine complex [(tren) Co(HN=C(H)(CO 2 )(CH 2 ) 3 NH 3 ] 3+ is formed; addition of nitromethanide affords the complex {(tren)Co [2-(nitromethyl)ornithinate]} 3+ from which the ligand could be released by reduction with ammonium sulfide.…”

Stereoretentive formylation of (S)-proline: new application of the self-regeneration of stereo-centres (SRS) principle via chelation to cobalt(iii)

“…However, in the formyl-prolinato- (22) and the hydroxymethyl-prolinato (23) complexes the increase primarily affects the angle N4-Co-N5 [97.7° (22); 96.5° (23)], while the angle N2-Co-N5 [90.7° (22); 91.3° (22)] is close to the ideal octahedral value in both instances. A common feature is the reduced N2-Co-N4 angle (ca.…”

“…Alkylation at the α-C centre of the lithium enolate intermediate 3 to give 4 also proceeded diastereoselectively. [14][15][16][17][18][19][20][21][22][23][24] However, the extent of concomitant activation of the neighbouring amine has not been elucidated. Hydrolytic removal of the auxiliary centre provided 5.…”