Formation of Coordinatively Unsaturated Cp*Ir-Amino Acid Complexes and Their Highly Diastereoselective Complexation Reactions

Abstract

Amino acid-metal complexes have been the subject of innumerable studies,1 prompted by the roles of metals in biochem-istry2 and amino acids in producing chiral catalysts.3 With very few exceptions,4 amino acid-metal complexes are coordinatively* saturated. For example, interaction of -unsubstituted amino acid salts with [Cp*IrCl(M-Cl)]21 leads to 18-electron complexes 2, the stereogenic element of the amino acid producing mixtures modestly enriched in diastereomer 2a (from 50:50 up to 92:8).5 Here we report that under modified conditions, amino acid derivatives 3 bearing electron-withdrawing groups Z on N afford air-stable 16-electron species 4, two members of which are characterized by X-ray diffraction. Furthermore, unlike ligand substitutions on 2 and related species, ligand addition reactions of chiral 4 proceed with high (¿25:1) stereoselectivity.Typically, a mixture of 3,6 dimer I,7 and anhydrous K2CO3 (molar ratio 1.00:0.50:2.0-3.0) was diluted with THF or CH3-CN until the concentration of 3 was 0.01-0.04 M. Nitrogen was bubbled through the mixture for 5-10 min, and the orange (THF) or yellow (CH3CN) mixture was stirred for 4-36 h, until the color deepened to red. The residue remaining after rotary evaporation was diluted with CH2C12 and filtered through Celite, and the filtrate was concentrated to afford air-stable brick-red to burgundy-red 4 in ¿90% yields.8 Surprisingly, elemental analysis, IR, and NMR data9 pointed to absence of coordination by THF, CH3CN, or N2 in the isolated products. The NMR data for glycine derivative 4a in CD2CI2 were particularly revealing, a sharp two-proton singlet being seen at 3.20 ppm

Extending the range of pentasubstituted cyclopentadienyl compounds: The synthesis of a series of tetramethyl(alkyl or aryl)cyclopentadienes (Cp ∗ R ), their iridium complexes and their catalytic activity for asymmetric transfer hydrogenation

“…[22,23,[36][37][38][39][40] All complexes adopt the three-legged piano-stool (half-sandwich) configuration, as expected. As is the case with Cp ⁄ Ir amino acid complexes previously reported, the iridium atom becomes a pseudo-tetrahedral chiral center upon coordination of the amino acid, resulting in diastereomers that differ at configuration of the metal when using an enantiomerically pure amino acid.…”

The Outer-Coordination Sphere: Incorporating Amino Acids and Peptides as Ligands for Homogeneous Catalysts to Mimic Enzyme Function

“…This instability is likely why these ligand motifs are observed less frequently. Complexes of 3b, or the analogous bridging dimetal complex, have been reported for many metals (22,35,36), but the chelate ligand (3c) is the most commonly reported binding motif and has been reported or inferred for amino acids complexed with gold, nickel, zinc, copper, cobalt, iron, rhenium, molybdenum, ruthenium, rhodium, platinum, iridium, and tungsten (22,28,29,(37)(38)(39)(40). The N,O chelate complexes have been shown to be sensitive to the presence of better ligands, resulting in ligand swapping or degradation, suggesting weak coordination (29).…”

“…However many of the complexes have additional stabilization such as a third ligation point within the ligand (42,43) or non natural amino acids providing rigidity or electronic stability in the molecule (38,44,45). Ligation through more than two sites on the amino acids has also been reviewed (39,46,47).…”

Molecular and Heterogenized Cp*Ir Water Oxidation Catalysts Bearing Glyphosate and Glyphosine as Ancillary and Anchoring Ligands

“…[35] Both molecular and heterogenized WOCs exhibited remarkable activity; importantly, structural and reactivity studies revealed, not unexpectedly, that IrÀ OP(O)(OH)R bond is significantly weaker than IrÀ OC(O)R one. This suggested that having a mixed terdentate dianionic O(P),N,(C)O-ligand [68] could be ideal since the phosphonate arm could easily dissociate and act as anchoring moiety, whereas the amino acidate (AA) fragment could remain tightly bound at iridium, [69][70][71][72][73][74] stabilizing the catalyst.…”

“…The search for ligands with such features straightly pointed toward glyphosate [N-(phosphonomethyl)glycine], a very controversial systemic herbicide and crop desiccant, [75][76][77] and its further methyl-phosphonated analogue glyphosine [N,N-bis (phosphonomethyl)glycine] (Scheme 1). Despite many Ir(AA)complexes have been reported [69][70][71][72][73][74] and successfully applied as catalysts [78][79][80][81][82][83][84] and anticancer agents, [85] to best of our knowledge, none of them bears coordinated glyphosate and glyphosine ligands. The transition metal coordination chemistry of these two molecules is, indeed, rather limited.…”