FORMATION AND STRUCTURE OF COORDINATIVELY UNSATURATED CP-ASTERISK-IR-AMINO ACID COMPLEXES - KINETIC AND THERMODYNAMIC CONTROL IN HIGHLY DIASTEREOSELECTIVE COMPLEXATION REACTIONS

摘要

Amino acid derivatives bearing an electron-withdrawing group Z on N (Z = tosyl, CO2-CH(2)Ph, or acetyl) serve as (N,O)-chelating, dianionic ligands to the fragment Cp*Ir. Six such complexes have been prepared, all of them coordinatively unsaturated yet air-stable. The structure of the N-tosylglycine derivative C19H24IrNO4S (5a) was analyzed at 20 degrees C. A planar chelate ring was revealed, and relatively short Ir-N and Ir-O bonds suggested stabilization of unsaturated Ir by pi-donation. Crystals of the (R)-N-tosylphenylglycine complex C25H28IrNO4S (5f) were monoclinic. Some distortion of the chelate ring was seen, and both aryl rings were syn, the angle between their mean planes being 19 degrees. Within seconds, red solutions of the unsaturated complexes 5 turn yellow on addition of ligands such as phosphines, CO, and primary aliphatic or heterocyclic amines. Ligands add to chiral complexes so as to place the amino acid side chain R and Cp* cis to each other on the metallacycle, suggesting preferred approach of the ligand to 5 from the side opposite R. For one PMe(3) adduct this was shown to be the result of kinetic control (greater than or equal to 50:1 selectivity at 25 degrees C) and thermodynamic control (40:1 selectivity after equilibration at 90 degrees C, half-life = 5 h). PPh(3) and amines exchange within minutes at 25 degrees C. The stereoselectivity of ligand addition was highest for smaller ligands. Comparing this result and previous work suggests that steric interactions between added ligand and the amino acid side chain R determine diastereoselectivity. [References: 82]