Computational Investigations of Chiral-at-Metal Complexes and Their Enantioselective Reactions

Abstract

Counter ion effects on the rates of catalytic reactions and enantioselectivities originated from hydrogen bond donor catalysts are computationally established. First, cat-ion/anion interactions are studied in salts of [(η5-C5H5)Ru(CO)(GBI)]+ and mer-[Co-(GBI)3]3+, where GBI is the chelate ligand 2-guanidinobenzimidazole (HN=C(NH2)NH- C=NC(CH)4CNH), with seven anions, namely F–, Cl–, Br–, I–, BF4–, PF6–, and BArf– (or B(3,5-C6H3(CF3)2)4–). In the gas phase, anion binding to the NH triad of the GBI ligand is favored. This association is maintained in CH2Cl2, except for the ruthenium PF6– and BArf– salts. Second, enantioselection in carbon-carbon bond forming reactions of chiral cationic ruthenium GBI catalysts is elucidated. [(η5-C5H5)Ru(CO)(GBI)]+ PF6– catalyzes the carbon-carbon bond forming addition of 1,3-dicarbonyl compounds to trans-β-nitro-styrene in the presence of a trialkyl amine, but enantioselectivities are computationally predicted to be modest. In contrast, (SRuRCRC)- and (RRuRCRC)-[(η5-C5H5)Ru(CO)(GBI- CH(CH2)4CHNMe2)]+ PF6–, which contain carbon stereocenters and an internal trialkyl-amine, effect highly enantioselective additions, and the mechanism of asymmetric induc-tion is elucidated.

Another major class of reactions elucidated involves the chiral rhenium Lewis acid [(η5-C5H5)Re(NO)(PPh3)]+. First, mechanisms of dichloromethane ligand substitution in [(η5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+ by various ligands or nucleophiles (:L), affording chiral Lewis base adducts [(η5-C5H5)Re(NO)(PPh3)(L)]+ with retention of rhenium confi-guration, are investigated. For weak nucleophiles, i.e., ethyl chloride and dichloromethane, an unprecedented mechanism type that involves neighboring group participation (NGP) of the PC=CH moiety of the PPh3 ligand is established. Strong nucleophiles, i.e., cyclohexanone and dimethyl sulfide, favor a classical concerted interchange (I) mechanism. Both mechanisms lead to retention of rhenium configuration. Second, DFT calculations are used to characterize Re...C multiple bonding and structural features and define the basis for the diastereoselectivity of CH3Li addition associated with carbon-bound rhenium σ-quinolinyl and σ-isoquinylinyl complexes (adducts of [(η5-C5H5)Re(NO)-(PPh3)]+) and their derivatives. The high attendant diastereoselectivity of CH3Li addition originates from the bulky PPh3 ligand.

Mechanisms and origins of enantiomer self-recognition of methylidene coupling of the chiral-at-rhenium complex [(η5-C5H5)Re(NO)(PPh3)(=CH2)]+ to the ethylene com-plex [(η5‑C5H5)Re(NO)(PPh3)(H2C=CH2)]+ are explored. First, two molecules of the methylidene complexes form non-covalent dimers stabilized by interactions of the PPh3 ligands. Then, carbon-carbon bond formation takes place to afford bimetallic μ-η2:η2-H2C...CH2 intermediates or [(η5-C5H5)Re(NO)(PPh3)(μ-η2:η2-H2C...CH2)(Ph3P)(ON)-Re(η5-C5H5)]2+, in which one rhenium is closer to the ligand centroid than the other. The lowest energy barrier of homochiral coupling (SRe/SRe or RRe/RRe) or enantiomer self-recognition of the methylidene complexes is lower than heterochiral coupling (SRe/RRe). Component analyses show that interactions of the PPh3 ligands play key roles in the enantiomer self-recognition.