Session: Organometallic Chemistry: Catalysis:

The 2-guanidinobenzimidazole (GBI) derivative complexes (S_RuR_CR_C)- and (R_RuR_CR_C)-[(η⁵-C₅H₅)Ru(CO)(GBICH(CH₂)₄CHNMe₂)]⁺ PF₆^– ((S_RuR_CR_C)- and (R_RuR_CR_C)-2⁺ PF₆^–) catalyze highly enantioselective additions of 1,3-dicarbonyl compounds to nitroalkenes. The mechanisms and origins of enantioselection of the addition of 2,4-pentanedione to trans-β-nitrostyrene have been elucidated by DFT calculations. To begin with, the GBI salt [(η⁵-C₅H₅)Ru(CO)(GBI)]⁺ PF₆^– (1⁺ PF₆^–) with only a ruthenium stereocenter was investigated. The external base (:NMe₃) is required. The NH groups of the GBI ligand hydrogen bond to 2,4-pentanedione, and then a proton of this dicarbonyl compound is transferred to :NMe₃. This salt does not contribute to enantioselection because the energies of carbon-carbon bond formation transition states leading to different enantiomers are comparable. In contrast, (S_RuR_CR_C)- and (R_RuR_CR_C)-2⁺ PF₆^– have an internal base, the :NMe₂ moiety in the GBI ligand scaffold. A proton of 2,4-pentanedione is transferred to the :NMe₂ moiety to form a HNMe₂⁺ group. Subsequently, this protonated amine group can act as the Brønsted acid site to activate and bind with trans-β-nitrostyrene. Interestingly, both (S_RuR_CR_C)- and (R_RuR_CR_C)-2⁺ PF₆^– afford mainly the R product enantiomer. Thus, the ruthenium configurations do not control the enantioselection.

Enantioselection in carbon-carbon bond forming reactions of ruthenium guanidinobenzimidazole second coordination sphere hydrogen bond donor catalysts

Enantioselection in carbon-carbon bond forming reactions of ruthenium guanidinobenzimidazole second coordination sphere hydrogen bond donor catalysts

Michael Hall

Texas A&M Univ

Appearances