The 2-guanidinobenzimidazole (GBI) derivative complexes (SRuRCRC)- and (RRuRCRC)-[(η5-C5H5)Ru(CO)(GBICH(CH2)4CHNMe2)]+ PF6– ((SRuRCRC)- and (RRuRCRC)-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, (SRuRCRC)- and (RRuRCRC)-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 (SRuRCRC)- and (RRuRCRC)-2+ PF6– afford mainly the R product enantiomer. Thus, the ruthenium configurations do not control the enantioselection.

Theoretical study of enantioselection in carbon-carbon bond forming reactions of ruthenium guanidinobenzimidazole second coordination sphere hydrogen bond donor catalysts

ACS Spring Meeting 2021

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