Fig. 6: Chiral counterion-directed sp3-carbon and chiral-at-metal stereochemistry applied to enantioselective catalysis.
a, The enantiomerization of LBB2RuCp(NCMe)+ is degenerate in the presence of an achiral counterion. In the presence of chiral anions, the ion-pair interactions break the degeneracy and bias the equilibrium towards one stereoisomer of the BB–metal complex, that is, K ≠ 1. Note: only the two lowest-energy stereoisomers are shown for clarity. Reagents and conditions: (i) LBB2RuCp(NCMe)·PF6, BuN4·Δ-TRISPHAT, CDCl3, r.t. (ii) LBB2RuCp(NCMe)·PF6, Na·(S)-BORBIN, CDCl3, r.t. b, Comparison of the partial 1H NMR spectra (400 MHz, CDCl3, 298 K) of LBB2RuCp(NCMe)·PF6 before (top row) and after (other rows) the addition of 1 equiv. of BuN4·Δ-TRISPHAT. Integration of the MeCN resonances shows that the initially racemic LBB2RuCp(NCMe)+ ion gradually becomes enriched in one stereoisomer (K = 1.2). c,d, The stereoinduction (c) arising from three phosphoramidite ligands was compared in an iridium-catalysed allylic substitution (d). We hypothesize that the chiral anion formed in situ biases the stereochemical equilibrium of the cationic LBB2Ir π-allyl intermediate, leading to its improved performance relative to the control ligands lacking dynamic sp3-C stereochemistry. Reagents and conditions: (iii) 1. [Ir(1,5-cod)Cl2] (4 mol%), ligand (16 mol%), THF, 30 min, r.t. 2. (R)-BDHP (10 mol%), r.t., 24 h. Δ-TRISPHAT, Δ-tris(tetrachloro-1,2-benzenediolato)phosphate(V); (S)-BORBIN, bis[(S)-1,1′-bis-2-naphtholato]borate; 1,5-cod, 1,5-cyclooctadiene; (R)-BDHP, (R)-1,1′-binaphthyl-2,2′-diyl hydrogen phosphate.
