Potassium and silver chiral cobaltate(III) complexes as precatalysts for asymmetric C–C bond formation

Chiral, coordinatively-saturated cobaltate(III) complexes of Schiff bases obtained from salicylaldehyde and the optically active amino acids, (S)-Val, (S)-Thr, and (S)-Trp, are formed as C2-symmetrical, octahedral, anionic Λ(S,S)-, and Δ(S,S)-diastereoisomeric complexes, which are easily separable by chromatography. The complexes are stereochemically inert and, thus, are not transformed into each other under normal conditions. The counter-cations of the complexes can be easily interchanged and the sodium, potassium, and silver salts of some of the complexes were prepared. The structures of diastereoisomeric Λ-[(Sal-(S)-Val)2Co(III)]−Ag+ and Δ-[(Sal-(S)-Val)2Co(III)]−Ag+ were established by single crystal X-ray analysis. All of the sodium and potassium complexes effectively catalyzed the reaction between benzaldehyde and Me3SiCN, but only the Λ-[(Sal-(S)-Trp)2Co(III)]−K+ complex gave rise to enantiomerically-enriched mandelonitrile with up to 77% ee. The silver salts catalyzed a Mukaiyama reaction with low enantioselectivity (ee in the range 6–27%).

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Sodium Λ-bis[N-salicylidene-(S)-tryptophanato]cobaltateC36H28CoNaN4O6ee >99%[α]D25=-4856.2 (c 0.029, MeOH)Source of chirality: synthesis from (S)-tryptophanAbsolute configuration: (Λ, S,S)

Sodium Δ-bis[N-salicylidene-(S)-tryptophanato]cobaltateC36H28CoNaN4O6ee >99%[α]D25=-5586 (c 0.032, MeOH)Source of chirality: synthesis from (S)-tryptophanAbsolute configuration: (Δ, S,S)

Sodium Δ-bis-[N-salicylidene-(S)-threoninato]-cobaltateC22H22N2O8CoNaee >99%[α]D25=-6406.6 (c 0.025, MeOH)Source of chirality: synthesis from (S)-threonineAbsolute configuration: (Δ, S,S)

Potassium Δ-bis-[N-salicylidene-(S)-threoninato]-cobaltateC22H22N2O8CoKee >99%[α]D25=-5995.2 (c 0.022, MeOH)Source of chirality: synthesis from (S)-threonineAbsolute configuration: (Δ, S,S)

Sodium Λ-bis-[N-salicylidene-(S)-valinato]-cobaltateC24H26N2O6CoNaee >99%[α]D25=-6184.6 (c 0.031, MeOH)Source of chirality: synthesis from (S)-valineAbsolute configuration: (Λ, S,S)

Sodium Δ-bis-[N-salicylidene-(S)-valinato]-cobaltateC24H26N2O6CoNaee >99%[α]D25=-3658.8 (c 0.034, MeOH)Source of chirality: synthesis from (S)-valineAbsolute configuration: (Δ, S,S)

Potassium Λ-bis-[N-salicylidene-(S)-valinato]-cobaltateC24H26N2O6CoKee >99%[α]D25=-5965.3 (c 0.031, MeOH)Source of chirality: synthesis from (S)-valineAbsolute configuration: (Λ, S,S)

Potassium Δ-bis-[N-salicylidene-(S)-valinato]-cobaltateC24H26N2O6CoKee >99%[α]D25=-3628.6 (c 0.030, MeOH)Source of chirality: synthesis from (S)-valineAbsolute configuration: (Δ, S,S)

Silver Λ-bis-[N-salicylidene-(S)-valinato]-cobaltateC24H26N2O6CoAgee >99%[α]D25=-5600 (c 0.029, MeOH)Source of chirality: synthesis from (S)-valineAbsolute configuration:(Λ, S,S)

Sodium Δ-bis[N-salicylidene-(S)-tryptophanato]cobaltateC36H28CoNaN4O6ee >99%[α]D25=-5586 (c 0.032, MeOH)Source of chirality: synthesis from (S)-tryptophanAbsolute configuration: (Δ, S,S)

Silver Δ-bis-[N-salicylidene-(S)-threoninato]-cobaltateC22H22N2O8CoAgee >99%[α]D25=-5978 (c 0.031, MeOH)Source of chirality: synthesis from (S)-threonineAbsolute configuration: (Δ, S,S)

Potassium Λ-bis[N-salicylidene-(S)-tryptophanato]cobaltateC36H28CoKN4O6ee >99%[α]D25=-4062.5 (c 0.032, MeOH)Source of chirality: synthesis from (S)-tryptophanAbsolute configuration: (Λ, S,S)

Silver Δ-bis-[N-salicylidene-(S)-valinato]-cobaltateC24H26N2O6CoAgee >99%[α]D25=-2769.2 (c 0.026, MeOH)Source of chirality: synthesis from (S)-valineAbsolute configuration: (Δ, S,S)