Tropos or Atropos? That is the Question!

Abstract

While non-racemic catalysts can generate non-racemic products with or without the non-linear relationship in enantiomeric excesses between catalysts and products, racemic catalysts inherently give only a racemic mixture of chiral products. Racemic catalysts can be enantioselectively evolved into highly activated catalysts by association with chiral activators. Asymmetric activation strategy can produce greater enantiomeric excess in the products, even when using a catalytic amount of chiral activator per chiral or racemic catalysts bearing atropisomeric (atropos: from Greek a meaning not, and tropos meaning turn) ligands, than the enantioselectivity attained by the enantiomerically pure catalyst on its own. Some recent applications of the asymmetric activation catalysis that employ not only atropos and racemic ligands but also tropos ligands without enantiomeric resolution are herein reported. The great success of the asymmetric catalysts with tropos ligands clearly illustrate that chirally rigid atropos ligands can be replaced by chirally flexible tropos ligands to give preferentially the thermodynamically favorable diastereomer of catalysts with higher chiral efficiency than does the minor isomer. The asymmetric activation concept now progress toward the use of racemic but tropos ligands rather than the use of atropos ones.

1 Prologue: Tropos or Atropos? That is the Question!

2 Asymmetric Activation of Atropos and Racemic
Catalysts

2.1 Carbon-Carbon Bond Forming (Ene, Aldol, and
Diels-Alder) Reactions

2.2 Hydrogenation

3 Asymmetric Activation of Racemic but Tropos Catalysts

3.1 Biphenol (BIPOL)

3.2 Biphenylphosphine (BIPHEP)

3.3 Bis(diphenylphosphino)ferrocene (DPPF)

3.4 Hydrogenation

3.5 Carbon-Carbon Bond Forming (Diels-Alder and Ene)
Reactions

3.6 Tropos vs. Atropos Nature of BIPHEP-Metal Catalysts

4 Epilogue: Tropos rather than Atropos! That is an Answer!

Key words

asymmetric catalysis - asymmetric activation - atropos - tropos - biphenylphosphine

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21

See ref. [14] Amounts of chiral poisons: 2.0 equiv for Al, 1.4 equiv for Rh, 20 equiv for Ru, 3.0 equiv for Ti, and 2.0 equiv for Ir.

22

Mikami, K.; Yusa, Y.; Korenaga, T. Org. Lett. in press.

37

Configuration of the dppf chirality is denoted by using the descriptors of chirality: (P), plus, clockwise and (M), minus, anticlockwise.

41

Geometry optimization was performed with Gaussian 98. Computational details, see: Yamanaka, M.; Mikami, K. submitted.

44

Epimerisation of (R)-RuCl₂(XylBIPHEP)/(S,S)-DPEN to (S)-RuCl₂(XylBIPHEP)/(S,S)-DPEN was very slow at room temperature in CD₂Cl₂, but accelerated in 2-propanol to give a 3:1 ratio of diastereomeric complexes within 2-3 hours (see ref. [28] ).