Topics in Current Chemistry (2020) 378:9
1 3
that an asymmetric oxidative phenol-indole [3 + 2] coupling reaction to access
chiral benzofuroindolines could be successfully accessed by Mn(III)/CPA relay
catalysis (Scheme 26). In comparison with the previous sequential CDC cascade
reactions involving in situ-generated but actually isolatable quinones or imines
[84], this binary catalytic system was capable of generating labile N-Boc quinone imines to enhance synthetic practicality. As such, it is inarguably a paradigm of asymmetric metal/chiral phosphoric relay catalysis.
8 Perspective
Asymmetric relay catalysis of metal complexes and CPA has proven an efficient strategy to achieve unprecedented enantioselective cascade transformations
that are otherwise inaccessible or disfavored by a single catalyst. Many robust
binary catalytic systems comprised of a diverse range of transition-metal complexes, such as (Hoveyda-)Grubbs catalysts, gold(I) complexes, and dirhodium
carboxylates, and (spiro) chiral phosphoric acids, have been applied widely to
the development of efficient methods to access a broad scope of enantioenriched
heteroatom-containing molecules. However, there still remain several limitations and unexplored directions, for example, catalyst compatibility issues still
pose constraints on the effective combination of versatile activation modes. We
envision that immobilization of these catalysts via rational design and the use
of flow chemistry might be possible approaches to conquer these challenges.
Besides, harnessing active intermediates with metal complexes and CPA is still
in its infancy, and attempts to trap the unexplored active intermediates are still
urgently needed, as exemplified by the beautiful radical-initiated cascade reactions reported by Liu and coworkers [80–82]. Moreover, the sustainability of
relay catalysis, such as performing reactions in aqueous media, and industrialization should be the future guiding principles. As this perspective has illustrated,
we hope that even more versatile transformations via this relay catalysis will
appear, offering a general platform to advance practical enantioselective organic
synthesis.
References
1. Noyori R (2002) Asymmetric catalysis: science and opportunities (Nobel lecture). Angew Chem
Int Ed 41(12):2008–2022
2. Heitbaum M, Glorius F, Escher I (2006) Asymmetric heterogeneous catalysis. Angew Chem Int
Ed 45(29):4732–4762. https ://doi.org/10.1002/anie.20050 4212
3. Fache F, Schulz E, Tommasino ML, Lemaire M (2000) Nitrogen-containing ligands for asymmetric homogeneous and heterogeneous catalysis. Chem Rev 100(6):2159–2231. https ://doi.
org/10.1021/cr990 2897
4. Cesar V, Bellemin-Laponnaz S, Gade LH (2004) Chiral N-heterocyclic carbenes as stereodirecting ligands in asymmetric catalysis. Chem Soc Rev 33(9):619–636. https ://doi.org/10.1039/
b4068 02p
Reprinted from the journal
200
1 3
that an asymmetric oxidative phenol-indole [3 + 2] coupling reaction to access
chiral benzofuroindolines could be successfully accessed by Mn(III)/CPA relay
catalysis (Scheme 26). In comparison with the previous sequential CDC cascade
reactions involving in situ-generated but actually isolatable quinones or imines
[84], this binary catalytic system was capable of generating labile N-Boc quinone imines to enhance synthetic practicality. As such, it is inarguably a paradigm of asymmetric metal/chiral phosphoric relay catalysis.
8 Perspective
Asymmetric relay catalysis of metal complexes and CPA has proven an efficient strategy to achieve unprecedented enantioselective cascade transformations
that are otherwise inaccessible or disfavored by a single catalyst. Many robust
binary catalytic systems comprised of a diverse range of transition-metal complexes, such as (Hoveyda-)Grubbs catalysts, gold(I) complexes, and dirhodium
carboxylates, and (spiro) chiral phosphoric acids, have been applied widely to
the development of efficient methods to access a broad scope of enantioenriched
heteroatom-containing molecules. However, there still remain several limitations and unexplored directions, for example, catalyst compatibility issues still
pose constraints on the effective combination of versatile activation modes. We
envision that immobilization of these catalysts via rational design and the use
of flow chemistry might be possible approaches to conquer these challenges.
Besides, harnessing active intermediates with metal complexes and CPA is still
in its infancy, and attempts to trap the unexplored active intermediates are still
urgently needed, as exemplified by the beautiful radical-initiated cascade reactions reported by Liu and coworkers [80–82]. Moreover, the sustainability of
relay catalysis, such as performing reactions in aqueous media, and industrialization should be the future guiding principles. As this perspective has illustrated,
we hope that even more versatile transformations via this relay catalysis will
appear, offering a general platform to advance practical enantioselective organic
synthesis.
References
1. Noyori R (2002) Asymmetric catalysis: science and opportunities (Nobel lecture). Angew Chem
Int Ed 41(12):2008–2022
2. Heitbaum M, Glorius F, Escher I (2006) Asymmetric heterogeneous catalysis. Angew Chem Int
Ed 45(29):4732–4762. https ://doi.org/10.1002/anie.20050 4212
3. Fache F, Schulz E, Tommasino ML, Lemaire M (2000) Nitrogen-containing ligands for asymmetric homogeneous and heterogeneous catalysis. Chem Rev 100(6):2159–2231. https ://doi.
org/10.1021/cr990 2897
4. Cesar V, Bellemin-Laponnaz S, Gade LH (2004) Chiral N-heterocyclic carbenes as stereodirecting ligands in asymmetric catalysis. Chem Soc Rev 33(9):619–636. https ://doi.org/10.1039/
b4068 02p
Reprinted from the journal
200