Topics in Current Chemistry (2020) 378:9
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conceptually novel multiple catalysis [7, 8]. The premier example of multiple catalysis is the combination of metal complexes and chiral organocatalysts, because, while
metal complexes are versatile in bond-breaking and bond-forming events, organocatalysts exhibit a high tolerance of functionalities with a unique level of stereocontrol.
Since the proof of concept was reported in 2001 [9, 10], research into asymmetric metal and organo combined catalysis has grown rapidly [11–14]. Based on the
behaviors of distinct catalysts, combined catalysis can be classified into cooperative [8, 11], relay [15], and sequential [16] catalysis (Fig. 1). Cooperative catalysis
describes the case where all the catalysts are involved in bond-breaking or forming events individually and simultaneously (Fig. 1a), while relay catalysis refers
to a one-pot cascade process in which individual reactions are independently promoted by distinct catalysts in a cascade manner (Fig. 1b). When catalysts become
significantly incompatible with each other, or with reaction conditions, some of the
catalysts or reagents must be added in a stepwise manner; this is named sequential
catalysis (Fig. 1c).
Since the seminal works by Terada [17] and Akiyama [18], chiral phosphoric
acids (CPA) have been established as powerful and privileged organocatalysts
[19–22]. The synergistic advantages of variable chiral backbones, tunable steric hindrance, bifunctional activator and good compatibility with metal complexes make
CPA superior for asymmetric metal/organo combined catalysis. Many excellent
reviews regarding metal/CPA combined catalysis have been published over the past
decades; however, most of them are focused on the classification of bond-forming
reaction [14, 21] or different catalyst combinations [15, 16, 23], and detailed discussion about the catalytic behaviors of metal complexes is still scarce (Scheme 1).
For instance, a variety of transition metal complexes, such as ruthenium, nickel and
gold, can promote a broad range of transformations featuring unsaturated hydrocarbon functionalization (e.g., olefin isomerization/metathesis, hydroamination/
hydroalkoxylation of alkynes). The resulting active intermediates (e.g., enamines,
imines and enols) are able to participate in subsequent CPA-catalyzed enantioselective chemical bond formation (Scheme 1a). Another successful example of asymmetric relay catalysis is the stepwise hydrogenation of imines motif (Scheme 1b).
Judicious selection of metal catalysts and reductants has provided efficient access
to enantioenriched heterocycles bearing multiple carbon stereocenters. Progress in
asymmetric relay catalysis of metal with CPA from 2008 to mid-2014 has been summarized by Patil [24], Gong [14, 15, 23], You [16] and Rueping [21]. Considering
that the last comprehensive reviews were published in 2014, and a number of exciting advances in this area have been presented during the past 5 years, we focus here
mainly on the progress in asymmetric relay catalysis of metal complexes with CPA
from 2014 to mid-2019.
2 Olefin Isomerization/Metathesis in Relay Catalysis
Metal-hydride-mediated olefin isomerization has been a widely applicable approach
to generate reactive intermediates for organic synthesis [25]. In 2008, Terada and
coworkers first demonstrated that RuH 2 (CO)PPh 3 was compatible with phosphoric
Reprinted from the journal
186
1 3
conceptually novel multiple catalysis [7, 8]. The premier example of multiple catalysis is the combination of metal complexes and chiral organocatalysts, because, while
metal complexes are versatile in bond-breaking and bond-forming events, organocatalysts exhibit a high tolerance of functionalities with a unique level of stereocontrol.
Since the proof of concept was reported in 2001 [9, 10], research into asymmetric metal and organo combined catalysis has grown rapidly [11–14]. Based on the
behaviors of distinct catalysts, combined catalysis can be classified into cooperative [8, 11], relay [15], and sequential [16] catalysis (Fig. 1). Cooperative catalysis
describes the case where all the catalysts are involved in bond-breaking or forming events individually and simultaneously (Fig. 1a), while relay catalysis refers
to a one-pot cascade process in which individual reactions are independently promoted by distinct catalysts in a cascade manner (Fig. 1b). When catalysts become
significantly incompatible with each other, or with reaction conditions, some of the
catalysts or reagents must be added in a stepwise manner; this is named sequential
catalysis (Fig. 1c).
Since the seminal works by Terada [17] and Akiyama [18], chiral phosphoric
acids (CPA) have been established as powerful and privileged organocatalysts
[19–22]. The synergistic advantages of variable chiral backbones, tunable steric hindrance, bifunctional activator and good compatibility with metal complexes make
CPA superior for asymmetric metal/organo combined catalysis. Many excellent
reviews regarding metal/CPA combined catalysis have been published over the past
decades; however, most of them are focused on the classification of bond-forming
reaction [14, 21] or different catalyst combinations [15, 16, 23], and detailed discussion about the catalytic behaviors of metal complexes is still scarce (Scheme 1).
For instance, a variety of transition metal complexes, such as ruthenium, nickel and
gold, can promote a broad range of transformations featuring unsaturated hydrocarbon functionalization (e.g., olefin isomerization/metathesis, hydroamination/
hydroalkoxylation of alkynes). The resulting active intermediates (e.g., enamines,
imines and enols) are able to participate in subsequent CPA-catalyzed enantioselective chemical bond formation (Scheme 1a). Another successful example of asymmetric relay catalysis is the stepwise hydrogenation of imines motif (Scheme 1b).
Judicious selection of metal catalysts and reductants has provided efficient access
to enantioenriched heterocycles bearing multiple carbon stereocenters. Progress in
asymmetric relay catalysis of metal with CPA from 2008 to mid-2014 has been summarized by Patil [24], Gong [14, 15, 23], You [16] and Rueping [21]. Considering
that the last comprehensive reviews were published in 2014, and a number of exciting advances in this area have been presented during the past 5 years, we focus here
mainly on the progress in asymmetric relay catalysis of metal complexes with CPA
from 2014 to mid-2019.
2 Olefin Isomerization/Metathesis in Relay Catalysis
Metal-hydride-mediated olefin isomerization has been a widely applicable approach
to generate reactive intermediates for organic synthesis [25]. In 2008, Terada and
coworkers first demonstrated that RuH 2 (CO)PPh 3 was compatible with phosphoric
Reprinted from the journal
186