Topics in Current Chemistry (2020) 378:9
1 3
acid [26], leading to a cascade allylic amine isomerization/Friedel-Craft type reaction [27–29]. A similar approach, long distance chain-walking/cyclization cascade
rendered by Ru–H/CPA relay catalysis, was recently accomplished by Saá and coworkers, giving 1,3-oxazaheterocycles 4 (Scheme 2) [30]. Although moderate enantioselectivity (62% ee) was obtained, this protocol eliminated the need for allylic
amines as necessary substrates.
Earth-abundant nickel complexes have also been used to combine with CPA for
relay catalysis [31]. In 2016, Moran and coworkers introduced this concept to create
a cascade isomerization of N-allylcarbamate 5 and enantioselective Friedel–Crafts
reaction with indole 6 by using NiCl 2 ·dme/triphos and CPA 7 (Scheme 3) [32].
Cationic metal complex and CPA can also work in a relay catalytic manner.
Murakami and coworkers found that a cationic iridium complex-promoted olefin transposition of (E)-1-alkenylboronates generated allylic boronates that were
able to undergo subsequent CPA-catalyzed allylation reaction of aldehydes [33]. In
addition, both cationic palladium (I) [34] and ruthenium(II) [35] complexes could
enable the double-bond transposition of 1,1-di(boryl)alk-3-enes to in  situ generate
1,1-di(boryl)alk-2-enes, which participated smoothly in CPA-catalyzed allylation of
R
N
(a) Enamine/Imine/Enolate Chemistry
(b) Hydrogenation of Imines
X = N, O
R
X
metathesis
isomerization
R''
X * R''
Nu
X- H
R
H-B*
nucleophilic
electrophilic
X
B*
Nu
X
* R
E
E
R'
EWG
electrophilic
R' *
EWG
Nu
Nu
H-B*
H-B*
R'
EWG
R
X
isomerization
[M]-H
ad di tio n
X metathesis
R
HN
*
H-B*
hydrogen
source
+
H 2 [M]
[M]
RNH 2
+
hydrogen
source
R''
X
oxidation
[O], H-B*
H-B*
Scheme  1 General strategies in asymmetric relay catalysis of metal complex with chiral phosphoric
acids (CPA)
Scheme 2 Long-distance chain-walking/cyclization cascade via RuH 2 (CO)(PPh 3 ) 3 and CPA relay catalysis
Reprinted from the journal
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