Topics in Current Chemistry (2019) 377:37
1 3
compound. In 2016, Hepburna and Melchiorre developed a conjugate amino radical addition process to alkenylpyridines by combining Brønsted acid catalysis with
photocatalysis [19]. An electrophilic pseudo-iminium ion intermediate could be
generated transiently after protonation of the alkenylpyridines with Brønsted acid. In
their initial investigations, the chiral product could be obtained with 35% ee when a
BINOL-based chiral acid OC-15 was used (Scheme 17).
In 2014, Jacobsen and Stephenson reported an enantioselective synthesis of
β-amino esters via the combination of photoredox and asymmetric anion-binding
catalysis [20]. As they proposed, the oxidation of tertiary amine by photocatalyst
would generate the iminium ion intermediate, and the following nucleophilic addition would occur in a stereoselective manner with the aid of a chiral H-bond donor
catalyst OC-16, affording the product with up to 99% ee (Scheme 18).
In 2018, Knowles et al. achieved the asymmetric synthesis of pyrroloindolines
by the combination of photoredox and chiral phosphate anion catalysis [21]. The
phosphate base would firstly bind to the indole N–H and then an indole radical
cation–phosphate ion pair would be generated via the oxidation by excited photocatalyst. It binds to TEMPO and closes the ring to furnish a bench-stable intermediate in an enantioenriched manner. Moreover, this intermediate could react with
a wide range of nucleophiles via a one-electron oxidation enabled by photoredox
catalysis, generating a lot of chiral pyrroloindolines and alkaloid natural products
(Scheme 19).
In 2015, Ooi et al. provided a redox-neutral enantioselective coupling reaction of aldimines and N-arylaminomethanes [22]. By using an Ir(III) photosensitizer and a chiral aryl amino phosphonium barfate under visible light irradiation,
they successfully produced relevant diamine products in 60–90% yields with
high enantioselectivities. They proposed a reductive quenching cycle in which
OC-14 (5 mol%)
PC-7 (5 mol%)
PhMe, 25
o
C
blue LEDs,14 h
81% yield, 35% ee
N
p-tol
44
N
N
p-tol
Me
Ph
Ar
O
Ar
O
P
O
OH
Ar = 2,4,6-(i-Pr) 3 C 6 H 2
OC-15
45
46
+
N
N
N
N
Ir
t-Bu
t-Bu
PC-7
PF 6
-
CF 3
CF 3
F
F
F
F
Scheme 17 Enantioselective conjugate amino radical addition to alkenylpyridines
Reprinted from the journal
78
1 3
compound. In 2016, Hepburna and Melchiorre developed a conjugate amino radical addition process to alkenylpyridines by combining Brønsted acid catalysis with
photocatalysis [19]. An electrophilic pseudo-iminium ion intermediate could be
generated transiently after protonation of the alkenylpyridines with Brønsted acid. In
their initial investigations, the chiral product could be obtained with 35% ee when a
BINOL-based chiral acid OC-15 was used (Scheme 17).
In 2014, Jacobsen and Stephenson reported an enantioselective synthesis of
β-amino esters via the combination of photoredox and asymmetric anion-binding
catalysis [20]. As they proposed, the oxidation of tertiary amine by photocatalyst
would generate the iminium ion intermediate, and the following nucleophilic addition would occur in a stereoselective manner with the aid of a chiral H-bond donor
catalyst OC-16, affording the product with up to 99% ee (Scheme 18).
In 2018, Knowles et al. achieved the asymmetric synthesis of pyrroloindolines
by the combination of photoredox and chiral phosphate anion catalysis [21]. The
phosphate base would firstly bind to the indole N–H and then an indole radical
cation–phosphate ion pair would be generated via the oxidation by excited photocatalyst. It binds to TEMPO and closes the ring to furnish a bench-stable intermediate in an enantioenriched manner. Moreover, this intermediate could react with
a wide range of nucleophiles via a one-electron oxidation enabled by photoredox
catalysis, generating a lot of chiral pyrroloindolines and alkaloid natural products
(Scheme 19).
In 2015, Ooi et al. provided a redox-neutral enantioselective coupling reaction of aldimines and N-arylaminomethanes [22]. By using an Ir(III) photosensitizer and a chiral aryl amino phosphonium barfate under visible light irradiation,
they successfully produced relevant diamine products in 60–90% yields with
high enantioselectivities. They proposed a reductive quenching cycle in which
OC-14 (5 mol%)
PC-7 (5 mol%)
PhMe, 25
o
C
blue LEDs,14 h
81% yield, 35% ee
N
p-tol
44
N
N
p-tol
Me
Ph
Ar
O
Ar
O
P
O
OH
Ar = 2,4,6-(i-Pr) 3 C 6 H 2
OC-15
45
46
+
N
N
N
N
Ir
t-Bu
t-Bu
PC-7
PF 6
-
CF 3
CF 3
F
F
F
F
Scheme 17 Enantioselective conjugate amino radical addition to alkenylpyridines
Reprinted from the journal
78