1 3
Topics in Current Chemistry (2019) 377:38
conditions allowed the authors to control the selective generation of the α,β- or β,γunsaturated ketones. Moreover, recently, Gong’s group demonstrated an asymmetric α-allylation approach of aldehyde with alkynes by combining hydridopalladium
and enamine catalysis [61]. The catalytic system comprised of an achiral palladium
complex, primary amine 67 and a chiral phosphoric acid 33. The reaction tolerated a
wide range of alkynes 65 and aldehydes 66, with various functionalities. The chemical transformation proceeds through the electrophilic π-allylpalladium intermediates (VII and VIII) combined with nucleophilic enamine intermediate. The reaction
provided the chiral α-quaternary aldehydes 68 in high yields and enantioselectivity (Scheme 14). Furthermore, ynals have also been coupled with aldehydes for the
preparation of stereoselective propargylic alcohols through a cross-aldol reaction
employing combined enamine and copper catalysis [62].
4 Reactions with Non‑activated Olefins
The reaction with simple non-activated olefins without pre-activation is a real challenge due to their inert nature making them resistant to most chemical reactions [63].
Within this theme, Dong and colleagues disclosed the intermolecular C-alkylation
of 1,2-diketones 70 with simple olefins 71 by employing the recyclable and directing
group aminopyridine 72 [64]. The aminopyridine reacts with the ketone, forming an
enamine intermediate, and the rhodium catalyst promotes C–H vinyl bond activation. Noteably, the reaction tolerated a wide range of simple olefins with various
ketones, providing alkylated products 73 in moderate-to-high yields (Scheme 15).
It is noteworthy that the directing group can be cleaved and recycled; moreover, the
reaction could be performed in one pot [64]. The same group further expanded their
O
H
57
CHCl 3 , –10
o
C
Ph
NO 2
58
Cat. 7 (10 mol%)
H
O
NO 2
Ph
59
d.r. 96:4 (syn/anti)
99% ee (syn)
p-TsOH (25 mol%)
EtOH (1.2 equiv.)
CHCl 3 , –10
o
C, 3 h
[PPh 3 Au]Cl (5 mol%)
AgBF 4 (5 mol%)
O
EtO
NO 2
Ph
80% yield
d.r. 92:8 (cis/trans)
60
Ph
O
O
O
I
TIPS
Ph
O
H
TIPS
Ph
O
TIPS
cat. 14 (100mol%)
AuCl (10 mol%)
toluene, r.t., 10 h
N
N
O
+
+
+
6
6 1
62 (20 mol%)
63
64
85% yield
63/64 = 5:1
Scheme 13 Enantioselective one-pot gold and enamine catalysis acetylation/cyclization reaction and the
direct α-vinylidenation of aldehyde 6
Reprinted from the journal
11
Topics in Current Chemistry (2019) 377:38
conditions allowed the authors to control the selective generation of the α,β- or β,γunsaturated ketones. Moreover, recently, Gong’s group demonstrated an asymmetric α-allylation approach of aldehyde with alkynes by combining hydridopalladium
and enamine catalysis [61]. The catalytic system comprised of an achiral palladium
complex, primary amine 67 and a chiral phosphoric acid 33. The reaction tolerated a
wide range of alkynes 65 and aldehydes 66, with various functionalities. The chemical transformation proceeds through the electrophilic π-allylpalladium intermediates (VII and VIII) combined with nucleophilic enamine intermediate. The reaction
provided the chiral α-quaternary aldehydes 68 in high yields and enantioselectivity (Scheme 14). Furthermore, ynals have also been coupled with aldehydes for the
preparation of stereoselective propargylic alcohols through a cross-aldol reaction
employing combined enamine and copper catalysis [62].
4 Reactions with Non‑activated Olefins
The reaction with simple non-activated olefins without pre-activation is a real challenge due to their inert nature making them resistant to most chemical reactions [63].
Within this theme, Dong and colleagues disclosed the intermolecular C-alkylation
of 1,2-diketones 70 with simple olefins 71 by employing the recyclable and directing
group aminopyridine 72 [64]. The aminopyridine reacts with the ketone, forming an
enamine intermediate, and the rhodium catalyst promotes C–H vinyl bond activation. Noteably, the reaction tolerated a wide range of simple olefins with various
ketones, providing alkylated products 73 in moderate-to-high yields (Scheme 15).
It is noteworthy that the directing group can be cleaved and recycled; moreover, the
reaction could be performed in one pot [64]. The same group further expanded their
O
H
57
CHCl 3 , –10
o
C
Ph
NO 2
58
Cat. 7 (10 mol%)
H
O
NO 2
Ph
59
d.r. 96:4 (syn/anti)
99% ee (syn)
p-TsOH (25 mol%)
EtOH (1.2 equiv.)
CHCl 3 , –10
o
C, 3 h
[PPh 3 Au]Cl (5 mol%)
AgBF 4 (5 mol%)
O
EtO
NO 2
Ph
80% yield
d.r. 92:8 (cis/trans)
60
Ph
O
O
O
I
TIPS
Ph
O
H
TIPS
Ph
O
TIPS
cat. 14 (100mol%)
AuCl (10 mol%)
toluene, r.t., 10 h
N
N
O
+
+
+
6
6 1
62 (20 mol%)
63
64
85% yield
63/64 = 5:1
Scheme 13 Enantioselective one-pot gold and enamine catalysis acetylation/cyclization reaction and the
direct α-vinylidenation of aldehyde 6
Reprinted from the journal
11