Pd-Catalyzed Umpolung of π-Allylpalladium Intermediates: Assembly of All-Carbon α-Vinyl Quaternary Aldehydes through C(sp3)-C(sp3) Coupling

Article abstract of DOI:10.1021/acscatal.8b03814

Construction of sterically congested all-carbon quaternary centers represents a formidable challenge in synthetic chemistry. The method described herein provides direct and facile access to a series of structurally diverse and synthetically useful aliphatic aldehydes, bearing an all-carbon α-vinyl quaternary center and a 1,5-diene functionality, through Pd-catalyzed umpolung of vinylethylene carbonates (VECs). The reaction features electrophilic-to-nucleophilic reactivity reversal of the VEC-derived π-allyl-palladium intermediate via an unusual β-hydride elimination process, and the resultant enolate is chemoselectively coupled with allylic acetate to form an α-vinyl aldehyde embedded with an all-carbon quaternary center.

Full text of DOI:10.1021/acscatal.8b03814

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Letter
Pd-Catalyzed Umpolung of #-Allylpalladium Intermediates: Assembly of
All-Carbon #-Vinyl Quaternary Aldehydes through C(sp3)-C(sp3) Coupling
Huifei Wang, Shuxian Qiu, Sasa Wang, and Hongbin Zhai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03814 • Publication Date (Web): 09 Nov 2018
Downloaded from http://pubs.acs.org on November 9, 2018
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Page 1 of 7
ACS Catalysis
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Pd-Catalyzed Umpolung of π-Allylpalladium Intermediates:
Assembly of All-Carbon α-Vinyl Quaternary Aldehydes through
C(sp3)-C(sp3) Coupling
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Huifei Wang,†,‡,‖ Shuxian Qiu,†,‖ Sasa Wang,† and Hongbin Zhai*,†,§
†State Key Laboratory of Chemical Oncogenomics, Shenzhen Engineering Laboratory of Nano Drug Slow-Release, Peking
University Shenzhen Graduate School, Shenzhen 518055, China
‡School of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China
§Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China
KEYWORDS: all-carbon quaternary center, decarboxylation, VECs, umpolung, palladium, allylic alkylation
ABSTRACT: Construction of sterically congested all-carbon quaternary centers represents a formidable challenge in synthetic
chemistry. The method described herein provides direct and facile access to a series of structurally diverse and synthetically useful
aliphatic aldehydes, bearing an all-carbon α-vinyl quaternary center and a 1,5-diene functionality, through Pd-catalyzed umpolung of
vinylethylene carbonates (VECs). The reaction
O
features electrophilic-to-nucleophilic reactivity
reversal of the VEC-derived π-allyl-palladium
intermediate via an unusual β-hydride
elimination process and the resultant enolate is
chemoselectively coupled with allylic acetate to
form an α-vinyl aldehyde embedded with an all-
carbon quaternary center.
L
R
"Pd"
E+
O
Pd
+
-
umpolung
O
O
O
AcO
R
R
CHO
CO2
R
VECs
nucleophile
electrophile
dienolate
high chemoselectivity
high functional diversity
all-carbon quaternary center
O
H
All-carbon quaternary centers with four different carbon
functional groups exist widely in biologically intriguing and
architecturally fascinating natural products as well as many
FDA-approved pharmaceuticals (Figure 1).1 Although synthetic
chemists have made tremendous efforts on developing novel
methodologies to access all-carbon quaternary centers in a
direct, efficient, and highly selective manner,2,3 there still
remain daunting challenges to the construction of acyclic all-
carbon α-vinyl quaternary aldehydes. The conventional
methods to those motifs usually required tedious steps and only
limited direct strategies have been reported.4,5 Consequently, it
is essential to develop novel approaches to rapidly construct all-
carbon α-vinyl quaternary aldehydes.
O
N
H
N
H
Me
Me
O
N
S
S
O
Me
Me
Me
N
N
S
N
H
S
N
O
O
O
O
O
H
H
N
O
H
O
Strychnine
Halenaquinone
11,11'-Dideoxyverticillin A
toxicity
cytotoxicity
PI3K inhibitor
MeO
MeO
MeO
OR
OH
N
N
O
O
HBr
H
MeO
MeN
HO
Razadyne
MeO
N

R = H, Morphine
R = Me, Codeine

Verapamil
calcium channel blocker
analgesic
AChE inhibitor
Organic cyclic carbonates, particularly vinylethylene
carbonates (VECs), have been witnessed in recent years as
privileged motifs enabling facile access to structurally diverse
and biologically potential building blocks.6 Notably, the
research groups of Zhang,7 Kleij, 8 Zhao,9 Glorius10 and others11
HO
NH2
N
OMe
CO2Et
Et
O
O
N
N
H
NH2
MeN
have accomplished
a
wide variety of Pd-catalyzed



Dextromethorphan
decarboxylative transformations of VECs (Scheme 1). The
common π-allylpalladium intermediate I, which contains a
nucleophilic alkoxide and an electrophilic π-allyl-Pd species
can serve as 1,3- or 1,5-dipole, leading to [3+2],7b-e,9c [5+2],10,11c
[5+3],11b or [5+4] cycloaddition9a-b,11a with high
Cytadre
Leritine
Talwin
analgesic
cough suppressant adrenocortical inhibitor
analgesic
Figure 1. Bioactive Natural Products and FDA-approved Drugs
with All-Carbon Quaternary Centers.
diastereo- or enantioselectivity (Scheme 1, eq 1). Alternatively,
the π-allylpalladium intermediate
I can also undergo
substitution at the terminal or internal position with certain
nucleophile, forging a new C-C,8c C-N,8a,d C-O,7a,8e,g C-S8b or C-
B bond8f (Scheme 1, eq 2). Compared with the wealth of reports
on VECs employed as an electrophile, the application of VECs
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ACS Catalysis
as a nucleophile is rare. Inspired by Trost’s pioneering
Page 2 of 7
while promising result was observed when the reaction was
performed at 50 ºC (entries and 6). Considerable
1
2
3
4
5
6
7
8
discovery on β-H elimination of π-allylpalladium intermediate
and umpolung of the reactivity12 (Scheme 1, eq 3) and due to
our continuing interest on novel decarboxylation strategies,13,14
we launched a research program on the palladium-catalyzed
reaction of VEC with allylic acetate as an electrophile. We
envisioned that VEC might display reversed reactivity and act
as a carbon-based nucleophile upon β-H elimination12,15 of the
π-allyl-Pd intermediate I, through which electrophilic π-allyl-
Pd intermediate I may be converted into nucleophilic dienolate
II. Note that during our current studies, the groups of Kleij8c
and Zhao9c disclosed their elegant work on Pd-catalyzed
umpolung of VECs in accomplishing homodimerization and
formal [4+2] cycloaddition, respectively (Scheme 1, eq 4),
representing the only two reports invloving reversed reactivity
of VECs to date. We herein describe our discovery of Pd-
catalyzed umpolung of VECs followed by allylation to forge
all-carbon α-quaternary aldehydes with high chemoselectivity
and good regioselectivity.
5
improvement, with complete suppression of homodimer 5 and
acetate 6, was noted when the reaction was conducted at 100 ºC
but the yield of the product was slightly decreased at 120 ºC
(entries 7 and 8). The reaction
Table 1. Optimization of Reaction Conditions
O
Pd cat. (2.5 mol %)
Ph
OHC
O
L
(7.5 mol %)
Me
O
+
+
AcO
Ph
THF (0.2 M), T, sealed-tube
CHO
Ph
9
1a
2a
3a
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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Cy
Me Me
P
OH
Ph
Cy
Cy
HO
OAc
L3
Ph
+
O
O
Ph
PPh2
PPh2
PPh2
PPh2
CHO
PPh2 PPh2
L1
L2
5
6
L4
entr
y
Pd cat.
L
T (ºC)
3a
3a:4:5:6
Reactions of VECs: serving as an electrophile (common)
1b
2b
Pd2dba3•CHCl3
L1
L1
rt
rt
29
34
34:37:12:17
41:39:11:19
[3+2], [5+2], [5+3] or [5+4]
cycloaddition
1)
O
O
L
Pd(OAc)2/bis-
sulfoxide
"Pd"
CO2
O
Pd
O
Nu
R
R
Nu
HO
3b
4b
5b
Pd(O2CCF3)2
Pd(PPh3)4
L1
L1
L1
rt
rt
0
25
31
20
31:48: 9:12
36:31:14:19
26:16:nd:58
HO
2)
or
VECs
(1,3- or 1,5-dipole)
-allyl-Pd intermediate
R
Ph
I
nucleophilic addition
nucleophile
electrophile
Pd(OAc)2/bis-
sulfoxide
Reactions of VECs: serving as a nucleophile (RARE)
3) Trost's early discovery:
6b
Pd(OAc)2/bis-
sulfoxide
L1
L1
L1
L1
L1
L1
50
43
62
58
74
73
65
45:50: 5:nd
73:27:nd:nd
73:27:nd:nd
80:20:nd:nd
77:23:nd:nd
70:30:nd:nd
H
"Pd"
Ph
OCO2Me
Ph
Ph
OCO2Me
Pd
OCO2Me
DMSO
L
L
-H
elimination
7b
Pd(OAc)2/bis-
sulfoxide
100
120
100
100
100
Ph
OCO2Me
O
8b
Pd(OAc)2/bis-
sulfoxide
4) Kleij's and Zhao's work
O
OH
O
Ph
O
OH
O
Ph
Ph
O
9
Pd(OAc)2/bis-
sulfoxide
O
Ph
homo-
O
Ph
palladium-titanium
relay catalysis
dimerization
Ph
CHO
10c
11d
Pd(OAc)2/bis-
sulfoxide
Kleij
Zhao
5) This work
O
O
O
L
R
Pd(OAc)2/bis-
sulfoxide
"Pd"
CO2
Pd
+
-
umpolung
E+
O
O
AcO
R
R
CHO
R
H
12
13
14
15
Pd(tBu3P)2
Pd(tBu3P)2
Pd(tBu3P)2
Pd(tBu3P)2
L1
L2
L3
L4
100
100
100
100
76
38
31
NR
87:13:nd:nd
52:48:nd:nd
39:61:nd:nd
VECs
dienolate
II
complete chemoselectivity
high functional diversity all-carbon quaternary center
Scheme 1. Pd-Catalyzed Umpolung of VECs
We commenced our studies with the reaction of phenyl-
substituted vinylethylene carbonate (1a) with 2 equiv of allylic
acetate (2a) using Pd2dba3•CHCl3 (2.5 mol %) as the pre-
catalyst and DPEPhos (L1) (7.5 mol %) as a ligand in THF at
room temperature (Table 1, entry 1). Gratifyingly, the desired
α-quaternary aldehyde (3a), was obtained in 29% yield,
accompanied by the enal 4, the homodimerization by-product
5, and the acetate 6. The formation of the first three compounds
(3a, 4, 5) implied that the reactivity of VEC had been
dominantly reversed as anticipated. Screening of the palladium
catalysts revealed that White’s catalyst16 perform better than
Pd(PPh3)4, Pd2dba3•CHCl3, or Pd(O2CCF3)2 (entries1-4). The
transformation was found to be sensitive to the reaction
temperature (entries 5-8). The reaction preferentially gave the
acetate 6 as the major product at a lower temperature (0 ºC),
aReaction optimization was performed using 0.2 mmol of 1a and 0.6 mmol
of 2a in THF for 4 h unless noted otherwise. Yields were determined by 1H
NMR using 1,3,5-trimethoxybenzene as an internal standard. b2a (0.4
mmol), 12 h. Na2SO4 (0.5 equiv) was added. Proton sponge (1.0 equiv)
was added. Pd2dba3•CHCl3 = tris(dibenzylideneacetone)dipalladium(0)
chloroform adduct. Pd(OAc)2/bis-sulfoxide = White’s catalyst. DPEphos
(L1) = oxydi-2,1-phenylene)bis(diphenylphosphine. Proton sponge = 1,8-
bis(dimethylamino)naphthalene.
c
d
efficiency was further enhanced by higher loading of acetate 2a
(3 equiv) and shorter reaction time (4 h) (entry 9). Addition of
Na2SO4 or proton sponge had no effect (entries 10 and 11). To
our delight, replacement of White’s catalyst with more electron-
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ACS Catalysis
rich and sterically hindered Pd(tBu3P)2 increased the yield of 3a
to 76% (entry 12). Ultimately, upon testing a series of other
ligands, DPEphos (L1) was identified as the optimal one
(entries 13-15).17
explored (Scheme 2). Substrates with an either electron-
donating (3b-3h) or electron-withdrawing (3i-3l) group at the
para position of the aromatic ring were compatible with the
reaction conditions, giving the desired products in moderate to
good yields. Additionally, m-substitution (3m, 3n), multi-
substitution (3o, 3p), or more conjugation (3q-3s) of the aryl
rings were all well-tolerated. Of particular interest, the reaction
of phenylvinyl substituted carbonate (3t) also proceeded
smoothly, producing the product in a comparable yield. In order
to further investigate the tolerance of structural diversity of the
transformation, a broad range of heteroaromatic rings were
embedded into the substrates and it was found that heteroaryl
1
2
3
4
5
6
7
8
9
O
Pd(tBu3P)2 (2.5 mol %)
CHO
CHO
O
DPEPhos (7.5 mol %)
+
O
Me
+
AcO
Ar
THF, 100 °C, 4 h
sealed-tube
Het
3
Het
1
2a
4
(minor)
Aromatic Moiety
10
11
12
13
14
15
16
17
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CHO
CHO
CHO
CHO
carbonates, those bearing
a furyl, thienyl, benzofuryl,
benzothienyl, or pyridyl group were all competent substrates
(3u-3ac). It is noteworthy that our method was also applicable
to a late-stage diversification of structurally intricate molecules.
The estrone analogue 3ad for example, reacted effectively and
furnished the product in a synthetically useful yield, although
higher catalyst loading and temperature were found to be
necessary.
Me
R
CF3
3k
CO2Me
3l
3m
, 70%
, 73%
, 61%
CHO
3a
, R = H, 72%
3b
3c
3d
3e
, R = Me, 80%
b
, R = Et, 62%
CHO
CHO
t
b
, R = Bu, 53%
, R = Ph, 60%
3f
, R = OMe, 58%
O
3g
3h
, R = OBn, 58%
, R = SMe, 76%
, R = F, 63%
CHO
O
OMe
Me
Me
MeO
OMe
Pd(tBu3P)2 (5 mol %)
DPEPhos (15 mol %)
O
Ar
Ar
OMe
3i
3j
+
b
AcO
3n
3o
3p
, 74%
, R = Cl, 79%
, 69%
, 58%
THF, 100 °C, 4 h
sealed-tube
1h
2'
7
CHO
SMe
Vinyl Moiety
SMe
CHO
CHO
CHO
CHO
CHO
CHO
SMe
Me
Ph
OMe
Me
3q
3s
, 63%
, 81%
CHO
3r
3t
, 63%
, 52%
SMe
SMe
7b
Heterocyclic Moiety
7a
7c
, 64%
, 58%
, 57%
CHO
S
CHO
S
OH
OH
OH
CHO
OMe
O
O
F
Cl
O
OMe
c
3u
3v
3w
3x
, 75%
, 63%
, 61%
, 78%
SMe
SMe
SMe
7f
b
b
7d
7e
, 60%
, 44%
, 52%
CHO
S
CHO
S
CHO
O
CHO
N
Scheme 3. Substrate Scope. a General reaction conditions: 1h
(0.2 mmol), 2' (0.6 mmol), Pd(tBu3P)2 (5 mol %), DPEPhos (15
b
OMe
mol %), 100 º C, 4 h. Isolated yields.
Yield of the
R
corresponding alcohol product over the two steps including
reduction with NaBH4.
3y
3z
, R = Me, 60%
, R = Cl, 75%
3aa
3ab
3ac
, 61%
, 60%
, 58%
Late-Stage Diversification of Complex Molecule
OMe
Me
Next, we explored the scope of the allylic acetate partners
with VEC 1h (Scheme 3).18 Under slightly reoptimized
conditions with higher catalyst loading, branched aryl-
substituted allylic acetates19 carrying either electron-rich (7a-
7d) or electron-deficient (7e-7f) groups showed good tolerance
and provided single linear product with exclusive E-selectivity,
implying that the nucleophilic addition take place preferentially
at the less sterically hindered terminal position of the π-allyl-Pd
intermediate.
H
3ad
estrone analogue
50%b
OHC
H
H
Scheme 2. Substrate Scope. a General reaction conditions: 1
(0.2 mmol), 2a (0.6 mmol), Pd(tBu3P)2 (2.5 mol %), DPEPhos
(7.5 mol %), 100 ºC, 4 h. Isolated yields. b Pd(tBu3P)2 (5 mol %),
DPEPhos (15 mol %), 110 ºC. Yield of the corresponding
alcohol product over the two steps including reduction with
NaBH4.
c
To demonstrate the practicality of the method, a gram-scale
experiment was performed, in which a comparable yield was
observed for the generation of α-quaternary aldehyde 3q
(Scheme 4). The synthetic utility of the obtained α-quaternary
aldehyde 3q was then extensively demonstrated in five types of
With the optimized conditions secured, we focused on
examination of the scope of the substrates in the Pd-catalyzed
umpolung of VECs. Various substituted carbonates were first
representative
derivatizations.
First,
Seyferth-Gilbert
ACS Paragon Plus Environment

ACS Catalysis
homologation of the aldehyde furnished alkyne 8a smoothly in
Page 4 of 7
A plausible mechanism of the Pd-catalyzed decarboxylative
generation of all-carbon α-quaternary aldehydes is proposed in
Scheme 5. On one hand, the π-allyl-Pd intermediate I22 is
initially generated from the vinylethylene carbonate 1 in the
presence of Pd(0) by oxidative addition, accompanied by CO2
extrusion. The electrophilic π-allyl-Pd intermediate I then
undergoes β-H elimination,12,15 delivering the nucleophilic
dienolate II. The palladium catalyst can either dissociate,
liberating the dienolate and regenerating Pd(0) as shown below,
or it can bind to the dienote II to undergo nucleophilic addition
prior to dissociation and catalyst regeneration. On the other
hand, π-allyl-Pd complex III, formed in situ from allyl acetate,
could be trapped by the dienolate II, furnishing the complex IV
with a newly-formed all-carbon quaternary center. Finally,
dissociation of complex IV delivers the aldehyde 3 with
simultaneous regeneration of the Pd(0) catalyst. As a side
reaction, nucleophilic addition of the dienolate II to π-allyl-Pd
intermediate I furnishes the homodimeric by-product 5.8c In
addition, protonation of dienolate II followed by isomerization
could generate the enal 4. The by-product 6 can be formed by
nucleophilic addition of acetate anion to the π-allyl-Pd
intermediate I, similar to previously reported nucleophilic
addition.6a
1
2
3
4
5
6
7
8
65% yield. The aldehyde could then be converted directly into
the carboxylic ester 8b and the hydrazone 8c under mild
conditions in excellent yields.20 The structure of hydrazone 8c
was unambiguously confirmed by single crystal X-ray
diffraction analysis.21 Next, one-step reductive amination of the
aldehyde
1q
2a
(4.75 mmol) +
CHO
standard
condition
[gram-scale]
9
2-Naph
, 82%
(Z:E = 1.4:1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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49
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58
59
60
2-Naph
e)
8e
a)
8a
, 65%
CHO
Cope
3q
0.931 g
(83%)
b)
NHPMB
d)
CO2Me
reductive
amina
t
i
on
2-Naph
8b
2-Naph
8d
c)
, 92%
, 77%
NNHTs
2-Naph
8c
In summary, by simply reacting vinyl cyclic carbonate with
allylic acetate in the presence of the sterically hindered
Pd(tBu3P)2 catalyst, we have accomplished a rare but useful
variant of Pd-catalyzed decarboxylative umpolung of VECs
providing direct access to a broad series of structurally diverse
and synthetically useful all-carbon α-vinyl quaternary
aldehydes. Given the long-standing challenges in construction
of all-carbon quaternary centers and the robustness of the novel
and efficient decarboxylative umpolung of VECs, it can be
anticipated with confidence that this protocol would enhance
synthetic utilities in the assembly of a wide range of
architecturally complex and bioactive building blocks.
[X-ray]
, 87%
Scheme 4. Gram-scale Synthesis and Representative
Derivatizations of 3q. Reagents and conditions: a) Bestmann
reagent (dimethyl (diazomethyl)phosphonate), MeOH, 0 º
C→rt, 0.5 h; b) I2, KOH, MeOH, 0 ºC→rt, 0.5 h; c) TsNHNH2,
Na2SO4, MeOH, rt, 3 h; d) PMBNH2, Na2SO4, DCE;
NaBH(OAc)3; e) mw, toluene, 150 ºC, 7 h.
O
CHO
O
Ph
O
Pd(0)
3
Ph
1
CO2
HOAc
AcO
Pd(0)
ASSOCIATED CONTENT
L
L
Pd
The Supporting Information is available free of charge on the ACS
Publications website.
Experimental procedures and characterization data for all
new compounds (PDF)
O
O
Ph
OAc
Ph
H
IV
L
Pd
L
I
H
OAc
Pd
L
L
X-ray crystallographic data for 8c (CIF)
III
OHC
Me

-H
OAc
Ph
eliminatio
AUTHOR INFORMATION
4
(by-product)
n
H+
Corresponding Author
HO
OAc
*E-mail: zhaihb@pkusz.edu.cn.
O
Ph
6
Ph
II
(by-product)
I
Author Contributions
‖ These authors contributed equally.
homodimer 5
(by-product)
Scheme 5. Catalytic Cycle of the Pd-Catalyzed Umpolung of
VECs.
Notes
The authors declare no competing financial interest.
afforded the corresponding amine 8d in 77% yield. In addition,
microwave mediated Cope rearrangement of the 1,5-diene 3q
took place at 150 ºC, providing the enals 8e (Z:E = 1.4:1) in
82% combined yield.
ACKNOWLEDGMENT
We thank the Shenzhen Science and Technology Innovation
Committee
(JCYJ20150529153646078
and
JSGG20160229150510483) and NSFC (21871018, 21732001,
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J. Enantioselective Route to Aryl(1,3-butadien-2-yl)methanols: Formal
21702010, and 21672017) for financial support. We are grateful to
Prof. Fayang Qiu of GIBH, CAS for enlightening discussions.
Synthesis of (-)-Sporochnol A. Heterocycles 2009, 77, 249-253. (d)
Kumar, R.; Halder, J.; Nanda, S. Asymmetric Total Synthesis of (R)-
α-Cuparenone, (S)-Cuparene and Formal Synthesis of (R)-β-
Cuparenone through Meinwald Rearrangement and Ring Closing
Metathesis (RCM) Reaction. Tetrahedron 2017, 73, 809-818. (e) Vital,
P.; Tanner, D. Efficient and Highly Enantioselective Formation of the
All-Carbon Quaternary Stereocentre of Lyngbyatoxin A. Org. Biomol.
Chem. 2006, 4, 4292-4298. (f) Simaan, M.; Delaye, P.-O.; Shi, M.;
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Stereocenters. Angew. Chem. Int. Ed. 2015, 54, 12345-12348.
(5) For selected direct strategies, see: (a) Haraguchi, R.; Kusakabe, A.;
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Synthesis of α-Quaternary Formimides and Aldehydes through
Umpolung Asymmetric Copper Catalysis with Isocyanides. J. Am.
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(17) For more details of screening of the ratio of Pd(tBu3)2/DPEPhos,
see the Supporting Information.
(18) Considering that hundreds of sulfur-containing drugs have been
approved by FDA and sulfur functional groups exist widely in
pharmaceuticals and natural products, we chose the sulfur-containing
VEC 1h to explore the scope of allylic acetates. When VEC 1a, without
substitution on the aromatic ring, was explored with acetate 2a’ under
the same conditions, the desired product was obtained in 49% yield.
(19) The linear aryl-substituted allylic acetate, cinnamyl acetate has
been explored with VEC 1h under the same condition, giving the
product 7a in a slightly reduced yield (46%).
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(21) The crystal structure of hydrazone 8c was deposited at the
Cambridge Crystallographic Data Centre (tracking number: 1862065).
(22) The 31P NMR experiments demonstrated that the bidentate
phosphine (DPEPhos) might replace the alkyl phosphines (tBu3P) to
form the diphosphine palladium complex. For more details, see the
Supporting Information.
Palladium-Catalyzed
Decarboxylative
Coupling
of
Vinyl
Benzoxazinanones with Arynes. Org. Lett. 2018, 20, 1417-1420. (b)
Wang, S.; Chen, X.; Ao, Q.; Wang, H.; Zhai, H. Decarboxylative
Csp3–Csp3 Coupling for Benzylation of Unstable Ketone Enolates:
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