Palladium-Catalyzed Oxidative Allylic Alkylation of N -Hydroxy-imides

Article abstract of DOI:10.1055/s-0039-1691508

A palladium-catalyzed oxidative C-H allylic alkylation of N -hydroxyimides has been developed. This transformation provided valuable N -allyloxypyrrolidinediones in moderate to excellent yields using operationally simple, ligand free, and mild reaction co

Full text of DOI:10.1055/s-0039-1691508

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
letter
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N. Ayyagari et al.
Letter
Synlett
Palladium-Catalyzed Oxidative Allylic Alkylation of N-Hydroxy-
imides
Narasimham Ayyagari◊
O
open air
Sunil Kumar Sunnam◊
Milind M. Ahire
Minxi Yang
H
O
N
O
R
+
HO N
R
Pd
O
O
20 examples
up to 85% yield
up to 1 gram scale
Kevin Ngo
C–H activation
R = alkyl, halide,
ether, ester,
ketone, nitrile
Jitendra D. Belani*
0
0
0
0-
0
0
0
3-4
8
7
6-0
0
4
4
Department of Pharmaceutical Sciences, School of Pharmacy,
Thomas Jefferson University, 901 Walnut St, Ste. 919, Philadelphia,
PA 19107, USA
jitendra.belani@jefferson.edu
◊ The authors contributed equally to the project
Received: 13.09.2019
Previous work
Accepted after revision: 13.11.2019
Published online: 29.11.2019
Intermolecular allylic alkylation
DOI: 10.1055/s-0039-1691508; Art ID: st-2019-r0489-l
C–C bond
R1
nitro alkanes, aryl boronic
acids, active methylenes,
ylides, allenes
C
N
(1)
(2)
Abstract A palladium-catalyzed oxidative C–H allylic alkylation of
N-hydroxyimides has been developed. This transformation provided
valuable N-allyloxypyrrolidinediones in moderate to excellent yields us-
ing operationally simple, ligand free, and mild reaction conditions. The
reaction tolerated broad and variable substituents on allylarenes and
N-hydroxyimides.
Pd
R1
R1
C–N bond
R1
amides, anilines, amines
R = alkyl or aryl
C–O bond
O
(3)
(4)
acids, alcohols, phenols
Key words allylic C–H activation, palladium, N-hydroxyimides, C–O
bond formation
Intramolecular allylic alkylation
X
Y
Pd
X
Y
R2
Construction of Csp3–oxygen, Csp3–Csp3, and Csp3–nitro-
gen bond can be efficiently achieved using the classic palla-
dium-catalyzed Tsuji–Trost allylic alkylation.1 The reaction
involves a Pd-η3--allyl complex, which undergoes attack
by various nucleophiles. The reaction, however, requires
allylic substrates to be pre-oxidized. Transition-metal-
catalyzed oxidative allylic alkylation is a privileged synthet-
ic transformation, which provides strategic advantages in
access of C–C and C–X (carbon–heteroatom) bonds with
minimum prefunctionalization.2 Direct oxidation or func-
tionalization of allylic Csp3–H bonds was first introduced by
the White group in 2004 using sulfoxide-promoted, catalyt-
ic Pd(OAc)2/benzoquinone (BQ)/AcOH -olefin allylic oxida-
tion systems.3a This chemo- and regioselective transforma-
tion proceeds via a serial ligand catalysis mechanism.3b The
reaction has now been expanded for the construction of C–C
bond (Scheme 1, eq. 1),4 C–N bond (Scheme 1, eq. 2 and 4),5
and C–O bond (Scheme 1, eq. 3 and 4)6 to provide function-
alized products.
R2
X, Y = O, N
This work
O
O
H
Pd
N
(5)
+
HO N
Ar
Ar
O
O
O
Scheme 1 Palladium-catalyzed allylic alkylation
try. They have been used in peptide synthesis7 and in radi-
cal and electrocatalytic reactions.8 It was thus envisioned
that nucleophiles such as N-hydroxysuccinimide (NHS,
pKa = 6.1)9a and N-hydroxyphthalimide (NHPI, pKa = 7.0)9b
have low basicity to allow their use in allylic substitutions.
Such oxygenation of allylic substrates has been reported
previously on allylic acetates using NHS and NHPI.10 Sepa-
rately benzylic and allylic hydrocarbons have been reported
to undergo radical-mediated oxygenation specifically with
NHPI.11 This work expands on the previous reports and
presents an alternative method utilizing nonradical process
that uses unfunctionalized allylic substrates. The N-allyl-
oxypyrrolidinedione products obtained in this reaction can
We have sought to extend the scope of oxidative allylic
C–H alkylation reaction using oxygen nucleophiles that
have heteroatoms directly attached to it. N-Hydroxyimides
are strategically very important reagents in organic chemis-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
N. Ayyagari et al.
Letter
Synlett
serve as convenient synthons for terminal oxygenation11a or
the installation of the –ONH2 group,12 which is an import-
ant moiety in biologically active molecules. Hydroxylamine
derivatives obtained easily from these pyrrolidinediones
exhibit important anticancer, antibacterial, and antimalari-
al activities.13 Separately, their structural features allow
them to be used as excellent directing groups in transition-
metal-catalyzed C–H activation reactions14a as well as ami-
nating reagents in C–H activation reagents.14b
Toward the goal of identifying suitable reaction condi-
tions for the transformation, preliminary evaluation was
conducted on commercially available 4-allyl anisole (1a) as
the allylic substrate (Table 1). A trial reaction with 1a (0.2
mmol scale) and N-hydroxysuccinimide (NHS, 2a, 2 equiv)
as the nucleophilic partner in the presence of Pd(OAc)2 (0.1
equiv) as the catalyst and benzoquinone (BQ, 2 equiv) as the
oxidant3a in 1,4-dioxane did not work (entry 1). A quick
survey of solvents using the above conditions proved that
acetonitrile was better than 1,4-dioxane and dichlo-
roethane (entries 2–4). To our delight, the desired product
3a was isolated in 23% yield using acetonitrile as the solvent
at 40 °C (entry 4). Increasing the reaction temperature to
75 °C improved the yields to 40% (entry 5). Commercially
available White catalyst in the presence of BQ provided a
low yield (entry 6). At this point, we wished to screen palla-
dium catalysts and oxidants for the reaction. Four addition-
al oxidants were screened including O2,15a PhI(OAc)2,15b
Cu(OAc)2 ,15c and Cu(OTf)2.15d The reaction works with oxy-
gen as the terminal oxidant in 46% yield (entry 7). PhI(OAc)2
was found to be less efficient oxidant providing the desired
product in only 29% yield (entry 8). Stoichiometric Cu(OAc)2
and Cu(OTf)2 were both tried with catalytic Pd(OTf)2 for the
Table 1 Optimization of Allylic Functionalization
O
N
H
O
catalyst (10 mol%)
oxidant
O
OAc
N
OH
+
+
O
solvent
temperature
time
MeO
MeO
MeO
O
1a
4a
2a
3a
Entrya
Catalyst
Oxidant (equiv)
NHS (equiv)
Solvent
Temp (°C)b
Yield (%)c
1
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
White catalyst
Pd(OAc)2
Pd(OAc)2
Pd(OTf)2
Pd(OTf)2
Pd(OAc)2
PdCl2
BQ (2)
2
2
2
2
2
2
2
2
2
2
2
2
2
1,4-dioxane
40
40
40
40
75
75
75
75
75
75
75
75
75
NR
NR
16
23
40
15
46
29
50
NR
62
NR
52
BQ (2)
DMSO/1,4-dioxane (1:1)
3
BQ (2)
DCE
4
BQ (2)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
5
BQ (2)
6
BQ (2)
7
O2 (1 atm)
PhI(OAc)2 (2)
Cu(OAc)2 (1)
Cu(OTf)2 (1)
Cu(OAc)2 (1)
Cu(OAc)2 (1)
Cu(OAc)2 (1)
8
9
10
11
12
13
Pd(OAc)2
MeCN
TEMPO (1 equiv)
14
–
Cu(OAc)2 (1)
Cu(OAc)2 (1)
2
3
MeCN
75
75
NR
70
15d
Pd(OAc)2
MeCN
without AcOH
16d
17e
Pd(OAc)2
Pd(OAc)2
Cu(OAc)2 (1)
Cu(OAc)2 (1)
3
3
MeCN
AcOH (0.5 equiv)
75
75
84
27
MeCN
AcOH (0.5 equiv)
a The reactions were carried out at a concentration of 0.1 M and heated for 24–28 h, 1a (0.05 mmol, 1 equiv), 2a (2 equiv), Pd catalyst (10 mol%). Abbreviations:
MeCN: acetonitrile; BQ: benzoquinone; DMSO: dimethylsulfoxide; TEMPO: 2,2,6,6-tetramethylpiperidine-1-oxyl radical; DCE: dichloroethane; NR: no reaction
observed by TLC.
b Temperature refers to inside temperature.
c Isolated yield.
d Conditions: 1a (0.05 mmol, 1 equiv), 2a (3 equiv), Pd(OAc)2 (10 mol%), Cu(OAc)2 (1 equiv), 75 °C.
e Reaction under strict anaerobic conditions.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E

C
N. Ayyagari et al.
Letter
Synlett
reaction. When Cu(OAc)2 was used as the oxidant, the de-
sired product was isolated in 50% yield (entry 9). No C–H
activation product was isolated when Cu(OTf)2 was the oxi-
dant in presence of catalytic Pd(OTf)2 (entry 10). Stoichio-
metric copper(II) acetate was found to be the best oxidant
in the presence of Pd(OAc)2 providing 3a in 62% yield (entry
11). Amongst the palladium catalysts, the more expensive
Pd(OTf)2 provided yields similar to Pd(OAc)2 when Cu(OAc)2
was the oxidant [entry 9 (50%) and entry 11 (62%)]. Catalyt-
ic PdCl2 did not work for the reaction (entry 12). Addition-
ally, we conducted a reaction in the presence of TEMPO (1
equiv) to investigate the reaction mechanism. The desired
product 3a was obtained in 52% yield confirming that the
reaction does not proceed through radical intermediates
(entry 13). Radical trapping products were not observed
even when the reaction was repeated with TEMPO (3
equiv); instead an increased product yield (65%) was ob-
served. The result confirmed that TEMPO was serving as a
co-oxidant for the reaction entry.16 The reaction does not
occur without palladium catalyst (entry 14). The effect of
equivalent ratios of allylbenzene 1a and NHS 2a was exam-
ined, and we found that the yield of 3a was significantly im-
proved after the equivalents of NHS (2a) were increased to
3.0 ( entry 15). Additionally, slight improvement in the
yield of 3a was observed after addition of acetic acid (entry
16).5d All reactions were conducted under aerobic condi-
tions, and no special precautions were taken to remove dis-
solved oxygen from the solvent. When dissolved oxygen
was completely removed using freeze-thaw cycles and the
reaction was conducted under strict anaerobic conditions,
the reaction yield fell to 27% percent (entry 17). It is note-
worthy to mention that the Z-isomer was not observed. Fi-
nally, the linear E-allylic acetate 4a (10–12%) was the only
1
byproduct formed and was characterized by H NMR and
13C NMR spectroscopy. The source of the acetate nucleop-
hile that results in formation of allylic acetate could be ace-
tic acid, palladium catalyst (Pd(OAc)2), or the oxidant
(Cu(OAc)2).
Once the reaction was optimized, a number of allyl ben-
zene substrates (1a–p, see Table 1 in Supporting Informa-
tion), either commercially available or prepared using
known protocols from the corresponding aryl bromides,
were subjected to the reaction conditions.17 Scheme 2 pro-
vides a summary of the scope of the developed protocol. A
O
H
O
Pd(OAc)2 (10 mol%), air
N
Cu(OAc)2⋅H2O (1 equiv)
O
+
R
HO N
R
O
AcOH (0.5 equiv)
24–28 h
75 °C
O
1a–t
2a–c
3a–t
O
O
O
O
N
N
N
N
O
O
O
O
O
O
O
O
O
MeO
Me
Me
3a, 84%
3b, 67%
3c, 78%
3d, 85%
O
O
O
O
N
Me
N
N
N
O
O
O
O
O
O
O
O
O
O
MeO
Me
CH3
3g, 77% (0.1 g scale)
70% (2.3 g scale)
O
Me
OMe
3e, 70%
3f, 59%
3h, 72%
O
O
O
O
O
N
N
N
N
N
O
N
O
O
O
O
O
O
O
O
CH3
F
Cl
3i, 61%
3j, 64%
3k, 28%
3l, 56%
O
O
O
O
N
O
N
O
O
O
O
O
O
N
OMe
F3C
NC
3m, 39%
3n, 78%
3o, 58%
3p, 44%
O
H
O
O
O
O
N
N
N
O
H
O
O
O
O
O
O
O
O
MeO
MeO
3q, 50%a
3r, 32%a
3s, 48%a
3t, 43%a
Scheme 2 Substrate scope for allylic C–H activation. Reagents and conditions: 1a (0.05 mmol, 1 equiv), 2a (3 equiv), Pd(OAc)2 (10 mol%), Cu(OAc)2
(1 equiv), AcOH (0.5 equiv), MeCN (0.1 M), 75 °C. a With 20 mol% Pd(OAc)2.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E

D
N. Ayyagari et al.
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variety of substituents including various electron-donating
and electron-withdrawing groups on allylbenzenes were
tested, and the corresponding products were obtained in
moderate to good yields (Scheme 2). Separately, NHS (3a–
p), NHPI (3q–s), and N-hydroxy-5-norbornene-2,3-dicar-
boxylic acid imide (3t) were evaluated as nucleophiles suc-
cessfully.
volves allyl C–H bond activation to afford a -allyl palladi-
um intermediate which is then attacked by the oxygen nuc-
leophile from N-hydroxyimide.
On the basis of control experiments and previous works
by the White group, the plausible reaction mechanism is
depicted in the Scheme 4. First, Pd(OAc)2 activates the allyl-
ic C–H bond of 1 to form η3--allyl palladium complex [I].
The electron-deficient complex I undergoes a nucleophilic
attack with NHS and generates complex II. Under the reac-
tion conditions complex II breaks to form desired com-
pound 3 and the Pd0, which is then oxidized by Cu(OAc)2 to
regenerate the active Pd(II) catalyst. Since only one equiva-
lent of 1e– oxidant Cu(II) was used, we believe that dis-
solved oxygen serves as the terminal oxidant.18 This trans-
formation constitutes the first example of Pd-catalyzed oxi-
dative allylic C–H bond activation followed by alkylation of
N-hydroxyimides.
In the optimization study, the 4-methoxy-1-allylben-
zene (1a) reacts with NHS and provided the desired product
in 84% yield (Table 1, entry 16). The unsubstituted allylben-
zene gave the respective product 3b in good yield (Scheme
2). The methyl-substituted allylbenzenes were found to be
good substrates for this transformation, and the corre-
sponding products 3c–f were obtained in good yields. In ad-
dition, electron-rich allylarenes 1g and 1h reacted smooth-
ly, and the expected products 3g and 3h were obtained in
good chemical yields. The developed reaction conditions
also tolerated halides on allylbenzenes and delivered the
oxidation products 3i and 3j in good yields. Allylbenzenes
with electron-withdrawing substituents such as ketone (1k
and 1l) at the ortho and para position of the phenyl ring, tri-
fluoromethane (1m), ester (1n), and nitrile (1o), afforded
the alkylation of NHS (3k–o) in moderate yields. The 2-al-
lylnaphthalene substrate (1p) performed well, furnishing
the product 3p in modest yield. Furthermore, we observed
10–15% of the allylic acetate formation in all of the above
substrates. Gratifyingly, the developed protocol gave a 70%
yield when tested on gram scale (Scheme 2, 3g).
Two experiments were conducted to investigate the
mechanism of the reaction (Scheme 3). To rule out allylic
acetate as an intermediate, we performed the reaction us-
ing E-allylic acetate 4a under optimized reaction condi-
tions. The linear oxidation product 3a was not observed un-
der these conditions (Scheme 3, eq. 1). Separately, a cross-
oxidation experiment was conducted where both 4-me-
thoxy-allylbenzene (1a) and E-allylic acetate (4b) were
used together. Interestingly, only the C–H activation prod-
uct 3a was the isolated in 70% yield (Scheme 3, eq. 2). The
compound 4b was recovered and we did not observe the
formation of 3b. This confirmed that the mechanism in-
H2O
H
1
1/2 O2
Cu(I)
Ar
Pd(II)
Cu(II)
AcOH
Pd(0)
[Pd](OAc)
O
Ar
I
O
N
Ar
O
O
O
N OH
[Pd](0)
3
2
N
O
Ar
O
HOAc
O
II
Scheme 4 Proposed catalytic cycle
In conclusion, we have developed novel, mild, and scal-
able Pd-catalyzed oxidative C–H allylic alkylation of N-hy-
droxyimides.19 Various substituted allylarenes can be toler-
ated in this reaction to provide the corresponding linear al-
lyloxypyrrolidinediones with moderate to excellent yields.
We are currently investigating the application of this meth-
od for the synthesis of a small library of bioactive com-
pounds.
O
N
optimized
reaction
conditions
O
OAc
O
(1)
O
N
OH
2a
MeO
MeO
O
4a
3a
not observed
O
optimized
reaction
conditions
O
O
N
N
O
O
MeO
O
1a
(2)
N
OH
O
MeO
3b
not observed
3a
70% yield
O
OAc
4b
H
Scheme 3 Control experiments
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–E

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Acknowledgment
J.D.B. gratefully acknowledges constant support of Dean Rebecca Fin-
ley in the College of Pharmacy at Thomas Jefferson University. J.D.B. is
also grateful to the Thomas Jefferson University for financial support.
(10) Miyabe, H.; Yoshida, K.; Yamauchi, M.; Takemoto, Y. J. Org.
Chem. 2005, 70, 2148.
Supporting Information
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https://doi.org/10.1055/s-0039-1691508.
S
u
p
p
orit
n
gInformati
o
n
S
u
p
p
orti
n
gInformati
o
n
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(17) (a) Lin, S.; Song, C. X.; Cai, G. X.; Wang, W. H.; Shi, Z. J. J. Am.
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(19) General Procedure for C–H Activation/C–O Bond Formation
To a solution of aryl benzene 1 (0.1 mmol, 1 equiv) in acetoni-
trile (2 mL) were added N-hydroxyimide (2, 3.0 equiv), cop-
per(II) acetate monohydrate (1.0 equiv), acetic acid (0.5 equiv),
and Pd(OAc)2 (0.1 equiv) in the same order and heated at 75 °C.
The reaction was conducted in a round-bottom flask equipped
with a reflux condenser. After 24–28 h, the reaction mass was
dried on a small mass of silica and was purified by flash chro-
matography using hexanes/EtOAc .
(E)-1-[(3-(4-Methoxyphenyl)allyl)oxy]pyrrolidine-2,5-dione
(3a)
Prepared according to the general procedure. Purification by
column chromatography (n-hexane/EtOAc, 4:1) gave 3a in 84%
yield as a white solid (mp 98–100 °C). 1H NMR (400 MHz,
CDCl3):  = 7.34–7.30 (d, J = 8.6 Hz, 2 H), 6.87–6.84 (d, J = 8.7 Hz,
2 H), 6.60 (d, J = 15.9 Hz, 1 H), 6.25–6.18 (dt, J = 15.8, 7.3 Hz, 1
H), 4.77 (dd, J = 7.3, 1.0 Hz, 2 H), 3.80 (s, 3 H), 2.65 (s, 4 H). 13C
NMR (100 MHz, CDCl3):  = 171.5, 160.0, 137.5, 128.4, 128.2,
119.2, 114.1, 77.7, 55.3, 25.4. HRMS: m/z calcd for C14H16NO4 [M
+ H+]: 261.1001; found: 261.1066.
(7) For use in peptide synthesis, see: (a) El-Faham, A.; Albericio, F.
Chem. Rev. 2011, 111, 6557. (b) Anderson, G. W.; Zimmerman, J.
E.; Callahan, F. M. J. Am. Chem. Soc. 1964, 86, 1839.
(c) Zimmerman, J. E.; Anderson, G. W. J. Am. Chem. Soc. 1967, 89,
7151.
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