Palladium-catalyzed interceptive decarboxylative addition of allyl carbonates with carbonyl group

Article abstract of DOI:10.1055/s-0033-1341201

The first palladium-catalyzed interceptive decarboxylative 1,4-addition of allyl carbonates with squarates is reported. Interestingly, the C-3 carbonyl group of N-substituted isatins undergoes smooth decarboxylative 1,2-addition with allyl carbonates. This transformation offers a straightforward method for the synthesis of spiro-oxepane-fused 2-oxindole. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1341201

1246▌
LETTER
letter
Palladium-Catalyzed Interceptive Decarboxylative Addition of Allyl Car-
bonates with Carbonyl Group
Interceptive Decarboxylative Addition of Allyl Carbonates
T. V. Baiju,a,b Nayana Joseph,b Jainu Ajit,b Praveen Prakash,b K. V. Radhakrishnan,*a,b Sunil Varughese,c
Yoshinori Yamamoto*d
a
Academy of Scientific and Innovative Research (AcSIR), New Delhi 110001, India
b
Organic Chemistry Section, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695019, India
Fax +91(471)2491712; E-mail: radhu2005@gmail.com
c
Inorganic Chemistry Section, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695019, India
d
WPI-AIMR (Advanced Institute for Materials Research), Tohoku University, Katahira 2-1-1, Aobaku, Sendai 980-8577, Japan
E-mail: yoshi@mail.tains.tohoku.ac.jp
Received: 10.02.2014; Accepted after revision: 19.03.2014
allylation of aldehydes using bis-π-allylpalladium inter-
Abstract: The first palladium-catalyzed interceptive decarboxyla-
mediates.13 Allylation of carbonyl groups using iridium,14
tive 1,4-addition of allyl carbonates with squarates is reported. In-
indium,15 zinc,16 and other metal reagents17 have been ex-
tensively studied. In 2004, Schaus and co-workers report-
terestingly, the C-3 carbonyl group of N-substituted isatins
undergoes smooth decarboxylative 1,2-addition with allyl carbon-
ates. This transformation offers a straightforward method for the ed the palladium(0)-catalyzed decarboxylative aldol
reaction of allyl-β-keto esters and aldehydes.18 But there
synthesis of spiro-oxepane-fused 2-oxindole.
are only limited reports on palladium-catalyzed intermo-
lecular decarboxylative coupling with carbonyl
groups.9c,8b This fact combined with our interest in al-
lylation using π-allyl–palladium complex prompted us to
investigate its reactivity on activated ketones such as
squarates and isatins. Herein, we report the catalytic inter-
ceptive decarboxylative 1,4-addition of allylcarbonates
with squarates and 1,2-addition of carbonyl group of ace-
naphthenequinone and isatins.
Key words: palladium, allylation, conjugate addition, allyl com-
plexes, spiro compounds, metathesis
The development of efficient methods to construct organ-
ic frameworks continues as a challenge in both academic
as well as industrial research applications. Since the
ground-breaking work of Tsuji,1 Saegusa,2 and Trost,3
palladium-catalyzed decarboxylative allylation has
emerged as an important method for the construction of
new carbon–carbon bonds. Intensive research efforts have
been devoted to transition-metal-catalyzed intramolecular
decarboxylative allylation by many research groups.4 Re-
cent advances toward interceptive decarboxylative al-
lylation include the reactions of allyl-β-keto esters,5
allylcarbonates,6 allyl carbamates,7 and allyl diphenylgly-
cinate esters8 with electrophilic olefins. Hayashi and co-
workers have extended the palladium-catalyzed IDcA to
malonate-derived valerolactones.9
Squarates, fascinating and versatile C-4 synthons, offer
the prospect of serving as useful starting materials for the
synthesis of a wide variety of compounds.19 The monoad-
dition and two-fold addition to the carbonyl group of
squarates by organolithium and Grignard reagents to syn-
thesize polycyclic compounds are well documented in the
literature.20 But there are only few reports on palladium-
catalyzed reactions involving squarates.19d,21 Our initial
studies involved the treatment of dibutyl squarate (1a) and
diallylcarbonate 2a in the presence of Pd(PPh3)4 (5 mol%)
in THF at room temperature. The reaction afforded 1,4-
addition product 2,4-bis(allyloxy)-3,4-dibutoxycyclobut-
2-enone (3aa) in 22% yield (Scheme 1). The structure of
the 1,4-addition product was characterized by usual spec-
troscopic analysis,22 and the regiochemistry was con-
We have recently disclosed our success on palladium-
catalyzed allylation reactions of isatylidenes10 and hepta-
fulvenes.11 Nucleophilic allylation of carbonyl groups
have attracted attention of a number of organic chemists.12
Yamamoto et al. reported the first palladium-catalyzed
O
O
O
Pd(PPh3)4
THF, r.t., 8 h
O
O
O
BuO
+
O
O
2a
BuO
OBu
1a
OBu
3aa (22%)
Scheme 1 Decarboxylative 1,4-addition of diallylcarbonate with dibutyl squarate
SYNLETT 2014, 25, 1246–1252
Advanced online publication: 10.04.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1341201; Art ID: ST-2014-B0112-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
Interceptive Decarboxylative Addition of Allyl Carbonates
1247
firmed by HMBC spectral analysis.23 To the best of our ing various allyl carbonates 2a–c. All the reactions pro-
knowledge this is the first report on palladium-catalyzed ceeded smoothly at room temperature to produce the
decarboxylative 1,4-addition of allyl carbonates to squa- desired ketals 3aa–cc in moderate yields (Table 1). Regi-
rate.
oselective 1,4-addition achieved using the alkyl squarate
1b,c, but the 1,4-addition is not observed for isopropyl
squarate 1d. The branched isopropyl group placed at the
olefinic carbon makes the 1,4-conjugate addition diffi-
cult.20f This palladium-catalyzed synthesis of ketals is
milder than the existing methods for acetalization, which
mostly involve acidic reagents.24
With this promising result in hand, we investigated the
generality of this reaction under the best conditions iden-
tified for the conversion of dibutyl squarate 1a into cy-
clobutenone derivative 3aa.23 The developed method was
successfully applied to other alkyl squarates 1b,c. The
scope of the reaction was substantially expanded by utiliz-
Table 1 Decarboxylative 1,4-Addition of Allyl Carbonates with Squaratesa
Entry
Squarate
Allyl carbonate
Product
Yield (%)
42b (62)c
O
O
O
O
O
O
O
BuO
O
O
O
1
O
O
O
O
BuO
OBu
OBu
O
2a
2a
2a
1a
3aa
3ba
O
O
O
MeO
O
2
3
59b (74)c
MeO
OMe
OMe
O
1b
O
O
O
EtO
O
21b (69)c
EtO
OEt
O
OEt
1c
3ca
O
O
O
O
4
5
n.r.d
O
O
i-PrO
Oi-Pr
2a
2b
1d
O
O
O
OMe
BuO
MeO
O
18b (67)c
45b (75)c
29b (63)c
BuO
OBu
OBu
1a
3ab
O
O
O
O
OMe
OMe
MeO
MeO
O
O
6
7
O
MeO
OMe
OMe
O
2b
2b
1b
3bb
O
O
O
EtO
MeO
O
EtO
OEt
OEt
1c
3cb
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1246–1252

1248
T. V. Baiju et al.
LETTER
Table 1 Decarboxylative 1,4-Addition of Allyl Carbonates with Squaratesa (continued)
Entry
8
Squarate
Allyl carbonate
Product
n.r.d
Yield (%)
O
O
O
OMe
O
i-PrO
Oi-Pr
2b
2c
1d
O
O
O
BuO
O
O
O
O
O
O
O
9
10
11
36b (69)c
43b (73)c
44b (71)c
O
O
BuO
OBu
OBu
O
1a
3ac
3bc
3cc
O
O
MeO
O
O
O
MeO
OMe
OMe
O
2c
1b
O
O
EtO
O
O
O
EtO
OEt
OEt
2c
1c
a Reaction conditions: squarate (1.0 equiv), allyl carbonate (2.0 equiv), catalyst (10 mol%), CH2Cl2 (2 mL), r.t., 12 h.
b Isolated yield.
c Yield based on recovered starting material.
d n.r. = no reaction.
We have carried out a straightforward synthetic transfor- counterion. Reductive elimination of C results in the for-
mation of 2,4-bis(allyloxy)-3,4-dibutoxycyclobut-2- mation of allylated product 3 and regenerates the catalyst.
enone (3aa). The ring-closing metathesis of compound
3aa with Grubbs second-generation catalyst afforded spi-
O
O
ro-4,7-dihydro-1,3-dioxepine-fused 2-butenone deriva-
tive 4 in 26% yield (Scheme 2). The formation of product
4 is presumed to form through the allyloxy rearrangement
followed by ring-closing metathesis.23
R2O
OR2
OR1
3
O
O
LnPd(0)
R1
O
O
O
O
Grubbs II
catalyst (10 mol%)
O
O
O
O
R1
O
BuO
BuO
O–
PdLn
–O
O
CH2Cl2, 8 h
OBu
3aa
A
R2O
OBu
4 (26%)
OR2
OR1
C
Scheme 2 Ring-closing metathesis of bisallyloxy cyclobutenone
CO2
O
O
Based on the results, we propose a plausible mechanism
as illustrated in Scheme 3. The catalytic cycle is initiated
by the oxidative addition of palladium(0) to allyl carbon-
ate 2 followed by decarboxylation to generate π-allyl–
palladium complex B.1,6 The alkoxy anion undergoes 1,4-
addition with squarate 1 to give cationic π-allyl–palladi-
um complex C having the oxyanion of squarate as the
PdLn
O–
R1
B
R2O
OR2
1
Scheme 3 Proposed catalytic cycle
Synlett 2014, 25, 1246–1252
© Georg Thieme Verlag Stuttgart · New York

LETTER
Interceptive Decarboxylative Addition of Allyl Carbonates
1249
O
O
O
O
O
Pd(PPh3)4
O
O
+
CH2Cl2, r.t., 12 h
O
2a
5
6a
(43%a, 66%b)
MeO
O
O
O
O
O
6b
(39%a, 59%b)
6c
(35%a, 51%b)
Scheme 4 Decarboxylative addition of allyl carbonate with acenaphthenequinone. a Isolated yield. b Yield based on recovered starting material.
To examine the possibility of interceptive decarboxyla- The method was also proved to be very convenient and
tive 1,2-addition we carried out the reaction of allyl car- general for the palladium-catalyzed decarboxylative 1,2-
bonates with electrophilic acenaphthenequinone under the addition of allyl carbonates 2a–c with the C-3 carbonyl
similar conditions. Allyl carbonates 2a–c reacted smooth- group of a range of isatins 7a–d. The reactions proceeded
ly with 5 to furnish the corresponding ketals 6a–c in mod- very efficiently under the same catalytic conditions to fur-
erate yields (Scheme 4).
nish the desired 1,2-addition products 8aa–dc in good
yields (Figure 2).
With a view to study the generality of 1,2-addition of al-
lylcarbonates, we turned our attention to the functional- The reaction of isatin 9 with diallyl carbonate 2a was con-
ization of carbonyl group of isatins. Isatins have been ducted under similar conditions. The reaction afforded 1-
widely used as precursors of spiro-oxindoles and many allylindoline-2,3-dione (10) in 52% yield along with 1-al-
natural products.25 In an initial attempt, we carried out the lyl-3,3-bis(allyloxy)indolin-2-one (8ca) in 8% yield
reaction of 1-ethyl isatin (7a) with diallyl carbonate 2a in (Scheme 6). From our investigations, it is noticeable that
the presence of 5 mol% Pd(PPh3)4 in THF at room temper- the C-3 functionalization occurs only in the N-protected
ature. The C-3 carbonyl group underwent smooth al- isatin.
lylation–oxyallylation to afford 3,3-bis(allyloxy)-1-
ethylindolin-2-one (8aa) in 57% yield (Scheme 5). The
structure of the product was established by NMR analysis,
mass spectrometry, and unambiguously confirmed by sin-
Finally, the synthetic utility of this chemistry is highlight-
ed by the synthesis of spiro-dioxepine-fused 2-oxindole
(Scheme 7). Ring-closing metathesis of 8aa using Grubbs
first-generation catalyst afforded dioxepine-fused spiro-
gle-crystal X-ray analysis of one of the derivatives 8da
oxindole in 76% yield. It is noteworthy that spirocyclic
(Figure 1).26
oxindole compounds are valuable pharmaceuticals.25 Spi-
ro-oxindoles have been reported to possess a wide range
of biological activities such as progesterone receptor
modulators,27 anti-HIV,28 anticancer,29 antimalarial,30 and
MDM2 inhibitor.31
In summary, we have developed for the first time a palla-
dium-catalyzed interceptive decarboxylative 1,4-addition
of allyl carbonates to squarates. Interceptive decarboxyla-
tive 1,2-addition of allyl carbonates with acenaphthene-
quinone and N-substituted isatins also described.
Furthermore, the ring-closing metathesis of the 1,2-addi-
Figure 1 ORTEP view of compound 8da
O
O
O
Pd(PPh3)4
O
O
O
+
O
THF, r.t., 12 h
N
N
O
2a
Et
Et
8aa
(57%a, 88%b)
7a
Scheme 5 Decarboxylative addition of diallyl carbonate with 1-ethyl isatin. a Isolated yield. b Yield based on recovered starting material.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1246–1252

1250
T. V. Baiju et al.
LETTER
O
O
MeO
O
O
MeO
O
O
O
O
Br
Br
O
O
O
O
O
N
N
N
N
N
Et
Et
Et
Me
Me
8aa (57%)
8ab (50%)
8ac (48%)
8ba (68%)
8bb (35%)
O
O
O
O
O
O
O
O
Br
O
O
N
O
O
N
N
N
Me
8bc (54%)
Bn
8da (76%)
Bn
8dc (42%)
8ca (38%)
Figure 2 Decarboxylative 1,2-addition of allylcarbonates with N-substituted isatin. Reagents and conditions: isatin (1.0 equiv), allylcarbonate
(2.0 equiv), catalyst (5 mol%), THF (2 mL), r.t., 12 h.
Typical Procedure for the Ring-Closing Metathesis of 2,4-
Bis(allyloxy)-3,4-dibutoxycyclobut-2-enone (3aa)
2,4-Bis(allyloxy)-3,4-dibutoxycyclobut-2-enone (3aa, 43 mg, 0.13
Grubbs I
catalyst
mmol) was dissolved in CH2Cl2 (6 mL). To this, Grubbs second-
generation catalyst (12 mg, 0.013 mmol) was added and stirred at
r.t. for 8 h. The reaction was monitored by TLC. The solvent was
evaporated in vacuo, and the residue on silica gel (100–200 mesh)
column chromatography using 5% EtOAc in hexane afforded the
product 4 in 26% (10 mg) yield.
O
O
O
O
O
O
CH2Cl2, r.t., 3 h
N
N
Et
Et
11 (76%)
8aa
Typical Procedure for the Ring-Closing Metathesis of 3,3-
Bis(allyloxy)-1-ethylindole-2-one (8aa)
Scheme 7 Ring-closing metathesis of bisallyloxy 2-oxindole
3,3-Bis(allyloxy)-1-ethylindolin-2-one (8aa, 25 mg, 0.09 mmol)
was dissolved in CH2Cl2 (4 mL). To this, Grubbs first-generation
catalyst (4 mg, 0.0046 mmol) was added and stirred at r.t. for 3 h.
The reaction was monitored by TLC. The solvent was evaporated in
vacuo, and the residue on silica gel (100–200 mesh) column chro-
matography using 10% EtOAc in hexane afforded the product 11 in
76% (17 mg) yield.
tion product of isatin furnished the corresponding spiro-
dioxepine-fused 2-oxindole in good yield. Investigations
to broaden the scope of this chemistry to other activated
carbonyl groups are currently in progress.
Acknowledgment
Typical Procedure for the Palladium-Catalyzed Decarboxyla-
tive Allylation of Dibutyl Squarate
BJU, NJ, and PP thank CSIR and UGC for a research fellowship. Fi-
nancial assistance from Department of Science and Technology
(DST No: SR/S1/OC-78/2009) and Council of Scientific and Indu-
strial Research, New Delhi [12th FYP project on catalysts for spe-
cialty chemicals (CSC-0125)] are greatly acknowledged. The
authors also thank Ms. Saumini Mathew, Mr. Saran P. Raveendran,
and Ms. S. Viji of CSIR-NIIST for recording NMR and mass spec-
tra.
Dibutyl squarate (1a, 40 mg, 0.18 mmol) and Pd(PPh3)4 (20 mg,
0.018 mmol) were taken in a Schlenk tube, degassed, and diallyl
carbonate 2a (50 mg, 0.35 mmol) was added followed by CH2Cl2 (2
mL). Argon gas was purged into the reaction mixture and stirred at
r.t. for 12 h. The reaction was monitored by TLC. The solvent was
removed under reduced pressure, and the residue on silica gel (100–
200 mesh) column chromatography using 3% EtOAc in hexane af-
forded 3aa in 42% yield (24 mg), and the unreacted dibutyl squarate
(13 mg) was recovered.
O
O
O
O
Pd(PPh3)4
O
O
O
+
O
+
O
THF, r.t., 12 h
N
N
N
O
H
9
2a
8ca (8%)
10 (52%)
Scheme 6 Decarboxylative addition of diallyl carbonate with isatin
Synlett 2014, 25, 1246–1252
© Georg Thieme Verlag Stuttgart · New York

LETTER
Interceptive Decarboxylative Addition of Allyl Carbonates
1251
3980. (e) Yatsumonji, Y.; Nishimura, T.; Tsubouchi, A.;
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5507.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
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ungIifoop
r
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J = 5.5 Hz, 2 H), 4.40 (t, J = 6.5 Hz, 2 H), 4.27 (d, J = 5.5
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J = 7.5 Hz, 3 H), 0.91 (t, J = 7.5 Hz, 3 H). 13C NMR (125
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LETTER
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Synlett 2014, 25, 1246–1252
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