Nickel-Catalyzed Carboxylation of Conjugated Dienes with Carbon Dioxide and DIBAL-H for the Synthesis of β,γ-Unsaturated Carboxylic Acids

Article abstract of DOI:10.1055/a-1336-8034

Conjugated dienes underwent Ni-catalyst-promoted 1,2-hydrocarboxylation in a 1:1 ratio with carbon dioxide under atmospheric pressure in the presence of diisobutylaluminum hydride (DIBAL-H) to give the corresponding β,γ-unsaturated carboxylic acids, without dimerization or oligomerization of the conjugated diene.

Full text of DOI:10.1055/a-1336-8034

SYNLETT
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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2020, 31, A–D
cluster
A
Modern Nickel-Catalyzed Reactions
Y. Luo et al.
Cluster
Synlett
Nickel-Catalyzed Carboxylation of Conjugated Dienes with
Carbon Dioxide and DIBAL-H for the Synthesis of ,-Unsaturated
Carboxylic Acids
Ying Luo
R2
R2
Ni catalyst
DIBAL-H
R2
H
COOH
R1
R1
Bun Chan
CO2
R1
R4
+
R4
+
R4
Tsutomu Fukuda
Gen Onodera
Masanari Kimura*
(1 atm)
hydrocarboxylation
in 1,2-addition
R3
R3
HOOC
H
R3
10 examples
up to 70% yield
moderate to high regioselectivity
Graduate School of Engineering, Nagasaki University,
Bunkyo-machi 1-14, Nagasaki 852-8521, Japan
masanari@nagasaki-u.ac.jp
Published as part of the Cluster
Modern Nickel-Catalyzed Reactions
MGeMraasadaiCulnoamartrreaiesSKsacpinmhoaonruoidr@lainongfaEgAnausgtaihnkoei-reur.iancg.j,pNagasaki University, Bunkyo-machi 1-14, Nagasaki 852-8521, Japan
Received: 15.11.2020
Accepted after revision: 14.12.2020
Published online: 14.12.2020
successfully developed a nickel-promoted oxidative cycliza-
tion of 1,3-dienes with carbon dioxide followed by hydroly-
sis of the oxo--allylnickel intermediate to produce ,-un-
saturated carboxylic acids (Scheme 1).10 In this case, a stoi-
chiometric amount of bis(cycloocta-1,5-diene)nickel
[Ni(cod)2] and 1,8-diazabyclo[5.4.0]undec-7-ene (DBU)
were required to enhance the reductive coupling of carbon
dioxide with the conjugated dienes through a 1,4-addition.
No catalytic version of a one-to-one coupling reaction of
conjugated dienes with carbon dioxide has yet been fully
developed, except for a Pd pincer-complex-catalyzed hy-
drocarboxylation of conjugated dienes with carbon dioxide
promoted by triethylaluminum as a hydride source to pro-
duce ,-unsaturated carboxylic acids.11
DOI: 10.1055/a-1336-8034; Art ID: st-2020-r0596-c
Abstract Conjugated dienes underwent Ni-catalyst-promoted 1,2-
hydrocarboxylation in a 1:1 ratio with carbon dioxide under atmospher-
ic pressure in the presence of diisobutylaluminum hydride (DIBAL-H) to
give the corresponding ,-unsaturated carboxylic acids, without di-
merization or oligomerization of the conjugated diene.
Key words nickel catalysis, carbon dioxide, dienes, hydrocarboxyl-
ation, diisobutylaluminum hydride, alkenoic acids
Carbon dioxide is a nontoxic, inexpensive, and abundant
resource that can serve as a one-carbon unit in organic syn-
thesis.1 In this regard, transition-metal-catalyzed coupling
reactions of carbon dioxide with various unsaturated hy-
drocarbons have attracted attention in recent years.2
Ni(cod)2 (1 equiv)
H+
DBU
O
Ph
Ph
Ni
O
CO2 (1 atm)
Among unsaturated hydrocarbons, conjugated dienes
form accessible and useful C4 elongation units in organic
synthesis.3 The transition-metal-catalyzed CO2-fixation re-
action with conjugated dienes has considerable potential as
a practical method for the preparation of unsaturated alco-
hols4 or carboxylic acids.5 However, there are formidable
challenges in C–C bond construction by this method be-
cause some conjugated dienes dimerize easily; for example,
in the presence of nickel(0), they can dimerize to produce
bis--allylnickel complexes.6 These bis--allylnickel inter-
mediates can often serve as reactive nucleophiles that com-
bine with carbonyl compounds such as aldehydes or ke-
tones.7 Furthermore, Ni-catalyzed two-to-one couplings of
dienes and carbon dioxide via bis--allylnickel species have
been reported by several groups as useful methods for pro-
ducing feedstocks from carbon dioxide.8
COOH
+
COOH
Ph
Ph
Scheme 1 Previously reported oxidative coupling of a conjugated diene
and CO2
A site-selective catalytic incorporation of several mole-
cules of carbon dioxide into conjugated dienes was recently
developed by Martin and co-workers, who developed a
bis(tetrabutylammonium) tetrabromonickelate [NiBr4(TBA)2]-
catalyzed dicarboxylation of conjugated dienes under atmo-
spheric pressure with carbon dioxide in the presence of Mn
as a reductant in DMA as a solvent.12 In this case, the -al-
lylnickel species derived by oxidative cyclization of a conju-
gated diene and carbon dioxide in a 1:1 ratio served as nuc-
leophilic intermediates to capture an additional molecule of
carbon dioxide at the -position to give adipic acid deriva-
tives.
Despite the problem of dimerization, some examples of
one-to-one couplings of conjugated dienes and carbon di-
oxide have been reported.9 For instance, Takimoto and Mori
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–D
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
Y. Luo et al.
Cluster
Synlett
a
As an alternative CO2-fixation process for the construc-
tion of a carbon framework, we have developed a Ni-cata-
lyzed one-to-one reductive coupling of conjugated dienes
with carbon dioxide in the presence of diisobutylaluminum
hydride (DIBAL-H) to give ,-unsaturated carboxylic acid
through a 1,2-addition, without dimerization of the conju-
gated diene (Scheme 2). The mild reaction conditions per-
mitted the use of a broad range of substrates, and the prod-
ucts were obtained in good yields with moderate to good
regioselectivities.
Table 1 Ni(0)-Promoted Hydrocarboxylation of Myrcene with CO2
Ni(cod)2/ligand
organometallic
HOOC
+
CO2
(1 atm)
1a
COOH
3a
2a
Entry
Ligand (mmol)
Organometallic
Yield (%) (2a/3a)
1
2
3
4
5
6
7
8
–
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
Et3Al
70 (5:1)
PPh3 (0.1)
34 (1:1)
R2
R2
R2
H
COOH
Ni catalyst
DIBAL-H
R1
PCy3 (0.1)
0
R1
R1
R4
R4
+
R4
CO2
(1 atm)
1,10-Phen (0.05)
25 (3:1)
R3
R3
H
R3
HOOC
IPr (0.05)
34 (3:1)
Scheme 2 Reductive coupling of conjugated dienes and CO2
–
–
–
0
0
0
9-BBN-H
Et3SiH
First, by using myrcene as a substrate, we examined the
effects of various ligands and organometallic compounds
on the desired reaction (Table 1). In the absence of any li-
gand, DIBAL-H produced a 70% yield of the hydrocarboxyl-
a Reaction conditions: Ni(cod)2 (0.05 mmol), myrcene (4 mmol), ligand, or-
ganometallic (1 mmol), CO2 (1 atm), hexane (2 mL), rt.
ation products 2a and 3a in a 5:1 ratio (Table 1, entry 1).13
A
combination of PPh3 and the DIBAL-H system gave the one-
to-one coupling products a 3:1 mixture of regioisomers 2a
and 3a in a modest 34% yield (entry 2). In the presence of
PCy3, a complicated mixture was obtained (entry 3). The
use of 1,10-phenanthroline (1,10-Phen) and IPr carbene li-
gands provided the desired products in low yields (entries 4
and 5). We also found that polar solvents such as 1,4-diox-
ane or diethyl ether led to a worsening of the reaction out-
come.14 It therefore appeared that competitive binding and
steric crowding of ligands or polar solvents might prevent
coordination of Ni. The use of other hydride sources such as
9-borabicyclo[3.3.1]nonane (9-BBN), Et3SiH, or triethylalu-
minum did not provide good results in terms of the forma-
tion of the desired products as complex mixtures were ob-
tained (entries 6–8). These results showed that the catalytic
activity of the Ni species is affected by the ligand and the
organometallic compound, and also demonstrated a signifi-
cant role for the hydride donor in the reaction. These re-
sults indicated that the conditions in entry 1 are the most
suitable for this coupling reaction. It appears that a combi-
nation of a Ni catalyst and a variety of ligands has a marked
effect on the formation of dimers and oligomers from con-
jugated dienes.
Next, by using this optimal strategy, we examined the
reactions of various commercially available conjugated
dienes (Table 2). Under similar catalytic conditions to those
shown in entry 1 of Table 1, a substrate containing a phenyl
group gave the desired product in a moderate yield as a 2:1
mixture of regioisomers (Table 2, entry 2).14 Notably, sub-
strates containing electron-donating groups, such as 4-me-
thoxybenzyl or benzodioxole gave good yields of the corre-
sponding products with improved regioselectivities (entries
3 and 4). In the case of alkyl substituents at the terminal
position of the conjugated diene, single isomers were ob-
tained in good yields (entries 5–8). In addition, cyclohexa-
1,3-diene gave a single isomer (entry 9). However, when
1,4-dimethylbuta-1,3-diene was used, the corresponding
carboxylic acid was almost undetectable and a highly in-
tractable mixture was formed (entry 10). Therefore, the
catalytic process is subtly affected by structural variations
in the substrate.
To verify the reactivity of conjugated dienes, we investi-
gated the reaction in the presence of benzaldehyde as an
electrophile instead of carbon dioxide, under a nitrogen at-
mosphere. The reaction of (E)-2-methyldeca-1,3-diene (1e)
with benzaldehyde in the presence of Ni(cod)2 catalyst and
DIBAL-H gave the corresponding homoallylic alcohol 2j in
50% yield as a mixture of diastereoisomers (Scheme 3). This
result suggests that the formation of the homoallylic alco-
hol under the present conditions proceeds through the
electrophilic attack of the aldehyde at the -position of an
allylic anion species, such as a -allylaluminum intermedi-
ate.15
Ni(cod)2 (0.05 mmol)
DIBAL-H (1 mmol)
PhCHO (0.5 mmol)
r.t., N2, overnight
5
5
n-hexane (2 mL)
r.t., N2, 24 h
Ph
OH
1e
50% (1.8:1.0)
2j
Scheme 3 Ni(0)-catalyzed three-component coupling reaction of a
conjugated diene, DIBAL-H, and benzaldehyde
Although it is premature to rationalize the reaction
mechanism, a plausible catalytic cycle is presented in
Scheme 4, based on our experimental results. We propose
that the reaction commences with the oxidative addition of
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C
Y. Luo et al.
Cluster
Synlett
Table 2 Ni(0)-Catalyzed Hydrocarboxylation of Conjugated Dienes with DIBAL-H and Carbon Dioxidea
R2
Ni(cod)2 (0.05 mmol)
DIBAL-H (1 mmol)
R2
R2
HOOC
R1
R4
R1
R1
R4
+
R4
CO2 (1 atm)
n-hexane (2 mL)
r.t, 24 h
R3
R3
H
R3
HOOC
1 (4 mmol)
2
3
Entry
R1
R2
R3
R4
Products
Yield (%) (ratio 2/3)
1
2b
3b
4b
5
H
H
H
H
H
H
H
H
H
1,1-dimethylbut-1-enyl
H
2a + 3a
2b + 3b
2c + 3c
2d + 3d
2e
70 (5:1)
40 (2:1)
58 (13:1)
48 (6:1)
52
Ph
H
H
H
H
H
H
H
H
4-MeOC6H4
H
1,3-benzodioxole
H
hexyl
Me
H
6
octyl
2f
50
7
Ph(CH2)2
H
2g
51
8
Cy
H
2h
67
9
cyclohexa-1,3-dienyl
Me
2i
46
10
H
H
Me
–
a Reaction conditions: Ni(cod)2 (0.05 mmol), conjugated diene (4.0 mmol), DIBAL-H (1 mmol), CO2 (1 atm), hexane (2 mL), rt, 24 h.
b A small amount of the 1,4-addition product was also formed; for detailed spectral data, see the Supporting Information.
DIBAL-H to Ni(0) metal to form a (i-Bu)2Al–Ni–H species;
this is followed by hydronickelation across the conjugated
DIBAL-H
Ni(0)
dienes to produce a -allylnickel complex.16 Subsequently,
the least-hindered -allylaluminum species II might be
formed by reductive elimination of the -allylnickel inter-
mediate I to regenerate the nickel(0) catalyst. Carbon diox-
H-Ni-Al(i-Bu)2
R3
ide then attacks at the -position of the allylaluminum in-
R3
termediate II via a six-membered-ring transition state III or
III′, predominantly producing the desired ,-unsaturated
carboxylic acid 2.17 The alternative -allylaluminum com-
plex VI obtained through the unfavorable hydronickelation
of (i-Bu)2Al–Ni–H toward the conjugated diene results in
the formation of carboxylic acid 3 as a minor regioisomer.
In conclusion, we have developed a Ni-catalyzed one-to-
one reductive coupling of conjugated dienes with carbon
dioxide in the presence of DIBAL-H in the presence of car-
bon dioxide at atmospheric pressure. The process proceeds
via an allylaluminum intermediate to produce a ,-unsatu-
rated carboxylic acid. The mild reaction conditions (room
temperature and atmospheric pressure) permit good yields
and high regioselectivities without oligomerization or po-
lymerization of the conjugated diene.
R1
Al(i-Bu)2
R3
II
R1
Ni
I
H
CO2
Al(i-Bu)2
R1
R1
C
R1
O
C
Al(i-Bu)2
R3
H+
O
O
C
O
Al(i-Bu)2
R1
R3
III
O
IV
HO2C
R3
R3
O
R3
H+
Al(i-Bu)2
C
Al(i-Bu)2
O
O
R1
R1
major
2
III'
IV'
R3
R3
R3
H-Ni-Al(i-Bu)2
R1
H
R1
H
R1
– Ni(0)
Ni
Al(i-Bu)2
Al(i-Bu)2
VI
V
Funding Information
CO2
R3
H+
R3
C
R1
Al(i-Bu)2
R3
R1
Al(i-Bu)2
O
O
C
R1
HO2C
This work was supported by a Grant-in-Aid for Scientific Research (B)
(JP18H01981) from JSPS and partly by a Grant-in-Aid for Scientific
Research on Innovative Areas, ‘Precise Formation of a Catalyst Having
a Specified Field for Use in Extremely Difficult Substrate Conversion
Reactions’ (18H04266) from MEXT. This work was also carried out as
O
O
3
VII
minor
VII
Scheme 4 Plausible mechanism for the catalytic hydrocarboxylation of
conjugated dienes with CO2
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–D

D
Y. Luo et al.
Cluster
Synlett
part of the research program on Artificial Photosynthesis at Osaka
Sato, Y.; Mori, M. J. Org. Chem. 2005, 70, 8605. (d) Kimura, M.;
Ezoe, A.; Mori, M.; Tamaru, Y. J. Am. Chem. Soc. 2005, 127, 201.
(e) Kojima, K.; Kimura, M.; Tamaru, Y. Chem. Commun. 2005,
4717. (f) Kimura, M.; Ezoe, A.; Mori, M.; Iwata, K.; Tamaru, Y.
J. Am. Chem. Soc. 2006, 128, 8559.
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Supporting Information
(8) (a) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2002, 124, 10008.
(b) Mori, Y.; Mori, T.; Onodera, G.; Kimura, M. Synthesis 2014,
46, 2287.
Supporting information for this article is available online at
https://doi.org/10.1055/a-1336-8034.
S
u
p
p
ortingI
n
formati
o
nS
u
p
p
ortingInformati
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(9) (a) Hoberg, H.; Jenni, K. J. Organomet. Chem. 1987, 322, 193.
(b) Gao, Y.; Iijima, S.; Urabe, H.; Sato, F. Inorg. Chim. Acta 1994,
222, 145.
References and Notes
(10) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2001, 123, 2895.
(11) Takaya, J.; Sasano, K.; Iwasawa, N. Org. Lett. 2011, 13, 1698.
(12) Tortajada, A.; Ninokata, R.; Martin, R. J. Am. Chem. Soc. 2018,
140, 2050.
(13) 2,7-Dimethyl-3-methyleneoct-6-enoic Acid (2a) and 2,6-
Dimethyl-2-vinylhept-5-enoic acid (3a): Typical Procedure
(Table 1, Entry 1)
(1) (a) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007, 2975.
(b) Zhang, L.; Hou, Z. Chem. Sci. 2013, 4, 3395. (c) Green Carbon
Dioxide: Advances in CO2 Utilization; Centi, G.; Perathoner, S.,
Ed.; Wiley: Hoboken, 2014. (d) Liu, Q.; Wu, L.; Jackstell, R.;
Beller, M. Nat. Commun. 2015, 6, 5933.
(2) For recent catalytic coupling reactions of carbon dioxide with
unsaturated hydrocarbon compounds, see: (a) Ostapowicz, T.
G.; Schmitz, M.; Krystof, M.; Klankermayer, J.; Leitner, W.
Angew. Chem. Int. Ed. 2013, 52, 12119. (b) Zhang, L.; Hou, Z. In
New and Future Developments in Catalysis: Activation of Carbon
Dioxide; Suib, S. L., Ed.; Elsevier: Amsterdam, 2013, Chap. 9, 253.
(c) Wu, L.; Liu, Q.; Fleischer, I.; Jackstell, R.; Beller, M. Nat.
Commun. 2014, 5, 3091. (d) Gaydou, M.; Moragas, T.; Juliá-
Hernández, F.; Martin, R. J. Am. Chem. Soc. 2017, 139, 12161.
(e) Zhang, L.; Hou, Z. Curr. Opin. Green Sustainable Chem. 2017,
3, 17. (f) Kuge, K.; Luo, Y.; Fujita, Y.; Mori, Y.; Onodera, G.;
Kimura, M. Org. Lett. 2017, 19, 854. (g) Ye, J.-H.; Miao, M.;
Huang, H.; Yan, S.-S.; Yin, Z.-B.; Zhou, W.-J.; Yu, D.-G. Angew.
Chem. Int. Ed. 2017, 56, 15416. (h) Yan, S.-S.; Fu, Q.; Liao, L.-L.;
Sun, G.-Q.; Ye, J.-H.; Gong, L.; Bo-Xue, Y.-Z.; Yu, D.-G. Coord.
Chem. Rev. 2018, 374, 439.
An oven-dried flask charged with Ni(cod)2 (13.7 mg, 0.05 mmol)
was subjected to three cycles of evacuation and filling with CO2.
Anhyd hexane (2 mL), myrcene (540 mg, 4.0 mmol), and a 19%
solution of DIBAL-H in hexane (1 mL, 1 mmol) were added
under a constant flow of CO2. The mixture was stirred at rt for
24 h, then 2 M aq NaOH was added and the mixture was diluted
with H2O and extracted with EtOAc (×3). The aqueous phases
were then neutralized with 2 M aq HCl and extracted with
EtOAc (×3). The combined organic phases were washed with
brine, dried (MgSO4), filtered, and concentrated under reduced
pressure. The residual oil was placed in a Kugelrohr (30 Torr, 90
°C) to remove the isovaleric acid byproduct. A 5:1 mixture of 2a
and 3a was obtained in the last bulb as a yellow liquid; yield:
0.13 g (70%).
2a
(3) (a) Sato, Y.; Takimoto, M.; Hayashi, K.; Katsuhara, T.; Takagi, K.;
Mori, M. J. Am. Chem. Soc. 1994, 116, 9771. (b) Takimoto, M.;
Hiraga, Y.; Sato, Y.; Mori, M. Tetrahedron Lett. 1998, 39, 4543.
(c) Löber, O.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc.
2001, 123, 4366. (d) Shibata, K.; Kimura, M.; Kojima, K.; Tanaka,
S.; Tamaru, Y. J. Organomet. Chem. 2001, 624, 348. (e) Kimura,
M.; Ezoe, A.; Tanaka, S.; Tamaru, Y. Angew. Chem. Int. Ed. 2001,
40, 3600. (f) Sato, Y.; Saito, N.; Mori, M. J. Org. Chem. 2002, 67,
9310. (g) Ezoe, A.; Kimura, M.; Inoue, T.; Mori, M.; Tamaru, Y.
Angew. Chem. Int. Ed. 2002, 41, 2784. (h) Parker, S. E.; Börgel, J.;
Ritter, T. J. Am. Chem. Soc. 2014, 136, 4857. (i) Maza, R. J.;
Davenport, E.; Miralles, N.; Carbó, J. J.; Fernández, E. Org. Lett.
2019, 21, 2251.
(4) (a) Gui, Y.-Y.; Hu, N.-F.; Chen, X.-W.; Liao, L.-L.; Ju, T.; Ye, J.-H.;
Zhang, Z.; Li, J.; Yu, D.-G. J. Am. Chem. Soc. 2017, 139, 17011.
(b) Mori, Y.; Shigeno, C.; Luo, Y.; Chan, B.; Onodera, G.; Kimura,
M. Synlett 2018, 29, 742.
(5) (a) Tenaglia, A.; Brun, P.; Waegell, B. J. Organomet. Chem. 1985,
285, 343. (b) Hoberg, H.; Gross, S.; Milchereit, A. Angew. Chem.
Int. Ed. 1987, 26, 571.
(6) (a) Jolly, P. W.; Wilke, G. The Organic Chemistry of Nickel; Aca-
demic Press: New York, 1974. (b) Keim, W. Angew. Chem. Int. Ed.
1990, 29, 235. (c) Lautens, M.; Klute, W.; Tam, W. Chem. Rev.
1996, 96, 49. (d) Kimura, M.; Tamaru, Y. In Modern Organonickel
Chemistry; Tamaru, Y., Ed.; Wiley-VCH: Weinheim, 2005, Chap.
5, 137.
(7) (a) Kimura, M.; Matsuo, S.; Shibata, K.; Tamaru, Y. Angew. Chem.
Int. Ed. 1999, 38, 3386. (b) Kimura, M.; Kojima, K.; Inoue, T.;
Tamaru, Y. Synthesis 2004, 3089. (c) Takimoto, M.; Kajima, Y.;
1H NMR (400 MHz, benzene-d6):  = 5.09–5.15 (m, 1 H). 4.99 (s,
1 H), 4.88 (s, 1 H), 3.01 (q, J = 7.0 Hz, 1 H), 2.12 (m, 4 H), 1.03 (s,
3 H), 1.51 (s, 3 H), 1.15 (d, J = 6.8 Hz, 3 H). 13C NMR (100 MHz,
CDCl3):  = 181.14, 147.45, 131.94, 123.67, 111.46, 45.57, 34.64,
26.30, 25.64, 17.66, 16.05.
3a
IR (neat): 3100, 2860, 2638, 1711, 1645, 1454, 1414, 1286,
1231, 1092, 901, 833 cm–1. 1H NMR (400 MHz, benzene-d6):  =
6.04 (dd, J = 17.6, 10.8 Hz, 1 H), 5.09–5.15 (m, 1 H), 5.02 (d, J =
17.6 Hz, 1 H), 4.98 (d, J = 10.8 Hz, 1 H), 2.00 (q, J = 8.2 Hz, 2 H),
1.76–1.83 (m, 1 H), 1.60–1.67 (m, 1 H), 1.63 (s, 3 H), 1.51 (s, 3
H), 1.23 (s, 3 H). 13C (100 MHz, CDCl3):  = 182.64, 140.94,
132.09, 123.62, 114.09, 48.33, 38.95, 25.60, 23.26, 20.16, 17.55.
MS (EI): m/z (%) = 182.1310 (28) [M+], 167 (4), 137..
(14) For the effects of solvents, see the Supporting Information.
(15) Shen, Z.-L.; Peng, Z.; Yang, C.-M.; Helberg, J.; Mayer, P.; Marek,
I.; Knochel, P. Org. Lett. 2014, 16, 956.
(16) (a) Ely, R. J.; Morken, J. P. J. Am. Chem. Soc. 2010, 132, 2534.
(b) Gao, F.; Hoveyda, A. H. J. Am. Chem. Soc. 2010, 132, 10961.
(c) Eisch, J. J.; Sexsmith, S. R.; Fichter, K. C. J. Organomet. Chem.
1997, 527, 301. (d) Eisch, J. J.; Ma, X.; Singh, M.; Wilke, G.
J. Organomet. Chem. 1990, 382, 273.
(17) (a) Duong, H. A.; Huleatt, P. B.; Tan, Q.-W.; Shuying, E. L. Org.
Lett. 2013, 15, 4034. (b) Wu, J.; Hazari, N. Chem. Commun. 2011,
47, 1069. (c) Johnson, M. T.; Johansson, R.; Kondrashov, M. V.;
Steyl, G.; Ahlquist, M. S. G.; Roodt, A.; Wendt, O. F. Organometal-
lics 2010, 29, 3521. (d) Takaya, J.; Iwasawa, N. J. Am. Chem. Soc.
2008, 130, 15254.
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