Highly Selective Carboxylative Cyclization of Allenylmethylamines with Carbon Dioxide Using N-Heterocyclic Carbene-Silver(I) Catalysts

Article abstract of DOI:10.1021/acs.orglett.5b00809

Silver(I) carboxylate complexes promote the carboxylative cyclization of allenylmethylamines to afford 5-alkenyl-1,3-oxazolidin-2-ones in 2-propanol. The use of an N-heterocyclic carbene ligand (IPr) under pressurized CO₂ is effective in suppre

Full text of DOI:10.1021/acs.orglett.5b00809

Letter
pubs.acs.org/OrgLett
Highly Selective Carboxylative Cyclization of Allenylmethylamines
with Carbon Dioxide Using N‑Heterocyclic Carbene-Silver(I) Catalysts
Kyohei Yamashita, Shun Hase, Yoshihito Kayaki,* and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama
2-12-1-E4-1, Meguro-ku, Tokyo 152-8552, Japan
S
* Supporting Information
ABSTRACT: Silver(I) carboxylate complexes promote the
carboxylative cyclization of allenylmethylamines to afford 5-
alkenyl-1,3-oxazolidin-2-ones in 2-propanol. The use of an N-
heterocyclic carbene ligand (IPr) under pressurized CO2 is
effective in suppressing the intramolecular hydroamination
that leads to 2,5-dihydropyrroles. The mechanism involving a
nucleophilic attack of the carbamate of the allene moiety and a
subsequent protonation was realized on the basis of experimental and theoretical results involving a model intermediate, the
alkenylgold(I) complex, which was synthesized from Au(OH)(IPr) and 1-methylamino-2,3-butadiene.
he formation of carbamic acids and their salts from carbon
dioxide (CO2) and amines is a fundamental step in the
an intramolecular hydroamination reaction competed with the
formation of urethane. A 65/10 urethane/amine selectivity was
attained, even under supercritical conditions (50 °C and 11.5
MPa). As an alternative means to access 5-alkenyl-1,3-
oxazolidin-2-ones, Ma and co-workers successively reported
the Pd-catalyzed three-component coupling reaction of
allenylmethylamines, aryl halides, and CO2 in the presence of
a stoichiometric amount of base and carboxylation of 2,3-
allenamides using K2CO3 or Cs2CO3.9 Given the interesting
catalytic properties of the above-mentioned group-11 metal
catalysts, we sought to develop more active and selective
catalysts that can operate under ambient conditions. Outlined
herein are our findings on the efficient carboxylative cyclization
promoted by silver(I) carboxylate complexes.
We initially examined the reaction of 1-(benzylamino)-2,3-
butadiene (1a) and pressurized CO2 (1.0 MPa) in the presence
of catalytic amounts of group-11 metal complexes (2.0 mol %)
in 2-propanol at 30 °C for 6 h (the standard conditions are
given in Table 1). The IPr-coordinated acetatosilver(I) complex
Ag(OAc)(IPr)10 [IPr = 1,3-bis(2,6-diisopropylphenyl)-imida-
zol-2-ylidene] underwent smooth carboxylation to afford the
cyclic urethanes 3-benzyl-5-vinyl-1,3-oxazolidin-2-one (2a) and
3-benzyl-6-methylene-1,3-oxazinan-2-one (3a) in 86 and 6%
yield, respectively, with a small amount (7%) of the
hydroamination11 product 1-benzyl-2,5-dihydropyrrole (4a)
(entry 1). In contrast, Au(OAc)(IPr)12 and Cu(OAc)(IPr)13
were found to have poor catalytic activity (entries 2 and 3).
Switching the acetato ligand to benzoate did not affect the
reaction outcome, but the addition of a chloro ligand
significantly deteriorated the catalytic performance (entries 4
and 5), suggesting that the weakly coordinating nature of the
carboxylato ligands rather than the chloro ligand will be crucial
T
transformation of CO2 into a range of urethanes and ureas as
value-added compounds.1 Among the possible reactions, the
addition of carbamic acids to C−C multiple bonds has been
regarded to be a simple and advantageous protocol for urethane
synthesis in terms of operational simplicity and atom
economy.2,3 We recently found that N-heterocyclic carbene
(NHC)-gold(I) complexes serve as highly effective catalysts in
alcoholic solvents for the carboxylative cyclization of propargyl-
amines to yield 5-alkylidene-1,3-oxazolidin-2-ones with perfect
regio- and stereoselectivities at ambient temperature and under
atmospheric pressure of CO2.4 At nearly the same time,
Yamada and co-workers reported the promotion of similar
carboxylation reactions by silver salts.3j,n−p
The related addition to a C−C double bond has been less
explored. In an original study reported in 1978 on a three-
component reaction of secondary amines, CO2, and enol
ethers, the urethane product was obtained in only 11% yield.5
The cyclization of allylic and homoallylic amines with CO2 was
examined with the use of stoichiometric iodine compounds or
catalytic amounts of guanidine bases.6 Very recently, Ca’ and
co-workers reported the direct synthesis of 1,3-oxazolidin-2-one
derivatives from CO2 and electrophilic allylamines attached to
electron-withdrawing ester and aryl groups in a mixture of
acetonitrile and methanol in supercritical CO2.7 In a previous
work, we established a palladium-catalyzed cyclization of
allenylmethylamines with dense CO2 (Scheme 1);8 however,
Scheme 1. Pd-Catalyzed Carboxylative Cyclization
Received: March 19, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00809
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Carboxylative Cyclization of 1a
Table 2. Pressure Effect on Urethane Selectivity
b
% yield
entry
pressure, MPa
2a
3a
4a
% urethane selectivity
1
2
3
4
5
0.1
1.0
3.0
5.0
7.0
70
82
85
88
89
7
5
5
5
6
18
10
5
81
90
95
96
99
b
% yield
4
entry
catalyst
solvent
2a
3a
4a
1
1
2
3
4
5
6
Ag(OAc)(IPr)
Au(OAc)(IPr)
Cu(OAc)(IPr)
Ag(OBz)(IPr)
AgCl(IPr)
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
toluene
86
0
6
7
a
Reaction conditions: 1a (1.0 mmol), Ag(OAc)(IPr) (0.02 mmol), 2-
propanol (1.0 mL) at 40 °C for 15 h. Determined by H NMR.
0
0
b
1
3
0
<1
7
85
7
6
product 4a was almost suppressed. The positive pressure effect
can be explained by the enhancement of CO2 uptake by the
amine substrate, thus more efficiently leading to the urethane
product.2d
The silver catalyst was also applicable to the carboxylative
cyclization of other allenylmethylamines at 30 °C under 1.0
MPa of CO2, as summarized in Table 3. The substrate with a p-
<1
1
<1
26
26
3
AgOAc
71
69
3
c
7
Ag(OAc)(ItBu)
Ag(OAc)(PPh3)
Ag(OAc)[(S)-binap]
Ag(OAc)(IPr)
Ag(OAc)(IPr)
Ag(OAc)(IPr)
Ag(OAc)(IPr)
Ag(OAc)(IPr)
Ag(OAc)(IPr)
Ag(OAc)(IPr)
2
8
<1
<1
<1
<1
3
d
9
3
3
10
11
12
13
14
15
2
2
THF
5
4
CH2Cl2
50
61
71
31
77
4
a
Table 3. Ag(OAc)(IPr) Catalyzed Reaction of 1 with CO2
CH3OH
tBuOH
4
31
4
6
CF3CH2OH
2-propanol
2
36
7
e
16
5
a
Standard conditions: the reaction was carried out with 1a (1.0 mmol)
and the catalyst (0.02 mmol, 2 mol %) in a solvent (1.0 mL) under 1.0
b
MPa of CO2 at 30 °C for 6 h. Determined by 1H NMR analysis using
c
d
e
durene as internal standard. 40 °C, 7.0 MPa, 15 h. 5.0 MPa. 0.1 mol
% of the catalyst for 96 h.
b
% yield
for generating active cationic silver species with a vacant
reaction site. Notably, the catalytic use of a silver acetate salt
resulted in the formation of 2a in 71% yield accompanied by
the side product 4a in 26% yield (entry 6), which supports the
important role of the IPr ligand in the enhancement of the
urethane/amine selectivity. Although related acetatosilver
complexes with other NHC ligands could also promote the
reaction, the urethane/amine selectivity was lowered (entry 7).
Silver acetate complexes bearing PPh3 or BINAP ligands
displayed limited activity for the cyclization (entries 8 and 9).
As listed in Table 1, the solvent had a pronounced effect on
both the conversion and product selectivity. Compared to
aprotic solvents such as toluene, THF, and CH2Cl2 (entries
10−12), alcoholic solvents markedly accelerated the reaction
(entries 1, 13, and 14), whereas the hydroamination product 4a
was competitively formed in CH3OH and CF3CH2OH (entries
13 and 15). These acidic alcohols would activate 1a, making the
allene unit more susceptible to attack by the amino group. Even
at a catalyst loading as low as 0.1 mol %, the carboxylation
steadily proceeded in 2-propanol to give 2a in 77% yield when
the reaction time was increased to 96 h (entry 16). A similar
beneficial role of methanol was demonstrated in the Au-
catalyzed carboxylative cyclization of propargylamines, where a
half-ester of carbonic acid,14 generated from alcohol and CO2,
facilitated the product-releasing step, i.e., the protonolysis of the
Au−C bond.
entry
1
amine
2
3
4
1b
1c
1d
1e
1f
82
82
80
81
83
87
41
32
5
5
6
4
5
5
7
9
8
5
c
2
3
4
5
6
d
d
d
d
1h
1i
n.d.
n.d.
n.d.
n.d.
40
e
7
e
8
1j
61
a
Reaction conditions: the reaction was carried out with 1 (1.0 mmol)
and Ag(OAc)(IPr) (0.02 mmol) in 2-propanol (1.0 mL) under 1.0
MPa of CO2 at 30 °C for 6 h. Determined by H NMR. 5.0 MPa.
b
c
1
d
e
Not detected. 7.0 MPa, 40 °C, 15 h. E/Z = 93/7.
methoxybenzyl group on the amine nitrogen provided the
desired five-membered urethane in 82% yield (1b, entry 1). An
electron-withdrawing CN group was also applicable to the
carboxylative cyclization to afford the corresponding urethane
2c in 82% yield under a higher CO2 pressure of 5.0 MPa. Other
substituents on the aryl group, including bromide, fluoride, and
acetal, were tolerated under the reaction conditions to give 2c−
2f in yields greater than 80% (entries 2−5).
The attachment of a methyl group at the α-position to the
amine facilitated the formation of the 5-membered cyclic
urethane 2g in 92% yield as a mixture of diastereomers (anti/
syn = 60/40), as shown in Scheme 2. The reaction of an
internal aminomethylallene (1h) containing an alkyl group at
the allene terminal also furnished the desired urethane 2h in
87% yield as a mixture of E/Z (93/7) isomers under similar
conditions (entry 6, Table 3). On the other hand, 1,1,3-
trisubstituted allenes, such as 1i and 1j, gave the urethanes in
moderate yields and competitively underwent intramolecular
As shown in Table 2, Ag(OAc)(IPr) exhibited sufficient
catalytic activity, even under atmospheric pressure of CO2, with
a good urethane selectivity of 81% (entry 1). Further
improvement in the urethane yield was successfully achieved
when the reaction was conducted under higher pressure. The
yields of 2a and 3a reached 89 and 6%, respectively, under the
condition of 7.0 MPa, and the formation of the hydroamination
B
DOI: 10.1021/acs.orglett.5b00809
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Carboxylative Cyclization of 1g
Scheme 4. Synthesis of Alkenylgold(I) Complexes 5 and 6 as
Model Intermediates in the Carboxylation Reaction
hydroamination to give 4i and 4j in 40 and 61% yield,
respectively, even under high pressure (7.0 MPa) of CO2
(entries 7 and 8, Table 3).
Following the related catalytic hydrofunctionalization of
allenes,15 we propose two catalytic cycles induced by a cationic
species through the facile dissociation of the acetate ligand on
the silver complex in a polar alcoholic solvent (Scheme 3). In
complex Au(OAc)(IPr) and 1a in 2-propanol under CO2 (1.0
MPa) in 89% yield without furnishing the free urethane 2,
indicating that the catalytic inefficiency of acetatogold is
ascribable to the poor reactivity of 5 toward protons.
Scheme 3. Plausible Catalytic Cycles
The colorless complex 5 was characterized by NMR
spectroscopy, elemental analysis, and X-ray crystallography.
The crystal structure (Figure 1) shows that the gold atom is
the case of intramolecular hydroamination, the mechanism
involves the initial formation of a cationic π-allene complex by
the coordination of the γ,δ-double bond of the amine substrates
followed by 5-endo-cyclization via an outer sphere attack of the
amino group. For the carboxylative cyclization, the nucleophilic
attack of the carbamate moiety generated from CO2 and the
amine substrate on another π-allene complex coordinated by
the β,γ-unsaturated bond probably leads to the corresponding
neutral alkenylsilver intermediates (A, B). Proton exchange
between the reaction medium or the carbamic acid and the
organosilvers regenerates the cationic species and liberates the
urethane products. These steps are deducible from our previous
study on the Au(I)-catalyzed carboxylative cyclization of
propargylamines.4
It is noteworthy that the selectivity in the coordination of the
allene substrates directly affects the amine/urethane ratios.
Namely, in the competition between the hydroamination and
carboxylative cyclization cycles in the reaction of the 1,1,3-
trisubstituted allenes 1i and 1j, the more electron-rich double
bond on the allene substrates will favor coordination to the
metal center and will be more susceptible to attack by the
amino group.16
Figure 1. ORTEP diagram of 5. Ellipsoids are shown at 30%
probability. Hydrogen atoms are omitted for clarity.
bound to an NHC-carbene and an alkenyl carbon in a linear
geometry. The gold−carbon bond lengths are 2.036 and 2.023
Å, which are within the typical range for related alkenylgold
complexes.17 Complex 5 displays a similar structure to the gold
complex 6 derived from 1-methylamino-2-butyne and CO2 that
was synthesized in our previous work.4 However, in the 13C
NMR spectra, the chemical shift of complex 5 (δ 175.4),
attributed to the alkenyl carbon adjacent to the gold center, was
found to be significantly shifted downfield compared to that of
6 (δ 130.4). These spectroscopic data suggest that complexes 5
and 6 show divergent reactivity toward electrophiles.
The clear differences in electronic structure of these
complexes as well as the enhanced electrophilicity of the silver
variant were supported by DFT calculations (see the
Supporting Information). These experimental and theoretical
results confirm that the polarized metal−carbon bond is crucial
for accomplishing the carboxylative cyclization.
To provide corroborative evidence of the nucleophilic attack
on the allene, stoichiometric reactions using NHC-Au(I)
complexes were investigated. The treatment of Au(OH)(IPr)
with an equimolar amount of 1a under a CO2 atmosphere in
dehydrated THF at 40 °C afforded the alkenylgold complex 5
in 83% yield after recrystallization from acetone/n-pentane
(Scheme 4). Moreover, 5 was similarly formed from the acetate
In conclusion, we have demonstrated that NHC-Ag(I)
carboxylate complexes serve as efficient catalysts, providing
1,3-oxazolidin-2-one derivatives from allenylmethylamines and
CO2 under relatively mild conditions with excellent selectivities.
The superiority of the Ag catalyst over the Au catalyst is in line
C
DOI: 10.1021/acs.orglett.5b00809
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2006, 36, 3029−3042. (c) Takeda, Y.; Okumura, S.; Tone, S.; Sasaki,
I.; Minakata, S. Org. Lett. 2012, 14, 4874−4877.
with the fact that the alkenylgold complex derived from 1a and
CO2 was less susceptible to the protonolysis reaction relevant
to the product-releasing step.
(7) Soldi, L.; Massera, C.; Costa, M.; Ca’, N. D. Tetrahedron Lett.
2014, 55, 1379−1383.
(8) Kayaki, Y.; Mori, N.; Ikariya, T. Tetrahedron Lett. 2009, 50,
6491−6493.
(9) (a) Li, S.; Ye, J.; Yuan, W.; Ma, S. Tetrahedron 2013, 69, 10450−
10456. (b) Chen, G.; Fu, C.; Ma, S. Org. Lett. 2009, 11, 2900−2903.
(10) Partyka, D. V.; Deligonul, N. Inorg. Chem. 2009, 48, 9463−9475.
(11) Silver-catalyzed intramolecular hydroamination of aminoallene:
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedure and analytical data for new com-
pounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
(a) Alvarez-Corral, M.; Munoz-Dorado, M.; Rodriguez-Garcia, I.
̃
Chem. Rev. 2008, 108, 3174−3198. (b) Claesson, A.; Sahlberg, C.;
Luthman, K. Acta Chem. Scand. 1979, 33b, 309−310. (c) Ohno, H.;
Toka, A.; Miwa, Y.; Taga, T.; Osawa, E.; Yamaoka, Y.; Fuji, N.; Ibuka,
T. J. Org. Chem. 1999, 64, 2992−2993. (d) Amombo, M. O.; Hausherr,
A.; Reissig, H.-U. Synlett 1999, 1871−1874. (e) Dieter, R. K.; Yu, H.
Org. Lett. 2001, 3, 3855−3858. (f) Billet, M.; Schoenfelder, A.; Klotz,
P.; Mann, A. Tetrahedron Lett. 2002, 43, 1453−1456. (g) Dieter, R. K.;
Chen, N.; Yu, H.; Nice, L. E.; Gore, V. K. J. Org. Chem. 2005, 70,
2109−2119. (h) Mitaev, B.; Brummond, K. M. Synlett 2006, 3100−
3104. (i) Arbour, J. L.; Rzepa, H. S.; Contreras-García, J.; Adrio, L. a.;
Barreiro, E. M.; Hii, K. K. Chem.Eur. J. 2012, 18, 11317−11324.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: ykayaki@o.cc.titech.ac.jp.
*E-mail: tikariya@apc.titech.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(12) Iglesias, A.; Muniz, K. Chem.Eur. J. 2009, 15, 10563−10569.
̃
This work was financially supported by JSPS KAKENHI Grant
Number 22225004, 24350079, and 26620143 and partly
supported by the GCOE Program. The authors thank the
Material Analysis Suzukake-dai Center, Technical Department,
Tokyo Institute of Technology for HRMS analysis. The
computations were performed using the resources of the
Research Center for Computational Science, Okazaki, Japan.
(13) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P.
Organometallics 2004, 23, 1191−1193.
(14) (a) West, K. N.; Wheeler, C.; McCarney, J. P.; Griffith, K. N.;
Bush, D.; Liotta, C. L.; Eckert, C. A. J. Phys. Chem. A 2001, 105, 3947−
3948. (b) Dibenedetto, A.; Aresta, M.; Giannoccaro, P.; Pastore, C.;
Papai, I.; Schubert, G. Eur. J. Inorg. Chem. 2006, 908−913. (c) Gohres,
́
J. L.; Marin, A. T.; Lu, J.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem.
Res. 2009, 48, 1302−1306.
(15) (a) Winter, C.; Krause, N. Gold-Catalyzed Addition Reaction to
Allenes. In Modern Gold Catalyzed Synthesis; Hashmi, A. S. K., Toste,
F. D., Ed.; Wiley-VCH: Weinheim, Germany, 2012; pp 363−389.
(b) Zhang, Z.; Liu, C.; Kinder, R. E.; Han, X.; Qian, H.; Widenhoefer,
R. A. J. Am. Chem. Soc. 2006, 128, 9066−9073. (c) Arcadi, A. Chem.
REFERENCES
■
(1) (a) Yang, Z.-Z.; He, L.-N.; Gao, J.; Liu, A.-H.; Yu, B. Energy
Environ. Sci. 2012, 5, 6602−6639. (b) Chaturvedi, D.; Chatruvedi, A.
K.; Mishra, V. Curr. Org. Chem. 2012, 16, 1609−1635. (c) Quaranta,
E.; Aresta, M. Carbon Dioxide As Chemical Feedstock; Aresta, M., Ed.;
Wiley-VCH: Weinheim, Germany, 2010; pp 121−167.
(2) Intermolecular addition of carbamic acids to alkynes: (a) Bruneau,
C.; Dixneuf, P. H. J. Mol. Catal. 1992, 74, 97−107. (b) Rohr, M.;
Geyer, C.; Wandeler, R.; Schneider, M. S.; Murphy, E. F.; Baiker, A.
Green Chem. 2001, 3, 123−125. (c) Jiang, J.-L.; Hua, R. Tetrahedron
Lett. 2006, 47, 953−955. (d) Kayaki, Y.; Suzuki, T.; Ikariya, T.
Chem.Asian J. 2008, 3, 1865−1870.
Rev. 2008, 108, 3266−3325. (d) Muller, T. E.; Hultzsch, K. C.; Yus,
̈
M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795−3892.
(e) Nishina, N.; Yamamoto, Y. Tetrahedron 2009, 65, 1799−1808.
(f) Patil, N. T.; Kavthe, R. D.; Shinde, V. K. Tetrahedron 2012, 68,
8079−8146. (g) Yang, W.; Hashmi, A. S. K. Chem. Soc. Rev. 2014, 43,
2941−2955. (h) Soriano, E.; Fernandez, I. Chem. Soc. Rev. 2014, 43,
3041−3105. (i) Munoz, M. P. Chem. Soc. Rev. 2014, 43, 3164−3183.
̃
(16) Kinder, R. E.; Zhang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10,
3157−3159.
(3) Intramolecular addition of carbamic acids to alkynes: (a) Mitsudo,
T.; Hori, Y.; Yamakawa, Y.; Watanabe, Y. Tetrahedron Lett. 1987, 28,
4417−4418. (b) Bacchi, A.; Chiusoli, G. P.; Costa, M.; Gabriele, B.;
Righi, C.; Salerno, G. Chem. Commun. 1997, 1209−1210. (c) Costa,
M.; Chiusoli, G. P.; Taffurelli, D.; Dalmonego, G. J. Chem. Soc., Perkin
Trans. 1 1998, 1541−1546. (d) Shi, M.; Shen, Y.-M. J. Org. Chem.
2002, 67, 16−21. (e) Feroci, M.; Orsini, M.; Sotgiu, G.; Rossi, L.;
Inesi, A. J. Org. Chem. 2005, 70, 7795−7798. (f) Kayaki, Y.;
Yamamoto, M.; Suzuki, T.; Ikariya, T. Green Chem. 2006, 8, 1019−
1021. (g) Maggi, R.; Bertolotti, C.; Orlandini, E.; Oro, C.; Sartori, G.;
Selva, M. Tetrahedron Lett. 2007, 48, 2131−2134. (h) Yoshida, M.;
Komatsuzaki, Y.; Ihara, M. Org. Lett. 2008, 10, 2083−2086. (i) Yoo,
W.-J.; Li, C.-J. Adv. Synth. Catal. 2008, 350, 1503−1506. (j) Yoshida,
S.; Fukui, K.; Kikuchi, S.; Yamada, T. Chem. Lett. 2009, 38, 786−787.
(k) Yoshida, M.; Mizuguchi, T.; Shishido, K. Chem.Eur. J. 2012, 18,
15578−15581. (l) Zhao, J.; Jiang, H. Tetrahedron Lett. 2012, 53,
6999−7002. (m) Zhao, J.; Huang, H.; Qi, C.; Jiang, H. Eur. J. Org.
Chem. 2012, 5665−5667. (n) Ishida, T.; Kikuchi, S.; Tsubo, T.;
Yamada, T. Org. Lett. 2013, 15, 848−851. (o) Ishida, T.; Kikuchi, S.;
Yamada, T. Org. Lett. 2013, 15, 3710−3713. (p) Ishida, T.; Kobayashi,
R.; Yamada, T. Org. Lett. 2014, 16, 2430−2433.
(17) Liu, L.-P.; Hammond, G. B. Chem. Soc. Rev. 2012, 41, 3129−
3139 and references cited therein.
(4) Hase, S.; Kayaki, Y.; Ikariya, T. Organometallics 2013, 32, 5285−
5288.
(5) Yoshida, Y.; Inoue, S. Chem. Lett. 1977, 6, 1375−1376.
(6) (a) Toda, T.; Kitagawa, Y. Angew. Chem., Int. Ed. Engl. 1987, 26,
334−335. (b) Egido, G. E.; Fernandez, I.; Munoz, L. Synth. Commun.
́
̃
D
DOI: 10.1021/acs.orglett.5b00809
Org. Lett. XXXX, XXX, XXX−XXX