[Cp?RhCl2]2-catalyzed alkyne hydroamination to 1,2-dihydroquinolines

Article abstract of DOI:10.1021/om501253v

[Cp?RhCl₂]₂ catalyzes the formation of 1,2-dihydroquinolines from the reaction of two terminal alkynes and an aniline. This reaction is believed to proceed via an alkyne hydroamination followed by an alkyne insertion.

Full text of DOI:10.1021/om501253v

Note
pubs.acs.org/Organometallics
[Cp*RhCl2]2‑Catalyzed Alkyne Hydroamination to
1,2‑Dihydroquinolines
Elumalai Kumaran and Weng Kee Leong*
Division of Chemistry and Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: [Cp*RhCl2]2 catalyzes the formation of 1,2-
dihydroquinolines from the reaction of two terminal alkynes
and an aniline. This reaction is believed to proceed via an
alkyne hydroamination followed by an alkyne insertion.
Scheme 1
he 1,2-dihydroquinoline unit occurs in a vast number of
Tpharmaceuticals and other biologically active com-
pounds,1 as well as several natural products, such as
angustureine and the alkaloids. 1,2-Dihydroquinolines are also
used as intermediates in organic and natural product synthesis,2
and they have the additional advantage that they can be
reduced to 1,2,3,4-tetrahydroquinolines, which also have great
importance as pharmaceuticals and agrochemicals.3 A straight-
forward synthetic route toward 1,2-dihydroquinolines would
therefore be of interest.4 The application of transition metal
mediated C−H bond functionalization in C−C and C−N bond
formation reactions is now widely recognized.5 In particular,
functional group directed arene C−H bond activation followed
by coupling with alkynes has been shown to be attractive for
the construction of valuable organic motifs.6 For instance,
imine has been used as a directing group for C−H bond
functionalization mediated by rhodium complexes, including
[Cp*RhCl2]2, 1.7
In our work on alkyne hydroamination reactions,8 we
recently reported that 1 can efficiently catalyze the reaction
of a terminal alkyne with an aniline to afford a ketimine; our
proposed catalytic cycle is shown in Scheme 1.8b
derivative 4aa in 55% isolated yield. The identity of the product
was established on the basis of NMR and mass spectroscopic
data, and the structure was confirmed by a single crystal X-ray
crystallographic study (Supporting Information). An optimiza-
tion study showed that an increase in the amount of catalyst
and/or NH4PF6 did not improve the yield significantly (Table
1, entries 2 and 3) but replacement of NH4PF6 with acids such
as TFA or HBF4, however, improved it dramatically (entries 4
and 5). The use of coordinating solvents like THF, IPA, or
methanol was detrimental (entries 9−11). Control experiments
showed that both 1 and HBF4 are necessary (entries 6 and 7).
The catalyst system is tolerant of a wide variety of substrates;
a complete list of the substrate scope tested is given in the
Supporting Information, and a selected list is presented in
The final step in this catalytic cycle is the formation of an
enamine which, we believed, rearranges rapidly under the
reaction conditions to the ketimine, the species isolated. It
occurred to us that the final imine−enamine equilibrium could
be shifted toward the enamine via protonation of the imine
nitrogen.9 A number of metal systems are known to catalyze
alkyne hydroamination with enamine to afford the 1,2-
dihydroquinoline unit,10 although they do have drawbacks
such as the need for an excess of the alkyne,10a,b or higher
temperatures.10c,d,f We therefore envisaged that we could switch
from the formation of ketimine to that of 1,2-dihydroquinoline
in our system through a change in the pH.
Indeed, the reaction of PhCCH, 2a, and 3-methoxyaniline,
3a, in an ∼2:1 molar ratio at 100 °C in the presence of 2 mol %
of 1 and 5 mol % of NH4PF6 afforded dihydroquinoline
Received: December 8, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/om501253v
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Note
species. Two of these are depicted in Scheme 2. Path A involves
an ortho alkenylation, a hydroamination, and then a [3,3]
Table 1. Optimization of Conditions for the Reaction 2a +
3a → 4aa
a
Scheme 2
additive
(mol %)
isolated yield
(%)
entry 1 (mol %)
solvent T (°C)
1
2
5
2
2
2
2
−
2
2
2
2
NH4PF6 (5)
NH4PF6 (10)
NH4PF6 (12)
HBF4 (5)
TFA (5)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCE
100
100
100
100
100
100
100
80
55
65
58
86
70
−
2
3
4
5
6
−
sigmatropic shift; metal-catalyzed hydroamination and ortho
C−H bond insertion of arylamines have both been well
studied.11 We have found, however, that 5 failed to react with
2a to afford 4aa, thus ruling out this pathway.
7
HBF4 (5)
HBF4 (5)
HBF4 (5)
HBF4 (5)
HBF4 (5)
−
78
8
9
THF
40
−
−
−
10
11
IPA
80
Path B involves the metal-catalyzed formation of an imine
and its insertion into a metal−alkynyl species, followed by
ortho C−H bond activated cyclization; a similar pathway has
been proposed previously for Ag and Au.10c,d Although we did
observe that 7 reacted with 2a to give 4aa, and that the reaction
required both 1 and HBF4·OEt2, the intermediate 8a was not
observed. This was also true for the less reactive aniline, which
would be expected to be arrested at the formation of the
analogue of 8a as the C−H bond activated cyclization would
not be favored without an activating group on the aniline; only
the corresponding imine was observed as a minor product.
Further confirmation of this was obtained through the
syntheses of the analogues 8b and 8c; under the same reaction
conditions, 8b did afford the corresponding 1,2-dihydroquino-
line 4aa′, but in a significantly reduced yield (compare Table 2
entry 7 and Supporting Information Table S3 entries 17−23),
while 8c failed to react (Scheme 3). These results served to rule
out path B.
MeOH
80
a
Conditions: 3a (1.0 mmol), 2a (2.1 mmol), 1, additive in a solvent (3
mL), heated for 24 h.
Table 2. Both electron-donating and electron-withdrawing
substituents on the phenylacetylene were tolerated, as were
Table 2. Selected Results for the Formation of 1,2-
Dihydroquinoline from the Reaction of Alkyne with Aniline
a
entry
R (alkyne, 2x)
Ph (2a)
R′ (aniline, 2y)
product, % yield
1
2
3
4
5
6
7
8
9
10
OMe (3a)
OMe (3a)
OMe (3a)
OMe (3a)
OMe (3a)
OMe (3a)
OMe (3a)
OEt (3b)
OiPr (3d)
OBn (3g)
CH3 (3j)
H (3k)
4aa, 86
4ga, 73
4ha, 71
4ja, 89
4na, 75
4pa, 85
4qa, 73
4ab, 83
4ad, 79
4ag, 80
4rj, 48
4rk, 34
HOCH2-4-C6H4 (2g)
HO-3-C6H4 (2h)
H3CO-4-C6H4 (2j)
Cl-4-C6H4 (2n)
F-4-C6H4 (2p)
−(CH2)2CH3 (2q)
Ph (2a)
Scheme 3
Ph (2a)
Ph (2a)
b
11
−(CH2)2CH3 (2q)
−(CH2)2CH3 (2q)
b
12
It is therefore likely that the reaction here proceeded via the
formation of an enamine intermediate followed by ortho C−H
bond activation and alkyne insertion (Scheme 4). Formation of
the intermediate B may be envisaged as proceeding via an
electrophilic substitution. The m-alkoxy and NH2 substituents
on the aniline make the position ortho to the amine group
electron-rich and thus favoring substitution there. Displacement
of the coordinated enaminyl alkene by an alkyne, followed by
alkyne insertion, affords the intermediate D. A 1,2-alkene
insertion of the enamine would give E, which can then undergo
protonolysis to yield the final product 4 and regenerate the
active catalyst.
Conditions: Arylamine (1.0 mmol), alkyne (2.1 mmol), 1 (0.02
mmol) and HBF4·OEt2 (0.05 mmol) in toluene (3 mL), heated at 100
°C for 24 h. Isolated yields. 30−40% ketimine was observed.
a
b
acid-sensitive groups like −OR and −OH. The reaction also
worked with aliphatic alkynes and various 3-alkoxyanilines.
Extension to other anilines, such as aniline and 3-methylaniline,
however, gave comparatively lower yields. The reaction failed
with anilines containing electron-withdrawing substituents at
the meta position (3-chloroaniline), secondary anilines (N-
methyl-3-methoxyaniline), and internal alkynes (PhCCPh).
Although the intermediacy of an enamine in such reactions
has been proposed for the other metal systems,10 we have also
sought to exclude other possible pathways, including ones in
which a part of the pathway does not involve a metal-bound
In conclusion, we have described a single-step synthesis of
1,2-dihydroquinolines from the reaction of arylamines with
terminal alkynes, catalyzed by the rhodium complex 1. This
B
DOI: 10.1021/om501253v
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Note
for 20 min. To the refluxing solution was added Fe powder (1.45 g, 25
mmol) portionwise, followed by FeCl3·6H2O (162 mg, 0.58 mmol).
After completion of the reaction, the mixture was filtered and diluted
with ether (25 mL) and water (15 mL), and the layers were separated.
The aqueous layer was extracted with ether (3 × 15 mL) and dried
over MgSO4, and the solvent was removed by evaporation. The crude
product obtained was purified by silica gel (60−120 mesh) column
chromatography to give pure 3b (256 mg, 52%). A similar procedure
was used for the other 3-alkoxyanilines. The amount of reagents used,
product formed (with yield), and HRMS for the products are given in
the Supporting Information.
Scheme 4
General Procedure for the Preparation of 1,2-Dihydroqui-
nolines. In a carius tube, 1 (12 mg, 0.02 mmol) and HBF4·OEt2 (7
mg, 0.05 mmol) were dissolved in toluene (3.0 mL). To this
suspension were added 3-methoxyaniline (123 mg, 1.0 mmol) and
PhCCH (214 mg, 2.1 mmol). The reaction mixture was heated at 100
°C for 24 h, and then the reaction solvent was rotary evaporated to
give the crude product as brown oil. Purification using flash column
chromatography on silica with ethyl acetate/hexane (5% v/v) as eluent
gave 4aa (281 mg, 86%). Similar procedures were used with the other
alkynes and anilines for other 4. The amount of reagents used, product
formed (with yield), and HRMS for the products are given in the
Supporting Information.
Synthesis of Compound 5. 3-Methoxyaniline (615 mg, 5.0
mmol), PhCCH (510 mg, 5.0 mmol), and montmorillonite K-10 (500
mg) were placed in a round bottomed flask equipped with magnetic
stirrer and reflux condenser. The reaction mixture was then heated at
140 °C for 12 h. After cooling to rt, the mixture was filtered and
washed through with diethyl ether (20 mL), and the solvent from the
combined filtrate and washings was removed by evaporation. The
crude product obtained was purified by silica gel (60−120 mesh)
column chromatography using hexane:ethyl acetate (90:10, v/v) as
eluent to obtain pure 5 (618 mg, 55%). HRMS: found (calcd)
226.1232 (226.1232).
reaction involved two separate catalytic cycles: the first
involving alkyne hydroamination to form a ketimine as reported
earlier, and the second involving transformation of the ketimine
through ortho C−H bond activation and alkyne insertion. The
acidic conditions employed favored the enamine in the imine−
enamine equilibrium which resulted from the first catalytic
cycle.
Reactions with Phenylacetylene. In a carius tube, 1 (10 mg,
0.016 mmol) and HBF4·OEt2 (7 mg, 0.04 mmol) were dissolved in
toluene (3 mL). To this suspension were added 5 (180 mg, 0.8 mmol)
and PhCCH (91 mg, 0.85 mmol). The mixture was heated at 100 °C
EXPERIMENTAL PROCEDURES
■
General. All reactions and manipulations were performed under
argon by using standard Schlenk techniques. The catalyst 1 was
prepared according to the published method.12 1H and 13C{1H} NMR
spectra were recorded in chloroform-d except 4ga and 4ha, which were
recorded in methanol-d4, on a JEOL ECA400 or ECA400SL
spectrometer; chemical shifts reported were referenced to residual
solvent resonances. High resolution mass spectra (HRMS) were
recorded in ESI mode on a Waters UPLC-Q-TOF mass spectrometer.
Crystallographic Analyses. Diffraction-quality crystals of 4aa and
4ga were obtained by slow evaporation of hexane/dichloromethane
solutions. X-ray data were collected on a Bruker AXS APEX system,
using Mo Kα radiation and at 103 K, with the SMART suite of
programs.13 Data were processed and corrected for Lorentz and
polarization effects with SAINT,14 and for absorption effects with
SADABS.15 Structural solution and refinement were carried out with
the SHELXTL suite of programs.16 The structures were solved by
direct methods and completed with difference maps following
refinement. Hydrogen atoms were placed in calculated positions and
refined with a riding model. There were two molecules in the
asymmetric unit of 4ga. One molecule exhibited disorder of the benzo
ring and the other exhibited disorder of the OMe and CH2OH groups.
These were modeled with two alternative sites of equal occupancies
each, with appropriate restraints.
1
for 24 h, and H NMR analysis showed that there was no product
formation.
An analogous reaction with 7 in place of 5 afforded, after flash
column chromatographic separation (ethyl acetate/hexane, 5% v/v) of
the crude, 4aa (55 mg, 86%). A similar reaction using 8b (212 mg, 0.8
mmol) but without the PhCCH afforded, after a similar workup, 4aa′
(89 mg, 42%). HRMS: found (calcd) 266.1541 (266.1545).
ASSOCIATED CONTENT
■
S
* Supporting Information
Details of optimization and substrate scope studies, character-
ization data of all new compounds, and X-ray crystallography
report. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/om501253v.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: +65 6592 7577. E-mail: chmlwk@ntu.edu.sg.
General Procedure for the Synthesis of 3-Alkoxyanilines. In
a 2-neck round bottomed flask, 3-nitrophenol (0.5 g, 3.59 mmol) was
dissolved in dry DMF (5 mL). To this solution were added 1-
iodoethane (316 μL, 3.95 mmol) and K2CO3 (0.5 g, 3.59 mmol), and
the reaction mixture was stirred at room temperature. After
completion of the reaction, reaction solvent was removed under
reduced pressure. Then the mixture was diluted with water, extracted
with chloroform (3 × 15 mL), and dried over MgSO4 and the solvent
evaporated.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Nanyang Technological
University and the Ministry of Education (Research Grant No.
M4011017). Assistance on the crystallographic studies by Dr
Rakesh Ganguly is acknowledged, and one of us (E.K.) thanks
the university for a Research Scholarship.
In a 2-neck round bottomed flask, the above obtained 3-
ethoxynitrobenzene was dissolved in ethanol (15 mL), followed by
the addition of acetic acid (675 μL). The reaction mixture was refluxed
C
DOI: 10.1021/om501253v
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Note
4849. (d) Kumaran, E.; How, K. T. S.; Ganguly, R.; Li, Y.; Leong, W.
K. Organometallics 2013, 32, 4149. (e) Kumaran, E.; Fan, W. Y.;
Leong, W. K. Org. Lett. 2014, 16, 1342. (f) Kumaran, E.; Leong, W. K.
Tetrahedron Lett. 2014, 55, 5495.
REFERENCES
■
(1) (a) Elmore, S. W.; Coghlan, M. J.; Anderson, D. D.; Pratt, J. K.;
Green, B. E.; Wang, A. X.; Staschko, M. A.; Lin, C. W.; Tyree, C. M.;
Miner, J. N.; Jacobson, P. B.; Wilcox, D. M.; Lane, B. C. J. Med. Chem.
2001, 44, 4481. (b) Lesson, P. D.; Carling, T. W.; Moore, K. W.;
Moseley, A. M.; Smith, J. D.; Stevenson, G.; Chan, T.; Baker, R.;
Foster, A. C.; Grimwood, S.; Kemp, J. A.; Marschall, G. R.; Hoogsteen,
K. J. Med. Chem. 1992, 35, 1954. (c) Chekal, B. P.; Freund, E.; Grill, C.
M.; Huthmann, E.; Juza, M.; Leshchinskaya, V.; Maloney, M. T.;
Miller, L. M., Jr.; Wang, Y.; Zeng, M. US Patent Appl. A1
20060020022, 2006.
(2) (a) Magnus, P.; Eisenbeis, S. A.; Fairhurst, R. A.; Iliadis, T.;
Magnus, N. A.; Parry, D. J. Am. Chem. Soc. 1997, 119, 5591. (b) Ye, K.-
Y.; Liu, W.-B.; Dai, L.-X.; Helmchen, G.; You, S.-L. J. Am. Chem. Soc.
2011, 133, 19006. (c) Shair, M. D.; Yoon, T. Y.; Mosny, K. K.; Chou,
T. C.; Danishefsky, S. J. J. Am. Chem. Soc. 1996, 118, 9509. (d) Kariba,
R. M.; Houghton, P. J.; Yenesew, A. J. Nat. Prod. 2002, 65, 566.
(e) Rakotoson, J. H.; Fabre, N.; Jacquemond-Collect, I.;
Hannedouche, S.; Fouraste, I.; Moulis, C. Planta Med. 1998, 64,
762. (f) Witherup, K. M.; Ransom, R. W.; Graham, A. C.; Bernard, A.
M.; Salvatore, M. J.; Lumma, W. C.; Anderson, P. S.; Pitzenberger, S.
M.; Varga, S. L. J. Am. Chem. Soc. 1995, 117, 6682.
́
(9) (a) Campora, J.; Hudson, S. A.; Massiot, P.; Maya, C. M.; Palma,
P.; Carmona, E. Organometallics 1999, 18, 5225−5237. (b) Carey, A.
R. E.; Fukata, G.; O’Ferrall, R. A. M.; Murphy, M. G. J. Chem. Soc.,
Perkin Trans. 2 1985, 1711−1722. (c) Fray, M. J.; Bull, D. J.; Cooper,
K.; Parry, M. J.; Stefaniak, M. H. J. Med. Chem. 1995, 38, 3524−3535.
(10) (a) Yi, C. S.; Yun, S. Y.; Guzei, I. A. J. Am. Chem. Soc. 2005, 127,
5782. (b) Yi, C. S.; Yun, S. Y. J. Am. Chem. Soc. 2005, 127, 17000.
(c) Luo, Y.; Li, Z.; Li, C.-J. Org. Lett. 2005, 7, 2675. (d) Liu, X.-Y.;
Ding, P.; Huang, J.-S.; Che, C.-M. Org. Lett. 2007, 9, 2645. (e) Zeng,
X.; Frey, G. D.; Kinjo, R.; Donnadieu, B.; Bertrand, G. J. Am. Chem.
Soc. 2009, 131, 8690. (f) Purkait, N.; Blechert, S. Adv. Synth. Catal.
2012, 354, 2079.
(11) (a) Kakiuchi, F.; Murai, S. In Activation of Unreactive Bonds and
Organic synthesis; Murai, S., Ed.; Springer: New York, 1999. (b) Jia, C.;
Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633. (c) Ritleng,
V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 633.
(12) White, C.; Yates, A.; Maitlis, P. M. Inorg. Synth. 2007, 29, 228−
234.
(13) SMART version 5.628; Bruker AXS Inc.: Madison, WI, 2001.
(14) SAINT+ version 6.22a; Bruker AXS Inc.: Madison, WI, 2001.
(15) Sheldrick, G. M. SADABS; 1996.
(3) (a) Katritzky, A. R.; Rachwal, S.; Rachwal, B. Tetrahedron 1996,
52, 15031. (b) Rauckman, B. S.; Tidwell, M. Y.; Jonson, J. V.; Roth, B.
J. Med. Chem. 1989, 32, 1942. (c) Anzini, M.; Cappelli, A.; Vomero, S.;
Giorgi, G.; Langer, T.; Hamon, M.; Merahi, N.; Emerit, B. M.;
Cagnotto, A.; Skorupska, M.; Mennini, T.; Pinto, J. C. J. Med. Chem.
1995, 38, 2692.
(16) SHELXTL version 5.1; Bruker AXS Inc.: Madison, WI, 1997.
(4) (a) Theoclitou, M.-E.; Robinson, L. A. Tetrahedron Lett. 2002, 43,
3907. (b) Kothandaraman, P.; Foo, S. J.; Chan, P. W. H. J. Org. Chem.
2009, 74, 5947. (c) Larock, R. C.; Hightower, T. R.; Hasvold, L. A.;
Peterson, K. P. J. Org. Chem. 1996, 61, 3584. (d) Murase, H.; Senda,
K.; Senoo, M.; Hata, T.; Urabe, H. Chem.Eur. J. 2014, 20, 317.
(5) (a) Colby, D. A.; Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010,
110, 624. (b) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879.
(c) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.;
Hartwig, J. F. Chem. Rev. 2010, 110, 890. (d) Alberico, D.; Scott, M.
E.; Lautens, M. Chem. Rev. 2007, 107, 174. (e) Lyons, T. W.; Sanford,
M. S. Chem. Rev. 2010, 110, 1147. (f) Arockiam, P. B.; Bruneau, C.;
Dixneuf, P. H. Chem. Rev. 2012, 112, 5879. (g) Ackermann, L. Chem.
Rev. 2011, 111, 1315. (h) Auckermann, L.; Vicente, R.; Kapdi, A. R.
Angew. Chem., Int. Ed. 2009, 48, 9792. (i) Kakiuchi, F.; Kochi, T.
Synthesis 2008, 3013.
(6) (a) Larock, R. C.; Yum, Y. K. J. Am. Chem. Soc. 1991, 113, 6689.
(b) Barluenga, J.; Aznar, F.; Palomero, M. A. Angew. Chem., Int. Ed.
2000, 39, 4343. (c) Dotz, K. H.; Stendel, J. Chem. Rev. 2009, 109,
3227. (d) Korivi, R. P.; Cheng, C. H. J. Org. Chem. 2006, 71, 7079.
(e) Sun, Z. M.; Chen, S. P.; Zhao, P. Chem.Eur. J. 2010, 16, 2619.
(f) Satoh, T.; Miura, M. Chem.Eur. J. 2010, 16, 11212. (g) Tang, Q.;
Xia, D.; Jin, X.; Zhang, Q.; Sun, X.; Wang, C. J. Am. Chem. Soc. 2013,
135, 4628. (h) Martin, R. M.; Bergman, R. G.; Ellman, J. A. J. Org.
Chem. 2012, 77, 2501. (i) Seo, J.; Park, Y.; Jeon, I.; Ryu, T.; Park, S.;
Lee, P. H. Org. Lett. 2013, 15, 3358. (j) Dooley, J. D.; Chidipudi, S. R.;
Lam, H. W. J. Am. Chem. Soc. 2013, 135, 10829. (k) Zhou, B.; Chen,
H.; Wang, C. J. Am. Chem. Soc. 2013, 135, 1264.
(7) (a) Guimond, N.; Fagnou, J. J. Am. Chem. Soc. 2009, 131, 12050.
(b) Colby, D. A.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2008,
130, 3645. (c) Lim, S. G.; Lee, J. H.; Moon, C. W.; Hong, J. B.; Jun, C.
H. Org. Lett. 2003, 5, 2759. (d) Parthasarathy, K.; Jeganmohan, M.;
Cheng, C. H. Org. Lett. 2008, 10, 325. (e) Parthasarathy, K.; Cheng, C.
H. J. Org. Chem. 2009, 74, 9359. (f) Fukutani, T.; Umeda, N.; Hirano,
K.; Satoh, T.; Miura, M. Chem. Commun. 2009, 34, 5141. (g) Too, P.
C.; Wang, Y. F.; Chiba, S. Org. Lett. 2010, 12, 5688. (h) Too, P. C.;
Chua, S. H.; Wong, S. H.; Chiba, S. J. Org. Chem. 2011, 76, 6159.
(i) Zhang, X.; Chen, D.; Zhao, M.; Zhao, J.; Jia, A.; Li, X. Adv. Synth.
Catal. 2011, 353, 719.
(8) (a) Kumaran, E.; Sridevi, V. S.; Leong, W. K. Organometallics
2010, 29, 6417. (b) Kumaran, E.; Leong, W. K. Organometallics 2012,
31, 1068. (c) Kumaran, E.; Leong, W. K. Organometallics 2012, 31,
D
DOI: 10.1021/om501253v
Organometallics XXXX, XXX, XXX−XXX