Activation of C-H Activation: The Beneficial Effect of Catalytic Amount of Triaryl Boranes on Palladium-Catalyzed C-H Activation

Article abstract of DOI:10.1021/acs.organomet.5b01017

Herein we report a novel approach to the acceleration of palladium-catalyzed C-H activation reactions. We demonstrated that the utilization of electron-deficient triaryl boranes as Lewis acidic cocatalysts of palladium enables the directed cross dehydrogenative coupling of aldehydes and anilides under mild reaction conditions. Study of the kinetic profile of the transformation reveals a unique, unexpectedly long induction period of the transformation.

Full text of DOI:10.1021/acs.organomet.5b01017

Article
pubs.acs.org/Organometallics
Activation of C−H Activation: The Beneficial Effect of Catalytic
Amount of Triaryl Boranes on Palladium-Catalyzed C−H Activation
Orsolya Tischler, Zsofia Bokanyi, and Zoltan Novak*
́
́
́
́
MTA-ELTE “Lendulet” Catalysis and Organic Synthesis Research Group, Institute of Chemistry, Eotvos Loran
́
d University, Paz
́
man
́
y
̈
̈
̈
Pet
́
er stny. 1/a, Budapest 1117, Hungary
S
* Supporting Information
ABSTRACT: Herein we report a novel approach to the
acceleration of palladium-catalyzed C−H activation reactions.
We demonstrated that the utilization of electron-deficient
triaryl boranes as Lewis acidic cocatalysts of palladium enables
the directed cross dehydrogenative coupling of aldehydes and
anilides under mild reaction conditions. Study of the kinetic
profile of the transformation reveals a unique, unexpectedly
long induction period of the transformation.
Electron-deficient boron compounds represent a powerful
and well-studied4 group of Lewis acids in synthesis (Scheme 1).
They are widely used in polymer chemistry5 and aldol and
Michael reactions.6 Most recently, they serve as excellent Lewis
acid partners for Frustrated Lewis Pair (FLP) systems.7
INTRODUCTION
■
Transition-metal-catalyzed C−H activation reactions are widely
used, efficient synthetic tools for the direct functionalization of
C−H bonds, even in complex molecules.1 These C−H bond
functionalization reactions typically require relatively high
temperature. For the sake of wider applicability, activation of
the catalytic system is desirable, which is in principle possible
either by enhancing the catalytic activity of the transition metal
or activation of the substrate itself. Based on the current
mechanistic understanding of the reaction, the barriers of the
elementary steps can be reduced through the development of
new catalysts, better reaction conditions, and beneficial
additives. Considering that the opening step of the C−H
activation process may occur via concerted metalation
deprotonation (CMD a.k.a. IES/AMLA) or electrophilic
metalation pathways, the utilization of transition-metal catalysts
Scheme 1. Synthetic Applications of B(C6F5)3
2
−
−
with noncoordinating anions such as BF4 or SbF6 , could
facilitate the directed ortho-metalation by the participation of
more electrophilic “naked” metal ion.
In the past few years, Pd-catalyzed C(sp2)-H activation
reactions on arenes bearing diverse ortho-directing groups have
been widely studied, including ortho acylations1n,3 using
different acyl equivalents. In continuation of our recent studies
on C−H activation,3j,k we aimed to examine the effect of Lewis
acids on the palladium-catalyzed cross-dehydrogenative cou-
pling of anilides and aldehydes, as a conceptually new approach
for the activation of the catalytic system. For our studies of
palladium-catalyzed oxidative coupling of anilides and alde-
hydes, we selected electron-deficient triarylboranes as well-
defined organic Lewis acids rather than inorganic compounds,
due to the simplification of the system and to avoid any redox
interaction of different metals. We hypothesize that the
presence of organic borane-based Lewis acidic additives in
the reaction may also accelerate the C−H activation reaction
via their interaction with the palladium catalyst.
We aimed to study the effect of various borane derivatives
with different electronic properties on the target palladium-
catalyzed transformation.
RESULTS AND DISCUSSION
■
The Lewis acid-free model reaction was performed as a
reference with the attempted coupling of acetanilide and 4-
fluorobenzaldehyde in the presence of Pd(OAc)2 catalyst in
t
dichloromethane (DCM) at 30 °C using BuOOH (5.5 M
solution in decane) as the oxidant. In the absence of Lewis acid,
the reaction is sluggish, and the desired acetamidobenzophe-
none was formed only in 8% conversion even after 24 h. We
tested four triarylborane derivatives with different properties as
Received: December 18, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b01017
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Lewis acid additives (Scheme 2). Among the mesityl
substituted borane-based Lewis acids, trimesityl or the bis-
Scheme 2. Conversions of the Same Reaction after 24 h with
a
the Triaryl Boranes as Additives
Figure 1. 19F NMR shifts of B(C6F5)3 (bottom) and effect of
additional Pd(OAc)2 (middle) or NH4OAc (top) in CD2Cl2 under N2
atmosphere.
a
Conversions were determined with GC.
tetrafluorophenyl derivative provide 5−7% conversion, whereas
the more-electron-deficient bis-perfluorophenyl-mesitylborane
provides coupled product in 94% conversion. Utilization of tris-
pentafluorophenyl borane as the most Lewis acidic BR3 additive
ensured the full conversion of the reaction in 24 h at 30 °C.
In comparison to organic boranes, we tested simple inorganic
boranes as hard Lewis acids. The reactions performed in the
presence of hard Lewis acidic boron trihalides such as BCl3 and
BBr3 did not provide the expected product. It is well-known
that halide anions have a significant deleterious effect on the
C−H activation step as these inorganic hard Lewis acids can
serve as halide sources in the reaction and block the catalytic
cycle. This comparative study revealed the special role and
strong influence of electron deficient triaryl boranes on the
palladium-catalyzed C−H activation.
amides.8 19F NMR studies were run to investigate the affinity of
the borane toward each reagent. When the borane was mixed
with one of the reagents, the formation of new borane species
was observed in solution by 19F NMR measurements (see SI,
Chapter 4).
Next, we prepared the borane−benzaldehyde complex and
measured the IR spectra of the compound in the solid form.
The characteristic peak of carbonyl group in the complex was
found at 1620 cm−1, which is in good agreement with the
literature data.8 However, when the IR spectra of the prepared
complex was taken in DCM or toluene solution, the
characteristic peak of carbonyl−borane interaction disappeared.
This comparative study revealed that the interaction between
the borane and the Lewis base is weaker in solution.
Nevertheless, we can still expect several possible interactions
of the borane species with the LB partners present. It was
demonstrated by Piers that the least basic component had the
lowest tendency to interact with the LA to form Lewis acid−
base adduct in the reaction mixture and the more Lewis basic
components straightforwardly displaces the weaker one from
their borane Lewis acid−base adduct.8 This well-documented
phenomenon, supports the preferred interaction of the more
Lewis basic acetate ion with the borane even in the presence of
less Lewis basic carbonyl compounds. However, interactions of
other Lewis basic components with the free borane in
equilibrium cannot be ruled out completely. Unfortunately,
the monitoring of the full reaction mixture to determine any
intermediates by 19F NMR was not feasible due to the
complexity of the NMR spectra.
To overcome the difficulties in finding and identifying
interactions of the borane with any reactants and possible
intermediates of the catalytic cycle, we aimed to examine the
role of the borane in the catalytic transformation (Scheme 3).
To achieve this, we prepared the stable bimetallic palladium
complex 4 and examined its reactivity toward aldehyde in the
presence and absence of borane under the standard reaction
conditions. The acetate-bridged palladacycle could not be
isolated, and therefore, we used the more stable complex 4 for
this study.9 We observed similar conversions of complex 4 with
the coupling of aldehyde (Scheme 3) either in the presence or
the absence of borane (50% and 49% conversions; 39% and
To simplify the mechanistic studies, we first used 19F NMR
to examine the interaction of Pd(OAc)2 and B(C6F5)3 to find
evidence for the supposed catalyst activation via acetate
abstraction. Upon the addition of the palladium catalyst to
the dichloromethane solution of the borane, new sets of two
multiplets appeared immediately next to the parent borane
ortho, meta, and para fluorine signals [133 ppm (ortho), 163
ppm (meta), 153 ppm (para).]. This appearance of new signals
in the NMR spectra refers to the formation of new borane
complex(es) in the solution (Figure 1. middle spectrum). To
confirm the existence of borane−acetate Lewis acid−base
interaction in the case of Pd(OAc)2-B(C6F5)3, we also treated
the tris-pentafluorophenyl borane with NH4OAc, under the
same reaction conditions. The obtained 19F-NMR spectra of
this mixture provided the same pattern of the 19F shifts such as
the palladium−borane mixture (Figure 1, top spectrum). The
pattern of the spectra obtained in these 19F NMR experiments
supports the hypothesis that the addition of Lewis acidic borane
facilitates the formation of a more electrophilic palladium
species via acetate abstraction and the formation of acetate−
borane Lewis acid−base adduct.
On the basis of the previous experiments, we suppose that
acetate could interact with the borane, but we should also
consider that there are several potential Lewis basic partners
present in the reaction mixture to which the borane species can
coordinate. It is known that the tris-pentafluorophenyl borane
forms Lewis acid−base adducts with both aldehydes and
B
DOI: 10.1021/acs.organomet.5b01017
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 3. Reaction of Dimeric Palladium Complex 4 with 4-
Fluorobenzaldehyde in the Absence and Presence of
Triarylborane
42% isolated yield of 3a after 20 h reaction time). The result of
this comparative study supports our suggestion that the borane
has influence only in the carbopalladation step.
After demonstrating the importance of triarylborane in the
C−H activation, we aimed to determine the optimal palladium/
borane ratio for the reaction of 1a and 2a. When 2.5 mol %
palladium was used, we observed low conversions independent
from the borane loading (0−30 mol % range). An insignificant
maximum was observed in the conversions when 7 mol %
B(C6F5)3 was applied. However, in this case the conversion was
only 23% (Figure 2, top).
Next, we studied the coupling of acetanilide 1a and 4-
fluorobenzaldehyde (2a) in the presence of 5 mol % Pd(OAc)2
in dichloromethane (DCM) at 30 °C. The borane loading
effect was examined in the range of 0−100 mol % (Figure 2.
middle). Interestingly, we found an optimum for the borane
loading. The transformation reached the highest conversion
after 24 h reaction time, when the reaction was carried out in
the presence of 7 mol % B(C6F5)3. The borane amount−
conversion curves show either the lower or the higher amount
of borane has a deleterious effect on the transformation. We
suppose that the Lewis acidic species may facilitate the
formation of the more electrophilic palladium species by its
interaction of the acetate ion brought by the metal, whereas
overloading is undesirable as it gives way to coordination of the
Lewis acid to several reactants blocking the optimal reaction
pathway.
Interestingly, when 10 mol % palladium acetate was used as
catalyst, the optimal borane loading was 30 mol % to reach
almost full conversion in 24 h. In this case, we also observed the
same phenomenon regarding the influence of different Lewis
acid/palladium catalyst ratios on the reaction. Low borane
loading does not facilitate the reaction, but overloading of the
borane leads again to inhibition of the reaction (Figure 2,
bottom).
The effect of the palladium/borane ratio on the conversion
measured after 24 h, initiated us to gain deeper insight into the
transformation. First, we monitored the coupling of the anilide
and the aldehyde in the presence of 5 mol % Pd(OAc)2 and 7
mol % borane by sampling and GC analysis. To our surprise,
the time−conversion curve gave an unusual profile (Figure 3).
We observed that the homogeneous catalytic reaction has an
unexpectedly long induction period. We found no trans-
formation in the first 6 h of the reaction, but after this period,
the coupling reaction started and completed in 24 h overall
reaction time.
Figure 2. Effect of B(C6F5)3 loading on the palladium-catalyzed
coupling: 2.5 mol % Pd(OAc)2 (top), 5 mol % Pd(OAc)2 (middle), 10
mol % Pd(OAc)2 (bottom). Conversions were measured after 24 h
and determined with GC.
product (1637 and 925 cm−1) enabled the monitoring of its
formation (Figure 4).
In situ IR spectroscopy studies supported the previous results
obtained by GC analysis. The unexpected progress of the
product formation including unusually long induction period
was also confirmed with this study. Our study showed that in
reaction with 5% Pd(OAc)2 and 7.2% B(C6F5)3 the initiation
In order to exclude the accidental effect of sampling on the
transformation, we followed the coupling reaction of acetanilide
and 4-fluorobenzaldehyde at 30 °C with the aid of real time IR
spectroscopic analysis. Fortunately, two IR bands of the
C
DOI: 10.1021/acs.organomet.5b01017
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Reaction conditions: acetanilide (0.5 mmol), 4-fluorobenzal-
dehyde (1 mmol, 105 μL), Pd(OAc)2 (0.025 mmol, 5.6 mg),
B(C6F5)3 (0.036 mmol, 18 mg), TBHP (5.5 M in decane, 4 Å
M.S.) (1 mmol, 200 μL), anh. DCM (1 mL), nitrogen
atmosphere, 30 °C, 24 h. Yields of isolated materials are
indicated.
After the demonstration of the significant effect of borane
type Lewis acids on the coupling, we aimed to explore the
generality of the phenomenon in this palladium-catalyzed C−H
activation reaction. We performed several coupling reactions of
diverse anilides and aldehydes using tris-perfluorophenylborane
as the borane-based Lewis acid under mild reaction conditions
(DCM, 30 °C). Acylation of the electron-rich acetanilide
derivatives with 4-fluorobenzaldehyde in the presence of borane
provided the appropriate benzophenone derivatives (3a, 3c, 3d,
and 3f) in 40−99% yield (Scheme 4). Ortho-substituted and
Figure 3. Progress of the formation of product 3a monitored by gas
chromatography.
Scheme 4. Reactions of Different N-Aryl Acetamides
electron-deficient anilides are more challenging substrates for
this palladium-catalyzed reaction. Ortho-subtituted substrates
afforded ortho-acylated products 3b and 3e in 21 and 32% yield,
respectively, in the Lewis acid-accelerated coupling. Anilides
bearing chloro and fluoro function, as well as the naphthyl
derivative 3g−j were also coupled with 4-fluorobenzaldehyde,
and the desired products were obtained in 31−65% yield.
Next, we examined the reactivity of different aldehydes
toward acetanilide in Lewis acid-accelerated C−H activation
reactions (Scheme 5). We found that benzaldehyde derivatives
bearing electron-donating or electron-withdrawing substituents
on the phenyl ring provide the appropriate products (3k−v) in
44−91% yield. 2-Naphtaldehyde, phenylacetaldehyde, and
aliphatic aldehydes also were compatible with the reaction
conditions and the benzophenone derivatives (3w−z) were
isolated in 17−63% yield.
We envisioned the following putative catalytic cycle for the
transformation (Scheme 6). The highly electrophilic palladium
monoacetate cation forms from Pd(OAc)2 via acetate
abstraction by the assistance of the Lewis acidic triarylborane.
In the following step, the amide-directed ortho C−H activation
takes place resulting the carbopalladated species. Next, we
propose that the monomeric palladium complex dimerizes and
a bimetallic palladium intermediate forms. In our earlier work,
we demonstrated by quantum chemical calculations that the
Figure 4. Appearance and evolution of characteristic IR bands of
product 3a: (top) 1637, (bottom) 925 cm−1 during the first 16 h of
the coupling reaction.
period takes about 5.5 h. After this period of time, the catalytic
process starts and the formation of the coupling product was
observed. Although , at this point, we are not able to explain the
occurrence of the unprecedentedly long induction period, with
the aid of IR data analysis, the formation and decomposition of
a transient species was suspected during the initial period of the
catalytic transformation. While we did not detect any other
species in the reaction mixture during this initial period, we
suppose that the formation and decomposition of this transient
species prior to the formation of the product is important for
the transformation.
D
DOI: 10.1021/acs.organomet.5b01017
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 5. Reactions of Different Aldehydes with Acetanilide
CONCLUSION
■
Summarizing our results, we have demonstrated for the first
time that addition of electron-deficient tris-perfluorophenylbor-
ane as a Lewis acid additive has a significant influence on a
palladium-catalyzed C−H activation reaction. We have found
that the Lewis acid acceleration facilitates palladium-catalyzed
cross dehydrogenative couplings of aromatic and aliphatic
aldehydes with N-arylacetamide derivatives to access amino-
benzophenones. On the basis of NMR, IR spectroscopic and
experimental results, we hypothesize that a specific affinity of
the borane toward the transition metal catalyst giving way to
the evolution of a more electrophilic palladium species as active
catalyst. This should be adjusted by fine-tuning of the
electrophilicity of the Lewis acid. We have revealed the
importance of the optimal borane/palladium catalyst ratio for
the coupling, and we identified a catalytic transformation with
unique and unusual induction period. The mechanistic studies
of the transformation and the extension of this Lewis acid effect
in the field of C−H activation reactions are in progress in our
laboratory.
EXPERIMENTAL SECTION
■
General Method. Under nitrogen atmosphere (in glove-
box), a flame-dried screw cap 4 mL vial with a stirring bar was
charged with amide (0.5 mmol, 1 equiv) and Pd(OAc)2 (0.025
mmol, 5.6 mg, 5 mol %) and the mixture was dissolved in
anhydrous dichloromethane (1 mL). Tris(pentafluoro)phenyl
borane (0.036 mmol, 18 mg, 7.2 mol %), 4-fluoro-
benzaldehyde (1 mmol, 105 μL, 2 equiv), and TBHP solution
(5.5 M in decane, 4 Å M.S.) (1 mmol, 200 μL, 2 equiv) were
added, and the vial was closed with a septum screw cap. The
reaction mixture was removed from the glovebox and stirred at
30 °C for 18 h under inert atmosphere. The mixture was
diluted with DCM, washed with 1 N HCl solution and water.
The collected organic phases were dried with anhydrous
sodium sulfate, and the solvent was removed in vacuum. The
product was purified by column chromatography and dried in
vacuum.
Scheme 6. Proposed Mechanistic Picture
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b01017.
Experimental procedures, optimization studies, character-
ization data, and NMR spectra for all compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: novakz@elte.hu.
Notes
■
formation of dimeric palladium complex provides an energeti-
cally more favorable pathway compared with the cycle based
upon a monomeric species.
Reaction conditions: acetanilide (0.5 mmol, 67.5 mg),
aldehyde (1 mmol), Pd(OAc)2 (0.025 mmol, 5.6 mg),
B(C6F5)3 (0.036 mmol, 18 mg), TBHP (5.5 M in decane, 4
Å M.S.) (1 mmol, 200 μL), anh. DCM (1 mL), nitrogen
atmosphere, 30 °C, 24 h. Yields of isolated materials are
indicated.
Benzoyl radical generated from the aldehyde with TBHP
reacts with the dimer palladium complex, and the formed
benzoyl palladium species provides the acetamido benzophe-
none derivative in a reductive elimination step.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This project was supported by the “Lendulet” Research
Scholarship of the Hungarian Academy of Sciences (LP2012-
■
̈
48/2012) and research grant by National Research, Develop-
́
ment and Innovation Office TET-10-1-2011-0245. The
generous donation of mesytilboranes, help, and technical
support of Dr. Tibor Soos and co-workers at the Hungarian
́
Academy of Sciences is gratefully acknowledged. The authors
E
DOI: 10.1021/acs.organomet.5b01017
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124−1126.
(f) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535−1539.
also thank Prof. Tim Peelen (Lebanon Valley College) for the
proofreading of this manuscript.
(g) Rokob, T. A.; Hamza, A.; Stirling, A.; Pap
́
ai, I. J. Am. Chem. Soc.
s, G.; Mehdi, H.; Papai, I.; Rokob, T.
yi, G.; Soos, T. Angew. Chem., Int. Ed. 2010, 49,
6559−6563. (i) Hamza, A.; Stirling, A.; Rokob, T. A.; Papai, I. Int. J.
2009, 131, 2029−2036. (h) Ero
̋
́
REFERENCES
■
A.; Kiraly, P.; Tarkan
́
́
́
́
(1) (a) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879−
2932. (b) Crabtree, R. H. Chem. Rev. 1985, 85, 245−269. (c) Ryabov,
A. D. Chem. Rev. 1990, 90, 403−424. (d) Mousseau, J. J.; Charette, A.
B. Acc. Chem. Res. 2012, 46, 412−424. (e) Liu, C.; Zhang, H.; Shi, W.;
Lei, A. Chem. Rev. 2011, 111, 1780−1824. (f) Wencel-Delord, J.;
Droge, T.; Liu, F.; Glorius, F. Chem. Soc. Rev. 2011, 40, 4740−4761.
(g) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731−
1770. (h) Lapointe, D.; Fagnou, K. Chem. Lett. 2010, 39, 1118−1126.
(i) Davies, D. L.; Donald, M. A. S.; Macgregor, S. A. J. Am. Chem. Soc.
2005, 127, 13754−13755. (j) Boutadla, Y.; Davies, D. L.; Macgregor,
S. A.; Poblador-Bahamonde, A. I. Dalton Trans. 2009, 5887−5893.
(k) Oxgaard, J.; Tenn, W. J.; Nielsen, R. J.; Periana, R. A.; Goddard, W.
A. Organometallics 2007, 26, 1565−1567. (l) Lyons, T. W.; Sanford,
M. S. Chem. Rev. 2010, 110, 1147−1169. (m) Ackermann, L. Chem.
Rev. 2011, 111, 1315−1345. (n) Pan, C.; Jia, X.; Cheng, J. Synthesis
2012, 44, 677−685. (o) Wu, X.-F. Chem. - Eur. J. 2015, 21, 12252−
12265.
́
Quantum Chem. 2009, 109, 2416−2425.
(8) Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko, M.
J. Organometallics 1998, 17, 1369.
(9) The Lewis acid also had a beneficial effect on the catalytic activity
of Pd(TFA)2 similarly to Pd(OAc)2-catalyzed coupling. See SI.
(2) (a) Nishikata, T.; Abela, A. R.; Lipshutz, B. H. Angew. Chem., Int.
Ed. 2010, 49, 781−784. (b) Shi, Z.; Boultadakis-Arapinis, M.; Glorius,
F. Chem. Commun. 2013, 49, 6489−6491.
(3) (a) Yuan, Y.; Chen, D.; Wang, X. Adv. Synth. Catal. 2011, 353,
3373−3379. (b) Li, C.; Wang, L.; Li, P.; Zhou, W. Chem. - Eur. J. 2011,
17, 10208−10212. (c) Yin, Z.; Sun, P. J. Org. Chem. 2012, 77, 11339−
́
11344. (d) Basle, O.; Bidange, J.; Shuai, Q.; Li, C.-J. Adv. Synth. Catal.
2010, 352, 1145−1149. (e) Wu, Y.; Li, B.; Mao, F.; Li, X.; Kwong, F.
Y. Org. Lett. 2011, 13, 3258−3261. (f) Fang, P.; Li, M.; Ge, H. J. Am.
Chem. Soc. 2010, 132, 11898−11899. (g) Jia, X.; Zhang, S.; Wang, W.;
Luo, F.; Cheng, J. Org. Lett. 2009, 11, 3120−3123. (h) Guin, S.; Rout,
S. K.; Banerjee, A.; Nandi, S.; Patel, B. K. Org. Lett. 2012, 14, 5294−
5297. (i) Lu, J.; Zhang, H.; Chen, X.; Liu, H.; Jiang, Y.; Fu, H. Adv.
Synth. Catal. 2013, 355, 529−536. (j) Szabo,
Nagy, T. Z.; Stirling, A.; Novak, Z. Adv. Synth. Catal. 2013, 355, 685−
691. (k) Szabo, F.; Simko, D.; Novak, Z. RSC Adv. 2014, 4, 3883−
́ ́
F.; Daru, J.; Simko, D.;
́
́
́
́
3886. (l) Wu, Y.; Choy, P. Y.; Mao, F.; Kwong, F. Y. Chem. Commun.
2013, 49, 689−691. (m) Weng, J.; Yu, Z.; Liu, X.; Zhang, G.
Tetrahedron Lett. 2013, 54, 1205−1207. (n) Chan, C.-W.; Zhou, Z.;
Yu, W.-Y. Adv. Synth. Catal. 2011, 353, 2999−3006. (o) Sun, M.; Hou,
L.-K.; Chen, X.-X.; Yang, X.-J.; Sun, W.; Zang, Y.-S. Adv. Synth. Catal.
2014, 356, 3789−3793. (p) Schmidt, B.; Elizarov, N.; Schilde, U.;
Kelling, A. J. Org. Chem. 2015, 80, 4223−4234. (q) Banerjee, A.;
Santra, S. K.; Guin, S.; Rout, S. K.; Patel, B. K. Eur. J. Org. Chem. 2013,
2013, 1367−1376. (r) Luo, F.; Yang, J.; Li, Z.; Xiang, H.; Zhou, X. Eur.
J. Org. Chem. 2015, 2015, 2463−2469.
(4) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218−
225. (b) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345−354.
(5) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991,
113, 3623−3625. (b) Foley, S. R.; Stockland, R. A.; Shen, H.; Jordan,
R. F. J. Am. Chem. Soc. 2003, 125, 4350−4361. (c) Deck, P. A.; Marks,
T. J. J. Am. Chem. Soc. 1995, 117, 6128−6129. (d) Bochmann, M.;
Lancaster, S. J.; Hursthouse, M. B.; Malik, K. M. A. Organometallics
1994, 13, 2235−2243. (e) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J.
J. Chem. Soc., Dalton Trans. 1996, 1809−1813. (f) Pellecchia, C.;
Pappalardo, D.; Oliva, L.; Zambelli, A. J. Am. Chem. Soc. 1995, 117,
6593−6594. (g) Wang, Q.; Quyoum, R.; Gillis, D. J.; Tudoret, M.-J.;
Jeremic, D.; Hunter, B. K.; Baird, M. C. Organometallics 1996, 15,
693−703.
(6) (a) Ishihara, K.; Funahashi, M.; Hanaki, N.; Miyata, M.;
Yamamoto, H. Synlett 1994, 1994 (11), 963−964. (b) Ishihara, K.;
Hananki, N.; Yamamoto, H. Synlett 1993, 1993 (08), 577−579.
(7) (a) Erker, G.; Stephan, D. W. Frustrated Lewis Pairs I, 1st ed.;
Springer: Berlin, 2013; Vol. 332. (b) Erker, G.; Stephan, D. W.
Frustrated Lewis Pairs II, 1st ed.; Springer: Berlin, 2013; Vol. 334.
(c) Stephan, D. W. Acc. Chem. Res. 2015, 48, 306−316. (d) Spies, P.;
Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.; Grimme, S.; Stephan,
D. W. Chem. Commun. 2007, 5072−5074. (e) Welch, G. C.; Juan, R.
F
DOI: 10.1021/acs.organomet.5b01017
Organometallics XXXX, XXX, XXX−XXX