Highly efficient and eco-friendly gold-catalyzed synthesis of homoallylic ketones

Article abstract of DOI:10.1021/cs500806m

We report a new catalytic protocol for the synthesis of γ,δ-unsaturated carbonyl units from simple starting materials, allylic alcohols and alkynes, via a hydroxalkoxylation/Claisen rearrangement sequence. This new process is more efficient (higher TON an

Full text of DOI:10.1021/cs500806m

Letter
pubs.acs.org/acscatalysis
Highly Efficient and Eco-Friendly Gold-Catalyzed Synthesis of
Homoallylic Ketones
Adrian Gomez-Suarez,† Danila Gasperini,† Sai V. C. Vummaleti,‡ Albert Poater,§ Luigi Cavallo,‡,∥
́
́
́
and Steven P. Nolan*,†
†EaStCHEM School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K.
‡KAUST Catalysis Center, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal
23955-6900, Saudi Arabia
§Institut de Química Computacional i Catal
Girona, Catalonia, Spain
̀
isi, Departament de Química, University of Girona, Campus de Montilivi sn, 17071
∥Dipartimento di Chimica e Biologia, Universita’ di Salerno, Via Papa Paolo Giovanni II, 84084 Fisciano, Italy
S
* Supporting Information
ABSTRACT: We report a new catalytic protocol for the synthesis of γ,δ−unsaturated carbonyl units from simple starting
materials, allylic alcohols and alkynes, via a hydroxalkoxylation/Claisen rearrangement sequence. This new process is more
efficient (higher TON and TOF) and more eco-friendly (increased mass efficiency) than the previous state-of-the-art technique.
In addition, this method tolerates both terminal and internal alkynes. Moreover, computational studies have been carried out in
order to shed light on how the Claisen rearrangement is initiated.
KEYWORDS: gold, hydroalkoxylation, Claisen rearrangement, homoallylic ketones, solvent-free
ince the establishment of the 12 principles of Green
Chemistry,1 almost 15 years ago, chemists have been
γ,δ−Unsaturated carbonyl units are important structural
motifs in natural products and also serve as highly versatile
building blocks in organic synthesis.7 Several strategies have
been developed for the synthesis of this type of organic
molecules (Scheme 1).8 Aponick and co-workers have recently
reported an elegant approach for the synthesis of this
S
interested in developing more efficient, robust, and environ-
mentally friendly synthetic protocols.2 One of the outcomes of
this quest has been the development of several metrics to assess
how green is any given chemical process (e.g., atom economy,
reaction mass efficiency, E-factor, etc).3 An additional target of
Green Chemistry has been to reduce to a minimum the use of
auxiliary substances.1 In this context, the development of
solvent-free reactions has greatly contributed to reduce the
environmental impact of chemical processes.4 In addition,
because the use of catalytic reagents is highly desirable in a
Green Chemistry context, terms such as turnover number
(TON) and turnover frequency (TOF) have shown most
informative in describing the efficiency of catalytic reactions.
Gold has become a powerful tool in organic chemistry,
allowing access to important structural motifs.5 Its use has
grown exponentially during the last 10 years, and new reports
are published highlighting its catalytic performance at an almost
daily rate. However, a search of the literature reveals that the
majority of gold-catalyzed transformations require relatively
high catalyst loadings (1−10 mol %),5 affording TON in the
range of 10−100. Protocols describing the use of lower catalyst
loadings are scarce.6 Therefore, the design of more efficient
catalytic processes involving gold is highly desirable.
Scheme 1. Strategies toward Homoallylic Ketones
Received: June 11, 2014
Revised: July 10, 2014
Published: July 15, 2014
© 2014 American Chemical Society
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interesting family of compounds using gold catalysis.8e They
proposed that homoallylic ketones could be synthesized
through a tandem hydroxalkoxylation/Claisen rearrangement
process, starting from internal alkynes and allylic alcohols
(Scheme 1E).8e Although this methodology offers an easy and
straightforward approach, it presents several disadvantages,
such as the following: (a) the use of high catalyst loadings (5
mol %), thus affording low TON (10−20) and TOF (0.5 to 1
h−1); (b) the need for an expensive silver salt (AgBF4)
activator; and (c) a 3:1 alkyne/alcohol ratio, thus affording a
reaction mass efficiency of 37.8%.8e
Scheme 2. Scope of the Reaction Using Internal Alkynes. [a]
Reaction Conditions: 1 (1 equiv), 2 (3 equiv), [Au] (0.2 mol
%), 120 °C. Isolated Yields, Average of Two Runs. [b] [Au]
(0.5 mol %). [c] [Au] (0.6 mol %)
We are interested in the development of more efficient,
economical, and environmentally friendly processes involving
NHC-AuI catalysts for the synthesis of valuable organic
molecules.9 Therefore, we hypothesized that it should be
possible to improve the elegant approach described by Aponick
in order to establish a greener and more robust protocol for the
synthesis of homoallylic ketones from allylic alcohols and
alkynes. In order to achieve this, we reasoned that the following
points needed to be addressed: (a) avoid the use of additives to
activate the gold catalyst, (b) reduce the environmental impact
of the reaction, (c) achieve high TON and TOF, and (d)
increase the mass efficiency. We began our investigations
focusing on the first two points. In order to avoid the use of
silver salts,10 which are light- and moisture-sensitive as well as
costly, we opted to use [Au(NHC)(MeCN)][X]11 (where X =
BF4, SbF6, or FABA) and [Au(NHC)NTf2]12 complexes during
our optimization studies. Additionally in order to reduce the
environmental impact of the reaction, we thought best to
remove the need for a reaction solvent and use the allylic
alcohol itself as the solvent. Initially, we reacted diphenylace-
tylene (1a) with allylic alcohol (2a), in a 1:16 ratio, using 1 mol
% of [Au(IPr)(MeCN)][BF4]. After 30 min at 120 °C, 98%
conversion to the desired homoallylic ketone 3aa was observed.
It is worth noting that even if a large excess of 2a was used, no
side products due to ketalization of the vinyl ether intermediate
were observed.13 Next, we decided to reduce the alkyne/
alcohol ratio to 1:3 and to reduce the catalyst loading to 0.3
mol %. After 30 min, 96% conversion to 3aa was observed.
Further optimization studies were conducted and can be found
in the Supporting Information. We concluded that the most
effective reaction conditions were 0.2 mol % of [Au(IPrCl)-
NTf2], 1:3 alkyne/alcohol ratio, at 120 °C.
Having established the optimized reaction conditions, we
next explored the scope and limitations of our methodology
(Scheme 2). Once more challenging substrates were tested, it
became obvious that longer reaction times were required.
Therefore, for practical reasons, we carried out reactions
between 16−21 h. In addition, when significant amounts of
ketone side products were observed, due to hydration of the
alkyne,6a,b,f,h,14 reactions were carried out under an argon
atmosphere in the presence of molecular sieves (4 Å). We
began our studies by reacting several allylic alcohols (2b−f)
with diphenylacetylene (1a). We were pleased to observe that
the use of the secondary allylic alcohol 2b afforded the
corresponding homoallylic ketone 3ab in excellent yields
(92%). However, if the steric hindrance about the reactive
centers is increased, the reaction does not proceed. The use of
α-vinylbenzyl alcohol only afforded traces of the addition
product (>5%) and no homoallylic ketone was observed after
24 h. Adding substituents to the α-position of the allylic alcohol
did not affect the reaction and it proceeded smoothly, affording
the desired product 3ac in good yields (78%). Varying the
substituent in the β−position of the allylic alcohol (2d−f),
afforded compounds 3ad−f in moderate to excellent yields
(50−93%). In addition, the observed diasteroselectivities were
in agreement with those reported by Aponick.8e Once the
substituents on the alcohol were evaluated, we next examined
the effect of modifying the alkyne. Changing 1a for a more
electron-rich alkyne, 1b, still afforded good yields (82%). We
were pleased to observe that even the use of a sterically
hindered alkyne (1c) was well tolerated and 3ca was obtained
in good yields (77%). Heteroaromatic substituted alkynes, such
as 1d, were also well tolerated by our methodology and
afforded the corresponding homoallylic ketone 3da in good
yields (63%). The reaction using dialkyl-substituted alkynes
proceeded smoothly with both primary and secondary allylic
alcohols, affording 3ea and 3eb in good yields (78 and 70%,
respectively). In order to assess the regioselectivity of the
reaction, three unsymmetrically substituted internal alkynes
were also tested, 1f−h. The use of 1-chloro-4-(phenylethynyl)-
benzene (1f) had no preferential effect on the regioselectivity
and a 1:1 ratio between the two possible isomers was observed
with both 2a and 2b. However, the reaction of 1-nitro-4-
(phenylethynyl)benzene (1g) and allyl alcohol 2a afforded only
one product 3ga in excellent yields, 98%. Finally, 1-methoxy-4-
(phenylethynyl)benzene (1h) was also tested. The reaction of
1h with 2a afforded a 3:1 ratio between both possible
regioisomers.
Encouraged by these results, we expanded the intermolecular
hydroxalkoxylation/Claisen rearrangement strategy to the use
of terminal alkynes (Scheme 3). This transformation proved to
be more challenging than the reaction using internal alkynes.
Therefore, the catalyst loading was increased to 0.5 mol %.
Gratifyingly, the reaction of allyl alcohol 2a with several
phenylacetylene derivatives, 1i−m, afforded the desired
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Scheme 3. Reaction between Allylic Alcohols and Terminal
Alkynes. [a] Reaction Conditions: 1 (1 equiv), 2 (3 equiv),
[Au] (0.5 mol %), 120 °C. Isolated Yields, Average of Two
Runs
Scheme 5. Proposed Reaction Mechanism
to determine whether this last step was thermally or gold-
catalyzed.8e After isolation of the corresponding vinyl ether
intermediate, the Claisen rearrangement in the presence and
absence of gold catalyst under the same reaction conditions
(THF, 65 °C, 18 h) was tested.8e Unfortunately, both reactions
afforded very similar conversions (40 and 30%, respectively).8e
As the experimental results reported by Aponick were quite
similar,8e we turned to computational analysis for support.
γ,δ−unsaturated ketones 3ia−3ma in moderate to good yields
(48−77%).
Both internal and terminal alkynes were tolerated by our
methodology, and the corresponding homoallylic ketones were
isolated in moderate to excellent yields. Once the scope of the
reaction was examined, TON and TOF were evaluated. The
new catalytic process afforded TON from 96 to 465 and TOF
ranging from 6 to 1396 h−1. To relate this to the previous state-
of-the-art method, the Aponick procedure afforded TONs from
10 to 20 and TOFs from 0.5 to 1 h−1.8e We were also interested
in assessing the mass efficiency of our methodology.3 By
changing the molar ratio between the alkyne and the allyl
alcohol to 1:3, instead of the 3:1 from the previous procedure,
we were able to increase the mass efficiency from 37.8 to
61.7%.15 In addition, we also assessed the recyclability of our
catalyst. Once the reaction involving 1a and 2a in the presence
of 0.2 mol % of [Au(IPrCl)(NTf2)] was complete, iterative
additions of both substrates (0.5 mmol of each) were
conducted. As a result, 5.22 mmol of 2a were converted over
14 h by using 2 μmol of [Au(IPrCl)(NTf2)], affording a TON
of 2610 and a TOF of 186 h−1 (Scheme 4).
COMPUTATIONAL STUDIES
■
We considered the thermodynamics and energy barrier for the
last step of the hydroalkoxylation/Claisen rearrangement
process shown in Scheme 5, to understand whether the
Claisen process is thermally or gold catalyzed. On the basis of
previous mechanistic studies,16 three different scenarios were
envisioned: (a) thermal rearrangement from compound III; (b)
gold mediated rearrangement from vinyl gold intermediate II;
(c) gold-catalyzed rearrangement from compound III via a
Au−O interaction (IV) or π-coordination of the Au to the
vinylic olefin (V). In order to best mimic the experimental
conditions, the substrates chosen for the in silico work were
diphenylacetylene (1a) and allyl alcohol (2a). Geometries were
optimized with the BP86 functional in connection with the SVP
basis set for main group atoms and the SDD ECP, together
with the associated triple-ζ valence basis set for Au. The
reported reaction free energies (ΔG in kcal/mol) were built
through single point energy calculations on the BP86/SVP
geometries using the M06 functional and the triple-ζ TZVP
basis set on main group atoms. Solvent effects were included
with the PCM model using THF as the solvent. To these M06/
TZVP electronic energies in solvent, zero point energy and
thermal corrections were included from the gas-phase
frequency calculations at the BP86/SVP level.
Scheme 4. Iterative Additions of 1a and 2a to the Catalytic
Reaction
Figure 1 shows the thermodynamics and energy barrier for
the Claisen step with and without gold for the above-
mentioned scenarios, and Figure 2 presents the corresponding
transition state (TS) structures. Focusing on energy, the
calculated ΔG of the reaction shows that it is clearly exergonic
in all the three cases. Moving to the energy barrier for the
transformation, ΔG‡, it is lower in the presence of the gold
catalyst, with a ΔG‡ of 21.2 and 24.5 kcal/mol from IV and II,
whereas in the absence of gold, the ΔG‡, 26.0 kcal/mol, is
clearly higher.
Once the scope and limitations of our methodology were
established we began to probe the reaction mechanism. A
typical hydroxalkoxylation mechanism was hypothesized for the
initial step of the tandem process (Scheme 5). First,
coordination of the gold catalyst to the alkyne (1) to form a
π−gold−alkyne complex (I), thus activating it toward
nucleophilic attack from the allyl alcohol (3) in anti-fashion.
This would afford a vinyl gold intermediate (II). Finally,
protodeauration of II would afford the vinyl ether species III.
Then, rearrangement of the latter would afford the desired
homoallylic ketone 3. Aponick and co-workers have attempted
This strongly suggests that there is a significant preference
for the isomerization step in the presence of gold, especially for
IV. Attempts to locate a transition state for the rearrangement
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Figure 1. Thermodynamics of the hydroalkoxylation step from intermediates II (blue lines), III (pink lines), and IV (orange lines). Free energies,
ΔG, are given in kcal/mol.
ASSOCIATED CONTENT
* Supporting Information
■
S
Optimization studies, characterization data for all the
compounds, computational details, and Cartesian coordinates.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: snolan@st-andrews.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The ERC (Advanced Investigator Award-FUNCAT), EPSRC
and Syngenta are gratefully acknowledged for support. Umicore
AG is acknowledged for their generous gift of materials. The
EPSRC National Mass Spectrometry Service Centre (NMSSC)
is gratefully acknowledged for HRMS analyses. S.P.N. is a Royal
Society Wolfson Research Merit Award holder. S.P.N. and L.C.
thank King Abdullah University of Science and Technology
(CCF project) for support. L.C. thanks the HPC team of Enea
for using the ENEA-GRID and the HPC facilities CRESCO in
Portici (Italy) for access to remarkable computational
Figure 2. Transition state structures along with the important bond
lengths in Å.
́
resources. A.P. thanks the Spanish MINECO for a Ramon y
Cajal contract (RYC-2009-05226) and European Commission
for a Career Integration Grant (CIG09-GA-2011-293900).
of π-complex V failed, suggesting that this route is not viable.
The geometry of the transition states connecting II, III, and IV
to the products are shown in Figure 2.
ABBREVIATIONS
■
NHC, N-heterocylic carbene; FABA, tetrakis(pentafluoro-
phenyl)borate; IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene; IPrCl, 4,5-dichloro-1,3-bis(2,6-diisopropylphenyl)-1H-
imidazol-2-ylidene
In summary, we have developed a new catalytic protocol for
the synthesis of homoallylic ketones from alkynes and allylic
alcohols via a hydroalkoxylation/Claisen rearrangement
sequence. This new procedure provides several advantages
over the existing state-of-the-art, such as (a) higher TONs and
TOFs, (b) higher mass efficiency, and (c) the absence of
additives to activate the gold catalyst. In addition, by removing
the need for auxiliary solvents, a more eco-friendly synthetic
protocol has been developed. Moreover, computational studies
revealed that the Claisen rearrangement step is, most likely,
aided by the presence of the gold catalyst. Ongoing studies are
aimed at exploring further this intriguing transformation in
conjunction with other tandem processes.
REFERENCES
■
(1) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: Oxford, U.K., 1998.
(2) (a) Irina, P. B.; Leonid, M. K. Russ. Chem. Rev. 2010, 79, 441.
̌
(b) Lipshutz, B.; Abela, A.; Boskovic, Z.; Nishikata, T.; Duplais, C.;
̌ ́
Krasovskiy, A. Top. Catal. 2010, 53, 985−990. (c) Parmeggiani, C.;
Cardona, F. Green Chem. 2012, 14, 547−564. (d) Jessop, P. G.;
Mercer, S. M.; Heldebrant, D. J. Energy Environ. Sci. 2012, 5, 7240−
7253. (e) Boisvert, L.; Goldberg, K. I. Acc. Chem. Res. 2012, 45, 899−
910. (f) Ciriminna, R.; Pagliaro, M. Org. Process Res. Dev. 2013, 17,
1479−1484.
2704
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Letter
(3) Constable, D. J. C.; Curzons, A. D.; Cunningham, V. L. Green
Chem. 2002, 4, 521−527.
(15) Calculations based on isolated yields of 3aa: 92% for our
methodology, 95% for Aponick.
(16) (a) Ganem, B. Angew. Chem., Int. Ed. 1996, 35, 936−945.
(b) Gourlaouen, C.; Marion, N.; Nolan, S. P.; Maseras, F. Org. Lett.
2009, 11, 81−84. (c) Vidhani, D. V.; Cran, J. W.; Krafft, M. E.;
Manoharan, M.; Alabugin, I. V. J. Org. Chem. 2012, 78, 2059−2073.
(d) Vidhani, D. V.; Cran, J. W.; Krafft, M. E.; Alabugin, I. V. Org.
Biomol. Chem. 2013, 11, 1624−1630. (e) Rehbein, J.; Hiersemann, M.
Synthesis 2013, 45, 1121−1159.
(4) (a) Singh, M. S.; Chowdhury, S. RSC Adv. 2012, 2, 4547−4592.
(b) Gawande, M. B.; Bonifacio, V. D.; Luque, R.; Branco, P. S.; Varma,
R. S. ChemSusChem 2013, 7, 24−44.
(5) (a) Hashmi, A. S. K. Gold Bull. 2004, 37, 51−65. (b) Hashmi, A.
S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896−7936.
(c) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180−3211. (d) Hashmi,
A. S. K.; Rudolph, M. Chem. Soc. Rev. 2008, 37, 1766−1775.
(e) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776−1782.
(f) Liu, L.; Xu, B.; Hammond, G. B. Beilstein J. Org. Chem. 2011, 7,
606−614. (g) Krause, N.; Winter, C. Chem. Rev. 2011, 111, 1994−
2009. (h) Corma, A.; Leyva-Perez, A.; Sabater, M. J. Chem. Rev. 2011,
111, 1657−1712. (i) Boorman, T. C.; Larrosa, I. Chem. Soc. Rev. 2011,
40, 1910−1925.
(6) (a) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed.
1998, 37, 1415−1418. (b) Mizushima, E.; Sato, K.; Hayashi, T.;
Tanaka, M. Angew. Chem., Int. Ed. 2002, 41, 4563−4565. (c) Comotti,
M.; Della Pina, C.; Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed.
2004, 43, 5812−5815. (d) Hashmi, A. S. K.; Weyrauch, J. P.; Rudolph,
M.; Kurpejovic,
(e) Zhou, C.-Y.; Chan, P. W. H.; Che, C.-M. Org. Lett. 2005, 8, 325−
328. (f) Marion, N.; Ramon, R. S.; Nolan, S. P. J. Am. Chem. Soc. 2008,
131, 448−449. (g) Nun, P.; Egbert, J. D.; Oliva-Madrid, M.-J.; Nolan,
, R. S.;
́
E. Angew. Chem., Int. Ed. 2004, 43, 6545−6547.
́
S. P. Chem.Eur. J. 2011, 18, 1064−1067. (h) Nun, P.; Ramon
́
Gaillard, S.; Nolan, S. P. J. Organomet. Chem. 2011, 696, 7−11.
(i) Bouhrara, M.; Jeanneau, E.; Veyre, L.; Coperet, C.; Thieuleux, C.
Dalton Trans. 2011, 40, 2995−2999. (j) Hashmi, A. S. K.; Loos, A.;
Doherty, S.; Knight, J. G.; Robson, K. J.; Rominger, F. Adv. Synth.
Catal. 2011, 353, 749−759. (k) Oliver-Meseguer, J.; Cabrero-
́
Antonino, J. R.; Domínguez, I.; Leyva-Perez, A.; Corma, A. Science
2012, 338, 1452−1455. (l) Xi, Y.; Wang, D.; Ye, X.; Akhmedov, N. G.;
Petersen, J. L.; Shi, X. Org. Lett. 2013, 16, 306−309. (m) Blanco
Jaimes, M. C.; Bohling, C. R. N.; Serrano-Becerra, J. M.; Hashmi, A. S.
̈
K. Angew. Chem., Int. Ed. 2013, 52, 7963−7966. (n) Canseco-
Gonzalez, D.; Petronilho, A.; Mueller-Bunz, H.; Ohmatsu, K.; Ooi, T.;
Albrecht, M. J. Am. Chem. Soc. 2013, 135, 13193−13203. (o) Gom
́
ez-
Suarez, A.; Oonishi, Y.; Meiries, S.; Nolan, S. P. Organometallics 2013,
́
32, 1106−1111. (p) Wang, Y.; Wang, Z.; Li, Y.; Wu, G.; Cao, Z.;
Zhang, L. Nat. Commun. 2014, 5, 3470. (q) Blanco Jaimes, M. C.;
Rominger, F.; Pereira, M. M.; Carrilho, R. M. B.; Carabineiro, S. A. C.;
Hashmi, A. S. K. Chem. Commun. 2014, 50, 4937−4940.
(7) (a) Yamamoto, I.; Tanaka, S.; Fujimoto, T.; Ohta, K. J. Org.
Chem. 1989, 54, 747−750. (b) Yoshida, J.-i.; Nakatani, S.; Isoe, S.
Tetrahedron Lett. 1990, 31, 2425−2428. (c) Chen, F.; Mudryk, B.;
Cohen, T. Tetrahedron 1994, 50, 12793−12810. (d) Hutton, T. K.;
Muir, K.; Procter, D. J. Org. Lett. 2002, 4, 2345−2347. (e) Agatsuma,
T.; Ogawa, H.; Akasaka, K.; Asai, A.; Yamashita, Y.; Mizukami, T.;
Akinaga, S.; Saitoh, Y. Bioorg. Med. Chem. 2002, 10, 3445−3454.
(8) (a) Yasuda, M.; Tsuji, S.; Shigeyoshi, Y.; Baba, A. J. Am. Chem.
Soc. 2002, 124, 7440−7447. (b) Hansford, K. A.; Dettwiler, J. E.;
Lubell, W. D. Org. Lett. 2003, 5, 4887−4890. (c) Yan, X.-X.; Liang, C.-
G.; Zhang, Y.; Hong, W.; Cao, B.-X.; Dai, L.-X.; Hou, X.-L. Angew.
Chem., Int. Ed. 2005, 44, 6544−6546. (d) Gribkov, D. V.; Hultzsch, K.
C.; Hampel, F. J. Am. Chem. Soc. 2006, 128, 3748−3759. (e) Ketcham,
J. M.; Biannic, B.; Aponick, A. Chem. Commun. 2013, 49, 4157.
(9) (a) Nolan, S. P. Acc. Chem. Res. 2011, 44, 91−100. (b) Gaillard,
S.; Cazin, C. S. J.; Nolan, S. P. Acc. Chem. Res. 2012, 45, 778−787.
(10) Schmidbaur, H.; Schier, A. Z. Naturforsch. 2011, 66b, 329−350.
́
(11) de Fremont, P.; Stevens, E. D.; Fructos, M. R.; Mar Diaz-
Requejo, M.; Perez, P. J.; Nolan, S. P. Chem. Commun. 2006, 2045−
2047.
(12) (a) Ricard, L.; Gagosz, F. Organometallics 2007, 26, 4704−4707.
́ ́
(b) Patrick, S. R.; Gomez-Suarez, A.; Slawin, A. M. Z.; Nolan, S. P.
Organometallics 2013, 33, 421−424.
́
(13) Corma, A.; Ruiz, V. R.; Leyva-Perez, A.; Sabater, M. J. Adv.
Synth. Catal. 2010, 352, 1701−1710.
(14) Leyva, A.; Corma, A. J. Org. Chem. 2009, 74, 2067−2074.
2705
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