In situ preparation of a multifunctional chiral hybrid organic-inorganic catalyst for asymmetric multicomponent reactions

Article abstract of DOI:10.1039/c3sc22310h

A chiral mesoporous organosilica material incorporating a urea based-cinchona derivative and propylamine groups was prepared by a co-condensation method. The multisite solid catalyst efficiently promoted the asymmetric multicomponent reaction of aldehydes, malonates and nitromethane.

Full text of DOI:10.1039/c3sc22310h

Chemical Science
View Article Online
View Journal
EDGE ARTICLE
In situ preparation of a multifunctional chiral hybrid
organic–inorganic catalyst for asymmetric
multicomponent reactions†
Cite this: DOI: 10.1039/c3sc22310h
a
b
c
b
*
´
´
´
´
Pilar Garcıa-Garcıa, Alexandre Zagdoun, Christophe Coperet, Anne Lesage,
a
a
*
Urbano Dıaz and Avelino Corma
Received 21st December 2012
Accepted 20th February 2013
A
chiral mesoporous organosilica material incorporating
a
urea based-cinchona derivative and
DOI: 10.1039/c3sc22310h
propylamine groups was prepared by a co-condensation method. The multisite solid catalyst efficiently
www.rsc.org/chemicalscience
promoted the asymmetric multicomponent reaction of aldehydes, malonates and nitromethane.
pace of innovation and progress in asymmetric homogeneous
catalysis contrasts profoundly with asymmetric heterogeneous
Introduction
Multicomponent reactions are the most efficient chemical
procedures for the synthetic preparation of organic
compounds.1–3 This approach is particularly attractive as it
enables the production of highly elaborated compounds
from raw materials in an economical, energy-saving, and
intensied manner, avoiding intermediate purication steps.
Asymmetric multicomponent reactions4 are of growing interest
and their potential have been recently expanded by the intro-
duction of chiral small organic molecules as highly efficient
catalysts.5 In this regard, and as far as we know, all of the
developed methodologies make use of homogeneous catalysts
and many of them involve nucleophilic addition to an in situ
formed imine.6 Another strategy exploits the ability of
chiral amine catalysts to combine two modes of catalytic
activation of carbonyl compounds (iminium and enamine
catalysis) easing highly enantioselective Michael-type multi-
component processes.7–9 Some other multicomponent reactions
mediated by chiral homogeneous organocatalysts have been
reported.10–13
procedures. A commonly used strategy in asymmetric hetero-
geneous catalysis involves the heterogenization of
a
successful homogeneous catalyst by covalent linkage to an
inorganic oxide support or organic polymer. It is frequently
observed that the new solid catalyst exhibits lower perfor-
mance compared to the homogeneous counterpart in terms of
activity, selectivity and enantioselectivity. Therefore, the
development of asymmetric heterogeneous catalyzed proce-
dures is an active area of research that can be of particular
interest if one could achieve highly enantioselective
reactions.15,16
With regards to asymmetric heterogeneous organocatalysis,
several reports have successfully accounted for the hetero-
genization of various catalysts17 such as proline,18,19 diaryl pro-
linol,20,21 MacMillan imidazolidinones,22,23 chiral binaphthyl
derived phosphoric acids,24,25 squaramides,26 and cinchona-
based organocatalysts.27–30
Nevertheless, the use of chiral solid catalysts has been
limited to two component reactions and applications to asym-
metric multicomponent procedures have been elusive. Here we
report our approach towards meeting this challenge. Speci-
cally, the unprecedented application of a chiral heterogeneous
catalyst in the enantioselective multicomponent reaction of
aldehydes, nitromethane and malonates (Fig. 1) is herein
documented.
In the domain of catalyst mediated reactions, heteroge-
neous catalysis is oen preferable to homogeneous catalysis
because of the easy separation of the catalyst from the reaction
medium and reuse. Furthermore, solid catalysts are specially
suited for the design of continuous stirred-tank reactor (CSTR)
or xed bed continuous ow processes.14 The extraordinary
a
´
´
´
Instituto de Tecnologıa Quımica, UPV-CSIC, Universidad Politecnica de Valencia,
Results and discussion
Avenida de los Naranjos s/n, 46022 Valencia, Spain. E-mail: pgargar@itq.upv.es;
acorma@itq.upv.es; Fax: +34 96 3877809; Tel: +34 96 3877800
We focused our attention on cinchona-based organocatalysts. It
has been recently established that derivatives bearing a (thio)urea
functional group are general and effective hydrogen
bond activators of imines, carbonyl and nitro compounds.31–33 The
unique ability of this organocatalyst to engage in general-base
plus hydrogen bonding catalyzed stereoselective transformations
makes it an interesting candidate for heterogenization.34,35
b
`
`
´
Centre de RMN a Tres Hauts Champs, Universite de Lyon (CNRS/ENS Lyon/UCB
Lyon 1), 69100 Villeurbanne, France
c
¨
ETH Zurich Department of Chemistry, Laboratory of Inorganic Chemistry, CH-8093,
¨
Zurich, Switzerland
† Electronic supplementary information (ESI) available: Experimental procedures,
complete characterization of the hybrid catalyst and products, NMR spectra, IR
spectra, HPLC traces. See DOI: 10.1039/c3sc22310h
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci.

View Article Online
Chemical Science
Edge Article
structure directing agent and synthetic conditions of around
90 ꢀC and either acidic or basic pH. However, in our case, such
conditions and the ulterior removal of the surfactant can
introduce irreversible transformations in the organic molecule
that may result in a loss of chirality and/or activity. Therefore,
we have used here a so synthetic method38 that involves a sol–
gel procedure catalyzed by very small amounts of NH4F, in the
absence of structural directing agents, at low temperature and
at neutral pH, to obtain the hybrid organic–inorganic meso-
porous material (see ESI† for details).
The presence of the organic component in the silica frame-
work was conrmed by elemental and thermogravimetrical
analysis and by NMR spectroscopy. The content of C and N in
the solid measured by elemental analysis (Table S1 in the ESI†)
is an evident probe of the incorporation of the bifunctional
catalyst in the material, as the latter was prepared in the
absence of structural directing agents. Moreover, the C/N molar
ratio (C/NHybCat ¼ 6.1) is near to the theoretical value
(C/Ntheoretical ¼ 6.4), indicating that the organic fragments are
preserved. Organocatalyst content (0.78 mmol gꢁ1) in the
material was established from the N% content.
Fig. 1 Asymmetric multicomponent reaction promoted by hybrid catalyst 4.
TMOS ¼ tetramethylorthosilicate.
Thermogravimetric analysis (Fig. S1 in the ESI†) was per-
formed in order to gain insight, not only on the organic content
present in the solid, but also on the thermal stability of the
organicꢀunits. Apart from the weight loss observed at around
50–100 C associated to the removal of physisorbed water, the
main weight loss corresponds to the organic moieties and
With this in mind, we synthesized rst the trialkoxysilylated
urea-based cinchona alkaloid derivative 3 (Scheme 1). The
known primary amine 5, prepared from quinine in two
steps,36,37 was reacted with 3,5-bis(triuoromethyl)phenyl
isocyanate in THF at room temperature to form the urea
derivative 6 in 84% yield. Subsequent reaction with 3-(trie-
thoxysilyl)propyl isocyanate afforded the silylated product 3
isolated aer column chromatography in 71% yield and used it
in the preparation of a hybrid organic–inorganic mesoporous
material (Fig. 1).
The incorporation of the organocatalyst in the structure of
the mesoporous silaceous material was achieved through co-
condensation of silylated precursor 3 and tetramethylorthosi-
licate (TMOS). The synthesis of mesoporous structured mate-
rials is normally done by means of a surfactant that acts as a
ꢀ
occurs at temperatures above 150 C.
The solid-state 13C cross-polarization magic angle spinning
(CPMAS) NMR spectrum of the solid is shown in Fig. 2a and
compared with the corresponding 13C spectrum of precursor 3
in MeOH-d4 (Fig. 2b). An assignment of the 13C resonances of
the solid material is proposed from a two-dimensional (2D)
1
short-range H–13C heteronuclear correlation (HETCOR) spec-
trum (Fig. 3A) and from the solution chemical shis of
compound 3 (Fig. S23–S30 in the ESI†). This assignment is
Fig. 2 (a) 13C-CP MAS NMR of HybCat 4. (b) 13C NMR of derivative 3 in MeOH-d4.
(c) 13C NMR of derivative 6 in MeOH-d4.
Scheme 1 Synthesis of trialcoxysilyl urea-based cinchona alkaloid derivative 3.
Chem. Sci.
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Edge Article
Chemical Science
material through the carbamate linkage or merely adsorbed in
the surface (in the form of derivative 6), with the aminopropyl
groups being the ones connected to the surface of the solid
material as a result of carbamate hydrolysis.
For doing so, several 1H–13C HETCOR spectra were recorded
with different cross-polarization (CP) contact times to probe
long-range spatial proximities between carbon and proton
nuclei (Fig. 3 and S6 in the ESI†). Interestingly, the correlation
spectrum of Fig. 3B (CP contact time of 2 ms) shows a clear
correlation between the aromatic protons and the carbon 3000
(dotted lines in the plot), suggesting that some of the parent
structures are intact in the solid. This was further evidenced by
the 1D 19F–29Si CPMAS spectrum that displays the silicon-29
resonances of both the Q and T sites at around ꢁ100 and ꢁ60
ppm (Fig. S7 in the ESI†). This latter resonance is a clear indi-
cation of spatial proximities between the CF3 groups and the
29Si T nuclei.
On the other hand, carbamate hydrolysis was observed for
derivative 3 at some extent when running the 1H NMR spectrum
in MeOH-d4 aer 24 hours, and also when a catalytic amount of
NH4F was added to the NMR tube equating the reaction
conditions followed for the preparation of the solid although in
the absence of tetramethylorthosilicate. With these results
together with the observation of the peak at 105 ppm in 13C
CPMAS NMR, we could conclude that the hybrid solid presents
the structure depicted in Fig. 1, with part of the cinchone-based
units covalently attached to the surface and some of them
strongly adsorbed to the silaceous surface. Additionally, as a
Fig. 3 Two-dimensional 1H–13C HETCOR spectra on HybCat 4 recorded with a CP
contact time of 0.25 ms (A) and of 2 ms (B). Other experimental details are given
in the ESI.† The one-dimensional carbon-13 CPMAS spectrum is shown above the
2D plots together with the assignment of the resonances.
reported in Fig. 3. The signal positions observed in 13C CPMAS consequence of the partial cleavage, the mesoporous material is
NMR (Fig. 2a) are in good agreement with those of derivative 3 functionalized as well with aminopropyl units which ultimately
(Fig. 2b), showing the peaks corresponding to the carbonyl results in a multisite solid catalyst that was envisioned to be
groups of the urea and carbamate moiety, the aromatic and the applied in a wide range of applications in asymmetric catalysis.
quinuclidine signals, which conrms the preservation of the As a rst illustration, the one reported herein bellow, as an
structure of the organic molecules during the experimental efficient promoter of the enantioselective multicomponent
conditions used for the preparation of the hybrid solid. The reaction of aldehydes, nitromethane and malonates.
signal at 18.7 ppm observed in liquid 13C NMR for derivative 3
The hybrid organic–inorganic material 4 was also analyzed
(Fig. 2b), corresponding to the methyls in the triethoxysilyl by nitrogen sorption volumetry. Table S2 in the ESI† reports the
groups is barely observed in solid 13C CPMAS NMR of the hybrid surface area, pore volume and mean pore diameter of the hybrid
material 4 (Fig. 2a), evidencing that hydrolysis of these groups catalyst. The N2 adsorption isotherm (Fig. S8 in the ESI†), shows
has occurred. 13C CPMAS NMR spectroscopy proves thereby the the characteristic pattern typical of less-ordered mesoporous
integrity of the organic moieties, but it is from 29Si NMR spectra materials with the change in the slope occurring at low partial
where it is established the real covalent incorporation of the pressure (P/P0 around 0.3). Distribution of pore diameter
organocatalyst. Silane precursor 3 exhibits only one peak at (Fig. S9 in the ESI†) reveals that the majority of the pores have a
˚
ꢁ45.6 ppm, while in the solid, the signal corresponding to size near to 30 A. From these data, it can be deduced that the
silicon atoms bonded to carbon units are shied to ꢁ57 and hybrid material presents no microporosity and the total surface
ꢁ67 ppm (Fig. S4 in the ESI†) showing the effective attachment and pore volume are due exclusively to the presence of
of organic linkers by covalent interaction between surface sila- mesoporous.
nol groups and terminal alkoxide units of organosilane.
Hybrid material 4 was also analyzed by FTIR spectrometry on
We were concerned however, by the presence of a peak at self-supporting wꢀafers. The wafer was outgassed at room
105 ppm in 13C CPMAS NMR spectrum of the solid material that temperature, 100 C, 200 C and 400 C before the IR spectra
was not observed for the organosilane precursor 3 (Fig. 2), and were recorded. The IR spectra at different temperatures are
could suggest that carbamate cleavage has occurred, as the free illustrated in the ESI (Fig. S10†) and compared with that of
phenol derivative 6 shows the same exact peak (105.9 ppm, in derivative 3 (Fig. S10,† bottom). It should be highlighted that
MeOH-d4, Fig. 2c) assigned to carbon 50 in the quinoline moiety. the carbonyl stretching band at around 1720 cmꢁ1 observed for
At this point, we decided to run extensive NMR character- the hybrid solid at room temperature and 100 ꢀC, was not
ization (see ESI, Fig. S5–S7†) with the aim of determining if the detected at 200 ꢀC, pointing out that organocatalyst degradation
cinchone derivative is truly covalently attached to the solid is occurring above 100 ꢀC and therefore conrming the data
ꢀ
ꢀ
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci.

View Article Online
Chemical Science
Edge Article
obtained in the thermogravimetrical analysis. In a different atom-economical way but also with high selectivity (see the ESI†
experiment the thermal stability over time was analyzed at 70 ꢀC for details: Knoevenagel adduct was also formed in 10% yield)
(that corresponds to the reaction temperature used in the and good enantiocontrol. Preliminary kinetic studies were per-
catalytic procedure shown below). This study revealed (see the formed and results are displayed in the ESI.†
ESI†) that the organic moiety endures the temperature condi-
tions used in the multicomponent transformation.
Precedents on the three-component reaction at hand are
exposed by McQuade et al.,51,52 who accomplished the trans-
Once secured organocatalyst integrity in the solid material, formation combining a microencapsulated primary amine and a
we envisioned applications as heterogeneous catalyst. At rst, nickel-based homogeneous catalyst. In this way, the product
the hybrid catalyst was tested in the Henry condensation of derived from benzaldehyde is reported to be formed in 80% yield
benzaldehyde 1a with nitromethane. Amine-functionalized aer 24 h reaction time.51 While highly promising, this meth-
mesoporous materials are known to promote such reaction.39–43 odology shows still limitations on the substrate scope, particu-
In our case, when using 10 mol% of organocatalyst loading, larly for aromatic aldehydes with electron-withdrawing groups.52
the desired nitroalkene 7 was formed aer 5 h reaction time in Furthermore, the asymmetric version, using a chiral nickel
51% yield (eqn (1)). Appreciable amounts of the Michael-type complex, is portrayed only with one substrate (3-methylbutyr-
addition product 8 were also observed. In addition, the hybrid aldehyde) that yields the product in 72% ee.52 Additionally, it
material 4 showcased activity in the asymmetric Michael-type should be noted that the nickel catalyst is a homogeneous system
addition of dimethylmalonate to nitroalkenes (eqn (2)) and the and accordingly, related issues of product purication and/or
product was formed in 76% yield with 91 : 9 enantiomeric ratio. catalyst recycling should be considered. Sartori et al.43 have also
Thiourea-based cinchone derivatives are known to catalyze the accounted for the production of compounds 2 from the corre-
latter enantioselective Michael type addition in the homoge- sponding aldehydes, by means of a continuous-ow two-step
neous fase.44,45 g-Nitro esters 2a (eqn (2)) are versatile building process using a combination of two column reactors packed with
blocks for the synthesis of important nitrogen containing different amino-functionalized silica catalysts. No enantiose-
bioactive compounds such as the antidepressant (R)-rolipram,46 lective version is reported in any case.
the selective serotonin reuptake inhibitor (3S,4R)-paroxetine,46
Interestingly, when the multicomponent reaction was
or the antispastic agent (R)-baclofren47 among many others.48–50 attempted with the homogeneous catalyst (data shown in the
ESI†), the desired product 2a was formed in less than 5% yield.
Furthermore, combination of the homogeneous catalyst with
˚
4 A MS or Silica 100 under otherwise similar conditions also
furnished the desired product in very low yield. The distinctive
reactivity offered by the hybrid catalyst could be attributed to
the presence of the aminopropyl groups on the solid material.
The primary amine sites would react with the aldehyde to
form an imine intermediate, more prone to be attacked (when
compared to the starting aldehyde) by the deprotonated nitro-
methane. Addition of nitromethane to the carbon-nitrogen imine
bond via an aza-Henry process would give a relatively unstable
supported b-nitroamine that aer b-scission produces the nitro-
alkene and regenerates the aminopropyl site.42 The bifunctional
(1)
(2)
The ability of the solid catalyst to promote the Henry cinchona-based organocatalyst would then be responsible for the
condensation and to impart high stereocontrol in the Michael- subsequent enantioselective Michael-type reaction.44,45
type reaction prompted us toward a more challenging target: the
To probe the role of the primary amine group in the multi-
creation of multifunctional compounds in a one-pot three component reaction, two silica materials were prepared by the
component reaction (as shown in Fig. 1) where the initially sol–gel method utilizing NH4F as a mineralizing agent. In the
formed nitroalkene could be trapped by dimethylmalonate in a rst place, aminopropyltriethoxysilane was co-condensed with
Michael-type addition. Accordingly, the chiral hybrid solid cata- TMOS. The resultant material proved highly efficient in the
lyst was used in 10 mol% loading at 80 ꢀC with a mixture of Henry condensation of aldehydes with nitromethane. Further-
benzaldehyde 1a, nitromethane and dimethylmalonate (data more, when combined with adsorbed organocatalyst 6, it
shown in the ESI†), and already in the rst attempt, the adduct 2a afforded adduct 2a in equal yield and enantioselectivity as
was obtained in 49% yield aer 8 h and with 84 : 16 enantiomeric shown for HybCat 4 under similar reaction conditions (see ESI†
ratio. Extensive optimization of reaction conditions, i.e. temper- for details). On the other hand, the silaceous material obtained
ature, catalyst loading, solvent, concentration and reagents ratio by co-condensation of diethylaminopropyltriethoxysilane and
are reported in Tables S3–S7 within the ESI,† and the best result TMOS showed no activity in the Henry condensation and
is displayed in Table 1 (entry 1). Under the optimized conditions, neither in the three-component reaction when combining it
the product 2a was isolated aer 36 h in 75% yield and good with the adsorbed catalyst 6. These facts additionally account
enantioselectivity (88 : 12 er). The obtained result is remarkable for the presence of primary amine groups in the HybCat 4.
since the approach not only provides straightforward access to
The adsorbed organic molecules 6 presented in HybCat 4
the complex and synthetically useful46–50 scaffolds in a rapid and could be removed (although not completely) by extensive
Chem. Sci.
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Edge Article
Chemical Science
Table 1 Asymmetric multicomponent reaction of aldehydes, nitromethane and malonates
a
Yield of isolated product. b Determined by HPLC on a chiral stationary phase. c 1 M concentration and 50 ꢀC. d Reaction in toluene for 48 h.
washing of the solid with a solution of AcOEt/MeOH/NH4OH 95/ This is certainly an advantage with respect to homogeneous
5/1 (see Fig. S12 in ESI†). The resultant material was also tested catalysis. In our case, the hybrid catalyst was easily
in the multicomponent reaction demonstrating comparable separated by centrifugation and reused for further repeating
performance to HybCat 4 in terms of both efficiency and runs.
enantioselectivity (data shown in the ESI†).
Table 2 shows that the catalytic performance of the hybrid
Adopting the favorable reaction conditions for the asymmetric solid remained unchanged throughout the three rst runs and
multicomponent reaction depicted in entry 1, Table 1, the catalyst deactivation occurs at run 4, probably due to organo-
generality of the method was demonstrated by evaluating a catalyst degradation and/or leaching aer successive submis-
variety of aldehyde and malonate substrates. As highlighted in sion of the solid material to the thermal conditions used in the
Table 1, there appears to be signicant tolerance toward struc- reaction (see ESI† for details).
tural and electronic variations of aromatic aldehydes enabling
the access to Michael adducts with good to high yields (70 to 90s)
and high optical purity up to 93 : 7 enantiomeric ratio. Different
substituents at ortho, meta and para positions can be accom-
modated. Interestingly, electronically different substituents are
allowed, ranging from electron donating groups to halogens and
to stronger electron withdrawing groups. The generality of the
method was also veried by using polyaromatic aldehydes (entry
14), as well as heteroaromatic substrates, such as furfural (entries
15 and 16). The enantioselectivity achieved in all cases is similar,
oscillating slightly between 85 : 15 and 93 : 7 enantiomeric ratio.
Aliphatic aldehydes (entries 17 and 18) probed more challenging,
though acceptable yields are achieved aer 48 h reaction time
forming the desired product in modest enantiomeric ratio
(72 : 28 to 76 : 24).
Table 2 Repeated runs using recycled hybrid solid as catalyst
Run
Yield (%)a
erb
Run 1
Run 2
Run 3
Run 4
90
93
85
54
93 : 7
91 : 9
93 : 7
92 : 8
a
Determined by 1H NMR using Ph3CH as internal standard.
Determined by HPLC on a chiral stationary phase.
Another important aspect of heterogeneous catalysis
concerns the stability of the catalyst during recycling runs.
b
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci.

View Article Online
Chemical Science
Edge Article
16 P. Wang, X. Liu, J. Yang, Y. Yang, L. Zhang, Q. Yang and C. Li,
J. Mater. Chem., 2009, 19, 8009–8014.
Conclusions
In summary, we have developed a new multifunctional hybrid
organic–inorganic catalyst based on a cinchona alkaloid and
aminopropyl groups using a uoride sol–gel route. The effective
incorporation of these organic units was conrmed by different
characterization techniques (elemental analysis, TGA, NMR).
The novel material was successfully applied to the asymmetric
multicomponent reaction between aldehydes, nitromethane
and malonates allowing direct access to highly functionalized
products in a rapid and atom-economical way and at the same
time, in good to high yields as well as good enantioselectivities.
The catalyst is stable, easily separable and can be reused several
times. We believe that the novel solid material prepared in this
work may provide new synthetic possibilities for multicompo-
nent transformations with the consequent preparation of chiral
and complex products.
17 M. Benaglia, in Handbook of Asymmetric Heterogeneous
Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp.
293–322.
`
18 D. Font, C. Jimeno and M. A. Pericas, Org. Lett., 2006, 8,
4653–4655.
¨
19 A. Zamboulis, N. J. Rahier, M. Gehringer, X. Cattoen, G. Niel,
C. Bied, J. J. E. Moreau and M. W. C. Man, Tetrahedron:
Asymmetry, 2009, 20, 2880–2885.
`
20 X. Fan, S. Sayalero and M. A. Pericas, Adv. Synth. Catal., 2012,
354, 2971–2976.
21 C. A. Wang, Z. K. Zhang, T. Yue, Y. L. Sun, L. Wang,
W. D. Wang, Y. Zhang, C. Liu and W. Wang, Chem.–Eur. J.,
2012, 18, 6718–6723.
`
22 P. Riente, J. Yadav and M. A. Pericas, Org. Lett., 2012, 14,
3668–3671.
23 J. Y. Shi, C. A. Wang, Z. J. Li, Q. Wang, Y. Zhang and
W. Wang, Chem.–Eur. J., 2011, 17, 6206–6213.
24 C. Bleschke, J. Schmidt, D. S. Kundu, S. Blechert and
A. Thomas, Adv. Synth. Catal., 2011, 353, 3101–3106.
25 M. Rueping, E. Sugiono, A. Steck and T. Theissmann, Adv.
Synth. Catal., 2010, 352, 281–287.
Acknowledgements
This work was supported by the Spanish Government (Con-
solider Ingenio 2010-MULTICAT (CSD2009-00050) and
MAT2011-29020-C02-01). P.G.-G. is grateful for a JAE-DOC
contract from CSIC co-funded by the ESF. The Severo Ochoa
program is thankfully acknowledged.
`
26 P. Kasaplar, P. Riente, C. Hartmann and M. A. Pericas, Adv.
Synth. Catal., 2012, 354, 2905–2910.
27 W. Wang, X. Ma, J. Wan, J. Cao and Q. Tang, Dalton Trans.,
2012, 41, 5715–5726.
28 D. Cancogni, A. Mandoli, R. P. Jumde and D. Pini, Eur. J. Org.
Chem., 2012, 1336–1345.
Notes and references
1 M. J. Climent, A. Corma and S. Iborra, RSC Adv., 2012, 2, 16–58. 29 R. P. Jumde, A. Mandoli, F. De Lorenzi, D. Pini and
´
´
2 A. Corma, U. Dıaz, T. Garcıa, G. N. Sastre and A. Velty, J. Am.
P. Salvadori, Adv. Synth. Catal., 2010, 352, 1434–1440.
30 S. H. Youk, S. H. Oh, H. S. Rho, J. E. Lee, J. W. Lee and
C. E. Song, Chem. Commun., 2009, 2220–2222.
Chem. Soc., 2010, 132, 15011–15021.
3 M. J. Climent, A. Corma and S. Iborra, Chem. Rev., 2010, 111,
1072–1133.
31 S. J. Connon, Chem.–Eur. J., 2006, 12, 5418–5427.
´
4 D. J. Ramon and M. Yus, Angew. Chem., Int. Ed., 2005, 44, 32 W.-Y. Siau and J. Wang, Catal. Sci. Technol., 2011, 1, 1298–1310.
1602–1634.
33 H. Miyabe and Y. Takemoto, Bull. Chem. Soc. Jpn., 2008, 81,
785–795.
´
5 G. Guillena, D. J. Ramon and M. Yus, Tetrahedron:
Asymmetry, 2007, 18, 693–700.
34 P. Yu, J. He and C. Guo, Chem. Commun., 2008, 2355–2357.
6 J. Yu, F. Shi and L.-Z. Gong, Acc. Chem. Res., 2011, 44, 1156– 35 O. Gleeson, G.-L. Davies, A. Peschiulli, R. Tekoriute,
1171.
Y. K. Gun'ko and S. J. Connon, Org. Biomol. Chem., 2011, 9,
7 Y. Huang, A. M. Walji, C. H. Larsen and D. W. C. MacMillan,
J. Am. Chem. Soc., 2005, 127, 15051–15053.
7929–7940.
36 B. Vakulya, S. Varga, A. Csampai and T. Soos, Org. Lett., 2005,
7, 1967–1969.
¨
8 D. Enders, M. R. M. Huttl, C. Grondal and G. Raabe, Nature,
2006, 441, 861–863.
9 P. Galzerano, F. Pesciaioli, A. Mazzanti, G. Bartoli and
P. Melchiorre, Angew. Chem., Int. Ed., 2009, 48, 7892–7894.
37 W. Chen, W. Du, Y.-Z. Duan, Y. Wu, S.-Y. Yang and
Y.-C. Chen, Angew. Chem., Int. Ed., 2007, 46, 7667–
7670.
10 D. B. Ramachary, N. S. Chowdari and C. F. Barbas, Angew. 38 U. Diaz, T. Garcia, A. Velty and A. Corma, J. Mater. Chem.,
Chem., Int. Ed., 2003, 42, 4233–4237. 2009, 19, 5970–5979.
11 D. B. Ramachary, K. Anebouselvy, N. S. Chowdari and 39 M. Lakshmi Kantam and P. Sreekanth, Catal. Lett., 1999, 57,
C. F. Barbas, J. Org. Chem., 2004, 69, 5838–5849.
227–231.
12 D. B. Ramachary and C. F. Barbas, Chem.–Eur. J., 2004, 10, 40 G. Sartori, F. Bigi, R. Maggi, R. Sartorio, D. J. Macquarrie,
5323–5331.
M. Lenarda, L. Storaro, S. Coluccia and G. Martra, J. Catal.,
2004, 222, 410–418.
13 C. G. Evans and J. E. Gestwicki, Org. Lett., 2009, 11, 2957–2959.
14 A. Corma and H. Garcia, Adv. Synth. Catal., 2006, 348, 1391– 41 Q. Wang and D. F. Shantz, J. Catal., 2010, 271, 170–
1412. 177.
15 X. Liu, P. Wang, Y. Yang, P. Wang and Q. Yang, Chem.–Asian 42 K. Motokura, M. Tada and Y. Iwasawa, Angew. Chem., Int. Ed.,
J., 2010, 5, 1232–1239. 2008, 47, 9230–9235.
Chem. Sci.
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Edge Article
Chemical Science
43 L. Soldi, W. Ferstl, S. Loebbecke, R. Maggi, C. Malmassari, 48 F. Xu, E. Corley, M. Zacuto, D. A. Conlon, B. Pipik,
G. Sartori and S. Yada, J. Catal., 2008, 258, 289–
295.
G. Humphrey, J. Murry and D. Tschaen, J. Org. Chem.,
2010, 75, 1343–1353.
44 J. Ye, D. J. Dixon and P. S. Hynes, Chem. Commun., 2005, 49 J.-M. Liu, X. Wang, Z.-M. Ge, Q. Sun, T.-M. Cheng and
4481–4483. R.-T. Li, Tetrahedron, 2011, 67, 636–640.
45 S. H. McCooey and S. J. Connon, Angew. Chem., Int. Ed., 2005, 50 P. Elsner, H. Jiang, J. B. Nielsen, F. Pasi and K. A. Jorgensen,
44, 6367–6370. Chem. Commun., 2008, 5827–5829.
46 P. S. Hynes, P. A. Stupple and D. J. Dixon, Org. Lett., 2008, 10, 51 S. L. Poe, M. Kobaslija and D. T. McQuade, J. Am. Chem. Soc.,
1389–1391. 2006, 128, 15586–15587.
47 T. Okino, Y. Hoashi, T. Furukawa, X. Xu and Y. Takemoto, 52 S. L. Poe, M. Kobaslija and D. T. McQuade, J. Am. Chem. Soc.,
ˇ
J. Am. Chem. Soc., 2004, 127, 119–125.
2007, 129, 9216–9221.
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci.