A surface-confined O=MnV(salen) oxene catalyst and high turnover values in asymmetric epoxidation of unfunctionalized olefins

Article abstract of DOI:10.1039/c2jm34847k

Given the widespread importance of chiral epoxides in chemical, biochemical and agricultural fields, an efficient synthesis that avoids wasteful use of the catalyst and assures its total recovery and reuse and high turnover values is highly desirable. We report on a thermally and temporally robust covalent monolayer of (salen)Mn(iii) molecules on silica functionalized substrates that resulted in an active heterogeneous catalyst for enantioselective epoxidation of 6-ciano-2,2-dimethyl-chromene, chosen as a model alkene, in high yields and high turnover numbers. No catalyst leaching was observed thus allowing continuous recycling. Experimental XPS evidence for the formation of the surface confined O=Mn^V(salen) oxene species has been obtained.

Full text of DOI:10.1039/c2jm34847k

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PAPER
A surface-confined O]MnV(salen) oxene catalyst and high turnover values in
asymmetric epoxidation of unfunctionalized olefins†
Valentina La Paglia Fragola,a Fabio Lupo,ab Andrea Pappalardo,a Giuseppe Trusso Sfrazzetto,a
a
a
a
Rosa M. Toscano, Francesco P. Ballistreri, Gaetano A. Tomaselli and Antonino Gulino
ab
*
*
Received 23rd July 2012, Accepted 10th August 2012
DOI: 10.1039/c2jm34847k
Given the widespread importance of chiral epoxides in chemical, biochemical and agricultural fields, an
efficient synthesis that avoids wasteful use of the catalyst and assures its total recovery and reuse and
high turnover values is highly desirable. We report on a thermally and temporally robust covalent
monolayer of (salen)Mn(III) molecules on silica functionalized substrates that resulted in an active
heterogeneous catalyst for enantioselective epoxidation of 6-ciano-2,2-dimethyl-chromene, chosen as a
model alkene, in high yields and high turnover numbers. No catalyst leaching was observed thus
allowing continuous recycling. Experimental XPS evidence for the formation of the surface confined
O]MnV(salen) oxene species has been obtained.
prototype (salen)Mn(III)-based monolayer, covalently bound to
quartz substrates,21 which resulted in a good chiral catalyst for
the asymmetric epoxidations22 in environmentally benign reac-
tions generating no Mn catalyst waste. Huge turnover values
have been obtained. No leaching of this thermally and tempo-
rally stable catalyst was observed. In addition, experimental
photoelectron evidence for the formation of the surface confined
O]MnV(salen) oxene active species was observed.
Introduction
The synthesis of chiral macrocyclic cavities is an increasing
research field because their metal complexes find applications as
selective receptors as well as bioinspired catalysts.1–3 Asymmetric
epoxidation of unfunctionalized prochiral olefins, catalyzed by
chiral (salen)Mn(III) complexes, has proven to be one of the most
useful reactions in organic syntheses since the obtained chiral
epoxides contain two new stereocentres and can be easily trans-
formed into a large variety of compounds active in the industrial,
biological, pharmaceutical and agricultural fields.4–11 However,
the separation and recycling of the expensive chiral catalyst are
the main drawbacks of this procedure.7,12,13 Moreover, exposure
to Mn in air at concentrations more than 5 mg mꢀ3 can lead to
manganism, a biphasic disorder mainly linked to impaired motor
skills, cognitive disorders, compulsive behaviors, and psychosis
which resembles Parkinson’s disease.14 Therefore, efforts have
been devoted to heterogenize the Jacobsen catalyst by anchoring
chiral (salen)Mn(III) complexes on solid surfaces in order to
enhance their stability, activity, selectivity, recovery and recy-
cling. Although several methods are known for this hetero-
genization,15–20 surface anchoring that assures no catalyst
leaching and high efficiency remains a challenging goal.11,15–19
Therefore, we have now developed a (salen)Mn(III) complex,
especially designed to maintain the Mn(III) environment unal-
tered after surface grafting. Then, we obtained a robust
Experimental details
The detailed description of the synthetic pathway for the cata-
lysts used in this work has been reported in the ESI.†
NMR experiments were carried out at 27 ꢁC on a Varian
UNITY Inova 500 MHz spectrometer (1H NMR at 499.88 MHz,
13C NMR at 125.7 MHz) equipped with a pulse field gradient
module (Z axis) and a tuneable 5 mm Varian inverse detection
probe (ID-PFG). 1H NMR peak assignments follow from
2D-COSY experiments. Gas chromatographic analyses of the
reaction mixtures were carried out on a gas chromatograph
equipped with
capability.
a flame ionization detector and program
ESI mass spectra were acquired on an ES-MS Thermo-Fin-
nigan LCQ-DECA using MeOH (positive ion mode).
X-ray photoelectron spectra (XPS) were collected at 45ꢁ rela-
tive to the surface plane with a PHI 5600 Multi Technique
System which gives a good control of the electron take-off angle
(base pressure of the main chamber is 3 ꢂ 10ꢀ10 Torr). Spectra
were excited with Al-Ka radiation and structures due to satellites
were subtracted from the spectra prior to data processing. XPS
peak intensities were obtained after Shirley background removal.
Spectral calibration was achieved by fixing the main C 1s peak at
285.0 eV. Experimental uncertainties in binding energies lie
a
ꢀ
Dipartimento di Scienze Chimiche, Universita di Catania, Viale A. Doria
6, 95125 Catania, Italy. E-mail: agulino@unict.it; Fax: +39-095-580138;
Tel: +39-095-7385067
bI.N.S.T.M. UdR of Catania, Viale A. Doria 6, 95125 Catania, Italy
† Electronic supplementary information (ESI) available: Synthesis
procedures, 1H–13
(Fig. S1–S9). See DOI: 10.1039/c2jm34847k
C
NMR, gCOSY and
N 1s XPS spectra
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within ꢃ0.4 eV. Simulation of the O 1s spectrum was carried out
by fitting the spectral profile with three symmetrical Gaussian
envelopes after subtracting the background. This process
involved data refinement, based on the method of the least
squares fitting, carried out until there was the highest possible
correlation between the experimental spectrum and the theoret-
ical profile. The R-factor (residual or agreement factor),
R ¼ [S(Fo ꢀ Fc)2/S(Fo)2]1/2, after minimization of the function
S(Fo ꢀ Fc)2 converged to R values #0.034.
Scheme 2 Covalent grafting of Mn_1 to silylated quartz substrates.
Reagents and conditions: quartz silylation: in a glove box under a N2
atmosphere, 0.3 : 100 v/v trichloro[4-(chloromethyl)phenyl]silane/n-
pentane, 25 ꢁC, 250; catalyst grafting: 4.4 ꢂ 10ꢀ4 M toluene solution of
the (salen)Mn(III), 90 ꢁC, 80 h.
UV-vis measurements were carried out on a UV-vis V-650
Jasco spectrometer and the spectra were recorded with a 0.2 nm
resolution at room temperature.
The solution under UV-vis shows two bands at 306.9 and
356.0 nm consistent with p / p* and n / p* transitions,
respectively.24 The UV-vis spectrum of Mn_1-SAM closely
resembles that of the solution with two bands at 305.8 and
354.0 nm. The Mn_1 molecule surface coverage, dsurf ¼ A3ꢀ1
(number of Mn_1 per cm2 in the SAM) was calculated using the
Beer–Lambert law (A ¼ 3lc, where A is the absorbance and 3, l
and c are the extinction coefficient, the thickness of the mono-
layer and the concentration of the (salen)Mn(III) molecules in the
monolayer, respectively).21 Taking into account the 3 value of
26 000 Mꢀ1 cmꢀ1 for the band at 356 nm of Mn_1 in toluene, the
Results and discussion
Scheme 1 shows the synthetic pathway for the protected 1-Mn
and related un-protected Mn_1 whose detailed description has
been reported in the ESI (see ESI, Fig. S1–S7†).
The self-assembled monolayer Mn_1-SAM was synthesized by
covalent grafting of Mn_1 to quartz substrates that were previ-
ously silylated (Scheme 2).21 The silylation was performed under
N2 with trichloro[4-(chloromethyl)phenyl]silane, a bi-functional
coupling agent that binds to both the substrate and the (salen)
Mn(III).21 The grafted molecules were found to be insoluble in
toluene, MeCN, THF, Et2O and EtOH. Moreover, they strongly
adhere to the substrate and neither can be removed by the
‘‘scotch-tape decohesion’’ test23 nor by abrasion with a task
wiper, as evidenced by UV-vis and XPS measurements.
Fig. 1 shows the absorption spectra of both the 1.23 ꢂ 10ꢀ7 M
toluene Mn_1 solution (black line) and the representative
Mn_1-SAM (red line).
calculated molecule density value is 3.94 ꢂ 1013 molecules per
2
cm2 with a footprint of 254 A per molecule. Fig. 2 shows
ꢁ
the X-ray photoelectron spectrum (XPS) of the representative
Mn_1-SAM in the Mn 2p energy region. Two well resolved spin–
orbit components appear at 641.5 and 653.3 eV (solid line),
together with a shake-up phenomenon, consistent with the
presence of Mn(III) species.25
The same spectral region has been analyzed after immersion of
the Mn_1-SAM for 10 min in a 0.14 M NaOCl solution in order
to activate the catalyst. The two spin–orbit components, now at
642.3 and 654.1 eV (dashed line), clearly show a 0.8 eV shift to
higher binding energy consistent with the formation of a new oxo
Mn species in which the Mn ion possesses a higher oxidation
state. The starting XPS spectrum was restored after the reaction
of the catalyst with a 0.14 M CH2Cl2 alkene (6-ciano-2,2-
dimethyl-chromene, vide infra) solution thus suggesting the Mn
reversible redox reaction. Fig. 3 shows the XPS spectra in the O
1s energy region for the as-synthesized Mn_1-SAM and after its
immersion in the NaOCl solution. Spectral fitting shows
Scheme 1 Synthetic pathway for the protected 1-Mn and un-protected
Mn_1 Reagents and conditions: (a) Cl2CHOCH3, SnCl4, CHCl3, 0 ꢁC, 300,
35%; (b) p-formaldehyde, TBABr, HCl, r.t., 3 days, 94%; (c) 6-amino-1-
hexanol, MW, 50, 94%; (d) KOH, K2CO3, TBABr, MW, 50, 51%; (e)
(1R,2R)-diphenylethylenediamine, EtOH, r.t, 24 h, 95%; (f) Mn(A-
cO)3$2H2O, r.t., 24 h, 98%; (g) hydrazine monohydrate, EtOH, reflux,
7 h, 98%.
Fig. 1 UV-vis spectra of a 1.23 ꢂ 10ꢀ7 M toluene solution (black line) of
the Mn_1 and of the representative Mn_1-SAM (red line).
20562 | J. Mater. Chem., 2012, 22, 20561–20565
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Fig. 4 Surface confined O]MnV(salen) complex on a siloxane-based
monolayer.
Fig. 2 Al Ka excited Mn_1-SAM in the Mn 2p energy region: the solid
black line represents the spectrum for the as-synthesized monolayer and
the red dashed line refers to the Mn_1-SAM after 10 min immersion in
0.14 M NaOCl aqueous solution, buffered with Na2HPO4 ¼ 0.05 M at
pH ¼ 11.2.
active oxidant is the oxene species, generated ‘‘in situ’’ by the
NaOCl oxygen donor.
The XPS spectrum for the Mn_1-SAM in the N 1s binding
energy region reveals two symmetric components at 398.8
and 400.2 eV having a 1 : 1 intensity ratio and indicates that
both –NH2 groups are involved in the surface grafting (see ESI,
Fig. S9†).
The protected 1-Mn and Mn_1-SAM were used to perform
homogeneous or heterogeneous epoxidation, respectively (Table
1) at 25 ꢁC of 6-ciano-2,2-dimethyl-chromene, chosen as a model
substrate since its epoxide is of potential value as a pharmaceu-
tical intermediate.22 In particular, for homogeneous reactions,
0.17 mmol of the alkene were dissolved in 4 mL of CH2Cl2 and
mixed with 4 mL of a 0.05 M Na2HPO4 buffer solution at pH ¼
11.2 containing 1.6 mmol of NaOCl. In all these homogeneous
experiments the total volume was 8 mL and the amount of
catalyst was 8.5 ꢂ 10ꢀ3 mmol.
Fig. 3 Al Ka excited Mn_1-SAM in the O 1s energy region: (a) as-
synthesized monolayer; (b) after 10 min immersion in the 0.14 M NaOCl
aqueous solution, buffered with Na2HPO4 ¼ 0.05 M at pH ¼ 11.2. Solid
black lines refer to the experimental profiles; dashed red, purple and green
lines refer to the 532.0, 530.7 and 530.1 eV Gaussian components,
respectively. The dashed blue lines superimposed to the experimental
profiles refer to the sum of the Gaussians.
Similar reactions were performed using the Mn_1-SAM of
2 ꢂ 1 cm size. Since the calculated density of the Mn_1-SAM is
3.94 ꢂ 1013 molecules per cm2 and the SAMs are covered in both
sizes, the overall catalysts on the substrate surface correspond to
1.58 ꢂ 1014 molecules ¼ 2.6 ꢂ 10ꢀ7 mmol. From these reactions
it emerged that the Mn_1 solution and the related Mn_1-SAM
always yield a total alkene conversion and high enantiomeric
interesting details. In fact, in both spectra the peaks at 532.0 eV
are due to the SiO2 substrate and ether oxygen of the anchored
Mn_1 molecule. The lower binding energy features at 530.1 eV
account for the oxygen of the salen ligand bound to Mn(III).25
Furthermore, the XPS spectrum of the Mn_1-SAM after
immersion in the NaOCl solution shows an additional feature at
530.7, which is absent in the spectrum of the as-synthesized
Mn_1-SAM, indicating the presence of another kind of oxygen.
The obtained binding energy is consistent with oxo species of Mn
in a formal oxidation state higher than three.25 The intensity ratio
of this last peak, obtained after a careful deconvolution of the
experimental spectrum, is 0.5 with respect to that at 530.1 eV. It
emerges that, apart from the two salen oxygen atoms per formula
unit, a new and additional oxygen atom is bound now to the Mn
and this indicates the presence of the O]MnV(salen) species
(Fig. 4). This observation is in agreement with the XPS
measurements that already evidenced an energy shift of the Mn
2p states to a higher binding energy upon immersion in NaOCl
solution. This experimental XPS observation is of great value
since it was not reported previously, and agrees well with earlier
proofs26 based on different investigations and confirms that the
Table 1 Results obtained for the epoxidation of 6-ciano-2,2-dimethyl-
chromene to 3,4-epoxy-6-ciano-2,2,-dimethyl-chromene using the Mn_1-
SAMa
Catalyst
Reaction time/h e. eb% Conv.b% TONc
TOFd (hꢀ1
)
Mn_1
0.5
1
81
81
81
72
72
72
71
30
65
100
20
65
100
100
6
13
20
12
13
10
2
Mn_1-SAM 24
130 800 5450
423 000 5875
6.5 ꢂ 105 5400
6.5 ꢂ 105 5400
72
120 ꢃ 10
Mn_1-SAM* 120 ꢃ 10
a
In all these experiments 0.17 mmol of 6-ciano-2,2-dimethyl-chromene
were dissolved in 4 mL of CH2Cl2; NaOCl was 1.6 mmol and the total
volume of the reacting solutions, buffered with 4 mL 0.05 M Na2HPO4
at pH ¼ 11.2, was 8 mL. The amount of the catalyst was 8.5 ꢂ 10ꢀ3
and 2.6 ꢂ 10ꢀ7 mmol for Mn_1, and Mn_1-SAM, respectively. Mn_1-
SAM* refers to results after 10 epoxidation cycles (Fig. 5) to test its
b
stability under basic reaction conditions. e. e determined by GC on a
dimethyl-pentyl-beta (DIMEPEBETA-086) chiral column, (25 m ꢂ
c
0.25 mm ID, 0.25 mm film). TON ¼ mmol of the overall products/
mmol of the catalyst. d TOF ¼ TON/reaction time (h).
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chains assure many freedom degrees to our grafted molecules
(Fig. 4). In fact, the present reduction in enantioselectivity from
solution (81%) to heterogeneous (72%) catalysis is small if the
fact that the catalyst is strongly bound to the surface is taken into
account. Therefore, we can assume that: (i) the monolayer is
sufficiently porous to allow the alkene to penetrate into it and
also (ii) the O]MnV group of the O]MnV(salen) points outside
the monolayer surface thus suggesting a rather ordered and
parallel Mn(salen) molecular plane orientation with respect to
the surface. A similar conclusion has been arrived at by Milstein
who reported on improved TON values in catalytic reactions
using molecular ordered Langmuir–Blodgett catalyst films.32
Concerning the O]MnV(salen) possible geometry, we synthe-
sized a complex of ligand 6 (Scheme 1) with UO2, as a model for
the O]MnV(salen) species, and performed NMR measurements
(see ESI, Fig. S8†).33,34 The 1H NMR spectrum shows a
symmetric pattern of signals thus suggesting a pseudo-octahedral
geometry of the UO2(salen) complex in agreement with previ-
ously reported data.35
Finally, a representative Mn_1-SAM, heated at 140 ꢁC to test
its thermal toughness and then reused at room temperature, did
not show any decrease in efficiency but it always required the
same time (120 ꢃ 10 h) to convert the alkene completely (Fig. 5).
The Mn_1-SAM was also tested after six months from the
synthesis showing no variation of performance and confirming
its temporal stability. In fact, XPS atomic concentration analysis,
performed for the as synthesized Mn_1-SAM and after 10
epoxidation cycles, always gave a Mn : N ratio of 1 : 4.
Fig. 5 Variation of the reaction time for the total conversion of 6-ciano-
2,2-dimethyl-chromene using the representative Mn_1-SAM. Cycles 6–10
show reaction times measured after 24 h heating of the Mn_1-SAM at
140 ꢁC and then cooling at room temperature before immersion in the
reaction environment.
excess values both in solution (80%) and under heterogeneous
conditions (>70%) (Table 1).
Similar high conversion and enantioselectivity values have
been recently reported for the epoxidation of alkenes with (salen)
Mn(III) phosphate complexes.26 An interesting observation is the
high present value of the turnover frequency under heteroge-
neous conditions (6.5 ꢂ 105) with respect to the solution (20)
after total disappearance of 6-ciano-2,2-dimethyl-chro-
mene.18,19,27 The TOF parameter27 displays a similar trend on
passing from homogeneous (10) to heterogeneous (5.4 ꢂ 103)
conditions. While the high reaction time (120 h) needed for the
total alkene conversion when using the Mn_1-SAM is due to the
low catalyst concentration (2.6 ꢂ 10ꢀ7 mmol), the impressive
TON value is consistent with the covalent nature of the mono-
layer that precludes any catalyst leaching or degradation. In fact,
no Mn was detected in the reacting solution investigated by
inductively coupled plasma mass spectrometry (ICP-MS). To the
best of our knowledge, the TON and TOF values reported in this
work are the highest observed in similar catalytic reactions. In
fact, high enantioselectivity was reported for the epoxidation of
6-cyano-2,2-dimethylchromene on the Mn(salen) catalyst,
immobilized via substituted phenyl sulfonic groups in meso-
porous materials, being the highest measured TOF value of only
14.8.28 Reactions, involving styrene epoxidation using solutions
of the meso-tetra(4-pyridyl)porphinatomanganese(III) chloride
catalyst resulted in TONs of approximately 1500.11 Recently, it
has been reported that an amphiphilic self-assembled polymeric
copper catalyst promoted the Huisgen 1,3-dipolar cycloaddition
of organic azides and terminal alkynes efficiently with a TON of
up to >200 000, and it was reused without loss of catalytic
activity or leaching of copper species.29 Also, a study on efficient
subnanometric gold-catalyzed hydrogen generation via formic
acid decomposition under ambient conditions revealed TON and
TOF values of 118 400 and 1590, respectively.30 An excellent
TOF value of 1740 for a heterogeneous catalyst under biphasic
conditions has been recently reported in a study on alkene
epoxidation reactions using an immobilized dimeric chiral
Mn(III) salen complex.31 In our present study we obtained TON
and TOF values of 6.5 ꢂ 105 and 5400, respectively. The reasons
for the present excellent TON and TOF values measured using
the Mn_1-SAM may reside in both the low catalyst concentra-
tion and in the adopted Mn_1 grafting geometry. An MM+
calculation (HyperChem 7.5) of the Mn_1 molecule dimensions
Preliminary epoxidation results performed with 1,2-dihy-
dronaphthalene and cis-b-methylstyrene substrates ensured
generality to the catalytic behaviour of the Mn_1-SAM since in
both cases we obtained high conversion, ee and TON values.
Conclusions
A new synthesized (salen)Mn(III) robust monolayer was cova-
lently fabricated on silica substrates and resulted in an active
catalyst for the asymmetric epoxidation of the model 6-ciano-
2,2-dimethyl-chromene. Good selectivity and high TON and
TOF values have been measured. No leaching of the catalyst as
well as no efficiency decrease were observed. In addition,
experimental XPS evidence for the formation of the surface
confined O]MnV(salen) oxene catalyst has been obtained. Our
present study pointed to the generation of a general platform that
will pave the way for heterogeneous epoxidation reactions of
alkenes generating no Mn catalyst waste and allowing contin-
uous recycling.
Acknowledgements
The authors thank the FIRB project ITALNANONET
(RBPR05JH2P), MIUR: P.R.I.N. 2008 and University of
Catania.
Notes and references
2
1 J. Lisowski, Inorg. Chem., 2011, 50, 5567–5576.
ꢁ
indicates a size of 156 A for the Mn(salen) molecular plane.
2 G. Haberhauer, Angew. Chem., Int. Ed., 2010, 49, 9286–9289.
3 A. Gonzalez-Alvarez, I. Alfonso, J. Cano, P. Diaz, V. Gotor,
V. Gotor-Fernandez, E. Garcia-Espana, S. Garcia-Granda,
2
Therefore, the adopted footprint of 254 A , calculated on the
ꢁ
basis of the UV-vis spectra, and the long aliphatic anchoring
20564 | J. Mater. Chem., 2012, 22, 20561–20565
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View Article Online
H. R. Jimenez and F. Lloret, Angew. Chem., Int. Ed., 2009, 48, 6055–
6058.
4 E. N. Jacobsen, in Catalytic Asymmetric Synthesis, ed. I. Ojima, VCH,
Weinheim, 1993, ch. 4.2, pp. 159–202.
5 K. Matsumoto and T. Katsuki, in Catalytic Asymmetric Synthesis, ed.
I. Ojima, J. Wiley & Sons, Inc., New Jersey, 3rd edn, 2010, pp. 839–
890.
21 A. Gulino, T. Gupta, M. Altman, S. Lo Schiavo, P. G. Mineo,
ꢀ
I. L. Fragala, G. Evmenenko, P. Dutta and M. E. van der Boom,
Chem. Commun., 2008, 2900–2902.
22 S. Liao and B. List, Angew. Chem., Int. Ed., 2010, 49, 628
–631.
23 A. D. Shukla, A. Das and M. E. van der Boom, Angew. Chem., Int.
Ed., 2005, 44, 3237.
6 A. F. Trindade, P. M. P. Gois and C. A. M. Afonso, Chem. Rev.,
2009, 109, 418–514.
24 M. Salavati-Niasari, S. N. Mirsattari and M. Bazarganipour,
Polyhedron, 2008, 27, 3653–3661.
7 T. P. Yoon and E. N. Jacobsen, Science, 2003, 299, 1691–1693.
8 I. W. E. E. Arends, Angew. Chem., Int. Ed., 2006, 45, 6250–6252.
9 D.-Y. Xie, S. B. Sharma, N. L. Paiva, D. Ferreira and R. A. Dixon,
Science, 2003, 299, 396–399.
25 I. Bar-Nahum, H. Cohen and R. Neumann, Inorg. Chem., 2003, 42,
3677–3684.
26 D. Feichtinger and D. A. Plattner, Angew. Chem., Int. Ed., 1997, 36,
1718–1719.
10 Q.-H. Xia, H.-Q. Ge, C.-P. Ye, Z.-M. Liu and K.-X. Su, Chem. Rev.,
2005, 105, 1603–1662.
27 R. L. Paddock and S. T. Nguyen, J. Am. Chem. Soc., 2001, 123,
11498–11499.
11 M. L. Merlau, M. Del Pillar Mejia, S. T. Nguyen and J. T. Hupp,
Angew. Chem., Int. Ed., 2001, 40, 4239–4242.
12 A. Corma and H. Garcia, Chem. Soc. Rev., 2008, 37, 2096–2126.
13 B. M. Rossbach, K. Leopold and R. Weberskirch, Angew. Chem., Int.
Ed., 2006, 45, 1309–1312.
28 H. Zhang and C. Li, Tetrahedron, 2006, 62, 6640–6649.
29 Y. M. A. Yamada, S. M. Sarkar and Y. Uozumi, J. Am. Chem. Soc.,
2012, 134, 9285–9290.
30 Q.-Y. Bi, X.-L. Du, Y.-M. Liu, Y. Cao, H.-Y. He and K.-N. Fan, J.
Am. Chem. Soc., 2012, 134, 8926–8933.
14 R. M. Bowler, S. Nakagawa, M. Drezgic, H. A. Roels, R. M. Park,
E. Diamond, D. Mergler, M. Bouchard, R. P. Bowler and
W. Koller, Neurotoxicology, 2007, 28, 298–311.
15 J. M. Thomas and R. Raja, Acc. Chem. Res., 2008, 41, 708–720.
16 S. Bell, B. Wustenberg, S. Kaiser, F. Menges, T. Netscher and
A. Pfaltz, Science, 2006, 311, 642–644.
31 T. Roy, R. I. Kureshy, N. H. Khan, S. H. R. Abdi, A. Sadhukhan and
H. C. Bajaj, Tetrahedron, 2012, 68, 6314–6322.
€
32 K. Tollner, R. Popovitz-Biro, M. Lahav and D. Milstein, Science,
1997, 278, 2100–2102.
33 A. Patti, S. Pedotti, F. P. Ballistreri and G. Trusso Sfrazzetto,
Molecules, 2009, 14, 4312–4325.
17 M. Wills, Science, 2006, 311, 619–620.
34 M. E. Amato, F. P. Ballistreri, A. Pappalardo, G. A. Tomaselli,
R. M. Toscano and J. D. Williams, Eur. J. Org. Chem., 2005, 3562–
3570.
35 F. P. Ballistreri, G. Lombardo, A. Pappalardo, F. Punzo,
A. Thompson, G. A. Tomaselli, R. M. Toscano and G. Trusso
Sfrazzetto, Dalton Trans., 2012, 41, 1951–1960.
18 R. Raja, J. M. Thomas, M. D. Jones, B. F. G. Johnson and
D. E. W. Vaughan, J. Am. Chem. Soc., 2003, 125, 14982–14983.
19 C. Bianchini and P. Barbaro, Top. Catal., 2002, 19, 17–32.
ꢂ
ꢂ
20 J. M. Fraile, J. I. Garcıa, C. I. Herrerıas, J. A. Mayoral and E. Pires,
Chem. Soc. Rev., 2009, 38, 695–706.
This journal is ª The Royal Society of Chemistry 2012
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