Exceptionally efficient catalytic hydrodechlorination of persistent organic pollutants: Application of new sterically shielded palladium carbene complexes

Article abstract of DOI:10.1039/c4dt02908a

A new sterically shielded carbene with branched aromatic substituents (9a) and two palladium halogenide complexes (11a,b) have been prepared. The single crystal X-ray structures of free carbene 9a and palladium carbene complexes 10b and 11a were determined. Very high catalytic efficiencies were evident for the sterically shielded palladium carbene complexes 10b and 11a,b when the latter complexes were employed as catalysts for hydrodechlorination of the chloroarenes p-dichlorobenzene and hexachlorobenzene. When optimized, the foregoing approach is significantly more effective than those of currently known transition metal carbene complexes. The most active catalysts were found to be the monocarbene complexes of palladium chloride and iodide, both of which feature highly branched aromatic substituents (11a,b).

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A. Evans, N. I. Korotkikh, A. H. Cowley , T. M. Pekhtereva, A. F. Popov and O. P. Shvaika, Dalton Trans.,
2014, DOI: 10.1039/C4DT02908A.
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ARTICLE
Exceptionally Efficient Catalytic
Hydrodechlorination of Persistent Organic
Pollutants: Application of New Sterically Shielded
Palladium Carbene Complexes
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Vagiz Sh. Saberov,a Daniel A. Evans,b Nikolai I. Korotkikh,a* Alan H. Cowley,b Tatyana
M. Pekhtereva,a Anatolii F. Popov,a and Oles P. Shvaikaa
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new sterically shielded carbene with branched aromatic substituents (9a) and two palladium
halogenide complexes (11a,b) have been prepared. The single crystal X-ray structures of free
carbene 9a and palladium carbene complexes 10b and 11a were determined. Very high
catalytic efficiences were evident for the sterically shielded palladium carbene complexes 10b
and 11a,b when the latter complexes were employed as catalysts for hydrodechlorination of the
chloroarenes p-dichlorobenzene and hexachlorobenzene. When optimized, the foregoing
approach is significantly more effective than those of currently known transition metal carbene
complexes. The most active catalysts were found to be the monocarbene complexes of
palladium chloride and iodide, both of which feature highly branched aromatic substituents
(
11a,b).
Introduction
Several transition metal carbene complexes have been
shown to be active with respect to hydrodehalogenation
processes. In particular, palladium carbene complexes are well
known to be suitable catalysts for the foregoing reactions. As
an example, Nolan et al. demonstrated that an in situ generated
palladium carbene catalyst based on Pd(dba)2, potassium
methoxide, and various imidazolium salts resulted in high
yields of hydrodehalogenation products upon reaction in
dioxane solution at 100 С (TON up to 28–50, TOF up to 28–
50 h-1).5
Haloarene compounds are members of a class of toxic waste
that is commonly referred to as persistent organic pollutants
(POPs). While haloarenes have useful applications in several
important areas such as pesticides, pharmaceuticals, and some
high-temperature fluids, these materials are highly toxic.
Furthermore, haloarenes are resistant to environmental
degradation and have accumulated in significant quantities
throughout the world. The compounds that require particularly
careful attention include DDT, hexachlorobenzene, dioxins,
polychlorodibenzofurans, and polychlorobiphenyls.1 Indeed, all
of the foregoing compounds were included in the list of the 12
most hazardous persistent organic pollutants that was presented
at the 2001 Stockholm Convention. Clearly, the safe
management of these toxic POPs represents an environmental
concern of paramount importance.
As described in subsequent publications by Nolan et al.,
complex
efficiencies for the reactions of chlorobenzene at ambient
temperature (TON 100, TOF 200 h-1).6 Moreover, complex
1 (Figure 1) was found to exhibit moderate catalytic
2
was shown to be active for the hydrodehalogenation of p-
dichlorobenzene at 60 С (TON 194, TOF 111 h-1).7
Interestingly, when microwave assistance was employed for the
Several approaches have been developed for haloarene
detoxification. Well established methods include combustion at
high temperatures, oxidation in supercritical water, catalytic
reduction with hydrogen, reduction by metals or nanometals,
biodegradation, and microwave techniques.2-4 Among the
foregoing methods, catalytic hydrodehalogenation offers the
most promise as a cost effective solution for the neutralization
of toxic POPs.
reaction with complex 2, an even higher rate of reaction could
be achieved at 120 С. Using the latter approach, virtually
quantitative yields for the hydrodechlorination of p-
chlorotoluene and p-dichlorobenzene can be achieved in only 2
min (TON 4000, TOF 120000 h-1 p-chlorotoluene; TON 3800,
TOF 114000 h-1 p-dichlorobenzene).7
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outlined in Scheme 1. First, the sterically hindered 2,6-
disubstituted aniline 6a was prepared by treatment of 4-
ethylaniline with benzhydrol in the presence of ZnCl2 and
concentrated aqueous HCl. The subsequent reaction of the
bulky aniline 6a with glyoxal in n-propanol solution in the
presence of formic acid resulted in the formation of the
corresponding diimine 7a. In the next step, cyclization of the
diimine 7a was accomplished by treatment with
paraformaldehyde in the presence of ZnCl2 and concentrated
aqueous HCl, thereby forming the 1,3-diarylimidazolium salt
8a. Finally, treatment of the imidazolium salt 8a with NaH in
THF solution afforded the free sterically shielded stable
carbene 9a (see Supporting Information).
In principle, the procedure that was employed for the
synthesis of the imidazolium salt 8a is similar to that described
by Marko et al.11 The known carbene 9b was generated from
the imidazolium salt 8b via reaction with potassium tert-
butoxide in THF solution. The subsequent reactions of the
foregoing carbenes with palladium(II) chloride or palladium
(II) iodide were carried out using a 1:1 molar ratio in THF
solution at room temperature according to the literature
procedure.12 The outcomes of these reactions were found to be
dependent on the steric demands of the nitrogen substituents.
On the one hand, the reaction of the more bulky dbep
substituted carbene 9a with palladium halogenides afforded the
mononuclear carbene complexes 11a,b. On the other hand,
treatment of the less bulky dipp-substituted carbene 9b with
PdI2 resulted in the formation of the diiodo-bridged dinuclear
palladium carbene complex 10b, and is reminiscent of the
corresponding reaction with PdCl2.12
Figure 1. Palladium and nickel carbene catalysts used for
hydrodehalogenation reactions
In addition to the three palladium carbene complexes
discussed above, two nickel(I) complexes (Figure 1) have also
been examined as potential catalysts for hydrodehalogenation
processes. Unfortunately, however, the nickel(I) complex
3
proved to be ineffective for both the hydrodebromination of p-
bromofluorobenzene and also for the hydrodechlorination of p-
chlorofluorobenzene, both of which are activated haloarenes
(TON 15-22 and TOF 4-44 h-1 at 25 С).8 Interestingly,
however, the nickel(I) complex
4 was found to be active for the
hydrodefluorination of fluorobenzenes.9 Taken collectively,
complexes 1-4 were found to exhibit inadequate catalytic
activities for the intended reactions.
Characterization of 6a-11b
Currently,
the
highest
efficiency
for
the
1
Compounds 6a-11b were characterized by means of H and 13
C
hydrodehalogenation of polyhalogenated arenes was obtained
by using the dichloro-bridged dinuclear palladium carbene
NMR spectroscopy and their compositions were established on
the basis of elemental analyses. The crystal structures of 9a
10b, and 11a were determined by single crystal X-ray
,
complex
5
that is displayed in Figure 1.10 This catalyst proved
to be highly active for the hydrodehalogenation of either
1,2,4,5-tetrachlorobenzene or 2,2,3,3-tetrachlorobiphenyl in
isopropanol solution at 80 С (TON 19650, TOF 819 h-1 and
TON 10000, TOF 417 h-1, respectively).10
diffraction (see Supporting Information).
Single Crystal X-ray Diffraction Studies
As evident from Figure 1, the existence of steric hindrance
adjacent to a carbenic carbon atom plays an important role with
respect to the reactivities of carbenes. With the overall
objective of developing more efficient catalysts for use in the
environmentally important haloarene hydrodehalogenation
reaction, the present work is focused on the development of
new highly sterically shielded palladium carbene complexes.
Colorless single crystals of the free carbene 9a were grown by
slow evaporation of a benzene solution of 9a and subsequently
examined by X-ray diffraction. The free carbene 9a crystallized
in the P21/c space group of the monoclinic crystal system with
two disordered benzene solvent molecules in the asymmetric
unit. The crystal structure of 9a, which is displayed in Figure 2,
features a planar C(1)-N(1)-C(2)-C(3)-N(2) five-membered
ring that is akin to those found for the majority of imidazol-2-
ylidene rings. The plane of the central imidazol-2-ylidene
moiety is oriented in an almost orthogonal fashion with respect
to the planes of the flanking aryl substituents (86.24 and
87.97). The bond angle at the carbene carbon atom is
101.5(2), and the existence of high bond orders of 1.61–1.63
for the С(1)-N(1) and C(1)-N(2) bonds and 1.50–1.53 for the
Results and Discussion
Syntheses of the stable, sterically shielded carbene 9a and
palladium carbene complexes 10b and 11a,b
The sterically shielded stable carbene 9a that features branched
aromatic N-substituents was synthesized in four steps as
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Scheme 1. Syntheses of sterically shielded carbenes and their respective palladium complexes (dbep = 2,6-dibenzhydryl-4-
ethylphenyl)
C(2)-N(1) and C(3)-N(2) bonds imply a substantial amount of palladium carbene complex 11a crystallized in the I2/a space
delocalization within the carbene ring structure. Interestingly, group of the monoclinic crystal system. Complex 11a also
the C(2)-C(3) bond (Table 1) features a noticeably shortened features a two-fold rotational axis that bisects the complex
bond length of 1.327(5) Å and a bond order of 2.051.
along the C(1)-Pd(1)-N(2) bond, hence only half of the
Dark red single crystals of 10b were grown by slow vapor complex is apparent in the asymmetric unit. In contrast to 10b
,
diffusion of hexanes into a chloroform solution of this complex 11a possesses a mononuclear structure. The bond
compound. As displayed in Figure 2, complex 10b possesses a angle at the carbene carbon (105.4(3)) is significantly larger
diiodo-bridged dinuclear structure of the overall composition than that exhibited by the free carbene 9a (101.5(2)).
(LPdI2)2.
Moreover, the imidazol-2-ylidene bond lengths are similar to
Complex 10b crystallizes in the P-1 space group of the those measured for the free carbene 9a (Table 1). Furthermore,
triclinic crystal system and features two diiodo-bridged
Table 1. Selected bond lengths (Å) for 9a 10b, and 11a
the angle between the plane of the imidazol-2-ylidene ring and
that of the flanking aryl substituent is more acute in 11a
(71.68) than that in the free carbene 9a (86.24 and 87.97). In
summary, palladium complexes 10b and 11a both adopt square
planar geometries and feature strong Pd-C bonds and noticeably
shortened Pd-Cl and Pd-I bonds (Supporting Information).
,
.
Bond
9a (Å)
10b (Å)
1.347(6)
1.352(6)
11a (Å)
C(1)-N(1)
C(1)-N(2)
C(1)-N(1a)
C(2)-C(3)
C(2)-C(2a)
С(2)-N(1)
С(3)-N(2)
C(1)-Pd(1)
1.364(4)
1.368(4)
1.359(3)
Catalytic Studies
1.359 (3)
-
1.340(5)
1.394(3)
-
The hydrodechlorination test reactions were performed by
heating each reaction mixture containing the haloarene with
potassium or sodium isopropoxide. The latter compounds had
been generated from potassium or sodium tert-butoxide (or
hydroxide) in the presence of the palladium catalyst in
isopropanol solution at 80 С. Each percent conversion was
based on one chlorine atom and calculated according to the
masses of the sodium or potassium chlorides. The most
efficient catalysis trials were examined by means of gas
chromatography. The catalytic efficiencies were estimated by
using the TON and TOF values (the general procedure is
described in the Supporting Information). A summary of the
catalytic efficiencies of carbene complexes 10a,b and 11a,b
with respect to the model compound p-dichlorobenzene and a
variety of bases is presented in Table 2.
1.327(5)
1.332 (7)
1.387(4)
1.381(4)
-
1.378 (6)
1.387(6)
1.985(4)
1.961(4)
dinuclear structures in the asymmetric unit. Interestingly,
complex 10b exhibits a distorted I(2)-Pd(1)-I(3)-Pd(2) four-
membered ring structure with a torsion angle of 157.522(16)°.
Furthermore, the planes of the imidazolium and adjacent
flanking dipp groups are oriented in an essentially orthogonal
manner (82.0° and 86.5°).
Yellow single crystals of the palladium carbene complex
11a were grown by slow evaporation of an acetonitrile solution
and examined by single crystal X-ray diffraction. The
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Figure 2. POV-Ray diagrams of the free carbene 9a (left) and palladium carbene complexes 10b (center) and 11a (right) with
thermal ellipsoids displayed at 50% probability. Compound 9a crystallized with two disordered benzene solvent molecules and
complex 10b crystallized with two dinuclear species in the asymmetric unit, only one of which is displayed above. Complex 11a
crystallized with two disordered acetonitrile solvent molecules. All solvent molecules and hydrogen atoms have been omitted for
clarity.
As evident from Table 2, the hydrodechlorination of p- chloride complex 10a had been replaced by the palladium
dichlorobenzene with potassium tert-butoxide in the presence iodide complex 10b (TON 44000, TOF 1830 h-1). Upon
of catalysts 10a,b and 11a,b proceeded very efficiently.
changing the base that had been used in the reaction with
sodium tert-butoxide (Table 2), a similar level of efficiency was
evident for complex 10b (TON 46000, TOF 1916 h-1).
However, the efficiency of 10b significantly exceeded that
observed for the palladium chloride complex 10a (TON 6400,
TOF 266 h-1).
Table 2. Catalytic efficiences of palladium catalysts 10a,b and
11a,b for the hydrodechlorination of p-dichlorobenzene
As the next step, the hydrodechlorination reactions of 10a,b
and 11a,b were carried out in the presence of sodium hydroxide
in an isopropanol solution. From the standpoint of haloarene
neutralization, the use of the inexpensive base sodium
hydroxide is of particularly importance with regard to
applications in real world technology. It was found that the
known complex 10a was somewhat more effective for the
reaction with sodium hydroxide (TON 19200, TOF 800 h-1),
than with either potassium or sodium tert-butoxide (TON
17600, TOF 734 h-1 and TON 6400, TOF 266 h-1, respectively).
Moreover, complex 10b (TON 42000, TOF 1750 h-1) was
significantly more effective than 10a in the presence of NaOH.
Overall, the efficiency of 10b is somewhat independent of
the identity of the bases that were tested for use in this reaction.
However, upon using 0.001 mol % of 10а, a decrease in
catalytic efficiency became evident (TON 10000, TOF 416 h-1
vs. 19200 and 800 h-1, respectively with 0.01 mol % of catalyst
10a). The foregoing decrease in efficiency is possibly due to
Cat.
Cat.
loading me
(mol
%)
Ti-
Con- TON
TOF
(h-1)
ROM
ver.
(%)
(per
Cl)
1
(h)
10a
10a
10a
10а
10а
10b
10b
10b
10b
0.01
0.01
0.01
0.01
0.001
0.01
0.001
0.01
0.001
8
63
88
32
96
5
100
22
100
23
30
100
21
30
70
90
12600
17600
6400
1560
734
266
800
416
834
1830
834
1916
2500
834
1750
2500
5833
7500
KOtBu
KOtBu
NaOtBu
NaOH
NaOH
KOtBu
KOtBu
NaOtBu
NaOtBu
NaOtBu
NaOH
NaOH
NaOH
KOtBu
KOtBu
24
24
24
24
24
24
24
24
24
24
24
24
24
24
19200
10000
20000
44000
20000
46000
60000
20000
42000
60000
140000
180000
10b* 0.001
10b
10b
10b* 0.001
11a
0.01
0.001
the equilibrium reaction of i-PrOH + NaOH
⇄ i-PrONa + H2O
that generates water and thereby deactivates this complex,
particularly at lower catalyst concentrations. Interestingly, the
catalytic efficiency of complex 10b increased in the presence
of a small quantity of lithium tert-butoxide (1 mol %) (TON
60000, TOF 2500 h-1 vs. 42000 and 1750 h-1 respectively for
complex 10b without the additive).
0.001
0.001
11b
*The reaction was run in the presence of 1% LiOtBu
The catalytic efficiences of the dipp-substituted complexes
10a,b were examined initially. The known complex 10a13
exhibited high TON and TOF values for the
hydrodechlorination of p-dichlorobenzene (TON 17600, TOF
1560 h-1). The catalytic efficiency increased when the known
The sterically shielded palladium carbene complexes with
branched aromatic N-substituents, 11a,b, proved to be
significantly more efficient than 10a,b. The catalytic trials that
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were carried out using 11a,b with potassium tert-butoxide
(Table 2) exhibited excellent efficiences and increased TON genation reaction (Scheme 2)
values of 140000 and 180000 and TOF values of 5833 h-1 and the pre-catalyst
with potassium isopropoxide (that was
7500 h-1, respectively.
generated from potassium or sodium tert-butoxide or
The most effective catalysts were found to be 10b and hydroxide) to form the alkoxide derivative . In the next step,
11a,b as determined on the basis of the model the latter complex underwent β-hydride elimination, thereby
hydrodechlorination reactions with p-dichlorobenzene. releasing acetone and generating the hydridic palladium species
Subsequently, 10b and 11a,b were examined for the . The subsequent reaction of the hydridic species with
hydrodechlorination of the known POP hexachlorobenzene. potassium tert-butoxide resulted in formation of the active
The pertinent catalytic data obtained in this study can be found palladium(0) complex . Subsequent activation of the C-Cl
in Table 3. The catalysts 10b and 11a,b displayed excellent bond in the haloarene compound
The proposed mechanism for the haloarene hydrodehalo-
14, 15
involves initial reaction of
A
B
,
C
C
D
E
by complex
D
resulted in
efficiencies for the hydrodechlorination of hexachlorobenzene oxidative addition and the formation of complex
with TON and TOF values that ranged from 88000–318000 and
3667 – 13350 h-1, respectively. Reactions that resulted in 100%
F
.
conversion, as estimated by mass analysis of the metal halide
precipitates, were confirmed by means of gas chromatography,
which did not detect any intermediates or initial haloarenes.
Table 3. Catalytic efficiences of palladium catalysts 10a,b and
11a,b for the hydrodechlorination of hexachlorobenzene
Cat. Cat.
load.
Ti-
me
(h)
Conv. TON
TOF
(h-1
ROM
(%)
(per
Cl)
1
)
(mol
%)
10b 0.01
10b 0.01
10b 0.003
11a 0.001
11a 0.005
11b 0.001
11b 0.004
24
24
24
24
24
24
24
100
66
44
35
100
53
60000
39600
88000
210000
120000
318000
150000
2500
1650
3667
8750
5000
13350 KOtBu
6250
KOtBu
KOtBu
NaOH
KOtBu
KOtBu
KOtBu
Scheme 2. Proposed mechanism for haloarene hydrodehalo-
genation
The reaction of complex
F
with potassium isopropoxide
afforded the alkoxide complex
G. Following this, an analogous
β-hydride elimination step occurred, thereby generating the
hydridic species . Finally, reductive elimination resulted in
formation of the protonated arene compound and regeneration
of the active catalyst palladium(0) complex
Based on previous studies of related sterically shielded
1,2,4-triazol-5-ylidene systems,16 an important stage in the
foregoing process may involve interaction of the initial
H
100
D
.
To the best of our knowledge, the catalytic efficiencies that
were determined for complexes 11a,b represent the highest
efficiencies reported to date for the hydrodehalogenation of
polyhaloarenes (TON 210000 – 318000, TOF 8750 – 13350 h-
1). Quantitative conversion per chlorine atom was achieved
after 24 h with catalyst concentrations of 0.004 – 0. 005 mol%.
When comparing the efficiencies of 10b and 11a,b for the
hydrodehalogenation of hexachlorobenzene, the best known
catalyst (10а) was found to exhibit lower catalytic efficiencies
for the reactions involving 1,2,4,5-tetrachlorobenzene (TON
19700, TOF 821 h-1), 2,2,3,3-tetrachlorobiphenyl (TON
10000, TOF 420 h-1), and decachlorobiphenyl (TON 1000, TOF
42 h-1).10 The best catalysts, 11a,b, exceeded the maximum
level of efficiency for 10a by a factor of 10 – 16 times.
Furthermore, catalysts 11a,b required only a 0.001 – 0.005 mol
percent concentration in order to function efficiently as
compared with 10a which is only active in the concentration
range of 0.01-0.04 mol percent.
precatalyst
A with the metal isopropoxide. This reaction should
be more facile for the iodide complexes than for the
corresponding chloride analogues. As might be expected, the
stabilities of these complexes depend on the extent of steric
congestion.
In summary, it is noteworthy that highest catalytic activities
for hydrodehalogenation to date have been achieved by the
development of new palladium complexes with sterically
shielded 1,3-diarylimidazol-2-ylidene ligands that feature two
bulky, branched aromatic N-substituents, namely 11a,b. A
plausible reason for the dramatic increases in catalytic activities
that were observed for the sterically shielded carbene
complexes 11a,b (in comparison with those of their dipp
substituted counterparts 10a,b
)
could be atrributed to
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stabilization of the Pd(0) complex
D on account of the steric
protection afforded by the bulky carbene ligand. Another
important contributor to this enhancement of catalytic
efficiency is the presence of iodide as the second ligand (10b,
1
T. I. Gorbunov, V. I. Saloutin, О. N. Chupakhin, Rus. Chem. Rev.,
2010, 79 , 565 (rus).
2
3
F. Alonso, I. Beletskaya, M. Yus, Chem. Rev., 2002, 102, 4009-4091.
C. L. Geiger, K. Carvalho-Knighton, S. Novaes-Card, P. Maloney, R.
DeVor, In Environmental Applications of Nanoscale and Microscale
Reactive Metal Particles / ACS Symposium Series; American
Chemical Society: Washington, DC, 2010.
11b) (chloride ligands typically exhibit lower catalytic
efficiencies than those of iodide-containing complexes). In
summary, the excellent activities that were observed for 11a,b
at very low mol percent catalyst loadings imply that 11a,b
represent a realistic and efficient technology for the remediation
of haloarene POPs.
4
5
6
7
8
9
W. M. Czaplik, S. Grupe, M. Mayer, A. J. von Wangelin, Chem.
Commun., 2010, 46, 6350.
M. S. Viciu, G. A. Grasa, S. P. Nolan, Organometallics, 2001, 20
,
3607.
Conclusions
O. Navarro, N. Marion, Y. Oonishi, R. A. Kelly, S. P. Nolan, J. Org.
Chem., 2006, 71, 685.
A new sterically shielded stable carbene of the imidazole series
with highly branched aromatic N-substituents (9a) has been
synthesized and structurally authenticated by means of a single
crystal X-ray diffraction study. The X-ray crystal structures of
the palladium carbene complexes 10b and 11a were also
determined. It was discovered that complexes 10b and 11a,b
serve as highly efficient catalysts for the hydrodehalogenation
of both hexachlorobenzene and p-dichlorobenzene. Moreover,
each catalyst proved to be effective at very low mol percent
catalyst loadings. Furthermore, the catalytic efficiencies of
complexes 10b and 11a,b were found to be significantly
superior for the hydrodehalogenation reaction of hexachloro-
benzene than those of p-dichlorobenzene. In summary,
complexes 11a,b represent excellent candidates for the
effective waste management of haloarene POPs. Finally, the
sterically shielded imidazol-2-ylidene palladium iodide
complex 11b was found to exhibit the highest efficiency for the
hydrodehalogenation reaction that has been reported to date and
significantly exceeds the efficiencies of the current literature
O. Navarro, H. Kaur, P. Mahjoor, S. P. Nolan, J. Org. Chem., 2004,
69, 3173.
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Acknowledgments
We thank the Ukrainian Academy of Sciences for financial
support (Grant № 140, 28.03.2013), and the generous financial
support provided by the Robert A. Welch Foundation (Grant F-
0003) (A.H.C.)
15 N. I. Korotkikh, O. P. Shvaika, Carbene and carbene complex cata-
lysis of organic reactions, Donetsk, DonNU, 2013, 372 p (ukr.).
16 N. V. Glinyanaya, V. Sh. Saberov, N. I. Korotkikh, A. H. Cowley, R.
R. Butorac, D. A. Evans, T. M. Pekhtereva, A. F. Popov, O. P.
Shvaika
,
Dalton Trans., 2014 (accepted), DOI: 10.1039/c4dt01353k
References
a The L.M. Litvinenko Institute of Physical Organic and Coal Chemistry,
Ukrainian Academy of Sciences, 70, R. Luxemburg, Donetsk, 83114,
Ukraine; Fax: (38+062)311-68-30; Tel: (38+062)311-68-35; E-mail:
nkorotkikh@ua.fm
b Department of Chemistry, The University of Texas at Austin, 1 University
Station A5300, Austin, Texas 78712-0165; Tel: (512)471-74-84; E-mail:
acowley@cm.utexas.edu
†
Electronic Supplementary Information (ESI) available: Experimental
procedures for the preparation of compounds 6a,7a,8a,9a,10b,11a,b and
for the catalytic reaction; 1H and 13C NMR spectra; X-ray crystallo-
graphic data and CIF files for 9a, 10b, 11a; CCDC 1009073 (9a),
1009074 (10b), 1009075 (11b).
6 | Chem. Sci., 2014, 00, 1-3
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