Novel palladium-aminocarbene species derived from metal-mediated coupling of isonitriles and 1,3-diiminoisoindoline: Synthesis and catalytic application in Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1021/om300020j

The reaction between metal-bound isonitriles in cis-[PdCl ₂(CNR¹)₂] [R¹ = cyclohexyl (Cy) 1, Bu^t2, C₆H₄(2,6-Me₂) (Xyl) 3, CMe₂CH₂CMe₃4] and 1,3-diiminoisoindoline (9) in CHCl₃ under reflux for 4 h provides the carbene species [Pd{C(N=C^a(C₆H₄CNHN^b))=N(H)R ¹}₂]^a-b (R¹ = Cy 10, 82% isolated yield) or cis-[PdCl{C(N=C^a(C₆H₄CNHN ^b))=N(H)R¹}(CNR¹)]^a-b (R¹ = Bu^t11, 78%; Xyl 12, 84%; CMe₂CH₂CMe ₃13, 79%), derived from the addition of two or one equivs of 9 to the starting 1-4, respectively. The corresponding integration of cis-[PdCl ₂(CNR¹)(PPh₃)] (R¹ = Cy 5, Bu ^t6, CMe₂CH₂CMe₃8) with 1 equiv of 9 in CHCl₃ under reflux for 4 h affords the carbene species cis-[PdCl{C(N=C^a(C₆H₄CNHN^b))=N(H) R¹}(PPh₃)]^a-b (R¹ = Cy 14, 84%; Bu^t15, 76%; CMe₂CH₂CMe₃16, 75%). Complexes 10-16 were characterized by elemental analyses (C, H, N), ESI ⁺-MS, IR, and 1D (¹H, ¹³C{¹H}) and 2D (¹H,¹H-COSY, ¹H,¹³C-HMQC/ ¹H,¹³C-HSQC, ¹H,¹³C-HMBC) NMR spectroscopies. In addition, the structures of aminocarbene complexes 10 and 12 were elucidated by X-ray diffraction. The catalytic properties of 10-16 in the Suzuki-Miyaura cross-coupling of aryl halides, viz., 4-R²C ₆H₄X (X = I, R² = OMe; X = Br, R² = Me, OMe, and NO₂), with phenylboronic acid (in EtOH as solvent, K₂CO₃ or Cs₂CO₃ as base, 80 °C) yielding biaryl species 4-R²C₆H₄Ph, were evaluated. Complexes 14-16 exhibit a high catalytic activity (yields up to 98%, TONs up to 9.8 × 10⁴, TOFs up to 3.9 × 10⁴).

Full text of DOI:10.1021/om300020j

Article
pubs.acs.org/Organometallics
Novel Palladium−Aminocarbene Species Derived from Metal-
Mediated Coupling of Isonitriles and 1,3-Diiminoisoindoline:
Synthesis and Catalytic Application in Suzuki−Miyaura Cross-
Coupling
Rogerio S. Chay,† Konstantin V. Luzyanin,*,†,‡ Vadim Yu. Kukushkin,‡,§ M. Fatima C. Guedes da Silva,†,⊥
́
́
and Armando J. L. Pombeiro*,†
†Centro de Química Estrutural, Complexo I, Instituto Superior Tec
1049-001, Lisbon, Portugal
́
nico, Technical University of Lisbon, Avenida Rovisco Pais,
‡St.Petersburg State University, 198504 Stary Petergof, Russian Federation
§Institute of Macromolecular Compounds of Russian Academy of Sciences, Bolshoii Pr. 31, 199004 St.Petersburg, Russian Federation
⊥Universidade Lusof
́
ona de Humanidades e Tecnologias, ULHT Lisbon, 1749-024 Lisbon, Portugal
S
* Supporting Information
ABSTRACT: The reaction between metal-bound isonitriles in
cis-[PdCl2(CNR1)2] [R1 = cyclohexyl (Cy) 1, But 2, C6H4(2,6-
Me2) (Xyl) 3, CMe2CH2CMe3 4] and 1,3-diiminoisoindoline (9)
in CHCl3 under reflux for 4 h provides the carbene species
[Pd{C(NCa(C6H4CNHNb))N(H)R1}2]a−b (R1 = Cy 10,
82% isolated yield) or cis-[PdCl{C(NCa(C6H4CNHNb))
N(H)R1}(CNR1)]a−b (R1 = But 11, 78%; Xyl 12, 84%;
CMe2CH2CMe3 13, 79%), derived from the addition of two or
one equivs of 9 to the starting 1−4, respectively. The
corresponding integration of cis-[PdCl2(CNR1)(PPh3)] (R1
=
Cy 5, But 6, CMe2CH2CMe3 8) with 1 equiv of 9 in CHCl3 under
reflux for 4 h affords the carbene species cis-[PdCl{C(N
Ca(C6H4CNHNb))N(H)R1}(PPh3)]a−b (R1 = Cy 14, 84%; But
15, 76%; CMe2CH2CMe3 16, 75%). Complexes 10−16 were characterized by elemental analyses (C, H, N), ESI+-MS, IR, and
1D (1H, 13C{1H}) and 2D (1H,1H-COSY, H,13C-HMQC/1H,13C-HSQC, H,13C-HMBC) NMR spectroscopies. In addition,
the structures of aminocarbene complexes 10 and 12 were elucidated by X-ray diffraction. The catalytic properties of 10−16 in
the Suzuki−Miyaura cross-coupling of aryl halides, viz., 4-R2C6H4X (X = I, R2 = OMe; X = Br, R2 = Me, OMe, and NO2), with
phenylboronic acid (in EtOH as solvent, K2CO3 or Cs2CO3 as base, 80 °C) yielding biaryl species 4-R2C6H4Ph, were evaluated.
Complexes 14−16 exhibit a high catalytic activity (yields up to 98%, TONs up to 9.8 × 104, TOFs up to 3.9 × 104).
1
1
Despite the appealing characteristics and similarities between
these two types of aminocarbene species, ADCs have so far
attracted much less attention as ancillary ligands in the cross-
coupling transformations as compared to the related N-
heterocyclic analogues. To date, only a few reports on catalytic
applications of palladium-ADCs in intermolecular Heck,25,26
Buchwald−Hartwig,25 Sonogashira,26,27 and Suzuki−
Miyaura26,28−31 cross-coupling and several intramolecular
cyclization reactions32−41 have been published, whereas the
corresponding [Pd]NHC species have found a broad
application as catalysts for such coupling reactions.4,6,7
INTRODUCTION
■
Acyclic diaminocarbenes (ADCs)1−3 are structural analogues of
the corresponding N-heterocyclic carbene species (NHCs);4−12
the latter compounds have revolutioned organometallic
catalysis (Scheme 1). ADCs exhibit a similar electronic
stabilization (delocalization of the electron density within the
carbene moiety) and possess comparable (equal bond lengths)
or slightly different (wider angles around the carbene carbon)
structural characteristics and coordination abilities as the related
NHCs.13−16 Metal complexes with ADC ligands1−3,13−15,17−24
are typically generated via two distinct synthetic protocols, i.e.,
by (i) a direct complexation of the generated in situ free
carbene species to a metal center1 or through (ii) a metal-
mediated nucleophilic attack on an isonitrile (Scheme 1).1,2 In
the latter approach, amines or hydrazines (sp3-N nucleophiles)
are commonly employed as nucleophiles.
Recently, we have reported on the first examples of novel-
type aminocarbene ligands derived from the coupling of
palladium-bound isonitriles with sp2-N [imines, e.g., 3-
Received: January 10, 2012
Published: March 5, 2012
© 2012 American Chemical Society
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Scheme 1. Complexes with N-Heterocyclic Carbene ([M]NHC) and Acyclic Diaminocarbene ([M]ADC) Ligands and Main
Routes for the Preparation of [M]ADC
Scheme 2. Generation of Novel-Type Aminocarbenes via Addition of 3-Iminoisoindolin-1-one (Route A) and N-
Phenylbenzamidine (Route B) to Metal-Bound Isonitrile
Scheme 3. Reaction of 1,3-Diiminoisoindoline (9) with the Isonitriles in cis-[PdCl2(CNR1)2] (1−4)
iminoisoindolin-1-one]42,43 or mixed sp2/sp3-N [amidines, e.g.,
N-phenylbenzamidine]27 type nucleophiles. Two of the studied
reactants, i.e., 3-iminoisoindolin-1-one and N-phenylbenzami-
dine, possess one amidine-type reaction center [HNC(NH),
Scheme 2] that in the reaction with metal-bound isonitrile
behaves as both a nucleophile (via the imine NH) and a
chelator (via the amide HN), affording a bidentate carbene-
amide ligand.
tautomeric equilibrium between the amino and imino forms
(Figure 1). Theoretical47,48 and X-ray diffraction46 studies
Following our ongoing project on the design of novel
carbene ligands,28,42−44 and in order to extend the family of
acyclic aminocarbene species, we decided to employ in the
reactions with various palladium−isonitrile species a structurally
related nucleophile bearing fused amidine and imine functions in
one molecule, viz., 1,3-diiminoisoindoline [HNCC6H4C(
NH)NH, see Scheme 3]. In addition, taking into account that
the catalytic properties of metal-ADC species are much less
explored as compared to the corresponding metal-NHCs, we
have also evaluated the catalytic efficiency of the newly
generated palladium carbenes in Suzuki−Miyaura cross-
coupling. The results of our study are disclosed in the sections
that follow.
Figure 1.
clearly indicate that the imino form possesses lower energy,
being the predominant one in solution, while the amino form is
prevalent in the solid state, where its stabilization is explained
mostly by intermolecular hydrogen bonding.
As electrophilic reagents, we addressed the known palladium-
(II) isonitrile complexes cis-[PdCl2(CNR1)2] [R1 = cyclohexyl
(Cy) 1, But 2, C6H4(2,6-Me2) (Xyl) 3] and cis-[PdCl2(CNR1)-
(PPh3)] [R1 = cyclohexyl (Cy) 5, But 6, C6H4(2,6-Me2) (Xyl)
7].49−53 In addition, we also prepared two new palladium
species, viz., cis-[PdCl2(CNCMe2CH2CMe3)2] (4) and cis-
[PdCl2(CNCMe2CH2CMe3)(PPh3)] (8), bearing 2-isocyano-
2,4,4-trimethylpentane ligands. The latter were synthesized via
slightly modified procedures applied for the preparation of 1−3
and 5−7, correspondingly, i.e., by replacement of two
acetonitrile ligands in [PdCl2(NCMe)2] by 2-isocyano-2,4,4-
RESULTS AND DISCUSSIONS
■
Metal-Mediated Coupling of 1,3-Diiminoisoindolines
with Palladium-Bound Isonitriles. For this study we
employed 1,3-diiminoisoindoline45 (9) as the nucleophile. In
accord with literature sources,46−48 this species exists in the
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Scheme 4. Reaction of 1,3-Diiminoisoindoline (9) with the Ligated Isonitrile in cis-[PdCl2(CNR1)(PPh3)] (5, 6, and 8)
trimethylpentane (2 equiv), affording 4 (90%), or by the
reaction of 4 with PPh3 (1 equiv) that furnishes 8 (87%).
In the initial experiments, we did not observe any reaction
between equimolar amounts of the uncomplexed isonitriles and
9. Under refluxing CHCl3, in the absence of any Pd complex,
only the starting materials were detected in the mixture after 2
d. This observation suggests that the addition of 9 to the
isonitrile CN triple bond described in the following sections has
a metal-mediated character.
The reactions between 1−8 and 9 in CHCl3 (Schemes 3, 4
and 1S of the Supporting Information) were studied at different
temperatures employing various ratios of reagents, and details
of these studies are provided in the Supporting Information.
The reaction between a metal-bound isonitrile in 1 and 2
equiv of 9 proceeds efficiently under CHCl3 reflux conditions
for 4 h (Scheme 3, route C), and the subsequent workup
provides the carbene species [Pd{C(NCa(C6H4CNHNb))
N(H)Cy}2]a−b (10) in 82% isolated yield. When 1 equiv of 9
was employed, complex 10 was formed in ca. 40% yield, and
unreacted starting isonitrile complex 1 (ca. 35%) was detected
in the reaction solution along with yet unidentified species
(three spots with close retention on TLC). The corresponding
reaction between a metal-bound isonitrile in each of 2−4 and 1
equiv of 9 proceeds in CHCl3 under reflux for 4 h (Scheme 3,
route D), and the subsequent workup provides the carbene
species cis-[PdCl{C(NCa(C6H4CNHNb))N(H)R1}-
(CNR1)]a−b (11−13) isolated in 78−84% yield. When 2
equivs of 9 were employed, complexes 11−13 were formed in
the same yield, although being strongly contaminated with
excess 9, 9·HCl, and other yet unidentified species (four spots
on TLC in each case).
Furthermore, the reaction between 5, 6, and 8 and 1 equiv of
9 in CHCl3 under reflux for 4 h (Scheme 4, route E) provides
the carbene species cis-[PdCl{C(NCa(C6H4CNHNb))
N(H)R1}(PPh3)]a−b (14−16) in 80−85% isolated yields.
When 2 equiv of 9 was employed, the reaction proceeds with
a similar rate, but the formed complexes 14−16 (ca. 80−85%)
were contaminated with unreacted 9, 9·HCl, and other
byproduct (five spots on TLC). The reaction between cis-
[PdCl2(CNXyl)(PPh3)] (7) and 1 equiv of 9 in CHCl3 under
reflux for 4 h affords a mixture of species (five spots on TLC),
which were not separated.
Characterization of Aminocarbene Complexes 10−16.
Complexes 10−16 were characterized by elemental analyses
(C, H, N), ESI+-MS, IR, and 1D (1H, 13C{1H}) and 2D
(1H,1H-COSY, 1H,13C-HMQC/1H,13C-HSQC, 1H,13C-
HMBC) NMR spectroscopies. In addition, the structures of
10 and 12 were elucidated by X-ray diffraction.
ion peaks and/or a fragmentation corresponding to the loss of
Cl’s from the molecular ion, viz., [M − nCl]+, with the
characteristic isotopic distribution.
The examination of the IR spectra of 10 and 14−16 revealed
no ν(CN) in the range between 2270 and 2150 cm−1,
indicating the complete transformation of the isonitrile species.
Meanwhile, the IR spectra of 11−13 exhibit one strong ν(C
N) stretching vibration at ca. 2210 cm−1, which is in agreement
with the presence of one unreacted isonitrile ligand in this
complex [the starting cis-[MCl2(CNR1)2] (M = Pd, Pt)
complexes usually display two overlapped stretches in the
2250−2150 cm−1 interval]. In the formed CN(H)R1 carbene
moiety, ν(N−H) bands emerge in the 3211−3052 cm−1 range,
while the corresponding very strong bands due to ν(CN)
appear between 1545 and 1522 cm−1. The medium/weak-
intensity bands in the 2988−2824 cm−1 range are characteristic
of νs(C−H) and νas(C−H) vibrations, while medium-intensity
bands due to δ(C−H from Ar) appear in the interval 778−692
cm−1.
1
The H NMR spectra of carbenes 10−16 display a broad
peak in the range δ 8.74−9.79, assigned to the M−Ccarbene

N(H)R1 proton, while the corresponding CcarbeneNH 13C
signals, in the 13C{1H} NMR spectra of 10−16, were found to
resonate in the 175−202 ppm range. These signals are
approximately 80−90 ppm downfield shifted in comparison
with the starting (isonitrile)MII complexes (e.g., 115 ppm for
CN in cis-[PdCl2(CNCy)2]) and agree with those previously
observed in the related palladium aminocarbene complexes
[PdCl{C(NCa (C6 R2 R3 R4 R5 CONb ))N(H)R1 }-
(CNR1)]a−b 42 Moreover, in the 1H NMR spectra of 11−16 the
.
presence of an additional distinct NH signalthat belongs to
the imine group of the unreacted iminoisoindoline moiety
was detected, and this suggests that 11−16 in solution are
stabilized in the imino form, in contrast to the solid state, where
the amino form was identified by X-ray diffraction (see below).
The 1H and 13C signal assignments were performed by
interpretation of gradient-enhanced two-dimensional 1H,1H-
COSY, 1H,13C-HMQC/1H,13C-HSQC, and 1H,13C-HMBC
NMR spectroscopy. Thus, the long-range shift correlation
2
3
experiments via JH,C and JH,C coupling (1H,13C-HMBC)
allowed the discrimination of the Ccarbene signal of the newly
formed carbene species from the CN carbon of the imine
moiety. The 31P{1H} NMR spectra of mixed carbene−
phosphine complexes 14−16 contain one sharp resonance at
ca. 25.0 ppm for the PPh3 ligand, indicating the presence of a
single isomer for each of 14−16.
The crystallographic data and processing parameters for 10
and 12 are summarized in Table 1S (see Supporting
Information), while the corresponding molecular structure
plots can be found in Figures 2 and 3, and bond lengths and
angles are given in the legends of these figures. In the
Compounds 10−16 gave satisfactory C, H, and N elemental
analyses, which are consistent with the proposed formulations
of the complexes, while ESI+-MS of 10−16 display molecular
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In both crystal structures of 10 and 12, the metal center
adopts a distorted square-planar geometry. In 10, both
CcarbeneN moieties of the two bidentate carbene ligands
{C(NCa(C6H4CNHNb))N(H)R1}a−b are in cis position to
each other. In the metallacycles formed upon coordination of
the bidentate carbene species to the metal center, the angles
around the metal [av 78.8°] are similar to those previously
observed in the related palladium complexes with chelating
carbenes [PdCl{C(NCa(C6R2R3R4R5CONb))N(H)Cy}-
42
(CNCy)]a−b
.
The carbene ligands are in the E-configuration
in 10 and in the Z-configuration in 12, and the Pd−Ccarbene
bond distances [av 2.002(3) (10) and 2.018(9) (12) Å] are
similar to those reported for the related palladium carbene
complexes [PdCl{C(NCa(C6R2R3R4R5CONb))N(H)Cy}-
(CNCy)]a−b (2.001(3) Å),42 cis-[PdCl2{C(OMe)N(H)-
Me}2] (1.953−1.972 Å),54 and cis-[PdCl2{C(NHCy)-
NHNHC(NHCy)}2] (1.958−1.964 Å).31
In each MCN2 fragment, one of the CN bonds has a double-
bond character [10: C11−N1 1.298(3), C21−N2 1.288(3) Å;
12: C2−N21 1.293(11), C5−N51 1.308(11) Å], with bond
distances in accord with those found in the above-mentioned
carbene complexes (1.29−1.33 Å),27,42 while the other CN
bonds have values of a typical single bond [10: C11−N33
1.414(4), C21−N43 1.411(3) Å; 12: C2−N31 1.434(12), C5−
N61 1.398(12) Å]. In 10, two hydrogen bonds were detected,
one being intramolecular and involving the disordered proton
from the CcarbeneN(H)R carbene groups [d(N1···N2)
2.654(3), ∠N1···Hn···N2 142(6)° (n = 1 or 2, see below)]
and the other being intermolecular and involving the
disordered NH proton of the iminoisoindoline groups
[d(N42···N32) 2.711(4), ∠(N42···HnB···N32) 172(6)° (n =
32) or 159(7)° (n = 42)]. These hydrogen bonds were
observed in the solid state, suggesting the presence of two
unsymmetrically protonated carbene ligands. However, in
Figure 2. View of 10 with the atomic numbering scheme and with
disordered models at the N1, N2, N32, and N42 protons. Thermal
ellipsoids are drawn at 50% probability. Selected bond lengths (Å) and
angles (deg): Pd1−C11 2.001(3), Pd1−C21 2.002(3), Pd1−N31
2.113(2), Pd1−N41 2.112(2), C11−N1 1.298(3), C11−N33
1.414(4), C21−N2 1.288(3), C21−N43 1.411(3), N33−C38
1.292(3), C38−N31 1.386(3), N43−C48 1.305(3), C48−N41
1.370(4), N31−C31 1.363(3), C31−N32 1.268(4), N41−C41
1.372(3), C41−N42 1.276(4), C11−Pd1−N31 78.27(10), C21−
Pd1−N41 78.81(9), N2−C21−N43 117.9(2), N1−C11−N33
117.8(2).
1
accord with the H and 13C{1H} NMR data, both carbene
ligands in 10 are similar in solution, therefore reinforcing the
disordered model incorporated in the crystal structure. In 12,
the iminoisoindoline moieties are caught up in both intra- and
intermolecular hydrogen bond interactions, the former with the
chloride ligand [d(N33···Cl1) 3.160(10) Å, ∠(N33−
H33A···Cl1) 166.3°; d(N63···Cl2) 3.187(10) Å, ∠(N63−
H63A···Cl2) 162.6°] and the latter with the N carbene atoms
[d(N21···N63) 2.930(11) Å, ∠(N21−H21···N63) 163.1°;
d(N51···N33) 2.898(12) Å, ∠(N51−H51···N33) 164.1°].
All the other bond lengths in 10 and 12 are of normal values
and agree with those reported for related palladium(II)
complexes.27,28,42
Application of the Prepared Aminocarbene Com-
plexes As Catalysts for Suzuki−Miyaura Cross-Coupling.
In recent reports, we28 and others26,30,31 have demonstrated
that palladium(II) complexes with ADC ligands display high/
moderate catalytic activity in Suzuki−Miyaura reaction. In
pursuit of the ongoing research in our laboratory on novel
efficient and green cross-coupling catalytic systems, we decided
to evaluate the catalytic properties of the aminocarbene
complexes, prepared in this study, toward that cross-coupling
reaction.
Figure 3. View of one of the two symmetry-independent molecules in
12 with the atomic numbering scheme. Thermal ellipsoids are drawn
with 50% probability. Selected bond lengths (Å) and angles (deg):
Pd1−Cl1 2.403(2), Pd1−C2 2.015(9), Pd1−N32 2.021(9), Pd1−C1
1.961(11), C2−N21 1.293(11), C2−N31 1.434(12), C1−N11
1.138(12), N31−C31 1.287(12), C31−N32 1.403(11), N32−C38
1.354(12), C38−N33 1.315(11), C2−Pd1−N32 79.2(4), Cl1−Pd1−
C1 85.4(3), Cl1−Pd1−N32 95.3(2), N21−C2−N31 113.4(8).
asymmetric unit of 12, there are two symmetry-independent
complex molecules, which differ in the PdNCC torsion angles,
averaging 173.5° and 162.1° (for Pd1- and Pd2-containing
molecules, correspondingly), thus showing that the latter is
As a model system, we have chosen the commonly used
reaction of 4-methoxybromobenzene (4-bromoanisole) with
phenylboronic acid, accomplishing 4-methoxybiphenyl. Pre-
liminary optimization of the reaction conditions performed as
previously reported by us for other Pd−aminocarbene species
[ref 28] allowed us to conclude that ethanol could be used as
clearly bent relatively to the former. In addition, the NCcarbene
N
angle in the Pd2 molecule is wider as compared to that of Pd1
[113.4(8)° against 115.0(8)°]. In 10, the PdNCC torsion and
the NCcarbeneN angles adopt average values of 177.3° and
117.8°, respectively.
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a
Table 1. Screening of the Catalytic Efficiency for 9−16 in the Suzuki−Miyaura Cross-Coupling System
yield with the selected base, %
entry
precatalyst
K2CO3
Cs2CO3
1
2
3
4
5
6
7
8
9
4
70
63
32
27
77
67
86
8
4
4
10
11
12
13
14
15
16
4
4
4
81
84
93
13
cis-[PdCl2(PPh3)2]
a
Base (1.5 × 10−4 mol, 1.5 equiv), 4-bromoanisole (1.0 × 10−4 mol, 1 equiv), phenylboronic acid (1.2 × 10−4 mol, 1.2 equiv); precatalysts (9−16, 1
× 10−8 mol); EtOH (1 mL).
a
Table 2. Screening of the Catalytic Efficiency for 9−16 in the Suzuki−Miyaura Cross-Coupling System
yield with the selected catalyst and base, %
14
15
16
entry
aryl halide
K2CO3
Cs2CO3
K2CO3
Cs2CO3
K2CO3
Cs2CO3
1
2
3
4
5
6
7
8
4-MeOC6H4I
4-MeOC6H4Br
4-MeC6H4Br
4-NO2C6H4Br
4-MeOC6H4I
4-MeOC6H4Br
4-MeC6H4Br
4-NO2C6H4Br
96
77
69
95
98
9
62
81
50
84
24
9
99
67
98
99
39
4
82
64
82
99
80
4
99
86
93
99
58
53
69
95
88
93
85
93
85
58
26
77
5
4
4
4
97
4
40
11
a
Base (1.5 × 10−4 mol, 1.5 equiv), aryl halide (1.0 × 10−4 mol, 1 equiv), phenylboronic acid (1.2 × 10−4 mol, 1.2 equiv); precatalysts 14−16 (1 ×
10−8 mol) for entries 1−4 and (1 × 10−9 mol) for entries 5−8; EtOH (1 mL).
an appropriate solvent for our catalytic runs. In refluxing EtOH,
For further screening of the catalytic activity, the most
the conversion of 4-bromoanisole is essentially complete after
ca. 2.5 h, furnishing the biaryl product (4-MeOC6H4Ph) in ca.
86−93% yield (for precatalyst 16), without visible catalyst
decomposition. In addition, K2CO3 and Cs2CO3 were found to
be the most appropriate bases for the studied catalytic system.
The efficiencies of all the prepared Pd−aminocarbene
complexes (10−16) in the Suzuki−Miyaura cross-coupling
reaction using the mentioned model coupling reaction were
estimated, and the results are summarized in Table 1.
The obtained data clearly demonstrated that the palladium
aminocarbene/phosphine species 14−16 are good catalyst
precursors (with either K2CO3 or Cs2CO3 as a base) of the
studied reaction, while the other complexes exhibit moderate
(10 or 11; both with the K2CO3 as a base) or low (12 or 13)
activity. In addition, we also compared the catalytic activity of
14−16 with the commonly used cis-[PdCl2(PPh3)2] (Table 1,
entry 9) and found that under our experimental conditions the
mixed aminocarbene/phosphine species are ca. 10 times more
efficient.
promising precatalysts (14−16) were used. At the next step, we
checked the effect of the substituent of the aryl halide and
found that various substituted aryl bromides 4-R2C6H4Br or
aryl iodides 4-R2C6H4I react with phenylboronic acid to give
excellent yields of biphenyl species, thus showing a valuable
versatility of our catalytic system (Table 2). Note, for entries
1−4 (catalyst loading 1 × 10−8 mol), the yields of the products
correspond to nearly quantitative conversions of the starting
materials, indicating that no side-reactions involving the
substrates occur under the described conditions.
Finally, we also examined the effect of catalyst loading in the
catalytic system and found that a maximum TON of 5.8 × 104
for the described model reaction (and a maximum TON of 9.8
× 104 with 4-R2C6H4I as a substrate) was obtained even at
catalyst loadings as low as 10−5 mol per mole of substrate.
Furthermore, it is important to mention that all the catalytic
runs were performed in nondried EtOH and in air. When the
runs were additionally performed under dinitrogen and in dried
EtOH, the obtained results were nearly identical with those
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description of additional experiments performed between 1−8 and 9
at different conditions.
presented in Tables 1 and 2, showing that the system is air/
humidity insensitive.
X-ray Structure Determinations. Single crystals of 10 and 12
were obtained as indicated below. Intensity data were collected using a
Bruker AXS-KAPPA APEX II diffractometer with graphite-mono-
chromated Mo Kα (λ 0.71073) radiation. Data were collected at 150 K
using omega scans of 0.5° per frame, and a full sphere of data was
obtained. Cell parameters were retrieved using Bruker SMART
software and refined using Bruker SAINT56 on all the observed
reflections. Absorption corrections were applied using SADABS.57
Structures were solved by direct methods by using the SHELXS-97
package58 and refined with SHELXL-97.59 Calculations were
performed using the WinGX System, Version 1.80.03.60 The hydrogen
atoms attached to carbon atoms were placed in calculated positions
and refined by using a riding model. In both structures the hydrogen
atoms bonded to the nitrogen atoms were located from the difference
Fourier synthesis and included, in the final refinement, at positions
calculated from the geometry of the molecules using the riding model,
with isotropic vibration parameters. However, in 10 the hydrogen
atom bonded to N1 and N2 and that bonded to N32 and N42 were
disordered over two positions by using the PART instruction; the
occupancies of H1 and H2 refined to a ratio of 49% and 51%, the same
occurring for the occupancies of H32B and H42B. Least-squares
refinements with anisotropic thermal motion parameters for all the
non-hydrogen atoms and isotropic for the remaining atoms were
employed. For 10 and 12, where disordered molecules in voids were
detected, PLATON/SQUEEZE61 was used to correct the data. Total
potential volumes of 574.4 (10) and 137.4 (12) Å3 were found and
119 (10) or 32 (12) electrons per unit cell worth of scattering were
located in the voids. CCDC-847294 and -847295 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthetic Work. Preparation of cis-[PdCl2(CNCMe2CH2CMe3)2]
(4). This complex was prepared using a modified procedure
previously employed for the preparation of cis-[PdCl2(CNR1)2]
(R1 = Cy 1, But 2, Xyl 3).49−53 Thus, a solution of 2-isocyano-
2,4,4-trimethylpentane (0.557 g, 4.0 mmol) in 5 mL of CHCl3
was slowly (ca. 30 min) added to a suspension of
[PdCl2(NCMe)2] (0.519 g, 2.0 mmol) in CHCl3 (15 mL)
under vigorous stirring. The reaction mixture was kept
additionally for 4 h at 20−25 °C and then filtered off to
remove some insoluble material. The filtrate was evaporated to
dryness under a stream of dinitrogen, producing a colorless
precipitate of 4. It was then washed with two 5 mL portions of
Et2O, two 5 mL portions of iPr2O, and again two 5 mL portions
of Et2O and dried in vacuo at 20−25 °C. Yield is 90% (0.820 g),
based on Pd. 1H NMR (DMSO-d6, δ): 1.73 (s, 4H, CH2), 1.56
(m, 12H, Me), 1.05 (s, 18H, Me). 13C{1H} NMR (DMSO-d6,
δ): 113.9 (CN), 62.8 (CH2), 51.9 (CMe2), 31.4 (CMe3),
30.6 (CMe2C), 29.8 (CMe3). IR (KBr, selected bands, cm−1):
2954 m, 2906 mw ν(C−H); 2247 s, 2231 s ν(CN); 1478 s δ
(C−H in But); 729 s δ(C−H). ESI+-MS, m/z: 455 [M + H]+,
419 [M − Cl]+. Anal. Calcd for C18H34N2Cl2Pd: C, 47.43; H,
7.52; N, 6.15. Found: C, 47.48; H, 7.45; N, 6.10.
FINAL REMARKS
■
The results of this work may be considered from at least two
perspectives. First, in the course of this study, we have
discovered that the metal-mediated coupling between 1,3-
diiminoisoindoline and isonitriles in cis-[PdCl2(CNR1)2] or cis-
[PdCl2(CNR1)(PPh3)] opens up a route to palladium
complexes containing novel aminocarbene ligands (Schemes
3 and 4). In this reaction, 1,3-diiminoisoindoline behaves as
both nucleophile (via the imine NH) and chelator (via the
amide HN), affording a bidentate carbene species.
Second, we found that the newly prepared aminocarbene
complexes display a high activity in Suzuki−Miyaura cross-
coupling of aryl bromides and iodides with phenylboronic acids.
In particular, the catalytic systems based on 14−16 [which
contain one aminocarbene and one phosphine ligand] with an
environmentally benign solvent (EtOH) exhibit higher
efficiencies in terms of yields/TONs (yields up to 93%,
TONs up to 5.8 × 104 with 4-MeOC6H4Br as a substrate) than
the majority of those reported for acyclic aminocarbenes. Thus,
the procedure introduced by Dhudshia and Thadani26 employs
toxic solvents (toluene/THF) and demands high catalyst
loading (0.5 mol % based on Pd), while the scheme described
by Slaughter and colleagues30,31 requires a high catalyst loading
(1 mol % based on Pd) and a long reaction time (24 h).
Furthermore, we believe that a superior catalytic activity of
mixed aminocarbene/phosphine complexes (14−16) as
compared to bis-aminocarbene (10), mixed aminocarbene/
isonitrile (11−13), and bis-phosphine (i.e., cis-[PdCl2(PPh3)2])
can be, at least partially, rationalized by a synergetic effect of
two catalytically important ligands, viz., aminocarbene and
phosphine species in 14−16. Despite the latter, the catalytic
system studied in this work exhibits comparable activity with
that based on structurally related acyclic aminocarbenes
reported recently by some of us [yields up to 88%, with a
TON up to 8.8 × 105].28,55
Currently, we are seeking novel metal-mediated couplings
between other sp2-N nucleophiles and isonitriles, with a
particular emphasis on those mediated by Pd, Pt, and Au
metal centers in view of potential catalytic applications of the
derived aminocarbene complexes of these metals.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. Solvents, PdCl2, PPh3, isoni-
triles, and all reagents for catalytic studies were obtained from
commercial sources and used as received, apart from chloroform and
ethanol, which were purified by conventional distillation over calcium
chloride and activated magnesium, correspondingly. 1,3-Diiminoisoin-
doline (9)45 and complexes [PdCl2(NCMe)2],52 cis-[PdCl2(CNR1)2]
(R1 = Cy 1, But 2, Xyl 3),49−53 and cis-[PdCl2(CNR1)(PPh3)] (R = Cy
5, But 6, Xyl 7)49−53 were prepared as previously reported. C, H, and
N elemental analyses were carried out by the Microanalytical Service
Reaction of 1 with 1,3-Diiminoisoindoline (9). Solid 1,3-
diiminoisoindoline (0.029 g, 0.20 mmol) was added to a solution of
cis-[PdCl2(CNCy)2] (0.040 g, 0.10 mmol) in CHCl3 (10 mL), and the
reaction mixture was refluxed for 4 h. During this period, the color of
the mixture gradually turned from light yellow to orange followed by
precipitation of a yellowish-orange solid. After 4 h, the reaction
mixture was filtered off, and the precipitate was washed with two 5-mL
portions of cold (5 °C) CHCl3 and with five 5 mL portions of Et2O
and then dried in vacuo at 20−25 °C. Yield of 10 was 82% (0.050 g),
based on Pd.
of the Instituto Superior Tec
́
nico. ESI+ mass spectra were obtained on
a Varian 500-MS LC ion trap mass spectrometer in MeOH (ion spray
voltage: +5 kV, capillary voltage: 30 V, RF loading: 100%). Infrared
spectra (4000−400 cm−1) were recorded on a BIO-RAD FTS
3000MX instrument in KBr pellets. 1D (1H, 13C{1H}, DEPT, and
31P{1H}) and 2D (1H,1H-COSY, H,13C-HMQC/1H,13C-HSQC and
1
1
Complex 10. H NMR (DMSO-d6, δ): 8.28, 8.23 (s, br, 2H, C
1H,13C-HMBC) NMR spectra were measured on Bruker Avance II+
500 MHz (UltraShield Plus Magnet) spectrometers at ambient
temperature. Supporting Information for this article contains Table
1S (crystallographic parameters for structures 10 and 12) and a
NH), 8.11 (s, br, 2H, CcarbNH), 7.96−7.71 (m, 8H, aryls), 4.54 (m,
2H, CH-Cy), 1.79−1.42 (m, 20H, CH2-Cy). 13C{1H} NMR (DMSO-
d6, δ): 197.3 (CcarbN), 176.3, 168.9 (CNH), 136.2, 134.6, 133.1,
132.2, 123.8, 122.1 (aryls), 54.6 (CH-Cy), 31.5, 30.7, 24.6 (CH2−-Cy).
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Article
IR (KBr, selected bands, cm−1): 3204 w ν(N−H); 2931 s, 2854 m
ν(C−H); 1686 w, 1618 mw ν(CN); 1545 s ν(N−Ccarb); 1437 s
δ(C−H from Cy); 1116 m, 1040 m ν(CC); 776 mw, 708 s δ(C−H
from aryl). ESI+-MS, m/z: 613 [M + H]+. Anal. Calcd for
C30H34N8Pd: C, 58.77; H, 5.59; N, 18.28. Found: C, 58.70; H, 5.52;
N, 17.99.
Reactions of 5, 6, and 8 with 1,3-Diiminoisoindoline (9). Solid
1,3-diiminoisoindoline (0.029 g, 0.20 mmol) was added to a solution
of cis-[PdCl2(CNR1)(PPh3)] (0.20 mmol) in CHCl3 (10 mL), and the
reaction mixture was refluxed for 4 h. During this period, the color of
the mixture gradually turned from light yellow to orange. The reaction
mixture was then filtered off to remove some insoluble material, the
filtrate was evaporated to dryness, and the product was extracted with
three 5 mL portions of CHCl3. The bright yellow solution was
evaporated at room temperature to dryness under a stream of N2,
washed with five 5 mL portions of Pri2O, one 1 mL portion of cold (5
°C) Et2O, and again five 5 mL portions of Pri2O, and then dried in
vacuo at 20−25 °C. Yields of 14−16 were 75−84%, based on Pd. In
the case of R1 = Xyl (7), no isolable product was formed.
Reactions of 2−4 and 1,3-Diiminoisoindoline (9). Solid 1,3-
diiminoisoindoline (0.029 g, 0.20 mmol) was added to a solution of
cis-[PdCl2(CNR1)2] (0.20 mmol) in CHCl3 (10 mL), and the reaction
mixture was refluxed for 4 h. During this period, the color of the
mixture gradually turned from light yellow to yellowish-orange. After 4
h, the reaction mixture was evaporated to dryness, and the solid
residue was extracted with five 5 mL portions of CH2Cl2. The extracts
were combined as a yellow-orange solution, which was evaporated to
dryness, and the residue was washed with two 5 mL portions of cold
(5 °C) Et2O and with five 5 mL portions of iPr2O and then dried in
vacuo at 20−25 °C. Yields of 11−13 were 78−84%, based on Pd.
Complex 11. Yield: 78%. 1H NMR (CDCl3, δ): 10.19 (s, 1H, C
NH), 9.13 (w, 1H, CcarbNH), 7.85−7.61 (m, 4H, aryls), 1.74−1.54
(m, 18H, Me). 13C{1H} NMR (DMSO-d6, δ): 182.2 (CcarbN),
181.2 and 170.5 (CNH), 135.4, 134.9, 134.6, 133.3, 125.7, and
122.7 (aryls), 59.9 and 59.1 (CMe3), 29.2, 29.4, 27.6, and 24.3 (CMe3).
IR (KBr, selected bands, cm−1): 3211 w ν(N−H); 2988 mw, 2924 m
ν(C−H); 2202 m ν(CN); 1654 mw, 1618 mw ν(CN); 1542 s
ν(N−Ccarb); 1440 s δ(C−H from But); 1116 m, 1133 s ν(CC); 768
m, 707 s δ(C−H from aryl). ESI+-MS, m/z: 452 [M + H]+. Anal.
Calcd for C18H24N5ClPd: C, 47.80; H, 5.35; N, 15.48. Found: C,
47.58; H, 5.42; N, 15.49.
1
Complex 14. Yield: 84%. H NMR (CDCl3, δ): 10.46 (s, 1H, C
NH), 9.22 (s, 1H, CcarbNH), 7.86−7.42 (m, 19H, aryls), 4.38 (s,
1H, CH-Cy), 1.90−1.42 (m, 10H, CH2-Cy). 13C{1H} NMR (CDCl3,
δ): 197.4 (CcarbN) 183.1, 171.0 (CNH), 135.4−125.0 (aryls),
55.8 (CH-Cy), 32.0, 24.9, 24.3 (CH2−Cy). 31P{1H} NMR (CDCl3, δ):
23.3. IR (KBr, selected bands, cm−1): 3054 w ν(N−H); 2930 m, 2853
m ν(C−H); 1686 w ν(CN); 1544 s, 1527 s ν(N−Ccarb); 1435 s
δ(N−H); 1410 s ν(C−P); 1153 w, 1038 m ν(CC); 778 w, 707 s
δ(C−H). ESI+-MS, m/z: 657 [M]+. Anal. Calcd for C33H32N4ClPPd:
C, 60.28; H, 4.91; N, 8.52. Found: C, 59.74; H, 4.70; N, 8.47.
1
Complex 15. Yield: 76%. H NMR (CDCl3, δ): 10.83 (s, br, 1H,
CNH), 9.29 (s, br, 1H, CcarbNH), 7.90−7.57 (s, 19H, aryls),
1.62−1.49 (m, 9H, Me). 13C{1H} NMR (CDCl3, δ): 200.6 (CcarbN)
176.1 (CNH), 134.2−123.6 (aryls), 58.0 (CMe3), 29.0, 28.7, 28.3
(CMe3). 31P{1H} NMR (CDCl3, δ): 24.3. IR (KBr, selected bands,
cm−1): 3052 w ν(N−H); 2972, 2932 w ν(C−H); 2226 w ν(CN);
1686 mw ν(CN); 1541 s ν(N−Ccarb); 1425 s ν(C−P); 1133 mw,
1041 w ν(CC); 778 w, 708 s δ(C−H). ESI+-MS, m/z: 631 [M]+.
Anal. Calcd for C31H30N4ClPPd: C, 58.97; H, 4.79; N, 8.87. Found: C,
58.37; H, 4.70; N, 8.92.
1
Complex 12. Yield: 84%. H NMR (CDCl3, δ): 9.48 (s, 1H, C
NH), 9.13 (w, 1H, CcarbNH), 7.82−7.60 (m, 4H, aryls) and 7.52−
7.28 (m, 6H, aryls), 2.12 (s, 6H) and 2.04 (s, 6H, Me). 13C{1H} NMR
(DMSO-d6, δ): 175.8 (CcarbN), 171.5 and 168.2 (CNH), 150.6,
135.5, 134.2, 133.4, 133.3, 132.9, 132.3, 131.4, 129.0, 127.5, 127.1,
126.0, 123.9, and 123.7 (aryls), 18.7 and 17.6 (Me). IR (KBr, selected
bands, cm−1): 3223 w ν(N−H); 2982 mw, 2928 m ν(C−H); 2192 m
ν(CN); 1642 mw, 1606 mw ν(CN); 1546 s ν(N−Ccarb); 1122 m,
1123 s ν(CC); 768 m, 707 s δ(C−H from aryl). ESI+-MS, m/z: 512
[M − Cl]+. Anal. Calcd for C26H24N5ClPd: C, 56.95; H, 4.41; N,
12.77. Found: C, 56.92; H, 4.42; N, 12.49.
1
Complex 16. Yield: 75%. H NMR (CDCl3, δ): 10.85 (s, 1H, C
NH), 8.23 (s, 1H, CcarbNH), 7.78−7.52 (m, 19H, aryls), 1.62 (s,
2H, CH2), 1.50 (s, 6H), 1.45 (s, 3H) and 1.10 (m, 6H, Me). 13C{1H}
NMR (CDCl3, δ): 195.8 (CcarbN) 176.5 (CNH), 134.8−124.2
(aryls), 65.4 (CMe2CH2), 51.4 (CH2), 31.9 (CH2CMe3), 15.6 and
14.3 (CMe2CH2 and CMe3). 31P{1H} NMR (CDCl3, δ): 24.5. IR
(KBr, selected bands, cm−1): 3057 w ν(N−H); 2960 w, ν(C−H);
1663 s ν(CN); 1522 m ν(N−Ccarb); 1436 s δ(C−H from Cy); 1420
m ν(C−P); 1133 mw, 1023 w ν(CC); 750 mw, 692 s δ(C−H from
aryl). ESI+-MS, m/z: 687 [M + H]+. Anal. Calcd for C35H38N4ClPPd:
C, 61.14; H, 5.57; N, 8.15. Found: C, 61.54; H, 5.58; N, 8.10.
General Procedure for the Catalytic Suzuki−Miyaura Cross-
Coupling (specific conditions are provided in Tables 1 and 2).
Selected base (1.5 × 10−4 mol, 1.5 equiv), aryl bromide (1.0 × 10−4
mol, 1.0 equiv) and phenylboronic acid (1.2 × 10−4 mol, 1.2 equiv)
were mixed in a 5 mL Schlenk tube connected to a reflux condenser
open to air. A solution of the precatalyst (1 × 10−8 mol) in nondried
EtOH (1 mL) was then added. For the tests under anaerobic reactions,
anhydrous EtOH was used, and the Schlenk tube was then connected
to a reflux condenser bearing a take-off with a connected dinitrogen
balloon. The Schlenk tube was placed in a preheated oil bath at 80 °C,
stirred for 2.5 h, and then cooled to 25 °C. The reaction mixture was
evaporated to dryness under a stream of dinitrogen followed by
addition of 1.0 equiv of 1,2-dimethoxyethane (NMR internal standard)
and extraction of the reaction mixture with three 0.20 mL portions of
CDCl3. All fractions were combined and subjected to 1H NMR
monitoring. The product peak assignments were based on authentic
samples or on published data,13,25,26,30,31,62 while quantifications were
performed upon integration of the selected peak of the product
relative to the peak of the standard. In some cases, the products were
isolated by extraction of the residue after evaporation of the reaction
mixture with CH2Cl2, followed by column chromatography on silica
gel (Fluka 40/60; 10:1 hexane/ethyl acetate, v/v).
Complex 13. Yield: 79%. 1H NMR (DMSO-d6, δ): 9.93 (s, br, 2H,
CNH), 8.76 (s, br, 2H, CcarbNH), 7.85−7.61 (s, 8H, aryls), 1.94
(s, 4H, CH2), 1.94 (s, 6H), 1.62 (s, 6H), 1.03 (s, 18H, Me from
C(Me2)CH2CMe3). 13C{1H} NMR (DMSO-d6, δ): 197.0 (CcarbN),
176.6, 171.0 (CNH), 134.6, 132.3, 125.1 (aryls), 63.3 (CMe2CH2),
53.0 (CH2), 33.0 (CMe3), 31.8 and 29.9 (CMe3 and CMe2CH2). IR
(KBr, selected bands, cm−1): 3186 w br ν(N−H); 2951 m br, ν(C−
H); 1686 s ν(CN); 1535 s ν(N−Ccarb); 1431 s br δ(N−H); 1133 s,
1087 s ν(CC); 777 m, 712 s δ(C−H from aryl). ESI+-MS, m/z: 564
[M]+. Anal. Calcd for C26H40N5ClPd: C, 55.32; H, 7.14; N, 12.41.
Found: C, 55.73; H, 7.04; N, 12.32.
Preparation of cis-[PdCl2(CNCMe2CH2CMe3)(PPh3)] (8). This
complex was prepared using a modified procedure previously
employed for the preparation of cis-[PdCl2(CNR1)(PPh3)] (R = Cy
5, But 6).49−53 Thus, a solution of PPh3 (0.262 g, 1.0 mmol) in Et2O
(2 mL) was added to solution of cis-[PdCl2(CNCMe2CH2CMe3)2] in
CHCl3 (10 mL). The reaction mixture was refluxed for 1 h and then
filtered off to remove some insoluble material. The filtrate was
evaporated to dryness under a stream of dinitrogen, producing a
colorless precipitate of 6. It was washed with two 5 mL portions of
Et2O, two 5 mL portions of iPr2O, and again two 5 mL portions of
1
Et2O and dried in vacuo at 20−25 °C. Yield is 87%, based on Pd. H
NMR (DMSO-d6, δ): 7.69−7.53 (m, 15H aryls); 1.32 (s, 2H, CH3),
1.12 (s, 6H, CH3), 0.86 (s, 9H, CH3). 13C{1H} NMR (DMSO-d6, δ):
133.9−129.1 (aryls), 62.0 (CMe2CH2), 51.6 (CH2), 31.2 (CMe3), 30.4
and 29.2 (CMe2CH2 and CMe3). 31P{1H} NMR (DMSO-d6, δ): 23.0.
IR (KBr, selected bands, cm−1): 2948 mw, 2905 w ν(C−H); 2228 s
ν(CN); 1481 m δ(C−H); 1412 s δ(C−P); 749 s δ(C−H). ESI+-
MS, m/z: 578 [M + H]+, 542 [M − Cl]+. Anal. Calcd for
C27H32NCl2PPd: C, 56.02; H, 5.57; N, 2.42. Found: C, 56.18; H,
5.45; N, 2.30.
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Article
(20) Wanniarachchi, Y. A.; Subramanium, S. S.; Slaughter, L. M. J.
Organomet. Chem. 2009, 694, 3297−3305.
ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
(21) Rosen, E. L.; Sung, D. H.; Chen, Z.; Lynch, V.; Bielawski, C. W.
Organometallics 2010, 29, 250−256.
(22) Ruiz, J.; Perandondes, B. F. Organometallics 2009, 28, 830−836.
(23) Collins, M. S.; Rosen, E. L.; Lynch, V. M.; Bielawski, C. W.
Organometallics 2010, 29, 3047−3053.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kluzyanin@ist.utl.pt; pombeiro@ist.utl.pt.
Notes
■
(24) Droge, T.; Wanniarachchi, Y. A.; Slaughter, L. M. Chem.
Commun. 2007, 3294−3296.
(25) Kremzow, D.; Seidel, G.; Lehmann, C. W.; Furstner, A. Chem.
Eur. J. 2005, 11, 1833−1853.
The authors declare no competing financial interest.
(26) Dhudshia, B.; Thadani, A. N. Chem. Commun. 2006, 6, 668−
670.
(27) Tskhovrebov, A. G.; Luzyanin, K. V.; Kuznetsov, M. L.;
Sorokoumov, V. N.; Balova, I. A.; Haukka, M.; Kukushkin, V. Yu.
Organometallics 2011, 30, 863−874.
ACKNOWLEDGMENTS
This work has been partially supported by the Fundaca
■
̧
o para a
̃
Cien
̂
cia e a Tecnologia (FCT), Portugal (including FCT
(28) Luzyanin, K. V.; Tskhovrebov, A. G.; Carias, M. C.; Guedes da
Silva, M. F. C.; Pombeiro, A. J. L.; Kukushkin, V. Yu. Organometallics
2009, 28, 6559−6566.
projects PTDC/QUI-QUI/098760/2008, PTDC/QUI-QUI/
109846/2009, and PEst-OE/QUI/UI0100/2011), as well as
Federal Targeted Program “Scientific and Scientific-Pedagogical
Personnel of the Innovative Russia in 2009−2013” (contract
P676 from 20/05/2010), Russian Fund of Basic Research
(grants 12-03-00076 and 11-03-12044), and Saint-Petersburg
State University for a research grant (2011−2013). K.V.L.
expresses gratitude to the FCT and the Instituto Superior
(29) Slaughter, L. M. Comm. Inorg. Chem. 2008, 29, 46−72.
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