Expanded ring diaminocarbene palladium complexes: Synthesis, structure, and Suzuki-Miyaura cross-coupling of heteroaryl chlorides in water

Article abstract of DOI:10.1039/c3dt32860k

A series of new 6- and 7-membered N-heterocyclic carbene (NHC) complexes of palladium (NHC)Pd(cinn)Cl (cinn = cinnamyl = 3-phenylallyl) were synthesized and characterized structurally in the solid state. The influence of ring size (5, 6 or 7) and bulkiness of N-aryl substituents (Mes = 2,4,6-trimethylphenyl, or Dipp = 2,6-diisopropylphenyl) in carbenes on palladium catalysed Suzuki-Miyaura cross-coupling was revealed. Due to the unique stereoelectronic properties of expanded ring NHCs, a versatile, highly efficient green protocol of coupling of heteroaromatic chlorides and bromides with boronic acids has been developed. High quantitative yields of biaryls were achieved with water as solvent, under air, using low catalyst and phase transfer agent loadings, and with mild and environmentally benign base NaHCO₃. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt32860k

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Expanded ring diaminocarbene palladium complexes:
synthesis, structure, and Suzuki–Miyaura cross-coupling
of heteroaryl chlorides in water†
Cite this: DOI: 10.1039/c3dt32860k
Eugene L. Kolychev,a Andrey F. Asachenko,a Pavel B. Dzhevakov,a Alexander A. Bush,a,b
Viacheslav V. Shuntikov,c Victor N. Khrustalevc and Mikhail S. Nechaev*a,b
A series of new 6- and 7-membered N-heterocyclic carbene (NHC) complexes of palladium (NHC)Pd(cinn)-
Cl (cinn = cinnamyl = 3-phenylallyl) were synthesized and characterized structurally in the solid state. The
influence of ring size (5, 6 or 7) and bulkiness of N-aryl substituents (Mes = 2,4,6-trimethylphenyl, or
Dipp = 2,6-diisopropylphenyl) in carbenes on palladium catalysed Suzuki–Miyaura cross-coupling was
revealed. Due to the unique stereoelectronic properties of expanded ring NHCs, a versatile, highly
efficient green protocol of coupling of heteroaromatic chlorides and bromides with boronic acids has
been developed. High quantitative yields of biaryls were achieved with water as solvent, under air, using
low catalyst and phase transfer agent loadings, and with mild and environmentally benign base NaHCO3.
Received 28th November 2012,
Accepted 19th February 2013
DOI: 10.1039/c3dt32860k
www.rsc.org/dalton
NHCs offer markedly different steric and electronic properties
when compared with their 5-membered ring counterparts.
Introduction
N-Heterocyclic carbenes (NHCs) are nowadays ubiquitous Expansion of the ring leads to a significant increase of donor
ligands in organometallic chemistry as well as catalysis due to properties, simultaneously the sterical demands of such
their unique properties.1 Steric and electronic properties of ligands increase dramatically.2 It is of particular interest to
NHCs can be tuned in a wide range by change of substituents explore the influence of stereoelectronic properties of
and backbones.2
expanded ring NHCs on the stability of palladium complexes
Palladium–NHC complexes exhibit a unique air and moisture in air and moisture and their catalytic properties under “non-
stability, strong metal–ligand bonds and efficient steric protec- conventional” conditions.
tion of metal. Such factors favor catalyst lifetime and efficiency.3
Here we report the synthesis and characterization of a
Numerous NHC–palladium catalytic systems for Suzuki–Miyaura series of new 6- and 7-membered NHC palladium(II) complexes
reaction were developed by the groups of Herrmann,4 Nolan,5 and their catalytic activity in Suzuki–Miyaura cross-coupling of
Organ,6 and Glorius.7 A number of sulfonate-, carboxylate- and heteroaryl chlorides in aqueous media under air.
ammonium-functionalized NHCs were used in palladium cata-
lyzed cross-coupling reactions in aqueous media.8
While the chemistry of imidazole-based NHCs is well docu-
Results and discussion
Synthesis
mented, expanded ring (six and seven) NHCs are new and
unexplored. Studies of their structure, chemical properties,
and catalytic applications are now appearing.9 Expanded ring
Recently we reported on the development of an efficient
method for the synthesis of expanded ring amidinium salts
bearing bulky aromatic substituents.9c This method was
further optimized (Scheme 1). Amidinium tetrafluoroborates
were obtained in excellent yields (80–90%) in two steps starting
from commercially available reagents in decagram-scale.
A series of palladium–carbene complexes were obtained
upon reaction of free carbenes with dimeric [Pd(cinn)Cl]2
(Scheme 2).10 Off-white to yellow powders were obtained upon
stirring at room temperature. These complexes were found to
be perfectly air- and moisture-stable and thus could be
handled in air.
aA.V. Topchiev Institute of Petrochemical Synthesis, RAS, Leninsky Prosp., 29,
Moscow 119991, Russian Federation. E-mail: nechaev@nmr.chem.msu.ru;
Fax: +7 (495)9392677
bDepartment of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory
1/3, Moscow 119991, Russian Federation
cA.N. Nesmeyanov Institute of Organoelement Compounds of RAS, Vavilov Str., 28,
Moscow 119991, Russian Federation
†Electronic supplementary information (ESI) available: Spectral data for the
cross-coupling products and additional crystallographic data. CCDC
784539–784543. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt32860k
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a half equivalent of [Pd(cinn)Cl]2 at room temperature for 24 h
leads to precipitation of one third of equivalent of AgBr. Fil-
tration through Celite gives a transparent solution from which
no precipitation was observed in 2 d. After evaporation, the
product was extracted with diethyl ether. Upon dissolution of
the residue in CH2Cl2, additional precipitation of AgBr was
observed. The combined yields of (NHC)Pd(cinn)Cl complexes
were 71–73% after three extractions.
The 13C NMR resonances of carbenic carbons in complexes
were observed at 213.8 ppm for six-membered ring carbenes,
and at 221.4–225.5 ppm for seven-membered ring carbenes (in
CDCl3). These values differ from those obtained for 5-mem-
bered saturated ring carbenes
4
(211.0 ppm) and
5
(212.1 ppm).10 Thus expansion of
a
carbene ring is
accompanied with deshielding of carbenic carbon, and sub-
sequent lower field 13C NMR shifts are observed. The same
trend was traced in Ru(II),15 Rh(I),13,16 Cu(I),9c,17 Cu(II),9i and
Ag(I)9a,c complexes.
Crystal structures
Scheme 1 Preparation of amidinium salts.
All new complexes were investigated by X-ray crystal structure
analysis. Their molecular structures along with the atomic
numbering schemes are shown in Fig. 1 and 2 and S1 and S3,†
and selected geometrical parameters are given in Table S1.†
In general, the structures of the complexes are similar. In
all cases, a distorted square-planar coordination around the
palladium center was observed, with the chloride ligand
located cis to the carbene. The allyl moiety is η3-coordinated to
Scheme 2 Preparation of palladium complexes.
Elemental analyses confirmed the compositions of the com-
plexes obtained. Contrary to five-membered ring carbenes,11
synthesis of complexes using in situ generated expanded ring
carbenes in reactions of amidinium salts and NaHMDS leads
to a significant decrease in yields and the formation of dark-
brown side products of unidentified nature.
Palladium complexes (6-Mes)Pd(cinn)Cl, (6-Dipp)Pd(cinn)Cl
and (7-Dipp)Pd(cinn)Cl were also obtained by the exchange
reaction of (NHC)AgBr with [Pd(cinn)Cl]2 (Scheme 2). Together
with migration to Cu(I)9c and Au(I),12 the observed reaction is
one of the rare examples of migration of aryl-substituted
expanded ring NHCs from Ag(I) to another metal. This obser-
vation contrasts with the previous observations of Buchmei-
ser,13 Herrmann,14 and our findings9c that expanded ring
NHCs does not migrate from Ag(I) to Pd(II).
Fig. 1 X-Ray structure of (6-Dipp)Pd(cinn)Cl (hydrogen atoms and the solvate
molecule are omitted for clarity, the alternative position of the disordered C3
atom is presented). Selected bond lengths (Å) and angles (°): Pd–C1: 2.042(4);
Pd–Cl1 2.3351(9); Pd–C29 2.113(4); Pd–C30 2.134(4); Pd–C31 2.204(4); N1–C1–
N2 117.1(3); C1–N–CAr 120.2(3), 121.2(3); C1–N–Cring 124.6(3), 126.0(3); CAr–N–
For more bulky 6-Dipp and 7-Dipp derivatives, transmetalla-
tion is reversible. Mixing of one equivalent of (NHC)AgBr with Cring 113.6(3), 113.7(3); Pd–C1–N 115.6(3), 126.1(3); C5–N1–N2–C17 4.0.
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with the two aryl ligands of the carbene ring, the palladium
coordination plane is not perpendicular to the N–Ccarb–N
plane, and the two Pd–Ccarb–N angles have different values
(Fig. 1 and 2). The geometries observed for the molecules of
complexes are stabilized by the short intramolecular CMe
–
H⋯Cl contacts (2.83, 2.61–2.78 and 2.57–2.63 Å for the five-,
six- and seven-membered NHC complexes, respectively) as well
as the crystal packing effects.
Catalytic tests in Suzuki–Miyaura reaction
Heteroaromatic compounds constitute a large and varied
family of organic compounds. The wide scope of utilization of
heteroaromatic compounds spans from medicinal chemistry
to organic electronics.18 Although Suzuki–Miyaura cross-coup-
ling is one of the most powerful methods of making C–C
bonds, the coupling of heteroaromatic compounds is still chal-
lenging. It is of particular interest to utilize heteroaromatic
chlorides in cross-coupling reactions.19
Substitution of organic solvents in cross-coupling reactions
with water is a challenging task. Utilization of water as a
solvent is ecologically friendly and might lead to significant
cost reductions on a large scale. However, most current cataly-
tic systems use mixtures of water with organic solvents.20
Several catalytic systems based on phosphine or nitrogen con-
taining ligands were developed for cross-coupling of heteroaro-
matic chlorides in aqueous media.21 However such catalytic
systems have significant drawbacks. To prevent rapid oxidation
Fig. 2 X-Ray structure of (7-Mes)Pd(cinn)Cl (hydrogen atoms are omitted for
clarity). Selected bond lengths (Å) and angles (°): Pd–C1: 2.0559(11); Pd–Cl1
2.3658(3); Pd–C24 2.1239(13); Pd–C25 2.1358(12); Pd–C26 2.2185(12); N1–C1–
N2 119.46(10); C1–N–CAr 117.22(9), 117.36(9); C1–N–Cring 123.31(10),
128.74(10); CAr–N–Cring 113.95(9), 116.92(10); Pd–C1–N 118.60(8), 121.91(8);
C6–N1–N2–C15 27.3.
the palladium, with the phenyl-substituted carbon atom trans of phosphine ligand, the solvent should be neatly degassed.
to the carbene ligand and the unsubstituted carbon atom trans Moreover, the product is contaminated with toxic phosphine
to the chlorine ligand. The Pd–Callyl bond opposite the NHC and products of side reactions of activations of C–P bonds.22
ligand is elongated by a strong trans-effect (Fig. 1 and 2). The Usually, reaction conditions are harsh: 12–24 h refluxing, large
Pd–Ccarb distances in the complexes fall comfortably within excess of strong base and boronic acid, high loadings of phase
the range obtained for palladium(II)–carbene complexes pre- transfer agents and catalysts. No examples of cross-coupling of
viously (1.91–2.10 Å)5b and increase upon passing from five- to heteroaromatic chlorides in pure water were reported to date.
seven-membered carbene rings.
N-Heterocyclic carbene complexes of palladium are well
The X-ray diffraction study reveals the characteristic sofa documented active catalysts of cross-coupling reactions.1a,b,23
and twist-sofa conformations for the saturated six- and seven- Several examples of Suzuki–Miyaura coupling in aqueous
membered carbene rings, respectively.9a,c,i Nitrogen atoms in media mediated by NHC palladium complexes were recently
five- and six-membered complexes adopt a nearly planar con- reported,8a,24 two reports were devoted to coupling of hetero-
figuration, as evidenced by the observation that the sum of the aromatic chlorides.24d,e In this work we set the target to
C–N–C angles is close to 360°. However, the two nitrogen develop an efficient catalytic system for Suzuki–Miyaura cross-
atoms of the seven-membered carbene rings have different coupling of heteroaromatic chlorides based on NHC–palla-
configurations: the nitrogen N1 atom is planar, while the nitro- dium complexes to meet the conditions of green chemistry:
gen N2 atom is slightly pyramidalized. Apparently, this fact water as an environmentally friendly solvent, low loadings of
could be explained by the steric reasons due to the presence of catalysts and co-catalysts, low or no excess of boronic acid.
a phenyl substituent at the allyl ligand. The torsion angle α
Catalytic tests were done in distilled water as solvent.
between the planes, defined by the CAr–N⋯N–CAr atoms, is NaHCO3 was used as a base and (Bu4N)Br as a phase transfer
different for five-, six- and seven-membered carbenes. This agent. Initially we tested the activities of palladium complexes
angle increases with the increasing of carbene ring size. The in the reaction of 3-chloropyridine with 4-tolylboronic acid
C–N–C angle distribution in the complexes is similar to that in (Table 1).
the six- and seven-membered silver complexes described pre-
viously.9a,c They change in the range of Cring–N–Ccarb > CAr–N– was isolated using flash-chromatography. NHCs bearing bulky
carb > Cring–N–CAr (Fig. 1 and 2). Dipp groups led to active pre-catalysts. (6-Dipp)Pd(cinn)Cl was
The phenyl-substituted allyl ligand in the complexes has found to be the most active, while (5)Pd(cinn)Cl exhibited
Reaction mixtures were refluxed in air for 1 h. The product
C
the trans conformation, with the phenyl plane twisted relative slightly lower activity. Less bulky Mes derivatives exhibit low
to the allyl plane. Due to the steric interactions of this ligand activities. The complex bearing 7-Dipp carbene led to only
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Table 1 Catalyst screening for Suzuki–Miyaura coupling of 3-chloropyridine
and 4-tolylboronic acida
Table 2 Suzuki–Miyaura coupling of heteroaryl bromides with 4-tolylboronic
acida
Entry
NHC
Isolated yield (%)
Entry
1
HetAr-Cl
Product
Time (h)
1
Isolated yield (%)
86
1
2
3
4
5
6
4
3
33
15
84
86
25
6-Mes
7-Mes
5
6-Dipp
7-Dipp
2
3
1
1
77
98
a Reaction conditions: 3.27 mmol of 3-chloropyridine (1 eq.),
4-tolylboronic acid (1.04 eq.), NaHCO3 (3.06 eq.), TBAB (0.095 eq.),
5 ml of distilled water and (NHC)Pd(cinn)Cl (0.5% mol); 1 h, reflux.
4
5
6
1
1
3
71
77
75
25% yield. We suppose that the decrease in catalytic activity is
due to steric overprotection of the palladium atom.
We have chosen (6-Dipp)Pd(cinn)Cl for further detailed
catalytic tests. A series of heteroaryl chlorides (Table 2) and
heteroaryl bromides (Table 3) were reacted with 4-tolylboronic
acid. In most cases reactions were run for 1 h in refluxing
water in air. Cross-coupling products were isolated in high
yields. All reactions are clean. No homo-coupling products
were detected.
Coupling of 3-chloropyridine and phenylboronic acid was
previously done by Nájera and coworkers using oxime-oxo pal-
ladacycles and bipyridine complexes of PdCl2 as pre-catalysts.
In their study, 1.5 fold excess of phenyl boronic acid and
K2CO3 as a base was used. Reaction mixtures were refluxed for
3–7 h to give 50–90% yield depending on catalyst loading and
the nature of the ligand at the palladium atom.21a,c Miyaura
and coworkers reported on the synthesis of a 4-tolyl derivative
in high yield upon heating of the reaction mixture to 80 °C for
16 h in the presence of K3PO4 as a base and 1 mol% of glyco-
7
8
9
3
3
3
90
61b
39b
a Reaction conditions: 3.27 mmol of heteroaromatic chloride (1 eq.),
4-tolylboronic acid (1.04 eq.; 2.08 eq. for entries 6–9), NaHCO3 (3.06 eq.),
TBAB (0.095 eq.), 5 ml of distilled water and (6-Dipp)Pd(cinn)Cl (0.5%
mol); reflux. b Monoarylated product was also isolated.
side-substituted phosphine–palladium complex.21d Using expected, bromides were more active than chlorides to give
(6-Dipp)Pd(cinnamyl)Cl, we were able to obtain high yield in high yields in 1 h (Table 3).
1 h using only a slight excess (0.04 eq.) of boronic acid
Next, we varied the nature of arylboronic acids. 2-Chloro-
(Table 2, entry 1). The coupling product of 4-chloropyridine pyridine was used for the catalytic test since 2-substituted pyri-
with phenylboronic acid can be obtained in 5621b or 70% dines are useful building blocks for organic synthesis.
yield.21d We obtained the corresponding product in 77% yield Synthesis of 2-arylated pyridines via heterocyclization is not
under softer conditions (Table 2, entry 2).
straightforward, while 2-chloropyridines are easily accessible.
Couplings of chloropyrimidines, chloropyridazines, and Additionally, it is known that 2-chloropyridines have a ten-
chloropyrazines in pure water were successfully done for the dency to hydrolyze under basic conditions.25 It was of interest
first time (Table 2, entries 3, 4, 8, 9).
to determine whether the hydrolysis of 2-chloropyri-
It is worth noting the coupling of dichlorides. In cases of dine affects cross-coupling reactions. It is worth mentioning
dichloropyrazine, only the monoarylated product was isolated that in the conditions reported by Nájera and coworkers, the
(Table 2, entries 4). While for 4,6-dichloropyrimidine and 3,6- reaction of phenylboronic acid with 2-chloropyridine results in
dichloropyridazine, mono- and diarylated products were separ- only 66% of the coupling product.21b Miyaura and coworkers
ately isolated (Table 2, entries 8 and 9). In cases of dichloro- obtained 2-(4-tolyl)pyridine in 88% yield.21d In addition,
pyridines, only diarylated products were obtained (Table 2, Plenio and coworkers obtained good yields in cross-coupling
entries 6 and 7).
of 2-chloropyridine with 4-tolylboronic acid, however they used
To widen the scope of substrates, we performed catalytic strong bases such as KOH and K2CO3 under reflux conditions
tests of (6-Dipp)Pd(cinn)Cl with heteroaryl bromides. As for 12 h.21f,24d
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Table 3 Suzuki–Miyaura coupling of heteroaryl bromides with 4-tolylboronic
acida
Table 4 Suzuki–Miyaura coupling of 2-chloropyridine with arylboronic acidsa
Entry
1
Ar–B(OH)2
Product
Isolated yield (%)
97
Entry
1
HetAr-Br
Product
Isolated yield (%)
97
2
3
4
99
82
98
2
3
99
99
4
5
6
89
97
98
5
6
86
90
7
8
82
92
a Reaction conditions: 3.27 mmol of 2-chloropyridine (1 eq.), boronic
acid (1.04 eq.), NaHCO3 (3.06 eq.), TBAB (0.095 eq.), 5 ml of distillated
water and (6-Dipp)Pd(cinn)Cl (0.5 mol%); reflux.
a Reaction conditions: 3.27 mmol of heteroaromatic bromide (1 eq.),
4-tolylboronic acid (1.04 eq.), NaHCO3 (3.06 eq.), TBAB (0.095 eq.), 5 ml
of distillated water and (6-Dipp)Pd(cinn)Cl (0.5 mol%); 1 h, reflux.
b. The reaction is carried out under air, no degasation of
water is needed.
c. Mild, cheap, environmentally benign NaHCO3 is used as
a base.
d. Low loadings of phase transfer agent (Bu4N)Br.
e. Only a slight excess of boronic acid is used.
f. The reaction is fast.
We found that in all cases, reactions were free from side
products. Target products were obtained in high or near quan-
titative yields (Table 4). The use of (6-Dipp)Pd(cinn)Cl is
efficient in cross-coupling of donor aryl boronic acids (Table 4,
entries 1, 2), as well as deactivated acceptor aryls (Table 4,
entries 3–5) and bulky (Table 4, entry 6) boronic acids.
g. The reaction is clean and no homo-coupling products are
formed.
Thus, we developed a versatile, robust, highly active cataly-
tic system for the coupling of heteroaromatic chlorides and
bromides. Boronic acids bearing donor, acceptor as well as
sterically bulky substituents can be efficiently used as coupling
partners. As an additional advantage, the precatalyst (6-Dipp)-
Pd(cinn)Cl is air- and moisture-stable and can be prepared in
multigram quantities in high yields.
Conclusions
A series of expanded ring NHC–palladium complexes were syn-
thesized and characterized structurally in the solid state.
These complexes can be obtained by direct interaction of free
carbenes with a palladium source or via transmetallation in
(NHC)AgCl.
Experimental
General information
The catalytic activities of the complexes in the Suzuki–
Miyaura cross-coupling reaction of heteroaryl chlorides and
bromides in aqueous media were tested. The (6-Dipp)Pd(cinn) Unless otherwise stated, all manipulations were carried out
Cl complex based on six-membered ring carbene bearing using standard Schlenk technique under argon atmosphere.
bulky Dipp substituents was found to be the most active pre- Dichloromethane was distilled prior to use over CaH2 in argon
catalyst. Catalytic reaction conditions were optimized to meet atmosphere. Diethyl ether was distilled over sodium-benzophe-
the requirements of “green chemistry”. The developed cross- none. Unsaturated amidinium precursors were prepared by the
coupling protocol has several prominent features:
method published by Hintermann.26 Free N-heterocyclic car-
a. The reaction takes place in water with no addition of benes were prepared from the corresponding amidinium salts
organic solvents.
by the method published by Cavell et al.9a Bis(palladium-
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(λ3-cinnamyl)chloride) was prepared using the published pro- precipitated product was filtered off and dried for a few days in
cedure.27 Other chemicals and solvents were obtained from a vacuumed desiccator over P2O5.
commercial sources and used without further purification.
General method of synthesis of silver complexes
NMR spectra were obtained on
a Bruker “Avance 400”
(400 MHz 1H, 101 MHz 13C) and Bruker “Avance 600” Silver complexes were prepared by a modified method reported
(600 MHz 1H, 151 MHz 13C). The chemical shifts are frequency earlier.9c A mixture of amidinium salt (1 eq.) and silver(I) oxide
referenced relative to the solvent peaks.28 Coupling constants J in a Schlenk flask was dissolved in dichloromethane (20 ml ×
are given in Hertz as positive values regardless of their real 1 mmol) in the absence of light. The reaction mixture was
individual signs. The multiplicity of the signals is indicated as stirred for 3 days, filtered through a short pad of Celite and
“s”, “d”, or “m” for singlet, doublet, or multiplet, respectively. evaporated at half. Methanol (5 ml × 1 mmol) was then added
The abbreviation “br” is given for broadened signals. Elemen- to the solution and the mixture was evaporated at 2/3. A white
tal analyses were performed on a Carlo Erba EA1108 CHNS-O precipitate of the target complex was filtered off and dried.
elemental analyzer at the Institute of Petrochemical Synthesis,
General methods of synthesis of palladium complexes
Russian Academy of Sciences.
Method A. A solution of N-heterocyclic carbene (1 eq.) in
diethyl ether (30 ml) was added to a suspension of Bis(palla-
X-Ray structure determination
Data were collected on a Bruker SMART APEX II CCD and a dium(η3-cinnamyl)chloride) (0.5 eq.) in a small amount of
Bruker SMART 1K CCD diffractometer (λ(MoKα)-radiation, diethyl ether. The reaction was stirred for 1 h, then filtered off
graphite monochromator, ω and φ scan mode) and corrected (if the solution is not clear), evaporated to dryness, grinded
for absorption using the SADABS program (versions 2.0129 and with hexane and the precipitate was filtered off and dried
2.0330). For details, see Table S2.† The structures were solved in vacuo. If the product is not pure according to the analytical
by direct methods and refined by full-matrix least squares data, it could be purified by recrystallization from hexane–
technique on F2 with anisotropic displacement parameters for DCM or by flash chromatography, using DCM as an eluent.
non-hydrogen atoms. The crystallographically independent
Method B. In air, the NHC–silver(I) complex (1 eq.) and bis-
parts of the unit cells of (6-Mes)Pd(cinn)Cl, (6-Dipp)Pd(cinn)Cl (palladium(η3-cinnamyl)chloride) (0.5 eq.) were dissolved in
and (7-Dipp)Pd(cinn)Cl contain one methylene chloride, one DCM (20 ml × 1 mmol) and stirred for 24 h in the absence of
diethyl ether, and a half hexane solvate molecule, respectively. light. The mixture was evaporated, dissolved in diethyl ether,
The capping-C3 carbon atoms of six-membered carbene rings filtered off through a short pad of Celite (for separation of the
in one of the two independent molecules of (6-Mes)Pd(cinn)Cl product solution from AgBr, unreacted starting silver complex
and in the molecule of (6-Dipp)Pd(cinn)Cl are disordered over and [Pd(cinn)Cl]2), and evaporated to give the pure palladium
two sites with the occupancies of 0.6 : 0.4 and 0.7 : 0.3, respect- complex. In the case of 6-Dipp and 7-Dipp, the pad of Celite
ively. The allyl ligand and hexane solvate molecule in (7-Dipp)- was extracted with DCM (for dissolution of the remaining
Pd(cinn)Cl are strongly disordered. The allyl ligand was suc- starting materials) and the filtrate was stirred for 24 h. Then
cessfully split by two positions with occupancies of 0.8 : 0.2, all processes were repeated two more times.
whereas the contribution to the scattering by the hexane
(4)Pd(cinn)Cl. Yield: 0.57 g, 48%, white powder (Method A).
solvent molecule was removed by the use of the utility Anal. Calcd for C30H35ClN2Pd: C, 63.72; H, 6.24; N, 4.95.
1
SQUEEZE in PLATON98.31 The hydrogen atoms in all com- Found: C, 63.94; H, 6.31; N, 5.03. H NMR (CDCl3, 400 MHz):
pounds were placed in calculated positions and refined within δppm 7.09–7.15 (m, 3H, Ph), 7.04–7.08 (m, 2H, Ph), 6.97 (br.s,
the riding model with fixed isotropic displacement parameters 2H, Ar-Mes-H), 6.95 (br.s, 2H, Ar-Mes-H), 5.09 (ddd, J = 18.6,
(Uiso(H) = 1.5Ueq(C) for the CH3-groups and Uiso(H) = 1.2Ueq(C) 12.5, 10.3 Hz, 1H, Allyl), 4.27 (d, J = 12.6 Hz, 1H, Allyl), 3.99 (d,
for the other groups). All calculations were carried out using J = 3.4 Hz, 4H, CH2-N), 3.27 (d, J = 6.3 Hz, 1H, Allyl), 2.46 (s,
the SHELXTL program.32 Crystallographic data for the com- 6H, CH3-Mes), 2.42 (s, 6H, CH3-Mes), 2.32 (s, 6H, CH3-Mes),
plexes have been deposited with the Cambridge Crystallo- 1.93 (d, J = 11.7 Hz, 1H, Allyl).
graphic Data Center. CCDC 784539–784543 contain the
supplementary crystallographic data for this paper.
13C NMR (151 MHz, CDCl3): δppm 211.2 (N-C-N), 138.1 (Ar),
136.4 (Ar), 129.4 (Ar), 128.3 (Ar), 127.4 (Ar), 126.6 (Ar), 109.7
(Allyl), 90.2 (Allyl), 51.3 (CH2-N), 46.8 (Allyl), 21.2 (CH3-Mes),
18.64 (CH3-Mes), 18.56 (CH3-Mes).
General method of synthesis of saturated amidinium salts
In air, to the mixture of formamidine (1.0 eq.), alkyldibromide
(5)Pd(cinn)Cl. Yield: 0.82 g, 63%, light-yellow powder
(1.5 eq.), and DIPEA (1.2 eq.) was added a small amount of (Method A). Anal. Calcd for C36H47ClN2Pd: C, 66.56; H, 7.29;
toluene which is necessary for easy mixing (approx. 15 ml to N, 4.31. Found: C, 67.13; H, 7.01; N, 4.24. 1H NMR (CDCl3,
0.1 mol of formamidine). The reaction mixture was heated at 600 MHz): δppm 7.38 (t, J = 7.8 Hz, 2H, p-Ar-Dipp-H), 7.25 (d, J
120 °C for 1 h, then dissolved in CH2Cl2, and washed in 3 × = 7.6 Hz, 4H, m-Ar-Dipp-H), 7.15–7.11 (m, 5H, Ph), 5.06 (dt, J =
100 ml of a diluted solution of potassium carbonate. The 13.1, 9.2, 1H, Allyl), 4.34 (d, J = 13.2 Hz, 1H, Allyl), 4.04 (s, 4H,
organic layer was dried and dissolved in acetone (200 ml × CH2-N), 3.45 (br.s, 4H, CH-iPr), 2.89 (br.s, 1H, Allyl), 1.58 (br.s,
0.1 mol). To the solution was added a solution of NaBF4 (2 eq.) 1H, Allyl), 1.47 (d, J = 5.9 Hz, 6H, CH3-iPr), 1.42 (d, J = 6.2 Hz,
in water (10 ml × 1 g) and acetone was evaporated; the 6H, CH3-iPr), 1.28 (d, J = 6.8 Hz, 12H, CH3-iPr). 13C NMR
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(CDCl3, 151 MHz): δppm 212.0 (N-C-N), 147.1 (Ar), 137.6 (Ar), 7.35; N, 4.28. 1H NMR (CDCl3, 600 MHz): δppm 7.31 (t, J =
136.3 (Ar), 129.0 (Ar), 128.2 (Ar), 127.2 (Ar), 126.6 (Ar), 124.1 7.6 Hz, 2H, p-Ar-Dipp-H), 7.23 (d, J = 7.6 Hz, 4H, m-Ar-Dipp-H),
(Ar), 109.0 (Allyl), 91.6 (Allyl), 54.0 (CH2-N), 46.0 (Allyl), 28.5 7.01–7.11 (m, 3H, Ph), 6.69–6.76 (m, 2H, Ph), 4.58 (dt, J = 21.1,
(CH-iPr), 26.6 (CH-iPr), 23.7 (CH3-iPr).
9.4 Hz, 1H, Allyl), 4.04 (br.s, 4H, CH2-N), 3.80 (d, J = 12.0 Hz,
(6-Mes)Pd(cinn)Cl. Yield 0.31 g, 62% (Method A); 0.88 g 1H, Allyl), 3.61 (br.s, 4H, CH-iPr), 2.35 (br.s, 4H, CH2CH2),
88% (Method B), light-yellow powder. Anal. Calcd for 1.45 (d, J = 6.7 Hz, 12H, CH3-iPr), 1.24 (d, J = 6.5 Hz, 12H, CH3-
C31H37ClN2Pd: C, 64.25; H, 6.44; N, 4.83. Found: C, 64.02; H, iPr) (two of the allyl protons were not observed). 13C NMR
6.59; N, 4.93. 1H NMR(DMSO-d6, 400 MHz): δppm 6.99–7.09 (m, (CDCl3, 151 MHz): δppm 225.5 (N-C-N), 146.4 (Ar), 145.7 (Ar),
3H, Ph), 6.95 (br.s, 2H, Ar-Mes-H), 6.87 (br.s, 2H, Ar-Mes-H), 139.2 (Ar), 128.2 (Ar), 127.8 (Ar), 127.4 (Ar), 125.9 (Ar), 124.1
6.68 (d, J = 6.4 Hz, 2H, Ph), 4.61 (ddd, J = 19.3, 11.8, 9.4 Hz, (Ar), 109.1 (Allyl), 85.6 (Allyl), 56.7 (Allyl), 48.3 (CH2-N), 31.7
1H, Allyl), 3.62 (d, J = 12.0 Hz, 1H, Allyl), 3.37 (s, 4H, CH2-N), (CH2CH2), 28.9 (CH-iPr), 28.8 (CH-iPr), 27.3 (CH-iPr), 25.5-
2.39–2.43 (m, 2H, Allyl), 2.34 (br.s, 6H, CH3-Mes), 2.28 (br.s, (CH-iPr), 23.9 (CH3-iPr).
12H, CH3-Mes), 2.15–2.23 (m, 2H, CH2) (one of the allyl
protons was not observed). 13C NMR(DMSO-d6, 101 MHz):
δppm 206.7 (N-C-N), 139.5 (Ar), 136.6 (Ar), 136.4 (Ar), 134.4 (Ar),
General method of Suzuki–Miyaura cross-coupling reaction in
water
129.3 (Ar), 128.9 (Ar), 128.7 (Ar), 128.0 (Ar), 127.7 (Ar), 127.0
In air, heteroaromatic halide (3.27 mmol, 1 eq.), boronic acid
(Ar), 125.5 (Ar), 109.5 (Allyl), 83.7 (Allyl), 48.3 (CH2-N), 45.9
(3.40 mmol, 1.04 eq.), NaHCO3 (0.84 g, 10 mmol, 3.06 eq.), TBAB
(allyl), 31.0 (CH2), 22.1 (CH3-Mes), 20.6 (CH3-Mes), 19.0 (CH3-
(0.1 g, 0.31 mmol, 0.095 eq.) and (6-Dipp)Pd(cinn)Cl (10.9 mg,
0.0164 mmol, 0.005 eq.) were mixed with 5 ml of distillated
water in a 25 ml round bottomed flask equipped with a magnetic
Mes), 17.5 (CH3-Mes), 14.0 (CH3-Mes). 13C NMR (151 MHz,
CDCl3) δppm 213.8 (N-C-N), 146.7 (Ar), 143.5 (Ar), 138.9 (Ar),
128.5 (Ar), 127.9 (Ar), 127.3 (Ar), 126.0 (Ar), 124.1 (Ar), 108.7
stirring bar and a reflux condenser. The reaction mixture was
(allyl), 86.7 (allyl), 65.7 (N-CH2), 49.3 (allyl), 31.5 (CH2), 28.6
refluxed for 1 h, cooled to room temperature and extracted with
(CH3-Mes), 26.9 (CH3-Mes), 23.6 (CH3-Mes), 22.6 (CH3-Mes),
3 × 20 ml CH2Cl2. The crude product obtained from evaporation
of the combined organic phase was purified by flash chromato-
graphy using hexane–CHCl3 or acetone–CHCl3 mixture as eluent.
20.8 (CH3-Mes), 15.2 (CH3-Mes).
(7-Mes)Pd(cinn)Cl. Yield: 0.94 g, 93% (Method B), light-
yellow powder. Anal. Calcd for C32H39ClN2Pd: C, 64.75; H,
6.62; N, 4.72. Found: C, 64.38; H, 6.31; N, 4.83. 1H NMR
(CDCl3, 400 MHz): δppm 7.07 (br.s, 2H, Ar-Mes-H), 6.96 (br.s,
5H, Ph), 6.75 (br.s, 2H, Ar-Mes-H), 4.63 (dd, J = 19.1, 11.7 Hz,
1H, Allyl), 4.09 (d, J = 11.0 Hz, 1H, Allyl), 3.60–3.90 (m, 4H,
Notes and references
CH2-N), 3.32 (d, J = 5.6 Hz, 1H, Allyl), 2.02–2.80 (m, 22H, CH3-
Mes + CH2CH2), 1.54 (d, J = 12.3 Hz, 1H, Allyl). 13C NMR
(CDCl3, 101 MHz): δppm 221.4 (N-C-N), 139.1 (Ar), 137.0 (Ar),
130.1 (Ar), 129.6 (Ar), 128.9 (Ar), 128.1 (Ar), 127.9 (Ar), 127.1
(Ar), 126.0 (Ar), 109.6 (Allyl), 86.0 (Allyl), 53.8 (Allyl), 48.5 (CH2-
N), 25.3 (CH2CH2), 20.9 (CH3-Mes), 20.02–20.37 (m, CH3-Mes),
19.55–20.02 (m, CH3-Mes), 18.10–19.02 (m, CH3-Mes).
(6-Dipp)Pd(cinn)Cl. Yield: 0.15 g, 34% (Method A); 0.85 g,
71% (Method B), light-yellow powder. Anal. Calcd for
C37H49ClN2Pd: C, 66.96; H, 7.44; N, 4.22. Found: C, 67.19; H,
7.53; N, 4.13. 1H NMR (CDCl3, 400 MHz): δppm 7.35 (t, J =
7.6 Hz, 2H, p-Ar-Dipp-H), 7.25 (d, J = 7.3 Hz, 4H, m-Ar-Dipp-H),
7.04–7.11 (m, 3H, Ph), 6.83–6.88 (m, 2H, Ph), 4.62 (dt, J = 12.2,
9.4 Hz, 1H, Allyl), 3.88 (d, J = 12.2 Hz, 1H, Allyl), 3.62–3.69 (t, J
= 5.6 Hz, 4H, CH2-N), 3.55 (dt, J = 13.4, 6.6 Hz, 4H, CH-iPr),
2.36 (ddd, J = 11.2, 6.0, 5.7 Hz, 2H, CH2), 1.42 (d, J = 6.6 Hz,
12H, CH3-iPr), 1.22 (d, J = 6.8 Hz, 12H, CH3-iPr), 1.18 (d, J =
6.6 Hz, 1H, Allyl) (one of the allyl protons was not observed).
13C NMR (CDCl3, 151 MHz): δppm 213.8 (N-C-N), 146.6 (Ar),
143.5 (Ar), 138.9 (Ar), 128.5 (Ar), 127.8 (Ar), 127.3 (Ar), 126.0
(Ar), 124.1 (Ar), 108.7 (Allyl), 86.7 (Allyl), 65.7 (Allyl), 49.3
(CH2-N), 31.5 (CH2), 28.6 (CH-iPr), 26.9 (CH-iPr), 23.6
(CH3-iPr), 22.5 (CH3-iPr), 20.8 (CH3-iPr), 15.2 (CH3-iPr).
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