Cyclopropenylidene carbene ligands in palladium catalysed coupling reactions: Carbene ligand rotation and application to the Stille reaction

Article abstract of DOI:10.1039/c1dt10109a

Reaction of [Pd(PPh₃)₄] with 1,1-dichloro-2,3- diarylcyclopropenes gives complexes of the type cis-[PdCl₂(PPh ₃)(C₃(Ar)₂)] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)₂] with 1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes trans-[PdCl₂(C ₃(Ar)₂)]₂ (Ar = Ph 7, p-(OMe)C ₆H₄8, p-(F)C₆H₄9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl₂(NCMe)(C₃(Ar)₂)] (Ar = Ph 10, p-(OMe)C₆H₄11 and p-(F)C₆H₄) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7 or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl ₂(PR₃)(C₃(Ar)₂)] (Ar = Ph, R = Cy 13, Ar = p-(OMe)C₆H₄, R = Ph 14, Ar = p-(F)C ₆H₄, R = Ph 15). Crystal structures of complexes 6·3.25CHCl₃, 10, 11·H₂O and 12-15 are reported. DFT calculations of complexes 10-12 indicate the barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond character in these species. Complexes 5-9 were tested for catalytic activity in C-C coupling (Mizoroki-Heck, Suzuki-Miyaura and, for the first time, Stille reactions) and C-N coupling (Buchwald-Hartwig amination) showing excellent conversion with moderate to high selectivity.

Full text of DOI:10.1039/c1dt10109a

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PAPER
Cyclopropenylidene carbene ligands in palladium catalysed coupling
reactions: carbene ligand rotation and application to the Stille reaction†
Ratanon Chotima, Tim Dale, Michael Green, Thomas W. Hey, Claire L. McMullin, Adam Nunns,
A. Guy Orpen, Igor V. Shishkov, Duncan F. Wass* and Richard L. Wingad
Received 19th January 2011, Accepted 18th February 2011
DOI: 10.1039/c1dt10109a
Reaction of [Pd(PPh3)4] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes
of the type cis-[PdCl2(PPh3)(C3(Ar)2)] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)2] with
1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes
trans-[PdCl2(C3(Ar)2)]2 (Ar = Ph 7, p-(OMe)C6H4 8, p-(F)C6H4 9). Alternatively, when the reactions were
performed in acetonitrile, the complexes trans-[PdCl2(NCMe)(C3(Ar)2)] (Ar = Ph 10, p-(OMe)C6H4 11
and p-(F)C6H4) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7
or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl2(PR3)(C3(Ar)2)] (Ar = Ph, R = Cy
13, Ar = p-(OMe)C6H4, R = Ph 14, Ar = p-(F)C6H4, R = Ph 15). Crystal structures of complexes
6·3.25CHCl3, 10, 11·H2O and 12–15 are reported. DFT calculations of complexes 10–12 indicate the
barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond
character in these species. Complexes 5–9 were tested for catalytic activity in C–C coupling
(Mizoroki–Heck, Suzuki–Miyaura and, for the first time, Stille reactions) and C–N coupling
(Buchwald–Hartwig amination) showing excellent conversion with moderate to high selectivity.
supported this thesis and also indicated such ligands are very
Introduction
robust to thermal and chemical decomposition.6 The reason for
N-heterocyclic carbenes are versatile supporting ligands for ho-
these attributes lies in the possibility of a contribution from a
mogeneous catalysis,1 their strong s-donor capabilities resulting
resonance form in which a 2p aromatic cationic cyclopropenium
in advantages over many other ligand types in a range of reactions,
moiety is formed (Scheme 1).
including C–C and C–N coupling.2 Numerous variations to the
structure of N-heterocyclic carbene ligands have been investigated,
often giving marked improvements in performance.3 It is perhaps
surprising that other types of carbene ligand are much less studied
as supporting ligands in catalysis; for example, carbocyclic carbene
ligands, i.e., with no heteroatom, are much rarer.4 This lack
of utility in catalysis is despite a wide range of such ligands
Scheme 1 Cyclopropenylidene ligands.
being reported in the synthetic organometallic literature. As early
as 1968, Ofele reported the stable carbocyclic carbene complex
[Cr(CO)5(2,3-diphenylcyclopropenylidene)].5 We were attracted to
this result, particularly the reported CO stretching frequencies
of the complex which suggest that the 3-membered ring carbene
ligand to be an exceptionally strong s-donor. Kawada and Jones
Certain palladium complexes of related ligands have been
reported.7,8 There has also been recent interest in free carbenes
of this type (when R = dialkylamino),9 extending also to chiral
derivatives.10 However, the potential of cyclopropenylidene car-
bene complexes in catalysis remained unrealised until our report
of the use of such complexes as highly active and efficient catalysts
for C–C coupling reactions.11 In the same year these results
were independently confirmed by Herrmann and co-workers.12
Following these promising results we, and Herrmann et al.,
extended the utility of cyclopropenylidene carbene complexes to
C–N coupling processes.13 In this full paper, we build on our
preliminary communications and report the synthesis of new
palladium complexes of cyclopropenylidene ligands which show
broad applicability in catalytic C–C and C–N bond forming
reactions.
¨
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS. E-mail: duncan.wass@bristol.ac.uk
† Electronic supplementary information (ESI) available: Relative energy
values for rotation of cyclopropenylidene ligands about the Pd–C1 bond
in complexes 10, 11 and 12. Catalysis data and experimental details
for selected Mizoroki–Heck, Suzuki–Miyaura and Buchwald–Hartwig
reactions. Crystal structures of complexes 13, 14 and 15. Cif files for all
structures are available. CCDC reference numbers 809246–809254. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10109a
5316 | Dalton Trans., 2011, 40, 5316–5323
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˚
Table 1 Selected bond distances (A) and angles ( ) for 10, 11·H2O
and 12
Results and discussion
1. Synthesis of ligands and palladium complexes
10
11·H2O
12
The carbene precursors 1,1-dichloro-2,3-diphenylcyclopropene
(1) and 1,1-dichloro-2,3-dimesitylcyclopropene (2) can be con-
veniently prepared from commercially available diphenylcy-
clopropenone and dimesitylcyclopropenone, respectively. The
aryl functionalised 1,1-dichloro-2,3-diarylcyclopropenes 3 and 4
(aryl = p-(OMe)C6H4 3 and p-(F)C6H4 4) are synthesised by
first preparing tetrachlorocyclopropene using a modification of
Tobey and West’s method14 and then a Friedel–Crafts alkylation
of the appropriate aromatic compounds to produce the diaryl-
cyclopropenone precursors.15 Reactions with thionyl chloride or
oxalyl chloride give 3 and 4 in moderate to good yields.
Pd1–C1
Pd1–N1
Pd1–Cl1
Pd1–Cl2
C1–C2
C1–C3
C2–C3/C2A
C1–Pd1–Cl1
Cl1–Pd1–Cl2
C2–C1–Pd1
Pd1–N1–C
C3–C1–C2
Cl1–Pd1–C1–C2
C1–C2–C–C (Ph tilt)
1.922(5)
2.074(5)
2.3030(9)
—
1.374(6)
—
1.354(8)
88.59(3)
177.19(6)
150.49(19)
180.00(1)
59.0(4)
-78.9(3)
9.4(8)
1.929(6)
2.079(6)
2.315(4)
—
1.374(7)
—
1.365 (9)
89.82(12)
179.6 (3)
150.2(2)
180.0
59.6(5)
-66.3(5)
1.4(11)
1.920(8)
2.087(7)
2.302(2)
2.297(2)
1.393(11)
1.368(11)
1.387(12)
88.0(3); 88.3(3)
175.49(9)
148.1(7); 151.6(7)
170.6(8)
60.3(6)
-39.9(12)
-6(2); 2(2)
The oxidative addition of 1,1-dichloro-2,3-diarylcyclopropenes
(1–4) to well-defined soluble Pd(0) sources enables the synthe-
sis of a range of palladium(II) cyclopropenylidene complexes
(Scheme 2). Herrmann and co-workers report a complementary
synthesis of similar compounds via direct addition to metallic
palladium;12 our method certainly suffers from lower atom effi-
ciency in comparison but has the advantage of being reproducible
and proceeding smoothly at lower temperatures and shorter
reaction times. We have previously reported the synthesis of
complexes 5 and 7 and indeed the conversion of 7 to 5 via a bridge
splitting reaction with triphenylphosphine.11,13 The reaction of 1,1-
dichloro-2,3-diarylcyclopropenes 3 and 4 with [Pd(dba)2] (dba =
dibenzylideneacetone) in benzene at 65 ◦C yields the binuclear
palladium complexes 8 and 9 respectively. When the oxidative
addition of 1, 3 and 4 to [Pd(dba)2] is performed using acetonitrile
as the solvent mononuclear palladium(II) complexes 10–12 are
formed, in which an acetonitrile ligand is bound trans to the
carbene. Crystals of complexes 10–12 were obtained and studied
by X-ray crystallography. The molecular structures are shown in
Figures 1–3 with bond distances and angles in Table 1.
Fig. 1 Thermal ellipsoid plot of 10, [PdCl2(MeCN)(cyclo-C3(C6H5)2)],
generated by molecular C2-symmetry. Displacement ellipsoids are shown
at the 50% probability level. All hydrogen atoms have been removed for
clarity.
Fig.
2
Thermal ellipsoid plot of 11·H2O, [PdCl2(MeCN)(cyclo-
C3(p-(OMe)C6H4)2)], generated by molecular C2-symmetry. Displacement
ellipsoids are shown at the 50% probability level. All hydrogen atoms have
been removed for clarity.
Fig.
3 Thermal ellipsoid plot of 12, [PdCl2(MeCN)(cyclo-
C3(p-(F)C6H4)2)]. Displacement ellipsoids are shown at the 50%
probability level. All hydrogen atoms have been removed for clarity.
Scheme 2 Routes to cyclopropenylidene palladium(II) complexes.
Complexes 10, 11·H2O and 12 crystallise in space groups C2/c,
Pbcn and P21/c, respectively. Complexes 10 and 11·H2O have
exact C2 symmetry with one half of the molecule present in
the asymmetric unit (Z¢ = 12 ). The palladium(II) centres of each
complex have a square planar geometry, and are coordinated by
the cyclopropenylidene ligand which lies trans to an acetronitrile
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Table 2 Selected bond distances (A) and angles ( ) for 6·3.25CHCl3
ligand and cis to two chloride ligands. In each case, the similarity
in bond lengths C1–C2 and C2–C3/C2A suggests the carbocyclic
ligand is best described by a cyclopropenium resonance form.
There is significant variation in the plane of the cyclopropenylidene
ligands to the square plane of the complex. In 10 this is
approximately perpendicular to the chloride ligand axis, being off-
set by ca 11◦ (torsion angle Cl1–Pd1–C1–C2 = -78.9◦). Whereas
cyclopropenylidene planes of complexes 11·H2O and 12 are off-
set from perpendicular to the chloride axis, by ca 23◦ and
50◦, respectively (torsion angles and bond lengths are shown in
Table 1).
6·3.25CHCl3
Pd1–C1
Pd1–P1
Pd1–Cl1 (trans P1)
Pd1–Cl2 (trans C1)
C1–C2
1.941(4)
2.2552(11)
2.3668(10)
2.3451(10)
1.392(6)
C1–C3
C2–C3
1.380(6)
1.372(5)
C1–Pd1–Cl1
C1–Pd1–P1
Cl1–Pd1–Cl2
C2–C1–Pd1
82.65(13)
94.83(13)
91.79(4)
Initially we speculated that variation in the cyclopropenylidene
plane may be due to differing p-bonding characteristics for the
various ligands; a more co-planar orientation of the carbocyclic
ring with the square plane of the complex would maximise
possible overlap between the cyclopropenylidene p-system and
the palladium dyz or dxz orbitals. Orienting the cyclopropenylidene
perpendicular to the square plane would maximise overlap with
the dxy orbital. In order to test this hypothesis the geometries in
the crystal structures were optimised computationally using DFT,
and then the cyclopropenylidene ligand was rotated about the Pd–
C1 bond in 15◦ intervals. The energy profiles for 10, 11·H2O and
12 are very shallow (Figure 4), with a barrier height between 2.5–
3 kcal mol-1 with a maximum when Cl1–Pd1–C1–C2 = 90◦ and
a minimum close to 40◦. There is little distinction between the
different ligands, implying the observed structural differences are
in fact a manifestation of packing effects. Certainly, in the case of
11·H2O and 12, intermolecular distances between phenyl rings of
145.5(3)
C3–C1–C2
59.3(3)
- 78.4(7)
65.9(6)
Cl1–Pd1–C1–C2
Cl1–Pd1–C1–C3
C1–C2–C4/C13–C5/C18 (Ph tilt)
25.4(10), 54.3(9)
˚
the cyclopropenylidene ligand are less than 3.6 A, suggesting p–p
stacking may have an influence.
Fig.
5
Thermal ellipsoid plot of complex 6·3.25CHCl3,
[PdCl2(cyclo-C3(Mes)2)(PPh3)]. Displacement ellipsoids are shown
at the 50% probability level. All hydrogen atoms and solvent molecules
have been omitted for clarity.
The plane of the cyclopropenylidene ring is approximately
perpendicular to the coordination plane (torsion angle Cl1–Pd1–
C1–C2 = -78.4◦). The mesityl substituents do not lie in the same
plane as the cyclopropenyl ring (torsion angles C1–C2–C4–C5 =
25.4◦ and C1–C3–C13–C18 = 54.3 ◦), but are tilted to reduce the
steric clash between carbon atoms at the internal ortho positions
(C12 and C19). The palladium–chlorine bond length trans to
the carbene (Pd1–Cl2) is slightly shorter than that trans to the
˚
phosphine (Pd1–Cl1) by 0.021 A. This is in agreement with the
previously reported structure of complex 5, and suggests that the
trans influence of the phosphine ligand is stronger than that of
the cyclopropenylidene. In 1979, Ibers and co-workers reported
the same trans influence effect in their complex, [PdCl2(2,3-
bis(dimethylamino)cyclopropenylidene)(PnBu3)].8 Pd–Cl bond
Fig. 4 B3LYP/6-31G*&LACV3P calculated energy barrier for rotation
about the Pd–C1 bond in complexes 10, 11 and 12.
The oxidative addition of 1,1-dichloro-2,3-diarylcyclopropenes
1 and 2 to [Pd(PPh3)4] gives cyclopropenylidene phosphine com-
plexes 5 (previously published and structurally characterised11)
and 6 (Scheme 2). Crystals of 6·3.25CHCl3 were obtained from
a saturated chloroform and hexane solution and crystallised in
˚
lengths trans to the carbene and the phosphine are 2.361(1) A
˚
and 2.385(1) A, respectively. The Pd-carbene bond length in
˚
6·3.25CHCl3 (Pd1–C1 1.941(4) A) is slightly shorter than the
¯
˚
space group P1 (see Figure 5 and Table 2 for bond distances and
equivalent bond length in Ibers’ complex (1.961(3) A) and in both
angles). The structure includes three molecules of chloroform,
and a fourth chloroform was located in the asymmetric unit with
25% occupancy, disordered over two positions. The palladium(II)
centre has square planar geometry, and is coordinated by the
cyclopropenylidene ligand cis to PPh3, both of which are trans
to a chloride ligand.
Herrmann’s cycloheptatrienylidene16 and cyclopropenylidene12
˚ ˚
A and 1.945(2) A, respectively), but
complexes (1.968(2)
slightly longer than our 2,3-diphenylcyclopropenylidene complex
(1.939(3) A).11 This suggests that there is little influence of electron
˚
donation from the mesityl groups on the cyclopropenylidene ring
compared to phenyl groups.
5318 | Dalton Trans., 2011, 40, 5316–5323
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Complexes of this type, containing mutually cis phosphine
and carbene ligands, can also be prepared by bridge splitting
reactions of 7–9 with suitable phosphine ligands (e.g., 5 and
13 were prepared by reaction of 7 with triphenylphosphine and
tricyclohexylphosphine respectively) or by reaction of a phosphine
with complexes 10–12 (e.g. 14 and 15 were prepared by reaction
of triphenylphosphine with 11 and 12 respectively) (Scheme 2).
Complexes 13, 14 and 15 were also structurally characterised.
The molecular structures are as expected and are included in the
supplementary information.
interesting trends emerge. Complex 7 has good performance across
a range of substrates with both high conversion and selectivity;
complexes 5, 6, 8 and 9 give rise to good conversion but are less
selective. Chloroarene substrates prove more challenging although
good conversions are observed when basic phosphines are added
as co-ligands; selectivity with these substrates is capricious but
modestly high selectivity is observed in selected examples.
We have previously reported ‘standard blank’ experiments using
palladium phosphine complexes in the absence of cyclopropenyli-
dene moieties.13 We have also reported how plots of conversion
with time reveal no catalyst initiation period in both Heck11
and Buchwald–Hartwig13 coupling. These results confirm that
the cyclopropenylidene ligand is influencing catalytic performance
but, given that so many ‘homogeneous’ palladium coupling
catalysts are in fact likely to be colloidal palladium metal,18 it is
prudent to avoid bold claims in this regard. To date, we have been
unable to detect bound cyclopropenylidene species during ongoing
catalytic runs using 1H or 13C NMR spectroscopy (although
detection of such species is hampered by the high concentration
of other species including substrates and products). Although it is
tempting to attribute the superior performance of these catalysts
to cyclopropenylidene complexes during the catalytic cycle, until
such species are detected, we cannot rule out a role for such ligands
in facilitating the initiation of the palladium(II) procatalysts to
ligand-free palladium(0) active species.
2. Catalytic results
A selection of cyclopropenylidene complexes were tested in four
well-known catalytic coupling reactions; Stille, Mizoroki–Heck
and Suzuki–Miyaura C–C cross coupling and Buchwald–Hartwig
C–N coupling reactions. We have published preliminary data for
all except the first of these reactions previously, and the new
palladium complexes described in this paper exhibit very similar
performance; the relevant catalysis data, including conversion and
yields, is presented in the supplementary information.†
We have not previously communicated the use of cyclo-
propenylidene catalysts in Stille cross coupling reactions, and a
range of substrates was investigated, as shown in Table 3. Some
Table 3 Catalytic activity of cyclopropenylidene palladium(II) complexes
in Stille coupling reactions
Conclusions
Palladium complexes supported by cyclopropenylidene ligands
may be conveniently synthesised by the addition of 1,1-dichloro-
2,3-diarylcyclopropenes to palladium(0) sources. Structural char-
acterisation of a number of these compounds reveals the coor-
dinated cyclopropenylidene ligand is best described as a cyclo-
propenium moiety. DFT calculations suggest that variations to
the torsion angle of these ligands with respect to the square plane
of the complex are not significant in terms of ligand bonding
effects. Such palladium complexes are effective in a wide range of
Pd-catalysed coupling reactions.
mol % cconversion
aEntry X Cat.
R
bphosphine Pd
(yield)/%
TON
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
7
7
7
7
7
7
8
8
8
8
9
9
5
5
6
6
7
7
7
7
7
7
8
8
8
9
9
OMe
OMe
PPh3
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
52(52)
66(66)
98(98)
98(98)
36(36)
54(54)
100(48)
100(50)
100(47)
100(51)
99(50)
100(42)
96(11)
94(19)
100(20)
94(15)
0(0)
84(84)
0(0)
98(98)
0(0)
76(76)
96(40)
100(27)
92(28)
95(6)
26
33
49
49
18
27
24
25
24
26
25
21
6
10
10
8
0
42
0
2
3
4
5
6
7
8
9
PtBu3
C(O)Me PPh3
C(O)Me PtBu3
CH3
CH3
PPh3
PtBu3
PPh3
PtBu3
OMe
OMe
C(O)Me PPh3
C(O)Me PtBu3
C(O)Me PPh3
C(O)Me PtBu3
OMe
C(O)Me none
OMe none
C(O)Me none
OMe
OMe
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Experimental
General procedures
none
All procedures were carried out under an inert (N2) atmosphere
using standard Schlenk line techniques or in an inert atmosphere
(Ar) glovebox. Chemicals were obtained from Sigma–Aldrich,
Fisher Scientific, Acros Organics, Fluka, or Alfa Aesar and used
without further purification unless otherwise stated. Tetrachloro-
cyclopropene was synthesised by the literature method.19 All
solvents were purified using an Anhydrous Engineering Grubbs-
type solvent system.20 NMR spectra were recorded on Jeol
ECP 300, Jeol Lambda 300, Jeol ECP 400 or Varian 400-MR
PPh3
PtBu3
C(O)Me PPh3
C(O)Me PtBu3
49
0
38
20
14
14
3
CH3
CH3
PPh3
PtBu3
C(O)Me PPh3
C(O)Me PtBu3
OMe
PtBu3
1
C(O)Me PPh3
spectrometers. H NMR chemical shifts were referenced relative
C(O)Me PtBu3
100(38)
19
to the residual solvent resonances in the deuterated solvent.
1
1
1
13C{ H}, 19F{ H}, and 31P{ H} NMR spectra were referenced
relative to TMS, CFCl3 and H3PO4 respectively at 23 ◦C. Mass
spectra were recorded on a VG Analytical Quattro (ESI) or a
VG Autospec (EI) spectrometer. Microanalyses were carried out
by the Microanalytical Laboratory of the School of Chemistry at
a Conditions: 1.0 mmol aryl halide, 1.1 mmol Bu3SnPh, 2 mL dioxane
diluent, 1.1 mmol CsF base, 90 ◦C, 18 h. b 1 mol equiv. of phosphine
with respect to amount of palladium used. c Conversion and yield were
determined by GC relative to the internal standard hexadecane. Products
were checked by NMR spectroscopy against authentic samples.17
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Table 4 Crystallographic data for structures 6·3.25CHCl3, 10, 11·H2O and 12
Compounds
6·3.25CHCl3
10
11·H2O
12
Colour, habit
Size/mm
Empirical formula
M
Colourless block
0.15 ¥ 0.11 ¥ 0.06
C42.25H40.25Cl11.75PPd
Yellow plate
0.07 ¥ 0.05 ¥ 0.02
C17H13Cl2NPd
408.58
Monoclinic
C2/c
10.4687(4)
18.5853(7)
8.5376(4)
90.00
110.274(2)
90.00
1558.2(11)
4
1.525
Yellow lath
0.12 ¥ 0.08 ¥ 0.02
C19H18Cl2NO2.5Pd
477.64
Orthorhombic
Pbcn
15.7371(8)
16.6448(9)
7.4293(4)
90.00
90.00
90.00
Yellow rod
0.078 ¥ 0.023 ¥ 0.02
C17H11Cl2F2NPd
444.57
Monoclinic
P21/c
13.0238(8)
7.3625(4)
17.0969(9)
90.00
92.697(3)
90.00
1637.6(16)
4
1.476
1101.90
Triclinic
Crystal system
¯
Space group
P1
˚
a/A
11.4288(6)
13.7141(8)
17.3825(10)
94.734(4)
106.928(3)
108.301(3)
2428.8(2)
2
˚
b/A
˚
c/A
a/◦
b/◦
g/◦
V/A3
1946.04(18)
4
1.243
Z
m/mm-1
1.091
100 (2)
T/K
120(2)
120(2)
120(2)
Reflections: total/independent
Rint
57 119/11 894
0.0827
0.0533
2.44, -1.93
1.507
8623/1776
0.0545
0.0418
0.58, -0.54
1.74
9707/2227
0.0564
0.0547
0.63, -0.73
1.63
15 754/3755
0.1310
0.0891
0.98, -0.89
1.80
Final R1 (observed data)
Largest peak, hole/eA-3
r
calc/g cm-3
the University of Bristol. All catalytic samples were analysed by
GC-FID, using a Varian L3800 gas chromatograph, fitted with a
Varian WC07 fused silica capillary column, 25 m ¥ 0.25 mm, ID
coating CP◦-Sil 5CB, DF = 0.25. Method:◦hold at 50 ◦C for 4 min,
10, 11·H2O and 12, and were individually optimized before torsion
angle restraints were applied, to build up the energy profile of
rotating the cyclopropenylidene ligand about the Pd–C1 bond.
◦
◦
heat to 58 C at 1 C min-1, heat to 200◦ C at 40 C min-1, hold
at 200 ◦C for 5 min, heat to 250 ◦C at 50 C min-1, hold at 250 ◦C
for 5 min.
Synthesis of diarylcyclopropenone precursors
Diarylcyclopropenone precursors were synthesised using a modi-
fication of Poloukhtine and Popik’s method.15
Crystal structure determination
2,3-di(p-methoxyphenyl)cyclopropenone.
A
solution
of
methoxybenzene (1.90 mL, 17.40 mmol) in
5
mL of
X-ray diffraction experiments for 10, 11·H2O and 12 were carried
dichloromethane was added to a suspension of aluminium
trichloride (1.40 g, 10.50 mmol) in tetrachlorocyclopropene
(1.00 mL, 8.70 mmol) at -78 ◦C. The reaction mixture was stirred
for 2 h at low temperature. The reaction mixture was allowed
to warm to room temperature and stirred for 30 min before ice
was added. The product was extracted into dichloromethane.
The organic layer was washed with water and brine, dried
over magnesium sulfate, filtered and concentrated in vacuo.
The crude solid was purified by precipitation from a 1 : 10
dichloromethane/hexane solution to give a colourless powder
(980 mg, 42%) (Found: C, 77.3; H, 5.4, Calc. for C17H14O3: C,
out at 120 K on a Bruker–Nonius Kappa CCD diffractometer,
˚
using Mo-Ka radiation (l = 0.71073 A) generated by a Bruker–
Nonius FR591 rotating anode. All data collections were per-
formed using a single crystal coated in paraffin oil and mounted
on a glass fibre. Intensities were integrated21 from several series of
exposures in j and w calculated by the COLLECT22 and DENZO
programs after unit cell determination by the DirAx program.
X-ray diffraction on 6·3.25CHCl3 was carried out at 100 K on
a Bruker Kappa Apex II CCD diffractometer. Intensities were
integrated23 from several series of exposures in j and w calculated
by the Apex II24 program after unit cell determination. Absorption
corrections were based on equivalent reflections using SADABS,
3
76.7; H, 5.3%); dH(300 MHz; CDCl3) 7.92 (4 H, d, JHH 8.9 Hz,
3
2
ArH), 7.06 (4 H, d, JHH 8.6 Hz, ArH), 3.91 (6 H, s, CH3); dC
and structures were refined against all Fo data with hydrogen
(75 MHz, CDCl3) 162.6 (ArC), 155.2 (C3 ring), 144.0 (CO), 133.4
(ArC), 117.0 (ArC), 114.6 (ArC), 55.5 (CH3); m/z (ESI): 267
(M+ + H).
atoms riding in calculated positions using SHELXTL. The high
Rint and R1 values for 12 reflect the fact that all crystals of 12
diffracted only weakly at high angle, despite data being collected
for an extended period. Crystal structure and refinement data are
given in Table 4.
2,3-di(p-fluorophenyl)cyclopropenone. Essentially the same
procedure was followed, only using fluorobenzene (6.50 mL,
69.00 mmol), aluminium trichloride (2.28 g, 17.10 mmol) and
tetrachlorocyclopropene (2.00 mL, 16.30 mmol). The reaction
mixture was stirred at 0 ◦C for 10 min and then increased to
50 ◦C for 2 h. The crude solid was purified by recrystallisation
from a methanol/diethyl ether solution to give colourless crystals
(1.90 g, 45%) (Found: C, 74.1; H 3.8, Calc. for C15H8F2O: C, 74.4;
H, 3.3%); dH(300 MHz, CDCl3) 7.98 (4 H, dd, 3JHH 5.3 Hz, ArH),
7.28 (4 H, t, 3JHH 8.6 Hz, ArH); dC(75 MHz, CDCl3) 165.0 (d, 1JCF
Computation
All geometries were optimised in the gas phase using the Jaguar
package25 and the standard Becke–Lee–Yang–Parr (B3LYP)26
hybrid density function. The Jaguar triple-z form of the standard
Los Alamos ECP basis set (LACV3P) was used to describe the
palladium atom, employing the 6-31G* basis for all other atoms.
The geometries for computation were taken from crystal structures
5320 | Dalton Trans., 2011, 40, 5316–5323
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257 Hz CF), 154.7 (C3 ring), 146.2 (CO), 133.7 (d, 3JCF 9 Hz, ArC)
120.4 (d, 4JCF 4 Hz, ArC), 116.8 (d, 2JCF 22 Hz, ArC); dF(283 MHz,
CDCl3) -103.2 (s); m/z (ESI): 243 (M+ + H).
5 mL of toluene and added dropwise to a solution of
tetrakis(triphenylphosphine)palladium(0) (416 mg, 0.36 mmol) in
15 mL of toluene. A colourless solid precipitated immediately and
the mixture was stirred for 5 h. The precipitate was collected by
filtration and dried in vacuo to give 5 as a white solid (200 mg,
87%). 5 can be recrystallised from CHCl3 (Found: C, 62.8; H,
4.15. Calc. for C33H26Cl2PPd: C, 63.0; H, 4.25%); dH(400 MHz,
CDCl3) 8.03 (4 H, m, ArH), 7.64 (8 H, m, ArH), 7.53 (4 H, m,
ArH), 7.25 (3 H, m, ArH), 7.15 (6 H, m, ArH); dP(121 MHz,
CDCl3) 27.6 (s, PPh3); m/z (ESI): 653 (M- + Na+), 595 (M+ - Cl).
Synthesis of 1,1-dichloro-2,3-diarylcyclopropenes
1,1-dichloro-2,3-diarylcyclopropenes 1–3 are synthesised using a
modification of Tobey and West’s method.14 4 was synthesised
following a modification of Herrmann’s method.12
1,1-dichloro-2,3-diphenylcyclopropene
(1). Diphenylcyclo-
propenone (500 mg, 2.40 mmol) was heated at 40 ◦C in excess
thionyl chloride (2 mL). The resulting solution was stirred and
the evolution of SO2 was observed. The reaction mixture was
stirred for approximately 2 h until gas evolution ceased. The
volatiles were removed in vacuo to give a beige powder. The solid
was dissolved in cyclohexane (6 mL) with heating and placed in a
freezer overnight to give 1 as pale yellow crystals (550 mg, 88%)
(Found: C, 69.0; H, 4.25, Calc. for C15H10Cl2: C, 69.0; H, 3.9%);
dH(300 MHz, CDCl3) 7.56 (10 H, m, ArH); dC(75 MHz, CDCl3)
131.4, 130.4, 129.5 (ArC), 128.4 (ArC), 125.9 (ArC), 92.7 (CCl2);
m/z (EI): 225 (M+ - Cl, 100%), 189.1 (M+ - 2Cl, 50).
[PdCl2(cyclo-C3(Mes)2)(PPh3)]
(6). 1,1-dichloro-2,3-dime-
sitylcyclopropene (2) (173 mg, 0.50 mmol) and tetrakis-
(triphenylphosphine)palladium(0) (578 mg, 0.50 mmol) were
weighed into a Schlenk flask. Toluene (10 mL) was added to give
a yellow solution and a precipitate immediately. The mixture was
stirred at 80 ◦C for 3 h. The precipitate was collected by filtration
and dried in vacuo to give 6 as a yellow solid. 6 can be recrystallised
from CHCl3 and hexane to give white crystals (130 mg, 35%)
(Found: C, 63.9; H, 4.7, Calc. for C39H37Cl2PPd·0.25CHCl3:
C, 63.4; H, 5.1%, chloroform present in the crystal structure);
dH(300 MHz, CDCl3) 7.76–7.69 (15 H, m, P(C6H5)3), 6.95 (4 H, s,
ArH), 2.31 (12 H, s, o-CH3), 2.18 (6 H, s, p-CH3); dP(121 MHz,
CDCl3) 27.6 (s, PPh3); m/z (ESI): 678.5 (M+ - Cl).
1,1-dichloro-2,3-dimesitylcyclopropene (2). An analogous pro-
cedure was followed, using mesitylcyclopropenone (523 mg, 1.80
mmol) to give 2 as a white powder (570 mg, 91%) (Found: C,
72.8; H, 6.45, Calc. for C21H22Cl2: C, 73.0; H, 6.4%); dH(400 MHz,
CDCl3) 6.99 (4 H, s, ArH), 2.38 (12 H, s, o-CH3), 2.35 (6 H, s, p-
CH3); dC(100 MHz, CDCl3) 151.2 (ArC), 140.6 (ArC), 139.7 (C3
ring), 130.1(ArC), 128.9 (ArC), 95.7 (CCl2), 21.5 (p-CH3), 20.7
(o-CH3); m/z (EI): 309 (M+ - Cl, 60%), 274 (M+ - 2Cl, 10).
[PdCl2(cyclo-C3(Ar)2)]2 complexes (7–9)
General procedure: The complexes were synthesised following a
modification of the literature method.13 A solution of 1,1-dichloro-
2,3-diarylcyclopropene (1, 3 or 4) in benzene (1 mL) was added
dropwise to a stirred suspension of palladium dibenzylideneace-
tone, [Pd(dba)2], in 15 mL of benzene at 65 ◦C. The reaction
mixture was refluxed overnight. The solid precipitate was collected
on a d4 glass filter. The product, [PdCl2(cyclo-C3(Ar)2)]2 (7–9), was
washed with diethyl ether and dried in air.
1,1-dichloro-2,3-di(p-methoxyphenyl)cyclopropene
(3). An
analogous procedure was followed, using 2,3-di(p-methoxy-
phenyl)cyclopropenone (320 mg, 1.20 mmol) to give 3 as a yellow
solid (330 mg, 85%) (Found: C, 62.95; H, 4.7; Cl, 21.6, Calc. for
C17H14Cl2O2: C, 63.6; H, 4.4; Cl, 22.1%); dH(300 MHz, CDCl3)
7.80 (4 H, d, 3JHH 8.8 Hz, ArH), 7.08 (4 H, d, 3JHH 8.8 Hz, ArH),
3.88 (6 H, s, CH3); dC(75 MHz, CDCl3) 161.9 (COCH3), 132.9
(C3 ring), 131.8 (ArC), 123.3 (ArC), 116.8 (ArC), 105.4 (CCl2),
55.6 (OCH3); m/z (EI): 320 (M+, 30%), 285 (M+ - Cl, 100), 250
(M+ - 2Cl, 5).
[PdCl2(cyclo-C3(C6H5)2)]2 (7). Reaction of 1,1-dichloro-2,3-
diphenylcyclopropene (1) (210 mg, 0.80 mmol) with [Pd(dba)2]
(440 mg, 0.80 mmol) gave 7 as a fine brown solid. (190 mg,
65%); m/z (ESI): 758 (M+ + Na+); HRMS (ESI): Calc. for
[C30H20Cl4Pd2]Na+: 754.8281, found 754.8305.
[PdCl2(cyclo-C3(p-(OMe)C6H4)2)]2 (8). Reaction of 1,1-
dichloro-2,3-di(p-methoxyphenyl) cyclopropene (3) (266 mg, 0.83
mmol) with [Pd(dba)2] (288 mg, 0.50 mmol) gave 8 as a brownish
solid (200 mg, 55%); m/z (ESI): 878 (M+ + Na+); HRMS (ESI);
Calc. for [C34H28Cl4O4Pd2]Na+: 874.8704, found 874.8734.
1,1-dichloro-2,3-di(p-fluorophenyl)cyclopropene (4). An excess
of oxalyl chloride (171 mL, 2.00 mmol) was added dropwise to
a stirred solution of the 2,3-di(p-fluorophenyl)cyclopropenone
(194 mg, 0.80 mmol) in 5 mL of dichloromethane at -78 ◦C. The
reaction mixture was allowed to warm to room temperature and
the evolution of CO2 was observed. The reaction was stirred
for approximately 4 h until gas evolution ceased. The volatiles
were removed in vacuo to give 4 as a white solid (220 mg, 92%)
(Found: C, 60.7; H, 2.8, Calc. For C15H8Cl2F2: C, 60.6; H, 2.7%);
dH(300 MHz, CDCl3) 7.85 (4 H, dd, 3JHH = 9.7 Hz, 3JFH = 3.6 Hz,
[PdCl2(cyclo-C3(p-(F)C6H4)2)]2 (9). Reaction of 1,1-dichloro-
2,3-di(p-fluorophenyl)cyclopropene (4) (277 mg, 0.93 mmol) with
[Pd(dba)2] (536 mg, 0.93 mmol) gave 9 as a black solid (220 mg,
59%); m/z (ESI): 831 (M+ + Na+); HRMS (ESI); Calc. for
[C30H16Cl4F4Pd2]Na+: 826.7904, found: 826.7923.
NMR spectroscopic data was not obtained due to the insolu-
bility of bimetallic complexes 7–9 in non-coordinating organic
solvents. The complexes were soluble in acetonitrile to give
[PdCl2(MeCN)(cyclo-C3(Ar)2)] complexes (10–12) via a bridge
splitting reaction. Alternatively complexes 10–12 can be synthe-
sised directly using the general procedure below.
3
ArH), 7.28 (4 H, t, JHH = 8.8 Hz, ArH); dC(75 MHz, CDCl3)
164.9 (ArC), 154.3 (ArC), 144.5 (C3 ring), 132.5 (ArC), 128.4
(ArC), 117.1 (CCl2); dF(283 MHz, CDCl3) - 106.0 (s); m/z (EI):
298 (M+, 10%), 263 (M+ - Cl, 65).
Synthesis of palladium(II) complexes
[PdCl2(cyclo-C3(Ph)2)(PPh3)] (5). 1,1-dichloro-2,3-diphenyl-
cyclopropene (1) (94.0 mg, 0.36 mmol) was dissolved in
General procedure: Acetonitrile (4 mL) was added to a mixture
of 1,1-dichloro-2,3-diarylcyclopropene (1, 3 or 4) and [Pd(dba)2]
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at 0 ◦C. The reaction mixture was stirred at 0 ◦C for 1 h, then
at room temperature for 72 h. The reaction mixture was passed
through Celite/sand and the filter was washed with acetonitrile.
The volatiles were removed in vacuo, and the crude solid was
recrystallised from acetonitrile, to give the [PdCl2(MeCN)(cyclo-
C3(Ar)2)] complex (10–12).
3.6%); dH(300 MHz, CD2Cl2); 8.15 (4 H, d, JHH = 5.5 Hz, o-
ArH), 8.12 (4 H, d, 3JHH = 5.5 Hz, m-ArH), 7.68 (6 H, m, ArH),
7.36 (6 H, m, ArH), 7.27 (6 H, m, ArH); dP(121 MHz, CD2Cl2)
27.7 (s, PPh3); dF(283 MHz, CD2Cl3) - 98.6 (s); HRMS (ESI);
Calc. for [C33H23PClF2Pd]+: 629.0223, found 629.0229.
3
Catalytic experiments
[PdCl2(MeCN)(cyclo-C3(Ph)2)] (10). Reaction of 1,1-dichloro-
2,3-diphenylcyclopropene (1) (136.0 mg, 0.52 mmol) with
[Pd(dba)2] (300.0 mg, 0.52 mmol) gave 10 as yellow crystals
(200.0 mg, 94%) dH(300 MHz, CD3CN) 8.58 (4 H, d, 3JHH = 8.4 Hz,
The catalytic procedure is given for entry 1 in Table 3. Catalysts
were added either as complex (5–9) or a mixture of complex (7–
9) with one equivalent (based on palladium) of the appropriate
phosphine ligand.
o-ArH), 7.90 (2 H, t, 3JHH = 7.5 Hz, p-ArH), 7.79 (4 H, t, 3JHH
7.7 Hz, m-ArH), 1.97 (3 H, s, CH3CN).
=
Stille reaction.17 A Schlenk flask was charged with cesium
fluoride (334 mg, 1.10 mmol), 4-bromoanisole (187 mg, 1.00
mmol), and the internal standard, hexadecane (100 mg, 0.40
mmol). Phenyltributyltin (404 mg, 1.10 mmol) and degassed
dioxane (2 mL) were added and the reaction was heated to
90 ◦C. Catalyst 7 (2 mol%) and triphenylphosphine (1 equiv. based
on Pd) was then added and the reaction stirred for 18 h. After
this time, the reaction mixture was allowed to cool, diluted with
diethyl ether and filtered through a pad of silica gel. The silica gel
was washed thoroughly with diethyl ether. The combined organic
layers were dried over magnesium sulfate. Conversion and yield
were determined by GC relative to the internal standard. Products
were checked by NMR spectroscopy against authentic samples.
[PdCl2(MeCN)(cyclo-C3(p-(OMe)C6H4)2)]
(11). Reaction
of
1,1-dichloro-2,3-bis(p-methoxyphenyl)cyclopropene
(3)
(100.0 mg, 0.31 mmol) with [Pd(dba)2] (178.0 mg, 0.31 mmol)
gave 11 as yellow crystals (36.0 mg, 25%) (Found: C, 48.1; H,
3.8; N, 2.9. Calc. for C19H17Cl2NO2Pd: C, 48.6; H, 3.7; N, 3.0%);
dH(300 MHz, CD3CN) 8.50 (4 H, d, 3JHH = 8.8 Hz, o-ArH), 7.25
(4 H, d, 3JHH = 8.8 Hz, m-ArH), 3.97 (6 H, s, OCH3), 2.14 (3 H, s,
CH3CN).
[PdCl2(MeCN)(cyclo-C3(p-(F)C6H4)2)] (12). Reaction of 1,1-
dichloro-2,3-bis(p-fluorophenyl)cyclopropene (4) (500.0 mg, 1.70
mmol) with [Pd(dba)2] (970.0 mg, 1.70 mmol) gave 12 as yellow
crystals (60.0 mg, 19%) dH(300 MHz, CD3CN) 8.65 (4 H, dd,
3JHH = 5.3, 9.0 Hz, o-ArH), 7.50 (4 H, t, 3JHH = 8.8 Hz, m-ArH);
dF(283 MHz, CD3CN) 100.3 (s).
Acknowledgements
Synthesis of [PdCl2(cyclo-C3(Ph)2)(PCy3)] (13). The bimetallic
palladium complex 7 (37.0 mg, 0.05 mmol) and tricyclohexylphos-
phine (34.0 mg, 0.12 mmol) were dissolved in acetonitrile in a
small vial. The reaction mixture was stirred for 30 min in the
glovebox and then filtered through a glass filter. The product
was recrystallised with hexane as a co-solvent to give 13 as
brown crystals (7.0 mg, 45%) (Found: C, 61.7; H, 7.3. Calc. for
C33H44Cl2PPd: C, 61.1; H, 6.8%); dH(300 MHz, CD2Cl2) 8.45 (4
We thank the EPSRC National Crystallography Service in
Southampton for collecting the diffraction data for three crystal
structures, specifically Peter Horton for 10 and 11.H2O, and Louise
Male for 12. CLM thanks CCDC and EPSRC for funding. RC
would also like to thank Ministry of Science and Technology,
Royal Thai government for funding. Natalie Fey (University of
Bristol) is thanked for useful discussions.
3
3
H, d, JHH 6.9 Hz, o-ArH), 7.99 (2 H, t, JHH 7.7 Hz, p-ArH),
3
7.89 (4 H, t, JHH 7.7 Hz, m-ArH), 2.3–1.8 (33 H, m, (cyclo-
Notes and references
C6H11)3; dP(121 MHz, CD2Cl2) 51.2 (s); HRMS (ESI): Calc. for
[C33H43PClPd]+ : 611.1845, found 611.1890.
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