Control over the E/Z selectivity of the catalytic dimerization of group 6 (Fischer) metal carbene complexes

Article abstract of DOI:10.1021/jo302130h

The systematic investigation of the effect of different catalysts and additives in the reaction of self-dimerization of alkoxychromium(0) (Fischer) carbene complexes resulted in the selection of Pd(P^tBu ₃)₂ to effect this

Full text of DOI:10.1021/jo302130h

Article
pubs.acs.org/joc
Control over the E/Z Selectivity of the Catalytic Dimerization of
Group 6 (Fischer) Metal Carbene Complexes
Gong M. Chu, Israel Fernandez,* and Miguel A. Sierra*
́
́
́
Departamento de Quımica Organica, Facultad de Ciencias Quımicas, Universidad Complutense, 28040 Madrid, Spain
́
S
* Supporting Information
ABSTRACT: The systematic investigation of the effect of different
catalysts and additives in the reaction of self-dimerization of
alkoxychromium(0) (Fischer) carbene complexes resulted in the
selection of Pd(PtBu3)2 to effect this transformation with good to
excellent E selectivities and acceptable to excellent chemical yields.
This catalyst will allow the control of the geometry in the synthesis of
polyconjugated olefins, one emerging application of these catalytic reactions.
mixture of the Z/E olefins 5 with concomitant regeneration of
the catalyst.
INTRODUCTION
■
Transmetalation processes from group 6 Fischer metal carbene
complexes to late transition metals (Pd, Pt, Cu, Ni, Rh, Au,
etc.) lead to new carbene complexes, exhibiting exceedingly
different chemical behavior with respect to the starting metal
complex.1 Thus, the new carbene complexes may show an
enhanced reactivity compared to the precursor complex2 or
they may undergo different reaction pathways, opening doors
to the synthesis of new classes of compounds (Scheme 1). This
The key intermediates in this catalytic cycle are the mono-
carbene complexes 3 and bis-carbene complexes 4. Very
recently,5 Albe
́
niz and Espinet isolated the first Pd
alkoxycarbene 7 from the reaction of the W(0) Fischer carbene
complex 6 and [PdCl2(NCMe)2].6 In addition, related Cu(I)7
Scheme 2. Catalytic Cycle for the Self-Dimerization of
Group 6 Carbene (Fischer) Complexes in the Presence of
Pd Catalysts
Scheme 1. Effect of Transmetalation in the Reactivity of
Group 6 Metal Carbene Complexes
fascinating process has been profusely exploited since our
original report on the self-dimerization of chromium(0) and
tungsten(0) carbene complexes promoted by Pd catalysts.3
The reaction mechanism for this transformation, which has
been proposed on the basis of experimental and theoretical
studies,3,4 is summarized in Scheme 2. Accordingly, a
transmetalation reaction of the carbene ligand from complex
1 to the palladium(0) catalyst leads to a new Pd−carbene
complex such as 3, through a heterobimetallic intermediate 2
that evolves to 3 by extrusion of the [M(CO)5] (M = Cr, W)
fragment, mediated by coordination of the solvent. Subsequent
transmetalation from a new molecule of the carbene complex 1
leads to the Pd(0) bis-carbene complex 4, which produces a
Received: September 27, 2012
© XXXX American Chemical Society
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or Au(I)8 mono-carbene complexes have been isolated and
fully characterized in similar transformations. Moreover, the
reaction of β-arylamino-substituted chromium(0) and tung-
sten(0) carbene complexes 8 with Pd reagents leads to the
mononuclear Pd bis-carbene complexes 9,9 the putative second
key intermediates shown in Scheme 2 (Scheme 3).
Table 1. Self-Dimerization Reactions of Complex 10 in the
Presence of Phosphines and Phosphites
a
b
c
entry
cat.
solvent
additive
E/Z
1
PdCl2(MeCN)2
PdCl2(MeCN)2
PdCl2(MeCN)2
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(OAc)2
THF
none
none
PtBu3
PPh3
PMe3
none
PtBu3
none
PtBu3
1/1.3
1/1.2
1/1.1
1/4.4
1/2.4
1/23b
1/3.5
1/1.5
1/1
2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
3
4
Scheme 3. Examples of Isolation of Mono- and Bis-Carbene
Pd Complexes by Transmetalation of Group 6 Fischer
Carbene Complexes
5
6
7
Pd(OAc)2
8
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(OAc)2
9
10
11
12
P(OMe)3
P(OEt)3
P(OMe)3
1/1.5
1/1.3
1/3.2
MeCN
a
b
5% of catalyst was used in all cases. Et3N (1 mmol/mmol of carbene
c
was used in all cases as coadditive). Determined by integration of the
1H NMR signals corresponding to MeO groups in the reaction
mixtures: 3.61 ppm for the Z isomer and 3.42 ppm for the E isomer.13a
The use of phosphites follows the same pattern as that
observed for phosphines. Thus, while no changes were
observed in the selectivity of the reactions using
PdCl2(MeCN)2 (entries 10 and 11), increased Z selectivity
was observed when Pd(OAc)2 was the catalyst (entry 12, Table
1).
The main shortcoming of the catalytic self-dimerization of
group 6 metal carbene complexes is the lack of E/Z selectivity.
This is highly important due to the emerging applications of
this process toward the synthesis of polyconjugated olefins,10
some of which have been patented as conjugated polymers and
devices.11 Therefore, the control of the E/Z selectivity is
required for this method to become of full utility in synthesis.
Reported herein is a study to accomplish this challenging goal.
N-heterocyclic carbene (NHC) species 12−15 (Figure 1)
were also tested as additives in the reaction of self-dimerization
RESULTS AND DISCUSSION
■
According to the previously reported experimental and
computational studies of the transmetalation process (see
above) and to the plethora of literature on the influence of
ligands in the outcome of Pd-catalyzed reactions,12 we
hypothesized that the ligand directly attached to the catalyst
may play a fundamental role in the E/Z selectivity of the self-
dimerization reactions of group 6 metal (Fischer) carbenes.
Thus, series of phosphines differing in their steric volume were
initially tested against the self-dimerization of complex 10
(Scheme 4, Table 1) using PdCl2(MeCN)2 as the catalyst. The
Figure 1. NHC additives used in this work.
Scheme 4. Self-Dimerization of Carbene Complex 10
of carbene complex 10. However, additives 12−14 did not
produce changes in the E/Z selectivities of the reaction that
were equal to those obtained in the absence of additive (E/Z =
1/1.3), while NHC 15 proportionates a slight increase in
selectivity (E/Z = 1/1.5).
The self-dimerization reactions of pentacarbonyl-
[ethoxyphenylcarbene]chromium(0) 16 were examined next
(Scheme 5) to determine the effect of the substituent at the
oxygen atom attached to the carbene carbon atom on the E/Z
selectivity of the corresponding olefins 17. The results are
collected in Table 2.
E/Z ratio of olefins 11 in the absence of an external added
phosphine was 1/1.3 in THF and was nearly independent of
the reaction solvent (1/1.2 in MeCN, entries 1 and 2 in Table
1). Addition of the bulky PtBu3 results in a small decrease of the
selectivity (1/1.1, entry 3), while the best selectivities were
observed for PPh3 (1/4.4) and PMe3 (1/2.4), both showing a
moderate selectivity. It is noteworthy that in all cases the
reaction was Z selective. Other sources of Pd were tested
against the addition of PtBu3. The use of Pd(OAc)2
proportionates a selectivity of 1/3.5, slightly increased from
that obtained without additive (1/2),3,4 while [Pd(allyl)Cl]2
gave essentially the same selectivity as PdCl2(MeCN)2.
Scheme 5. Self-Dimerization of Ethoxycarbene Complex 16
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Table 2. Self-Dimerization Reactions of Complex 16 in the
Presence of Phosphines and Phosphites
Table 3. Self-Dimerization Reactions of Complexes 10 and
16 in the Presence of Catalysts Incorporating Phosphine
Ligands
a
b
c
entry
cat.
solvent
additive
E/Z
a
b
c
entry
cat.
complex
additive
E/Z
1
PdCl2(MeCN)2
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(OAc)2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
none
PtBu3
PMe3
none
PtBu3
none
PtBu3
none
PtBu3
none
PtBu3
1.2/1
9.4/1
1/1.4
1/1.7
7/1
2
1
2
3
4
5
6
7
8
9
Pd(PPh3)4
10
10
10
10
16
16
16
16
none
none
none
none
none
none
PtBu3
none
none
1/4.1
1/1.8
9.4/1
5.7/1
1/2.6
1/1.2
2.4/1
15.7/1
1/2.2
3
PdCl2(PPh3)2
Pd(PtBu3)2
Pd(dppe)2
4
5
Pd(OAc)2
6
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
Pd2(dba)3
1/1.3
2.7/1
1/2.4
1.7/1
1/2.8
1/1.7
1/1.8
1/1.1
1/1.3
1/1.1
1/1.7
1/1.1
Pd(PPh3)4
7
PdCl2(PPh3)2
PdCl2(PPh3)2
Pd(PtBu3)2
Pd(dppe)2
8
9
Pd2(dba)3
10
11
12
13
14
15
16
17
Pd(dba)2
16
a
b
Pd(dba)2
5% of catalyst was used in all cases. Et3N (1 mmol/mmol of carbene
Pd(OAc)2
P(OMe)3
P(OEt)3
P(OMe)3
PPh3
was used in all cases as coadditive, except for Pd(0) catalysts).
c
1
PdCl2(MeCN)2
PdCl2(Me3CN)2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
Determined by integration of the H NMR signals in the reaction
mixtures.
was tested next in the self-dimerization of complex 16 (Table
4). The selectivity is maintained using higher loads of catalyst
(up to 20%), while it steadily decreases with lower catalyst
loads (reaching 1/1 ratio for catalyst loads of 1%). Therefore,
the optimum balance of catalyst load/selectivity was 5% mol of
catalyst/mol of carbene complex.
AsPh3
MeCN
BiPh3
a
b
5% of catalyst was used in all cases. Et3N (1 mmol/mmol of carbene
c
was used in all cases as coadditive). Determined by integration of the
1H NMR signals corresponding to CH2O groups in the reaction
mixtures: 3.80 ppm for the Z isomer and 3.57 ppm for the E isomer.13b
To test these conclusions, different ethoxychromium(0)
complexes 18−25 were reacted with PdCl2(MeCN)2 in the
The results in Table 2 show a different trend than that
obtained for complex 10. In the absence of additives, the use of
PdCl2(MeCN)2 as the catalyst affords the E isomer as the
major isomer (although still close to an equimolar ratio).
Strikingly, addition of PtBu3 resulted in an excellent increase in
the E selectivity for Pd(OAc)2 and PdCl2(MeCN)2 catalysts
(up to 9.4/1 when the latter catalyst was used). The use
[Pd(allyl)Cl]2 leads to poorer but still noticeable E selectivities
(E/Z = 2.7/1). Other additives (phosphites, PPh3, AsPh3,
BiPh3)3 give selectivities close to 1/1. In contrast to complex
10, the use of NHCs carbenes 12−15 (Figure 1) in the self-
dimerization reaction proportionates low, albeit noticeable,
changes in selectivities. Thus, NHC 12 was nearly unselective
(E/Z = 1/1.1), NHCs 14 and 15 were slightly Z selective (E/Z
= 1/1.4 and 1/1.3, respectively), and NHC 13 was slightly E
selective (E/Z 1.3/1).
Table 4. Self-Dimerization Reactions of Complex 16 in the
Presence of Different Amounts of Pd(PtBu3)2
a
b
entry
cat. load (%)
E/Z
1
2
3
4
5
6
20
10
5
13.6/1
14.0/1
15.7/1
4.6/1
1.6/1
1/1
2.5
1
0.5
a
b
Referenced to carbene complex. Determined by integration of the
1H NMR signals in the reaction mixtures.
absence of additives or in the presence of 5% of PtBu3 (Table
5). From the data in Table 4, it becomes obvious that the trend
observed in Tables 2 and 3 also holds in complexes 18−22,
with the E isomer formation being the main reaction product in
the presence of PtBu3 (the exception is complex 23, forming a
1/2.2 E/Z mixture). To demonstrate the ability of Pd(PtBu3)2
as catalyst for the selective preparation of E olefins, the self-
dimerization reaction was conducted in a preparative scale
using Pd(PtBu3)2 as the catalyst (Scheme 6). While yields were
usually acceptable (complexes 16, 19−21, and 23−25) in some
cases (complexes 18 and 22) the corresponding olefins were
obtained together with variable amounts of the corresponding
esters due to oxidation of the starting materials14 (probably as a
consequence of the instability of the intermediate Pd mono- or
bis-carbene complexes having bulky phosphine ligands).
Despite that, it is clear in all cases that the use of Pd(PtBu3)2
leads to the favored formation of the corresponding E olefin.
Finally, pentacarbonyl[(benzyloxy)(methyl)carbene]-
chromium(0) (33) was reacted in the presence of Pd(PtBu3)2
(5%) under the usual reaction conditions. The self-dimerization
product 34 was obtained in a 70% yield as a 1/1.6 E/Z mixture
(Scheme 7). Although the selectivity of the reaction was low, it
The use of Pd(0) catalysts having monodentate phosphine
(PdCl2(PPh3)2, Pd(PPh3)4, and Pd(PtBu3)2) and bidentate
diphosphine ligands (Pd(dppe)2) was studied next (the
combination of PdCl2(PPh3)2/PtBu3 as the additive was also
tested for comparison reasons). The results of these reactions
with complexes 10 and 16 are collected in Table 3. For both
complexes, the best results were obtained with Pd(PtBu3)2
(which affords a remarkable 15.7/1 E/Z selectivity for carbene
complex 16). The use of Pd(PPh3)4 led also to an increased
selectivity, but now favoring the Z isomer. Pd(dppe)2 catalyst
produces good E selectivities (5.7/1) with carbene complex 10
while the selectivity decreases and inverts for complex 16 (1/
2.2). The use of PdCl2(PPh3)2 with added PtBu3 reversed the
selectivity observed in the absence of the additive (1/1.2 vs 2.4/
1), but the E selectivity is considerably lower than that obtained
with Pd(PtBu3)2.
Results in Tables 1−3 pointed to PtBu3 (as additive or
directly as Pd(PtBu3)2) and the use of an ethoxy carbene
instead of a methoxy carbene complex as the optimum
combination for obtaining high E selectivities. The influence
of the amount of the Pd(PtBu3)2 catalyst in the E/Z selectivity
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alkoxychromium(0) (Fischer) carbene complexes, it can be
concluded that the E/Z selectivity of the process can be indeed
controlled with the proper selection of these variables. Thus,
the use of Pd(PtBu3)2 and chromium(0) ethoxycarbene
complexes is recommended in order to obtain good to
excellent E selectivities and acceptable to excellent chemical
yields in most cases. This catalyst will allow the control of the
geometry in the synthesis of polyconjugated olefins, one of the
main emerging applications of these reactions.
Table 5. Catalytic Self-Dimerization Reactions of Fischer
Carbene Complexes 18−25
a
b
c
entry
cat.
complex
additive
E/Z
1
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(PtBu3)2
18
18
18
19
19
19
20
20
20
21
21
21
22
22
22
23
23
23
24
none
PtBu3
none
none
PtBu3
none
none
PtBu3
none
none
PtBu3
none
none
PtBu3
none
none
PtBu3
none
none
none
1/1
2
5.8/1
6.8/1
1/1.1
6/1
3
4
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(PtBu3)2
5
6
8.1/1
1.5/1
1.5/1
11/1
1/1.5
7.6:1
10.8/1
1/1.4
1.7/1
2.2/1
1/2.5
1/2.2
6/1
7
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(PtBu3)2
EXPERIMENTAL SECTION
■
8
General Procedures. All reactions were carried out under an
argon atmosphere. All solvents used in this work were purified by
distillation and were freshly distilled immediately before use.
Acetonitrile (MeCN) and triethylamine (Et3N) were distilled from
calcium hydride, whereas tetrahydrofuran (THF) and diethyl ether
(Et2O) were purified using a Pure Solv PS-MD-5 system. Flame-dried
glassware was used for moisture-sensitive reactions. Silica gel (Merck:
230−400 mesh) was used as stationary phase for purification of crude
reaction mixtures by flash column chromatography. Identification of
products was made by thin-layer chromatography (Kieselgel 60F-254).
UV light (λ 254 nm) and 5% phosphomolybdic acid solution in 95%
EtOH were used to develop the plates. NMR spectra were recorded at
9
10
11
12
13
14
15
16
17
18
19
20
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(PtBu3)2
PdCl2(MeCN)2
PdCl2(MeCN)2
Pd(PtBu3)2
PdCl2(CH3CN)2
PdCl2(CH3CN)2
Pd(PtBu3)2
Pd(PtBu3)2
Pd(PtBu3)2
1
25 °C in CDCl3, at 300 MHz for H and 75 MHz for 13C. Chemical
2.1/1
2.0/1
shifts are given in ppm relative to CDCl3 (1H, 7.27 ppm; 13C, 77.0
25
ppm). IR spectra were taken on a MIR (8000−400 cm−1
)
a
b
5% of catalyst was used in all cases. Et3N (1 mmol/mmol of carbene
was used in all cases as coadditive, except for Pd(PtBu3)2 catalyst).
spectrometer as solid films by slow evaporation of the solvent using
the ATR (attenuated total reflectance) technique. MS spectra
(HRMS) were acquired on a QTOF mass spectrometer. All
commercially available products were used without further purification.
The following metal carbenes were prepared according to previously
described methods: pentacarbonyl[(methoxy)(phenyl)carbene]-
chromium(0),15 pentacarbonyl[(ethoxy)(phenyl)carbene]-
chromium(0),16 pentacarbonyl[(p-bromophenyl)(ethoxy)carbene]-
c
1
Determined by integration of the H NMR signals corresponding to
CH2O groups in the reaction mixtures.
Scheme 6. Self-Dimerization of Complexes 16 and 18−25
Catalyzed by Pd(PtBu3)2
chromium(0),17 pentacarbonyl[(p-methoxyphenyl)(ethoxy)carbene]-
1 8
c h r o m i u m ( 0 ) ,
p e n t a c a r b o n y l [ ( m -
tolyl)(ethoxy)carbene]chromium(0),19 pentacarbonyl[(ethoxy)-
(phenyl)carbene]tungsten(0),20 pentacarbonyl[(2E)-1-ethoxy-3-phe-
nyl-2-propen-1-ylidene]chromium(0),21 pentacarbonyl[(benzyloxy)-
(methyl)carbene]chromium(0).22
Pentacarbonyl[(p-trifluoromethylphenyl)(ethoxy)carbene]-
chromium(0) (20). To a solution of 1-bromo-4-(trifluoromethyl)-
benzene (0.77 g, 3.42 mmol) in 4 mL of dry Et2O was added 2.1 mL
(3.42 mmol) of n-BuLi (1.6 M in hexanes) at −78 °C. After it was
stirred for 30 min, the solution was transferred via cannula at 0 °C to a
suspension of Cr(CO)6 (0.75 g, 3.42 mmol) in 8 mL of Et2O at room
temperature. The mixture was refluxed for 3 h, the solvent was
removed under reduced pressure, and the residue was dissolved in 6.3
mL of degassed water. Et3OBF4 (1.13 g, 5.95 mmol) was added slowly
to the solution at 0 °C and stirred for 10 min. The mixture was
extracted with EtOAc and dried over MgSO4. The solvent was
removed in vacuo. The crude product was purified by flash
chromatography (hexane) to yield 0.8 g (59%) of a dark red solid;
mp 46−48 °C. 1H NMR (300 MHz, ppm): δ 7.69 (d, J = 7.6 Hz, 2H),
7.26 (d, J = 7.6 Hz, 2H), 4.98 (br s, 2H), 1.69 (t, J = 6.7 Hz, 3H). 13C
NMR (75 MHz, ppm): δ 348.4, 223.8, 215.7, 156.3, 131.2, 130.0,
125.4, 122.1, 77.5, 15.1. IR (ATR): ν 2064, 1928 cm−1. HRMS (ESI):
m/z calcd for C15H9CrF3O6 [M + H]+ 394.9840, found 394.9850.
Pentacarbonyl[(p-(p-bromophenyl)phenyl)(ethoxy)-
carbene]chromium(0) (22). To a solution of 4,4′-dibromobiphenyl
(1.42 g, 4.54 mmol) in 5.4 mL of dry Et2O was added 2.8 mL (4.54
mmol) of n-BuLi (1.6 M in hexanes) at −78 °C. After it was stirred for
30 min, the solution was transferred via cannula at 0 °C to a
suspension of Cr(CO)6 (1 g, 4.54 mmol) in 2.7 mL of Et2O at room
temperature. The mixture was refluxed for 3 h, the solvent was
removed under reduced pressure, and the residue was dissolved in 5.4
mL of degassed water. Et3OBF4 (1.47 g, 7.72 mmol) was added slowly
to the solution at 0 °C and stirred for 10 min. The mixture was
extracted with EtOAc and dried over MgSO4. The solvent was
Scheme 7. Self-Dimerization of the Aliphatic Carbene
Complex 33
is worthy noting that while complex 33 is prone to experience
β-elimination processes to yield enol ether 35 in the presence
of several Pd(0) catalysts,3b Pd(PtBu3)2 behaves in this case as
Pd(PPh3)4, which gave 34 as a 1/1.1 E/Z mixture in 87%
yield.3b
CONCLUSIONS
From the systematic investigation of the effects of different
catalysts and additives in the reaction of self-dimerization of
■
D
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removed in vacuo. The crude product was purified by flash
catalyst (0.025 mmol), and Et3N (70 μL, 0.49 mmol) were reacted for
6 h, to yield compound 26 as a white solid (100 mg, 96%, E/Z = 1/1);
mp 77−79 °C. Z isomer: 1H NMR (300 MHz) δ 7.32 (d, J = 8.7 Hz,
4H), 7.05 (d, J = 8.7 Hz, 4H), 3.77 (q, J = 7.0 Hz, 4H), 1.32 (t, J = 7.0
Hz, 6H); 13C NMR (75 MHz) δ 142.1, 134.1, 131.6, 131.1, 121.5,
chromatography (hexane) to yield 570 mg (26%) of a dark red
1
solid; mp 108−110 °C. H NMR (300 MHz, ppm): δ 7.68−7.41 (m,
8H), 5.07 (q, J = 7.1 Hz, 2H), 1.72 (t, J = 7.1 Hz, 3H). 13C NMR (75
MHz, ppm): δ 346.5, 224.0, 216.3, 152.5, 141.9, 138.6, 132.0, 128.6,
126.5, 124.5, 122.4, 77.2, 15.2. IR (ATR): ν 2064, 1928 cm−1. HRMS
(ESI): m/z calcd for C20H13BrCrO6 [M + H]+ 480.9386, found
480.9392.
1
66.1, 15.5; IR (ATR): ν 1274, 1107, 1013, 820. E isomer: H NMR
(300 MHz) δ 7.65 (d, J = 8.7 Hz, 4H), 7.53 (d, J = 8.7 Hz, 4H), 3.52
(q, J = 7.1 Hz, 4H), 1.13 (t, J = 7.1 Hz, 6H); 13C NMR (75 MHz) δ
143.8, 133.5, 131.2, 130.0, 121.5, 66.6, 15.3. IR (ATR): ν 1150, 1105,
1014, 834 cm−1. HRMS (ESI): m/z calcd for C18H19Br2O2 [M + H]+
425.9780, found 425.9776.
Pentacarbonyl[(p-bromophenoxyphenyl)(ethoxy)carbene]-
chromium(0) (23). To a solution of bis(4-bromophenyl) ether (1.49
g, 4.54 mmol) in 5.4 mL of dry Et2O was added 2.8 mL (4.54 mmol)
of n-BuLi (1.6 M in hexanes) at −78 °C. After it was stirred for 30
min, the solution was transferred via cannula at 0 °C to a suspension of
Cr(CO)6 (1 g, 4.54 mmol) in 2.7 mL of Et2O at room temperature.
The mixture was refluxed for 3 h, the solvent was removed under
reduced pressure, and the residue was dissolved in 5.4 mL of degassed
water. Et3OBF4 (1.47 g, 7.72 mmol) was added slowly to the solution
at 0 °C and stirred for 10 min. The mixture was extracted with EtOAc
and dried over MgSO4. The solvent was removed in vacuo. The crude
product was purified by flash chromatography (hexane) to yield 1.1 g
(47%) of a dark red solid; mp 84−86 °C. 1H NMR (300 MHz, ppm):
δ 7.59 (d, J = 8.8 Hz, 2H), 7.51 (d, J = 8.8 Hz, 2H), 7.00−6.95 (m,
4H) 5.14 (q, J = 7.1 Hz, 2H), 1.72 (t, J = 7.1 Hz, 3H). 13C NMR (75
MHz, ppm): δ 342.0, 223.8, 216.5, 160.1, 154.8, 148.4, 133.0, 128.0,
121.7, 117.2, 117.0, 77.2, 17.9. IR (ATR): ν 2059, 1926 cm−1. HRMS
(ESI): m/z calcd for C20H13BrCrO7 [M + H]+ 496.9335, found
496.9357.
General Procedure for Carbene Ligand Dimerization.
Method A. To a solution of the corresponding alkoxycarbene complex
(1 mmol) in MeCN (6 mL/mmol carbene complex) were added the
PdCl2(MeCN)2 catalyst (5%) and the corresponding additive (0.1
mmol) and Et3N (1 mmol/mmol of carbene complex). The reaction
mixture was stirred at room temperature until total disappearance of
the starting material (checked by TLC). The solvent was then
removed under reduced pressure. The residue was dissolved in EtOAc,
the solution was filtered through a short pad of Celite, and finally the
solvent was removed again under reduced pressure. An E/Z isomer
mixture was obtained. The crude product was then purified by flash
column chromatography (hexane/EtOAc 100/1). This method was
used for characterizing both E and Z isomers.
Method B. Carbene complex 18 (200 mg, 0.49 mmol) and
Pd(PtBu3)2 (13 mg, 0.025 mmol) were reacted for 18 h, to yield
compound 26 as a white solid (18 mg, 18%, E/Z = 6.8/1).
(Z)- and (E)-1,2-Diethoxy-1,2-bis(p-methoxyphenyl)ethylene
(27). Method A. Carbene complex 19 (50 mg, 0.14 mmol), Pd catalyst
(0.007 mmol), and Et3N (20 μL, 0.14 mmol) were reacted for 4 h, to
yield compound 27 as a pale yellow solid (20 mg, 89%, E/Z = 1/1.1);
mp 78−80 °C. Z isomer: 1H NMR (300 MHz) δ 7.12 (d, J = 8.7 Hz,
4H), 6.71 (d, J = 8.7 Hz, 4H), 3.77 (s, 6H), 3.76 (q, J = 7.0 Hz, 4H),
1.31 (t, J = 7.0 Hz, 6H). E isomer: 1H NMR (300 MHz) δ 7.69 (d, J =
8.7 Hz, 4H), 6.93 (d, J = 8.7 Hz, 4H), 3.85 (s, 6H), 3.54 (q, J = 7.0 Hz,
4H), 1.14 (t, J = 7.0 Hz, 6H); 13C NMR (75 MHz) δ 159.0, 143.6,
142.0, 132.0, 131.3, 129.9, 128.3, 113.9, 113.7, 110.0, 66.6, 66.1, 55.6,
55.5, 16.0, 15.7. IR (ATR) ν 1271, 1106, 1073, 815, 759 cm−1. Exact
mass data could not be collected for this compound under any of the
conditions tested. The reasons for this anomalous result are not
known.
Method B. Carbene complex 19 (200 mg, 0.56 mmol) and
Pd(PtBu3)2 (7 mg, 0.014 mmol) were reacted for 18 h, to yield
compound 27 as a pale yellow solid (57 mg, 62%, E/Z = 8.1/1).
(Z)- and (E)-1,2-Diethoxy-1,2-bis(p-trifluoromethylphenyl)-
ethylene (28). Method A. Carbene complex 20 (50 mg, 0.17 mmol),
Pd catalyst (0.085 mmol), and Et3N (24 μL, 0.17 mmol) were reacted
for 2 h, to yield compound 28 as a pale yellow solid (11 mg, 33%, E/Z
= 1.5/1); mp 112−114 °C. Z isomer: 1H NMR (300 MHz) δ 7.54 (d,
J = 8.1 Hz, 4H), 7.29 (d, J = 8.1 Hz, 4H), 3.80 (q, J = 7.0 Hz, 4H),
1.35 (t, J = 7.0 Hz, 6H; 13C NMR (75 MHz) δ 142.8, 138.8, 130.0,
1
129.9, 128.7, 125.1 (q, J = 3.9 Hz), 66.5, 15.6. E isomer: H NMR
(300 MHz) δ 7.90 (d, J = 8.1 Hz, 4H), 7.67 (d, J = 8.1 Hz, 4H), 3.54
(q, J = 7.0 Hz, 4H), 1.14 (t, J = 7.0 Hz, 6H); 13C NMR (75 MHz) δ
144.3, 137.9, 129.6 (q, J = 32.2 Hz), 129.5, 126.0, 125.0 (q, J = 3.8
Hz), 66.9, 15.2. IR (ATR): ν 1327, 1116 cm−1. HRMS (ESI): m/z
calcd for C20H19F6O2 [M + H]+ 405.1284, found 405.1301.
Method B. To a solution of the corresponding alkoxycarbene
complex (1 mmol) in MeCN (6 mL/mmol of carbene complex) was
added Pd(PtBu3)2 (5%). The reaction mixture was stirred at room
temperature until total disappearance of the starting material (checked
by TLC). The solvent was then removed under reduced pressure. The
residue was dissolved in EtOAc, the solution was filtered through a
short pad of Celite, and finally the solvent was removed again under
reduced pressure. A mixture of E and Z isomers was obtained, together
with variable amounts of the corresponding ester formed either by
oxidation of the starting carbene complex or by oxidation of Pd
carbene intermediates.14 The crude product was then purified by flash
column chromatography (hexane/EtOAc 100/1).23
Method B. Carbene complex 20 (200 mg, 0.51 mmol) and
Pd(PtBu3)2 (13 mg, 0.026 mmol) were reacted for 24 h, to yield
compound 28 as a pale yellow solid (66 mg, 64%, E/Z = 11/1).
(Z)- and (E)-1,2-Diethoxy-1,2-bis(m-tolyl)ethylene (29). Meth-
od A. Carbene complex 21 (100 mg, 0.29 mmol), Pd catalyst (0.014
mmol), and Et3N (40 μL, 0.29 mmol) were reacted for 16 h, to yield
compound 29 as a yellow oil (40 mg, 94%, E/Z = 1/1.5). Z isomer: 1H
NMR (300 MHz) δ 7.57−6.93 (m, 16H, Z + E), 3.79 (q, J = 7.0 Hz,
(Z)-, and (E)-α,β-Diethoxystilbene (17).13b Method A. Carbene
complex 16 (200 mg, 0.61 mmol), Pd catalyst (0.031 mmol) and Et3N
(91 μL, 0.61 mmol) were reacted for 3 h, to yield compound 17 as a
1
4H), 2.23 (s, 6H), 1.33 (t, J = 7.0 Hz, 6H). E isomer: H NMR (300
MHz) δ 7.56 (s, 2H), 7.53 (d, J = 7.8 Hz, 2H), 7.29 (dd, J = 7.8, 7.4
Hz, 2H), 7.11 (d, J = 7.4 Hz, 2H), 3.57 (q, J = 7.0 Hz, 4H), 2.40 (s,
6H), 1.16 (t, J = 7.0 Hz, 6H); 13C NMR (75 MHz) δ 142.6, 137.3,
137.2, 135.3, 130.1, 130.0, 128.9, 128.2, 128.0, 127.8, 127.6, 127.0,
126.6, 125.4, 66.4, 65.8, 21.6, 21.3, 15.6, 15.2. IR (ATR): ν 1280, 1105
cm−1. HRMS (ESI): m/z calcd for C20H25O2 [M + H]+ 297.1849,
found 297.1860.
1
white solid (70 mg, 82%, E/Z = 1.2/1). Z isomer: H NMR (300
MHz) δ 8.1−7.1 (m, 10H), 3.80 (q, J = 7.2 Hz, 4H), 1.15 (t, J = 7.2
Hz, 6H); 13C NMR (75 MHz) δ 144.1, 135.4, 129.8, 127.9, 127.5,
1
66.4, 15.2; IR (ATR): 1599, 1259 cm−1. E isomer: H NMR (300
MHz) δ 8.1−7.1 (m, 10H), 3.57 (q, J = 7.1 Hz, 4H), 1.15 (t, J = 7.1
Hz, 6H); 13C NMR (75 MHz) δ 142.6, 134.7, 128.3, 127.8, 127.3,
65.9, 15.2. IR (ATR) 1665, 1351 cm−1.
Method B. Carbene complex 16 (200 mg, 0.61 mmol) and
Pd(PtBu3)2 (16 mg, 0.008 mmol) were reacted for 7 h, to yield
compound 17 as a white solid (80 mg, 99%, E/Z = 15.7/1).
Method B. Carbene complex 24 (50 mg, 0.11 mmol) and
Pd(PtBu3)2 (0.006 mmol) were reacted for 28 h, to yield compound
17 as a white solid (12 mg, 79%, E/Z = 2.1/1).
Method B. Carbene complex 21 (200 mg, 0.59 mmol) and
Pd(PtBu3)2 (15 mg, 0.03 mmol) were reacted for 5 h, to yield
compound 29 as a pale yellow oil (39 mg, 45%, E/Z = 10.8/1).
(Z)- and (E)-1,2-Bis[(p-bromophenyl)-p-phenyl]-1,2-dieth-
oxyethylene (30). Method A. Carbene complex 22 (100 mg, 0.21
mmol), Pd catalyst (0.011 mmol), and Et3N (29 μL, 0.21 mmol) were
reacted for 16 h, to yield compound 30 as a pale yellow solid (19 mg,
1
32%, E/Z = 1/1.4); mp 63−65 °C. Z + E isomers: H NMR (300
MHz) δ 7.89−7.30 (m, 32H, Z + E) 3.88 (q, J = 7.0 Hz, 4H, Z), 3.66
(q, J = 7.0 Hz, 4H, E), 1.39 (t, J = 7.0 Hz, 6H, Z), 1.23 (t, J = 7.0 Hz,
(Z)- and (E)-1,2-Bis(p-bromophenyl)-1,2-diethoxyethylene
(26). Method A. Carbene complex 18 (200 mg, 0.49 mmol), Pd
E
dx.doi.org/10.1021/jo302130h | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
6H, E); 13C NMR (75 MHz) δ 144.4, 139.7, 138.9, 134.2, 132.1,
132.0, 131.9, 130.3, 130.0, 129.0, 128.9, 128.7, 128.6, 128.5, 126.9,
126.5, 126.4, 121.6, 66.7, 66.2, 15.7, 15.4; IR (ATR) ν 1480, 1263,
1115, 1073, 816 cm−1; HRMS (ESI) m/z calcd for C30H27Br2O2 [M +
H]+ 579.0354, found 579.0359.
(2) Selected general reviews in the chemistry of group 6 metal
carbenes: (a) Dotz, K. H.; Fischer, H.; Hofmann, P.; Kreissel, R.;
̈
Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag
Chemie: Deerfield Beach, FL, 1983. (b) Dotz, K. H. Angew. Chem., Int.
̈
Ed. Engl. 1984, 23, 587. (c) Wulff, W. D. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K.,
1991; Vol. 5, p 1065. (d) Schwindt, M. A.; Miller, J. R.; Hegedus, L. S.
J. Organomet. Chem. 1991, 413, 143. (e) Rudler, H.; Audouin, M.;
Chelain, E.; Denise, B.; Goumont, R.; Massoud, A.; Parlier, A.;
Method B. Carbene complex 22 (200 mg, 0.42 mmol) and
Pd(PtBu3)2 (11 mg, 0.021 mmol) were reacted for 6 h, to yield
compound 30 as a pale yellow solid (36 mg, 30%, E/Z = 2.2/1).
(Z)- and (E)-1,2-Bis[(p-bromophenyl ether)-p-phenyl]-1,2-
diethoxyethylene (31). Method A. Carbene complex 23 (200 mg,
0.4 mmol), Pd catalyst (0.02 mmol), and Et3N (56 μL, 0.4 mmol)
were reacted for 16 h, to yield compound 31 as a yellow oil (33 mg,
28%, E/Z = 1/2.5). Z isomer: 1H NMR (300 MHz) δ 7.44 (d, J = 8.9
Hz, 2H), 7.17 (d, J = 8.9 Hz, 2H), 6.88 (d, J = 8.9 Hz, 2H), 6.81 (d, J
= 8.9 Hz, 2H), 3.83 (q, J = 7.1 Hz, 4H), 1.35 (t, J = 7.1 Hz, 6H). E
isomer: 1H NMR (300 MHz) δ 7.74 (d, J = 8.9 Hz, 2H), 7.47 (d, J =
8.9 Hz, 2H), 7.02 (d, J = 8.9 Hz, 2H), 6.96 (d, J = 8.9 Hz, 2H), 3.59
(q, J = 7.1 Hz, 4H), 1.18 (t, J = 7.1 Hz, 6H); 13C NMR (75 MHz) δ
156.3, 156.1, 143.4, 133.1, 132.8, 131.7, 131.3, 130.1, 129.9, 128.8,
122.0, 120.8, 120.7, 118.9, 118.3, 118.2, 117.2, 115.9, 66.5, 66.1, 15.7,
15.3. IR (ATR): ν 1487, 1240 cm−1. HRMS (ESI): m/z calcd for
C30H27O4Br2 [M + H]+ 609.0271, found 609.0304.
́
Pacreau, A.; Rudler, M.; Yefsah, R.; Alvarez, C.; Delgado-Reyes, F.
Chem. Soc. Rev. 1991, 20, 503. (f) Grotjahn, D. B.; Dotz, K. H. Synlett.
̈
̈
1991, 381. (g) Wulff, W. D. In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 12, p 470. (h) Hegedus, L. S. In
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 12, p 549.
(i) Harvey, D. F.; Sigano, D. M. Chem. Rev. 1996, 96, 271. (j) Hegedus,
L. S. Tetrahedron 1997, 53, 4105. (k) Aumann, R.; Nienaber, H. Adv.
Organomet. Chem. 1997, 41, 163. (l) Alcaide, B.; Casarrubios, L.;
Domínguez, G.; Sierra, M. A. Curr. Org. Chem. 1998, 2, 551.
(m) Sierra, M. A. Chem. Rev. 2000, 100, 3591. (n) De Meijere, A.;
Schirmer, H.; Duetsch, M. Angew. Chem., Int. Ed. 2000, 39, 3964.
Method B. Carbene complex 23 (200 mg, 0.4 mmol) and
Pd(PtBu3)2 (2.6 mg, 0.005 mmol) were reacted for 22 h, to yield
compound 31 as a white solid (72 mg, 59%, E/Z = 6/1).
(o) Barluenga, J.; Fananas
́
, F. J. Tetrahedron 2000, 56, 4597.
ez, J.; Fananas, F. J. J. Organomet. Chem. 2001,
624, 5. (q) Barluenga, J.; Santamaría, J.; Tomas, M. Chem. Rev. 2004,
104, 2259. (r) Herndon, J. W. Coord. Chem. Rev. 2006, 250, 1889.
̃
(p) Barluenga, J.; Flor
́
́
̃
́
(E,Z,E)- and (E,E,E)-3,4-Diethoxy-1,6-diphenyl-1,3,5-hexa-
trienes (32).3b Method B. Carbene complex 25 (100 mg, 0.28
mmol) and Pd(PtBu3)2 (0.005 mmol) were reacted for 16 h, to yield
compound 32 as a dark brown solid (23 mg, 51%, E/Z = 2/1). Z + E
́
(s) Sierra, M. A.; Gom
J. 2007, 13, 736. (t) Sierra, M. A.; Fernan
Commun. 2008, 4671. (u) Dotz, K. H.; Stendel, J., Jr. Chem. Rev. 2009,
́
ez-Gallego, M.; Martínez-Alvarez, R. Chem. Eur.
́
dez, I.; Cossío, F. P. Chem.
1
̈
isomers: H NMR (300 MHz) δ 7.42 (d, J = 7.6 Hz, 8H), 7.31−7.16
́
109, 3227. (v) Fernan
Res. 2011, 44, 479.
(3) (a) Sierra, M. A.; Mancheno, M. J.; Sae
́
dez, I.; Cossıo, F. P.; Sierra, M. A. Acc. Chem.
(m, 12H), 7.07 (d, J = 16.1 Hz, 2H), 6.99 (d, J = 15.5 Hz, 2H), 6.81
(d, J = 15.5 Hz, 2H), 6.72 (d, J = 16.1 Hz, 2H), 3.97 (q, J = 7.0 Hz,
4H), 3.84 (q, J = 7.0 Hz, 4H), 1.36 (t, J = 7.0 Hz, 6H), 1.34 (t, J = 7.0
Hz, 6H).
́
z, E.; del Amo, J. C. J. Am.
Chem. Soc. 1998, 120, 6812. (b) Sierra, M. A.; del Amo, J. C.;
Mancheno, M. J.; Gomez-Gallego, M. J. Am. Chem. Soc. 2001, 123,
̃
(Z)- and (E)-1,2-Dibenzyloxy-1,2-dimethyethene (34).3b
Method B. Carbene complex 33 (50 mg, 0.15 mmol) and Pd(PtBu3)2
(0.002 mmol) were reacted for 22 h, to yield compound 34 as a
mixture of isomers (14 mg, 70%, E/Z = 1/1.6). Compound 34 was
unstable for further purification. Z + E isomers: 1H NMR (300 MHz)
δ 7.40−7.23 (m, 20H), 4.71 (s, 4H), 4.56 (s, 4H), 1.77 (s, 6H), 1.71
(s, 6H).
́
̃
851.
(4) (a) Fernan
Sierra, M. A. Chem. Eur. J. 2008, 14, 11222. (b) Lop
Fernandez, I.; Mancheno, M. J.; Gom
Sierra, M. A. Eur. J. Org. Chem. 2011, 3293.
(5) Meana, I.; Toledo, A.; Albeniz, A. C.; Espinet, P. Chem. Eur. J.
2012, 18, 7658.
́
dez, I.; Mancheno, M. J.; Vicente, R.; Lop
́
ez, L. A.;
̃
́
ez-Alberca, M. P.;
́
ez-Gallego, M.; Casarrubios, L.;
́
̃
́
ASSOCIATED CONTENT
(6) Isolation and characterization of the mononuclear Pd amino-
carbene complex from the transmetalation of a tungsten(0) (Fischer)
carbene with [PdBrPf(MeCN)2] followed by reaction with PMe3 has
■
S
* Supporting Information
Figures giving 1H and 13C NMR spectra of the new compounds
prepared in this work. This material is available free of charge
via the Internet at http://pubs.acs.org.
previously been reported; see: (a) Alben
Manrique, R.; Perez-Mateo, A. Angew. Chem., Int. Ed. 2002, 41, 2363.
(b) Albeniz, A. C.; Espinet, P.; Manrique, R.; Perez-Mateo, A. Chem.
Eur. J. 2005, 11, 1565.
(7) (a) Barluenga, J.; Lop
́
iz, A. C.; Espinet, P.;
́
́
́
AUTHOR INFORMATION
Corresponding Author
*E-mail: israel@quim.ucm.es (I.F.); sierraor@quim.ucm.es
(M.A.S.). Fax: +34-913944310. Tel: +34-913944310.
Notes
́
́
ez, L. A.; Lober, O.; Tomas
Granda, S.; Alvarez-Rua, C.; Borge, J. Angew. Chem., Int. Ed. 2001, 40,
́
, M.; Garcıa-
̈
■
́
́
3392. (b) Barluenga, J.; Barrio, P.; Vicente, R.; Lop
M. J. Organomet. Chem. 2004, 689, 3793.
(8) Fananas-Mistral, M.; Aznar, F. Organometallics 2009, 28, 666.
(9) (a) Lopez-Alberca, M. P.; Mancheno, M. J.; Fernandez, I.;
Gomez-Gallego, M.; Sierra, M. A.; Torres, R. Org. Lett. 2007, 9, 1757.
(b) Lopez-Alberca, M. P.; Mancheno, M. J.; Fernandez, I.; Gomez-
Gallego, M.; Sierra, M. A.; Torres, R. Chem. Eur. J. 2009, 15, 3595.
(10) Lage, M. L.; Curiel, D.; Fernandez, I.; Mancheno, M. J.; Gomez-
́ ́
ez, L. A.; Tomas,
́
̃
́
́
̃
The authors declare no competing financial interest.
́
́
́
́
̃
ACKNOWLEDGMENTS
■
́
́
̃
Support for this work under grants CTQ-2010-20414-C02-01/
BQU and CSD2007-00006 (Programa Consolider Ingenio
2010) from the MEyC (Spain) and CAM (S2009/PPQ-1634-
AVANCAT) is acknowledged. A generous loan of Pd catalysts
from Johnson Matthey Inc. is gratefully acknowledged. G.M.C.
thanks the MEyC (Spain) for a FPI fellowship. I.F. is a Ramon
y Cajal fellow.
Gallego, M.; Molina, P.; Sierra, M. A. Organometallics 2011, 30, 1794.
(11) Facchetti, A.; Marks, T. J.; Huang, H.; Chen, Z. PCT Int. Appl.
WO 2011119374 A1 20110929, 2011.
(12) Representative recent examples: (a) Ohmura, T.; Oshima, K.;
Taniguchi, H.; Suginome, M. J. Am. Chem. Soc. 2010, 132, 12194.
(b) Tsvelikhovsky, D.; Buchwald, S. L. J. Am. Chem. Soc. 2010, 132,
́
14048. (c) Sole,
6950.
́ ́
D.; Fernandez, I.; Sierra, M. A. Chem. Eur. J. 2012, 18,
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■
(1) Gom
Res. 2005, 38, 44.
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F
dx.doi.org/10.1021/jo302130h | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
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(23) The reported E/Z ratios reflected the ratio of olefins obtained
during the catalytic self-dimerization process. To discard, as proposed
by a reviewer, the possibility of the Pd catalytic species reacting with
the originally formed olefins and therefore affecting the final E/Z ratio,
three separate experiments were carried out. Thus, a 1/1 E/Z mixture
of olefins 17 (15 mg) was reacted in the presence of 1.3 mg of
Pd(PtBu3)2 in 0.4 mL of MeCN for 48 h. The resulting mixture
contains a 1/1 E/Z mixture of olefins 17. The olefins were recovered
quantitatively. Two additional experiments were carried out with
olefins (Z)-11 and (E)-26. Both compounds were recovered unaltered
under the reactions conditions used for 17. These experiments prove
that the obtained olefins are configurationally stable under the catalytic
conditions used.
G
dx.doi.org/10.1021/jo302130h | J. Org. Chem. XXXX, XXX, XXX−XXX