Synthesis of bis-N-alkyl imidazolium salts and their palladium(0)(NHC) (η2-MA)2 complexes

Article abstract of DOI:10.1002/aoc.2866

New N-Alkyl-substituted imidazolium salts as well as a series of their corresponding [Pd(NHC)(MA)₂] complexes have been obtained by three routes in good yield. The previously reported synthesis for the analogous N-aryl substituted [Pd(NHC)(MA)₂] complexes has been improved. The N-alkyl-substituted [Pd(NHC)(MA)₂] complexes are thermally more labile than their N-aryl counterparts. Catalytic transfer semi-hydrogenation of phenylpropyne resulted in good to excellent chemo- and stereo- selectivity conversion into (Z)-phenylpropene. The size of the alkyl substituents correlates with the rate of hydrogenation in the sense that more bulky substituents give rise to faster transfer hydrogenation rates. Copyright

Full text of DOI:10.1002/aoc.2866

Full Paper
Received: 21 February 2012
Revised: 27 March 2012
Accepted: 29 March 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2866
Synthesis of bis-N-alkyl imidazolium salts and
their palladium(0)(NHC)(h2-MA)2 complexes
Dorette S. Tromp*, Peter Hauwert and Cornelis J. Elsevier
New N-Alkyl-substituted imidazolium salts as well as a series of their corresponding [Pd(NHC)(MA)2] complexes have been
obtained by three routes in good yield. The previously reported synthesis for the analogous N-aryl substituted [Pd(NHC)(MA)2]
complexes has been improved. The N-alkyl-substituted [Pd(NHC)(MA)2] complexes are thermally more labile than their N-aryl
counterparts. Catalytic transfer semi-hydrogenation of phenylpropyne resulted in good to excellent chemo- and stereo- selectivity
conversion into (Z)-phenylpropene. The size of the alkyl substituents correlates with the rate of hydrogenation in the sense that more
bulky substituents give rise to faster transfer hydrogenation rates. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: carbene; palladium; alkyne; semi-hydrogenation
Here we describe the results of our investigation on the influence
of substitution of the N-aryl by N-alkyl substituents in the NHC
Introduction
N-Heterocyclic carbenes (NHCs) have gained a special position
among activating and stabilizing spectator ligands and their
coordination chemistry and applications have grown tremen-
dously over the past decade.[1–6] NHC ligands combine strong
donating powers with some aptness for accepting electron
density, the latter when such is required due to accumulating
electronic charge at the metal to which the NHC coordinates. This
unique feature classifies this class of compounds as excellent
adaptive ligands for a range of transition metals (TMs) in high
as well as in low oxidation states.
The structure and electronic properties of TM(NHC) complexes have
been tuned in order to improve their performance in catalysis: in
particular, Pd(NHC) complexes that play a significant role in catalytic
reactions such as C–H, C–C and C–N bond-forming reactions.[7–13]
Especially for applications in catalysis, N-aryl-substituted NHCs have
often been applied because of their relative ease of handling and
isolation as compared to N-alkyl analogues. It is well known that the
ligands, on the stability of the complexes and their activity and
selectivity in the transfer semi-hydrogenation of alkynes. Because
the alkyls are more electron-donating substituents, we expect a more
electron-rich carbene atom. Hence the N-alkyl complexes with
electron-rich Pd0 will be more labile, which will influence the
selectivity and reactivity of the transfer hydrogenation. The synthesis
of several N-alkyl substituted imidazolium salts, their carbenes (IAlk)
and their [Pd0(IAlk)(MA)2] complexes is reported.
Several new routes are investigated in order to obtain the
[Pd0(IAlk)(MA)2] complexes in high yield. One of these routes
is the transmetallation of a silver(I) complex to a Pd0 complex,
first reported in our group.[22] Finally the complexes are applied
in the transfer semi-hydrogenation of 1-phenylpropyne to (Z)-1-
phenylpropene and the results are compared with the N-aryl
compound [Pd0(IMes)(MA)2].
introduction of alkyl groups alters the electronic and steric properties Results and Discussion
of the NHC ligand involved and this may be of consequence for the
stability of the ensuing TM(NHC) complexes.
Synthesis of Imidazolium Salts
In our group there is a strong interest in (transfer) semi-
hydrogenation of unsaturated compounds and we have made
several contributions to this challenging research field.[14–16]
In this light, we have become particularly interested in the use of
TM(NHC) complexes. TM(NHC) complexes that have been employed
in hydrogenation reactions are mostly based on Ru,[17] Rh,[18,19] and
Ir,[19,20] while the use of Pd(NHC) systems in hydrogenation reactions
of alkynes and alkenes has only been recently reported.[15,21–24] In
this area, [Pd0(IAr)(Z2-maleic anhydride)2] complexes of type X
(Scheme 1) have been developed which show good activity and
selectivity in the semi-hydrogenation of alkynes to Z-alkenes.[15]
These complexes also exhibit good activity and selectivity in the
transfer semi-hydrogenation of 1-phenyl-1-propyne with triethy-
lammonium formate (HCO2H/NEt3) as hydrogen donor[21,24] and
for these transfer semi-hydrogenations only N-aryl substituted
complexes of type X have been used thus far.
A range of N-alkyl-substituted imidazolium salts were used as
ligands for the synthesis of [Pd(IAlk)(MA)2] complexes. Imidazo-
lium salt 1a was synthesized from 1-methylimidazole and iodo-
methane.[25] Imidazolium salts 1b–h were prepared in two-step
reactions according to literature procedures. The synthetic strat-
egy involves a condensation reaction between an alkyl amine
and glyoxal, followed by reaction with formaldehyde under acidic
conditions (Scheme 2).[26–29] The salts were isolated as hygroscopic
solids in moderate to high yield.
*
Correspondence to: Dorette S. Tromp, Molecular Inorganic Chemistry, van‘t
Hoff Institute for Molecular Sciences, University of Amsterdam, Science park
904, 1098 XH Amsterdam, The Netherlands. E-mail: D.S.Tromp@uva.nl
Molecular Inorganic Chemistry, van‘t Hoff Institute for Molecular Sciences,
University of Amsterdam, Science park 904, 1098 XH Amsterdam, The Netherlands
Appl. Organometal. Chem. 2012, 26, 335–341
Copyright © 2012 John Wiley & Sons, Ltd.

D. S. Tromp et al.
Scheme 1. [Pd0(IAr)(MA)2] complexes for the semi-hydrogenation of alkynes
Scheme 2. Synthesis of N-alkyl-substituted imidazolium salts.
For the newly synthesized imidazolium salts 1b, 1e and 1h (1e
is known with other counterions) imidazolium C2 proton signals
are observed at 10.99, 10.72 and 10.89 ppm respectively, and
the C2 carbon signals are found at 139 ppm for 1b and 1e and
141 ppm for 1h. These values are comparable to the known
imidazolium salts 1a, 1c–d and 1f–1g.[25,30,31] Owing to their
moisture sensitivity, the imidazolium salts must be stored and
handled under an inert atmosphere. The imidazolium salts 1c,
1d and 1g were obtained from commercial sources.
Direct metallation (method A)
When the labile precursor [Pd(nbd)(MA)] is used, method A only
works well for 1b–d and complexes 4b–d were synthesized in
73%, 85% and 43% yield respectively. Formation of 4f and 4g
gave undefined compounds where no Pd was detected in their
structure. [Pd2(dvtms)3] was used to obtain complex 4e in high
yield (80%). For comparison, 4d was also synthesized with [Pd2
(dvtms)3] and obtained in somewhat higher yield (54%), as when
[Pd(nbd)(MA)] was used; thus for method A [Pd2(dvtms)3] was
shown to be a better Pd0 source.
Carbene synthesis and isolation prior to the addition of a Pd0
precursor (method B)
Synthesis of the [Pd(IAlk)(MA)2] Complexes
Complexes of type X are normally synthesized by deprotonation
of the imidazolium salts, followed by addition of the labile [Pd
(nbd)(MA)] as Pd0 source and the addition of an alkene.[15]
Contrary to this, in order to synthesize the [Pd(IAlk)(MA)2]
complexes, different methods must be used to obtain the desired
[Pd(IAlk)(MA)2] complexes, depending on the nature of the alkyl
substituents. The most direct method is the deprotonation of
the imidazolium salt in the presence of a Pd0 precursor; i.e. the
imidazolium salt and Pd0 precursor are mixed in the solvent, after
which the base is added (method A). If this method does not
work, the carbene could be isolated first before addition of the
Pd0 precursor (method B). When direct metallation did not work
and the isolated carbene was not stable, a silver(I) complex was
made and transmetallated to the desired Pd0 complex (method C).
Following new insights into the reaction we decided to resort to
bispalladium-tris(divinyltetramethyl-disiloxane) [Pd2(dvtms)3] as
Pd0 precursor for the synthesis of some of the Pd(NHC) complexes.
The use of this precursor for the synthesis of low-valent Pd com-
plexes has been developed by Pörschke[32,33] and applied in the syn-
thesis of Pd(NHC)-1,6-diene complexes by the group of Beller.[34,35]
Our group and that of Cavell have extended it to the synthesis of
low-valent Pd complexes with an expanded-ring carbene.[36,37]
For some alkyl-substituted NHCs [Pd2(dvtms)3] is used in meth-
ods A and B. All the different methods used are outlined in
Scheme 3.
Except for 2b and 2h the free carbenes of the used imidazolium salts
are known.[29,30,38] Carbene 2g was the first isolated N-heterocyclic
carbene by the group of Arduengo.[39] Isolation of carbene 2a led
to an unstable oil as described in the literature[40] and could not
be used for this method. The new carbene 2b (yield: 90%) and
2d–g were isolated prior to the addition of the Pd0 precursor. 1H NMR
of 2b showed the disappearance of the imidazolium (C2) protons
and 13C NMR showed a downfield shift of the C2 atom from
140 ppm in 1b to 216 ppm in 2b, indicative for these types of
carbenes.[41]
Isolation of 2g showed that C2 or C4 deprotonation occurs
randomly under the same conditions. For 2d and 2f method B
failed. For 2b and 2e method B worked well, albeit carbene 2e
was only reacted with [Pd2(dvtms)3]. Complex 4b was formed
with both [Pd(nbd)(MA)] and [Pd2(dvtms)3] in 52% and 92%
yields, respectively. Complex 4e was formed in 80% yield. The
C2 carbene of 2g was used in method B with both precursors
but 4g gave, as in method A, an undefined compound. Also here,
in Method B, [Pd2(dvtms)3] seems to be a better Pd0 source.
Synthesis of [(IAlk)2AgI][AgICl2] complexes and transmetallation
(method C)
The use of silver(I) complexes of NHCs for transmetallation to
PdII(NHC) complexes is well investigated.[42,43] Access to these
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 335–341

Palladium(0)(NHC)(Z2-MA)2 complexes
Scheme 3. Methods A–C for the synthesis of the [Pd(IAlk)(MA)2] complexes 4a–e.
Analysis of the [Pd(IAlk)(MA)2] complexes
Analysis of the Pd0 complexes 4a–4g showed that complexes 4f
and 4g gave undefined compounds where no Pd is detected in
their structures. It seems that for 4f and 4g a mixture of both
C2- and C4-coordinated species are being formed and the
reaction of these bulky NHCs with Pd0 precursors should be
further investigated. These complexes are not further used in this
research. In 1H NMR a small upfield shift of ~0.14 ppm of the
C4/C5 protons is observed in complexes 4a–e but the best
evidence for coordination of the NHC ligands to palladium stems
from the large downfield shift in 13C NMR of the C2 carbons from
~140 ppm in 1a–e to ~175 in 4a–e. These chemical shifts are
quite normal and expected for C2 carbene carbons. A shift of
the C2 carbon from 183 ppm in 3a to 176 ppm in 4a is observed.
This shift is caused by the additional p-acceptor ligands (MA) in
complex 4a. 1H NMR of 4b–e showed coordination of MA to
palladium, which is deduced from an upfield shift from 7.0 ppm
for free MA to a broad signal at 4.8 ppm of coordinated MA. In
4a the same upfield shift is observed, but no broadening of the
signal of coordinated MA is observed, contrary to the other
complexes. This could be due to the free rotation of MA, which
is possible for this compound because of the small CH3 groups
on the nitrogen atoms. Complexes 4a–e are stable in the solid
state, but they gradually decompose if they are kept in solution,
even under inert atmosphere, and therefore no crystals could
be isolated for X-ray analysis.
Scheme 4. Synthesis of silver(I) complexes 3a and 3b with Ag2O.
silver(I)(NHC)2 complexes is straightforward through the reaction
of the relevant imidazolium salts with Ag2O (Scheme 4).
Deprotonation takes place at the C2 position and other (relatively
acidic) protons are generally not affected. Complexes 3a and 3b were
synthesized according to literature procedures[44,45] and were formed
in high yield (95% and 97% respectively). The complexes could be
handled in air without decomposition. The silver(I) complexes 3f
and 3g could not be formed and formation of complexes 4f and
4g via the transmetallation reaction (method C) failed.
In 13C NMR, a shift for C2 from 137.84 ppm in 1a to 182.87 ppm in
complex 3a was found. No C2–Ag coupling was observed but the
downfield shift indicates coordination of AgI to C2.[44] In 3b a double
doublet for the C2 was found at 181.64 ppm with a 1J109Ag of 267.9 Hz
and a 1J107Ag of 238.5 Hz respectively, similar to the values reported in
the literature.[43] The absence of C2–Ag coupling in 3a contrary to 3b
is interpreted in terms of fluxional behavior on the NMR time-scale of
the complexes.[44] AgI(NHC) complexes that are static or show
dynamic behavior that is slow on the NMR time-scale (like 3b) will
show C2–Ag coupling in 13C NMR spectra.[43,44]
Catalytic Transfer Semi-hydrogenation
In order to assess the catalytic activity of complexes 4, they were
used in the catalytic transfer semi-hydrogenation reaction devel-
oped in our group.[14–16,21,22,24] Several homogeneous Pd com-
plexes are known for alkyne semi-hydrogenation, but mostly give
over-reduction and/or isomerization after full conversion.[23,46–48]
Transmetallation of complex 3a with [Pd(nbd)(MA)] and an
excess of MA led to complex 4a in 84% yield. For 3b transmetal-
lation failed and complex 4b could only be synthesized by
methods A and B.
Appl. Organometal. Chem. 2012, 26, 335–341
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

D. S. Tromp et al.
With the aryl-substituted complexes of type X this reaction shows
unique reactivity of Pd(NHC)(alkene) complexes, as it catalyzes the for-
mation of Z-alkene in high selectivity, generally without over-reduction
to the saturated product after full conversion of the alkyne.[21,24]
The catalytic activity of the Pd0 complexes 4b–d was tested
in the transfer semi-hydrogenation of 1-phenyl-1-propyne
(Scheme 5 and Table 1). Complexes 4a and 4e were also
tested but decomposed under catalytic conditions.
When performing the catalytic reaction in MeCN at 70ꢀC, the
reaction had not proceeded to full conversion after 21 h. The
selectivity measured at this point was excellent (Z-selectivity of
>97%), comparable with the aryl-substituted Pd(IMes)(MA)2]
complex 5. Extrapolation of these data to full conversion led to
similar Z-selectivity. (Table 1, entries 1–3). To overcome the low
reaction rate, the catalytic reactions were performed in refluxing
MeCN. Although the reaction rates and conversions were higher,
the selectivity of catalysts 4b–d at 82ꢀC is lower than in the
reaction at 70ꢀC, as well as that of complex 5 (entries 4–7). In this
case, some over-reduction and isomerization were observed after
full conversion. For aryl-substituted [Pd(NHC)(MA)] complexes
this effect of temperature on selectivity had not been observed
before. This temperature effect could be explained by the
lower stability of complexes 4 compared to type X. The alkyl-
substituted complexes are more labile and tend to decompose
at elevated temperatures.
two rate-determining steps: (i) decarboxylation from an anionic [Pd
(NHC)(alkyne)(formate)] species and subsequent addition of the
hydride to the Pd(alkyne) moiety; and (ii) protonolysis of the [Pd
(NHC)(alkenyl)] species by triethylammonium cation.[21] Larger sub-
stituents on the carbene ligand will not only favor the dissociation
of MA and solvent molecules, but also increase the rate of the de-
carboxylation as well as the hydride migration, since the steric re-
pulsion in the intermediate species is decreased. Bulky ligands also
lead to a more crowded and therefore less stable alkenyl complex.
The effect of steric repulsion is also reflected in the formation of
allylbenzene for the larger substituents. The alkyl-substituted car-
bene ligands described in this paper give similar results in selectiv-
ity (see Table 1, entry 3 vs. 1 and 2 and entry 7 vs. 4–6). At full con-
version they tend to give more over-reduction and isomerization
than their N-aryl-substituted counterparts. However, these new
[Pd(IAlk)(MA)2] complexes still outperform most other reported
catalysts for this reaction.[14,16,23,47–49] Furthermore, the results of
the present study are in line with the proposed mechanism for the
catalytic cycle involving [Pd(IMes)(MA)2] as the catalyst.[21]
Conclusion
We have synthesized two new N-alkyl-substituted imidazolium
salts and synthesized a series of new [Pd(IAlk)(MA)2] complexes
in good yield. The previously reported synthesis for the [Pd(IAr)
(MA)2] complexes was improved and several methods were
developed in order to obtain the alkyl complexes. The formed
[Pd(IAlk)(MA)2] are more labile than their aromatic counterparts,
which was expressed in the more laborious synthesis of the
alkyl-substituted complexes. The reaction of the N-substituted
NHC ligands with bulky alkyl groups with Pd0 precursors should
be further investigated.
Looking at the effect of steric environment on activity and
selectivity, we see that complexes 4c and 4d with less steric
N-alkyl substituent (isopropyl and cyclohexyl) give lower turn-
over frequencies, while the more steric bulk in 4b (isoheptyl) leads
to higher reaction rates (see Table 1, entries 4–6). For an explana-
tion we refer to the mechanism of this reaction (see supporting
information for the scheme of the mechanism). Mechanistic stud-
ies of the transfer semi-hydrogenation involving N-aryl-substituted
complexes like [Pd(IMes)(MA)2] have shown that this reaction has
Catalytic transfer semi-hydrogenation was performed as a
probe for the catalytic activity of the new complexes, which
Scheme 5. Transfer hydrogenation of alkynes catalyzed by [Pd0(NHC)(alkene)2] complexes.
Table 1. Catalytic transfer semi-hydrogenation of 1-phenylpropynea
Entry
cat
Temp. (ꢀC)
Conversion (%)b
TOF (mol molꢁ1 hꢁ1
)
Initial selectivity (%)
Selectivity (%)
Z / E / allyl / alk.
Z / E / allyl / alk. at full conv.
1
4b
4d
5
70
70
70
82
82
82
82
64
25
89
81
37
49
90
2.6
1.5
97 / 1 / 2 / 0
98 / 2 / 0 /0
98 / 1 / 1 / 0
85 / 3 / 3/ 9
89 / 2 / 0 /10
92 / 2 / 2/ 4
93 /5/ 0 /2
97 / 2 / 0 / 1c
99 / 1 / 0 / 0c
97 / 2 / 0 / 1c
61/10/0/29
2
3
4.9
4
4b
4c
4d
5
15.3
6.1
5
75/8/0/17
6
7[21]
5.0
43/19/0/38
25.6
80 / 10 / 0 / 10
aReaction conditions: 0.14 M alkyne, 0.58 M HCO2H/NEt3, 1.5 ꢂ 10ꢁ3 M catalyst and 0.14 M p-xylene (internal standard) in 14 ml of MeCN at given
temperature.
bEntries 1–3 conversion at 21 h, entries 4–6 conversion at 5 h, entry 7 at 3.5 h.
cEntries 1–3: selectivity at full conversion determined by extrapolation.
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 335–341

Palladium(0)(NHC)(Z2-MA)2 complexes
showed good to excellent chemo- and stereo-selectivity to the
Z-alkene. The size of the alkyl substituents was found to correlate
with the rate of hydrogenation; more bulky substituents lead to
higher reaction rates. This can be rationalized by considering the
proposed mechanism for the [Pd(IAr)(MA)2] analogues, considering
that large N-substituents such as the N-isoheptyl substituent in the
case of [Pd(IAlk)(MA)2] strongly enforce the dissociation of MA from
the coordination sphere of palladium. The catalytic performance of
the [Pd(IAlk)(MA)2] species are comparable to that of their (easier
to synthesize) [Pd(IAr)(MA)2] analogues.
The suspension was filtered on a glass filter and the white
residue was washed with dry THF and dried under vacuum. Yield:
1
white solid, 2.11 g, 89%. H NMR (300.1 MHz, CD2Cl2): d (ppm)
10.99 (s, 1H, NCHN), 7.32 (s, 2H, HC¼CH), 4.21 (t, 2H, CH(iPr)2,
3J = 6.9 Hz), 2.30–2.23 (m, 4H, CH(CH3)2), 0.91 (d, 12H, CH3,
3
3J = 6.6 Hz), 0.81 (d, 12H, CH3, J = 6.6 Hz). 13C NMR (75.47 MHz,
CD2Cl2): d (ppm) 139.97 NCN, 121.30 C¼C, 72.70 C(iPr)2, 29.37 C
(CH3)2, 20.28 CH3, 18.07 CH3. MS (FAB+): m/z observed: 265.26 [M
Cl]+ for C17H33N2.
1,3-Dineopentylimidazoliumchloride (1e)
Synthesis as in 1b; 3.4 ml of an anhydrous 4 M solution of HCl
(13.46 mmol) in dioxane, 0.34 g (11.21 mmol) paraformaldehyde.
N,N′-neopentyldiazabutadiene 1.76 g (8.97 mmol) in 20 ml of dry
THF. Stirring for 16 h. From the black solution a grey solid was
isolated; 0.5 g (23%). 1H NMR (300.1 MHz, CD2Cl2): d (ppm)
10.72 (s, 1H, NCHN), 7.35 (s, 2H, HC¼CH), 4.21 (s, 4H, CH2), 1.05
(s, 18H, CH3). 13C NMR (75.47 MHz, CD2Cl2): d (ppm) 139.4 NCN,
122.74 C¼C, 60.78 CH2, 32.39 C*, 26.87 CH3. MS (FAB+): m/z ob-
served: 209.20 [M ꢁ Cl]+ for C13H25N2.
Experimental
General
The synthesis of the N,N′-alkylsubstituted-1,4-diazabutadiene
ligands, the precursors for the NHC salts, and NHC salts 1a–h
were prepared according to the literature. As Pd0 precursor,
[Pd(nbd)(MA)] was used and prepared according to a modified
procedure.[50,51] The [Pd2(dvtms)3]/dvtms solution was a gift
from Umicore.
1,3-Dicyclopropylimidazolium chloride (1h)
Complexes 4b–e were made via modified methods based on the
method published by Sprengers.[15] The structures of compounds
1–4 can be found in Scheme 3. The procedure for transfer hydroge-
nation has been described by Hauwert.[21,24] All reactions were
carried out in an inert atmosphere. Solvents were dried and freshly
distilled prior to use. Et2O, pentane and THF were distilled from so-
dium wire. Dichloromethane and acetonitrile were distilled from
CaH2. Chemicals were purchased from commercial sources and
were used as received, except for the salts 1c, 1d and 1g which
were directly transferred to a dry Schlenk tube and stored under
nitrogen. Gas chromatographic analyses were performed with a
Carlo Erba HRGC 8000 Top instrument using a VB-1 or DB-5 capillary
column and p-xylene as internal standard.
Synthesis as for 1b; 6 ml of an anhydrous 4 M solution of HCl (24
mmol) in dioxane, 0.59 g (19.64 mmol) paraformaldehyde. N,N′-
cyclopropyl diazabutadiene 2.14 g (15.71 mmol) in 50 ml of dry
THF. Stirring for 16 h. From a sticky gum a very small amount of
the product as a beige solid was obtained. Yield: 100 mg, 3%.
1H NMR (300.1 MHz, CD2Cl2): d (ppm) 10.89 (s, 1H, NCHN), 7.18
(s, 2H, HC¼CH), 3.87–3.79 (m, 2H, CH), 1.44–1.38 (m, 4H, CH2),
1.27–1.20 (m, 4H, CH2). 13C NMR (75.47 MHz, CD2Cl2): d (ppm)
141 NCN, 123 C¼C, 32 CH, 8 CH2. MS (FAB+): m/z observed:
149.11 [M ꢁ Cl]+ for C9H13N2.
Synthesis of the N-Alkyl-Substituted Imidazolidenes
1H and 13C NMR spectra were recorded at appropriate frequen-
cies on a Varian Mercury 300 (1H, 300.13 MHz; 13C, 75.47 MHz), a
Bruker DRX300 (1H, 300.11 MHz; 13C, 75.47 MHz), a Bruker AMX
400 (1H, 400.13 MHz; 13C, 100.61 MHz) and a Varian Inova 500
(1H, 499.86 MHz; 13C, 125.70 MHz) spectrometer. 13C spectra were
measured with 1H decoupling. Mass spectrometry was carried
out using a JEOL JMS SX/SX 102A four-sector mass spectrometer.
The samples were loaded in a matrix solution (3-nitrobenzyl
alcohol) on to a stainless steel probe and bombarded with xenon
atoms with an energy of 3 keV. During the high-resolution fast atom
bombardment–mass spectrometric measurements, a resolving
power of 10 000 (10% valley definition) was used.
A general procedure for the synthesis of the carbenes 2b and
2d–g follows. The salts were suspended in dry THF, KOtBu was
added and the reaction mixture stirred for 1 h. Filtration of the
mixture (via a cannula) on celite and evaporation of THF yielded
white solids.
1,3-Di(2,4-dimethyl-3-pentyl)imidazolidene (2b)
White solid; 0.4 g, 90% yield. 1H NMR (300.1 MHz, CD2Cl2): d (ppm)
6.83 (s, 2H, HC¼CH), 3.67–3.58 (m, broad, 2H, CH(iPr)2), 2.27 (o, 4H,
CH(CH3)2, 3J = 6.9 Hz), 0.88 (d, 12H, CH3, 3J = 6.9 Hz), 0.84 (d, 12H,
3
CH3, J = 6.3 Hz). 13C NMR (75.47 MHz, CD2Cl2): d (ppm) 216.35
NCN, 120.72 C¼C, 73.63 C(iPr)2, 31.78 C(CH3)2, 22.84 CH3, 19.77
CH3.
Synthesis of the Alkyl-Substituted Imidazolium Salts
1,3-Di(adamantyl)imidazolidene (2g)
C2 carbene: white solid, 0.14 g, 78%. 1H NMR (300.1 MHz, CD2Cl2):
d (ppm) 7.05 (s, 2H, CH¼CH), 2.14 (br, 15H, adamantyl), 1.76 (br,
15H, adamantyl).
1,3-Di(2,4-dimethyl-3-pentyl)imidazolium chloride (1b)
To 2.97 ml of an anhydrous 4 M solution of HCl (11.88 mmol) in
dioxane, 0.3 g (9.90 mmol) paraformaldehyde was added and
heated at 80 ꢀC until the paraformaldehyde was completely
dissolved. The solution was then cooled to room temperature.
In a Schlenk tube under nitrogen 2 g (7.92 mmol) of N,N′-2,4-
dimethyl-3-pentyl diazabutadiene (dab) in 50 ml of dry THF was
stirred and cooled to 0ꢀC.
C4 carbene: white solid, quantitative yield. 1H NMR (300.1 MHz,
CD2Cl2): d (ppm) 10.26 (s, 1H, CH¼N), 7.55 (s, 1H, CH¼N), 2.30 (br,
15H, adamantyl), 1.80 (br, 15H, adamantyl).
Synthesis of the Bis(IAlkyl)Ag(I) Complexes
The solution of paraformaldehyde in HCl/dioxane was then
added dropwise to the THF/dab solution at 0ꢀC. The reaction
mixture was warmed to room temperature and stirred for 32 h.
A white suspension appeared during the reaction.
[Bis(1,3-dimethyl)imidazol-2-ylidene)Ag(I)][AgCl2] (3a)
To 10 ml of dry CH2Cl2, 0.49 g (2.19 mmol) of 1a was added and
stirred at room temperature. Ag2O was added (0.25 g, 1.08 mmol)
Appl. Organometal. Chem. 2012, 26, 335–341
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

D. S. Tromp et al.
Method B with [Pd(nbd)(MA). [Pd(nbd)(MA)] (280 mg, 0.95 mmol)
was suspended with 4 drops of free nbd in 5 ml dry THF (yellow
suspension). 250 mg (0.95 mmol) of the free carbene 2b in 5 ml
dry THF was added by syringe. The reaction mixture directly col-
ored to an orange suspension and became a clear orange
solution after a few minutes. MA (0.46 g, 4.73 mmol) was dis-
solved in a minimum of dry THF and added by syringe to the or-
ange solution. The reaction mixture colored red and became a
suspension. After 1 h of stirring, the mixture was filtered on celite
and the filtrate was evaporated. This yielded a solid which was
washed 4 times with dry Et2O. White solid, 280 mg, 52 %.
and the reaction mixture was stirred for 16 h. This resulted in a
white suspension. Filtration of the suspension on a filter yielded
a white solid; 0.65 g, 95%. 1H NMR (300.1 MHz, DMSO-d8): d
(ppm) 7.40 (s, 4H, HC¼CH), 3.79 (s, 12H, CH3). 13C NMR (75.47
MHz, DMSO-d8): d (ppm) 182.87 C–Ag(I), 123.80 C¼C, 38.88
CH3. MS (FAB+): m/z observed: 301.0 [(NHC)2Ag109]+, m/z
299.0 [(NHC)2Ag107 +
] for C10H16N4Ag.
[Bis(1,3-di(2,4-dimethyl-3-pentyl)imidazol-2-ylidene)Ag(I)][AgCl2] (3b)
To 15 ml of dry CH2Cl2, 0.2 g (0.66 mmol) of 1b was added and
stirred at room temperature. Ag2O was added (0.08 g, 0.33 mmol).
The reaction mixture was stirred for 16 h. After filtration on celite
the filtrate was evaporated to yield 0.26 g, 97% of a white to gray
Method B with [Pd2(dvtms)3]. In 15 ml of dry THF 0.5 g (1.66 mmol)
of 1b and 0.37 g (3.32 mmol) of KOtBu were stirred for 1.5 h. The
reaction mixture was filtered on celite and the THF was evapo-
rated and the carbene 2b isolated. Toluene (15 ml) was added
to the residue and 2 g [Pd2(dvtms)3]/dvtms solution (8.67% Pd,
1.65 mmol Pd) was added dropwise. After 2.5 h 0.8 g (8.23 mmol)
MA was added to the reaction mixture, followed by additional
stirring of 1 h. The reaction mixture turned to a purple suspen-
sion. The toluene was evaporated and the residue washed with
Et2O to yield a purple solid (0.86 g, 92%).
1
solid. H NMR (300.1 MHz, CD2Cl2): d (ppm) 7.01 (s, 4H, HC¼CH),
3
4.02 (t, 4H, CH(iPr)2 3J= 7.5 Hz), 2.22 (o, 8H, CH(CH3)2, J=6.9 Hz),
0.96 (d, 24 H, CH3, 3J= 6.9 Hz), 0.84 (d, 24 H, CH3, 3J=6.9 Hz),
13C NMR (75.47 MHz, CD2Cl2): d (ppm) 181.64 (dd, C–Ag(I),
1J109Ag = 267.9 Hz, 1J107Ag = 238.5 Hz), 119.65 C¼C, 73.71 C
(iPr)2, 29.58 C(CH3)2, 20.39 CH3, 17.88 CH3. MS (FAB+): m/z
observed: 635,41 [(NHC)2Ag109]+, m/z 637.42 [(NHC)2Ag107 +
]
for C10H16N4Ag.
[Pd(1,3-diisopropylimidazol-2-ylidene)(ꢀ2maleicanhydride)2] (4c)
Synthesis of the (IAlkyl)Pd(0)(MA)2 Complexes
Method A with [Pd(nbd)(MA)]. In 15 ml of THF 0.39 g (2.07 mmol) of
1c and 3 drops of free nbd were added. After stirring for a few sec-
onds, 0.61 g (2.07 mmol) [Pd(nbd)(MA)] was added and gave a
green suspension. After stirring for a few seconds, 0.28 g (2.5 mmol)
KOtBu was added and the reaction mixture turned to an orange
and later brownish suspension. After 1 h the reaction mixture was
clear, orange to brown and 1 g (10 mmol) MA was added. The
reaction mixture turned immediately to pink/purple. After half an
hour of additional stirring the mixture was filtered over celite and
the purple filtrate was evaporated to yield a purple solid, 0.80 g,
85%. 1H NMR (300.1 MHz, CD2Cl2): d (ppm) 7.27 (s, 2H, HC¼CHNHC),
4.78 (s, 4H, HC¼CHMA), 4.1 (q, 2H, CH(CH3)2, 3J = 6.9 Hz), 1.42 (d, 12H,
CH3, 3J = 6.9 Hz). 13C NMR (75.47 MHz, CD2Cl2): d (ppm) 172.98 NCN,
167.37 C¼O, 119.41 C¼CNHC, 65.22 C¼CMA, 53.81 C(CH3)2, 23.43
CH3.
[Pd(1,3-dimethylimidazol-2-ylidene)(ꢀ2maleicanhydride)] (4a)
Via transmetallation, method C. In a Schlenk tube 0.38 g (1.28 mmol)
of [Pd(nbd)(MA)] with 1–2 drops of norbornadiene (nbd) was
suspended in 10 ml of dry THF. Complex 3a was added to this
green suspension. The reaction mixture immediately turned
black. After 1.5 min the reaction mixture turned gray. After 1 h
0.13 g (1.28 mmol) maleic anhydride was added. After 2 h the
reaction mixture was filtered through celite, giving a light-yellow
filtrate. Evaporation of THF gave a yellow to orange solid.
Washing with ether and drying under vacuum yielded 0.22 g of
light-pink solid, 85%. 1H NMR (300.1 MHz, CD2Cl2): d (ppm)
7.17 (s, 2H, HC¼CHNHC), 4.79 (s, 4H, HC¼CHMA), 3.51 (s, 6H,
CH3). 13C NMR (75.47 MHz, CD2Cl2): d (ppm) 176.15 NCN,
167.66 C¼O, 124.58 C¼CNHC, 65.16 C¼CMA, 37.35 CH3.
MS (FAB+): m/z observed: 357.04 [M ꢁ MA + H]+ for C13H19N2O3Pd.
E.A. Calcd C: 44.90, H: 4.43, N: 6.16. Found: C: 44.90 H: 4.54 N: 6.04.
[Pd(1,3-di(2,4-dimethyl-3-pentyl)imidazol-2-ylidene)
(ꢀ2maleicanhydride)2] (4b)
[Pd(1,3-dicyclohexylimidazol-2-ylidene)(ꢀ2maleicanhydride)2] (4d)
Method A with [Pd(nbd)(MA)]. In 6 ml of dry THF, 0.24 g (0.797 mmol)
of 1b and a few drops of nbd were suspended. [Pd(nbd)(MA)]
(0.23 g, 0.797 mmol) was added and the suspension turned
green. Extra THF (94 ml) was added.
Method A with [Pd(nbd)(MA)]. A solution was made of 500 mg
(1.56 mmol) of 1d in 15 ml dry THF with 4–10 drops of free
nbd. The precursor [Pd(nbd)(MA)] (463 mg, 1.56 mmol) was
added and while stirring the reaction mixture turned from a
yellow to gray suspension. Then 210 mg (1.87 mmol) of KOtBu
was added and the reaction mixture turned from yellow to
orange/green. An additional amount of 10 ml dry THF was added,
resulting in a clear orange to green solution. After 1.5 h an extra
amount of 202 mg (1.80 mmol) KOtBu was added to complete
the reaction. After 1 h of stirring, 460 mg (4.69 mmol) of MA
was added and the reaction mixture turned directly to purple.
The mixture was stirred overnight and filtrated on celite.
THF was evaporated and gave a pink solid. Washing with
dry Et2O gave a light-pink solid, 0.36 g, 43%. 1H NMR
(300.1 MHz, CD2Cl2): d (ppm) 7.24 (s, 2H, HC¼CHNHC), 4.77
(s broad, 4H, HC¼CHMA), 3.65–3.58 (m, 2H, CHcy), 2.10–1.23
(m, 20H, CH2cy). 13C NMR (75.47 MHz, CD2Cl2): d (ppm) 172.90
NCN, 167.30 C¼O, 119.76 C¼CNHC, 65.02 C¼CMA, 61.31 (NCHcy),
25.25 (o-CH2), 24.84 (m- + p-CH2). MS (FAB+): m/z observed: 409.10
Hereafter 0.15 g (1.12 mmol) of KOtBu was added; the mixture
turned to orange and was stirred for 1.5 h. Addition of 0.10 g
(1.04 mmol) maleic anhydride gave a bright-orange solution
which was filtered on celite after 0.5 h. The filtrate was evapo-
rated and washed with dry Et2O to yield 0.33 g (73%) off-white
1
solid. H NMR (300.1 MHz, CD2Cl2): d (ppm) 7.27 (s, 2H, HC¼CH),
3
4.87 (s, broad, 4H, HC¼CHMA), 3.63 (t, 2H, CH(iPr)2, J = 7.2 Hz),
2.20 (o, 4H, CH(CH3)2, 3J = 6.9 Hz), 0.94 (broad, 12H, CH3), 0.92
(broad, 12H, CH3). 13C NMR (100.6 MHz, CD2Cl2): d (ppm)
176.23 NCN, 167.14 C¼O, 121.07 C¼C, 71.85 C(iPr)2, 67.88
C¼CMA, 39.96 C(CH3)2, 20.66 CH3, 17.52 CH3. MS (FAB+):
m/z observed: 441.20 [M ꢁ MA ꢁ CO(2nd
molecule) + H]+
MA
for C20H35N2O2Pd. E.A. Calcd C: 52.96 H: 6.40 N: 4.94.
Found(A): C: 51.24 H: 6.71 N: 4.92. Found(B): C: 51.27 H:
6.71 N: 5.01.
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 335–341

Palladium(0)(NHC)(Z2-MA)2 complexes
[M ꢁ MA ꢁ CO(2nd MA molecule) +H]+ for C18H25N2O2Pd. E.A.; Calcd C:
51.64 H: 5.28 N: 5.24. Found (A) C: 52.15 H: 5.87 N: 5.45 Found (B)
C: 51.97 H: 5.93 N: 5.51.
[8] E. A. B. Kantchev, C. J. O’Brien, M. G. Organ, Angew. Chem. Int. Ed.
2007, 46, 2768.
[9] D. M. Khramov, E. L. Rosen, J. A. Er, P. D. Vu, V. M. Lynch, C. W. Bielawski,
Tetrahedron 2008, 64, 6853.
[10] O. Esposito, A. Lewis, P. B. Hitchcock, S. Caddick, F. G. N. Cloke, Chem.
Commun. 2007, 1157.
[11] S. Caddick, F. G. N. Cloke, G. K. B. Clentsmith, P. B. Hitchcock,
D. McKerrecher, L. R. Titcomb, M. R. V. Williams, J. Organomet. Chem.
2001, 617, 635.
Method A with Pd2(dvtms)3. In a dried Schlenk tube 323 mg (1.007
mmol) of 1d and 226 mg (2.014 mmol) KOtBu were added and
placed under vacuum for 15 min. Dry THF (5 ml) was added
and the reaction mixture was stirred for 2 h at room temperature.
Then 1.22 g [Pd2(dvtms)3]/dvtms solution (8.67% Pd, 1.004 mmol
Pd) was added dropwise via a syringe. The reaction mixture
turned red to brown upon addition. After 30 min 0.49 g (5 mmol)
MA was added and the reaction mixture turned red. After 30 min
of additional stirring the reaction mixture was filtered through
celite and the solvent evaporated, the residue washed with
Et2O and dried under vacuum. Yield; 0.30 g light-pink solid (54%).
[12] C. W. K. Gstottmayr, V. P. W. Bohm, E. Herdtweck, M. Grosche, W. A.
Herrmann, Angew. Chem. Int. Ed. 2002, 41, 1363.
[13] D. S. McGuinness, K. J. Cavell, B. W. Skelton, A. H. White, Organome-
tallics 1999, 18, 1596.
[14] A. M. Kluwer, T. S. Koblenz, T. Jonischkeit, K. Woelk, C. J. Elsevier,
J. Am. Chem. Soc. 2005, 127, 15470.
[15] J. W. Sprengers, J. Wassenaar, N. D. Clement, K. J. Cavell, C. J. Elsevier,
Angew. Chem. Int. Ed. 2005, 44, 2026.
[16] M. van Laren, C. J. Elsevier, Angew. Chem. Int. Ed. 1999, 38, 3715.
[17] H. M. Lee, D. C. Smith, Z. J. He, E. D. Stevens, C. S. Yi, S. P. Nolan,
Organometallics 2001, 20, 794.
[18] L. Yang, A. Kruger, A. Neels, M. Albrecht, Organometallics 2008, 27, 3161.
[19] G. Dyson, J.-C. Frison, A. C. Whitwood, R. E. Douthwaite, Dalton Trans.
2009, 7141.
[20] S. Nanchen, A. Pfaltz, Chemistry 2006, 12, 4550.
[21] P. Hauwert, R. Boerleider, S. Warsink, J. J. Weigand, C. J. Elsevier,
J. Am. Chem. Soc. 2010, 132, 16900.
[22] S. Warsink, P. Hauwert, M. A. Siegler, A. L. Spek, C. J. Elsevier, Appl.
Organomet. Chem. 2009, 23, 225.
[23] V. Jurcik, S. P. Nolan, C. S. L. Cazin, Chemistry 2009, 15, 2509.
[24] P. Hauwert, G. Maestri, J. W. Sprengers, M. Catellani, C. J. Elsevier,
Angew. Chem. Int. Ed. 2008, 47, 3223.
[Pd(1,3-dineopentylimidazol-2-ylidene)(ꢀ2maleicanhydride)2] (4e)
Method A with [Pd2(dvtms)3]. In a dried Schlenk tube 207 mg (0.846
mmol) of 1e and 182 mg (1.62 mmol) KOtBu were placed under
vacuum for 20 min. Then 15 ml of dry THF was added and stirred
for 2 h. The white suspension became a clear and light-yellow so-
lution. Then 1.03 g Pd(dvtms)/dvtms solution (8.67% Pd, 0.848
mmol Pd) was added dropwise via a syringe. The reaction mix-
ture turned to a brown clear solution upon reaction time. After
30 min MA was added (0.41 g, 4.23 mmol) and the reaction mix-
ture turned from dark red/brown to a red/orange and later pink
suspension. This was stirred for 1 h and then THF was evaporated
and the residue washed with dry Et2O and dried under vacuum to
yield a light-pink solid; 0.34 g (80%). 1H NMR (300.1 MHz, CD2Cl2):
d (ppm) 7.25 (s, 2H, ¼CH) 4.72 (s, 4H, HC¼CHMA), 3.60 (s, 4H,
CH2), 0.95 (s, 18H, CH3). 13C NMR (75.47 MHz, CD2Cl2): d (ppm)
177.46 NCN, 167.48 C¼O, 122.63 C¼CNHC, 67.04 C¼CMA, 61.39 CH2,
32.83 C*, 27.90 CH3. MS (FAB+): m/z observed: 453.20 [M ꢁ C(CH3)3]+
[25] W. A. Herrmann, C. Kocher, L. J. Goossen, G. R. J. Artus, Chemistry
1996, 2, 1627.
[26] J. M. Kliegman, R. K. Barnes, Tetrahedron 1970, 26, 2255.
[27] L. H. Staal, L. H. Polm, K. Vrieze, F. Ploeger, C. H. Stam, Inorg. Chem.
1981, 20, 3590.
[28] L. Delaude, M. Szypa, A. Demonceau, A. F. Noels, Adv. Synth. Catal.
2002, 344, 749.
[29] S. Saravanakumar, M. K. Kindermann, J. Heinicke, M. Köckerling,
Chem. Commun. 2006, 640.
[30] M. Niehues, G. Erker, G. Kehr, P. Schwab, R. Frohlich, O. Blacque,
H. Berke, Organometallics 2002, 21, 2905.
[31] W. A. Herrmann, V. P. W. Bohm, C. W. K. Gstottmayr, M. Grosche,
C. P. Reisinger, T. Weskamp, J. Organomet. Chem. 2001, 617, 616.
[32] J. Krause, G. Cestaric, K.-J. Haack, K. Seevogel, W. Storm, K.-R.
Pörschke, J. Am. Chem. Soc. 1999, 121, 9807.
[33] J. Krause, K.-J. Haack, G. Cestaric, R. Goddart, K.-R. Pörschke, Chem.
Commun. 1998, 1291.
for C17H19N2O6Pd, 385.10 [M ꢁ MA ꢁ CO(2nd
molecule) +H]+ for
MA
C16H27N2O2Pd.
General Procedure for Transfer Semi-hydrogenation of
Alkynes
A stock solution was made of 100 mmol Et3N, 100 mmol HCO2H
and 20 mmol p-xylene in 100 ml dry MeCN. The tube of a parallel
reactor was equipped with a stirring bar, evacuated and put
under an inert atmosphere (N2), then filled with 2 mmol 1-phenyl
propyne, 1 mol% catalyst (20 mmol), 10 ml of the stock solution
and 4 ml dry MeCN. The reaction mixture was heated at 70ꢀC. Sam-
ples were withdrawn periodically for gas chromatographic analysis
(0.05 ml reaction mixture in 0.950 ml EtOH). At the end of the reaction
the mixture was allowed to cool down to room temperature. Water
was added and the mixture extracted with pentane, dried with
MgSO4, filtered and evaporated and analyzed by 1H NMR.
[34] R. Jackstell, S. Harkal, H. Jiao, A. Spannenberg, C. Borgmann,
D. Röttger, F. Nierlich, M. Elliot, S. Niven, K. Cavell, O. Navarro,
M. S. Viciu, S. P. Nolan, M. Beller, Chemistry 2004, 10, 3891.
[35] M. Beller, M. G. Andreo, A. Zapf, R. Karch, I. Kleinwächter, O. Briel,
German Patent DE 10062577, 2002.
[36] J. J. Dunsford, K. J. Cavell, Dalton Trans. 2011, 40, 9131.
[37] P. Hauwert, PhD thesis, University of Amsterdam, 2011.
[38] A. J. Arduengo, H. Bock, H. Chen, M. Denk, D. A. Dixon, J. C. Green,
W. A. Herrmann, N. L. Jones, M. Wagner, R. West, J. Am. Chem. Soc.
1994, 116, 6641.
[39] J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113, 361.
[40] J. Arduengo, H. V. R. Dias, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1992, 114, 5530.
[41] D. Tapu, D. A. Dixon, C. Roe, Chem. Rev. 2009, 109, 3385.
[42] J. B. Lin, C. S. Vasam, Coordin. Chem. Rev. 2007, 251, 642.
[43] C. Garrison, W. J. Youngs, Chem. Rev. 2005, 105, 3978.
[44] H. M. J. Wang, I. J. B. Lin, Organometallics 1998, 17, 972.
[45] W. Z. Chen, F. H. Liu, J. Organomet. Chem. 2003, 673, 5.
[46] R. Shen, T. Chen, Y. Zhao, R. Qiu, Y. Zhou, S. Yin, X. Wang, M. Goto,
L.-B. Han, J. Am. Chem. Soc. 2011, 133, 17037.
[47] M. Costa, P. Pelagatti, C. Pelizzi, D. Rogolino, J. Mol. Catal. A 2002, 178, 21.
[48] Tani, N. Ono, S. Okamoto, F. Sato, J. Chem. Soc. Chem. Commun.
1993, 386.
References
[1] F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122.
[2] F. Boeda, S. P. Nolan, Annu. Rep. Prog. Chem. B 2008, 104, 184.
[3] N. M. Scott, S. P. Nolan, Eur. J. Inorg. Chem. 2005, 1815.
[4] W. A. Herrmann, Angew. Chem. Int. Ed. 2002, 41, 1290.
[5] D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev. 2000,
100, 39.
[49] H. Lindlar, Helv. Chim. Acta 1952, 35, 446.
[6] T. Weskamp, V. P. W. Böhm, W. A. Herrmann, J. Organomet. Chem.
2000, 600, 12.
[7] M. G. Organ, G. A. Chass, D. C. Fang, A. C. Hopkinson, C. Valente,
Synthesis-Stuttgart 2008, No. 17, 2776.
[50] Itoh, F. Ueda, K. Hirai, Y. Ishii, Chem. Lett. 1977, 877.
[51] A. M. Kluwer, C. J. Elsevier, M. Buhl, M. Lutz, A. L. Spek, Angew. Chem.
Int. Ed. 2003, 42, 3501.
Appl. Organometal. Chem. 2012, 26, 335–341
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc