N-Alkylfluorenyl-substituted N-heterocyclic carbenes as bimodal pincers

Article abstract of DOI:10.1039/c5dt00980d

Two N-heterocyclic carbene precursors having their nitrogen atoms substituted by the expanded 9-ethyl-9-fluorenyl group, namely imidazolinium chloride 6 and imidazolium chloride 7, have been synthesized in high yields from fluorenone (1). The key intermed

Full text of DOI:10.1039/c5dt00980d

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N-Alkylfluorenyl-substituted N-heterocyclic
carbenes as bimodal pincers†
Cite this: DOI: 10.1039/c5dt00980d
a
a
a
b
Matthieu Teci, Eric Brenner,* Dominique Matt,* Christophe Gourlaouen and
Loïc Toupet
c
Two N-heterocyclic carbene precursors having their nitrogen atoms substituted by the expanded
9
-ethyl-9-fluorenyl group, namely imidazolinium chloride 6 and imidazolium chloride 7, have been syn-
thesized in high yields from fluorenone (1). The key intermediate of their syntheses is the new primary
amine 9-ethyl-9-fluorenylamine (3), which was prepared in 75% yield. Both salts were readily converted
into the corresponding PEPPSI-type palladium complexes, 8 and 9 (PEPPSI: pyridine-enhanced pre-
catalyst preparation stabilisation and initiation). Despite rotational freedom of the ethylfluorenyl moieties
about the N–C(fluorenyl) bond in their cationic precursors, the carbene ligands of the Pd(II)-complexes 8
and 9 both behave as bimodal pincers in solution and in the solid state, the resulting confinement being
2
essentially due to (weak) attractive anagostic interactions between the CH (fluorenyl) groups and the
metal centre. Unlike in 8 and 9, there was no indication for similar anagostic interactions in the
imidazolylidene chlorosilver complex 11, which could be obtained from 7. In the solid state, however,
2
1
1 adopts a remarkable “open sandwich” structure, with the two alkylfluorenilidene planes η -bonded to
Received 11th March 2015,
Accepted 15th April 2015
the silver, this constituting a further bimodal pincer-type bonding mode of this ligand class. Complexes 8
and 9 were assessed in Suzuki–Miyaura cross-coupling reactions. The imidazolylidene complex 9
displayed high activity towards unencumbered aryl chlorides. Its activity is comparable to that of the
previously reported, highly efficient benzimidazolylidene analogue 10.
DOI: 10.1039/c5dt00980d
www.rsc.org/dalton
3
4
3i,5
propylphenyl, adamantyl, 2,6-diisopentylphenyl,
2,6-diiso-
Introduction
6
7
8
propyl-4-tritylphenyl,
mesitylmethyl,
phenanthrenyl,
9
10
The current increasing interest in N-heterocyclic carbenes calixarenyl, resorcinarenyl, 2,6-di(diphenylmethyl)-4-meth-
NHCs) as metallocatalyst components is primarily due to the oxyphenyl ) have been shown to result in much higher activi-
1
1
(
strong donor ability of these ligands which, in most cases, ties than those obtained with analogs bearing smaller
1
exceeds that of tertiary phosphines. Their particular electronic N-substituents. In fact, the high degree of crowding created
properties alone do not, however, necessarily confer optimised with the former ligands not only facilitates the final reductive
reactivity on their metal complexes. Improved ligand efficiency elimination step of the coupling reaction, but also favours
is often achieved, exactly as in phosphine chemistry, through monoligation and increases the lifetime of key intermediates.
subtle control of the NHC sterics. For example, in palladium- Further, Glorius et al. have exposed the beneficial role in these
catalysed Suzuki–Miyaura cross-coupling reactions, N-hetero- reactions of bulky N-substituents that display structural flexi-
cyclic carbenes with N atoms substituted by side-expanded aryl bility, thereby enabling their adaptation to the steric require-
2
12
rings (with N-substituents such as, e.g., mesityl, 2,6-diiso- ments of each individual step of the catalytic cycle.
In some recent publications, we have described a series of
benzimidazole-based NHCs bearing nitrogen-grafted alkyl-
1
3
fluorenyl (AF) substituents (Fig. 1). In their complexes, the
metal centre was shown to be firmly protected in a permanent
manner by the alkyl groups as a consequence of the orientat-
ing properties of the large, planar fluorenylidene unit com-
bined with a high rotational barrier of the AF group about the
N–C(fluorenyl) bond. In the case of Pd(II) complexes obtained
with these ligands, anagostic CH⋯M interactions between the
metal and the two alkyl chains were shown to contribute to
the high steric protection. In the present study we describe the
a
Laboratoire de Chimie Inorganique Moléculaire et Catalyse, Institut de Chimie
UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal, 67070 Strasbourg
(
Cedex, France. E-mail: dmatt@unistra.fr, eric.brenner@unistra.fr
b
Laboratoire de Chimie Quantique, Institut de Chimie (UMR 7177 CNRS),
Université de Strasbourg, 1 rue Blaise Pascal, F-67008 Strasbourg, France
c
Institut de Physique de Rennes, UMR 6251 CNRS, Université de Rennes 1,
Campus de Beaulieu, 35042 Rennes cedex, France
†
CCDC 875396, 901467 and 1004245. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c5dt00980d
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Results and discussion
Syntheses of the N-heterocyclic carbene precursors 6 and 7
The syntheses of imidazolinium chloride 6 and imidazolium
chloride 7 required the preparation of the (new) primary
amine 3, which was obtained by applying a one-flask pro-
1
4
cedure inspired from the literature (Scheme 1). Thus, reac-
tion of fluorenone with lithium bis(trimethylsilyl)amide (1.3
equiv.) in refluxing THF afforded a pale orange solution,
which was subsequently treated with EtMgBr in excess. Silyl
imine 2, formed in the first step of this sequence, was not iso-
lated. After workup, amine 3 was obtained in 75% yield. Reac-
tion of 3 with glyoxal in propanol at 70 °C then gave diimine 4
in high yield. Reduction of 4 with BH3 in THF, followed by
Fig. 1 Carbene ligands used in this study: benzimidazol-2-ylidene (A),
imidazolin-2-ylidene (B) and imidazol-2-ylidene (C).
3
reaction of the resulting diamine with HC(OEt) –HCl led to 6
in 90% overall yield. Applying Hintermann’s nonclassical pro-
1
5
tocol for the synthesis of imidazolium salts, 4 was further
reacted with paraformaldehyde and Me SiCl (4 : HCHO :
SiCl ratio 1 : 1 : 1) in AcOEt, giving 7 in 93% yield. In the
H NMR spectra of both salts, 6 and 7, the signal of the NCHN
3
coordinative properties of related ligands, which instead of Me
3
1
the benzimidazolylidene moiety contain the smaller imidazoli-
nylidene and imidazolylidene ring moieties. These were proton appears, as expected, at relatively low field (10.25 (6)
expected to confer rotational freedom to the AF groups and and 11.85 (7) ppm). Furthermore, the 2D ROESY spectra of
accordingly to modify the confining properties of the ligand. both salts revealed correlations between the methylene
In the following, the term bimodal pincer designates any tri- protons of the Et groups and the protons connected to all
dentate ligand containing a strongly coordinating atom (in our three C atoms of the NHC ring, this being a clear indication of
case a carbenic C atom) and two other donor atoms (or func- freely rotating alkylfluorenyl groups. Both salts are strongly
tions) able to interact in a non covalent manner with the com- hygroscopic and therefore need to be kept in
a dry
plexed metal ion.
atmosphere.
Scheme 1 Synthesis of imidazolinium salt 6 and imidazolium salt 7.
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Syntheses of metal complexes
The PEPPSI-type complex 8 was obtained in 75% yield by the
following two-step, one-flask synthesis: (a) reaction of imidazo-
linium salt 6 with LiHDMS in THF at room temperature; (b)
after 1 h, addition of [PdCl (pyridine) ] to the reaction mixture
2 2
with subsequent heating for 18 h at 65 °C (Scheme 2). The
related imidazolylidene complex 9 was obtained in 85% yield
by refluxing a suspension of precursor 7, K
pyridine for 18 h. The C NMR spectrum of both complexes
2
CO
3 2
and PdCl in
1
3
shows a signal corresponding to a carbenic C atom, respect- Fig. 2 Molecular structure of complex 8. Pd⋯H(Et) anagostic bond
separations: 2.61, 2.78, 2.60, 2.72 Å. Important bond distances (Å):
C(carbene)–Pd 1.951(4), Pd–N 2.100(3). Pd–Cl 2.311(1) and 2.329(1).
ively at 174.9 (8) and 144.2 (9) ppm.
The NCCH signals of 8 and 9 are considerably downfield
2
shifted with respect to those of the corresponding precursor
salts (Δδ = 1.59 (8 vs. 6), and 1.42 (9 vs. 7) ppm). The observed
downfield shifted signals, together with relatively “standard”
1
J(CH) values (all close to 130 Hz) are indicative of anagostic
1
6
C–H⋯Pd interactions, that is of interactions being essen-
tially electrostatic in nature (with the dz orbital being not sig-
2
nificantly involved). 2D NMR experiments further revealed that
while for the precursors 6 and 7 the ethyl-fluorenylidene
groups freely rotate about the corresponding N–C(fluorenyli-
dene) bonds, the same groups are blocked in 8 and 9, with the
ethyl groups being permanently turned towards the “Pd(pyri-
dine)” unit. The particular orientation of the ethyl groups was
confirmed by single crystal X-ray analyses carried out for each Fig. 3 Molecular structure of complex 9. Pd⋯H(Et) anagostic bond
separations: 2.74, 2.62, 2.60, 2.80 Å. Other important bond lengths (Å):
C(carbene)–Pd 1.961(2), Pd–N 2.093(2). Pd–Cl 2.301(6) and 2.322(6).
complex (Fig. 2 and 3). It is noteworthy that this orientation is
the same as the one seen in the solid state for the Et groups of
the previously reported benzimidazolylidene analogue 10
(Fig. 4). The Pd–C(carbene) and Pd–N bond lengths in 8 and 9
Fig. 4 Anagostic interactions (red) observed in the palladium com-
plexes 8–10.
are comparable to those found in related Pd-PEPPSI
1
j,5
complexes.
How does an ethylfluorenyl-substituted imidazolylidene
1
6a
ligand behave towards a metal ion, such as Ag(I), less prone
to form anagostic interactions? To answer this question, imid-
azolium salt 7 was reacted with Ag O in refluxing acetonitrile.
2
This reaction led to silver complex 11 in high yield (Scheme 3).
Complex 11 is highly stable under light. The 2D ROESY
spectrum of 11 displayed cross peaks between the CH CH
2 3
and the two protons of the NHC ring, thus suggesting that AF
groups freely rotate about the corresponding N-C(fluorenyl)
Scheme 2 Synthesis of palladium complexes 8 and 9.
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from the metal. However, the interaction energy of each
fluorenylidene group with the silver atom is of only ca. 2 kcal
−
1
mol . Molecules of type A and B are further supramolecularly
linked (in the solide state) through H⋯Cl bonds involving H
atoms of the fluorenylidene moieties and the imidazolyl rings.
The two Ag–C(carbene) bond lengths (2.102(5) and 2.110 (6) Å)
1
7
lie in the range expected for [Ag(NHC)X] complexes.
It is noteworthy that in the palladium complexes 8 and 9,
the AF planes are turned away from the metal as a result of
anagostic interactions between the CH CH protons and the
2 3
palladium atom. These interactions are seemingly dominant
over other possible weak interactions in the palladium com-
plexes. Overall, the present study shows that alkylfluorenyli-
dene substituted NHCs may behave as carbene-centered
bimodal pincers functioning in two possible bonding modes,
Scheme 3 Synthesis of silver complex 11.
bonds. In contrast to those of 8–10, the chemical shift of the one involving two weak (i.e. mainly electrostatic) CH⋯M inter-
2
CH
2
CH
3
protons of 11 (2.75 ppm) is normal, this ruling out actions, the other involving two weak η -arene⋯M interactions.
It is further interesting to note that determination of the
significant Ag⋯H(Et) interactions.
1
8
Crystals of 11 suitable for X-ray diffraction analysis were percent buried volume (pbv) of the carbene ligand resulted
obtained by slow diffusion of pentane into a solution of the in markedly different values (35.8% and 58.3%, respectively)
complex. In the solid state two distinct molecules, A and B, are according to which structure, 9 or 11, was used for the calcu-
present (Fig. 5). Their most striking feature concerns the orien- lation. This makes the relevance of the pbv concept question-
tation of the fluorenylidene planes, which, in keeping with the able, as in fact it does not take into account time dependent
NMR data (vide supra), are both bent towards the metal atom, conformational changes. Note that the two NHC’s of the
thus resulting in an “open sandwich” structure. The X-ray data literature with the highest reported pbv are a NHC which caps
19
(2-Np)
indicate that molecule A has both fluorenylidene moieties a cyclodextrin unit (pbv = 58.5%), and IPr*
η -coordinated to the metal (see Fig. 5; shortest Ag⋯C dis- 57.4%).
(pbv =
2
20
tances: 2.93 and 3.02 Å for fluorenylidene A1; 3.10 and 3.12 Å
for fluorenylidene A2). A similar π-arene–metal interaction can Suzuki–Miyaura cross coupling
also be seen in molecule B, but only for one of its two ethyl-
fluorenyl groups (shortest Ag⋯C distances: 3.00 and 3.18 Å).
DFT calculations (see computational details) revealed that con-
formers having the fluorenylidene planes bent towards the
silver atom (exactly as observed for 11 in the solid state) are
favoured over those in which these planes are turned away
Complexes 8 and 9 were assessed in Suzuki–Miyaura cross
coupling between phenyl boronic acid and various aryl chlor-
ides. The catalytic runs were performed in dioxane at 80 °C
using 1 mol% of complex and 2 equiv. of Cs CO per aryl
2 3
chloride. Preliminary tests carried out with p-tolyl chloride
revealed that the activity of the imidazolylidene complex 9 was
6
times higher than that of the electronically richer complex 8
(
Scheme 4), but we have no rationalisation for the observed
activity difference. In fact the performance of 9 for this particu-
lar reaction compares with that of the previously reported
1
3a
benzimidazolidene complex 10,
which is one of the fastest
Suzuki–Miyaura cross coupling catalyst reported so far for this
reaction. The latter complex turned out to be two times more
active than its analogue with IPr.
In view of the lower activity of 8, tests with other aryl chlor-
ides (see below) were achieved with 9 only. To allow compari-
Fig. 5 Silver complex 11 existing in the solid state as two distinct mole-
cules with “sandwiched” metal atoms. The red arrows indicate CC bonds
π-interacting with the metal centres (Ag⋯C(arom.) separations
2.93–3.18 Å). Important distances (Å): Ag–C(carb) 2.102(5) (A) and 2.110
Scheme 4 Suzuki–Miyaura
cross-coupling
of
PhB(OH)
2
with
(
6) (B); Ag–Cl 2.342(1) (A) and 2.379(2) (B).
p-tolylchloride using palladium complexes 8–10.
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son, the same tests were also carried out with 10. The relative highly Suzuki–Miyaura cross coupling catalyst, but only for
activity of these complexes turned out to be dependant on the unencumbered aryl chlorides.
substrate.
Thus, aryl chlorides bearing electron-withdrawing groups in
para position resulted in activities similar to those obtained
for p-tolyl chloride (Table 1, entries 3 and 4), whether 9 or 10
Conclusion
was used. In contrast, for p-chloroanisole, the coupling reac- In summary, a useful synthetic route for the preparation of
tion occurred ca. 3 times faster with 9 than with 10 (Table 1, imidazolinium salt 6 and imidazolium salt 7 has been
entry 2). This possibly reflects the better donor properties of described. The key intermediate for these syntheses is the
the carbene ligand of 9, which consequently facilitates the oxi- primary amine 3, which was obtained according to a one-flask
dative addition step. With o-substituted arylchlorides, the procedure starting from fluorenone. Both salts were converted
activity of both catalysts dropped significantly (Table 1, entries into the PEPPSI-type palladium complexes 8 and 9 by applying
2
c,21
5
and 6), suggesting that the encumbrance of the carbene the classical procedure developed by Organ.
While
ligands hinders the approach of the reagent. A similar complex 8 showed low efficiency, complex 9 based on a NHC
decrease in activity was observed with 3-chloropyridine. Prob- with weaker donor properties behaved as a fast catalyst for the
ably competition occurs in that case between the expected oxi- cross coupling of phenyl boronic acid with p-substituted aryl
dative addition step and (reversible) coordination of the chlorides, its activity being comparable to the best catalysts
arylchloride through its nitrogen atom, this slowing down the reported to date. In solution as well as in the solid state, ana-
coupling reaction. Overall, complex 9 may be regarded as a gostic (mainly electrostatic) interactions were seen in both
complexes between the CH (fluorenyl) groups and the palla-
2
dium centre, thus confirming that these carbene ligands may
behave as bimodal pincers. As suggested by the solid state
structure of the “open sandwich” silver complex 11, another
Table 1 Efficiency of complexes 9 and 10 in Suzuki–Miyaura cross- type of bimodal pincer may be envisaged with such carbenes.
a
coupling of aryl chlorides
Overall, our findings unambiguously demonstrate that alkyl-
fluorenyl-substituted NHCs constitute a new class of ligands
displaying variable encumbrance.
Experimental section
General procedures
b
Yield (%)
Entry
Product
All commercial reagents were used as supplied. The syntheses
were performed in Schlenk-type flasks under dry nitrogen. Sol-
vents were dried by conventional methods and distilled
9
10
1
2
3
4
99
99
1
13
1
immediately prior to use. Routine H and C{ H} NMR
1
spectra were recorded on a FT Bruker AVANCE 300 ( H:
96
96
98
29
90
96
1
3
1
3
00.1 MHz, C: 75.5 MHz) instrument at 25 °C. H NMR spec-
tral data were referenced to residual protonated solvent
1
3
(
3
CHCl , δ 7.26), C chemical shifts are reported relative to
deuterated solvent (CDCl , δ 77.16). In the NMR data given
3
hereafter, Cq denotes a quaternary carbon atom. Flash chrom-
22
atography was performed as described by Still et al., employ-
ing Geduran SI (E. Merck, 0.040–0.063 mm) silica. Routine
thin-layer chromatography analyses were carried out by using
plates coated with Merck Kieselgel 60 GF254. Elemental ana-
lyses were performed by the Service de Microanalyse, Institut
de Chimie (UDS-CNRS), Strasbourg. Melting points were deter-
mined with a Büchi 535 capillary melting-point apparatus and
are uncorrected. Palladium complex 10 was prepared following
5
6
9 (24 h)
20
15
23
1
3a
a procedure described in the literature.
9-Ethyl-9H-fluoren-9-yl)amine (3). LiHMDS (1 M in THF,
1.7 mL, 21.7 mmol) was added to a stirred solution of fluore-
none (1) (3.0 g, 16.7 mmol) in THF (10 mL) at room tempera-
(2 mmol), [Pd] (1 mol%), dioxane (3 mL), 3 h. ture. The reaction mixture was then heated at 65 °C for 18 h.
7
20
20
(
2
a
Reaction conditions: aryl chloride (1 mmol), phenylboronic acid
CO
Isolated yields; average of two runs.
(
1.5 mmol), Cs
2
3
b
The resulting dark solution was then cooled to 0 °C and ethyl-
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magnesium bromide (1.5 M in THF, 33.5 mL, 50.3 mmol) was was added and he mixture was heated at 80 °C for 18 h. A
added dropwise. The mixture was heated at 65 °C during 4 h white precipitate was observed. The suspension was then
and then cooled to 0 °C. Water (100 mL) was slowly added and cooled to room temperature, and petroleum ether was added
the mixture was extracted with AcOEt (3 × 100 mL). The com- (ca. 20 mL). The precipitate was collected by filtration and
bined organic layers were dried over Na SO and the solvent washed with petroleum ether (3 × 15 mL) to afford 6 (507 mg,
2
4
1
removed under vacuum. The crude product was purified by 90%) as a hygroscopic white solid; mp > 230 °C. H NMR
flash chromatography (SiO ; AcOEt) to afford 3 as a pale yellow (CDCl , 300 MHz): δ 10.25 (s, 1 H, NCHN), 7.99–7.92 (m, 4 H,
oil (2.62 g, 75%). H NMR (CDCl , 300 MHz): δ 7.63–7.56 (m, ArH), 7.68–7.61 (m, 4 H, ArH), 7.46–7.38 (m, 8 H, ArH), 3.27 (q,
2
3
1
3
3
3
2
2
7
H, ArH), 7.50–7.45 (m, 2 H, ArH), 7.39–7.30 (m, 4 H, ArH),
J = 7.4 Hz, 4 H, CH
2
CH
3
), 3.01 (s, 4 H, NCH
2
), 0.41 (t, J = 7.4
, 75 MHz), δ 156.1
.5 Hz, 3 H, CH3). C{ H} NMR (CDCl , 75 MHz): δ 150.8 (NCHN), 142.2 (arom. Cq), 140.7 (arom. Cq), 130.0 (arom. CH),
3
3
13
1
2 2 2 3 3
.10 (q, J = 7.5 Hz, 2 H, CH ), 2.02 (br s, 2 H, NH ), 0.47 (t, J = Hz, 6 H, CH CH ). C{ H} NMR (CDCl
13
1
3
(
arom. Cq), 139.9 (arom. Cq), 128.1 (arom. CH), 127.8 (arom. 129.1 (arom. CH), 124.5 (arom. CH), 120.3 (arom. CH), 73.6
CH), 123.0 (arom. CH), 120.0 (arom. CH), 66.0 (Cq), 34.1 (Cq), 45.2 (NCH ), 29.4 (CH CH ), 7.9 (CH CH ). Anal. Calcd
CH ), 8.8 (CH ). Anal. Calcd for C H N (209.29): C, 86.08; for C H ClN (491.08): C, 80.71; H, 6.36; N, 5.70. Found: C,
2
2
3
2
3
(
2
3
15 15
33 31
2
H, 7.22; N, 6.69. Found: C, 86.15; H, 6.92; N, 6.49.
N,N′-(Ethane-1,2-diylidene)bis(9-ethyl-9H-fluoren-9-amine)
80.91; H, 6.36; N, 5.40.
1,3-Bis(9-ethyl-9H-fluoren-9-yl)imidazolium chloride (7).
(4). To a solution of amine 3 (2.47 g, 11.8 mmol) in n-propanol TMSCl (189 µL, 1.50 mmol) was slowly added to a mixture of
(23 mL) was added glyoxal (40%, 0.675 mL, 5.89 mmol). The diimine 4 (635 mg, 1.44 mmol) and paraformaldehyde (45 mg,
solution was then heated at 70 °C for 18 h under vigorous stir- 1.50 mmol) in AcOEt (5 mL). The mixture was then heated at
ring. A white precipitate was observed. The suspension was 70 °C for 18 h. A white precipitate was observed. The suspen-
allowed to reach room temperature and was then concentrated sion was then cooled to room temperature and petroleum
under vacuum. The residue was recrystallized from n-propanol ether was added (ca. 20 mL). The precipitate was collected by
to afford pure diimine 4 as a white crystalline solid (2.33 g, filtration and washed with petroleum ether (3 × 15 mL) to
1
9
0%); mp > 220 °C. H NMR (CDCl
3
, 300 MHz): δ 7.74 (s, 2 H, afford compound 7 (654 mg, 93%) was obtained as a hygro-
1
NCH), 7.68–7.61 (m, 4 H, ArH), 7.37–7.23 (m, 12 H, ArH), 2.28 scopic white solid; mp 212 °C. H NMR (CDCl
q, J = 7.5 Hz, 4 H, CH ), 0.47 (t, J = 7.5 Hz, 6 H, CH ).
, 75 MHz): δ 159.3 (NCH), 146.6 (arom. Cq),
40.6 (arom. Cq), 128.4 (arom. CH), 127.8 (arom. CH), 124.7 (dd, 4 H, J = J′ = 7.5 Hz, ArH), 6.37 (s, 2 H, NCH), 3.47
3
, 500 MHz),
3
3
13
3
(
{
C
δ 11.85 (s, 1 H, NCHN), 7.87 (d, J = 7.5 Hz, 4 H, ArH), 7.71 (d,
J = 7.5 Hz, 4 H, ArH), 7.46 (dd, J = J′ = 7.5 Hz, 4 H, ArH), 7.40
2
3
1
3
3
3
H} NMR (CDCl
3
3
3
1
(
3
3
arom. CH), 120.3 (arom. CH), 78.4 (Cq), 32.8 (CH ), 8.4 (CH ). (q, 4 H, J = 6.3 Hz, CH ), 0.50 (t, 6 H, J = 6.3 Hz, CH3).
2
3
2
1
3
1
Anal. Calcd for C32
H
28
N
2
(440.59): C, 87.24; H, 6.41; N, 6.36.
C{ H} NMR (CDCl
3
, 125 MHz), δ 143.0 (arom. Cq), 140.3
Found C: 87.15; H, 6.35; N, 6.39.
N,N′-Bis(9-ethyl-9H-fluoren-9-yl)ethane-1,2-diamine (5). BH3 CH), 124.7 (arom. CH), 120.6 (arom. CH), 119.8 (NCH), 75.7
1 M in THF, 3 mL, 3.00 mmol) was added dropwise to a (Cq), 30.5 (CH ), 8.0 (CH ). Anal. Calcd for C33 29ClN
stirred solution of diimine 4 (1.0 g, 2.27 mmol) in THF (489.06): C, 81.05; H, 5.98; N, 5.73. Found: C, 81.38; H, 5.97;
10 mL) at 0 °C. The reaction mixture was allowed to reach N, 5.55.
room temperature and stirred for 1 h. Water was slowly added trans-[1,3-Bis(9-ethyl-9H-fluoren-9-yl)imidazolin-2-ylidene]-
40 mL) and the reaction mixture was extracted with AcOEt (pyridine)palladium(II) dichloride (8). To a suspension of imi-
3 × 40 mL). The combined organic layers were dried over dazolinium salt 6 (491 mg, 1.00 mmol) in THF (5 mL) cooled
SO and the solvent was removed under reduced pressure. at 0 °C, was added LiHMDS (1 M in THF, 1 mL, 1.00 mmol).
The light yellow crude diamine was taken in CH Cl and char- The suspension was allowed to reach room temperature and
coal was added. The black mixture was heated to reflux and was stirred for 1 h. To the resulting orange solution was added
then allowed to reach room temperature. After filtration [PdCl (pyridine) ] (335 mg, 1 mmol) and the mixture was
(arom. Cq), 137.7 (NCHN), 130.4 (arom. CH), 129.2 (arom.
(
2
3
H
2
(
(
(
Na
2
4
2
2
2
2
through Celite the filtrate was concentrated under reduced stirred at room temperature for 18 h. The mixture was filtered
pressure to afford pure diamine 5 as a white solid (997 mg, through Celite and the collected solid washed with CH Cl (ca.
2
2
1
3
9
9%); mp 174 °C. H NMR (CDCl
3
, 400 MHz): δ 7.62 (d, J = 20 mL). The filtrate was concentrated under reduced pressure
7
.3 Hz, 4 H, ArH), 7.35–7.24 (m, 12 H, ArH), 2.04–1.96 (m, 6 H, and the residue purified by flash chromatography (SiO
2
;
overlapping signals, 4 H CH CH and 2 H NH), 1.85 (s, 4 H, CH Cl /petroleum ether, 50 : 50) to afford 8 as a yellow solid
2
3
2
2
3
13
1
1
NCH
00 MHz): δ 148.1 (arom. Cq), 141.0 (arom. Cq), 128.0 (arom. δ 9.11–9.06 (m, 2 H, o-NC
CH),127.5 (arom. CH), 123.4 (arom. CH), 119.8 (arom. CH), 7.84–7.77 (m, 1 H, p-NC H ), 7.64–7.57 (m, 4 H, ArH),
2
), 0.41 (t, J = 7.5 Hz, 6 H, CH
2
CH
3
). C{ H} NMR (CDCl
3
,
(534 mg, 75%); mp > 230 °C. H NMR (CDCl
3
, 300 MHz),
1
5 5
H
), 7.95–7.89 (m, 4 H, ArH),
5
5
7
1.0 (Cq), 43.5 (NCH
Calcd for C32 (444.62): C, 86.44; H, 7.25; N, 6.30. Found: m-NC
C, 86.20; H, 7.32; N, 6.55. NCH ), 0.36 (t, J = 7.2 Hz, 6 H, CH CH ). C{ H} NMR
,3-Bis(9-ethyl-9H-fluoren-9-yl)imidazolinium chloride (6). (CDCl
2 2 3 2 3
), 33.9 (CH CH ), 8.3 (CH CH ). Anal. 7.44–7.31 (m, 10 H, overlapping signals, 8 H ArH and 2 H
3
H
32
N
2
5 5 2 3
H ), 4.86 (q, J = 7.2 Hz, 4 H, CH CH ), 2.57 (s, 4 H,
3
13
1
2
2
3
1
3
, 75 MHz), δ 174.9 (NCN), 151.7 (arom. CH), 146.0
Diamine 5 (512 mg, 1.15 mmol) was dissolved under magnetic (arom. Cq), 140.6 (arom. Cq), 138.1 (arom. CH), 128.9 (arom.
stirring in HC(OEt) (3 mL). HCl (12 M, 116 μL, 1.39 mmol) CH), 128.8 (arom. CH), 124.9 (arom. CH), 124.7 (arom. CH),
3
Dalton Trans.
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1
19.8 (arom. CH), 75.2 (Cq), 45.7 (NCH
2
), 33.8 (CH
2
CH
3
), 9.06 the intensity of the methyl signal of the product [δ(Me) =
(CH CH ). Anal. Calcd for C H Cl N Pd (711.04): C, 64.19; 2.41 ppm] with that of the internal reference [δ(Me) =
2 3 38 35 2 3
H, 4.96; N, 5.91. Found: C, 64.42; H, 5.08; N, 5.66.
3.78 ppm]. In some experiments the product was isolated chro-
trans-[1,3-Bis(9-ethyl-9H-fluoren-9-yl)imidazol-2-ylidene]- matographically. The isolated yield turned out to be very close
pyridine)palladium(II) dichloride (9). A suspension of imida- (deviation less than 5%) to that determined by NMR.
(
zolium salt (7) (1.23 g, 2.51 mmol), finely crushed K
1.72 g, 12.4 mmol), and PdCl (534 mg, 3.01 mmol) in pyri- of palladium complex (0.01 mmol), phenylboronic acid
dine (5 mL) was heated at 80 °C for 18 h under vigorous stir- (183 mg, 1.50 mmol) and Cs CO (652 mg, 2 mmol) was sus-
2
CO
3
Procedure applied for the runs shown in Table 1. A mixture
(
2
2
3
ring. The mixture was cooled to room temperature, filtered pended in dioxane (3 mL). After addition of the arylchloride
through Celite and the collected solid washed with CH Cl (ca. (1 mmol), the mixture was vigorously stirred at 80 °C for 3 h.
0 mL). The filtrate was evaporated to dryness and the residue The hot mixture was filtered through Celite and the collected
purified by flash chromatography (SiO ; CH Cl /petroleum solid washed with CH Cl (ca. 20 mL). The filtrate was evapo-
ether, 50 : 50) to afford 9 as a yellow solid (1.51 g, 85%); mp > rated to dryness and the residue purified by flash chromato-
2
2
2
2
2
2
2
2
1
2
20 °C. H NMR (CDCl , 500 MHz), δ 9.14–9.12 (m, 2 H, graphy (SiO ; AcOEt/petroleum ether, 0.5 : 99.5) to afford the
3
2
3
4
o-NC
(
7
1
5
H
5
), 7.82 (tt, J = 7.6 Hz, J = 1.7 Hz, 1 H, p-NC
d, J = 7.5 Hz, 4 H, ArH), 7.66 (d, J = 7.5 Hz, 4 H, ArH), by NMR after their isolation. The NMR spectra were compared
.44–7.40 (m, 2 H, m-NC H ), 7.37 (ddd, J = J′ = 7.5 Hz, J = to those reported in the literature.
5 5
.2 Hz, 4 H, ArH), 7.31 (ddd, J = J′ = 7.5 Hz, J = 1.2 Hz, 4 H,
5
H
5
), 7.75 desired product. All products were unambiguously identified
3
3
3
3
4
10b
3
3
4
3
X-ray crystallography
Crystal data for complex 8. Crystals suitable for X-ray diffrac-
δ 151.8 (arom. CH), 147.6 (arom. Cq), 144.2 (NCN), 140.1 tion were obtained by slow diffusion of ether into a dichloro-
arom. Cq), 138.1 (arom. CH), 129.2 (arom. CH), 129.0 (arom. methane solution of the complex: C38 Pd, M = 710.99,
CH), 125.0 (arom. CH), 124.8 (arom. CH), 122.8 (arom. CH), orthorhombic, space group P2 2 2 , a = 11.6715(2), b =
ArH), 5.85 (s, 2 H, NCH), 4.99 (q, J = 7.3 Hz, 4 H, CH
2
), 0.47
3
13
1
(
t, J = 7.3 Hz, 6 H, CH3). C{ H} NMR (CDCl , 75 MHz),
3
(
2 3
H35Cl N
1
1 1
3
1
20.0 (arom. CH), 75.8 (Cq), 33.1 (CH
for C38
Pd (709.02): C, 64.37; H, 4.69; N, 5.93. Found: 4, μ = 0.760 mm− , F(000) = 1456. Crystals of the compound
C, 64.52; H, 4.62; N, 5.65. were mounted on a Oxford Diffraction CCD Sapphire 3 Xcali-
2 3
), 8.9 (CH ). Anal. Calcd 13.6877(2), c = 20.5488(4) Å, β = 90.00, V = 3282.79(10) Å , Z =
1
H
2 3
33Cl N
trans-[1,3-Bis(9-ethyl-9H-fluoren-9-yl)imidazol-2-ylidene]- bur diffractometer. Data collection with Mo-Kα radiation (λ =
silver(I) chloride (11). A mixture of imidazolium salt 7 0.71073 Å) was carried out at 120 K. 23 091 reflections were col-
(
210 mg, 0.43 mmol) and Ag O (119 mg, 0.516 mmol) in lected (2.64 < θ < 27.00°), 7149 were found to be unique and
2
CH
3
CN (8 mL) was stirred at reflux for 24 h, protected from 5389 were observed (merging R = 0.0620). The structure was
2
3
light. The mixture was cooled to room temperature, filtered solved with SHELXS-97. Final results: R
2
1 2 1
, R , wR , wR , Goof;
through Celite and the collected solid washed with CH Cl (ca. 0.0592, 0.0378, 0.0631, 0.0598, 0.869. Residual electron density
2
2
3
2
0 mL). The filtrate was evaporated to dryness to afford 11 as a minimum/maximum = −0.460/0.899 e Å−
.
1
white solid (230 mg, 90%); mp 215 °C decomposition.
NMR (CDCl , 300 MHz), δ 7.77 (d, J = 7.6 Hz, 4 H, ArH), 7.46 tion were obtained by slow diffusion of ether into a dichloro-
dd, J = J′ = 7.6 Hz, 4 H, ArH), 7.29 (dd, J = J′ = 7.6 Hz, 4 H, methane solution of the complex: C38
ArH), 7.22 (d, J = 7.6 Hz, 4 H, ArH), 7.02 (s, 2 H, NCH), 2.75 orthorhombic, space group P2 2 2 , a = 11.6469(2), b =
q, 4 H, J = 7.2 Hz, CH ), 0.39 (t, 6 H, J = 7.2 Hz, CH ).
3
H} NMR (CDCl , 75 MHz), δ 145.8 (arom. Cq), 140.3 (arom. μ = 0.765 mm , F(000) = 1448. Crystals of the compound were
H
Crystal data for complex 9. Crystals suitable for X-ray diffrac-
3
3
3
3
3
3
(
H
33Cl
2
N
3
Pd, M = 708.97,
3
1 1 1
3
3
13
3
(
{
C
13.7077(2), c = 20.4229(3) Å, β = 90.00, V = 3260.56(9) Å , Z = 4,
2
3
1
−1
Cq), 129.7 (arom. CH), 128.5 (arom. CH), 123.4 (arom. CH), mounted on a Oxford Diffraction CCD Sapphire 3 Xcalibur
21.0 (arom. CH), 119.0 (NCH), 73.9 (Cq), 32.2 (CH ), 8.2 diffractometer. Data collection with Mo-Kα radiation (λ =
1
2
(
CH
Anal. Calcd for C33
.70. Found: C, 66.32; H, 4.81; N, 4.65.
3
). The signal of the carbenic C atom was not detected. 0.71073 Å) was carried out at 120 K. 27 148 reflections were col-
28AgClN (595.92): C, 66.51; H, 4.74; N, lected (2.65 < θ < 27.00°), 7099 were found to be unique and
6467 were observed (merging R = 0.0335). The structure was
H
2
4
2
3
2 1 2 1
solved with SHELXS-97. Final results: R , R , wR , wR , Goof;
General procedure for Suzuki–Miyaura cross-couplings
Procedure applied for the runs shown in Scheme 4. A minimum/maximum = –0.251/0.476 e Å
mixture of palladium complex (0.01 mmol), phenylboronic Crystal data for complex 11. Crystals suitable for X-ray diffr-
acid (183 mg, 1.50 mmol) and Cs CO (652 mg, 2 mmol) was action were obtained by slow diffusion of pentane into a
suspended in dioxane (3 mL). After addition of p-tolylchloride dichloromethane solution of the complex: C H AgClN , M =
0
.0289, 0.0244, 0.0526, 0.0517, 0.969. Residual electron density
−
3
.
2
3
3
3
28
2
(
126 mg, 1 mmol), the mixture was vigorously stirred at 80 °C 1191.79, triclinic, space group P ˉ1 , a = 10.2170(7), b =
3
for 1 h. The hot mixture was filtered through Celite. 1,4- 13.5390(10), c = 20.073(2) Å, β = 78.226(8), V = 2660.9(4) Å , Z =
Dimethoxybenzene (69 mg, 0.5 mmol; internal standard) was 4, μ = 0.883 mm− , F(000) = 1216. Crystals of the compound
then added to the filtrate. The solvent was removed under were mounted on a Oxford Diffraction CCD Sapphire 3 Xcali-
1
1
reduced pressure, and the crude mixture was analysed by H bur diffractometer. Data collection with Mo-Kα radiation (λ =
NMR spectroscopy. The yields were determined by comparing 0.71073 Å) was carried out at 140 K. 21 631 reflections were col-
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Dalton Trans.

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lected (3.05 < θ < 27.00°), 11 591 were found to be unique and
( j) A. Chartoire, X. Frogneux, A. Boreux, A. M. Z. Slawin
and S. P. Nolan, Organometallics, 2012, 31, 6947–6951;
(k) C. Valente, S. Çalimsiz, K. H. Hoi, D. Mallik, M. Sayah
and M. G. Organ, Angew. Chem., Int. Ed., 2012, 51, 3314–
3332; (l) S. Meiries, K. Speck, D. B. Cordes, A. M. Z. Slawin
and S. P. Nolan, Organometallics, 2013, 32, 330–339;
(m) D. J. Nelson and S. P. Nolan, Chem. Soc. Rev., 2013, 42,
6723–6753; (n) G. Le Duc, S. Meiries and S. P. Nolan,
Organometallics, 2013, 32, 7547–7551; (o) N. Sahin,
D. Sémeril, E. Brenner, D. Matt, I. Özdemir, C. Kaya and
L. Toupet, Eur. J. Org. Chem., 2013, 4443–4449;
6
846 were observed (merging R = 0.0708). The structure was
2
3
solved with SHELXS-97. Final results: R
2
1 2 1
, R , wR , wR , Goof;
0
.1315, 0.0645, 0.1176, 0.0941, 1.019. Residual electron density
−
3
minimum/maximum = −0.621/0.759 e Å
.
CCDC-875396 (for 8), 901467 (for 9) and 1004245 (for 11)
contain the supplementary crystallographic data for this
paper.
Computational details
2
4
Calculations were performed using the ADF 2013 package.
All electrons Slater type orbitals were used with all-electron
(
p) C. Valente, M. Pompeo, M. Sayah and M. G. Organ,
Org. Process Res. Dev., 2014, 18, 180–190;
q) M. N. Hopkinson, C. Richter, M. Schedler and
F. Glorius, Nature, 2014, 510, 485–496.
2
5
triple-ζ quality basis sets at DFT level with PBE functional.
Dispersive interactions were taken into account applying
(
26
Grimme corrections. Scalar relativistic effects were included
2
7
2 (a) G. A. Grasa, M. S. Viciu, J. K. Huang, C. M. Zhang,
M. L. Trudell and S. P. Nolan, Organometallics, 2002, 21,
through ZORA Hamiltonian. Full geometry optimization was
performed on the complexes. Interaction energy between the
silver atom of 11 and the fluorenyl moiety was calculated by
two methods. First, the ethylfluorenyl groups were rotated
around the corresponding N–C bond moving the fluorenyli-
dene plane from a position in which it is bent towards the
metal centre to the opposite one (calculation 1). Secondly, we
computed the substitution energy of a NHC-complex having
two propyl chains instead of ethylfluorenyl groups by the NHC
bearing two ethylfluorenyl groups (calculation 2). Both calcu-
lations showed that the complex where interaction between
the silver atom and the fluorenyl groups occurs is more stable
2
866–2873; (b) R. Singh, M. S. Viciu, N. Kramareva,
O. Navarro and S. P. Nolan, Org. Lett., 2005, 7, 1829–1832;
c) J. Nasielski, N. Hadei, G. Achonduh, E. A. B. Kantchev,
C. J. O’Brien, A. Lough and M. G. Organ, Chem. – Eur. J.,
010, 16, 10844–10853; (d) S. Dastgir, K. S. Coleman,
A. R. Cowley and M. L. H. Green, Organometallics, 2010, 29,
858–4870; (e) Y. Q. Tang, J. M. Lu and L. X. Shao, J. Orga-
(
2
4
nomet. Chem., 2011, 696, 3741–3744.
3
(a) W. A. Herrmann, V. P. W. Böhm, C. W. K. Gstöttmayr,
M. Grosche, C. P. Reisinger and T. Weskamp, J. Organomet.
Chem., 2001, 617, 616–628; (b) O. Navarro, H. Kaur,
P. Mahjoor and S. P. Nolan, J. Org. Chem., 2004, 69, 3173–
ΔE = 3.2 kcal mol− for calculation 1; ΔE = 3.8 kcal mol for
1
−1
(
calculation 2).
3180; (c) C. Burstein, C. W. Lehmann and F. Glorius,
Tetrahedron, 2005, 61, 6207–6217; (d) N. Hadei,
E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Org. Lett.,
Acknowledgements
2005, 7, 1991–1994; (e) N. Marion, O. Navarro, J. G. Mei,
This work was supported by the Ministère de l’Enseignement
Supérieur et de la Recherche for a grant to M.T. We gratefully
acknowledge the CNRS and the University of Strasbourg for
their financial support.
E. D. Stevens, N. M. Scott and S. P. Nolan, J. Am. Chem.
Soc., 2006, 128, 4101–4111; (f) C. Fleckenstein, S. Roy,
S. Leuthausser and H. Plenio, Chem. Commun., 2007, 2870–
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L. Cavallo, S. P. Nolan, A. M. Z. Slawin and C. S. J. Cazin,
Organometallics, 2010, 29, 1443–1450; (h) M. T. Chen,
D. A. Vicic, M. L. Turner and O. Navarro, Organometallics,
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