Synthesis of N,O-heterocyclic carbene and coordination to rhodium(I) and copper(I)

Article abstract of DOI:10.1016/j.poly.2009.05.067

5,7-Di-tert-butyl-3-phenyl benzoxazolium tetrafluoroborate 1 could be prepared by simple reaction of the corresponding aminophenol with triethyl orthoformate under acidic conditions. Both rhodium(I) and copper(I) complexes with benzoxazol-2-ylidene ligand

Full text of DOI:10.1016/j.poly.2009.05.067

Polyhedron 29 (2010) 30–33
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis of N,O-heterocyclic carbene and coordination to rhodium(I) and copper(I)
*
Stéphane Bellemin-Laponnaz
Institut de chimie, Université de Strasbourg – CNRS, 1 rue Blaise Pascal, 67000 Strasbourg, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 6 June 2009
5,7-Di-tert-butyl-3-phenyl benzoxazolium tetrafluoroborate 1 could be prepared by simple reaction of
the corresponding aminophenol with triethyl orthoformate under acidic conditions. Both rhodium(I)
and copper(I) complexes with benzoxazol-2-ylidene ligand were then efficiently synthesised in a
straightforward and smooth manner involving the reaction of benzoxazolium salt 1 with metal precursor
and an external base. The complexes have been fully characterised and used in metal-catalysed hydrosi-
lylation of ketones, where they showed poor catalytic activity, presumably due to low stability of the
complexes under those conditions.
Keywords:
N-heterocyclic carbene
Rhodium
Copper
Coordination chemistry
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
by indirect methods via isocyanide derivatives [7,11]. However in
2005, Biffis and co-workers reported the synthesis of palladium
N-heterocyclic carbenes (NHCs) have been established as a class
of priviledged ligand in transition metal coordination chemistry
since Arduengo and collaborators first isolated stable NHC species
in 1991 [1,2]. They have provided the base for extensive research
efforts in the development of catalysts and during the last decade,
numerous applications of NHC complexes as phosphine mimics
have been reported [3,4]. However, the reactivity of NHC com-
plexes is different from that of the corresponding phosphine com-
plexes and these differences may be connected with the nature of
the metal–carbene bond, which remain subject of debate [2d].
NHCs and phosphines have different topography so that, to
date, electronic and steric properties of phosphines can be readily
varied whereas such easy modularity is less accessible for N-het-
erocyclic carbenes. Indeed, tuning the electronic components of
NHCs may be of great interest for the development of highly active
catalysts. The strategies are as follows: (i) attachment of electron-
donating or electron-withdrawing groups [5], (ii) incorporation of
additional heteroatoms into the backbone (e.g. nitrogen, boron
and phosphorus) [6], (iii) substitution of heteroatoms (O, S and
even P) [7].
bis-N,O-heterocyclic carbene complexes from oxazolium salts and
palladium precursor [12]. The oxazolium precursors were prepared
by direct alkylation of oxazole with methyl iodide or benzylic bro-
mides. These bidentate ligands were found to catalyse the Heck
type coupling reaction.
In this contribution, we report a straightforward synthesis of
benzoxazolium salt directly obtained from the hydroxyaniline.
The synthesis and coordination chemistry of the benzoxazol-2-yli-
dene as ligand with Rh(I) and Cu(I) is also discussed. Finally, some
preliminary experiments on hydrosilylation reaction have been
carried out.
2. Results and discussion
2.1. Synthesis of the Ligand
Biffis, Cavell and collaborators prepared oxazolium salts
through the alkylation of oxazole with primary alkyl halides, a
procedure that does not allow synthesis of N-aryl substituted
oxazolium salts. We found that such N-aryl substituted precursors
may be obtained directly from N-aryl 2-amino phenol. Therefore,
3,5-di-tert-butyl-2-hydroxy-N-phenylaniline [13] was reacted
with one equivalent of tetrafluoroboric acid and excess of triethyl
orthoformate. Stirring at room temperature the solution led to
precipitation of benzoxazolium salt 1 (Scheme 1) as a white solid,
which was obtained after workup in 60% yield. The oxazolium
proton (OCHN) resonance is characteristically downfield shifted
(d = 10.25 ppm) and the carbon resonance was found to be at
d = 155.4 ppm. Molecular structure of 1 was also confirmed by
X-ray diffraction studies as shown in Fig. 1.
Tuning the properties of the NHCs by substitution of nitrogen by
a sulphur atom or an oxygen atom is an attractive approach that
still remains underexploited so far (Chart 1). N,S-heterocyclic car-
bene complexes have been used with success in arylation of allylic
alcohols [8], C–C coupling reactions [9] or olefin metathesis [10].
However, there are fewer examples of N,O-heterocyclic carbenes
probably because such carbene complexes were originally obtained
* Tel.: +33 (0)3 9024 1542; fax: +33 (0)3 9024 5001.
E-mail address: bellemin@unistra.fr
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.067

S. Bellemin-Laponnaz / Polyhedron 29 (2010) 30–33
31
N
S
O
N
N
N
R
R
R
R
NHC
NSHC
NOHC
Chart 1.
2.2. Coordination to rhodium(I) and copper(I)
For the coordination of precursor 1 as an N,O-heterocyclic car-
bene to rhodium or copper, the strategy based on the ligand trans-
fer by a silver carbene complex was unsuccessful [14]. Reaction of
1 with 0.5 equivalent of Ag2O leads to a mixture of undefined com-
pounds. On the other hand, the in situ reaction of 1 with
[RhCl(cod)]2 and tBuOK was found to be successful. The reaction
was carried out in THF at room temperature for 15 h. Filtration
over celite and recrystallisation from THF/pentane gave the neutral
rhodium complex 2 as yellow crystals in 91% yield (Scheme 2). The
product slowly decomposes in solution but is stable in the solid
state and under nitrogen. In the 13C NMR spectrum, the resonance
of the carbene carbon nuclei of 2 was observed as a doublet at
d = 214.4 [1J(103Rh13C) = 57.1 Hz]. Two signals for the olefin hydro-
gen atoms CHcod were observed by 1H NMR spectroscopy indicat-
ing free rotation of the carbene around the C–Rh bond on the
NMR timescale.
X-ray structure analysis confirms the molecular structure of 2
(Fig. 2). The coordination geometry around the metal centre is a
distorted square plane with the N,O-heterocyclic carbene ring ori-
ented almost orthogonally. The Ccarbene–Rh bond length is
1.988(2) Å and Ccod–Rh bond lengths are within the expected
range. The Rh–Ccod distances trans to the carbene carbon atom
are longer than those trans to the chloride [2.212(2)/2.244(2) ver-
sus 2.102(3)/2.121(2) Å]. This trans influence is also reflected in the
difference in the C@C double bond lengths of the coordinated cod
ligand [1.373(4) versus 1.403(3) Å].
Fig. 1. Molecular structure of benzoxazolium 1. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (°): C(1)–N(1), 1.310(5); C(1)–O(1),
1.317(4); N(1)–C(1)–O(1), 112.8(3).
BF4
Rh(cod)Cl 2
tBu
N
O
N
O
tBuOK
C
Rh
Cl
H
thf, r.t.
tBu
- NaBF4
1
2
Scheme 2. Preparation of rhodium complex 2 from benzoxazolium 1.
In order to define the donating capacity of the carbene, we trea-
ted the rhodium complex 2 with CO gas in an attempt to form the
corresponding rhodium carbonyl complex. When a solution of the
complex was subject to CO gas bubbling, the yellow solution
instantaneously turned to deep orange, which gradually decom-
poses. Proton NMR analysis showed a mixture of undefined prod-
ucts and unfortunately any attempt to isolate well-defined
products were unsuccessful.
A similar procedure was used for the synthesis of the copper(I)
complex. We first reacted CuCl with LiOtBu overnight in THF prior
to addition of the benzoxazolium salt 1 in one portion (Scheme 3).
The resulting suspension was then stirred at room temperature for
12 h. Removal of the formed white salt by filtration and evapora-
tion of the solvent lead to the copper complex 3 in 65% yield
(Scheme 3). Proton and carbon NMR analysis confirmed the forma-
tion of the N,O-heterocyclic carbene. In the 13C NMR spectrum, the
resonance of the carbene carbon nuclei of 3 was observed at
d = 200.0 ppm. Recrystallisation of the complex proved to be diffi-
cult and the X-ray data obtained were of insufficient quality for a
refinement of the structure. However, the basic connectivities were
Fig. 2. Molecular structure of compound 2. Hydrogen atoms and a solvent molecule
(THF) are omitted for clarity. Selected bond lengths (Å) and angles (°): C(1)–Rh(1),
1.988(2); C(1)–N(1), 1.341(3); C(1)–O(1), 1.358(3); C(22)–C(23), 1.403(3); C(27)–
C(26), 1.373(4); C(22)–Rh(1), 2.102(3); C(23)–Rh(1), 2.121(2); C(26)–Rh(1),
2.212(2); C(27)–Rh(1), 2.244(2); Cl(1)–Rh(1)–C(1), 89.2(2); N(1)–C(1)–Rh(1)–
Cl(1), À95.1.
BF4
NH
OH
OH
(i)
1) HBF4
N
O
H
2)(EtO)3CH
OH
1
Scheme 1. Synthesis of benzoxazolium salt 1 from 3,5-di-tert-butylcatechol. Reaction conditions: (i) aniline, NEt3, heptane, reflux [13].

32
S. Bellemin-Laponnaz / Polyhedron 29 (2010) 30–33
tBu
1
Cl
Cl
O
N
tBu
N
O
CuCl + LiOtBu
Cu
C
C
Cu
thf, r.t.
- LiBF4
tBu
tBu
3
Scheme 3. Synthesis of the copper(I) complex 3.
verified and the complex was found to be a dimer in the solid state
with bridging chlorides (see Electronic Supplementary Information
for a view).
125.1, 119.2, 107.9, 35.9 (–CMe3), 35.0 (–CMe3), 31.4 (–C(CH3)3),
29.8 (–C(CH3)3). HRMS (ESI) m/z (%): Calc. for C21H26NO ([MÀBF4]+)
308.2014, found: 308.2009 (100). Anal. Calc. for C21H26BF4NO: C,
63.82; H, 6.63; N, 3.54. Found: C, 63.98; H, 6.78; N, 3.47%.
We tested the rhodium and copper complexes for their activity
in the hydrosilylation reaction in order to evaluate their potential
in catalysis. Standardised reaction conditions were applied (i.e.
acetophenone, Ph2SiH2, THF and CH2Cl2 as solvent, 1 mol% of pre-
catalyst). The conversions were found to be below 2% after 48 h
with both complexes, which is in contrast with NHC systems that
usually display good activity under the same experimental condi-
tions [15,16]. Although no black solids were observed, it appeared
that the solutions gradually turned deep dark, which suggest us
that a slow decomposition of the complexes occurs.
In summary, we have illustrated that the benzoxazolium salt 1
is a suitable precursor for the synthesis of N,O-heterocyclic carbene
complexes. Its coordination capabilities have been established in
the synthesis of rhodium(I) and copper(I) complexes. Preliminary
results in hydrosilylation of ketones indicate that such systems ex-
hibit low catalytic activity. Future work will explore the potential
of this class of ligands with other transition metals (i.e. Pd, Pt).
3.2.2. [(C21H26NO)Rh(COD)Cl] (2)
In a glove box, a THF solution (5 mL) of [Rh(cod)Cl]2 (100.0 mg,
0.20 mmol) and the benzoxazolium salt 1 (160.0 mg, 0.40 mmol)
was precooled at À35 °C. tBuOK (48.0 mg, 0.42 mmol) was then
added in one portion. The reaction mixture was stirred overnight
at room temperature to yield a yellow solution, which was filtered
over celite and evaporated to dryness. The product was recrystal-
lised from THF/pentane layering to give 2 as yellow crystals in
91% yield (210 mg, 0.38 mmol). 1H NMR (300 MHz, CDCl3):
d = 8.10 (m, 2H), 7.68 (m, 3H), 7.26 (d, J = 1.8 Hz, 1H), 7.05 (d,
J = 1.8 Hz, 1H), 5.13 (m, 2H, CHcod), 3.35 (m, 2H, CHcod), 2.12 (m,
4H, CH2), 1.90 (m, 4H, CH2), 1.56 (s, 9H, tBu), 1.30 (s, 9H, tBu).
13C NMR (75 MHz, CDCl3): d 214.4 (d, J = 57.1 Hz, NCO), 149.9,
148.9, 135.6, 135.0, 132.2, 129.5, 129.4, 126.9, 119.1, 105.3, 102.4
(d, J = 6.2 Hz), 69.3 (d, J = 14.9 Hz), 35.3, 34.7, 32.6 (CH2(cod)), 31.7
(C(CH3)3), 30.0 (C(CH3)3), 28.5 (CH2(cod)). HRMS (ESI) m/z: Calc. for
C29H37NORh ([MÀCl]+) 518.1930, found: 518.1926. Anal. Calc. for
C29H37ClNORh: C, 62.88; H, 6.73; N, 2.53. Found: C, 63.02; H,
6.79; N, 2.45%.
3. Experimental
3.1. General procedures
All experiments were carried out under N2 using standard
Schlenk techniques or in a Mbraun Unilab glovebox. THF, dichloro-
methane and pentane were first dried through a solvent purifica-
tion system (MBraun SPS) and stored for at least a couple of days
over activated molecular sieves (4 Å) in a glovebox prior to use.
CD2Cl2 and C6D6 were purchased from Eurisotope (CEA, Saclay,
France), degassed under a N2 flow and stored over activated molec-
ular sieves (4 Å) in a glovebox prior to use. 3,5-Di-tert-butyl-2-hy-
droxy-N-phenylaniline was synthesised according to literature
procedures. NMR spectra were recorded on Bruker AC 300 MHz
NMR spectrometers at ambient temperature. 1H and 13C chemical
shifts are reported versus SiMe4 and were determined by reference
to the residual 1H and 13C solvent peaks. Elemental analysis and
mass analysis were performed by the Service de Microanalyse
and Service de masse of Université de Strasbourg.
3.2.3. [(C21H26NO)CuCl] (3)
In a glove box, a THF solution (5 mL) of CuCl (40.0 mg,
0.4 mmol) and LiOtBu (33.0 mg, 0.41 mmol) was stirred at room
temperature. After 12 h, the resulting yellow solution was pre-
cooled at À35 °C prior to addition of a solution of 160 mg of ben-
zoxazolium 1 dissolved in 2 mL of THF. After stirring overnight,
the colourless solution was filtered over celite and the solvent
was removed under vacuum. Toluene (5 mL) was added and the
solution was filtered again over celite. Evaporation of the solvent
and recristallisation from THF/pentane afforded 3 as colourless
crystalline solid (104 mg, 0.26 mmol, 65% yield). 1H NMR
(300 MHz, CD2Cl2): d = 7.69 (m, 5H), 7.48 (d, J = 1. Hz), 7.21 (d,
J = 1.8 Hz), 1.56 (s, 9H), 1.33 (s, 9H). 13C NMR (75 MHz, CD2Cl2): d
200.0 (NCO), 150.6, 148.4, 136.1, 134.4, 131.1, 130.4, 125.8,
121.3, 106.5, 35.3 (C(CH3)3), 34.6 (C(CH3)3), 31.2 (C(CH3)3), 29.6
(C(CH3)3). Anal. Calc. for C21H25ClCuNO: C, 62.06; H, 6.20; N, 3.45.
Found: C, 62.34; H, 6.45; N, 3.52%. Note: We have been unable to
obtain satisfactory crystal for X-ray diffraction studies. However,
the basic connectivities were verified and the complex was found
3.2. Preparation of the compounds
3.2.1. 5,7-Di-tert-butyl-3-phenyl benzoxazolium tetrafluoroborate (1)
HBF4 (1.39 mL, 48% w/w) was added dropwise to a solution of
3,5-di-tert-butyl-2-hydroxy-N-phenylaniline (3.17 g, 10.6 mmol)
in 50 mL of MeOH. After 30 min of stirring, the solvent was re-
moved under vacuum and (EtO)3CH (30 mL) was added. The result-
ing solution was stirred at room temperature under N2 overnight
to give a white suspension. The solid was filtered, washed with
diethyl ether and dried in vacuo, giving 2.52 g of 1 (60% yield).
1H NMR (300 MHz, CDCl3): d 10.25 (s, 1H), 7.86–7.72 (m, 6H),
7.36 (s, 1H), 1.58 (s, 9H), 1.38 (s 9H). 13C{1H} NMR (75 MHz, CDCl3):
d 155.4 (NCHO), 153.7, 146.1, 138.2, 132.3, 131.0, 130.2, 129.1,
to be
Electronic supporting information displays
molecule (C21H25ClCuNO, space group: P21/c, a = 11.90930(10),
a
dimer in the solid state with bridging chlorides.
a
view of the
b = 14.1890(2),
c = 23.8492(4),
b = 89.9970,
cell
volume:
4030.06 Å3).
3.3. Crystal structure determination
Single crystals of the compounds 1, 2 and 3 were mounted on a
Nonius Kappa-CCD area detector diffractometer (Mo
Ka

S. Bellemin-Laponnaz / Polyhedron 29 (2010) 30–33
33
Table 1
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk
Details of the crystal structure determinations of the compounds 1 and 2.
1
2
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2009.05.067.
Formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
C21H26NOBF4
395.24
C33H45ClNO2Rh
626.06
monoclinic
P21=c
9.3524(2)
12.1837(3)
27.1533(4)
100.7890(10)
triclinic
ꢀ
P1
References
10.7077(6)
15.5708(11)
16.1510(13)
80.737(4)
89.455(4)
2524.0(3)
4
1.040
0.083
À13 to 13, À15 to 20,
À20 to 20
1.3–27.4
173(2)
832
40 530
11 392, 0.0755
11 392/0/512
1.057
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)
0.679
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set) h, k, l
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T (K)
F000
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À13 to 9, À15 to 17,
À38 to 34
1–30.0
173(2)
1312
23 030
8418, 0.05
8418/0/350
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(F)]
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1.645 and À1.111
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Appendix A. Supplementary data
CCDC 723327 and 723328 contain the supplementary crystallo-
graphic data for 1 and 2. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the