Thiazole-annelated imidazolium salts: A new architecture for N-heterocyclic carbenes

Article abstract of DOI:10.1055/s-2008-1067147

A short synthesis of thiazole-annelated imidazolium salts is presented. Deprotonation of these salts allowed the formation of new N-heterocyclic carbenes (NHCs) and the investigation of their respective electronic properties, which were determined by X-ray crystal structure analysis and IR spectroscopy of the corresponding iridium carbonyl complexes. Finally, the ability of these NHCs to form the corresponding palladium complexes was demonstrated. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1067147

PAPER
2221
Thiazole-Annelated Imidazolium Salts: A New Architecture for
N-Heterocyclic Carbenes
T
hiazole-Annelat
l
ed Imid
a
azolium Sa
u
lts dia Lohre, Roland Fröhlich, Frank Glorius*
Institute of Organic Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany
Fax +49(251)8333202; E-mail: glorius@uni-muenster.de
Received 5 May 2008
Dedicated to Professor Reinhard W. Hoffmann on the occasion of his 75th birthday
Based on the different geometry of five-membered com-
pared to six-membered rings, the steric demand of car-
benes 2 should lie between those of monocyclic NHCs
like IMes and imidazo[1,5-a]pyridine-3-ylidenes (1).
Abstract: A short synthesis of thiazole-annelated imidazolium
salts is presented. Deprotonation of these salts allowed the forma-
tion of new N-heterocyclic carbenes (NHCs) and the investigation
of their respective electronic properties, which were determined by
X-ray crystal structure analysis and IR spectroscopy of the corre-
sponding iridium carbonyl complexes. Finally, the ability of these
NHCs to form the corresponding palladium complexes was demon-
strated.
To the best of our knowledge, fully unsaturated five-ring
annelated NHCs (2) have not previously been made. In
this communication, we report the first synthesis and in-
vestigation of NHCs of type 2 with X = S.
Key words: N-heterocyclic carbene (NHC), imidazolium salt, thia-
zole, NHC–metal complex
X
R3
N
N
N
N
N
N
R1
R1
Mes
Mes
R2
R2
The development of N-heterocyclic carbenes (NHCs) and
their application in transition-metal catalysis, organoca-
talysis and other areas of chemistry has already been a tre-
mendous success story.1,2,3 Many favorable characteristics
render NHCs to be attractive ligands for catalysis.1 First of
all, they are electron-rich donor ligands and generally
form intriguingly stable complexes with many metals. In
addition, most NHCs successfully employed in catalysis
can be considered to be sterically demanding. Moreover,
the shape of NHCs differs from other ligands, and mono-
dentate NHCs can easily influence the metal’s coordina-
tion sphere. Interestingly, it has been found that the steric
demand of NHCs can more easily be tuned than their elec-
tronic character.1b
IMes
2
1
(X = S: this paper)
Figure 1 Structural comparison of monocyclic NHC IMes and bi-
cyclic annelated NHCs 1 and 2
We selected the thiazole-annelated system (X = S), since
sulphur has been shown to exhibit interesting carbene sta-
bilizing properties in thiazolium salt derived NHCs.2
Thus, the symmetrical imidazolium salt 6 was selected as
the initial target. Our synthetic endeavour commenced
with the synthesis of bisthiazole 5, easily available by re-
action of dithiooxalamide (3) and a-bromopinacolone (4)
in refluxing ethanol (Scheme 1).7 However, treatment of 5
under conditions developed for the formation of imidazo-
lium salts from bisimines or bioxazolines failed to provide
any significant amounts of 6. Interestingly, N,O-acetal 7
formed in high yield and was unequivocally characterized
by single-crystal X-ray structural analysis (Figure 2).8
Unfortunately, 7 could not be transformed into the desired
imidazolium salt 6 by either heating in tetrahydrofuran
Arguably, 1,3-disubstituted imidazolylidenes IMes, IiPr
and their saturated analogues are the most commonly ap-
plied NHCs in transition-metal catalysis (Figure 1). Still,
in many cases, other NHCs are optimal and the design and
synthesis of NHCs with new electronic and steric proper-
ties is highly desirable and should lead to an increasing
number of applications in catalysis.4 Recently, we and
others designed a class of annelated NHCs 1 and we suc-
cessfully applied them in Suzuki–Miyaura cross-coupling
reactions.5,6 As a result of the bicyclic structure of 1, the
R2 substituent is placed in close proximity to a carbene-
bound metal, thus allowing significant shielding of the
metal (Figure 1). Using the same substituents (R1,
R2 = Mes), the steric demand is estimated to be signifi-
cantly larger compared to IMes.
SYNTHESIS 2008, No. 14, pp 2221–2228
x
x.
x
x
.
2
0
0
8
Advanced online publication: 18.06.2008
DOI: 10.1055/s-2008-1067147; Art ID: C02008SS
© Georg Thieme Verlag Stuttgart · New York
Figure 2 Structure of 7 obtained by X-ray crystal structure analysis8

2222
C. Lohre et al.
PAPER
S
S
N
N
t-Bu
t-Bu
OTf
S
NH2
S
Br
S
N
S
N
(i)
(ii)
6
t-Bu
H2N
O
t-Bu
t-Bu
53%
89%
S
S
3
4
5
N
N
t-Bu
t-Bu
OTf
O
t-Bu
O
7
Scheme 1 Reagents and conditions: (i) EtOH, reflux, 4 h; (ii) AgOTf, chloromethyl pivalate, CH2Cl2, 40 °C, 16 h.
(80 °C) or dioxane (120 °C). In addition, activation of the and, subsequently, the AgCl precipitate was allowed to
pivalate leaving group by the Lewis acid BF3·OEt2 or by settle. The supernatant was then added to the bisimine and
the nucleophilic catalyst 4-(N,N-dimethylamino)pyridine the solution was stirred at 40 °C in the dark. Imidazolium
(DMAP) was unsuccessful (Scheme 1).
salts 10 and 13 were thus obtained in good overall yields.
It has already been noted that the counterion of imidazoli-
um salts has a dramatic influence on the solubility.1,4
Thus, despite the ionic character of the triflate products,
purification by chromatography was the method of
choice. The structure of the imidazolium salts was un-
equivocally established by X-ray structural analysis of
single crystals (Figure 3).
Since the corresponding cyclization of bisimines to imida-
zolium salts proceeds smoothly, the cyclization of the
mixed imine thiazoles 9 and 12 was then investigated
(Scheme 2). Selective deprotonation of 4-methylthiazole
(8) and benzothiazole (11) at the 2-position, was followed
by formylation with N,N-dimethylformamide (DMF) and
subsequent imine formation by heating the aldehydes in
the presence of mesityl amine. Employing a recently de- With these imidazolium salts in hand, the formation of the
veloped protocol4c resulted in the formation of the desired corresponding carbenes and their properties was investi-
cyclized imidazolium salts. Namely, chloromethyl piv- gated. First of all, the imidazolium triflates 10 and 13 were
alate and AgOTf were stirred in dichloromethane at ambi- stirred in tetrahydrofuran with potassium tert-butoxide for
ent temperature for 45 minutes with the exclusion of light five minutes, after which time elemental sulphur was add-
S
S
N
S
N
1. (i)
H
(iii)
N
N
2. (ii)
63%
Mes
N
Mes
66%
OTf
10
8
9
S
H
S
N
S
N
1. (i)
2. (ii)
81%
(iii)
N
N
Mes
N
Mes
53%
OTf
11
12
13
Scheme 2 Reagents and conditions: (i) n-BuLi, Et2O, –78 °C, 1 h; DMF, 0–20 °C, 3 h; (ii) 2,4,6-trimethylaniline, EtOH, reflux, 3 h; (iii)
AgOTf, chloromethyl pivalate, CH2Cl2, r.t.; 40 °C, 16 h.
S
S
S
(i)
(ii)
N
N
N
N
N
N
Mes
Mes
Mes
78%
(overall)
OTf
10
S
14
15
S
S
S
(i)
(ii)
N
N
N
N
N
N
Mes
Mes
Mes
83%
(overall)
OTf
S
16
13
17
Scheme 3 Reagents and conditions: (i) t-BuOK, THF, r.t., 5 min; (ii) S8, r.t., 12 h.
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York

PAPER
Thiazole-Annelated Imidazolium Salts
2223
Figure 3 Structures of 10 and 13 obtained by X-ray crystal structure analysis.8 Selected bond lengths (Å) and angles (°): 10: N5–C4, 1.406(4);
C4–C3, 1.339(5); C3–S2, 1.733(4); S2–C1, 1.729(3); C1–N5, 1.378(4); N5–C6, 1.341(4); C6–N7, 1.325(4); N7–C8, 1.384(4); C8–C1,
1.357(4); N7–C6–N5, 106.8(3); C6–N5–C1, 109.4(2); C6–N7–C8, 110.8(3); 13: C1–C2, 1.389(8); C2–C3, 1.374(7); C3–C4, 1.380(7); C4–
C5, 1.392(7); C5–C6, 1.394(7); C6–C1, 1.360(8); N7–C4, 1.410(5); C4–C5, 1.392(7); C5–S9, 1.745(5); S9–C8, 1.727(5); C8–N7, 1.390(6);
C8–C12, 1.348(7); C12–N11, 1.379(6); N11–C10, 1.333(5); C10–N7, 1.336(5); N7–C10–N11, 107.3(4); C10–N7–C8, 109.4(4); C10–N11–
C12, 109.7(4).
ed. The resulting formation of thiones 15 and 17 in good have to be measured in solution (Table 1). Comparison of
yields was a strong indication that the NHCs had formed the obtained data suggests that carbene 14, exhibiting
in situ (Scheme 3).9 More information on the free car- lower nCO and a lower nCOav, is slightly more electron-rich
benes was obtained by conducting the deprotonation in an than benzannulated carbene 16. Interestingly, 14 and the
NMR tube with perdeuterated tetrahydrofuran as the sol- well known IMes exhibit similar electronic properties.
vent. Although the 1H and 13C NMR spectra of NHCs pro- Based on work by Nolan, the nCOav can be correlated with
vide some information on the characteristics, these data Tolman’s electronic parameter {TEP; TEP (cm–1) =
did not allow a direct correlation with the electronic prop- 0.847x + 336 [x = nCOav(cm–1)]} (Table 1), in order to ob-
erties of these carbenes. The signal of the carbene carbon tain a convenient and comparable measure for the ligands’
was found at d = 207.50 ppm and at d = 202.32 ppm, for electronic character.11d
14 and 16, respectively. It is interesting to note that the
As expected, the single-crystal X-ray structure of complex
18 (Figure 4) reveals a square-planar coordination geo-
metry of the iridium. Comparison of the two iridium–CO
corresponding 13C NMR chemical shifts of most other
NHCs occur at lower field.6
Many different methods for the determination of the elec- bond lengths reveals that the Ir–C bond trans to the NHC
tronic properties of NHC ligands in transition-metal com- is significantly longer compared to the one cis to the
plexes have been applied.10 The most widely accepted NHC. The bond length of the iridium–NHC bond
methodology, however, is the IR-spectroscopic analysis [2.075(4) Å] corresponds to those known in the literature.
of the CO stretching frequencies of (L)Ni(CO)3,
(L)Rh(CO)2Cl, (L)Ir(CO)2Cl or related complexes.11,12
S
S
s-Donor ligands (L) lead to an increased electron-density
on the metal center that causes stronger back-bonding
from the transition-metal to the p* of the CO-ligands and,
thus, a weakening of the CO bond, which is detectable by
IR-spectroscopy. However, care has to be taken in the
comparison, since not only have different transition met-
als been used, but also different vibrational frequencies
were taken into account (frequency of the trans-CO ligand
vs. averaged frequency of all CO ligands).12 Since an ex-
tensive range of reference data exists for (NHC)Ir(CO)2Cl
complexes,7,11d the corresponding iridium complexes of
carbenes 14 and 16 were therefore prepared.
1. (i)
N
N
N
N
Mes
Mes
2. (ii)
OTf
Cl Ir CO
CO
41%
10
18
S
S
1. (i)
N
N
N
N
2. (ii)
Mes
Mes
OTf
13
Cl Ir CO
62%
CO
19
The imidazolium salts 10 and 13 were deprotonated with
potassium tert-butoxide and the generated carbenes were
transferred to [Ir(COD)Cl]2 to give air-stable intermedi-
ates. The COD/CO-ligand exchange was realized by bub-
bling a slow stream of carbon monoxide through a
solution of the COD complex in dichloromethane at 0 °C,
resulting in the formation of dicarbonyl complexes 18 and
19, respectively (Scheme 4). It is important to note that for
reasons of comparability, the IR spectra (and thus the nCO)
Scheme
4
Reagents and conditions: (i) t-BuOK, THF,
[Ir(COD)Cl]2, r.t., 16 h; (ii) CO, CH2Cl2, 0 °C.
The importance of palladium-NHC complexes in cataly-
sis has been amply demonstrated.1b Thus, we explored the
ability of the new NHC 14 to form a stable palladium
complex. We decided to synthesize a palladium–PEPPSI
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York

2224
C. Lohre et al.
PAPER
Figure 4 Structure of 18 obtained by X-ray crystal structure analy-
sis.8 Selected bond lengths (Å) and angles (°): C1–C2, 1.345(6); C1–
S5, 1.749(5); S5–C4, 1.739(5); C4–N3, 1.391(5); N3–C2, 1.411(5);
C4–C8, 1.352(7); C8–N7, 1.390(6); N7–C6, 1.360(5); C6–N3,
1.367(6); Ir–C12 (cis to NHC), 1.855(6); Ir–C13 (trans to NHC),
1.880(5); Ir–Cl14, 2.3479(13); Ir–C6, 2.075(4); C6–Ir–C13,
174.6(2); C6–Ir–C12, 92.60(19); C6–Ir–Cl14, 87.19(12).
Figure 5 Structure of 20 obtained by X-ray crystal structure analy-
sis.8 Selected bond lengths (Å) and angles (°) for molecule A: C4–S5,
1.740(6); S5–C1, 1.736(7); C1–C2, 1.336(9); C2–N3, 1.413(8); N3–
C6, 1.360(8); C6–N7, 1.354(8); N7–C8, 1.407(7); C8–C4, 1.330(9);
C4–N3, 1.397(8); C6–Pd, 1.962(2); Pd–Cl1, 2.307(2); Pd–N12,
2.103(5); C6–Pd–N12, 177.5(3); C6–Pd–Cl1, 87.5(2).
Table 1 IR Data of Complexes 18 and 19 and the Corresponding
IMes Complex Obtained by Measurement in Dichloromethane Solu-
tion
Complex
18
n
CO (cm–1)
n
CO (cm–1)
nCOav (cm–1) TEPa (cm–1)
2065.8
2069.0
2066.4
1982.3
1985.9
1979.8
2024.1
2027.4
2023.1
2050.4
2053.2
2049.6b
In conclusion, a new class of thiazole-annelated NHCs
was synthesized in a few steps and their electronic prop-
erties were characterized by several methods. For the first
time, NHCs with a fully conjugated bicyclic [3.3.0] back-
bone have been prepared. The catalytic properties of these
NHCs in organocatalysis and transition-metal catalysis
are under investigation and will be reported in due course.
19
IMes
a TEP value calculated by linear regression on the basis of the
Ir(CO)2Cl(NHC) complexes.11d
b The real TEP value derived from Ni(CO)3IMes was found to be
2050.7.3b
All commercially available compounds were used as received and
all reactions were conducted in dried glassware equipped with a
magnetic stirring bar under an atmosphere of argon. Solvents were
distilled or dried prior to use over the indicated drying agents:
CH2Cl2 (CaH), THF (sodium/benzophenone), DMF (p.a. grade
dried, over molecular sieves 4 Å), hexane (degassed, HPLC grade
without further purification).
complex (PEPPSI = Pyridine-Enhanced Precatalyst Prep-
aration, Stabilization and Initiation), since PEPPSI com-
plexes are known to be good precatalysts that easily enter
the catalytic cycle.13 Heating the imidazolium salt 10 in
the presence of PdCl2 and K2CO3 in neat 3-chloropyridine
gave access to the desired complex 20 (Scheme 5). Single
crystals were grown and the identity of 20 was unequivo-
cally determined by X-ray structural analysis (Figure 5).
In the structure obtained, the palladium exhibits a square-
planar geometry and the resulting coordination plane of
the palladium is virtually orthogonal to the plane of the
NHC bicycle. The palladium–carbene bond length
[1.962(2) Å] has a value typical for palladium–NHC com-
plexes.
Column chromatography was performed on Merck silica gel 60
(0.040–0.063 mm). 1H and 13C NMR spectra were recorded on ei-
ther an ARX 300 or an AMX 400 spectrometer (Bruker). Standards
for the calibration of the chemical shifts were residual protonated
CHCl3 (1H: d = 7.26 ppm, 13C: d = 77.0 ppm) or THF (1H: d = 1.73
ppm, 13C: d = 25.5 ppm) unless otherwise noted. IR spectra were re-
corded on a FT-IR 3100 Excalibur Series (Varian Associated).
HRMS were recorded either on MicroTof (Bruker), a Finnigan LTQ
FT or TSQ 700. Elemental analyses were recorded on Vario EL III
(Elementar Analysensystem GmbH).
S
4-Methylthiazole-2-carbaldehyde
To a solution of compound 8 (500 mg, 5.00 mmol, 1.0 equiv) in an-
hydrous THF (20 mL) was added, dropwise, n-BuLi (3.75 mL, 1.6
M in hexane, 6.00 mmol, 1.2 equiv) at –78 °C. The solution was
stirred for 1 h then DMF (0.77 mL, 10.00 mmol, 2.0 equiv) was add-
ed. After 3 h stirring (–78 °C to 20 °C), the mixture was acidified
with aq HCl (4 M, 15 mL). The organic phase was separated and ex-
tracted with aq HCl (4 M, 3 × 20 mL), the aqueous phase was made
basic with K2CO3 and extracted with CH2Cl2 (3 × 20 mL). The com-
bined organic phases were dried over Na2SO4. After removal of the
N
N
S
Mes
(i)
Cl Pd Cl
N
N
N
Mes
49%
OTf
Cl
20
10
Scheme 5 Reagents and conditions: (i) K2CO3, PdCl2, 3-chloropy-
ridine, 80 °C, 16 h.
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York

PAPER
Thiazole-Annelated Imidazolium Salts
2225
solvent in vacuo, the residue was purified by column chromatogra-
Anal. Calcd for C17H16N2S: C, 72.82; H, 5.75; N, 9.99. Found: C,
72.79; H, 5.79; N, 9.75.
phy (n-pentane–EtOAc, 5:1) to give the desired product.14
Yield: 90%; yellow solid; Rf = 0.43 (n-pentane–EtOAc, 1:1).
Synthesis of Imidazolium Triflates; General Procedure
To a suspension of AgOTf (1.5 equiv) in CH2Cl2 (0.3 M) was added
chloromethyl pivalate (1.5 equiv), and the resulting suspension was
stirred for 45 min in the dark. The supernatant was transferred via
syringe to the bisimine (1 equiv), and the resulting solution was
stirred in a sealed tube in the dark at 40 °C for 16 h. The solution
was cooled to r.t., quenched with MeOH (5 mL/mmol) and the sol-
vent was evaporated in vacuo. The resulting oil was purified by
chromatography on silica gel (CH2Cl2–MeOH, 98:2) to give the im-
idazolium triflate as yellow crystals.7
IR (ATR): 3106, 2926, 2848, 2670, 1686, 1503, 1442, 1299, 1233,
964, 868, 742, 657, 632, 553 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.59 (s, 3 H, CH3), 7.34 (s, 1 H,
ArH), 9.98 (s, 1 H, CHO).
13C NMR (75.5 MHz, CDCl3): d = 17.0 (CH3), 121.0 (CAr), 156.3
(CAr), 165.2 (CAr), 183.8 (CHO).
HRMS (ESI): m/z [M + Na]+ calcd for C5H5NNaOS+: 149.9990;
found: 149.9992.
Formation of Bisimines; General Procedure
4,4¢-Di-tert-butyl-3-(2,2-dimethylpropionyloxymethyl)-
[2,2¢]bithiazol-3-ylium Trifluoromethanesulfonate (7)
Yield: 89%; pale-yellow crystals; Rf = 0.24 (CH2Cl2–MeOH, 10:1).
Carbaldehyde (1 equiv) and 2,4,6-trimethylaniline (2 equiv) were
dissolved in EtOH (3 mL per mmol) and heated to reflux for 3 h.
The solvent was removed in vacuo and the residue was purified by
column chromatography (n-pentane–EtOAc–Et3N, 100:10:1) to
give the desired product.15
IR (ATR): 3121, 3100, 2970, 2940, 2909, 2874, 2348, 1740, 1546,
1480, 1355, 1225, 1194, 1162, 1120, 1077, 1029, 1015, 1003, 912,
871, 821, 636 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.11 (s, 9 H, tBu), 1.40 (s, 9 H,
tBu), 1.57 (s, 9 H, tBu), 1.85 (s, 2 H, CH2), 7.55 (s, 1 H, ArH), 8.28
(s, 1 H, ArH).
4,4¢-Di-tert-butyl[2,2¢]bithiazolyl (5)
Yield: 53%; pale-yellow crystals; Rf = 0.70 (n-pentane–EtOAc,
4:1).
13C NMR (100 MHz, CDCl3): d = 26.8 (tBu), 30.1 (tBu), 30.3 (tBu),
35.5 (Cq), 35.7 (Cq), 38.9 (Cq), 72.5 (NCH2O), 120.0 (CAr), 122.1
(CAr), 150.0 (CAr), 158.7 (CAr), 166.0 (CAr), 171.5 (CAr), 176.1
(C=O).
IR (ATR): 3119, 2970, 2874, 2357, 1739, 1701, 1545, 1481, 1369,
1260, 1225, 1133, 1029, 912, 821, 636, 618, 574 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.37 (s, 18 H, t-Bu), 6.95 (s, 2 H,
ArH).
13C NMR (75.5 MHz, CDCl3): d = 30.0 (t-Bu), 34.9 (t-Bu), 117.0
(CAr), 158.6 (CAr), 165.6 (N=C).
+
HRMS (ESI): m/z [M – OTf–]+ calcd for C20H31N2O2S2 : 395.1821;
found: 395.1819.
+
HRMS (ESI): m/z [M + H]+ calcd for C14H21N2S2 : 281.1141;
found: 281.1144.
3-Methyl-6-(2,4,6-trimethylphenyl)-6H-imidazo[5,1-b]thiazol-
4-ylium Trifluoromethanesulfonate (10)
Yield: 66%; pale-yellow crystals; Rf = 0.39 (CH2Cl2–MeOH, 10:1).
[(E)-1-(4-Methylthiazol-2-yl)methylidene]-2,4,6-trimethylphe-
nylamine (9)
Yield: 63%; yellow solid; Rf = 0.63 (n-pentane–EtOAc, 1:1).
IR (ATR): 3130, 1547, 1524, 1485, 1388, 1266, 1250, 1222, 1188,
1158, 1028, 861, 782, 661, 635 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.04 (s, 6 H, o-Me), 2.34 (s, 3 H,
p-Me), 2.67 (s, 3 H, Me), 7.00 (s, 2 H, 2 × ArH), 7.10 (s, 1 H, ArH),
7.47 (s, 1 H, ArH), 9.58 (s, 1 H, NCHN).
13C NMR (100 MHz, CDCl3): d = 12.9 (Me), 17.3 (Me), 21.2 (Me),
113.0 (NCHN), 117.1 (CAr), 122.9 (CAr), 126.6 (CAr), 126.9 (CAr),
129.7 (CAr), 131.2 (CAr), 134.2 (CAr), 141.6 (CAr).
IR (ATR): 2954, 2922, 2853, 1627, 1590, 1456, 1376, 1316, 1258,
1204, 1014, 852, 823, 759, 729, 670, 632, 568 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.15 (s, 6 H, o-Me), 2.28 (s, 3 H,
p-Me), 2.54 (s, 3 H, N-Me), 6.89 (s, 2 H, ArH), 7.07 (s, 1 H, ArH),
8.35 (s, 1 H, N=CH).
13C NMR (100 MHz, CDCl3): d = 17.3 (Me), 18.5 (Me), 21.0 (Me),
117.5 (CAr), 122.0 (CAr), 127.4 (CAr), 129.0 (CAr), 129.1 (CAr), 134.2
(CAr), 147.1 (CAr), 154.8 (CAr), 156.3 (CAr), 166.6 (N=CH).
HRMS (ESI): m/z [M – OTf–]+ calcd for C15H17N2S+: 257.1107;
found: 257.1109.
HRMS (ESI): m/z [M + Na]+ calcd for C14H16N2NaS+: 267.0932;
found: 267.0930.
2-(2,4,6-Trimethylphenyl)-2H-benzo[d]imidazo-[5,1-b]thiazol-
9-ylium Trifluoromethanesulfonate (13)
Yield: 53%; yellow crystals; Rf = 0.36 (CH2Cl2–MeOH, 10:1).
[(E)-1-(Benzothiazol-2-yl)methylidene]-2,4,6-trimethylphenyl-
amine (12)
Yield: 81%; yellow solid; Rf = 0.85 (n-pentane–EtOAc, 1:1).
IR (ATR): 3316, 3055, 2989, 2952, 2877, 2792, 1667, 1579, 1467,
1393, 1290, 1265, 1182, 896, 736, 706, 629, 584 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.11 (s, 6 H, o-Me), 2.37 (s, 3 H,
p-Me), 7.05 (s, 2 H, 2 × ArH), 7.10 (s, 1 H, ArH), 7.40 (d, J = 8.3
Hz, 1 H, ArH), 7.64–7.68 (m, 1 H, ArH), 7.78–7.81 (m, 1 H, ArH),
8.68–8.74 (m, 1 H, ArH), 10.47 (s, 1 H, NCHN).
13C NMR (100 MHz, CDCl3): d = 17.3 (Me), 21.4 (Me), 114.3
(NCHN), 117.7 (CAr), 122.8 (CAr), 124.1 (CAr), 128.0 (CAr), 129.3
(CAr), 129.7 (CAr), 129.8 (CAr), 130.1 (CAr), 131.9 (CAr), 132.5 (CAr),
134.2 (CAr), 141.6 (CAr).
IR (ATR): 3371, 3007, 2968, 2914, 2856, 2732, 2361, 1628, 1807,
1468, 1443, 1307, 1251, 1208, 1156, 1010, 854, 877, 744 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.20 (s, 6 H, o-Me), 2.31 (s, 3 H,
p-Me), 6.93 (s, 2 H, ArH), 7.48–7.55 (m, 3 H, ArH), 7.96 (d, J = 8.2
Hz, 1 H, ArH), 8.54 (s, 1 H, N=CH).
13C NMR (75.5 MHz, CDCl3): d = 18.3 (Me), 20.82 (Me), 122.8
(CAr), 124.3 (CAr), 126.6 (CAr), 127.3 (CAr), 127.6 (CAr), 129.7 (CAr),
132.1 (CAr), 135.3 (CAr), 136.3 (CAr), 147.3 (CAr), 154.5 (CAr), 157.5
(CAr), 168.2 (N=CH).
HRMS (ESI): m/z [M – OTf–]+ calcd for C18H17N2S+: 293.1107;
found: 293.1107.
HRMS (ESI): m/z [M + H]+ calcd for C17H17N2S+: 281.1112; found:
281.1111.
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York

2226
C. Lohre et al.
PAPER
Formation of Free Carbenes; General Procedure
tinued overnight. The solvent was evaporated and the residue was
purified by chromatography on silica gel (n-pentane–EtOAc, 10:1)
to give the corresponding product.
To a mixture of the imidazolium salt 10 or 13 (1.0 equiv) and
t-BuOK (1.10 equiv) in a standard NMR tube was added THF-d8
(1 mL per 0.1 mmol) affording the free carbenes in solution.
[(NHC)Ir(COD)Cl]
Yield: 89%; yellow solid; Rf = 0.70 (n-pentane–EtOAc, 1:1).
Carbene 14
1H NMR (300 MHz, THF-d8): d = 1.97 (s, 6 H, o-Me), 2.30 (s, 3 H,
p-Me), 2.47 (s, 3 H, Me), 6.52 (s, 1 H, ArH), 6.94 (s, 3 H, ArH).
IR (ATR): 2915, 2877, 2830, 1734, 1608, 1558, 1488, 1360, 1327,
1241, 1159, 1038, 1000, 971, 884, 852, 817, 735, 644, 611 cm–1.
13C NMR (75.5 MHz, THF-d8): d = 13.9 (Me), 18.1 (Me), 21.3
(Me), 109.0 (CAr), 111.5 (CAr), 128.3 (CAr), 129.5 (CAr), 130.3 (CAr),
133.8 (CAr), 135.8 (CAr), 138.2 (CAr), 140.4 (CAr), 207.5.
1H NMR (400 MHz, THF-d8): d = 1.10–1.79 [m, 2 H, (CH2)COD],
2.01–2.25 [m, 6 H, (CH2)COD], 2.28 (s, 6 H, o-Me), 3.17–3.26 (m,
2 H, CHCOD), 3.31 (s, 3 H, p-Me), 4.09 (s, 3 H, Me), 4.38–4.54 (m,
2 H, CHCOD), 6.39 (s, 1 H, ArH), 6.83 (s, 1 H, ArH), 6.93 (s, 1 H,
ArH), 7.05 (s, 1 H, ArH).
13C NMR (75.5 MHz, CDCl3): d= 17.9, 19.6, 21.1, 28.2, 30.1, 50.6,
81.6, 109.7, 112.5, 127.9, 129.5, 132.8, 134.2, 138.0, 138.8, 171.5
(NCIrCl).
Carbene 16
1H NMR (300 MHz, THF-d8): d = 1.98 (s, 6 H, o-Me), 2.25 (s, 3 H,
p-Me), 6.92 (s, 2 H, 2 × ArH), 7.33–7.44 (m, 2 H, ArH), 7.85–7.92
(m, 2 H, ArH), 8.04 (s, 1 H, ArH).
13C NMR (75.5 MHz, THF-d8): d = 18.6 (Me), 21.2 (Me), 111.1
(CAr), 122.9 (CAr), 124.2 (CAr), 126.3 (CAr), 126.9 (CAr), 130.6 (CAr),
137.1 (CAr), 137.3 (CAr), 137.9 (CAr), 139.4 (CAr), 153.7 (CAr),
163.73 (CAr), 166.84 (CAr), 202.32.
HRMS (ESI): m/z [M – Cl–]+ calcd for C23H28IrN2S+: 557.1596;
found: 557.1592.
[(NHC)Ir(COD)Cl]
Yield: 58%; yellow solid; Rf = 0.69 (n-pentane–EtOAc, 1:1).
IR (ATR): 3054, 2994, 2936, 2833, 2816, 1264, 759, 735, 631 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.18–1.26 [m, 2 H, (CH2)COD],
1.36–1.88 [m, 6 H, (CH2)COD], 1.98 (s, 6 H, o-Me), 2.39 (s, 6 H, p-
Me), 2.78–2.86 [m, 2 H, CHCOD], 3.19–3.22 (m, 2 H, CHCOD), 6.87
(s, 1 H, ArH), 6.90 (s, 1 H, ArH), 7.02 (s, 1 H, ArH), 7.38–7.52 (m,
1 H, ArH), 7.54–7.59 (m, 2 H, ArH), 9.92 (d, J = 7.8 Hz, 1 H, ArH).
Synthesis of Thiourea Derivatives; General Procedure
A suspension of compound 10 or 13 (1.0 equiv) in THF (1.00 mL
per 0.1 mmol) was treated with t-BuOK (1.4 equiv). After stirring
for 5 min, elemental sulphur (S8, 1.4 equiv) was added and stirring
was continued overnight. The solvent was removed in vacuo and the
residue was purified by flash chromatography (n-pentane–EtOAc,
10:1) affording thiones 15 or 17 as yellow solids.9
13C NMR (100 MHz, CDCl3): d = 17.7, 19.3, 20.9, 28.2, 29.9, 31.7,
34.7, 51.0, 52.3, 83.0, 84.9, 112.6, 119.7, 123.2, 126.3, 128.2,
129.4, 129.8, 131.7, 134.3, 136.9, 139.2, 174.40 (C-Ir).
3-Methyl-6-(2,4,6-trimethylphenyl)-6H-imidazo[5,1-b]thiazole-
5-thione (15)
Yield: 78%; yellow solid; Rf = 0.73 (n-pentane–EtOAc, 1:1).
HRMS (ESI): m/z [M – Cl–]+ calcd for C26H28ClIrN2S+: 593.1602;
found: 593.1596.
IR (ATR): 2987, 2957, 2878, 2392, 2348, 1669, 1472, 906, 730, 705
cm–1.
1H NMR (400 MHz, CDCl3): d = 2.07 (s, 6 H, o-Me), 2.34 (s, 3 H,
p-Me), 2.97 (s, 3 H, Me), 6.18 (s, 1 H, ArH), 6.58 (s, 1 H, ArH),
6.98 (s, 2 H, ArH).
13C NMR (75.5 MHz, THF-d8): d = 15.6 (Me), 18.6 (Me), 21.5
(Me), 107.7 (CAr), 109.3 (CAr), 129.1 (CAr), 129.5 (CAr), 129.8 (CAr),
133.9 (CAr), 137.0 (CAr), 139.6 (CAr), 158.9 (C=S).
Formation of [(NHC)Ir(CO)2Cl] Complexes; General Proce-
dure
CO gas (balloon) was passed through an ice-cold solution of the
complex 21 or 22 in CH2Cl2 (5 mL) for 15 min. Solvent was evap-
orated and the residue was washed several times with cold hexane
to afford the complex.11c
+
HRMS (ESI): m/z [M + Na]+ calcd for C15H16N2NaS2 : 311.0653;
[(NHC)Ir(CO)2Cl] (18)
Yield: 41%; yellow solid; Rf = 0.76 (CH2Cl2–MeOH, 10:1).
found: 311.0651.
2-(2,4,6-Trimethylphenyl)-2H-benzo[d]imidazo[5,1-b]thiazole-
1-thione (17)
Yield: 83%; pale-yellow solid; Rf = 0.69 (n-pentane–EtOAc, 1:1).
IR (ATR): 3095, 2923, 2858, 2056, 1971, 1609, 1552, 1489, 1435,
1385, 1339, 1260, 1187, 1160, 1093, 1042, 854, 819, 741, 662, 615
cm–1.
IR (solution in CH2Cl2): 2065.81, 1982.33 (C=O) cm–1.
IR (ATR): 3052, 2985, 2876, 2360, 1613, 1508, 1420, 1265, 737,
563, 561, 551 cm–1.
1H NMR (400 MHz, THF-d8): d = 2.03 (s, 6 H, o-Me), 2.28 (s, 3 H,
p-Me), 6.61 (s, 1 H, ArH), 6.94 (s, 2 H, 2 × ArH), 7.28–7.38 (m,
3 H, ArH), 7.48 (dd, J = 17.6, 8.1 Hz, 1 H, ArH).
13C NMR (100 MHz, CDCl3): d = 18.4 (Me), 22.1 (Me), 109.3
(CAr), 117.0 (CAr), 124.0 (CAr), 125.8 (CAr), 126.4 (CAr), 126.9 (CAr),
129.8 (CAr), 132.5 (CAr), 135.0 (CAr), 135.7 (CAr), 137.1 (CAr), 139.8
(CAr), 157.5 (C=S).
1H NMR (400 MHz, CDCl3): d = 2.06 (s, 6 H, o-Me), 2.40 (s, 3 H,
p-Me), 3.01 (s, 3 H, Me), 6.61 (s, 1 H, ArH), 7.02 (s, 1 H, ArH),
7.04 (s, 2 H, ArH).
13C NMR (100 MHz, CDCl3): d = 17.9 (Me), 18.8 (Me), 21.4 (Me),
111.8 (CAr), 112.9 (CAr), 130.0 (CAr), 131.4 (CAr), 131.6 (CAr), 132.3
(CAr), 134.0 (CAr), 136.0 (CAr), 140.0 (CAr), 165.5 (C=O), 167.9
(C=O), 180.6 (C-Ir).
HRMS (ESI): m/z [M – Cl–]+ calcd for C17H16IrN2O2S+: 505.0562;
found: 505.0566.
+
HRMS (ESI): m/z [M + Na]+ calcd for C18H16N2NaS2 : 347.0647;
found: 347.0649.
[(NHC)Ir(CO)2Cl] (19)
Yield: 62%; yellow solid; Rf = 0.82 (CH2Cl2–MeOH, 10:1).
Synthesis of [(NHC)Ir(COD)Cl] Complexes; General Proce-
dure
A suspension of compound 10 or 13 (1.8 equiv) in THF (1 mL per
0.1 mmol) was treated with t-BuOK (2.0 equiv). After stirring for
10 min, [Ir(COD)Cl]2 (1.0 equiv) was added and stirring was con-
IR (ATR): 3687, 3599, 3164, 3055, 2982, 2957, 2855, 2926, 2534,
2348, 2278, 2068, 1985, 1671, 1590, 1251, 1277, 1243, 1137, 895,
651 cm–1.
IR (solution in CH2Cl2): 2068.95, 1985.93 (C=O) cm–1.
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York

PAPER
Thiazole-Annelated Imidazolium Salts
2227
1H NMR (400 MHz, CDCl3): d = 1.98 (s, 6 H, o-Me), 2.39 (s, 3 H,
p-Me), 6.99 (s, 2 H, ArH), 7.06 (s, 1 H, ArH), 7.44 (m, 2 H, ArH),
7.64 (m, 1 H, ArH), 9.41 (m, 1 H, ArH).
13C NMR (100 MHz, CDCl3): d = 17.8 (C, Me), 18.9 (Me), 21.3
(Me), 113.3 (CAr), 120.0 (CAr), 123.6 (CAr), 127.6 (CAr), 129.1 (CAr),
129.6 (CAr), 131.0 (CAr), 131.7 (CAr), 133.7 (CAr), 133.9 (CAr), 136.0
(CAr), 136.3 (CAr), 140.1 (CAr), 167.7 (C=O), 168.1 (C=O), 180.5
(C-Ir).
442.47; colorless crystal 0.35 × 0.10 × 0.05 mm; Unit cell dimen-
sions: a = 12.8722(5) Å, b = 18.9645(8) Å, c = 17.1399(7) Å,
b = 104.237(3)°; V = 4055.6(3) Å3; D (calculated) = 1.449 g cm–3;
m = 2.843 mm–1, empirical absorption correction (0.436 £ T £
0.871); Z = 8; monoclinic, space group P21/c (No. 14); l = 1.54178
Å; T = 223(2) K; w and j scans, 32123 reflections collected ( h, k,
l), [(sin q)/l] = 0.60 Å–1; 6649 independent (Rint = 0.053) and 4080
observed reflections [I ≥ 2 s(I)]; 602 refined parameters, R = 0.072,
wR2 = 0.205; max. (min.) residual electron density 0.56 (–0.37)
e Å–3; two almost identical molecules in the asymmetric unit, one
anion refined with split positions and geometrical restraints, hydro-
gen atoms calculated and refined as riding atoms.
HRMS (ESI): m/z [M – Cl–]+ calcd for C20H16IrN2O2S+: 541.0562;
found: 541.0566.
Formation of (NHC)–PdCl2–(3-Chloropyridine) Complex (20)
A mixture of PdCl2 (21.8 mg, 0.123 mmol, 1 equiv), imidazolium
salt 10 (50 mg, 0.123 mmol, 1.0 equiv) and K2CO3 (84 mg, 0.651
mmol, 5.0 equiv) was suspended in 3-chloropyridine (0.5 mL) and
stirred at 80 °C for 16 h. After cooling to ambient temperature, the
reaction mixture was diluted with CH2Cl2 and filtered through a
short pad of Celite®. Solvents were removed in vacuo and the resi-
due was purified by chromatography on silica gel (CH2Cl2–MeOH,
50:1) to give the desired product.13
X-ray Crystal Structure Analysis of 188
CCDC 68506. Empirical formula C17H16ClIrN2O2S; Formula
weight 540.03; yellow crystal 0.50 × 0.40 × 0.20 mm; Unit cell di-
mensions: a = 8.1949(1) Å, b = 11.0607(2) Å, c = 19.9746(3) Å,
b = 92.430(1)°; V = 1808.90(5) Å3; D (calculated) = 1.983 g cm–3;
m = 7.655 mm–1, empirical absorption correction (0.114 £ T ≤
0.310); Z = 4; monoclinic, space group P21/c (No. 14); l = 0.71073
Å; T = 223(2) K; w and j scans, 11467 reflections collected ( h, k,
l), [(sin q)/l] = 0.66 Å–1; 4288 independent (Rint = 0.060) and 3765
observed reflections [I ≥ 2 s(I)]; 221 refined parameters, R = 0.033,
wR2 = 0.089; max. (min.) residual electron density 2.35 (–1.23) e Å–3;
close to Ir, hydrogen atoms calculated and refined as riding atoms.
Yield: 49%; yellow solid; Rf = 0.84 (CH2Cl2–MeOH, 10:1).
IR (ATR): 2921, 2852, 1462, 1347, 1187, 747, 740, 716, 640, 625,
567 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.18 (s, 6 H, o-Me), 2.31 (s, 3 H,
p-Me), 3.45 (s, 3 H, Me), 6.62 (s, 1 H, ArH), 6.94 (s, 1 H, ArH),
6.99 (s, 2 H, ArH), 7.60–7.63 (m, 2 H, ArH), 8.67 (dd, J = 6.5, 4.3
Hz, 1 H, ArH), 8.75 (d, J = 2.1 Hz, 1 H, ArH).
13C NMR (75.5 MHz, CDCl3): d = 17.5 (Me), 19.4 (Me), 21.8 (Me),
112.4 (CAr), 115.1 (CAr), 124.8 (CAr), 129.5 (CAr), 131.6 (CAr), 132.5
(CAr), 133.5 (CAr), 136.4 (CAr), 136.9 (CAr), 138.0 (CAr), 139.7 (CAr),
149.4 (CAr), 150.0 (CAr), 151.0 (CAr).
X-ray Crystal Structure Analysis of 208
CCDC 68507. Empirical formula C20H20Cl3N3PdS; Formula weight
547.20; light-yellow crystal 0.35 × 0.06 × 0.05 mm; Unit cell di-
mensions: a = 16.5972(2) Å, b = 11.9804(2) Å, c = 21.8986(1) Å;
V = 4354.34(12) Å3; D (calculated) = 1.669 g cm–3; m = 1.328 mm–1,
empirical absorption correction (0.654 £ T £ 0.937); Z = 8; ortho-
rhombic, space group Pca21 (No. 29); l = 0.71073 Å; T = 223(2) K;
w and j scans, 23959 reflections collected ( h, k, l), [(sinq)/
l] = 0.66 Å–1; 8304 independent (Rint = 0.071) and 5718 observed
reflections [I ≥ 2 s(I)]; 524 refined parameters, R = 0.048,
wR2 = 0.102, Flack parameter –0.45(3); max. (min.) residual elec-
tron density 0.89 (–1.32) e Å–3; two almost identical molecules in
the asymmetric unit, disorder of the chloropyridine ligand of mole-
cule B refined with split positions, hydrogen atoms calculated and
refined as riding atoms.
HRMS (ESI): m/z [M – Cl–]+ calcd for C20H20Cl2N3PdS+: 509.9790;
found: 509.9786.
X-ray Crystal Structure Analysis of 78
CCDC 68504. Empirical formula C21H31F3N2O5S3; Formula weight
544.66; light-yellow crystal 0.40 × 0.30 × 0.15 mm; Unit cell di-
mensions: a = 9.2232(2) Å, b = 16.6824(2) Å, c = 36.5487(6) Å;
V = 5623.57(17) Å3; D (calculated) = 1.287 g cm–3; m = 0.315 mm–1
empirical absorption correction (0.884 £ T £ 0.954); Z = 8; ortho-
rhombic, space group Pbca (No. 61); l = 0.71073 Å; T = 223(2) K;
w and j scans, 37182 reflections collected ( h, k, l), [(sin q)/
l] = 0.66 Å–1, 6566 independent (Rint = 0.051) and 4133 observed
reflections [I ≥ 2 s(I)]; 316 refined parameters; R = 0.048,
wR2 = 0.138; max. (min.) residual electron density 0.30 (–0.27) e Å–3;
hydrogen atoms calculated and refined as riding atoms.
Acknowledgment
We thank the Fonds der Chemischen Industrie, the Alfried-Krupp
von Bohlen und Halbach-Stiftung and the Deutsche Forschungsge-
meinschaft for generous financial support.
References
X-ray Crystal Structure Analysis of 108
CCDC 68503. Empirical formula C16H17F3N2O3S2; Formula weight
406.44; colorless crystal 035 × 0.35 × 0.25 mm; Unit cell dimen-
sions: a = 19.5592(5) Å, b = 13.6605(3) Å, c = 22.2651(6) Å,
b = 91.592(1)°; V = 5946.7(3) Å3; D (calculated) = 1.362 g cm–3;
m = 2.854 mm–1 empirical absorption correction (0.435 £ T £
0.536); Z = 12; monoclinic, space group P21/c (No. 14);
l = 1.54178 Å; T = 223(2) K; w and j scans, 566606 reflections
collected ( h, k, l), [(sin q)/l] = 0.60 Å–1; 10659 independent
(Rint = 0.057) and 8617 observed reflections [I ≥ 2 s(I)]; 715 refined
parameters, R = 0.063, wR2 = 0.174; max. (min.) residual electron
density 1.07 (–0.37) e Å–3; three almost identical molecules in the
asymmetric unit, hydrogen atoms calculated and refined as riding
atoms.
(1) (a) Herrmann, W. A. Angew. Chem. Int. Ed. 2002, 41, 1290.
(b) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew.
Chem. Int. Ed. 2007, 46, 2768. (c) N-Heterocyclic carbenes
in synthesis; Nolan, S. P., Ed.; Wiley-VCH: Weinheim,
2006. (d) N-Heterocyclic carbenes in transition metal
catalysis; Glorius, F., Ed.; Springer: Berlin, 2007.
(2) For an excellent review on applications in organocatalysis,
see: Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev.
2007, 107, 5606.
(3) For an excellent overview on the variety of NHC structures
and their coordination chemistry, see: (a) Hahn, F. E.;
Jahnke, M. C. Angew. Chem. Int. Ed. 2008, 47, 3122. See
also: (b) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem.
2005, 1815.
X-ray Crystal Structure Analysis of 138
CCDC 68505. Empirical formula C19H17F3N2O3S2; Formula weight
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York

2228
C. Lohre et al.
PAPER
(9) Arduengo, A. J. III Acc. Chem. Res. 1999, 32, 913.
(10) See for example: (a) Magill, A. M.; Cavell, K. J.; Yates, B.
F. J. Am. Chem. Soc. 2004, 126, 8717. (b) Huang, J.;
Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics
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(11) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313. (b) Chianese,
A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H.
Organometallics 2003, 22, 1663. (c) Chianese, A. R.;
Kovacevic, A.; Zeglis, B. M.; Faller, J. W.; Crabtree, R. H.
Organometallics 2004, 23, 2461. (d) Kelly, R. A. III;
Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.;
Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan,
S. P. Organometallics 2008, 27, 202.
(12) Additional reference data and a pertinent discussion of NHC
donor abilities can be found in the supporting information of
the following publication: Fürstner, A.; Alcazaro, M.;
Krause, H.; Lehmann, C. W. J. Am. Chem. Soc. 2007, 129,
12676.
(13) (a) Organ, M. G.; Abdel-Hadi, M.; Avola, S.; Dubovyk, I.;
Hadei, N.; Kantchev, E. A. B.; O’Brien, C. J.; Sayah, M.;
Valente, C. Chem. Eur. J. 2008, 14, 2443. (b) O’Brien, C.
J.; Kantchev, E. A. B.; Valente, C.; Hadei, N.; Chass, G. A.;
Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem. Eur. J.
2006, 12, 4743.
(14) Katritzky, A. R.; He, H.-Y.; Long, Q.; Cui, X.; Level, J.;
Wilcox, A. L. ARKIVOC 2000, (iii), 240.
(15) Kennedy, D. F.; Messerle, B. A.; Smith, M. K. Eur. J. Inorg.
(4) For the design and application of the IBiox series of NHC
ligands with different steric demand, see: (a) Glorius, F.;
Altenhoff, G.; Goddard, R.; Lehmann, C. Chem. Commun.
2002, 2704. (b) Altenhoff, G.; Goddard, R.; Lehmann, C.
W.; Glorius, F. Angew. Chem. Int. Ed. 2003, 42, 3690.
(c) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F.
J. Am. Chem. Soc. 2004, 126, 15195. (d) Altenhoff, G.;
Würtz, S.; Glorius, F. Tetrahedron Lett. 2006, 47, 2925.
(5) (a) Burstein, C.; Lehmann, C. W.; Glorius, F. Tetrahedron
2005, 61, 6207. (b) Alcarazo, M.; Roseblade, S. J.; Cowley,
A. R.; Fernandez, R.; Brown, J. M.; Lassaletta, J. M. J. Am.
Chem. Soc. 2005, 127, 3290.
(6) For related NHCs, see: Nonnenmacher, M.; Kunz, D.;
Rominger, F.; Oeser, T. Chem. Commun. 2006, 1378; and
references cited therein.
(7) Wong, W.-Y.; Chan, S.-M.; Choi, K.-H.; Cheah, K.-W.;
Chan, W.-K. Macromol. Rapid Commun. 2000, 21, 453.
(8) CCDC 685503–685507 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
Fax: +44(1223)336033; E-mail: deposit@ccdc.cam.ac.uk].
Data sets were collected with Nonius KappaCCD
diffractometers, in the case of Mo-radiation equipped with a
rotating anode generator. Programs used: (a) COLLECT –
data collection software: Nonius B. V., 1998 (b) Data
reduction Denzo-SMN: Otwinowski, Z.; Minor, W. Methods
in Enzymology 1997, 276, 307. For absorption correction
SORTAV, see: (c) Blessing, R. H. Acta Crystallogr. Sect. A
1995, 51, 33. (d) Blessing, R. H. J. Appl. Crystallogr. 1997,
30, 421. (e) Denzo: Otwinowski, Z.; Borek, D.; Majewski,
W.; Minor, W. Acta Crystallogr., Sect. A 2003, 59, 228. (f)
Structure solution SHELXS-97: Sheldrick, G. M. Acta
Crystallogr., Sect. A 1990, 46, 467. (g)Structurerefinement
SHELXL-97: Sheldrick, G. M. Acta Crystallogr., Sect. A
2008, 64, 112. (h) Graphics SCHAKAL: Keller, E.
Universität Freiburg; 1997.
Chem. 2007, 1, 80.
Synthesis 2008, No. 14, 2221–2228 © Thieme Stuttgart · New York