A new paradigm in N-heterocyclic carbenoid ligands

Article abstract of DOI:10.1016/j.jorganchem.2005.09.015

We report a new class of very readily prepared chiral N-heterocyclic carbenoid ligand that also contains two different types of chirality: an asymmetric centre and an atropoisomeric unit, but contained in two separate N-substituents, such that both can ea

Full text of DOI:10.1016/j.jorganchem.2005.09.015

Journal of Organometallic Chemistry 690 (2005) 6210–6216
www.elsevier.com/locate/jorganchem
A new paradigm in N-heterocyclic carbenoid ligands
*
*
Philip C. Bulman Page , Benjamin R. Buckley, Steven D.R. Christie , Mark Edgar,
Andrew M. Poulton, Mark R.J. Elsegood, Vickie McKee
Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
Received 1 September 2005; accepted 1 September 2005
Available online 20 October 2005
Abstract
We report a new class of very readily prepared chiral N-heterocyclic carbenoid ligand that also contains two different types of chi-
rality: an asymmetric centre and an atropoisomeric unit, but contained in two separate N-substituents, such that both can easily be var-
ied. In addition to its simplicity and flexibility, this approach has potential advantages over systems containing only atropoisomeric units,
because the inclusion of an additional fixed asymmetric centre means that the atropoisomers are diastereoisomers and therefore chem-
ically distinct entities. Complexation to a metal centre increases the barrier to rotation in the atropoisomeric unit so that the two dia-
stereoisomers may be separable by simple methods such as standard chromatography. In addition, a novel cis-substituted palladium
complex has been characterised by X-ray crystallography.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Carbene; Benzimidazolylidene; Imidazolylidene; Atropoisomerism; Chirality; Asymmetry
1. Introduction
regarding a new class of very readily prepared chiral N-het-
erocyclic carbenoid ligand that also contains two different
types of chirality: an asymmetric centre and an atropoiso-
meric unit, but contained in two separate N-substituents,
such that both can easily be varied. In addition to its sim-
plicity and flexibility, this approach has potential advanta-
ges over systems containing only atropoisomeric units,
because the inclusion of an additional fixed asymmetric
centre means that the atropoisomers are diastereoisomers
and therefore chemically distinct entities. We further con-
jectured that complexation to a metal centre might increase
the barrier to rotation in the atropoisomeric unit so that
the two diastereoisomers would in principle be separable
by simple methods such as standard chromatography.
Diver has recently reported the use of C2 symmetric
carbenes based on a benzimidazole structure [7], and this
prompts us to describe our own results in this area. Such
a design of ligand is attractive because both of the enanti-
omers are in principle available, and because the ligands
can be readily accessed through a short synthetic sequence;
for example, the bis-a-methylbenzylamine-derived benzim-
idazolium salt 1 was easily prepared as shown in Schemes 1
The use of N-heterocyclic carbenoid (NHC) ligands in
organic and inorganic chemistry has grown exponentially
over the last 10 years [1]. The majority of reactions have,
however, been concerned with relatively simple achiral
carbenes containing aniline-derived amine units. Several
approaches to chiral carbene ligands have appeared over
this time: for example Hermann [2] and Grubbs [3] have
both employed chiral N-substituted imidazolium salts as
a means of accessing enantiomerically enriched ligands
and complexes. More recently, interest in the area of chiral
N-heterocyclic carbene ligands has increased [4], but the
preparation and use of chiral benzimidazolylidene exam-
ples remain rare (vide infra). Of particular relevance to this
paper is HoveydaÕs use of axial chirality in an N-heterocy-
clic carbene metal complex [5]; planar chiral elements have
also been used [6]. We report here our initial findings
*
Corresponding authors. Tel.: + 44 1509 222581.
E-mail addresses: p.c.b.page@lboro.ac.uk (P.C. Bulman Page),
s.d.christie@lboro.ac.uk (S.D.R. Christie).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.09.015

P.C. Bulman Page et al. / Journal of Organometallic Chemistry 690 (2005) 6210–6216
6211
Me
Ph
NH
Me
Ph
NH
Br
Br
H N
Ph
H N
Ph
2
2
BINAP, NaOtBu
toluene, 110˚C,
87%
BINAP, NaOtBu
mesitylene, 150˚C,
60%
NH
Ph
Br
2
Me
3
H N
2
Ph
BINAP, NaOtBu
mesitylene, 150˚C,
63%
Scheme 1.
and 2. Nevertheless, an in situ double Buchwald–Hartwig
coupling provided the diamine product in only 63% yield,
and the use of two sequential Buchwald–Hartwig couplings
proved frustrating as the second reaction consistently gave
a lower yield than expected; reaction of (S)-a-methylben-
zylamine with 1,2-dibromobenzene gave the monosubsti-
tuted product 2 in an excellent 87% yield, but attempts to
introduce the second amine gave the desired product 3 in
no more than a respectable 60% yield in our hands.
Generation of a palladium–carbene complex 4 was
achieved by heating the imidazolium salt in the presence
of Pd2(dba)3 under the conditions described by Faller
and Crabtree (Scheme 2) [8]. Single crystal X-ray analysis
(Fig. 1) showed that in this trans complex the two imidaz-
olylidene rings are almost orthogonal to the central plane
containing the Pd atom, and are close to coplanarity, in
each case with the two benzyl substituents oriented so that
their two phenyl groups project over opposite faces of the
imidazolylidene ring. The phenyl groups of adjacent benzyl
substituents in different imidazolylidene rings are also ori-
ented to project towards different faces of the ring system.
We were interested to assess whether this unsatisfactory
yield in the second coupling reaction was a function of ste-
ric interactions between the monosubstituted amine sub-
strate and the nucleophile, or due to electronic factors.
We therefore used a structurally very different amine, 2-iso-
propyl aniline, as the coupling partner. A fascinating addi-
tional possibility was the enhanced potential in this system
for atropoisomerism in the imidazolium carbenoid as a re-
sult of hindered rotation around the new carbon–nitrogen
Fig. 1. Pd–carbene complex of the bis-a-methylbenzylamine-derived
benzimidazolylidene 4 [9].
bond. We were intrigued to find that the second Buchwald–
Hartwig coupling proceeded well with this substituted ani-
line, producing the differentially substituted bis-amine 5 in
an excellent 95% yield. Insertion of the final carbon atom
was achieved by reaction with triethyl orthoformate under
acidic conditions, producing the benzimidazolium salt 6 in
84% yield (Scheme 3).
Examination of the benzimidazolium salt 6 by 1H NMR
spectroscopy at room temperature did indeed show two
sets of signals corresponding to the presence of two isomers
in an approximately 1:1 ratio. The distinct central benz-
imidazole proton signals at 10.892 and 10.833 ppm
(dt = 23.6 Hz at 400 MHz) coalesce at 55 ꢁC, which corre-
sponds to an experimentally derived conformational bar-
rier of DGà = 68.8 kJ molꢀ1 [10]. The two diastereotopic
methyl signals of the isopropyl unit remained distinct at
lower temperatures, only reaching near-coalescence at
115 ꢁC, which corresponds to DGà = 80 kJ molꢀ1. The rel-
ative energy difference between the two isomers was ob-
tained using the classic Boltzman equation and the ratio
of the two benzimidazole protons, giving DG =
0.5 kJ molꢀ1
.
Nuclear Overhauser effect (nOe) experiments provided
further evidence of this hindered rotation. Irradiation of
the benzimidazole proton shows an enhancement in the
Me
Me
Ph
NH
Me
Me
Ph
Me
Ph
Ph
N
N
-
+ Cl
N
CH(OEt)3
Pd2(dba)3
Cl Pd Cl
Ph
Ph
HCl, 80˚C,
88%
NH
Ph
N
THF,
51%
,
N
N
Me
Me
Ph
Me
1
4
Scheme 2.

6212
P.C. Bulman Page et al. / Journal of Organometallic Chemistry 690 (2005) 6210–6216
Me
Ph
Me
Ph
NH
N
Cl-
Me
Ph
N+
iPr-C6H4-NH2
NH
Br
CH(OEt)3
NH
BINAP, NaOtBu
toluene, 110˚C
95%
HCl, 80˚C
84%
6
5
Scheme 3.
proton, methyl and phenyl group of the a-methyl benzyl-
amine substituents, suggesting that this side chain is freely
rotating at room temperature. The enhancement in the o-
isopropylaryl group is, however, much more specific: the
isopropyl methine proton is enhanced, but the methyl
groups are very little affected, again suggesting that rota-
tion around the aryl-isopropyl C–C bond is severely re-
stricted (Fig. 2).
We thus attribute the lower conformational barrier to
atropoisomerism by rotation about the C–N bond between
the 2-isopropylphenyl group and the central heterocyclic
ring (Fig. 1), and the higher rotational barrier to rotation
about the isopropyl group C2–arene bond. This conclusion
is supported by molecular modelling, which suggests a very
high barrier (>49 kJ molꢀ1, semi-empirical, AM1 level) in
both cases [11]. Ab initio calculations at the UHF/6-
31G* level for a fully optimized structure suggest that
the 2-isopropylaniline group lies nearly orthogonal to the
benzimidazole ring, with DG = 0.5 kJ molꢀ1 between the
two atropoisomeric structures (Fig. 3) [12].
H
With the carbene precursor 6 in hand, we next investi-
gated the formation of metal complexes of the carbenoid
ligand. Reaction with Pd(0) as before did indeed provide
a palladium–carbene complex in 44% yield, and 1H
NMR spectroscopy at room temperature indicated the
presence of a mixture of three conformational isomers in
a 1:1:1 ratio (Scheme 4). The signals in the proton NMR
N
N
H
H
Fig. 2. The nOe enhancement in the differentially substituted benzimi-
dazolium salt.
Fig. 3. Two conformations of the o-isopropropyl aryl group with respect to the benzimidazolium unit.
Ph
N
N
Me
N+
N
Pd2(dba)3
N
N
Pd
THF,
44%
,
Ph
Cl-
Cl
Cl
Ph
Scheme 4.

P.C. Bulman Page et al. / Journal of Organometallic Chemistry 690 (2005) 6210–6216
6213
In conclusion, we have prepared a new type of N-hetero-
cyclic carbene ligand that contains two different types of
chirality, an asymmetric centre and an atropoisomeric axis,
contained in separate N-substituents that are readily var-
ied. We believe this is the first time this has been achieved
with a benzimidazole skeleton. A significant barrier to
rotation results in two atropoisomeric diastereoisomers
being produced, and this is carried forward into the result-
ing metal complexes. In addition, a novel cis-substituted
palladium complex has been characterized by X-ray
crystallography.
2. Experimental
Fig. 4. Pd–carbene complex of the differentially substituted benzimidaz-
olylidene 7 [14].
2.1. (2-Bromophenyl)-(S)-a-methylbenzylamine (2)
Pd2(dba)3 (0.081 g, 0.088 mmol) and ( )-BINAP
(0.11 g, 0.18 mmol) were dissolved in toluene (6 ml) and
the solution degassed for 15 min before being heated at
110 ꢁC for 10 min (solution turns from deep purple to dark
orange). Upon cooling sodium tert-butoxide (0.43 g,
4.51 mmol), (S)-a-methylbenzylamine (0.67 g, 5.5 mmol)
and 1,2-dibromobenzene (0.52 g, 2.2 mmol) were added
and the reaction mixture heated under reflux for 1.5 h.
The solution was allowed to cool and filtered through a
pad of celite. Solvents were removed under reduced pres-
sure and the crude material purified by column chromatog-
raphy eluting with light petroleum/ethyl acetate (99:1).
Colourless oil (0.53 g, 87%). mmax (film)/cmꢀ1 3050, 2964,
1734, 1596, 1507, 1449, 1320, 1282, 1203, 1016, 741, 699,
665; dH (400 MHz; CDCl3) 1.57 (3H, d J 6.8 Hz), 4.51
(1H, q, J 6.8 Hz), 6.29 (1H, dd, J 1.5 and 8.2 Hz), 6.42
(1H, dt, J 1.5 and 7.4 Hz), 6.89 (1H, dt, J 1.5 and
7.4 Hz), 7.12–7.16 (1H, m), 7.21–7.26 (4H, m), 7.32 (1H,
dd, J 1.5 and 7.8 Hz); dC (100 MHz; CDCl3) 25.2, 53.6,
109. 7, 112.7, 117.8, 125.8, 127.1, 128.4, 128.8, 132.3,
144.0, 144.6.
spectrum were unaffected by heating the sample, suggesting
that the rotational barrier about the C–N bond is signifi-
cantly greater than for the uncomplexed adduct, as a result
of the large steric effect of the metal. However, selective
recrystallization did provide a single complex 7 by 1H
NMR spectroscopy, and this was suitable for X-ray crys-
tallographic analysis.
Single crystal X-ray analysis (Fig. 4) showed that this is,
unexpectedly, a cis complex, most cis Pd complexes of N-
heterocyclic carbenes having chelated structures [13]. The
solid-state structure is C2 symmetric, with the phenyl
groups of the benzylamine units orientated away from
the metal centre, and the methine protons of the isopropyl
groups orientated towards the imidazolylidene moiety, as
expected. The two imidazolylidene rings are not in this case
orthogonal to the central plane containing the Pd atom.
We have in the past successfully prepared complexes of
NHC ligands with cobalt alkyne complexes [15]. Applica-
tion of this procedure using our new chiral ligand, by gen-
eration of the free carbene followed by reaction with the
cobalt complex of diphenylacetylene, gave two new iso-
meric complexes in a 1:1 ratio, Scheme 5. Separation of
these complexes by silica gel chromatography now proved
possible, and 1H NMR spectroscopy of the separated com-
plexes indicates that the two complexes are indeed diaste-
reoisomeric; we again attribute this to the presence of
atropoisomers differentiated by rotation about the C–N
bond.
2.2. (+)-N- (2-Isopropylphenyl)-N0-(S)-a-methylbenzyl-
1,2-diaminobenzene (5)
Pd2(dba)3 (0.081 g, 0.088 mmol) and ( )-BINAP
(0.11 g, 0.18 mmol) were dissolved in mesitylene (6 ml)
and the solution degassed for 15 min before being heated
at 150 ꢁC for 10 min (solution turns from deep purple to
-
Cl
Ph
Me
Ph
+
Ph
N
N
Ph
(CO)
(OC) Co
Co
Ph
Me
Ph
2
3
N
KOtAmyl, hexane, 65˚C
(OC) Co
Co(CO)
3
3
N
Scheme 5.

6214
P.C. Bulman Page et al. / Journal of Organometallic Chemistry 690 (2005) 6210–6216
dark orange). Upon cooling sodium tert-butoxide (0.43 g,
4.51 mmol), 2-isopropyl-phenylamine (0.74 g, 5.5 mmol)
and (2-Bromo-phenyl)-(S)-a-methylbenzylamine (0.50 g,
2.2 mmol) were added and the reaction mixture heated at
150 ꢁC for 12 h. The solution was allowed to cool and fil-
tered through a pad of celite. Solvents were removed under
reduced pressure and the crude material purified by column
chromatography eluting with light petroleum/ethyl acetate
(99:1). Yellow oil (0.69 g, 95%). [a]D + 93.2 (c 1.49,
CHCl3); mmax (film)/cmꢀ1 3060, 2960, 1598, 1511, 1450,
1322, 1277, 1307, 745, 700; dH (400 MHz; CDCl3) 1.33
(3H, d, J 6.8 Hz), 1.35 (3H, d, J 6.8 Hz), 1.43 (3H, d, J
6.4 Hz), 3.12 (1H, sep, J 6.8 Hz), 4.30–4.70 (1H, m),
6.61–6.70 (1H, m), 6.46–6.56 (1H, m), 6.60–6.68 (2H, m),
6.87–6.93 (2H, m), 6.96 (1H, dd, J 1.2 and 7.6 Hz), 7.07
(1H, dt, J 1.6 and 8.0 Hz), 7.16–7.40 (6H, m); dC
(100 MHz; CDCl3) 22.7, 23.0, 27.6, 31.6, 53.5, 112.6,
116.2, 117.5, 120.3, 124.6, 125.5, 125.9, 126.6, 127.2,
128.6, 128.7, 129.2, 134.9, 135.0, 142.5, 142.7; m/z (ES+)
330 (M+ 100%), 225 (24%) 208 (71%), 105 (100%);
330.2091; C23H26N2 requires 330.2096.
diluted with diethyl ether (30 ml) and filtered. The off white
solid obtained was washed with diethyl ether and the residual
solvents removed under high reduced pressure. Colour-
less powder (0.30 g, 88%). M.p. 241.4–243.7 ꢁC, (lit.7
242–243 ꢁC); [a]D + 22.9 (c 1.07, MeOH), (lit.7 [a]D + 24.0
(c 1.0 MeOH); mmax (film)/cmꢀ1 3026, 2988, 2945, 2904,
1546, 1442, 1385, 1242, 750, 703; dH (250 MHz; CHCl3)
2.45 (6H, d, J 7.2 Hz), 6.30 (2H, q, J 7.0 Hz), 7.30–7.45
(10H, m) 7.50–7.57 (4H, m), 12.46 (1H, s); dC (100 MHz;
CHCl3) 20.9, 59.5, 114.5, 126.7, 126.8, 128.2, 129.7,
133.1, 135.0, 142.7; m/z (FAB+) 327 (M+ 100%), 223
(14%), 119 (59%), 105 (85%); 327.1861; C24H25N2 (cation)
requires 327.1861.
2.5. (ꢀ)-(S)-3-(2-Isopropylphenyl)-1-(S)-a-
methylbenzylbenzimidazolium chloride (6)
(+)-N-(2-Isopropyl-phenyl)-N0-(S)-a-methylbenzyl-1,2-
diaminobenzene (0.50 g, 1.5 mmol) was dissolved in
CH(OEt)3 (10 ml), and 12.1 N HCl (0.12 ml, 1.5 mmol)
added dropwise over 5 min. The resulting solution was stir-
red at room temperature for 30 min under nitrogen and
then heated to 80 ꢁC until condensation was observed on
the neck of the flask. At this point the rubber septum
was removed and the solution was allowed to stir open
to the air for 2 h. After cooling to room temperature, the
suspension obtained was diluted with diethyl ether
(30 ml) and filtered. The off white solid obtained was
washed with diethyl ether and the residual solvents re-
moved under high reduced pressure. Colourless powder
(0.475 g, 84%). M.p. 178.3–180.1 ꢁC; [a]D ꢀ30.6 (c 1.15,
CHCl3); mmax (film)/cmꢀ1 3050, 2964, 1614, 1547, 1493,
1421, 1327, 1276, 1203, 1079, 1030, 753, 729, 703; dH
(400 MHz; DMSO 100 ꢁC, mixture of isomers) 1.05 (6H,
d, J 6.8 Hz), 2.10 (3 H, d, J 7.2 Hz), 2.40–2.60 (1H, m),
6.31 (1H, q, J 6.4 Hz), 7.32–7.41 (4H, m), 7.45–7.56 (3 H,
m), 7.58–7.74 (5 H m), 7.84–8.20 (1H, m), 10.46 (1H, s);
dC (100 MHz; CDCl3, mixture of isomers) 19.8, 20.4,
23.3, 24.5, 24.6, 28.4, 28.5, 58.3, 113.4, 113.5, 114.8,
115.0, 127.05, 127.09, 127.11, 127.15, 127.47, 127.50,
127.55, 127.66, 127.84, 127.90, 128.04, 128.24, 129.00,
129.10, 129.34, 129.38, 129.78, 129.89, 129.92, 129.98,
132.04, 132.06, 137.3, 137.4, 145.51, 145.57. ; m/z (ES+)
341 (M+ 45%), 238 (60%), 237 (100%), 105 (75%);
341.2009; C24H25N2 (cation) requires 341.2012.
2.3. (+)-N,N0-bis-(S)-a-Methylbenzyl-1,2-diaminobenzene
(3)
Pd2(dba)3 (0.036 g, 0.04 mmol) and ( )-BINAP (0.05 g,
0.08 mmol) were dissolved in mesitylene (6 ml) and the
solution degassed for 15 min before being heated at
150 ꢁC for 10 min (solution turns from deep purple to dark
orange). Upon cooling sodium tert-butoxide (0.39 g,
4.0 mmol), (S)-a-methyl benzylamine (0.61 g, 5.0 mmol)
and 1,2-dibromobenzene (0.26 g, 1.0 mmol) were added
and the reaction mixture heated under reflux for 16 h.
The solution was allowed to cool and filtered through a
pad of celite. Solvents were removed under reduced pres-
sure and the crude material purified by column chromatog-
raphy eluting with light petroleum/ethyl acetate (99:1).
Yellow oil (0.20 g, 63%). [a]D + 177.6 (c 0.94 CHCl3),
(lit.7 [a]D + 182 (c 0.76 CHCl3); mmax (film)/cmꢀ1 3054,
2962, 2865, 1597, 1508, 1490, 1443, 1432, 738, 699; dH
(250 MHz; CDCl3) 1.59 (6H, d J 6.7 Hz), 4.49 (2H, q, J
6.5 Hz), 6.42–6.46 (2H, m), 6.55–6.59 (2H, m), 7.22–7.42
(10H, m); dC (100 MHz; CDCl3) 25.4, 53.5, 113.7, 118.8,
125.8, 126.6, 128.7, 136.3, 145.4.
2.4. (+)-N,N0-bis-(S)-a-Methylbenzyl benzimidazolium
chloride (1)
2.6. (ꢀ)-bis-[N,N0-bis-(S)-a-Methylbenzyl-
benzimidazolylidene] palladium dichloride (4)
(+)-N,N0-Bis-(S)-a-methylbenzyl-1,2-diaminobenzene
(0.30 g, 0.95 mmol) was dissolved in CH(OEt)3 (10 ml), and
12.1 N HCl (0.08 ml, 0.95 mmol) added dropwise over
5 min. The resulting solution was stirred at room tempera-
ture for 30 min under nitrogen and then heated to 80 ꢁC
until condensation was observed on the neck of the flask.
At this point the rubber septum was removed and the solu-
tion was allowed to stir open to the air for 2 h. After cool-
ing to room temperature, the suspension obtained was
Pd2(dba)3 (0.136 g, 0.15 mmol) was dissolved in THF
(10 ml) under a nitrogen atmosphere, and (+)-N,N0-Bis-
(S)-a-methylbenzylbenzimidazolium chloride (0.095 g,
0.26 mmol) added. The reaction mixture was heated un-
der reflux for 3 h, allowed to cool to room temperature,
and a scoop of silica added. The solvent was removed
under reduced pressure to give the reaction mixture ad-
sorbed onto silica. Column chromatography eluting with

P.C. Bulman Page et al. / Journal of Organometallic Chemistry 690 (2005) 6210–6216
6215
dichloromethane/light petroleum (5:1) gave the product
as a colourless solid. Recrystallized from dichlorometh-
ane/light petroleum, colourless crystals (0.053 g, 51%).
M.p. 273.1 ꢁC (dec.); [a]D –344.6 (c 0.91, CHCl3); mmax
(film)/cmꢀ1 3057, 3030, 2975, 1605, 1495, 1448, 1396,
1379, 1282, 1079, 1056, 741, 700; dH (250 MHz; CHCl3)
2.07 (12H, d, J 7.2 Hz), 6.75–6.85 (8H, m), 7.20–7.40
(12H, m), 7.45 (4H, q, 7.0 Hz), 7.65–7.69 (8H, m); dC
(100 MHz; CHCl3) 17.8, 59.1, 112.7, 122.11, 127.6,
127.8, 128.7, 133.2, 139.1, 182.0.
mmax (film)/cmꢀ1 3089, 2963, 2925, 2869, 2052, 2003,
1990, 1955, 1712, 1600, 1494, 1261, 1025, 691;
2.9. First eluting diastereoisomer
dH (400 MHz; CDCl3) 1.07 (3H, d, J 6.8 Hz), 1.31 (3H,
d, J 6.8 Hz), 1.74 (3H, d, J 6.8 Hz), 2.26 (1H, sept, J
6.8 Hz), 6.29 1H, (q, J 6.8 Hz), 6.4–6.48 (1H, m), 6.50–
6.76 (4H, m), 6.95–7.04 (3H, m), 7.09–7.16 (4H, m),
7.20–7.28 (3H, m), 7.30–7.39 (2H, m), 7.52–7.57 (4H, m);
dC (100 MHz; CDCl3); 18.8, 23.4, 23.9, 29.0, 58.4, 86.4,
89.5, 111.3, 112.6, 122.1, 122.4, 125.8, 125.9, 126.2, 126.7,
127.3, 127.4, 128.0, 128.1, 128.2, 128.9, 129.7, 129.8, 130.5.
2.7. (ꢀ)-bis-N-(2-Isopropyl-phenyl)-N0-(S)-a-
methylbenzyl-1,2-diamino-benzimidazolylidene palladium
dichloride (7)
2.10. Second eluting diastereoisomer
Pd2(dba)3 (0.07 g, 0.08 mmol) was dissolved in THF
(10 ml) under a nitrogen atmosphere, and (+)-N,N0-Bis-
dH (400 MHz; CDCl3; 1.05 (3H, d, J 6.8 Hz), 1.23 (3H,
d, J 6.8 Hz), 1.24 (3H, d, J 6.8 Hz), 2.20 (1H, sept, J
6.8 Hz), 6.02 (1H, m), 6.44 (1H, q, J 6.8 Hz), 6.51–6.63
(1H, m), 6.71–6.99 (5H, m), 7.01–7.12 (2H, m), 7.13–7.34
(8H, m), 7.37–7.63 (5H, m); dC (100 MHz; CDCl3); 18.0,
32.1, 23.5, 27.9, 58.5, 85.1, 87.9, 112.0, 112.2, 122.2,
122.4, 125.2, 125.6, 126.0, 126.9, 127.0, 127.1, 127.8,
128.0, 128.3, 129.0, 129.4, 129.5, 130.2.
(S)-a-methylbenzylbenzimidazolium
chloride
(0.05 g,
0.14 mmol) added. The reaction mixture was heated under
reflux for 3 h, allowed to cool to room temperature, and a
scoop of silica added. The solvent was removed under re-
duced pressure to give the reaction mixture adsorbed onto
silica. Column chromatography eluting with dichlorometh-
ane/light petroleum (5:1) gave the product as a colourless
solid. Recrystallized from dichloromethane/light petro-
leum, colourless crystals (0.05 g, 44%). M.p. 284.3 ꢁC
(dec.); [a]D –139.7 (c 0.73, CHCl3); mmax (film)/cmꢀ1 3060,
2960, 1603, 1490, 1448, 1380, 1284, 1067, 1031, 745, 665;
dH (400 MHz; CHCl3) 0.75 (6H, d, J 6.9 Hz), 1.06 (6H,
d, 7.2 Hz), 1.59 (6H, d, 6.7 Hz), 3.11 (2H, quin, 6.5 Hz),
6.02 (2H, dd, J 0.8 and 7.6 Hz), 6.46 (2H, dt, 1.2 and
7.6 Hz), 6.52 (2H, d, 8.0 Hz), 6.65 (2H, d, J 8.0 Hz), 6.75
(2H, d, J 8.8 Hz), 7.14–7.28 (8H, m), 7.34–7.37 (4H, m),
7.44 (2H, t, J 7.6 Hz), 7.67 (2H, dd, J 1.2 and 7.6 Hz),
7.72 (2H, q, J 7.2 Hz); dC (100 MHz; CHCl3) 23.9, 24.3,
27.9, 53.4, 59.0, 111.1, 113.6, 123.3, 123.4, 126.1, 126.4,
127.4, 128.1, 128.9, 130.9, 131.9, 134.6, 137.6, 137.5,
139.3, 149.3, 172.4; m/z (FAB+) 820 (5%), 784 (6%), 445
(6%), 339 (100%), 235 (75%), 105 (93%); 820.2629;
C48H48N4PdCl (M+ ꢀ Cl) requires 820.2618.
Acknowledgements
This research has enjoyed the support of Loughborough
University and the EPSRC. We are also indebted to the
EPSRC Mass Spectrometry Service, Swansea.
References
[1] For a recent review, see: A.J. Arduengo, Acc. Chem. Res. 32 (1999)
913.
[2] (a) W.A. Herrmann, L.J. Goossen, C. Ko¨cher, G.R.J. Artus, Angew.
Chem., Int. Ed. Engl. 35 (1996) 2805;
(b) W.A. Herrmann, L.J. Goossen, G.R.J. Artus, C. Ko¨cher,
Organometallics 16 (1997) 2472.
[3] J. Seiders, D.W. Ward, R.H. Grubbs, Org. Lett. 3 (2001) 3225.
[4] For a recent review, see: M.C. Perry, K. Burgess, Tetrahedron:
Asymmetry 14 (2003) 951.
[5] J.J. Van Veldhuizen, J.E. Campbell, R.E. Giudici, A.H. Hoveyda, J.
Am. Chem. Soc. 127 (2005) 6877.
[6] C. Bolm, T. Focken, G. Raabe, Tetrahedron: Asymmetry 14 (2003)
1733;
2.8. Dicobaltpentacarbonyl-3-(2-isopropylphenyl)-1-(S)-a-
methylbenzyl-benzimidazolium–diphenylacetylene
H. Seo, H. Park, B.Y. Kim, J.H. Lee, S.U. Son, Y.K. Chung,
Organometallics 22 (2003) 618.
[7] F.M. Rivas, U. Riaz, A. Giessert, J.A. Smulik, S.T. Diver, Org. Lett.
3 (2001) 2673.
(+)-N,N0-Bis-(S)-a-methylbenzylbenzimidazolium chlo-
ride (0.074 g, 0.19 mmol) was dissolved in hexane (10 ml)
under a nitrogen atmosphere, and potassium-tert-pentox-
ide (0.22 ml, 0.38 mmol) (25% solution in toluene) added
dropwise. The resulting solution was stirred for 30 min at
room temperature. Dicobalt hexacarbonyl-diphenylacety-
lene (0.176 g, 0.38 mmol) was then added as a solid. The
solution was then heated to 65 ꢁC for 40 min. The solution
was then filtered through a pad of celite and silica, and then
solvent was removed under reduced pressure. Column
chromatography eluting with diethyl ether/light petroleum
(5:95) gave the product as a dark purple liquid. Two sepa-
rate diastereoisomers (1:1) (0.103 g, 68% combined yield);
[8] S. Grundemann, M. Albrecht, A. Kovacevic, J.W. Faller, R.H.
¨
Crabtree, J. Chem. Soc., Dalton Trans. 10 (2002) 2163.
[9] Crystal data for 1: Colourless, air stable, block-shaped crystal,
0.28 · 0.21 · 0.20 mm3. C46H44Cl2N4Pd, M = 830.15, a = 12.3796(5) A,
˚
3
˚
˚
c = 25.0253(15) A, U = 3835.2(3) A , tetragonal, space group P41212,
Z = 4. Bruker SMART 1000 CCD diffractometer, Mo Ka radiation,
˚
k = 0.71073 A. 22,178 data measured, 4549 unique with 3893
observed with I > 2r(I), Rint = 0.053. Semi-empirical absorption
correction from equivalent data, l = 0.662 mmꢀ1. Final R = 0.0385
(for 3893 observed data, based on F), and wR2 = 0.0820 (for all
unique data, based on F2). Absolute structure parameter ꢀ0.06(3).
Crystallographic data for the structural analysis has been deposited

6216
P.C. Bulman Page et al. / Journal of Organometallic Chemistry 690 (2005) 6210–6216
with the Cambridge Crystallographic Data Centre, CCDC No.
279711 for compound 1.
[10] R.J. Abraham, J. Fisher, P. Loftus, Introduction to NMR Spectros-
copy, Wiley, Chichester, 1988.
Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challa-
combe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C.
Gonzalez, J.A. Pople, Gaussian 03, Revision C.02, Gaussian Inc.,
Wallingford, CT, 2004.
[11] Spartan Õ04 for Macintiosh, Wavefunction Inc., Irvine, CA.
[12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C.
Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B.
Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin,
R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma,
G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D.
Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G.
Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T.
[13] For chelating carbene complexes that are cis, see: W.A. Herrmann,
M. Elison, J. Fischer, C. Kocher, G.R.J. Artus, Angew. Chem., Int.
Ed. Engl. 34 (1995) 2371;
M.G. Gardiner, W.A. Herrmann, C.P. Reisinger, J. Schwarz, M.
Spiegler, J. Organomet. Chem. 572 (1999) 239;
W.A. Herrmann, J. Schwarz, M.G. Gardiner, Organometallics 18
(1999) 4082;
L. Jafarpour, J. Huang, E.D. Stevens, S.P. Nolan, Organometallics
18 (1999) 3760, 18.
[14] A preliminary structural determination was performed for compound
2. Although the basic connectivity has been established unequivo-
cally, unresolved twinning problems preclude the full reporting of this
structure at the present time.
[15] A.M. Poulton, S.D.R. Christie, R. Fryatt, S.H. Dale, M.R.J.
Elsegood, D.M. Andrews, Synlett (2004) 2103.