An improved synthesis of bis(1,3-di-N-tert-butylimidazol-2-ylidene)palladium(0) and its use in C-C and C-N coupling reactions

Article abstract of DOI:10.1016/S0022-328X(00)00724-5

A new, improved synthesis of [Pd{CN(^tBu)(CH)₂N(^tBu)}₂] (1) and its use as a catalyst in coupling reactions, including aminations, is presented. An interesting side product formed in the synthesis of 1, [Pd(η³-C₄H₇){(CN(^tBu)(CH) ₂N(^tBu)}Cl] (2), is also discussed.

Full text of DOI:10.1016/S0022-328X(00)00724-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 617–618 (2001) 635–639
An improved synthesis of
bis(1,3-di-N-tert-butylimidazol-2-ylidene)palladium(0) and its use in
C–C and C–N coupling reactions
a,
a,
a
Stephen Caddick *, F. Geoffrey N. Cloke * Guy K.B. Clentsmith ,
a
b
a
Peter B. Hitchcock , Darren McKerrecher , Lisa R. Titcomb ,
a
Meredith R.V. Williams
a
The Chemistry Laboratory, CPES, Uni6ersity of Sussex, Brighton, BN1 9QJ, UK
b
AstraZeneca, Alderley Park, Macclesfield, Cheshire, SK10 4TF, UK
Received 1 September 2000; accepted 28 September 2000
Abstract
A new, improved synthesis of [Pd{CN( Bu)(CH) N( Bu)} ] (1) and its use as a catalyst in coupling reactions, including
¸
¹¹¹¹¹¹¹º
t
t
2
2
¸
¹¹¹¹¹¹¹º
3
t
t
aminations, is presented. An interesting side product formed in the synthesis of 1, [Pd(h -C H ){(CN( Bu)(CH) N( Bu)}Cl] (2), is
also discussed. © 2001 Elsevier Science B.V. All rights reserved.
4
7
2
Keywords: N-heterocyclic carbenes; Aminations; Coupling reactions; Palladium complexes
1. Introduction
opportunities to devise new classes of Pd catalysts with
improved efficiency.
Palladium mediated transformations have found
In recent years it has become apparent that the
ligating properties of N-heterocyclic carbenes ex-
hibit some similarity to phosphines. Recent theoreti-
cal and experimental work on the complex
widespread applications in organic synthesis and in
particular zerovalent Pd catalysts have enjoyed remark-
able popularity as reagents for a diverse range of
coupling reactions [1]. Perhaps the most commonly
employed catalyst for C–C coupling reactions is
¸
¹¹¹¹¹¹¹º
t
t
[
Pd{CN( Bu)(CH) N( Bu)} ] (1) and its platinum ana-
2
2
logue showed that the metal-ligand bonding occurs
predominantly through s donation from the carbene
lone pairs to the metal centre [5]. A number of reports
have successfully demonstrated that a range of such
carbene ligands can be incorporated into Pd(II) com-
plexes and offer an alternative class of pre-catalysts,
with improved stability (and potential turnover capac-
ity), for C–C coupling reactions [6]. Until recently there
had been no report of a two-coordinate Pd(0) carbene
species, presumed to be the active catalyst in the latter
[
Pd(PPh ) ] which can be readily prepared from PdCl
3
4
2
and PPh in the presence of hydrazine hydrate and is
3
also commercially available [2]. Whilst this (or indeed a
Pd(II) pre-catalyst) can be used as an effective reagent
for coupling reactions, the labile nature of the Pd-P
bonds in this system means that a variety of species
may be present in the reaction mixture [3,4]. Moreover
this may have an impact on the catalyst turnover and
thus efficiency. With these factors in mind it would
seem that alternative ligand environments may offer
[7,8]. We present results of a study which describes a
simple one-pot procedure for the preparation of such a
species and some preliminary work which shows, for
the first time, that an isolated Pd(0) carbene complex
can catalyse an amination coupling reaction.
*
Corresponding author. Tel.: +44-1273-678734; fax: +44-1273-
6
0
78734.
E-mail address: s.caddick@sussex.ac.uk (S. Caddick).
022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00724-5

6
36
S. Caddick et al. / Journal of Organometallic Chemistry 617–618 (2001) 635–639
3
2. Synthesis
the isolation of a 1:2 mixture of 1 and [Pd(h -
¸¹¹¹¹¹¹¹º
t t
C H ){(CN( Bu)(CH) N( Bu)}Cl] (2); carrying out these
4
7
2
Recent results from our laboratories showed that two
reactions at elevated temperature (90°C) resulted in
coordinate palladium (0) carbene species could be read-
ily prepared 6ia metal vapour synthesis (MVS) [7].
Herrmann et al. have also shown that such species can
reduction of the starting material to palladium metal.
3
Reaction of [(Pd(h -C
H
)Cl)
] with two equivalents of
4
7
2
t
t
[CN( Bu)(CH) N( Bu)] yielded 2 in quantitative yield.
2
be
prepared
from
bis(tri-ortho-tolylphosphine)-
Crystals of 2 suitable for X-ray diffraction studies were
obtained from a toluene solution at ambient tempera-
ture. Fig. 1 shows the molecular structure of 2, with
selected bond lengths and angles collated in Table 1.
From consideration of bond angles, the gross geometry
around the Pd(II) centre is best regarded as pseudo-
square planar; the palladium–carbon bond trans to the
chlorine atom is significantly shorter than the bond
palladium(0) and the free carbene by displacement of
the phosphine ligands [8]. This procedure requires the
3
synthesis of tri-ortho-tolylphosphine and [Pd(h -
5
C H )(h -C H )] as precursors to the bis-phosphine pal-
3
5
5
5
ladium complex; the synthesis of the latter from
tri-ortho-tolylphosphine and [Pd(dba) ] has also been
2
reported [9]. Given the potential interest in new cata-
lysts of this type, we sought to devise a more simple
preparative route to these complexes. A classic synthe-
sis of zerovalent palladium phosphine complexes in-
volves addition of a nucleophile to the bound allyl unit
trans to the carbene ligand (Pd–C(12)=2.121(4) A
Pd–C14=2.158(4) A). This is consistent with the pre-
,
cf.
,
3
viously reported triphenylphosphine analogue [Pd(h -
C H )(PPh )Cl] [11,12]. The palladium–carbene bond
4
7
3
3
length in 2 is 2.061(3) A
in previously reported carbene palladium(II) complexes,
which range from 1.95 to 2.07 A; the bond distances
,
which is comparable with those
of a Pd(II) complex, typically [{Pd(h -C H )Cl} ] or
3
5
2
3
[
{Pd(h -C H )Cl} ], in the presence of stoichiometric
4
7
2
,
phosphine ligand to trap the Pd(0) centre which results
from attack at the allyl ligand [10]. Thus our initial
and angles within the carbene ligand in 2 are also
3
comparable to those found in related compounds [13].
attempts to prepare 1 involved treatment of [{Pd(h -
3
The geometry and bond lengths in the h -C H group
4
7
C H )Cl} ] with either one equivalent per Pd of either
4
7
2
in 2 are essentially identical (within estimated S.D.s) to
those found in [Pd(h -C H )(PPh )Cl] [11,12]. Variable
sodium methoxide or potassium tert-butoxide in the
presence of [CN( Bu)(CH) N( Bu)] at 0°C, which led to
3
¸
¹¹¹¹¹¹¹º
t
t
4
7
3
2
temperature nuclear Overhauser effect (nOe) experi-
ments on the terminal allyl protons showed no evidence
for fluxional behaviour in the allyl group up to a
temperature of 70°C, in contrast to the previously
reported phosphine analogue [14,15]: irradiation of
each of the terminal allyl protons did not lead to any
enhancement effects in the other terminal allyl protons.
t
However, irradiation of one set of Bu protons showed
t
an enhancement of 2.5% in the other Bu group at 25°C
increasing to 33% at 55°C, indicating slow rotation
about the palladium–carbene bond on the NMR
timescale.
3
Fig¸.
{
1.
Molecular
structure
of
[Pd(h -C H )-
¹¹¹¹¹¹º
4
7
t
t
However, addition of sodium dimethylmalonate to
(CN( Bu)(CH) N( Bu)}Cl] (2). Thermal ellipsoids are shown at the
2
3
5
0% probability level.
[{Pd(h -C H )Cl} ] in the presence of two equivalents
¸
¹¹ ¹4 ¹7¹¹¹ 2º
t
t
of [CN( Bu)(CH) N( Bu)] leads to formation of the
2
Table 1
Selected
desired bis-carbene palladium(0) complex in a one-pot
procedure. Complex 1 is obtained in 60% yield after
filtration of the reaction mixture followed by recrystalli-
sation from THF at −50°C (Scheme 1).
3
bond
lengths
and
angles
in
[Pd(h -C H )-
4
7
¸
¹¹¹¹¹¹¹º
t
t
{
(CN( Bu)(CH) N( Bu)}Cl] (2)
2
Bond
Distance (A
,
)
Group
Angle (°)
Pd–C(1)
2.061(3)
2.121(4)
2.163(4)
2.158(4)
2.378(1)
1.362(4)
1.381(5)
1.348(5)
1.389(5)
1.370(4)
C(1)–Pd–C(12)
C(12)–Pd–C(14)
C(12)–Pd–C(13)
C(13)–Pd–C(14)
C(14)–Pd–Cl
102.90(15)
68.03(18)
38.76(15)
37.38(17)
97.33(14)
91.53(10)
104.7(3)
110.3(3)
107.9(3)
106.3(3)
110.7(3)
Pd–C(12)
Pd–C(13)
Pd–C(14)
Pd–Cl
C(1)–N(2)
N(2)–C(3)
C(3)–C(2)
C(2)–N(1)
N(1)–C(1)
3
. Catalysis
Complex 1 has very recently been shown to act as
C(1)–Pd–Cl
single component catalyst in the Suzuki cross-coupling
reaction [8] and here we are able to report that 1 is also
able to catalyse a Sonogashira type cross-coupling reac-
tion with an efficiency comparable to that of
N(1)–C(1)–N(2)
C(1)–N(2)–C(3)
N(2)–C(3)–C(2)
C(3)–C(2)–N(1)
C(2)–N(1)–C(1)
[
Pd(PPh ) ] (Scheme 2).
3 4

S. Caddick et al. / Journal of Organometallic Chemistry 617–618 (2001) 635–639
637
¸
¹¹¹¹¹¹º
t
t
Scheme 1. Synthesis of [Pd{CN( Bu)(CH) N( Bu)} ] (1).
2
2
Scheme 2. Sonogashira type coupling using 1.
The bromoeneyne, 3, shown in Scheme 2 is a com-
mon building block for our investigations into the
synthesis of enediyne natural products and their ana-
logues [16,17]. Ethyl-2-iodo-3-bromopropenoate was
found to couple to trimethylsilylacetylene using 1 to
afford 3 in 85% isolated yield. Particularly encouraging
is the good conversion observed using only 1 mol% of
The complex 1 has been used in a Sonogashira type
coupling and, for the first time, in amination reactions
of p-chlorotoluene. Further investigations into the use
of 1 in couplings with other aryl halides and amines are
ongoing.
1
which is comparable with the use of [Pd(PPh ) ]
5. Experimental
3
4
[
16,17].
Catalytic amination of chloroarenes has been effected
Unless otherwise stated, all experimental procedures
were carried out using standard high vacuum and
Schlenk techniques, under an atmosphere of dry argon,
or under dinitrogen in an MBraun or a Miller–Howe
glove box. Glassware was dried in an oven at 150°C
prior to use. THF was distilled from potassium metal
and toluene from sodium metal under dinitrogen prior
t
by palladium-phospine systems, e.g. [Pd(dba) ]/P Bu
2
3
n
for p-chlorotoluene with di( butyl)amine (1 mol% Pd,
0°C, 12 h, 88% yield) [18]. Recent work by Nolan et
7
al. and Hartwig et al. has shown that imidazolium and
imidazolinium salts with bulky substituents can also be
used as additives in palladium catalysed reactions to
effect the coupling of an otherwise inert aryl chloride to
an amine [19,20]. We have also commenced preliminary
investigations into the use of 1 as a catalyst for such
couplings. These investigations have shown for the first
time that such two coordinate palladium(0) complexes
can act as useful catalysts for these reactions (Scheme
to use; d -benzene was dried over molten potassium
6
then vacuum transferred to an ampoule under argon
prior to use. NMR spectra were recorded at 295 K on
a Bruker DPX 300 MHz spectrometer, with chemical
shifts (l) reported in ppm, relative to the residual
proton chemical shifts of the internal deuterated solvent
1
13
3): p-chlorotoluene can be coupled to morpholine and
( H and C) set relative to external TMS, and relative
7
piperidine in 95% and 70% yields, respectively, which
compare favourably with reported yields (82% and
to external LiCl ( Li). Coupling constants are quoted in
Hz. Electron impact mass spectra were recorded on a
VG Autospec mass spectrometer. Elemental analyses
were carried out by Medac Ltd., Brunel Science Centre,
Surrey, UK. Morpholine, piperidine and p-chloro-
toluene were purchased from Aldrich and passed
96%, respectively) using the imidazolium salt route to
generate the catalyst in situ [19].
4. Conclusions
An¸ improved
one-pot
synthesis
of
¹¹¹¹¹¹¹º
t
t
[
Pd{CN( Bu)(CH) N( Bu)} ] (1) has been developed
2
2
3
and
an
intermediate,
[Pd(h -C H )-
¸
¹¹¹¹¹¹¹º
4
7
t
t
{
(CN( Bu)(CH) N( Bu)}Cl] (2), in the formation of 1
2
has been isolated and fully characterised. No evidence
for fluxionality of the allyl group in 2 is observed at
Scheme 3. Amination of p-chlorotoluene with morpholine and pipe-
70°C in contrast to the triphenylphosphine analogue.
ridine.

6
38
S. Caddick et al. / Journal of Organometallic Chemistry 617–618 (2001) 635–639
−
3
through a column of basic alumina prior to use. Potas-
u(Mo–K )=0.71073 A
,
, Z=4, Dcalc=1.51 Mg m
,
a
−
1
sium tert-butoxide was purchased from Aldrich and
v(Mo–K )=1.27 mm , T=173 K. Data was col-
a
−
5
sublimed twice at 157°C, 1×10
dioxane was purchased from Aldrich and stored in an
ampoule under argon over 4 A molecular sieves.
mbar. Anhydrous
lected on an Enraf-Nonius CAD4 diffractometer using
a crystal of dimensions 0.3×0.3×0.2 mm. A total of
4832 unique reflections were measured of which 4067
had ꢀF ꢀ\2|(F ). The final residuals were R =0.045
,
¸
¹¹¹¹¹¹¹º
t
t
3
2
2
[
CN( Bu)(CH) N( Bu)] [7] and [(Pd(h -C H )Cl) ] [21]
2
4
7
2
1
were synthesised according to literature procedures.
(for I\2|(I)) and wR =0.121 for all data.
2
For a procedure for the synthesis of (Z)-ethyl-2-
bromo-3-iodopropenoate, its coupling with trimethyl-
silylacteylene and for characterisation data for (Z)-
5
.1. Synthesis of bis(1,3-di-tert-butylimidazol-2-
¸¹¹¹¹¹¹¹º
t
t
ylidene)palladium(0) [Pd{CN( Bu)(CH) N( Bu)} ] (1)
ethyl-2-bromo-3-iodopropenoate
and
(Z)-ethyl-2-
2
2
bromo-3-ethynyl-(trimethylsilyl)propenoate (3) see
Refs. [16,17].
¸
¹¹¹¹¹¹¹º
t
t
3
[
CN( Bu)(CH) N( Bu)] (183 mg, 1.02 mmol), [(Pd(h -
2
C H )Cl) ] (100 mg, 0.254 mmol) and sodium dimethyl-
4
7
2
malonate (78 mg, 0.506 mmol) were placed in an
ampoule equipped with a Rotaflo stopcock and THF
added (30 ml). The mixture was heated at 90°C for 16
h with stirring after which the solution was brown.
After cooling to room temperature, the mixture was
filtered from some free palladium metal and the solu-
tion concentrated. Crystallisation at −50°C yielded the
product as a yellow solid in 60% yield (143 mg).
5.3. Coupling of p-chlorotoluene with morpholine and
piperidine
5.3.1. General procedure
The catalyst (7.4 mg, 0.16 mmol) and KO Bu (133
t
mg, 1.19 mmol) were weighed into a tube in a glovebox
and capped with a septum. Dioxane (5 ml) was added.
Chlorotoluene (100 mg, 0.79 mmol) followed by the
amine (0.95 mmol) were injected into the tube. The
mixture was heated at 100°C for 16 h. The product was
purified by absorption of the crude reaction mixture
onto silica followed by flash chromatography eluting
with a solvent gradient starting with hexane and finish-
ing with 5% EtOAc–hexane.
Elemental analysis. Anal. Found: C, 56.67; H, 8.54;
N, 12.22. Calc. for [C H N Pd]: C, 56.58; H, 8.63; N,
2
2
40
4
1
2.00%.
1
H-NMR (C D ): l 2.12 (18H, s, NC(CH
6
3)3), 6.74
6
6
13
1
(
2H, s, NCH
NC(CH ) ), 57.35 (s, N C6 (CH ) ), 113.95 (s, N C6 C).
3 3
6 CH6 N). C{ H}-NMR (C D ): l 31.96 (s,
6
6
6
3
3
5
.2. Synthesis of chloro(1,3-di-tert-butylimidazol-2-
The respective coupling products were found to be
identical by H-NMR with literature data for 1-(4-
methylphenyl)piperidine [22] and 1-(4-methylphenyl)-
morpholine [23].
3
1
ylidene)(methylallyl)palladium(II) [Pd(p -C H )-
¸
¹¹¹¹¹¹¹º
4
7
t
t
{(CN( Bu)(CH) N( Bu)}Cl] (2)
2
3
To a Schlenk tube containing [(Pd(h -C H )Cl) ] (260
¸
¹¹¹¹¹¹¹º
4
7
2
t
t
mg, 0.66 mmol) and [CN( Bu)(CH) N( Bu)] (237 mg,
2
1.32 mmol) toluene (20 ml) was added. The solids
dissolved to give an orange solution. The solution was
concentrated and the product crystallised at ambient
6. Supplementary material
temperature in 90% yield.
Crystallographic data for the structures reported in
this paper have been deposited with the Cambridge
Crystallographic Data Centre, CCDC no. 148362 for
complex 2. Copies of this information may be obtained
free of charge from the Director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
1
H-NMR (C D ): l 1.55 (s, 9H, NC(CH
6
3)3), 1.69 (s,
6
6
9
1
1
H, NC(CH
6
) ), 1.71 (s, 3H, CH CCH
6
CH ), 2.15 (s,
3
3
2
3
2
H, broad), 3.05 (m, 1H), 3.24 (s, 1H, broad), 4.07 (m,
1
3
1
H) (CH
6
CCH CH ), 6.69 (m, 2H, NCCH
6
6
). C{ H}-
2
3
2
NMR (C D ): l 22.73 (s, CH CC
6 H CH ) 31.73 (s,
H ) ), 52.04 (s,
(CH ) ), 58.53 (s,
6
6
2
3
2
NC(C
6
H ) ), 31.99 (s, NC(C
6
3
3
3 3
CH CCH C
6
H2), 57.96 (s, NC
6
2
3
3 3
NC
NCC
6 (CH ) ), 67.91 (s, CH CCH C6 H2), 118.06 (s,
3 3 2 3
6
H), 118.12 (s, NC
6 CH), 127.29 (s, CH C6 CH CH ),
2 3 2
+
1
M
78 (s, NC
6
N). EI MS: m/z 378 (33%, M ), 341 (7%,
+
+
+
–Cl), 286 (100%, M –C H and Cl), 229 (93%,
Acknowledgements
4
7
t
M -C H , Cl and Bu). C H ClN Pd (378): Calc.: C,
4
7
15 27
2
4
7.87, H, 7.18, N, 7.45. Anal. Found: C, 47.72, H, 6.95,
The authors thank the EPSRC and AstraZeneca for
financial support and are grateful to Dr. A. Abdul-Sada
for recording the mass spectra and to Dr. A. Avent for
recording NMR spectra.
N, 7.52. Crystal data: C H ClN Pd, monoclinic, FW
3
9
15
27
2
77.24, space group P2 /c (no. 14); a=10.951(4), b=
1
3
.987(4), c=15.400(6) A, i=98.8°, U=1664(1) A ,
, ,

S. Caddick et al. / Journal of Organometallic Chemistry 617–618 (2001) 635–639
639
[
9] F. Paul, J. Patt, J.F. Hartwig, Organometallics 14 (1995) 3030.
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