N-Heterocyclic Olefin-Ligated Palladium(II) Complexes as Pre-Catalysts for Buchwald–Hartwig Aminations

Article abstract of DOI:10.1002/chem.201901376

New N-heterocyclic olefins (NHOs) are described with functionalization on the ligand heterocyclic backbone and terminal alkylidene positions. Various Pd^II–NHO complexes have been formed and their use as pre-catalysts in Buchwald–Hartwig aminations was explored. The most active system for catalytic C?N bond formation between hindered arylamine and arylhalide substrates was accessed by combining a backbone methylated NHO with [Pd(cinnamyl)Cl]₂ in the presence of NaOtBu as a base. In these active systems evidence suggests that catalysis is mediated by colloidal palladium metal, highlighting a different coordination ability of NHOs in comparison with commonly used N-heterocyclic carbene co-ligands.

Full text of DOI:10.1002/chem.201901376

A Journal of
Accepted Article
Title: N-Heterocyclic Olefin Ligated Palladium(II) Complexes as Pre-
Catalysts for Buchwald-Hartwig Aminations
Authors: Ian Watson, André Schumann, Haoyang Yu, Emma Davy,
Robert McDonald, Michael Ferguson, Christian Hering-
Junghans, and Eric Rivard
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To be cited as: Chem. Eur. J. 10.1002/chem.201901376
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N-Heterocyclic Olefin Ligated Palladium(II) Complexes as Pre-
Catalysts for Buchwald-Hartwig Aminations
Ian C. Watson,a André Schumann,b Haoyang Yu,a Emma C. Davy,c Robert McDonald,a Michael J.
Ferguson,a Christian Hering-Junghans,b,* Eric Rivarda,*
R2
R3
R1
N
R2
Abstract: New N-heterocyclic olefins (NHOs) are described with
functionalization on the ligand heterocyclic backbone and terminal
alkylidene positions. Various Pd(II)-NHO complexes have been
formed and their use as pre-catalysts in Buchwald-Hartwig
aminations was explored. The most active system for catalytic C-N
bond formation between hindered arylamine and arylhalide
substrates was accessed by combining various NHOs with
[Pd(cinnamyl)Cl]2 in the presence of NaOtBu as a base. In these
active systems our evidence suggests that catalysis is mediated by
colloidal palladium metal, highlighting a different coordination ability
of NHOs in comparison with commonly used N-heterocyclic carbene
co-ligands.
R2
N
R3
N
R2
R2
N
R3
N
R1
R1
R2
R1
I
II
III
Chart 1. Generic structures of NHCs (I), aNHCs (II) and CAACs (III).
Owing to their strong σ-donating properties and their easily
tuneable steric and electronic properties, NHCs have joined
phosphines as ligands of choice in palladium-catalyzed cross-
coupling reactions.[5,6] The most commonly explored Pd(II)-
containing pre-catalysts for cross-coupling are outlined in Chart
2, and include the 1:1 PdCl2-ligand complex (A),[7] palladium-allyl
species (B),[8] and generally active “PEPPSI” complexes bearing
an NHC and 3-chloropyridine (3-Cl-pyr) in a mutually trans
orientation (C).[9]
Introduction
Since the discovery of bottleable N-heterocyclic carbenes
(NHCs) by Arduengo and co-workers (Chart 1, I),[1] these
carbon-based donors have been used with great success as
ancillary ligands in metal-mediated catalysis.[2] These studies
were followed by the development of abnormal N-heterocyclic
carbenes (aNHCs, II) that strongly coordinate metals through
anionic backbone (C4 or C5) positions.[3] Furthermore,
replacement of one ring-positioned N atom in an NHC for an sp3-
hybridized carbon atom yields cyclic(alkyl)amino carbenes
(CAACs, III), which are better π-acids when compared with
NHCs (Chart 1).[4]
R2
R1
N
R1
Cl
Dipp
Cl
R2
N
N
N
Cl
R1
R2
N
N
Dipp
Pd Cl
Pd
Pd
Pd
R1
Dipp
N
Cl
Cl
Cl
N
Cl
N
R3
Dipp
A
B
C
Chart 2. Widely investigated Pd(II) pre-catalysts bearing NHC co-ligands; Dipp
= 2,6-iPr2C6H3.
[a]
I. C. Watson, H. Yu, Dr. R. McDonald, Dr. M. J. Ferguson, Prof. Dr.
E. Rivard
N-Heterocyclic olefins (NHOs) represent an emerging
class of carbon-based donors that each contain a polarized,
ylidic, alkylidene unit (=CH2 or =CR2) terminally linked to an N-
heterocyclic carbene fragment (see Scheme 1 for contributing
resonance forms).[10,11] The first isolable example of an NHO,
(MeCNMe)2C=CH2, was described by Kuhn and co-workers in
1993,[10,11] with nucleophilic/donor ability at the terminal carbon
atom demonstrated.[10-13] Moreover, N-Heterocyclic olefins are
considered to be softer σ-donors than NHCs[13] and might yield
stable coordination complexes with the soft Pd(0) centers found
during cross-coupling catalysis. While the seminal work by
Kuhn and co-workers introduced various NHO•M(CO)5
complexes to the community (M = Cr, Mo and W),[10b] the
Department of Chemistry
University of Alberta
11227 Saskatchewan Dr., Edmonton, Alberta, T6G 2G2, Canada
E-mail: erivard@ualberta.ca
A. Schumann, Prof. Dr. C. Hering-Junghans
Leibniz Institute for Catalysis
University of Rostock
Albert Einstein Str. 29a, 18059 Rostock, Germany
Prof. Dr. E. C. Davy
Department of Physical Sciences
Quest University
3200 University Boulevard, Squamish, British Columbia, V8B 0N8,
Canada
[b]
[c]
Supporting information and the ORCID identification numbers for the
authors of this article can be found under:
https:doi.org/10.1002/chem.xxxxxxxxx.
1
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number of metal complexes comprising NHOs as ligands is still
limited, with examples of Au, Ir and Rh complexes now
known.[12a,13-15] NHOs have also been used to stabilize reactive
main group environments,[12b,16] and an exciting new direction is
their use in organocatalysis.[11,17,18]
Dipp
Me
Dipp
1) 2 KOtBu
2) BrCH2CH=CH2
Me
Me
N
N
CH2
(iii)
(iv)
H
Cl
THF, RT
- KCl, KBr
- 2 HOtBu
N
N
Me
Dipp
Dipp
1
H
N
H
N
H
N
H
N
H
N
H
N
1) 2 KOtBu
2) H3C-I
Dipp
Dipp
N
N
R
R
R
R
R
R
CH2
Cl
H
THF, RT
- KCl, KI
- 2 HOtBu
N
N
Dipp
Dipp
2
Scheme 1. Dominant canonical forms of N-heterocyclic olefins (NHOs).
1) 2 KOtBu
2) H3C-I
Dipp
Dipp
N
N
CH2
(v)
H
Cl
THF, RT
- KCl, KI
- 2 HOtBu
N
N
In this paper, we describe the synthesis of new N-
heterocyclic olefin ligands, including those bearing extended
backbone π-conjugation and functionalization at the terminal
alkylidene group. In addition it is shown that some NHO-Pd(II)
complex combinations are viable pre-catalysts for the selective
Buchwald-Hartwig C-N cross-coupling of hindered substrates,
with evidence for heterogeneous catalysis modulated by Pd
nanoparticles.
Dipp
Dipp
3
Dipp
Dipp
I
I
R
R
R
R
n
N
N
n
(vi)
3
toluene, RT
N
N
- 2 [RIPrH]Cl
Dipp
Dipp
4 (R = H, n = 2)
5 (R = Me, n = 2)
6 (R = H, n = 1)
7 (R = Me, n = 1)
Results and Discussion
Scheme 3. Structurally modified NHOs (1-7) examined in the current study.
Synthesis of N-Heterocyclic Olefins (NHOs) and their
Respective Pd(II) Complexes.
In a recent paper,[13] high yielding one-pot protocols were
introduced to form the bulky NHOs MeIPrCH2 and IPrCH2 [MeIPr =
In order to expand the range of NHOs available and
introduce possibly new (stabilizing) binding modes with late
transition metals, a variety of modified NHOs were prepared
(Scheme 3). For the first ligand candidate we synthesized the
butadiene-NHO MeIPr=CH-CH=CH2 (1). The structurally related
species IPr=CH-CH=CH2 was prepared by Jacobi von Wangelin
and co-workers with nucleophilic character at the exocyclic α-
and γ-C atoms postulated.[19,20] In a modified procedure, the
imidazolium salt [MeIPrH]Cl was combined with allyl bromide in
the presence of 2 equiv. of KOtBu to give MeIPr=CH-CH=CH2 (1)
in a 84 % yield as a yellow crystalline solid (Figure 1).
Placement of Me groups at the backbone of 1 was designed to
suppress possible C-H activation at the 4- or 5-positions in the
presence of Pd(II) complex and base; related NHO ligand
activation has been recently noted by Schumann and Hering-
Junghans.[21]
An N-heterocyclic olefin bearing a π-extended acenaphthene
backbone[22] IPr(BIAN)CH2 (2) was also prepared using a
procedure analogous to that used to obtain 1 (Scheme 3).
IPr(BIAN)CH2 (2) was isolated as a deep blue air- and moisture-
sensitive powder (87 % yield), and X-ray quality crystals were
obtained from a benzene/hexanes mixture at 23 °C (Figure 2).[23]
In a similar fashion, the saturated analogue of IPrCH2, SIPrCH2
(3) (SIPr = [(H2CNDipp)2C]), previously reported by Ghadwal
and co-workers,[24] was obtained as a colorless crystalline solid
in 70 % yield using the modified one-pot procedure outlined in
Scheme 3.
(MeCNDipp)2C; IPr
= (HCNDipp)2C; Dipp =
2,6-iPr2C6H3].
Depending on the approach used, either MeI (reaction i,
Scheme 2) or the alkylchlorosilane ClCH2SiMe3 (reaction ii,
Scheme 2) can be used as methylene sources.
Dipp
Dipp
1) 2 KOtBu
2) H3C-I
Me
Me
Me
Me
N
N
(i)
H
CH2
Cl
THF, RT
- KCl, KI
- 2 HOtBu
N
N
Dipp
Dipp
MeIPrCH2
Dipp
Dipp
N
N
Cl
SiMe3
(ii)
CH2
- ClSiMe3
toluene
N
N
Dipp
Dipp
IPrCH2
Scheme 2. Established synthetic routes towards MeIPrCH2 (i, top) and IPrCH2
(ii, bottom).
2
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Figure 1. Molecular structure of MeIPr=CH-CH=CH2 (1) with thermal ellipsoids
shown at a 30 % probability level. Hydrogen atoms on the backbone and on
the Dipp groups are omitted for clarity. Selected bond lengths [Å] and angles
[°] with values belonging to a second molecule in the asymmetric unit in
square brackets: C1A–C2A 1.328(3) [1.338(2)]; C2A−C3 1.411(3) [1.282(1)];
C3−C4 1.369(3) [1.369(3)]; N1−C4−N2 104.17(15) [104.17(15)]; C1A-C2A-C3
127.1(3) [125.6(11)].
Figure 3. Molecular structure of IPr=C(CH2)4 (4) with thermal ellipsoids shown
at a 30 % probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [°] with values belonging to a second molecule in
the asymmetric unit in square brackets: N1A−C1A 1.414(2) [1.4096(19)],
N2A−C1A 1.420(2) [1.4068(19)], C1A−C4A 1.343(2) [1.353(2)]; C4A−C5A
1.522(2) [1.521(2)]; C6A−C7A 1.446(4) [1.522(2)]; N1A−C1A−N2A 103.98(13)
[103.48(13)], C4A−C5A−C6A 103.65(17) [103.79(13)].
A
series of NHOs bearing ring-fused cycloalkane
substituents (compounds 4-7, Scheme 3, reaction vi) were
generated in a one-pot procedure by treatment of the requisite
1,5-diiodoalkane with three equivalents of carbene (IPr or MeIPr)
in toluene. The resulting NHOs were soluble in organic solvents,
facilitating their separation from the insoluble imidazolium salt
by-product ([IPrH]I or
[
MeIPrH]I) via filtration. The crystal
structures of the ring-fused compounds IPr=C(CH2)4 (4) and
IPr=C(CH2)3 (6) (Figures 3 and S13)[23] revealed identical ylidic
C=C distances of 1.343(2) Å and 1.3432(21) Å respectively. The
hydrocarbon five-membered ring (=C(CH2)4) in 4 is non-planar,
while the related four-membered ring in IPr=C(CH2)3 (6) is planar.
A recent theoretical study revealed a high proton affinity of ring-
fused NHOs, with their basicity reaching the high-end of
“superbasicity”.[25]
To evaluate possible differences in donor capability amongst
the NHOs 1-4 and 6, computations at the B3LYP/6-31G+(d,p)
level of density functional theory (DFT) were carried out.[23] As
expected, these NHOs possess exocyclic double bonds with
substantially polarized terminal C=C π-components, leading to
accumulation of negative charge on the exocyclic carbon atom.
For compounds 2 and 3 the charge on the terminal CH2 carbon
atom was computed to be −0.69e and −0.67e, respectively (as
determined by a natural population analysis, NPA). In contrast,
the corresponding degree of C=C bond polarization in the
bicyclic NHOs 4 and 6 is less pronounced, as reflected by lower
NPA charges of −0.23e and −0.24e, respectively, and almost
non-polar π-components of the corresponding C=C double
bonds according to Natural Bond Order (NBO) analysis (see
ESI).[23] In MeIPr=CH-CH=CH2 (1) the largest negative charge
(−0.53e) is found on the terminal exocyclic carbon atom,
suggesting preferential metal ligation via an end-on mode (vide
infra).
Figure 2. Molecular structure of IPr(BIAN)CH2 (2) with thermal ellipsoids
shown at a 30 % probability level. Hydrogen atoms on the backbone and on
the Dipp groups are omitted for clarity. Selected bond lengths [Å] and angles
[°]: C1−C4 1.336(3), N1−C1 1.405(2), N2−C1 1.403(2); N2−C1−N1 105.34(17).
The majority of the NHOs described in this study have been
crystallographically characterized, and selected examples are
presented in Figures 1-3.[23] MeIPr=CH-CH=CH2 (1) shows bond
alternation within the exocyclic =CH-CH=CH2 group as
evidenced by shorter C4-C3 [1.369(3) Å] and C2A-C1A
[1.328(3) Å] distances (Figure 1) compared to the central bond
C3-C2A [1.411(3) Å]. The exocyclic =CH-CH=CH2 unit in 1 is in
the same plane as the proximal 5-membered imidazole ring. The
structure of the deep blue IPr(BIAN)CH2 (2) was also determined
by X-ray crystallography (Figure 2) and the exocyclic C1-C4
linkage [1.336(3) Å] is of a typical length for an N-heterocyclic
olefin;[11,12b] likewise standard metrical parameters for the
backbone saturated SIPrCH2 (3) were noted (Figure S6).[23]
To determine whether the bicyclic NHOs 4-7 introduced
above were able to act as formal two-electron donors, the
methylated analogue MeIPr=C(CH2)4 (5) was combined with
3
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MeOTf. As expected, this reaction afforded the methylated salt
[
[(MeIPrCH2)PdCl(µ-Cl)]2 (9) (Scheme 5). The most drastic
structural change within the NHO ligand upon coordination is
elongation of the once terminal C=C bond from a length of
1.349(2) Å[13] to a single bond C1-C4 distance of 1.453(3) Å in 9,
consistent with transfer of exocyclic C=C π-electron density from
MeIPrCH2 to Pd. The resulting coordinative Pd1−C4 distance of
2.026(2) Å in 9 is ca. 0.07 Å longer than in the corresponding
NHC-capped PdCl2 complex [(IPr)Pd(µ-Cl)]2 [1.955(3) Å], which
retains an similar overall geometry as in 9.[8,26]
MeIPrC(Me)(CH2)4]OTf (8) (Scheme 4 and Figure 4).
Dipp
Dipp
OTf
Me
Me
Me
Me
N
MeOTf
N
toluene, RT
N
N
Me
Dipp
Dipp
5
8
Me
Dipp
Dipp
Scheme 4. The methylation of MeIPr=C(CH2)4 (5) by treatment with MeOTf.
N
Me
Me
Cl2Pd(NCPh)2
toluene, RT
N
Me
CH2
1/2
N
H2C
N
Dipp
Dipp
Cl Pd
Cl
Me
Pd
Cl
Cl
Dipp
xs 3-Cl-pyr
CH2Cl2, RT
Me
9
Dipp
N
CH2
N
N
Me
Dipp
N
CH2
Dipp
Me
10
Cl Pd Cl
N
Cl
Scheme 5.Synthesis of [(MeIPrCH2)PdCl(µ-Cl)]2 (9) and [(MeIPrCH2)PdCl2(3-Cl-
pyr)] (10).
Prior work by Organ and co-workers revealed that their
(NHC)PdCl2(3-Cl-pyr) “PEPPSI” complexes were active in C-N
bond forming catalysis, and they selectively achieved either
mono- or diarylation of primary amines (ArNH2) depending on
the choice of NHC and base.[27] Given the lower steric bulk of
NHOs in relation to NHCs and possibly enhanced soft-soft NHO-
Pd(0) interactions during catalysis, we prepared the potential
pre-catalyst [(MeIPrCH2)PdCl2(3-Cl-pyr)] (10) by addition of 3-
chloropyridine to a solution of 9 in CH2Cl2 (Scheme 5). After
work-up of the reaction mixture, including product
recrystallization from CH2Cl2/hexanes (-30 °C), yellow X-ray
quality crystals of [(MeIPrCH2)PdCl2(3-Cl-pyr)] (10) were obtained
in 67 % yield (Figure 5). The ligating Pd−C interaction in
[(MeIPrCH2)PdCl2(3-Cl-pyr)] (10) [2.043(6) Å] is the same within
experimental error as in the pyridine-free precursor 9, while the
trans-disposed Pd-N3-Cl-pyr bond has a length [2.137(6) Å] that is
similar as the related Pd-N distance of 2.137(2) Å in the N-
heterocyclic carbene complex [(IPr)PdCl2(3-Cl-pyr)].[9a]
Figure 4. Molecular structure of [MeIPrC(Me)(CH2)4]OTf (8) with thermal
ellipsoids shown at a 30 % probability level. Hydrogen atoms have been
omitted for clarity. Selected bond lengths [Å] and angles [°]: C1−N1 1.3542(17),
C1−N2 1.3526(18), C1-C6A 1.546(9); C6A-C11A 1.508(8); C8A-C9A 1.531(8);
N2−C1−N1 105.85(12); C1-C6A-C11A 107.7(5); C6A-C10A-C9A 103.9(5).
MeIPr=CH-CH=CH2 (1) shows four distinct resonances for
butadiene =CH-CH=CH2-group with the CH proton at the vinylic
position being downfield shifted compared to NHOs 2 and 3.
1
NHOs 1-7 show H NMR resonances (in C6D6) consistent with
the formulated structures with upfield-positioned terminal
methylene =CH2 resonances ranging from 2.42 to 2.72 ppm,
with the most deshielded environment arising from the π-
electron-rich NHO IPr(BIAN)CH2 (2). The resulting 13C{1H} NMR
shifts for the NHC-appended methylene carbon atoms (=CR2)
range from 48.3 ppm in IPr(BIAN)CH2 (2) to 75.3 ppm for the
bicyclic NHO MeIPr=C(CH2)4 (5).
IPr(BIAN)CH2 (2) adopts parallel coordination chemistry as
outlined for MeIPrCH2, which enabled the stepwise formation of
the red complex [{IPr(BIAN)CH2}PdCl(µ-Cl)]2 (11) (Figure
With an expanded library of N-heterocyclic olefin ligands in
hand, we then decided to explore their coordinating ability
towards Pd(II) centers, with the ultimate goal of accessing
suitable pre-catalysts for C-N bond formation (amination). The
first example of such a complex was prepared by combining a
slight molar excess of MeIPrCH2 with trans-[Cl2Pd(NCPh)2] in
toluene, leading to the deposition of a red crystalline precipitate.
This product was identified by X-ray crystallography (Figure
S25)[23]
and
its
yellow
3-chloropyridine
adduct
[{IPr(BIAN)CH2}PdCl2(3-Cl-pyr)] (12) (Scheme 6). The synthesis
of the dimeric NHO-PdCl2 adduct 11 proceeded in a low isolated
yield of 31 %, and despite repeated attempts, this compound
routinely contained ca. 10 % unknown impurities; thus the 3-
chloropyridine adduct 12 was prepared from in situ generated 11.
Despite the change in the structure of the coordinating NHO, the
S20)[23]
as
the
centrosymmetric
µ-Cl-bridged
dimer
4
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metrical parameters involving the Pd center in the IPr(BIAN)CH2
complexes 12 were similar to its MeIPrCH2 analogue (Figure 6).
Our attempts to yield isolable Pd(II) complexes between the
ring-fused NHOs 4-7 and Pd(II) precursors gave no reaction in
each case; this observation is likely due to the small steric
pocket that would result upon coordinating 4-7 to Pd (vide supra).
Dipp
N
Dipp
Cl2Pd(NCPh)2
toluene, RT
N
CH2
1/2
N
H2C
N
Dipp
Cl
Dipp
Cl Pd
Pd
Cl
Cl
Dipp
xs 3-Cl-pyr
CH2Cl2, RT
11
Dipp
CH2
CH2
N
N
N
N
Dipp
Dipp
Cl Pd Cl
N
12
Cl
Scheme 6. Synthesis of [{IPr(BIAN)}PdCl(µ-Cl)]2 (11) and [{IPr(BIAN)}PdCl2(3-
Cl-pyr)] (12).
MeIPr=CH-CH=CH2 (1) was also complexed with Pd(II)
centers in order to verify if η1-coordination occurs via the α- or γ-
position of the NHO, or whether an allyl-type η3-coordination
mode prevails. It was also hoped that during catalysis, the
presence of an added olefinic unit could lead to Pd(0) complex
stabilization via metal to C=C π* back-bonding. When complex 1
was combined in toluene with trans-[Cl2Pd(NCPh)2], the
Figure 5. Molecular structure of [(MeIPrCH2)PdCl2(3-Cl-pyr)] (10) with thermal
ellipsoids shown at a 30 % probability level. Hydrogen atoms have been
omitted for clarity. Selected bond lengths [Å] and angles [°]: N1−C2 1.354(5),
N2−C2 1.351(5), C1−C2 1.448(7), Pd1−N3 2.137(6), Pd1−C1 2.043(6);
N2−C2−N1 107.0(3), C2−C1−Pd1 116.7(3).
formation
of
a
red
precipitate
(presumably
[(MeIPrCHCHCH2)PdCl(µ-Cl)]2, vide infra) was observed. This
compound was difficult to purify in a consistent fashion (c.f.
compound 11 above), thus crude samples of this complex were
subsequently combined with an excess of 3-chloropyridine to
yield [(MeIPrCHCHCH2)PdCl2(3-Cl-pyr)] (13) as an analytically
pure red solid in a 44 % yield (Scheme 7). As shown in Figure 7,
coordination of the NHO 1 to Pd is achieved through the less
sterically hindered γ-position with a Pd-C distance of 2.0393(18)
Å.
Figure 6. Molecular structure of [{IPr(BIAN)}PdCl2(3-Cl-pyr)] (12) with thermal
ellipsoids shown at a 30 % probability level. Hydrogen atoms have been
omitted for clarity. Selected bond lengths [Å] and angles [°]: N1−C2 1.368(3),
N2−C2 1.373(3), C1−C2 1.447(3), Pd1−C1 2.045(2), Pd1−N3 2.147(2);
N1−C2−N2 107.41(19), C1−Pd1−N3 171.65(9), C2−C1−Pd1 120.09(17).
Figure 7. Molecular structure of [(MeIPrCHCHCH2)PdCl2(3-Cl-pyr)] (13) with
thermal ellipsoids shown at a 30 % probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [°]: N1−C1 1.353(2),
N2−C1 1.348(2), C1−C4 1.433(2); C4−C5 1.353(2); C5−C6 1.453(2); C6−Pd
2.0393(18); Pd−N3 2.1487(1); N2−C1−N1 106.28(13), C5−C6−Pd 103.23(12).
5
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promoted the reaction quickly, with complete conversion after 20
minutes under the same conditions. The role of solvent on this
cross-coupling was also explored, and it was found that THF
(Table 3) consistently gave better yields for the p-toluidine/4-
chlorotoluene coupling in relation to reactions conducted in 1,4-
dioxane or toluene. It should also be mentioned that the use of
pre-dried THF from a commercial solvent purification system
further dried over sodium and benzophenone and distilled gave
the best yields, whereas if one does not take care to exclude
water/oxygen from the THF, then a lowering of conversion
occurred (< 47 % yield for conditions in entry 4, Table 3).
Me
Me
Dipp
N
N
Dipp
Dipp
1) Cl2Pd(NCPh)2
toluene, RT
Me
Me
N
CH2
2) xs 3-Cl-pyr
CH2Cl2, RT
N
Cl Pd Cl
N
Dipp
13
Cl
Table 1. Optimization of palladium source for the cross-coupling of p-toluidine
and 4-chlorotoluene.
Scheme 7. Synthesis of [(MeIPrCHCHCH2)PdCl2(3-Cl-pyr)] (13).
0.5 mol. % [Pd]
H
1 mol. % MeIPrCH2
Cl H2N
N
1.5 eq. NaOtBu
THF, 80 °C
Me
Me
Me
Me
Catalytic Buchwald-Hartwig Aminations.
Motivated by the ability of (NHC)PdCl2(3-Cl-pyr) complexes to
act as effective pre-catalysts for cross-coupling,[9,27] we focused
our initial C-N bond catalysis screening on the (NHO)PdCl2(3-Cl-
pyr) analogues 12 and 13. Upon examining cross-coupling with
the test substrates p-toluidine (4-methylaniline) and 4-
chlorotoluene, no catalytic activity in the presence of the pre-
catalysts 12 and 13 was observed, neither in THF, toluene, nor
1,4-dioxane. With the goal of preparing more active
homogeneous Pd(0) complexes in situ, we explored catalyst
mixtures derived from mixing the Pd sources [Pd(cinnamyl)Cl]2,
Pd2(dba)3 (dba = dibenzylideneacetone), Pd(OAc)2 or PdCl2 with
two equivalents of the common NHO donor, MeIPrCH2 in THF
(80 °C, NaOtBu). As outlined in Table 1, this general procedure
led to the efficient catalytic coupling of p-toluidine and 4-
chlorotoluene. Based on an average of three runs per Pd source,
it was found that the highest conversion, along with the best
reproducibility, occurred with the [Pd(cinnamyl)Cl]2/MeIPrCH2 pre-
catalyst mixture (93 ± 5 % conversion after 1 hr at 80 °C). We
also explored the addition of NEt3 to facilitate the reduction of
Pd(II) complexes to Pd(0),[28] however only marginal
improvement in the case of PdCl2 as a metal source was found
(Table 1, entries 5 and 6).
Entry
Pd source[a]
Yield [%][c]
1
2
3
4
5
6
[Pd(cinnamyl)2Cl]2
Pd2(dba)3
93(5)
85(15)
41(7)
39(6)
31(15)
44(4)
Pd(OAc)2
[a]
Pd(OAc)2
PdCl2
[a]
PdCl2
[a] 0.5 mol% NEt3 were used [b] 1 mol% was used [c] Yield determined by 1H
NMR using 1,3,5-trimethoxybenzene as an internal standard. (+/-) in
parentheses for triplicate runs.
Table 2. Optimization of the NHO ligand for the cross-coupling of p-toluidine
and 4-chlorotoluene.
0.5 mol. % [Pd(cinnamyl)Cl]2
H
Cl H2N
1 mol. % NHO
N
1.5 eq. NaOtBu
THF, 80 °C
Me
Me
Me
Me
Entry
NHO[a]
Yield [%][b]
We then set to explore the influence of different NHO
ligands on the catalytic activity when partnered with
[Pd(cinnamyl)Cl]2 as a common Pd source (Table 2). In these
trials, the influence of the saturation of the imidazole backbone
(c.f. entry 4), on appending π-extended units (entries 2 and 5),
and upon substitution at the terminal methylidene group (=CH2
vs. a ring-fused =C(CH2)4 unit; entries 1 and 3) were examined.
It was found that NHOs with an unsaturated backbone showed
comparably excellent catalytic activity (> 93 % conversion after 1
hr, 80 °C, NaOtBu), with the exception of IPr(BIAN)CH2, which
only facilitated the coupling of p-toluidine with 4-chlorotoluene up
to a conversion of 9 ± 2 % (Table 2, entry 5). As a result,
MeIPrCH2 was selected as the ligand of choice for all future
cross-coupling trials as it is a commonly used NHO in our group
that can be synthesized easily on a > 20 g scale. We also
compared the ability of MeIPr, the NHC analogue to MeIPrCH2, to
perform the cross-coupling of p-toluidine and 4-chlorotoluene.
We found that the MeIPr/[Pd(cinnamyl)Cl]2 pre-catalyst system
1
2
3
4
5
6
MeIPrCH2
93(5)
96(4)
97(3)
7(7)
MeIPrCHCHCH2
MeIPrC(CH2)4
SIPrCH2
IPr(BIAN)CH2
MeIPr
9(2)
99(1)[c]
[a]
1
mol% was used; [b] Yield determined by 1H NMR using 1,3,5-
trimethoxybenzene as an internal standard. (+/-) in parentheses for triplicate
runs; [c] Conversion completed after 20 minutes.
After our initial catalyst screening trials led us to select
[Pd(cinnamyl)Cl]2/MeIPrCH2 as our preferred pre-catalyst for
Buchwald-Hartwig aminations, we were interested in expanding
the scope of this reaction. The motivating postulates behind
6
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10.1002/chem.201901376
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exploring NHOs as ligands for this chemistry were: a) the soft
nature of N-heterocyclic olefin (NHO) donors might help stabilize
Pd(0) intermediates during catalysis, and b) that the lower steric
bulk of NHOs compared to N-heterocyclic carbenes could
facilitate Pd-X/amine exchange (X = halide).[28]
protocol, we scaled up the coupling of 4-Cl-toluene with 4-
toluidine, starting from 10 mmol of the arylhalide and were able
to isolate 1.84 g of di(p-tolyl)amine (94% yield).
Table 3. Optimization of the solvent used for the cross-coupling of p-toluidine
and 4-chlorotoluene.
0.5 mol. % [Pd(cinnamyl)Cl]2
H
1 mol. % MeIPrCH2
Cl H2N
N
1.5 eq. NaOtBu
solvent, 80 °C
Me
Me
Me
Me
Entry
Solvent
Yield [%][a]
1
2
3
4
1,4-dioxane
THF
63(2)
93(5)
17(5)
47(7)
Toluene
THF[b]
[a] Yield determined by 1H NMR using 1,3,5-trimethoxtbenzene as an internal
standard. (+/-) in parentheses for triplicate runs. [b] THF not distilled from
sodium and benzophenone.
We then focused our efforts on the coupling of sterically
encumbered arylhalides with bulky arylamines (Scheme 8). In all
cases the selective formation of secondary diarylamines
occurred. For example, mesitylamine (MesNH2) was coupled
with 4-bromotoluene to give the mono-coupled product Mes(p-
tolyl)NH in a 88 % isolated yield. We noted that further
increasing the steric hindrance on the amine position resulted in
invariant results. In the coupling of diisopropylaniline (DippNH2)
with 4-bromotoluene full conversion could not be achieved; even
after heating for 2 days at 80 °C only 12% of the product Dipp(p-
tolyl)NH could be isolated after purification by flash
chromatography. Interestingly, the coupling of the sterically more
demanding bromomesitylene (MesBr) with DippNH2 proceeded
smoothly, and full conversion was noted after
1 hr (as
determined by 1H NMR spectroscopy); after work-up,
Mes(Dipp)NH was isolated in a 93 % yield. Having established
the coupling of sterically demanding substrates, we turned to 9-
bromoanthracene (BrAnth) to show that π-extended functional
groups could be coupled. Accordingly p-toluidine was coupled
with 9-bromoanthracene to exclusively afford the mono-coupled
product N-(4-methylphenyl)anthracen-9-amine, Anth(p-tolyl)NH,
in a 98 % yield. To investigate the effect of electron-donating
and electron-withdrawing groups, cross-coupling was performed
between
4-bromoanisole
and
4-bromo-1-fluorobenzene,
respectively, and 4-toluidine (Scheme 8). The yields of these
reactions were 99 % and 98 %, respectively, showing that the
presence of either electron-donating or electron-withdrawing
groups does not hinder the effectiveness of our system. We then
expanded the scope of this reaction to include the coupling of a
Scheme 8. The substrate scope investigated during this study. DippNH2 was
redistilled under vacuum prior to use. Each reaction was conducted in
duplicate with average isolated yields reported (avg. deviation in yield was +/-
2 %). See Table S1 for more details.[23]
primary alkylamine (p-methylbenzylamine),
a
secondary
alkylamine (morpholine), and the secondary aniline PhNHMe
with 4-chlorotoluene; in all cases, successful monocoupling was
found to give the expected products in > 80 % isolated yield
(Scheme 8). In order to further test the performance of our
7
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[Pd(cinnamyl)Cl]2 (0.5 mol. %) as a palladium source, and MeIPrCH2 (1 mol. %)
as a ligand. PMe3 was added at time = 30 min leading to a halt catalysis.
To gain a greater understanding of the system, we fitted the
kinetic data to the Finke-Watzky model.[32] This model is based
on a two-step process for nanoparticle formation: 1) a slow,
continuous nucleation step, and 2) an autocatalytic surface
growth step. This model was chosen to fit our kinetic data since
it relates directly to physical properties of the reaction, yielding
rate constants k1 and k2 with each distinct step in nanoparticle
growth. Assuming that Buchwald-Hartwig amination is fast
compared to these two steps, we can use the disappearance of
reagent to follow nanoparticle formation.[33] As shown in Figure
10, there is reasonable agreement between the two-step Finke-
Watzky model and the experimentally derived concentration of
[Pd(cinnamyl)Cl]2 precursor, with an R2 value of 0.988. This
suggests, in accordance with other evidence provided, that
nanoparticles are being formed which, in turn, perform catalytic
Buchwald-Hartwig amination. It is worth noting, however, that
the induction period is not entirely flat, so there is some catalytic
activity before the rate of catalysis increases. For further kinetic
analyses and the plot of the concentration of 9-bromoanthracene
over time, please see the Supporting Information.[23]
Figure 8. Plot of percent yield over time in the reaction of p-toluidine and 4-
chlorotoluene at 80 °C in THF with 1.5 equiv. of NaOtBu as
a base,
[Pd(cinnamyl)Cl]2 (0.5 mol. %) as a palladium source, and MeIPrCH2 (1 mol. %)
as a ligand. Elemental mercury was added at time = 30 min leading to a halt in
catalysis.
While the observed cessation of catalysis upon addition of
Hg (Figure 8) can indicate that a mercury-palladium amalgam
formed and thereby rendering catalytically active Pd
nanoparticles inert, reactions involving a homogeneous Pd(0)
species and mercury is also possible.[30] As such, an additional
catalyst poisoning experiment using substoichiometric amounts
of PMe3 was conducted. By using substoichiometric amounts of
poisoning phosphine in relation to the Pd present, further
support can be offered for the presence of catalytic palladium
colloids since the bulk of palladium present in these
nanoparticles is buried in the core of the particles, so small
amounts of poisoning ligand will halt catalysis.[31] Again, PMe3
was added 30 minutes into the reaction time and a similar halt in
catalysis was noted (Figure 9), thus adding further support for
the initial presence of catalytically active Pd nanoparticles.
Figure 10. A plot of the concentration of [Pd(cinnamyl)Cl]2 vs. time (h) during
the cross-coupling of 9-bromoanthracene and p-toluidine as observed by in
situ 1H NMR spectroscopy with 1,3,5-trimethoxybenzene as an internal
standard. Using the Finke-Watzky model, disappearance of pre-catalyst
[Pd(cinnamyl)Cl]2 and thus formation of Pd nanoparticles can be tracked by
correlating the disappearance of 9-bromoanthracene. Only every third data
point is shown in the plot, for clarity.
Satisfied with the evidence supporting heterogeneous
catalysis, we examined the fate of the NHO ligand after
palladium nanoparticle formation. One possibility would be
Heck-type coupling between a tolyl group, derived from 4-
chlorotoluene, and MeIPrCH2 ligand.[34] Accordingly, one
equivalent each of MeIPrCH2 and 4-chlorotoluene were combined
in the presence of 0.5 mol. % [Pd(cinnamyl)Cl]2 and 1.5
equivalents of NaOtBu (at 80 °C in THF); however only
unreacted starting materials were found by 1H NMR
Figure 9. Plot of percent yield over time of the reaction of p-toluidine and 4-
chlorotoluene at 80 °C in THF with 1.5 equiv. of NaOtBu as
a base,
8
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10.1002/chem.201901376
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spectroscopy. We also combined stoichiometric amounts of p-
toluidine, 4-chlorotoluene, and MeIPrCH2 with a half equivalent of
[Pd(cinnamyl)Cl]2 and 1.5 equivalents of NaOtBu (at 80 °C in
THF). We found that the only NHO-containing species after this
reaction was free MeIPrCH2. This leads us to believe that the
NHO is left unchanged after Pd mediated cross-coupling.
each case. A variety of [(NHO)PdCl2(3-Cl-pyr)] complexes were
prepared, however, these species proved to be ineffective in
Buchwald-Hartwig cross-coupling between arylchlorides and
primary arylamines. A suitable catalyst system containing Pd
nanoparticles was obtained by combining the readily accessible
NHO MeIPrCH2 with the Pd source [Pd(cinnamyl)Cl]2 in the
presence of NaOtBu in THF at 80 °C. This system was active for
the coupling of a wide range of arylhalides with arylamines
(including high conversion with bulky substrates), while also
avoiding over-arylation to tertiary triarylamines. Catalyst
poisoning experiments revealed that addition of Hg or
substoichiometric amounts of PMe3 halts catalysis, thus pointing
to the observed catalysis being heterogeneous in nature, a
feature that is likely more common in cross-coupling reactions
than previously noted.
We sought to isolate the palladium nanoparticles and image
them using various transmission electron microscopy (TEM)
techniques. After performing the cross-coupling of p-toluidine
and 4-chlorotoluene as per the conditions in Scheme 8, the
resulting suspended Pd nanoparticles were isolated by
centrifugation and washed 3 times with water to remove the
sodium chloride. The resulting particles were re-suspended in
anhydrous ethanol,[35] then drop-cast on a TEM grid, the solvent
was then removed under vacuum followed by heating to 300 °C
overnight to remove excess organic material. A control sample
was prepared by drop-casting the anhydrous ethanol Pd
nanoparticle suspension onto a TEM grid and removing solvent
under vacuum overnight, to ensure that heating the grid did not
dramatically affect the particles. The resulting spherical
nanoparticles have high contrast compared to the grid, and thus
were readily observable by TEM. 300 particles were measured
averaging 4.80 ± 0.84 nm in size (Figures S37 to S39)[23] and
are similar to the control sample prepared by vacuum drying
(4.72 ± 0.91 nm). Lattice fringes with a spacing of 0.22 nm were
obtained in both high-resolution and high angle angular dark-
field (HAADF) scanning mode (Figure 11) and indexed to Pd
[111] faces.[36] The composition of nanoparticles was further
confirmed with an energy dispersive X-ray (EDX) detector. A
good overlap of the nanoparticles in the dark field image with
palladium signal mapping was observed (Figure S42).[23]
Experimental Section
Materials and Instrumentation: All reactions were performed using
standard Schlenk line techniques under an atmosphere of nitrogen or in
an inert atmosphere glovebox (MBruan Labmaster 100). Solvents were
dried using a Grubbs-type solvent purification system manufactured by
Innovative Technology, Inc. and stored under an atmosphere of nitrogen
and over 4 Å molecular sieves prior to use. [MeIPrH]Cl,[37] [SIPrH]Cl,[38]
[13]
[IPr(BIAN)H]Cl,[39] IPr,[40] MeIPr,[41] and MeIPrCH2
were prepared
according to literature procedures. [Pd(cinnamyl)(µ-Cl)]2 was purchased
from Sigma-Aldrich and used as received. Allyl bromide was purchased
from Alfa Aesar and freeze-thaw degassed before use. NaOtBu and
KOtBu were purchased from Sigma-Aldrich and used as received. 3-
Chloropyridine was purchased form Oakwood Chemicals and distilled
before storing in
a glovebox before use. 1,5-Diiodopentane, 1,4-
diiodobutane and methyl iodide were purchased from Sigma-Aldrich and
freeze-thaw degassed before use. Methyl trifluoromethylsulfonate was
purchased from Oakwood Chemicals and used as received.
Bis(benzonitrile) palladium (II) dichloride was purchased from Strem
Chemicals and used as received. 1H, 13C{1H} and 19F{1H} NMR spectra
were recorded on 500 MHz and 700 MHz Varian Inova spectrometers
and referenced externally to SiMe4 (1H, 13C{1H}) and FCCl3 19F{1H}).
(
Elemental analyses were performed by the Analytical and
Instrumentation Laboratory at the University of Alberta. Melting points
were measured in sealed glass capillaries under nitrogen using
MelTemp melting point apparatus and are uncorrected.
a
Transmission Electron Microscopy: TEM images were obtained from
a JEOL JEM-ARM200CF Transmission Electron Microscope. Samples
were prepared from the Buchwald-Hartwig amination of p-toluidine and 4-
chlorotoluene as earlier described. Samples were centrifuged (3000 rpm,
5 min.) to separate the Pd nanoparticles from the supernatant. The
isolated Pd nanoparticles were then washed 3 times with water, and then
suspended in anhydrous ethanol. This suspension was then drop-cast
onto a holey carbon grid, placed under vacuum for 3-4 hrs, followed by
heating to 300 °C overnight to remove organic material.
Figure 11. An HRTEM image of
a Pd nanoparticle isolated after the
completion of aryl amination of p-toluidine and 4-chlorotoluene under the
conditions shown in Scheme 8. Left: An HAADF image showing Pd
nanoparticles as well as their lattice fringes. Right: A STEM image depicting
Pd nanoparticles and their lattice fringes.
X-Ray Crystallography: Crystals of appropriate quality for X-ray
diffraction studies were removed from either a Schlenk tube under a
stream of nitrogen, or from a vial (glove box) and immediately covered
with a thin layer of hydrocarbon oil (Paratone-N). A suitable crystal was
then selected, attached to a glass fiber, and quickly placed in a low-
temperature stream of nitrogen. All data were collected using a Bruker
APEX II CCD detector/D8 diffractometer using Mo Kα or Cu Kα radiation,
with the crystal cooled to −100 °C or −80 °C, respectively. The data were
Conclusions
A series of structurally distinct N-heterocyclic olefins (NHOs)
have been prepared, including analogues with extended π-
frameworks. In line with prior work involving NHOs, metal
coordination through the terminal C atoms (η1) was found in
9
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corrected for absorption through Gaussian integration from indexing of
the crystal faces. Structures were solved using the direct methods
programs SHELXT-2014,[42] and refinements were completed using the
program SHELXL-2014.[43] Hydrogen atoms were assigned positions
based on the sp2- or sp3-hybridization geometries of their attached
carbon atoms, and were given thermal parameters 20 % greater than
those of their parent atoms.
(ArC), 149.1 (ArC), 157.6 (NCN). UV/vis (THF): λmax (ε) = 286 nm (6.4 x
105 L mol-1 cm-1), 406 nm (1.86 x 104 L mol-1 cm-1), 686 nm (1.03 x 104 L
mol-1 cm-1). Anal. Calcd. for C33H46N2: C, 86.64; H, 8.04; N, 5.32. Found:
C, 86.02; H, 8.00; N, 5.17 %. Mp (°C): > 260.
Synthesis of SIPrCH2 (3): A 20 mL scintillation vial was charged with
[SIPrH]Cl (0.4271 g, 1.000 mmol), KOtBu (0.2468 g, 2.200 mmol) and 5
mL of THF was added. The mixture was stirred for 10 minutes, after
which MeI (0.1419 g, 1.100 mmol) in 2 mL of THF was added dropwise
and stirred overnight. The resulting precipitate was allowed to settle and
the supernatant was filtered through a pad of Celite. The volatiles were
removed from the filtrate in vacuo to give 3 as a white solid (0.2671 g,
70 %). X-ray quality crystals were obtained from a saturated toluene
solution at -25 °C for 24 hrs. 1H NMR (500 MHz, C6D6): δ = 7.23 (t, 2H,
3JHH = 7.2 Hz, ArH), 7.14 (d, 4H, 3JHH = 7.5 Hz, ArH), 3.40 (s, 4H, NCH2),
Computational Methods: Density Functional theory (DFT) calculations
(full geometry optimization) were carried out on 1-3, 4 and 6 starting from
the geometry of their respective X-ray structures. Geometry optimizations
were carried out using the Gaussian09 program package:[44] B3LYP[45]
functional with a 6-31+G(d,p) basis set[46] for C, H, and N. The optimized
structures were in reasonable agreement with the observed molecular
structures. All stationary points were characterized by frequency
analyses. For all calculated molecules and intermediates there are no
imaginary frequencies. The optimized structures were also subjected to
natural bond orbital (NBO) analyses using the NBO 6.0 program.[47]
3
3.37 (sept, 4H, JHH = 6.8 Hz, CH(CH3)2), 2.42 (s, 2H, C=CH2), 1.36 (d,
3
3
12H, JHH = 6.7 Hz, CH(CH3)2), 1.27 (d, 12H, JHH = 6.7 Hz, CH(CH3)2).
13C{1H} NMR (126 MHz, C6D6): δ = 156.1 (NCN), 149.5 (ArC), 137.2
(ArC), 128.5 (ArC), 124.5 (ArC), 51.8 (NCH2), 50.4 (C=CH2), 28.7
(CH(CH3)2), 24.7 (CH(CH3)2), 24.5 (CH(CH3)2. Anal. Calcd. for C28H40N2:
C, 83.11; H, 9.96; N, 6.92. Found: C, 82.92; H, 10.07; N, 6.77 %. Mp
(°C): 132-134.
Synthesis of MeIPr=CH-CH=CH2 (1): To a mixture of [MeIPrH]Cl (0.445 g,
0.984 mmol) and KOtBu (0.232 g, 2.07 mmol) in 10 mL of THF was
added dropwise a solution of allyl bromide (0.127 g, 1.05 mmol) in 2 mL
of THF. The resulting suspension was stirred at room temperature for
12 hrs. The precipitate was allowed to settle and the deep yellow
supernatant was decanted away and filtered through a plug of Celite. The
volatiles were evaporated under vacuum from the filtrate and the yellow
residue was extracted with toluene (10 mL). Filtration of the toluene
extract followed by evaporation of the toluene gave MeIPr=CH-CH=CH2
(1) (0.285 g, 84 %) as a bright yellow solid. X-ray quality crystals of 1
were obtained by slowly evaporating a saturated hexanes solution over a
period of 24 hrs at room temperature. 1H NMR (500 MHz, C6D6): δ =
7.27-7.20 (m, 2H, ArH), 7.15-7.11 (m, 4H, ArH), 5.70-5.60 (m, 1H, C-
CH=CH2), 4.29 (dd, 1H, 3JHH = 16.1 Hz, 2JHH = 2.8 Hz, cis-CH=CH2), 4.03
Synthesis of IPr=C(CH2)4 (4): To a solution of IPr (0.335 g, 0.861 mmol)
in 10 mL of toluene was added dropwise 1,5-diiodopentane (0.098 g,
0.30 mmol) in 2 mL of toluene. The resulting suspension was stirred at
room temperature for 8 hrs. The colorless precipitate ([IPrH]I) was
allowed to settle and the yellow supernatant was separated by filtration
through a pad of Celite. The volatiles were removed from the filtrate
under vacuum and the yellow residue was extracted with hexanes
(10 mL). Evaporation of the hexanes afforded 4 (0.110 g, 84 %) as a
bright yellow solid. X-ray quality crystals of 4 were obtained by slowly
evaporation (under N2) of a saturated hexanes solution over a period of
24 hrs at room temperature. 1H NMR (498 MHz, C6D6): δ = 7.20 (t, 2H,
3JHH = 7.7 Hz, ArH), 7.07 (d, 4H, 3JHH = 7.7 Hz, ArH), 5.77 (s, 2H, NCH),
3
3
2
(d, 1H, JHH = 11.5 Hz, C=CH-CH), 3.97 (dd, 1H, JHH = 10.7 Hz, JHH
=
3
2.8 Hz, trans-CH=CH2), 3.26 (sept, 2H, JHH = 6.8 Hz, CH(CH3)2), 3.16
(sept, 2H, 3JHH = 6.9 Hz, CH(CH3)2), 1.52-1.47 (m, 6H, H3C-CN), 1.43 (d,
6H, 3JHH = 6.8 Hz, CH(CH3)2), 1.38 (d, 6H, 3JHH = 6.8 Hz, CH(CH3)2), 1.18
3
3.59 (sept, 2H, JHH = 6.9 Hz, CH(CH3)2), 1.80-1.73 (m, 4H, CH2), 1.37-
3
1.32 (m, 4H, CH2), 1.32 (d, 12H, JHH = 7.0 Hz, CH(CH3)2), 1.25 (d, 12H,
3
3
(d, 6H, JHH = 6.9 Hz, CH(CH3)2), 1.17 (d, 6H, JHH = 6.9 Hz, CH(CH3)2).
13C{1H} NMR (125 MHz, C6D6): δ = 9.1 (H3C-CN), 9.5 (H3C-CN), 23.8
(CH(CH3)2), 24.1 (CH(CH3)2), 24.8 (CH(CH3)2), 28.9 (CH(CH3)2), 29.0
(CH(CH3)2), 72.2 (CH=CH2), 95.3 (=CH-CH), 116.6 (NC-CH3), 117.2
(NC-CH3), 124.4 (ArC), 124.6 (ArC), 129.7 (ArC), 129.8 (ArC), 132.4
(ArC), 132.6 (CH=CH2), 134.4 (ArC), 146.0 (ArC), 148.7 (ArC), 149.2
(ArC), 206.5 (NCN). Anal. Calcd. for C28H42N2B2: C, 84.16; H, 9.71; N,
6.13. Found: C, 83.49; H, 9.74; N, 5.91 %. Mp (°C): 146-149.
3JHH = 7.0 Hz, CH(CH3)2). 13C{1H} NMR (125 MHz, C6D6): δ = 22.8
(CH(CH3)2), 25.0 (CH(CH3)2), 28.2 ((CH2)4), 28.7 (CH(CH3)2), 29.9
((CH2)4), 75.3 (C=C), 116.8 (NC-H), 123.5 (ArC), 128.7 (ArC), 137.8
(ArC), 138.4 (ArC), 148.3 (NCN). Anal. Calcd. for C32H44N2: C, 84.16; H,
9.71; N, 6.13. Found: C, 83.47; H, 9.56; N, 6.64 %. Mp (°C): 105 (dec.).
Synthesis of MeIPr=C(CH2)4 (5): To a solution of MeIPr (0.417 g, 1.00
mmol) in 10 mL of toluene was added dropwise 1,5-diiodopentane
(0.118 g, 0.364 mmol) in 2 mL of toluene. The resulting suspension was
stirred at room temperature for 8 hrs. The colorless precipitate was
allowed to settle and the yellow supernatant separated by filtration
through a pad of Celite. The volatiles were evaporated from the filtrate
under vacuum and the yellow residue was extracted with hexanes
(10 mL). Evaporation of hexanes afforded 5 (0.135 g, 77 %) as a bright
yellow solid. X-ray quality crystals of 5 were obtained by placing a
saturated hexanes solution in the freezer at -30 °C over a period of 24
hrs. 1H NMR (498 MHz, C6D6): δ = 7.22 (t, 2H, 3JHH = 7.6 Hz, ArH), 7.08
Synthesis of IPr(BIAN)CH2 (2): [IPr(BIAN)H]Cl (0.105 g, 0.192 mmol)
and KOtBu (0.044 g, 0.39 mmol) were combined in 10 mL of a 1:1
mixture of toluene/THF at room temperature and the mixture was stirred
for 30 mins. To the yellow suspension was added MeI (0.031 g,
0.21 mmol) and the color immediately changed to deep blue. Stirring was
continued for 12 hrs and the precipitate was allowed to settle. The
supernatant was removed by filtration through a plug of Celite. The
volatiles were removed from the filtrate in vacuo and IPr(BIAN)CH2 (2,
0.088 g, 87 %) was obtained as a blue solid. X-ray quality crystals of 2
were obtained from a saturated benzene solution layered with hexanes at
room temperature after 24 hrs. 1H NMR (498 MHz, C6D6): δ = 7.33 (t, 2H,
3
3
(d, 4H, JHH = 7.6 Hz, ArH), 3.48 (sept, 2H, JHH = 6.9 Hz, CH(CH3)2),
3
1.75-1.69 (m, 4H, CH2), 1.37 (d, 12H, JHH = 7.0 Hz, CH(CH3)2), 1.36-
1.30 (m, 4H, CH2), 1.20 (d, 12H, JHH = 7.0 Hz, CH(CH3)2). 13C{1H} NMR
3
3
3JHH = 7.7 Hz, ArH), 7.27 (d, 4H, JHH = 7.7 Hz, ArH), 7.15-7.14 (m, 2H,
(125 MHz, C6D6): δ = 9.9 (NC-CH3), 23.6 (CH(CH3)2), 24.3 (CH(CH3)2),
28.1 ((CH2)4), 28.5 (CH(CH3)2), 30.0 ((CH2)4), 73.5 (C=C(CH2)2), 116.8
(NC-H), 123.4 (ArC), 128.7 (ArC), 136.5 (ArC), 139.6 (ArC), 149.4 (NCN).
Anal. Calcd. for C34H48N2: C, 84.24; H, 9.98; N, 5.78. Found: C, 83.41; H,
9.91; N, 5.76 %. Mp (°C): 117 (dec.).
Napth-H), 6.88-6.85 (m, 2H, Napth-H), 6.67 (d, 2H, 3JHH = 7.0 Hz, Napth-
3
H), 3.59 (sept, 4H, JHH = 7.0 Hz, CH(CH3)2), 2.73 (s, 2H, C=CH2), 1.39
(d, 12H, 3JHH = 7.0 Hz, CH(CH3)2), 1.16 (d, 12H, 3JHH = 7.0 Hz, CH(CH3)2).
13C{1H} NMR (125 MHz, C6D6): δ = 24.0 (CH(CH3)2), 24.4 (CH(CH3)2),
29.2 (CH(CH3)2), 48.3 (CH2), 118.5 (ArC), 124.8 (ArC), 126.1 (ArC),
127.5 (ArC), 128.9 (ArC), 129.4 (ArC), 129.7 (ArC), 132.0 (ArC), 133.5
10
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10.1002/chem.201901376
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FULL PAPER
Synthesis of IPr=C(CH2)3 (6): To a solution of IPr (1.017 g, 2.621 mmol)
in 10 mL of toluene was added dropwise 1,4-diiodobutane (0.280 g,
0.903 mmol) in 2 mL of toluene. The resulting suspension was stirred at
room temperature for 12 hrs. The colorless precipitate ([IPrH]I) was
allowed to settle and the yellow supernatant separated by filtration
through a pad of Celite. The volatiles were removed from the filtrate
under vacuum and the yellow residue was extracted with hexanes (10
mL). Evaporation of hexanes resulted afforded 6 as a solid (0.300 g,
74 %) as a bright yellow solid. X-ray quality crystals of 6 were obtained
by placing a saturated hexanes solution in the freezer at -30 °C over a
period of 24 hrs. 1H NMR (498 MHz, C6D6): δ = 7.20 (t, 2H, 3JHH = 7.7 Hz,
ArH), 7.07 (d, 4H, 3JHH = 7.7 Hz, ArH), 5.73 (s, 2H, H-CN), 3.53 (sept, 4H,
3JHH = 6.9 Hz, CH(CH3)2), 2.16 (t, 4H, 3JHH = 7.5 Hz, CH2), 1.79 (quint, 2H,
3JHH = 7.5 Hz, CH2), 1.41 (d, 12H, 3JHH = 6.9 Hz, CH(CH3)2), 1.25 (d, 12H,
3JHH = 6.9 Hz, CH(CH3)2). 13C{1H} NMR (125 MHz, C6D6): δ = 20.0 (CH2),
23.0 (CH(CH3)2), 24.7 (CH(CH3)2), 28.2 (CH2), 28.8 (CH(CH3)2), 69.7
(C=C(CH2)2), 115.6 (NC-H), 123.3 (ArC), 128.8 (ArC), 136.2 (ArC), 137.2
(ArC), 149.2 (NCN). Anal. Calcd. for C31H42N2: C, 84.11; H, 9.56; N, 6.33.
Found: C, 83.29; H, 9.62; N, 6.14 %. Mp (°C): 84 (dec.). Despite
subsequent recrystallizations, an impurity of ca. 6 % free IPr was present.
See Figures S14 and S15 for copies of the 1H and 13C{1H} NMR
spectra.[23]
solution of trans-[Cl2Pd(NCPh)2] (0.035 g, 0.091 mmol) in 1 mL of toluene
at room temperature. A red crystalline solid precipitated from the solution
after stirring for 2 hrs. This precipitate was isolated by filtration and
washed with 2 mL of fresh toluene. The collected solid was re-dissolved
in a minimum amount of CH2Cl2 and the resulting solution was layered
with hexanes. X-ray quality red crystals of 9 (0.061 g, 72 %) were then
obtained after storing this layered solution at -30 °C for 24 hrs. 1H NMR
(498 MHz, CDCl3): δ = 7.54 (t, 4H, 3JHH = 7.6 Hz, ArH), 7.37 (d, 8H, 3JHH
3
= 7.8 Hz, ArH), 2.69 (sept, 8H, JHH = 6.9 Hz, CH(CH3)2), 2.42 (s, 4H,
CCH2Pd), 1.86 (s, 12H, NC-CH3), 1.52 (d, 24H, 3JHH = 6.7 Hz, CH(CH3)2),
3
1.08 (d, 24H, JHH = 6.7 Hz, CH(CH3)2). 13C{1H} NMR (125 MHz, CDCl3):
δ = 10.2 (NC(CH3)), 10.7 (CCH2Pd), 25.2 (CH(CH3)2), 25.7 (CH(CH3)2),
29.0 (CH(CH3)2), 125.6 (NC(CH3)), 129.1 (ArC), 131.2 (ArC), 132.4 (ArC),
145.8 (ArC), 146.7 (NCN). Anal. Calcd. for C74H100N4Pd2Cl4 (9•2 C7H8): C,
63.47; H, 7.20; N, 4.00. Found: C, 62.86; H, 7.18; N, 3.96 %. Mp (°C):
163 (dec.).
Synthesis of [(MeIPrCH2)PdCl2(3-Cl-pyr)] (10): To
a solution of
[(MeIPrCH2)PdCl((µ-Cl)]2 (0.020 g, 0.014 mmol) in 2 mL of CH2Cl2 was
added 3-chloropyridine (0.010 g, 0.08 mmol) at room temperature.
Stirring was continued for 2 hrs and afterwards the volatiles were
removed under vacuum. The residue was dissolved in 0.2 mL of CH2Cl2
and filtered through a plug of Celite. The filtrate was concentrated to
incipient crystallization and layered with hexanes. After placing the
sample at -30 °C for 24 hrs, pale yellow crystals of 10 formed (0.015 g,
67 %) that were suitable for X-ray crystallographic analysis. 1H NMR (498
Synthesis of MeIPr=C(CH2)3 (7): To a solution of MeIPr (0.133 g, 0.294
mmol) in 10 mL of toluene was added dropwise 1,4-diiodobutane
(0.038 g, 0.123 mmol) in 2 mL of toluene. The resulting suspension was
stirred at room temperature for 12 hrs. The colorless precipitate
([MeIPrH]I) was allowed to settle and the yellow supernatant separated by
filtration through a pad of Celite. The volatiles were removed from the
filtrate under vacuum and the yellow residue was extracted with hexanes
(5 mL) and filtered. Evaporation of the hexanes gave 7 (0.050 g, 82 %)
3
3
MHz, CDCl3): δ = 8.62 (d, 1H, JHH = 2.2 Hz, 3-Cl-pyr), 8.54 (d, 1H, JHH
= 5.4 Hz, 3-Cl-pyr), 7.57 (t, 2H, 3JHH = 7.8 Hz, ArH), 7.56-7.53 (m, 1H, 3-
Cl-pyr), 7.39 (d, 4H, 3JHH = 7.8 Hz, ArH), 7.09 (dd, 1H, 3JHH = 8.0 Hz, 3JHH
= 5.7 Hz, 3-Cl-pyr), 2.60 (sept, 4H, 3JHH = 6.7 Hz, CH(CH3)2), 2.60 (s, 2H,
CCH2-Pd), 1.96 (s, 6H, NC-CH3), 1.44 (d, 24H, 3JHH = 6.7 Hz, CH(CH3)2),
as a bright yellow solid. 1H NMR (498 MHz, C6D6): δ = 7.22 (t, 2H, 3JHH
=
1.13 (d, 12H, JHH = 6.7 Hz, CH(CH3)2). 13C{1H} NMR (126 MHz, CDCl3):
3
3
3
7.7 Hz, ArH), 7.08 (d, 4H, JHH = 7.7 Hz, ArH), 3.44 (sept, 4H, JHH = 7.0
δ = 10.4 (NC(CH3)), 25.0 (CH(CH3)2), 25.3 (CH(CH3)2), 28.8 (CH(CH3)2),
124.2 (ArC), 125.5 (ArC), 126.1 (ArC), 129.9 (ArC), 131.2 (ArC), 136.6
(ArC), 146.8 (ArC), 149.4 (ArC), 150.5 (ArC), 163.8 (NCN). Anal. Calcd.
for C34H45Cl3N3Pd: C, 58.26; H, 6.43; N, 5.80. Found: C, 58.51; H, 6.20;
N, 5.82 %. Mp (°C): 178 (dec).
3
Hz, CH(CH3)2), 2.11 (t, 4H, JHH = 7.5 Hz, CH2), 1.76 (quint, 2H,
3
3JHH = 7.5 Hz, CH2), 1.53 (s, 6H, H3C-CN), 1.45 (d, 12H, JHH = 7.0 Hz,
3
CH(CH3)2), 1.21 (d, 12H, JHH = 7.0 Hz, CH(CH3)2). 13C{1H} NMR (125
MHz, C6D6): δ = 9.4 (H3C-CN), 19.7 (1C, CH2), 23.8 (CH(CH3)2), 24.2
(CH(CH3)2), 28.6 (2C, CH2), 28.7 (CH(CH3)2), 68.1 (C=C(CH2)2), 115.9
(NC-H), 123.3 (ArC), 128.8 (ArC), 134.6 (ArC), 138.2 (ArC), 149.8 (NCN).
Anal. Calcd. for C33H46N2: C, 84.20; H, 9.85; N, 5.95. Found: C, 83.48; H,
9.87; N, 5.69 %. Mp (°C): 108 (dec.).
Synthesis of [[{IPr(BIAN)CH2}PdCl(µ-Cl)]2 (11):
A
solution of
IPr(BIAN)CH2 (0.063 g, 0.12 mmol) in 2 mL of toluene was added
dropwise to a solution of trans-[Cl2Pd(NCPh)2] (0.046 g, 0.12 mmol) in 1
mL of toluene at room temperature. A red crystalline solid precipitated
from the solution after stirring for 2 hrs. The precipitate was isolated by
filtration and washed with 2 mL of fresh toluene. The collected solid was
re-dissolved in a minimum amount of CH2Cl2, and the resulting solution
was layered with hexanes. X-ray quality red crystals of 11 (0.030 g,
31 %) were obtained from this layered solution after cooling at -30 °C for
24 hrs. Despite obtaining crystalline material, the bulk sample routinely
contained ca. 10 % impurity (Figure S26).[23] 1H NMR (498 MHz, CDCl3):
δ = 7.84 (d, 4H, 3JHH = 6.9 Hz, Naph-H), 7.82 (t, 4H, 3JHH = 7.9 Hz, ArH),
Synthesis of [MeIPrC(Me)(CH2)4]OTf (8). To a solution of MeIPr=C(CH2)4
(0.050 g, 0.10 mmol) in 5 mL of hexanes was added dropwise a solution
of MeOTf (0.020 g, 0.12 mmol) in 1 mL of hexanes at room temperature.
The resulting suspension was stirred at room temperature for 12 hrs. The
colorless precipitate was isolated by filtration and washed with hexanes
(2 mL). Afterwards the precipitate was dissolved in 1 mL of CH2Cl2 and
the resulting solution layered with hexanes, which resulted in the
formation of colorless X-ray quality crystals of 8 after 24 hrs at -30 °C
3
3
3
3
(0.030 g, 47 %). 1H NMR (399 MHz, CDCl3): δ = 7.65 (t, 2H, JHH = 7.8
7.59 (d, 8H, JHH = 7.9 Hz, ArH), 7.46 (dd, 4H, JHH = 6.9 Hz, JHH = 8.1
Hz, ArH), 7.40 (d, 4H, 3JHH = 7.8 Hz, ArH), 2.34 (sept, 4H, 3JHH = 6.7 Hz,
CH(CH3)2), 2.08 (s, 6H, H3C-CN), 1.56-1.30 (m, 8H, CH2), 1.35-1.20 (m,
4H CH(CH3)2), 1.33 (d, 12H, 3JHH = 6.7 Hz, CH(CH3)2), 1.22 (d, 12H, 3JHH
= 6.7 Hz, 12H, CH(CH3)2), 1.11 (s, 3H, -CH3). 13C{1H} NMR (176 MHz,
CDCl3): δ = 10.5 (H3C-CN), 21.1 (C-CH2-CH2), 23.5 (CH(CH3)2), 24.5
(CH(CH3)2), 25.3 (CH(CH3)2), 29.2 (CH(CH3)2), 37.4 (C-CH2-CH2), 46.3
(NC-C), 125.7 (ArC), 129.9 (ArC), 130.5 (ArC), 132.5 (ArC), 145.5 (ArC),
151.4 (NCN). 19F{1H} NMR (376 MHz, CDCl3): δ = -78.0 (s). Anal. Calcd.
for C36H51F3N2O3S: C, 66.64; H, 7.92; N, 4.32; S, 4.94. Found: C, 66.52;
H, 7.92; N, 4.24; S, 4.63 %. Mp (°C): >300.
Hz, Naph-H), 7.02 (d, 4H, JHH = 6.9 Hz, Naph-H), 2.95 (sept, 8H, JHH =
6.9 Hz, CH(CH3)2), 2.57 (s, 4H, CCH2-Pd), 1.49 (d, 24H, JHH = 6.7 Hz,
CH(CH3)2), 0.98 (d, 24H, 3JHH = 6.7 Hz, CH(CH3)2).
3
3
3
[{IPr(BIAN)CH2}PdCl2(3-Cl-pyr)] (12): To
a
solution of crude
[{IPr(BIAN)CH2}PdCl(µ-Cl)]2 (0.034 g, 0.021 mmol) in 2 mL of CH2Cl2
was added 3-chloropyridine (0.015 g, 0.13 mmol) at room temperature.
Stirring was continued for 2 hrs and the solvent was then removed under
vacuum. The residue was dissolved in 0.2 mL CH2Cl2 and filtered
through a plug of Celite. The filtrate was concentrated to incipient
crystallization and layered with hexanes. After placing the sample at -
30 °C for 24 hrs 12 (0.012 g, 32 %) was obtained in the form of yellow
crystals suitable for X-ray crystallographic analysis. 1H NMR (498 MHz,
Synthesis of [(MeIPrCH2)PdCl(µ-Cl)]2 (9):
A
solution of MeIPrCH2
(0.053 g, 0.12 mmol) in 2 mL of toluene was added dropwise to a
11
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10.1002/chem.201901376
Chemistry - A European Journal
FULL PAPER
CDCl3): δ = 8.80 (d, 1H, 3JHH = 2.4 Hz, 3-Cl-pyr), 8.70 (dd, 1H, 3JHH = 5.5,
3JHH = 1.4 Hz, 3-Cl-pyr), 7.81 (d, 2H, 3JHH = 8.2 Hz, Naph-H), 7.69 (t, 2H,
3JHH = 7.9 Hz, ArH), 7.56-7.53 (m, 1H, 3-Cl-pyr), 7.50 (d, 4H,
grants to E.R.; CREATE fellowship to I.W.). E.R. also thanks the
Alexander von Humboldt Foundation for a fellowship. C. H.-J.
thanks Prof. M. Beller for his support, the Alexander von
Humboldt Foundation for funding (Returning Fellowship) and the
Max Buchner Research Foundation for a fellowship. Sean Liew
and Kate Powers are also acknowledged for early synthetic work
on this project. Prof. Rylan Lundgren (University of Alberta) is
also acknowledged for helpful advice.
3
3
3JHH = 7.9 Hz, ArH), 7.40 (dd, 2H, JHH,1 = 8.2 Hz, JHH,1 = 7.0 Hz, Naph-
3
H), 7.11 (dd, 1H, JHH = 8.1 Hz, 3JHH = 5.5 Hz, 3-Cl-pyr), 7.02 (d, 2H, 3JHH
= 7.0 Hz, Naph-H), 3.15 (sept, 8H, 3JHH = 6.9 Hz, CH(CH3)2), 2.86 (s, 4H,
3
3
CCH2-Pd), 1.44 (d, 12H, JHH = 6.7 Hz, CH(CH3)2), 1.01 (d, 12H, JHH
=
6.7 Hz, CH(CH3)2). 13C{1H} NMR (126 MHz, CDCl3): δ = 23.9 (CH(CH3)2),
25.8 (CH(CH3)2), 29.4 (CH(CH3)2), 122.6 (Naph-C), 124.3 (pyr-C), 127.7
(Naph-C), 129.2 (ArC), 129.8 (ArC), 129.9 (ArC), 130.9 (Naph-C), 132.0
(ArC), 131.7 (ArC), 132.0 (ArC), 136.6 (pyr-C), 146.6 (pyr-C), 149.6 (pyr-
C), 150.7 (pyr-C), 168.3 (NCN). Anal. Calcd. for: C, 63.71; H, 5.67; N,
5.14. Found: C, 61.72; H, 5.65; N, 5.01. Despite repeated attempts,
combustion analysis gave consistently low carbon values; see Figures
S27-S28 for copies of the NMR spectra.[23] Mp (°C): 183-185 (dec.).
Keywords: N-Heterocyclic olefins • Buchwald-Hartwig amination
• Pd complexes • Nanoparticle catalysis
[1]
[2]
A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113,
361-363.
[(MeIPrCHCHCH2)PdCl2(3-Cl-pyr)] (13): A solution of MeIPr=CH-CH=CH2
(0.110 g, 0.241 mmol) in 1 mL toluene was added dropwise a solution of
trans-[Cl2Pd(NCPh)2] (0.070 g, 0.18 mmol) in 5 mL of toluene at room
temperature. After 2 hrs a red precipitate formed, which was isolated by
filtration and washed with 2 mL of toluene. This solid was then dissolved
in 1 mL of CH2Cl2 and 3-chloropyridine (0.027 g, 0.24 mmol) was then
added. The resulting mixture was filtered through Celite and the volatiles
were removed in vacuo from the filtrate. The crude product was then
recrystallized from an acetonitrile/hexanes mixture that was cooled to -
30 °C for 24 hrs, affording red X-ray quality crystals of 13 (0.0712 g,
46 %). 1H NMR (498 MHz, CDCl3): δ = 8.86 (br, 1H, 3-Cl-pyr), 8.76 (br,
1H, 3-Cl-pyr), 7.63 (t, 2H, 3JHH = 8.0 Hz, ArH), 7.57 (d, 1H, 3JHH = 8.0 Hz,
S. P. Nolan, C, Cazin, Science of Synthesis: N-Heterocyclic Carbenes
in Catalytic Organic Synthesis, Vol. 1 (Eds.: S. P. Nolan, C. Cazin),
Thieme Gruppe, Stuttgart, pp 161-182.
[3]
a) S. Gründemann, A. Kovacevic, M. Albrecht, J. W. Faller, R. H.
Crabtree, J. Am. Chem. Soc. 2002, 124, 10473−10481; b) H. Lebel, M.
K. Janes, A. B. Charette, S. P. Nolan, J. Am. Chem. Soc. 2004, 126,
5046−5047; c) O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht,
Chem. Rev. 2009, 109, 3445−3478.
[4]
[5]
M. Melaimi, R. Jazzar, M. Soleilhavoup, G. Bertrand, Angew. Chem.
2017, 129, 10180-10203; Angew. Chem. Int. Ed. 2017, 56, 10046-
10068.
a) T. Dröge, F. Glorius, Angew. Chem. 2010, 122, 7094–7107; Angew.
Chem. Int. Ed. 2010, 49, 6940-6952; b) M. N. Hopkinson, C. Richter, M.
Schedler, F. Glorius, Nature 2014, 510, 485−496.
3
3-Cl-pyr, br), 7.42 (d, 4H, JHH = 8.0 Hz, ArH), 7.13 (br, 1H, 3-Cl-pyr),
6.08-5.99 (m, 1H, CH=CH), 5.92 (d, 1H, 3JHH = 15.5 Hz, CH=CH), 3.01 (d,
3
3
[6]
[7]
[8]
M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1,
953−956.
2H, JHH = 9.3 Hz, CHCH2Pd), 2.45 (sept, 4H, JHH = 6.9 Hz, CH(CH3)2),
3
1.98 (s, 6H, NCCH3), 1.36 (d, 12H, JHH = 7.0 Hz, CH(CH3)2), 1.25 (d,
12H, JHH = 7.0 Hz, CH(CH3)2). 13C{1H} NMR (125.7 MHz, CDCl3): δ =
3
M. S. Viciu, R. M. Kissling, E. D. Stevens, S. P. Nolan, Org. Lett. 2002,
4, 2229−2231.
9.4 (NCCH3), 19.7 (CH=CH-CH2), 23.8 (CH(CH3), 24.3 (CH(CH3)2),
CH(CH3)2, 29.2 (CH(CH3)2, 97.7 (CH=CH-CH2), 125.3 (3-Cl-pyr), 125.7
(ArC), 128.6 (3-Cl-pyr), 132.2 (ArC), 145.2 (3-Cl-pyr), 146.3 (3-Cl-pyr),
156.8 (CH=CH-CH2). Anal. Calcd. for C51H39Cl3N3Pd: C, 59.14; H, 6.57;
N, 5.61. Found: C, 59.14; H, 6.57; N, 5.79. Mp(°C): 154-156 (dec).
a) M. S. Viciu, R. F. Germaneau, O. Navarro-Fernandez, E. D. Stevens,
S. P. Nolan, Organometallics 2002, 21, 5470−5472; b) N. Marion, O.
Navarro-Fernandez, J. Mei, E. D. Stevens, N. M. Scott, S. P. Nolan, J.
Am. Chem. Soc. 2006, 128, 4101−4111.
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Buchwald-Hartwig Cross-Coupling Procedure: Preparation of the
reaction mixtures were conducted in
a glovebox under an argon
atmosphere. A 0.100 M stock solution of MeIPrCH2 and a 0.0125 M stock
solution of [Pd(cinnamyl)Cl]2 were prepared. To a mixture of 1.00 mmol
arylhalide, 1.20 mmol arylamine and 144 mg (1.50 mmol) NaOtBu in 2
mL of THF in a vial was added 100 µL (0.0100 mmol) of the MeIPrCH2
stock solution and 400 µL (0.00500 mmol) of the [Pd(cinnamyl)Cl]2 stock
solution were added. Molecular sieves (4 Å) were added and the vial was
capped using a cap with a PTFE septa. The reaction mixture was stirred
for 1 hr at 80 °C. The reaction mixture was sampled via syringe for NMR
analysis. To isolate the product, after cooling to room temperature the
vial was opened to air, filtered, then evaporated using a rotary evaporator.
The products were isolated by column chromatography (silica, n-
hexane/ethyl acetate 10:1) or flash chromatography (silica, n-hexane
eluent).[23]
[10] a) N. Kuhn, H. Bohnen, J. Kreutzberg, D. Bläser, R. Boese, J. Chem.
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Bläser, R. Boese, Chem. Ber. 1994, 127, 1405−1407; c) For a related
study on triazolylidene-based NHOs, see: D. Enders, K. Breuer, G.
Raabe, J. Runsink, J. H. Teles, J.–P. Melder, K. Ebel, S. Brode, Angew.
Chem. 1995, 107; 1119-1122; Angew. Chem., Int. Ed. Engl. 1995, 34,
1021−1023.
[11] M. M. D. Roy, E. Rivard, Acc. Chem. Res. 2017, 50, 2017-2025.
[12] a) A. Fürstner, M. Alcarazo, R. Goddard, C. W. Lehmann, Angew.
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McDonald, E. Rivard, Chem. Commun. 2011, 47, 6987–6989; c) C. E. I.
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Acknowledgements
[13] K. Powers, C. Hering-Junghans, R. McDonald, M. J. Ferguson, E.
Rivard, Polyhedron 2016, 108, 8−14.
This work was supported by the donors of The American
Chemical Society Petroleum Research Fund (ACS-PRF), the
Canada Foundation for Innovation, the Faculty of Science at the
University of Alberta, and the Natural Sciences and Engineering
Research Council (NSERC) of Canada (Discovery and CREATE
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Layout 2:
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Ian C. Watson, André Schumann,
Haoyang Yu, Prof. Dr. Emma C. Davy,
Dr. Robert McDonald, Dr. Michael J.
Ferguson, Prof. Dr. Christian Hering-
Junghans,* Prof. Dr. Eric Rivard,*
Page xx. – Page xx
N-Heterocyclic olefins (NHOs) with functionalized backbones and modifications to
the terminal alkylidene positions are described. Various Pd(II)-NHO complexes
have been synthesized and their effectiveness as pre-catalysts in Buchwald-
Hartwig aminations was explored. Evidence suggests that catalysis is mediated by
colloidal palladium metal, which highlights the different coordination ability of NHOs
in comparison with the commonly used N-heterocyclic carbene co-ligands.
N-Heterocyclic Olefin Ligated
Palladium(II) Complexes as Pre-
Catalysts for Buchwald-Hartwig
Aminations
14
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