N-heterocyclic carbene C,S palladium(II) π-allyl complexes: Synthesis, characterization, and catalytic application in allylic amination reactions

Article abstract of DOI:10.1021/om400110t

A series of five-membered N-heterocyclic carbene C,S palladium(II) π-allyl complexes were successfully developed and characterized. Structural analyses of these complexes revealed that the organopalladium chelates adopt a skew-envelope conformation with a

Full text of DOI:10.1021/om400110t

Article
pubs.acs.org/Organometallics
N‑Heterocyclic Carbene C,S Palladium(II) π‑Allyl Complexes:
Synthesis, Characterization, and Catalytic Application In Allylic
Amination Reactions
Deepa Krishnan, Meiyi Wu, Minyi Chiang, Yongxin Li, Pak-Hing Leung,* and Sumod A. Pullarkat*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: A series of five-membered N-heterocyclic
carbene C,S palladium(II) π-allyl complexes were successfully
developed and characterized. Structural analyses of these
complexes revealed that the organopalladium chelates adopt a
skew-envelope conformation with a trans disposition of the
substituents on the metal chelate rings. Utilizing these C,S
palladium(II) π-allyl complexes as catalysts, a catalytic system for the allylic amination reaction has been developed. A series of
C−N bond formations between amines and unsymmetrically substituted allylic carbonates could be catalyzed efficiently by
complex ( )-9 in a regioselective manner.
phosphine-free allylic amination reaction, which to the best of
our knowledge has not been reported to date. Although a few
chelating NHC−Pd complexes (excluding sulfur donors) have
been reported for the C−N bond formation reaction, these
studies also highlight the inevitability of employing PPh3 to
generate the active Pd0 species in their catalytic cycle.12 We
herein report the synthesis of sulfur-functionalized NHC
complexes of palladium, 9−12, that exhibit catalytic activity
in the allylic amination reaction, most notably in the absence of
any phosphine as additive. A detailed study of the role of the
cationic π-allyl palladium(II) complexes and in particular ( )-9
in allylic amination was undertaken in this work.
INTRODUCTION
■
The discovery of N-heterocyclic carbenes (NHCs) by
1
2
̈
Wanzlick and Ofele in the 1960s focused significant attention
on NHCs particularly as ancillary ligands in various transition-
metal-mediated catalytic reactions.3 The increased popularity of
NHC ligands is attributed to the high inherent stability of these
metal−carbene complexes to heat, moisture, and oxygen.
Furthermore, their electronic and steric properties are easily
tuned, which make NHCs a viable alternative to phosphines in
various metal-based homogeneous catalytic reactions. The ease
in the synthetic preparation of functionalized NHCs with a
wide range of classical donors (E  P,4 N,5 O6) provides an
avenue for the construction of different polydentate ligands
with unique coordination chemistry and their potential
application in various metal-catalyzed reactions.7 In comparison
with other donor-functionalized NHCs, the synthetic method-
ologies targeting sulfur-functionalized NHC metallacycle
complexes are rare in the literature, although their potential
in various catalytic scenarios such as C−C, C−O, and C−N
bond formations have been demonstrated.8
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Palladium(II) π-Allyl
Complexes. The synthesis of palladium(II) π-allyl complexes
9−12 were accomplished via the transmetallation method13
(Scheme 1) with addition of 0.55 equiv of Ag2O to substituted
thioether−imidazolium bromides 1−4 under the exclusion of
light followed by the reaction of silver carbenes 5−8 with
Over the past few decades, transition metals such as Pd, Ni,
Pt, Mo, W, Rh, and Ir have been successfully employed as
catalysts in allylic substitution reactions.7f,9 However, Pd-
catalyzed allylation via a π-allyl intermediate, through oxidative
addition of allylic compounds, with successive nucleophilic
attack at the allyl fragment yielding an active Pd0 species to
afford the allylated product, is the widely used method for the
construction of C−C, C−O, and C−N bonds in organic
synthesis.3q,10 The formation of Pd0 species is generally
achieved by the addition of activators, mostly phosphines,
and consequently, a wide range of phosphine-based palladium
complexes have been developed for allylic substitution
reactions.11 Thus, our interest lies in the development of
sulfur-functionalized NHC metallacycle complexes for the
[Pd(η3−C3H5)(COD)]+BF4 .
−
The complexation reactions proceeded smoothly to afford
cationic palladium complexes 9−12 in 90−92% isolated yields.
The complexes were isolated as white-gray powders and were
fully characterized by NMR techniques (1H, 13C), mass
spectrometry, and single-crystal X-ray crystallography.
Structural Investigation of Palladium Complex 9.
Colorless needlelike crystals of complex 9 were obtained by
slow diffusion of diethyl ether into a dissolved diastereoisomer
mixture of palladium(II) π-allyl complex 9 in acetone solution
Received: February 8, 2013
Published: April 3, 2013
© 2013 American Chemical Society
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Scheme 1. Synthesis of Palladium(II) π-Allyl Complexes
at −20 °C. The molecular structure of 9 was determined by
single-crystal X-ray diffraction analysis. It is noteworthy that the
parent complex may exist as a mixture of up to four
diastereomers, as it contains one chiral carbon center and a
coordinated sulfur stereogenic center. Interestingly, the crystal
selected for analysis was that of one diastereomer, i.e., (RS,SC)*-
9, in its enantiomerically pure form (space group P212121,
absolute structure parameter 0.00(6)). It is evident that in the
solid state the five-membered palladacycle (RS,SC)*-9 adopts
the λ(SC)-ring conformation where the phenyl substituent on
the prochiral sulfur atom adopts the axial (Ph) position due to
the axial trans disposition of the phenyl group at C13. The
molecular structure with the numbering scheme is presented in
Figure 1. The driving force for the Ph substituent at C13 to
and the phenyl substituent at S1 is 101.4°. The longer Pd−C
bond trans to the carbene carbon (Pd−C26 = 2.167 Å) in
comparison to the Pd−C bond trans to the sulfur atom (Pd−
C28 = 2.123 Å) reflects the strong trans influence of the
carbene. Moreover, for the bidentate complex (RS,SC)*-9, the
M−S−C bond angles of C20−S1−Pd1 and C13−S1−Pd1 were
found to be 105.2 and 96.8°, which are smaller than the
observed angle for a tetrahedral geometry, and this accounts for
the aforementioned steric factors. Selected bond lengths and
bond angles are given in Table 1.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
(RS,SC)*-9
Pd1−C1
Pd1−S1
Pd1−C28
Pd1−C26
Pd1−C27
C1−N1
2.030(9)
2.366(2)
2.123(8)
2.167(9)
2.177(10)
1.368(11)
1.339(10)
1.836(10)
1.000
N1−C1−Pd1
N2−C13−C14
C1−Pd1−C28
N2−C1−Pd1
N1−C1−N2
N2−C13−H13
C1−Pd1−S1
C20−S1−C13
C20−S1−Pd1
N2−C13−S1
C13−S1−Pd1
C1−Pd1−C26
137.6(6)
115.3(8)
105.1(3)
118.2(6)
104.0(7)
109.7
C1−N2
82.7(2)
101.4(4)
105.2(3)
106.2(6)
96.8(3)
172.0(3)
C13−S1
C13−H13
C13−N2
C13−C14
C20−S1
1.482(11)
1.510(12)
1.809(9)
The four possible λ-ring conformations of the palladacycle 9
are shown in Figure 2. It needs to be noted that the isomers
shown in Figure 2a are not expected to be formed in this
current scenario, due to the equatorial−equatorial Ph−Ph
repulsions in play.
Figure 1. Molecular structure of (RS,SC)*-9;. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms except H13 and
−
the BF4 counteranion are omitted for clarity.
adopt the axial position is attributed to the steric repulsive
interaction with the Ph substituent on the S1 atom within the
PdCS coordination plane. Upon coordination, S1 becomes a
stereogenic center that was formed with R absolute
configuration, and this selectivity can be explained by the
lower steric interactions due to the relative trans disposition of
the neighboring Ph(α) group in the case of the isomer studied
by X-ray crystallography.
The monocationic complex exhibits the usual distorted-
square-planar geometry through coordination of palladium(II)
ion, with a Ccarbene−Pd1 bond distance of 2.030 Å along with a
C1−Pd1−S1 bite angle of 82.7°. The C20−S1−C13−C14
torsion angle between the phenyl substituent at the α-carbon
Figure 2. The possible isomers of complex ( )-9 in λ-ring
conformation.
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1
Figure 3. Variable-temperature H NMR spectra for complex ( )-9 in CD2Cl2.
Scheme 2. Allylation Reaction of ( )-1-Phenylethanamine
The NMR spectroscopic characterization of ( )-9 was
catalytic activity of the racemic complexes 9−12 was evaluated
in the allylation reaction of 1-phenylethanamine. The
palladium-catalyzed allylation of 1-phenylethanamine with
cinnamyl ethyl carbonate (Scheme 2) was investigated under
various conditions.
In the first series of experiments, catalytic amounts of
complexes 9−12 (∼2 mol %) were reacted with a mixture of
( )-1-phenylethanamine (14g; 0.727 mmol) and cinnamyl
ethyl carbonate (13a; 0.242 mmol) in dry THF (2 mL) at 50
°C for 15 h. Under these conditions, a complete conversion was
observed with complex ( )-9, whereas complexes ( )-10,
( )-11, and ( )-12 exhibited only moderate reactivity (Table
2).
Substrate Scope of Aliphatic Amines. We further
screened a series of primary and secondary aliphatic amines
for their reactivity toward amination of cinnamyl ethyl
carbonate (13a; 0.242 mmol) using ( )-9 (∼2 mol %) in
dry THF (2 mL) at 50 °C (Scheme 3), and the results are
summarized in Table 3.
1
assisted by a combination of H and 13C NMR spectroscopy
experiments. Data from a variable-temperature 1H NMR
spectroscopic investigation done for complex ( )-9, in the
temperature range +23 to −57 °C (Figure 3), was collected
l
using a dichloromethane-d2 solution. At 23 °C, the H NMR
spectrum shows the presence of two diastereomers of 9 in
solution.
As the temperature is lowered, each resonance signal tends to
broaden as the rate of the interconversion process diminishes
and finally undergoes a collapse at −17 °C. This temperature
was considered as the coalescence point, where the signal is at
its broadest and before any splitting was observed. The
resonances of the allyl protons and aromatic protons provide
evidence for the reduced rate of the dynamic process at lower
temperatures. Hence, the low-temperature spectra displayed in
Figure 3, indicating the chemical exchange of syn and anti
protons of the allyl group, explicates the interconversion of a ⇄
a′ or b ⇄ b′ isomers (Figure 2), and further, the
interconversion of the rotamers, i.e. a ⇄ b′ or b ⇄ a′, was
not possible, as the latter requires higher temperatures. Hence,
the palladacycle 9 obtained in this work was obtained in a
regioselective manner with the λ(SC)-ring conformation. The
aforementioned studies, while providing structural information
for the synthesized complexes in both the solid and solution
states, also yielded important insights for the future design of
chiral variants.
Table 2. Reaction of Cinnamyl Ethyl Carbonate (13a) and
a
( )-1-Phenylethanamine (14g)
time
(h)
temp
(°C)
b
entry
[C]
additive
conv. (%) (15g + 16g)
c
1
2
3
4
( )-9
none
none
none
none
15
15
15
15
50
50
50
50
96
c
( )-10
( )-11
( )-12
79
c
75
Catalytic Activity of Complexes 9−12. After obtaining
insights into the coordination behavior of sulfur in palladium-
(II) π-allyl complexes from the aforementioned solid and
solution state structural studies, we undertook an investigation
to access their performances in catalytic allylic amination. The
c
72
a
b
Conditions: 0.242 mmol, 50 °C, 2 mL of THF was used. Isolated
c
1
yield. Determined by H NMR; contains both 15g (mono-allylated
product) and 16g (bis-allylated product).
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Scheme 3. Reaction of Mono- and DiSubstituted Amines with (E)-Cinnamyl Carbonate
a
Table 3. Reaction of Amines 14a−j with (E)-Cinnamyl Carbonate (13a)
a
b
c
Conditions: 0.242 mmol, 50 °C, 2 mL of THF was used. Isolated yield. Isolated yield of 15 (mono-allylated product) and 16 (bis-allylated
product).
The results collected in Table 3 show that both the primary
NMR spectra, a doublet with a coupling range of 16 Hz in the
range of 6.5 ppm for Ha (trans coupling) and a sextuplet (Hb)
with a coupling of 6 Hz in the range of 6.2−6.3 ppm, followed
by a doublet with a coupling of 6 Hz in the range of 3.14−3.16
ppm from the two Hc atoms, pointing out that trans linear
products were obtained from trans substrates, with complete
stereoretention.
Substrate Scope of Allylic Carbonates. With the
efficient reaction conditions for aliphatic amines toward allylic
amination reaction established, we further investigated the
scope and limitations of various α/γ mono- and disubstituted
allylic carbonates. Under the previously optimized conditions,
the reactions of morpholine and various α/γ mono- and
disubstituted carbonates 13b−g were efficiently promoted by
catalyst ( )-9 to provide the desired allylated products in high
yields. Morpholine was used as the amine during the entire
course of substrate studies on allylic carbonates in order to
and secondary aliphatic amines had a strong influence on the
reaction rate and the product yields. The amination of cinnamyl
ethyl carbonate (13a) worked well for the range of secondary
amines tested, 14a−f (entries 1−6, Table 3), giving generally
high yields of the corresponding allylated amine products.
Conversely, primary amines 14g−j (entries 7−10, Table 3)
gave moderate yields. This difference in reactivity could be
related to the nucleophilicity of the corresponding amines. The
reaction of naphthalen-1-ylmethanamine (14i) (entry 9, Table
3) and phenylmethanamine (14j) (entry 10, Table 3) under
optimized conditions gave a higher percentage of bis-allylated
product in comparison to 1-phenylethanamine (14g) (entry 7,
Table 3) and its 1-(napththalen-1-yl)ethanamine counterpart
(14h) (entry 8, Table 3), and these differences in reactivity
could be attributed to the basicity of the corresponding amines.
Additionally, all (E)-cinnamyl allyl amines gave similar 1H
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a
Table 4. Reaction of Morpholine (14d) with Allylic Carbonates 13b−g
a
b
c
1
Conditions: 0.242 mmol, 50 °C, 2 mL of THF was used. Isolated yield. Determined by H NMR spectroscopy, contains both 17f,g and 18f,g
d
e
isomers, E/Z mixture of 13c (E/Z = 4/1) was used. ∼4 mol % of ( )-9 was used.
obtain a clear picture of the regioselective mechanism and
thereby avoiding the bis-allylated products. The results for
amination of allylic carbonates 13b−g with morpholine (14d)
using ( )-9 are summarized in Table 4.
[Pd(η3-alkenyl)] intermediate complexes, which led to the
formation of the mixture of isomers. The loss of stereo-
chemistry of the α-substituted allylic carbonate 13f and highly
branched α,α-disubstituted allylic carbonate 13g was due to
rapid σ−η3−σ interconversion of the palladium(II) allyl
intermediate complex in comparison to the slower rate of the
amination reaction. With the highly substituted carbonate 13g
(entry 6, Table 4), although the reaction proceeded more
slowly and was complete in 96 h, quantitatively high chemical
yields were achieved with mixtures of isomers. The reaction for
the unsubstituted allylic carbonate is relatively fast, and high
yields of the desired product were obtained. From the above
results, it is significant that all the reactions proceeded with
complete conversion, and hence we conclude that complex
( )-9 was indeed successful in inducing good regioselectivity
for a range of substrates tested (aliphatic amines (primary and
secondary alkyl substituted amines) and α-/γ-substituted allylic
carbonates) toward allylic amination under mild and simple
conditions, without any aid of activators such as triphenyl-
phosphine, carboxylic acids, etc.
At 50 °C, all allylic carbonates examined, 13a−g, underwent
amination, giving the corresponding N-allylamines in overall
isolated yields ranging from 68 to 98%. The reaction of allyl
ethyl carbonate (13b) (entry 1, Table 4]) proceeded in a
manner similar to that for the γ-phenyl-substituted carbonate
13a (entry 4, Table 3) with full conversion. The γ-methyl-
substituted carbonate 13c (entry 2, Table 4) containing 20% of
cis isomer underwent full conversion to yield the allylated
products with a trans:cis ratio of 4:1, without any branched
isomer; hence, we conclude that the trans substrate ionized
initially to a more thermodynamically stable syn-[Pd(η3-
alkenyl)] intermediate complex, which then preferentially led
to the formation of the trans linear product. (E)-3,7-
Dimethylocta-2,6-dienyl ethyl carbonate (13d) (entry 3,
Table 4) gave exclusively the trans linear product with 98%
yield. The allylation of morpholine with the α,γ-disubstituted
carbonate 13e (entry 4, Table 4) also proceeded smoothly
under the optimized conditions. Treatment of morpholine with
the α-substituted allylic carbonate 13f and highly branched α,α-
disubstituted allylic carbonate 13g (entries 5 and 6, Table 4)
gave mixtures of the stereo- and regioisomeric amines 17f,g and
18f,g in 70% and 30% yields, respectively. The ratio of the
product distribution shows that the products were derived via
the π-allyl intermediate, where nucleophilic substitution is
possible on both the C-1 and C-3 positions. The branched
substrates ionized partially to syn-[Pd(η3-alkenyl)] and anti-
CONCLUSION
■
In summary, heterobidentate C,S ligands and their correspond-
ing palladium(II) π-allyl complexes were successfully designed,
developed, and completely characterized. Our studies showed
that in the solid state the sulfur-functionalized NHC PdII allyl
complex ( )-9 adopts the λ(SC)-ring conformation. The
preferred catalyst ( )-9 has been structurally determined by
single-crystal X-ray diffraction analysis of one of its
diastereomers. This investigation along with a solution NMR
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synthesis of compound 2 was followed. Yield: 0.886 g (82%). 1H
NMR (400 MHz, CDCl3): δ 10.52 (s, 1H, NCHN), 8.09 (s, 1H,
NCHS), 7.89−7.92 (m, 2H, SArH), 7.60−7.64 (m, 2H, ArH), 7.56 (s,
1H, NCHCHN), 7.49 (s, 1H, NCHCHN) 7.42−7.44 (m, 3H,
ArH), 7.29−7.33 (m, 3H, SArH), 1.47 (s, 9H, tBu). 13C NMR (100
MHz, CDCl3): δ 135.9, 134.0, 133.9, 130.3, 130.1, 129.6, 129.4, 129.3,
128.0, 119.4, 119.3, 67.1, 60.3, 29.8. HRMS: calcd for C20H23N2SBr
[M − Br]+ 323.16, found 323.15.
1-(tert-Butyl)-3-((tert-butylthio)(phenyl)methyl)-1H-imida-
zol-3-ium Bromide (4). To a stirred solution of 3-(bromo(phenyl)-
methyl)-1-tert-butyl-1H-imidazol-3-ium bromide (1 g, 2.688 mmol) in
dry dichloromethane (50 mL) at −10 °C was added sodium 2-
methylpropane-2-thiolate (0.3 g, 2.688 mmol). The procedure given
for the synthesis of compound 2 was followed. Yield: 0.8 g (78%). 1H
NMR (400 MHz, DMSO-d6): δ 9.86 (s, 1H, NCHN), 8.34 (s, 1H,
NCHCHN), 8.17 (s, 1H, NCHCHN), 7.38−7.48 (m, 5H, ArH),
7.12 (s, 1H, NCHS), 1.61 (s, 9H, StBu), 1.29 (s, 9H, tBu). 13C NMR
(100 MHz, DMSO-d6): δ 137.5, 133.9, 129.1, 129.0, 126.4, 121.7,
121.3, 64.0, 60.3, 46.5, 30.1, 28.9. HRMS: calcd for C18H27N2SBr [M
− Br]+ 303.19, found 303.19.
Silver Carbene 5. Silver(I) oxide (0.234 g, 1.006 mmol) was
added to a solution of 3-mesityl-1-(phenyl(phenylthio)methyl)-1H-
imidazol-3-ium bromide (0.85 g, 1.831 mmol) in CH2Cl2 (25 mL),
and the suspension was stirred for 4 h at room temperature in the
dark. The mixture was filtered off through a small Celite plug and
concentrated to afford the crude product. Yield: 0.9 g (89%). 1H NMR
(400 MHz, CDCl3): δ 7.87−7.90 (m, 4H, ArH), 7.80−7.82 (m, 2H,
NCHCHN), 7.67−7.71 (m, 2H, SArH), 7.58−7.60 (m, 2H, SArH),
7.53−7.56 (m, 4H, ArH), 7.48−7.50 (m, 4H, SArH), 7.44−7.46 (m,
2H, ArH), 7.33−7.34 (m, 1H, NCHCHN), 7.12 (s, 2H, SArH),
6.96 (s, 1H, NCHCHN), 6.85−6.91 (m, 6H, NCHS; ArH), 2.29 (s,
6H, ArMe), 1.90 (s, 3H, ArMe), 1.81 (s, 3H, ArMe), 1.67 (d, 3H,
ArMe), 1.62 (d, 3H, ArMe). 13C NMR (100 MHz, CD3Cl):δ 139.6,
136.1, 134.5, 134.4, 133.9, 129.9, 129.6, 129.5, 129.4, 129.3, 129.3,
129.2, 126.6, 72.6, 21.0, 17.8, 17.6, 17.5. HRMS: calcd for
C50H48AgN4S2 [M + H]+ 877.25, found 877.24.
analysis revealed factors that contribute to stereoselectivity
factors in such synthetic protocols. An efficient and convenient
catalytic system has been successfully developed for allylic
amination reactions employing ( )-9. The catalyst was shown
to promote allylic amination for a wide range of allylic
carbonates as well as aliphatic amines, for the production of
allylamines in a highly regioselective manner and typically in
good yields in the absence of activators. Development of chiral
variants and their application in asymmetric synthesis is
currently being pursued in our laboratory.
EXPERIMENTAL SECTION
■
General Methods. All reactions are carried under an inert
atmosphere of argon using Schlenk line techniques in oven-dried
glassware. Tetrahydrofuran was distilled from sodium benzophenone;
dichloromethane and acetonitrile were distilled from calcium hydride
immediately prior to use. All other solvents and reagents were used as
−
received. [Pd(η3-C3H51)(COD)]+BF4 was prepared according to the
literature procedures. H NMR and 13C NMR (400 and 100 MHz;
500 and 125 MHz) were recorded in CDCl3, CD2Cl2, CD3OD, and
DMSO solutions using Bruker Avance 400 and 500 NMR
spectrometers. Mass spectra were obtained on a Finnigan LCQ
DECA XP MAX instrument with ESI mode. High-resolution mass
spectra were obtained using a Water Q-Tof premier also with ESI
mode. It is to be noted that, due to the hygroscopic nature of these
compounds, elemental analysis data were not reproducible.
3-Mesityl-1-(phenyl(phenylthio)methyl)-1H-imidazol-3-ium
Bromide (1). To a stirred solution of 3-(bromo(phenyl)methyl)-1-
mesityl-1H-imidazol-3-ium bromide (1 g, 2.304 mmol) in dry
dichloromethane (50 mL) at −10 °C was added sodium
benzenethiolate (0.304 g, 2.304 mmol). The reaction mixture was
warmed to room temperature and stirred for another 4 h. On
completion of the reaction, the reaction mixture was filtered and the
filtrate was evaporated, followed by tituration with diethyl ether, to
yield the desired crude product. The pure product was purified by
column chromatography using silica gel and chloroform−methanol
100/3) as eluent. Yield: 0.856 g (80%). 1H NMR (400 MHz, CDCl3):
Silver Carbene 6. Silver(I) oxide (0.264 g, 1.137 mmol) was
added to a solution of 1-tert-butyl-3-(phenyl(p-tolylthio)methyl)-1H-
imidazol-3-ium bromide (0.86 g, 2.067 mmol) in CH2Cl2 (25 mL),
and the procedure given for the synthesis of compound 5 was
followed. Yield: 0.92 g (86%). 1H NMR (400 MHz, CDCl3): δ 7.47−
δ 10.87 (s, 1H, NCHN), 8.76 (s, 1H, NCHS), 7.97−7.98 (d, JH−H
=
=
6.8 Hz, 2H, ArH), 7.88 (s, 1H, NCHCHN), 7.77−7.90 (d, JH−H
7.6 Hz, 2H, SArH), 7.44−7.49 (m, 3H, ArH), 7.32−7.36 (t, JH−H = 7.6
Hz, 2H, SArH), 7.27−7.29 (m, 1H, SArH), 6.95 (s, 1H, NCH
CHN), 6.91 (s, 1H, ArH), 6.88 (s, 1H, ArH), 2.29 (s, 3H, ArMe), 1.93
(s, 3H, ArMe), 1.51 (s, 3H, ArMe).13C-NMR (100 MHz, CDCl3): δ
141.4, 137.8, 134.4, 134.1, 133.9, 133.2, 130.3, 130.3, 130.0, 129.8,
129.7, 129.5, 129.1, 127.8, 123.3, 120.2, 66.7, 21.1, 17.6, 17.0. HRMS:
calcd for C25H25N2SBr [M − Br]+ 385.17, found 385.91.
7.48 (m, 2H, SArH), 7.38−7.43 (m, 3H, ArH), 7.30−7.31 (d, JH−H
=
2.0 Hz, 1H, NCHCHN), 7.20−7.22 (m, 2H, ArH), 7.10−7.14 (m,
3H, NCHCHN; SArH), 6.91 (s, 1H, NCHS), 2.31 (s, 3H, SArMe),
1.56 (s, 9H, tBu).13C NMR (100 MHz, CDCl3): δ 139.79, 136.04,
134.62, 130.61, 129.42, 129.10, 127.17, 126.97, 119.76, 117.31, 74.10,
57.74, 31.64, 21.21. HRMS: calcd for C21H25N2SAgBr [M − Br]+
443.07, found 443.07.
1-(tert-Butyl)-3-(phenyl(p-tolylthio)methyl)-1H-imidazol-3-
ium Bromide (2). To a stirred solution of 3-(bromo(phenyl)methyl)-
1-tert-butyl-1H-imidazol-3-ium bromide (1 g, 2.688 mmol) in dry
dichloromethane (50 mL) at −10 °C was added sodium 4-
methylbenzenethiolate (0.393 g, 2.689 mmol). The reaction mixture
was warmed to room temperature and stirred for another 4 h. On
completion of the reaction, the reaction mixture was filtered and the
filtrate was evaporated, followed by tituration with diethyl ether to
yield the desired crude product. The residue was chromatographed on
silica gel with of methanol−chloroform (100/3) as eluent to yield the
desired product 2 as white hygroscopic solids. Yield; 0.864 g (80%).
1H NMR (400 MHz, CD2Cl2): δ 10.60 (s, 1H, NCHN), 8.06 (s, 1H,
NCHS), 7.86−7.88 (m, 2H, SArH), 7.52 (s, 1H, NCHCHN),
7.44−7.49 (m, 5H, ArH), 7.15 (s, 1H, SArH), 7.12−7.13 (m, 2H,
SArH; NCHCHN), 2.30 (s, 3H, SArMe), 1.44 (s, 9H, tBu). 13C
NMR (100 MHz, CD2Cl2): δ 140.0, 136.2, 134.4, 134.2, 130.3, 130.2,
129.3, 127.9, 126.7, 119.3, 119.1, 67.2, 29.6, 20.9, −0.4. HRMS: calcd
for C21H25N2SBr [M − Br]+ 337.17, found 337.17.
Silver Carbene 7. Silver(I) oxide (0.280 g, 1.2 mmol) was added
to a solution of 1-tert-butyl-3-(phenyl(phenylthio)methyl)-1H-imida-
zol-3-ium bromide (0.88 g, 2.189 mmol) in CH2Cl2 (25 mL), and the
procedure given for the synthesis of compound 5 was followed. Yield:
1
0.958 g (86%). H NMR (400 MHz, CDCl3): δ 7.48−7.51 (m, 2H,
ArH; NCHCHN), 7.41−7.43 (m, 3H, ArH), 7.32−7.35 (m, 2H,
SArH; ArH), 7.14 (s, 1H, NCHCHN), 6.99 (s, 1H, NCHS), 1.56 (s,
9H, tBu).13C NMR (100 MHz, CD3Cl): δ 136.0, 134.5, 130.7, 129.9,
129.5, 129.4, 129.2, 127.0, 119.8, 117.3, 73.9, 57.8, 31.7, −0.03.
HRMS: calcd for C20H23N2SAgBr [M − Br]+ 430.34, found 430.37.
Silver Carbene 8. Silver(I) oxide (0.267 g, 1.15 mmol) was added
to a solution of 1-(tert-butyl)-3-((tert-butylthio)(phenyl)methyl)-1H-
imidazol-3-ium bromide (0.8 g, 2.09 mmol) in CH2Cl2 (25 mL), and
the procedure given for the synthesis of compound 5 was followed.
1
Yield: 0.90 g (88%). H NMR (400 MHz, CDCl3): δ 7.56−7.57 (d,
JH−H = 2 Hz, 1H, NCHCHN), 7.28−7.35 (m, 5H, ArH), 7.23 (d,
JH−H = 2.0 Hz, 1H, NCHCHN), 6.91 (s, 1H, NCHS), 1.73 (s, 9H,
StBu), 1.33 (s, 9H, tBu).13C NMR (100 MHz, CDCl3): δ 138.5, 129.0,
128.7, 126.5, 119.1, 118.4, 68.5, 58.1, 46.3, 31.8, 31.6, 30.9. HRMS:
calcd for C18H27N2SAgBr [M − Br]+ 409.09, found 409.1.
1-(tert-Butyl)-3-(phenyl(phenylthio)methyl)-1H-imidazol-3-
ium Bromide (3). To a stirred solution of 3-(bromo(phenyl)methyl)-
1-tert-butyl-1H-imidazol-3-ium bromide (1 g, 2.689 mmol) in dry
dichloromethane (50 mL) at −10 °C was added sodium
benzenethiolate (0.354 g, 2.689 mmol). The procedure given for the
2394
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Organometallics
Article
Palladium Complex ( )-9 (Mixture of Two Diastereomers in
1:1 Ratio).
= 2.1 Hz, 1H), 7.45−7.46 (d, JH−H = 2.0 Hz, 0.5H), 7.36−7.43 (m,
6H), 7.32−7.36 (m, 2H), 7.22−7.27 (m, 2H), 7.10−7.12 (m, 2H),
7.05−7.07 (s, 0.5H), 6.88−6.91 (s, 0.5H), 5.75−5.87 (septet, JH−H
=
6.8 Hz, 1H, Hc), 5.61−5.73 (septet, JH−H = 6.7 Hz, 0.5H, H′c), 4.77−
4.82 (d, JH−H = 7.1 Hz, 0.5H, H′e), 4.61−4.66 (d, JH−H = 7.3 Hz, 1H,
He), 4.46−4.53 (m, 1.5H, Ha and H′a), 3.69−3.76 (d, JH−H = 13.5 Hz,
1H, Hd), 3.46−3.54 (m, 1H, H′d), 3.28 (m, 0.3H, H′b), 3.16−3.22 (d,
JH−H = 13.5 Hz, 1H, Hb), 1.85 (s, 4.5H, tBu), 1.83 (s, 9H, tBu), 1.70
(s, 4.5H, tBu), 1.51 (s, 4H, StBu), 1.41 (s, 9H, StBu), 1.37 (s, 4.5H,
StBu). 13C-NMR (100 MHz, CD2Cl2): δ 177.8, 177.7, 137.6, 136.3,
136.0, 133.8, 129.6, 129.5, 129.3, 129.2, 129.2, 126.4, 126.3, 126.2,
121.1, 121.0, 120.8, 119.9, 119.6, 119.0, 118.9, 70.8, 67.3, 67.1, 67.1,
65.5, 59.1, 59.0, 55.0, 53.8, 47.1, 30.9, 30.9, 30.4, 30.3, 30.2, 29.5.
HRMS: calcd for C21H33N2PdSBF4 [M − BF4]+ 450.13, found 450.13.
General Procedure for Allylic Amination. The appropriate
palladium(II) π-allyl complex (3.02 mg, ∼2 mol %, 0.02 equiv), was
added to a dry screw-cap reaction tube containing cinnamyl ethyl
carbonate (50.0 mg, 0.242 mmol, 1 equiv), followed by primary or
secondary amines (0.727 mmol, 3 equiv) in dry THF (2 mL). The
whole reaction tube was heated to 50 °C until the reaction reached
completion (determined through thin-layer chromatography). After
the disappearance of starting material, the reaction mixture was cooled
to room temperature and the THF was removed under reduced
pressure. The residue was then diluted with diethyl ether and filtered
through a pad of silica gel. The ether layer was extracted with water
and dried over MgSO4. The layer was concentrated and purified
through silica gel column chromatography to give the desired
amination product, determined through 1H NMR, 13C NMR and
high-resolution mass spectroscopy.
To a stirred solution of the complex silver carbene complex (5 ; 0.9 g,
1.58 mmol) in dichloromethane (20 mL) was added [Pd(η3-
C3H5)(COD)]+BF4−, (0.560 g, 1.58 mmol) under an argon
atmosphere. The mixture was stirred in the dark for 4 h at room
temperature, filtered through Celite, and concentrated, and the residue
was titurated with hexane to afford complex 9, as a grayish white
powder. Yield: 0.87 g (90%). 1H NMR (500 MHz, CD3OD): δ 7.65−
7.72 (d, JH−H = 8.0 Hz, 2H), 7.52−7.59 (m, 8H), 7.37−7.49 (m, 10H),
7.30−7.35 (m, 2H), 7.26−7.22 (m, 6H), 7.16−7.19 (s, 2H), 5.52−
5.39 m, 2H, Hc and H′c), 4.36−4.46 (d, JH−H = 7.3 Hz, 2H, He and
H′e), 3.24−3.29 (m, 2H, H′a and H′d), 3.09−3.14 (d, JH−H = 13.5 Hz,
1H, Hd), 3.02−3.06 (d, JH−H = 7.3 Hz, 1H, Ha), 2.84−2.90 (d, JH−H
=
12.6 Hz, 1H, H′b), 2.63−2.71 (dd, JH−H = 13.6 Hz, 1H, Hb), 2.43 (s,
6H, ArMe), 2.18 (s, 3H, ArMe), 2.05−2.14 (d, JH−H = 15 Hz, 6H,
ArMe), 1.99 (s, 3H, ArMe). 13C NMR (100 MHz, CDCl3): δ 179.9,
140.2, 136.6, 135.6, 135.3, 134.9, 134.5, 133.2, 133.1, 131.3, 130.3,
130.2, 130.1, 130.0, 129.5, 129.4, 129.3, 129.1, 129.0, 128.6, 127.3,
123.9, 123.8, 122.2, 122.1, 119.8, 119.1, 75.0, 69.4, 69.2, 60.6, 60.1,
21.2, 17.7, 17.6, 17.6. HRMS: calcd for C28H31N2PdSBF4 [M − BF4]+
533.13, found 533.12.
Single-Crystal X-ray Crystallographic Studies. Crystal data for
(Rs,S)-9 were collected on a Bruker X8 CCD diffractometer with Mo
Kα radiation (graphite monochromator). SADABS absorption
corrections were applied. All non-hydrogen atoms were refined
anisotropically, while hydrogen atoms were introduced at calculated
positions and refined riding on their carrier atoms. CCDC 853452
contains supplementary crystallographic data for (Rs,S)-9. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Palladium Complex ( )-10 (Mixture of Two Diastereomers
in 1:1 Ratio). The complex was prepared in a manner analogous to
that described for ( )-9, using silver carbene complex (6; 0.92 g, 1.76
mmol) in dichloromethane (20 mL) and [Pd(η3-C3H5)-
−
(COD)]+SbF6 (0.602 g, 1.76 mmol) under an argon atmosphere.
Yield: 0.91 g (90%), as a white powder. 1H NMR (400 MHz,
CD2Cl2): δ 7.33−7.43 (m, 12H), 7.19−7.28 (m, 6H), 7.12−7.16 (m,
3H), 7.00 (s, 1H, NCHS), 6.41−6.60 (m, 2H), 5.59−5.77 (m, 2H, Hc
and H′c), 4.67−4.72 (dd, JH−H = 5.6 Hz, 2H, He and H′e), 4.31−4.41
(d, JH−H = 6.2 Hz, 2H, Ha and H′a), 3.45−3.70 (dd, JH−H = 12.8 Hz,
2H, Hd and H′d), 3.11−3.25 (dd, JH−H = 13.0 Hz, 2H, Hb and H′b),
2.37 (s, 3H, SArMe), 2.35 (s, 3H, SArMe), 1.83−1.84 (d, JH−H = 3.6
Hz, 18H, tBu). 13C NMR (100 MHz, CD2Cl2): δ 177.8, 177.5, 142.9,
142.7, 134.5, 133.6, 133.3, 130.8, 130.8, 130.4, 130.3, 129.4, 129.3,
127.3, 126.9, 121.1, 120.8, 119.5, 119.5, 119.3, 70.7, 70.5, 68.3, 68.1,
59.3, 59.2, 31.0, 30.9, 21.1, 21.0. HRMS: calcd for C24H31N2PdSBF4
[M − BF4]+ 484.12, found 484.12.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures giving spectral characterization data of allylated amines
and 1H and 13C NMR spectra of palladium(II) π-allyl
complexes ( )-9−( )-12 and a CIF file giving crystallographic
data for (Rs,S)-9. This material is available free of charge via the
Internet at http://pubs.acs.org.
Palladium Complex ( )-11 (Mixture of Two Diastereomers
in 1:1 Ratio). The complex was prepared in a manner analogous to
that described for ( )-9, using silver carbene complex (7; 0.958 g,
1.889 mmol) in dichloromethane (20 mL) and [Pd(η3-C3H5)-
AUTHOR INFORMATION
Corresponding Author
*Tel: (65) 6316 8905. Fax: (+65) 6316 6984. E-mail:
pakhing@ntu.edu.sg (P.-H.L.); sumod@ntu.edu.sg (S.A.P.).
■
−
(COD)]+SbF6 (0.646 g, 1.889 mmol) under an argon atmosphere.
Yield: 0.94 g (90%), as a white powder. 1H NMR (400 MHz,
CD2Cl2): δ 7.33−7.70 (m, 18H), 7.22−7.32 (b, 2H), 7.10−7.21 (m,
3H), 7.01−7.06 (s, 1H, NCHS), 6.71−6.78 (m, 2H), 5.58−5.82 (m,
2H, Hc and H′c), 4.62−4.78 (dd, JH−H = 6.4 Hz, 2H, He and H′e),
Notes
The authors declare no competing financial interest.
4.35−4.45 (d, JH−H = 7.3 Hz, 2H, Ha and H′a), 3.50−3.72 (dd, JH−H
=
ACKNOWLEDGMENTS
13.0 Hz, 2H, Hd and H′d), 3.14−3.27 (d, JH−H = 13.4 Hz, 2H, Hb and
H′b), 1.73−1.93 (d, JH−H = 4.1 Hz, 18H, tBu). 13C NMR (100 MHz,
CD2Cl2): δ 177.7, 177.5, 134.4, 133.6, 133.4, 131.8, 131.7, 130.4,
130.4, 130.2, 130.1, 129.4, 129.3, 127.3, 126.9, 121.1, 120.8, 119.6,
119.5, 119.3, 70.9, 70.6, 68.5, 68.2, 59.3, 59.3, 31.0, 30.9, 29.0. HRMS:
calcd for C23H29N2PdSBF4 [M − BF4]+ 470.12, found 470.13.
Palladium Complex ( )-12 (Mixture of Three Diastereomers
in 2:1:1 Ratio). The complex was prepared in a manner analogous to
that described for ( )-9, using silver carbene complex (8; 0.9 g, 1.844
mmol) in dichloromethane (20 mL) and [Pd(η3-C3H5)-
■
We thank Nanyang Technological University for support of this
research work and also funding research scholarship to D.
Krishnan.
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