Well-defined N-heterocyclic carbene silver halides of 1-cyclohexyl-3- arylmethylimidazolylidenes: Synthesis, structure and catalysis in A 3-reaction of aldehydes, amines and alkynes

Article abstract of DOI:10.1039/c0dt01074j

Structurally well-defined N-heterocyclic carbene silver chlorides and bromides supported by 1-cyclohexyl-3-benzylimidazolylidene (CyBn-NHC) or 1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene (CyNaph-NHC) were synthesized by reaction of the correspondi

Full text of DOI:10.1039/c0dt01074j

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PAPER
Well-defined N-heterocyclic carbene silver halides of
1-cyclohexyl-3-arylmethylimidazolylidenes: synthesis, structure
and catalysis in A3-reaction of aldehydes, amines and alkynes†
Yanbo Li, Xiaofeng Chen, Yin Song, Ling Fang and Gang Zou*
Received 20th August 2010, Accepted 2nd December 2010
DOI: 10.1039/c0dt01074j
Structurally well-defined N-heterocyclic carbene silver chlorides and bromides supported by
1-cyclohexyl-3-benzylimidazolylidene (CyBn-NHC) or 1-cyclohexyl-3-naphthalen-2-
ylmethylimidazolylidene (CyNaph-NHC) were synthesized by reaction of the corresponding
imidazolium halides with silver(I) oxide while cationic bis(CyBn-NHC) silver nitrate was isolated under
similar conditions using imidazolium iodide in the presence of sodium nitrate. Single-crystal X-ray
diffraction revealed a dimeric structure through a nonpolar weak-hydrogen-bond supported Ag–Ag
bond for 1-cyclohexyl-3-benzylimidazolylidene silver halides [(CyBn-NHC)AgX]2 (X = Cl, 1; Br, 2) but
a monomeric structure for N-heterocyclic carbene silver halides with the more sterically demanding
1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene ligand (CyNaph-NHC)AgX (X = Cl, 4; Br, 5).
Cationic biscarbene silver nitrate [(CyBn-NHC)2Ag]+NO3 3 assumed a cis orientation with respect to
-
the two carbene ligands. The monomeric complexes (CyNaph-NHC)AgX 4 and 5 showed higher
catalytic activity than the dimeric [(CyBn-NHC)AgX]2 1 and 2 as well as the cationic biscarbene silver
nitrate 3 in the model three component reaction of 3-phenylpropionaldehyde, phenylacetylene and
piperidine with chloride 4 performing best and giving product in almost quantitative yield within 2 h at
100 ◦C. An explanation for the structure–activity relationship in N-heterocyclic carbene silver halide
catalyzed three component reaction is given based on a slightly modified mechanism from the one in
literature.
to explore the catalytic chemistry of NHC silver complexes.
Introduction
However, even for the most common silver chloride, struc-
turally characterized single site two coordinate NHC complexes
(NHC)AgCl still remain rare, except for a handful of examples
supported by very sterically demanding NHC ligands.1,7,8 Herein
we report synthesis and structure of well-defined N-heterocyclic
carbene silver halides, including single site two coordinate silver
halides (NHC)AgX supported by readily available 1-cyclohexyl-
3-arylmethyl imidazolylidenes and their catalytic activities in the
A3-reaction of aldehydes, amines and alkynes.
N-Heterocyclic carbene (NHC) silver complexes have attracted
increasing interests due to their structural diversity, wide appli-
cations as carbene transfer reagent and their antimicrobial and
anticancer properties1 since Arduengo and co-workers2 isolated
the first NHC silver complex in 1993. However, the NHC silver
complexes have found few applications in silver catalyzed organic
transformations3 except for scattered examples, such as diboration
of alkynes and alkenes,4 carbomagnesiation of terminal alkynes5
and reaction of aldehydes, amines and alkynes (A3).6 In fact, the
features of structural diversity, especially the polynuclear or cluster
structures, and good carbene transfer property of NHC silver
complexes are undesirable in transition metal catalysis because
the former could hamper identification of the true catalytic species
while the later would lead to decomposition of the corresponding
catalysts. Therefore, structurally well-defined stable NHC silver
complexes, especially the single site ones, are highly required
Results and discussion
1-Cyclohexyl-3-benzylimidazolylidene silver complexes
The NHC silver halides, 1-cyclohexyl-3-benzylimidazolylidene
(CyBn-NHC) silver chloride (1) and bromide (2) were syn-
thesized by reaction of the corresponding 1-cyclohexyl-3-
benzylimidazolium salts with silver oxide (Ag2O) in CH2Cl2
following a procedure reported in literature (Scheme 1).9
However, reaction of 1-cyclohexyl-3-benzylimidazolium iodide
with Ag2O under the otherwise identical conditions led to the
formation of NHC silver chloride complex 1 in CH2Cl2, through
Department of Fine Chemicals, East China University of Science &
Technology, Shanghai, China. E-mail: zougang@ecust.edu.cn; Fax: +86-
21-64253881; Tel: +86-21-64252390
† CCDC reference numbers 790416, 801886–801889. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0dt01074j
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Scheme 1 Synthesis of NHC silver halides 1 and 2.
an I/Cl exchange.9,10 In chloride-free solvents THF or methanol, a
structurally unidentified white solid formed but almost insoluble in
common organic solvents. Elemental analyses indicated the solid
consisted of one NHC ligand with seven silver iodide molecules. A
rationale for the failure in isolating the corresponding CyBn-NHC
silver iodide possibly lies in the stronger interaction (soft acid-
soft base interaction) of silver ion with iodide than NHCs, which
makes silver iodide form cluster itself rather than be supported by
NHCs. In other words, if the coordination of an anion to silver
is much weaker than a NHC, then NHC-supported silver cations
[(NHC)2Ag]+ would be likely to form. To test this, one equivalent
sodium nitrate was deliberately added to the reaction mixture of 1-
cyclohexyl-3-benzylimidazolium iodide and silver(I) oxide in THF
to supply a weakly coordinating anion and a biscarbene silver
Fig. 1 Crystal structure (30% probability) of [(CyBn-NHC)AgCl]2 1.
-
nitrate [(CyBn-NHC)2Ag]+NO3 3 was obtained in 37% yield.
Spectra and elemental analyses provided little information on
the molecular structure of these NHC silver complexes. Thus,
single crystals of the NHC silver complexes 1, 2 and 3 were grown
by slow evaporation of their acetonitrile solutions and X-ray
diffraction analysis was performed. A dimeric structure through
a ligand-unsupported nonpolar Ag–Ag bond having crystallogr-
phically imposed twofold symmetry, thus a trans conformation,
was found for NHC silver chloride 1 (Fig. 1) and bromide 2
while the two NHC ligands assume a cis orientation in cationic
biscarbene silver nitrate complex 3 (Fig. 2). Complexes 1 and 2
are isomorphous, thus structure of 2 is not shown. The selected
bond lengths and angles of complexes 1, 2 and 3 are compiled in
Table 1.
The lengths of the coordination bonds between the CyBn-
Fig. 2 Crystal structure (30% probability) of [(CyBn-NHC)2Ag]+NO3
(anion omitted).
3
-
˚
NHC ligand and silver ion, C(1)–Ag(1) in 1 (2.079 A) and 2
˚
(2.085 A), are close to the values observed in NHC silver chlorides
with similar structure, such as 1,3-dialkyl imidazolylidene silver
8
˚
˚
chlorides [(EtEt-NHC)AgCl]2 (2.064 A),11 [(iBuiBu-NHC)AgCl]2,
˚
(2.060–2.087 A), [(CyCy-NHC)AgCl]2 (2.075–2.096 A) and bis-
carbene silver cation [Ag(NHC)2]+ with non-coordinating anions
(2.073–2.089 A), as well as the average value observed in
[Ag(NHC)2] [AgX2] (2.08 A). However, the Ag–Ag distances in
12
˚
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for complexes 1, 2
+
-
1
˚
and 3
˚
the dimeric CyBn-NHC silver complexes 1 (2.8886 A) and 2
(2.8564 A) are not only significantly shorter than sum (3.44 A)
of the van der Waals radii of silver,13 but also compared to the
Bond
1
2
3
˚
˚
C(1)–Ag
Ag–X
Ag–Ag
2.079(4)
2.085(7)
2.071(2), 2.077(2)
˚
metal-metal distances in the open-shell metallic silver (2.889 A),
2.3296(11)
2.8886(7)
169.00(11)
132.5(3)
123.4(3)
82.74(4)
2.4045(12)
2.8564(12)
166.6(2)
134.3(5)
122.7(5)
80.74(3)
103.61(19)
indicating a genuine attractive interaction between silver atoms.14
The Ag–Ag distances in the dimeric complexes of 1 and 2 are also
shorter than those typically observed for ligand-unsupported po-
lar Ag–Ag interaction, such as in [(MeMe-NHC)2Ag]+[AgX2]- of
C(1)–Ag–Xa
Ag–C(1)–N(1)
Ag–C(1)–N(2)
Ag–Ag–X
Ag–Ag–C(1)
177.96(8)
128.36(16), 127.89(16)
127.42(17), 128.05(15)
101.44(11)
˚
N,N¢-dimethylimidazolylidene silver chloride (3.188 A) and bro-
9
˚
mide (3.2082 A) although longer than a Ccarbene-supported Ag–Ag
interaction (2.705 A) in (NHC)Ag+(m-NHC)Ag+(DMSO)m.
a C–Ag–C for 3.
15
˚
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 4 and 5
Compared with the NHC silver halides having ligand-
unsupported nonpolar Ag–Ag interaction similar to 1 and
2, such as 1,3-dialkylimidazolylidene silver chlorides [(iBuiBu-
Bond
4
5
˚
˚
˚
NHC)AgCl]2, (3.108 A, 3.124 A), [(CyCy-NHC)AgCl]2 (3.0650 A,
C(1)–Ag
Ag–X
C(1)–Ag–X
Ag–C(1)–N(1)
Ag–C(1)–N(2)
2.090(8)
2.3824(16)
166.5(2)
130.3(6)
126.0(5)
2.090(5)
2.4331(7)
174.18(12)
126.4(3)
128.6(3)
8
11
˚
˚
3.0181 A), [(EtEt-NHC)AgCl]2 (3.0553 A), 1,3,4,5-tetramethyl
imidazolylidene silver chloride [(meNHC)AgCl]2 (3.0673 A)
8
˚
and complex [(2,6-bis(n-butylcarbenemethyl)-pyridine)AgCl]2
16
˚
(3.269 A), the Ag–Ag interaction in 1 and 2 is also obviously
stronger. The strong nonpolar Ag–Ag interaction and trans
conformation implied that the Ag–Ag interaction in complexes
1 and 2 could not be truly unsupported. In fact, a couple of intra-
and intermolecular Csp3-H–A and Csp2-H–A (A = Cl, Br, Ag)
weak hydrogen bonds17 could be identified in the crystal packing
diagram of complexes 1 and 2. Further, these weak hydrogen bonds
appeared not only to provide supports for the conformation and
the nonpolar Ag–Ag interaction in dimeric structure of 1 and 2,
but also to drive the dimer to self-assembly into a supramolecular
architecture in crystal (Fig. 3).
And more, Nolan et al.8 has shown that replacement of
the isobutyl or cyclohexyl groups on the N-heterocyclic car-
bene ligands with more sterically demanding isopropyl groups
in the 1,3-bialkylimidazolylidene silver chlorides made them
change from dimers through a ligand-unsupported Ag–Ag bond
to monomeric complexes (NHC)AgCl. Inspired by these, a
more sterically demanding ligand 1-cyclohexyl-3-naphthalen-2-
ylmethylimidazolylidene was designed to support monomeric
NHC silver halides.
Similar to complexes 1 and 2, 1-cyclohexyl-3-naphthalen-2-
ylmethylimidazolylidene silver chloride 4 was synthesized by
reaction of 1-cyclohexyl-3-naphthalen-2-ylmethyl imidazolium
chloride with Ag2O in CH2Cl2 in 64% yield. However, reaction of
1-cyclohexyl-3-naphthalen-2-ylmethylimidazolium bromide with
Ag2O under the otherwise identical conditions led to the formation
of chloride 4, through an unexpected Br/Cl exchange, implying
1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene silver chlo-
ride is more thermally stable than its bromide analogue. The
desired bromide 5 had to be prepared by conduction of the reaction
in acetonitrile to prevent the Br/Cl exchange from taking place.
Single-crystal X-ray diffraction analysis on complexes 4 and 5
revealed the desired monomeric structure with a two coordinate
silver ion (CyNaph-NHC)AgX for 4 (Fig. 4) and 5. The structure
of 5 is essentially identical to 4, thus being omitted. The selected
bond lengths and angles of complexes 4 and 5 are compiled in
Table 2.
Fig. 3 A weak-hydrogen-bond supported supramolecular architec-
˚
ture of
1 and 2 in crystal. Bond lengths (A) and angles
(◦): C4–H4B–X, 2.88(2.88), 164.5(168.5); C4–H4B–Ag 3.07(3.09),
115.9(115.6); C7–H7–Ag 3.02(3.12), 111.6(107.2); C8–H8–Cl 3.14(3.09),
136.6(140.3); C11–H11–Ag 2.65(2.64), 125.2(125.1); C16–H16A–Cl
2.84(2.88), 163.9(164.0).
Fig. 4 Crystal structure (30% probability) of (CyNaph-NHC)AgCl 4.
˚
The C(carbene)-Ag bond lengths (2.090 A) in complexes 4 and
1-Cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene silver
halides
5 are slightly longer than those reported for some monomeric
two-coordinate (NHC)AgCl complexes of sterically demand-
ing NHCs, such as in 1,3-dimesimidazolylidene silver chlo-
ride (2.056 A), 1,3-di(2,6-4-tolyl)imidazolylidene silver chlo-
ride (2.062 A) and 1,3-didodecylimidazolylidene silver chloride
Generally, weak hydrogen bonds C–H–A are readily destroyed by
a slight change of the molecular structure. Breaking the weak-
hydrogen-bond net in the dimeric structure of complexes 1 and 2
could lead to disassembly of the (NHC)AgX dimers.
˚
˚
˚
(2.067 A), but close to those in highly sterically demanding NHC
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Table 3 Catalytic activity of NHC silver complexes 1–5 in A3 reaction
supported silver chlorides, such as 1,3-di(2-phenyl)imidazol-
˚
ylidene silver chloride (2.079 A), 1,3-di(2,6-bis(diphenylmethyl)-4-
˚
methylphenyl) imidazolylidene silver chloride (2.081 A) and 1,3-
˚
diadamantylimidazolylidene silver chloride (2.094 A), while the
˚
Ag–Cl bond length (2.3824 A) in 4 is slightly longer than all in
the above sterically demanding NHC supported silver chloride
R1
R2
Ph
T/◦C t/h Yield (%)a
7,8
Entry Cat.
˚
(2.314–2.345 A), indicating the 1-cyclohexyl-3-naphthalen-2-
ylmethylimidazolylidene is rather sterically demanding. The C(1)–
Ag and Ag–X bond lengths in 4 and 5 are also comparable
to, if not longer than, those observed in [(CyBn-NHC)AgX]2 1
and 2 (Table 1) which have a more crowded environment around
silver ion, consistent with the more sterically demanding property
of 1-cyclohexyl-3-naphthalen-2-ylmethyl imidazolylidene than 1-
cyclohexyl-3-benzylimidazolylidene. All the NHC silver halides 1,
2, 4 and 5 have a near linear C–Ag–X geometry with bond angles
from 166–174◦, falling into the ranges reported for the NHC silver
halides.1
1
2
3
4
5
6
1
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
4-ClC6H4
100
100
100
100
100
100
100
100
100
100
50
2
2
2
2
2
2
2
2
2
2
12
72
12
6
79
61
77
99
84
80
64
49
61
78
81
69
31b
64
77
49
2
3
4
5
AgCl
7
8
AgBr
AgI
9
AgNO3
Ag2O
10
11
12
13
14
15
16
4
4
4
4
4
4
Ph
Ph
Ph
r.t.
100
n-C6H13 100
Ph
Ph
Catalysis
100
100
12
12
Piperonyl
Propargylamines are important skeletons or synthetically versatile
building blocks for the preparation of many nitrogen-containing
biologically active compounds.18 In 2003, Li et al. reported the first
silver catalyzed three-component A3 reaction of aldehydes, amines
and alkynes in water to efficiently provide propargylamines.19
Owning to the importance of propargylamines and the high
synthetic efficiency of the A3 reaction, many efforts have been
made to improve the three-component reaction by use of silver
nanoparticles20 and supported silver salt (AgI) catalysts,21 as well
as development of alternative transition-metal based catalysts
such as copper,22 gold,23 iridium24 and zinc25 etc.. Recently,
N-heterocyclic carbene silver halides have been reported to show
high catalytic activity in the A3 reaction,6 in which in situ-generated
polymer-supported complexes were claimed to be more active
than the parent NHC silver halides. However, the intrinsic nature
of the in situ-generated polymer-supported complexes hampers
elucidation of the true catalytic species. With the structurally
well-defined NHC silver complexes 1–5 in hands, we investigated
the structure–activity relationship of NHC silver-based catalysts
in the A3 reaction. The reaction of 3-phenylpropionaldehyde,
phenylacetylene and piperidine was chosen as the model. Using
NHC silver halides 1, 2, 4 or 5 as the catalyst or combining
the corresponding imidazolium halides with silver(I) oxide to
generate catalysts in situ, the model A3 reaction occurred very
slowly adapting the conditions reported by Wang et al.6 and only
trace amount of product could be detected by TLC in 12 h at
room temperature in CH2Cl2 although the reaction proceeded
significantly faster in refluxing CH2Cl2.‡ Considering most of
metal-catalyzed A3 reactions were run at elevated temperature
(~100 ◦C) and to avoid the possible Br/Cl exchange of the NHC
silver bromide in chloride solvents, dioxane was chosen as the
solvent for the model reaction (Table 3).
a Isolated yield. b Run in water.
NHC silver halides 1 and 2 were just comparable to the simple
inorganic silver halides of AgCl and AgBr (Table 3, entries 1,
2, 6 and 7). However, the more sterically demanding NHC,
1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene, supported
monomeric silver halides 4 and 5 displayed remarkably higher
activities than simple silver halides AgX as well as the dimeric
NHC silver halides 1 and 2 supported by the less sterically
demanding 1-cyclohexyl-3-benzylimidazolylidene (Table 3, entries
4 and 5). In fact, the reaction catalyzed by complex 4, (CyNaph-
NHC)AgCl, completed in 2 h giving the product in an almost
quantitative yield. It was reported that cationic or high oxidation
state metal compouds, thus strong Lewis acids, showed high
activities in the A3 reaction.23 However, the cationic biscarbene
silver nitrate 3 did not display better performance than the silver
halides possibly due to steric hindrance of the silver in the
biscarbene cation [(NHC)2Ag]+, thus implying the importance
of accessible coordination of silver to catalyze the A3 reaction.
The reaction proceeded rather slowly at low temperature (Table 3,
entries 11 and 12). Alkyl alkynes, such as octyne, and aromatic
aldehydes, both electron-rich and -deficient ones, also reacted,
albeit slowly, under similar conditions (Table 3, entries 14–16).
Unlike the reaction in water,19 the catalytic activities of both NHC
supported and simple silver halides AgX decreased from chloride
to bromide or iodide in dioxane and even nitrate and oxide gave
better performance than silver iodide (Table 3, entries 1–10).
The fact of higher catalytic activity of chloride 4 than the
corresponding bromide 5 and the cationic biscarbene silver
[(NHC)2Ag]+ nitrate 3 along with the formation of 4 from the
precursor imidazolium bromide for 5 in CH2Cl2 during synthesis
indicated the true catalytic species in the NHC silver-catalyzed
A3 reaction would be a structurally stable and coordinatively
unsaturated N-heterocyclic carbene silver halide (NHC)AgX
rather than a silver cation. A slightly modified mechanism from the
originally proposed one by Li19 is shown in Scheme 2 to explain the
observed structure–activity relationship in the NHC silver halide
catalyzed A3 reaction.
All the NHC silver halides and nitrate 1–5 could efficiently
catalyze the A3 reaction giving the desired product in good to
excellent yields in dioxane at 100 ◦C in air. The activities of
‡ We could not repeat the results reported in ref. 6 at room temperature
using 1,3-dibenzylimidazolylidene silver chloride as catalyst. When the
reaction was conducted in refluxing CH2Cl2 for 12 h (~50 ◦C bath
temperature), the desired product was isolated in 55% yield.
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1-Cyclohexyl-3-benzylimidazolylidene silver chloride (1)
To a solution of 1-cyclohexyl imidazole (1.5 g, 10 mmol) in
20 mL toluene was added benzyl chloride (1.5 g, 12 mmol). The
resulting mixture was heated to reflux until 1-cyclohexyl imidazole
was completely consumed. The salt separated upon cooling was
washed with toluene until free of benzyl chloride to provide
1-cyclohexyl-3-benzylimidazolium chloride quantitatively, which
was dissolved in 50 mL CH2Cl2 and subjected to Ag2O (3.5 g,
15 mmol) at room temperature. The resulting mixture was stirred
for 24 h in dark. Then, the solid materials were filtered off and
the filtrate was concentrated under reduced pressure. The residue
was washed with ether and recrystallized in acetonitrile to provide
Scheme 2 A plausible mechanism for (NHC)AgX catalyzed A3 reaction.
◦
1
1 as colorless crystals, 1.77 g (46%). M.p.: 157–159 C. HNMR
◦
(CDCl3, 500 MHz, 25 C), d (ppm): 7.40–7.47 (m, 3H), 7.35 (d,
The catalyst (NHC)AgX reacts with alkyne and amine to form
the silver alkynide and the amine hydrogen halide salt at first. The
latter then condenses with aldehyde to generate iminium halide,
which reacts with the previously generated silver alkynide to afford
the desired product and regenerate the catalyst (NHC)AgX. The
structural stability and coordinative accessibility of silver ion in
NHC silver halides appeared to play a crucial role in the catalysis.
Based on this, it is easy to explain the highest catalytic activity of
chloride 4 (CyNaph-NHC)AgCl among these investigated silver
compounds since it posses the most stable structure yet with open
coordination environment around silver ion.
J = 5.5 Hz, 2H), 7.05 (d, J = 2.0 Hz, 1H), 6.90 (d, J = 1.5 Hz, 1H),
5.30 (s, 2H), 4.26–4.31 (m, 1H), 2.05–1.20 (m, 10H). Anal. for 1:
Calcd for C16H20N2ClAg (%): C, 50.09; H, 5.25; N, 7.30. Found
(%): C, 50.46; H, 5.41; N, 6.97.
1-Cyclohexyl-3-benzylimidazolylidene silver bromide (2)
A procedure similar to the synthesis of 1 was adapted with benzyl
bromide (2.1 g, 12 mmol) to afford 2 as colorless crystals (2.23 g,
◦
◦
1
52%). M.p.: 108–111 C. HNMR (CDCl3, 500 MHz, 25 C), d
(ppm): 7.42–7.48 (m, 3H), 7.30 (d, J = 6.0 Hz, 2H), 7.00 (d, J =
2.0 Hz, 1H), 6.90 (d, J = 1.5 Hz, 1H), 5.35 (s, 2H), 4.28–32 (m,
1H), 2.05–1.20 (m, 10H). Anal. for 2: Calcd for C16H20N2BrAg
(%): C, 44.89; H, 4.71; N, 6.54. Found (%): C, 45.26; H, 4.70; N,
6.27.
Conclusions
Structurally well-defined N-heterocyclic carbene silver chlorides,
bromides and nitrate supported by 1-cyclohexyl-3-arylmethyl
imidazolylidene have been synthesized and crystallographically
characterized. The structures of the 1-cyclohexyl-3-arylmethyl
imidazolylidene silver halides have been tuned from dimer
[(NHC)AgX]2 through non-polar silver-silver interaction to
monomer (NHC)AgX with a two coordinate silver ion by
replacement of benzyl group on the imidazole moiety to more
sterically demanding naphthalene-2-ylmethyl group. The catalytic
activities of these structurally well-defined N-heterocyclic carbene
silver complexes in the model three-component reaction of 3-
phenylpropionaldehyde, phenylacetylene and piperidine indicated
that both the structural stability and accessible coordination to
silver of the NHC silver catalyst played key roles in the NHC silver
catalyzed A3 reaction. Further work on these structurally well-
defined N-heterocyclic carbene silver halides and applications of
readily available sterically demanding 1-cyclohexyl-3-naphthalen-
2-ylmethylimidazolylidene ligand in transition metal catalysis is in
progress in our laboratory.
Bis(1-cyclohexyl-3-benzylimidazolylidene) silver nitrate (3)
A procedure similar to the synthesis of 1 was adapted with benzyl
iodide (2.6 g, 12 mmol), Ag2O (3.5 g, 15 mmole) and NaNO3
(0.9 g) but using THF in placement of CH2Cl2 to afford 3 (1.24 g,
37%). as a ◦pale crystal. M.p.: 149 ◦C (dec.) 1HNMR (CDCl3, 500
M Hz, 25 C), d (ppm): 7.43–7.50 (m, 6H), 7.30 (d, J = 6.1 Hz,
4H), 7.00 (d, J = 2.0 Hz, 2H), 6.90 (d, J = 1.6 Hz, 2H), 5.40 (s,
4H), 4.27–4.31 (m, 2H), 2.00–1.20 (m, 20H). Anal. for 3: Calcd for
C32H40N5O3Ag·H2O (%):C, 57.49; H, 6.33; N, 10.48. Found (%):
C, 57.92; H, 6.20; N, 10.39.
1-Cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene silver
chloride (4)
A procedure similar to the synthesis of 1 was adapted with
2-(chloromethyl)naphthalene (2.1 g, 12 mmol) to afford 4 as
colorless crystals (2.77 g, 64%). M.p.:178–180 ◦C. 1HNMR
(CDCl3, 500 MHz, 25 ◦C), d (ppm): 7.84–7.82 (m, 3H), 7.73
(s, 1H), 7.52–7.50 (m, 2H), 7.34 (dd, J1 = 2.5 Hz, J2 = 2.0 Hz,
1H), 7.01 (d, J = 2.5 Hz, 1H), 6.94 (d, J = 2.0 Hz, 1H), 5.41 (s,
2H), 4.34–4.28 (m, 1H), 2.08–1.44 (m, 10H). Anal. for 4: Calcd
for C20H23N2ClAg (%): C, 55.38; H, 5.11; N, 6.46. Found (%): C,
55.01; H, 5.23; N,6.56.
Experimental
General
1-Cyclohexylimidazole,26 2-(chloromethyl)naphthalene and 2-
(bromomethyl)naphthalene27 were prepared according to the
previously reported procedures. All the other chemicals were com-
mercially available and were used as received. Melting points were
1-Cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene silver
bromide (5)
R
recorded on a SWGꢀX-4 capillary melting point apparatus. 1H and
13CNMR spectra were recorded on a Bruker Avance Spectrometer.
Elemental analyses and mass spectra were performed at the Center
for Analysis, ECUST.
A
procedure similar to the synthesis of
1 was adapted
but using CH3CN in placement of CH2Cl2 with
2050 | Dalton Trans., 2011, 40, 2046–2052
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Table 4 Crystal data for N-cyclohexyl-N¢-arylmethylimidazolylidene silver complexes
1
2
3
4
5
Formula
C32H40Ag2Cl2N4
767.32
Monoclinic
C32H40Ag2Br2N4
856.24
Monoclinic
C32H42AgN5O4
668.58
C20H22AgClN2
433.72
C20H22AgBrN2
478.18
Formula weight
Crystal system
Space group
Triclinic
Triclinic
Triclinic
¯
¯
¯
C2/c
C2/c
P1
P1
P1
˚
a/A
10.1326(10)
16.0862(16)
19.984(2)
90
10.2059(10)
16.0854(16)
19.955(2)
90
10.4641(7)
13.1306(9)
13.3806(9)
67.602(1)
71.583(1)
76.576(1)
1599.59(19)
2
8.6763(10)
9.2220(11)
12.2495(15)
91.356(2)
105.679(2)
97.504(2)
933.78(19)
2
9.5958(14)
10.0014(14)
10.5225(15)
78.320(2)
68.860(2)
84.966(2)
922.3(2)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
94.549(2)
90
3247.0(6)
4
1552
3523
95.573(2)
90
3262.5(6)
4
1696
3696
3
˚
V/A
Z
F(000)
696
6768
440
3428
476
3548
No of reflns
No of reflns gt
No of params
Goodness of fit
Final R, Rw (I > 2s(I))
2781
182
1.099
0.0501, 0.1380
0.001, 0.000
2445
181
1.041
0.0721, 0.2378
0.001, 0.000
5977
387
0.993
0.0340, 0.0813
0.002, 0.000
2885
217
1.069
0.0773, 0.2171
0.000, 0.000
3023
218
1.023
0.0539, 0.1375
0.000, 0.000
-3
˚
Drmax,min/e A
2-(bromomethyl)naphthalene (2.6 g, 12 mmol) to afford 5
7.57–7.56 (m, 2H), 7.38–7.37 (m, 5H), 4.80 (s, 1H), 2.59 (s, 4H),
1.68–1.58 (m, 4H), 1.51–1.50 (m, 2H).
◦
1
as colorless crystals (2.71 g, 57%). M.p.: 150–152 C. HNMR
◦
(CDCl3, 500 MHz, 25 C), d (ppm): 7.84–7.82 (m, 3H), 7.74 (s,
1-(1-(Benz[1,3]dioxol-5-yl)-3-p◦henylprop-2-ynyl)
piperidine28.
1H), 7.52–7.50 (m, 2H), 7.34 (dd, J1 = J2 = 2.0 Hz, 1H), 7.01 (d,
J = 2.5 Hz, 1H), 6.94 (d, J = 2.5 Hz, 1H), 5.41 (s, 2H), 4.33–4.27
(m, 1H), 2.09–1.20 (m, 10H). Anal. for 5: Calcd for C20H23N2BrAg
(%): C, 50.24; H, 4.64; N, 5.86. Found (%): C, 50.51; H, 4.33; N,
5.74.
1HNMR (CDCl3, 400 MHz, 25 C), d (ppm): 7.51–7.49 (m, 2H),
7.33–7.29 (m, 3H), 7.14 (s, 1H),7.10 (d, J = 8 Hz, 1H), 6.78 (d, J =
8 Hz, 1H), 5.95 (s, 2H), 4.68 (s, 1H), 2.56–2.53 (m, 4H), 1.62–1.55
(m, 4H), 1.44–1.42 (m, 2H).
1-(1-Phenylundec-4-yn-3-yl)piperidine. 1HNMR
(CDCl3,
500 MHz, 25 ◦C), d (ppm): 7.28–7.24 (m, 2H), 7.21–7.15 (m,
3H), 3.24 (q, J = 8.0 Hz, 1H), 2.86–2.79 (m, 1H), 2.72–2.66
(m, 1H), 2.60–2.56 (m, 2H), 2.40–2.36 (m, 2H), 2.25–2.22 (m,
2H), 1.97–1.83 (m, 2H), 1.66–1.49 (m, 6H), 1.47–1.42 (m, 4H),
X-Ray crystallography
Single crystals of 1–5 were sealed in thin-walled glass capillaries,
and data collection was performed at room temperature on a
Bruker SMART diffractometer with grapite-monochromated Mo-
1
1.35–1.30 (m, 4H), 0.898 (t, J = 8.5 Hz, 3H). CNMR (CDCl3,
125 MHz, 25 ◦C), d (ppm): 142.09, 128.60, 128.28, 125.74, 86.02,
77.72, 57.35, 35.41, 33.02, 31.38, 29.15, 28.57, 26.20, 24,66,
22.65, 18.72, 14.10. HREI-MS (70 eV) m/z: calcd. For C22H33N+:
311.2613, Found: 311.2610.
˚
Ka radiation (l = 0.71073 A). The SMART program package
was used to determine the unit-cell parameters. The absorption
correction was applied using SADABS. The structures were
solved by direct methods and refined on F2 by full-matrix
least-squares techniques with anisotropic thermal parameters for
non-hydrogen atoms. Hydrogen atoms were placed at calculated
without further refinement of the parameters. All calculations were
carried out using the SHELXS-97 program. Crystallographic data
and refinement for 1–5 are listed in Table 4.
Acknowledgements
Financial support from the National Nature Science Foundation
of China (No. 20972049) are gratefully acknowledged.
Typical procedure for silver-catalyzed A3 reaction
Notes and references
Aldehyde (1.0 mmol), piperidine (1.2 mmol), alkyne (1.5 mmol)
and silver catalyst (3 mol%) were heated in 3 mL dioxane at 100 ◦C
for given time in air. The products were purified directly by flash
chromatography with ethyl acetate and hexane as eluents.
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