Gold(I) and palladium(II) complexes of 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene "click" carbenes: Systematic study of the electronic and steric influence on catalytic activity

Article abstract of DOI:10.1021/om400773n

The synthesis of a small family of six electronically and sterically modified 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene gold(I) chloride complexes is described. Additionally, the corresponding trans-[PdBr₂(iPr ₂-bimy)(1,3,4-trisub

Full text of DOI:10.1021/om400773n

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Gold(I) and Palladium(II) Complexes of 1,3,4-Trisubstituted 1,2,3-
Triazol-5-ylidene “Click” Carbenes: Systematic Study of the Electronic
and Steric Influence on Catalytic Activity
James R. Wright,†,§ Paul C. Young,‡,§ Nigel T. Lucas,† Ai-Lan Lee,*,‡ and James D. Crowley*,†
†Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
‡Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
S
* Supporting Information
ABSTRACT: The synthesis of a small family of six electronically
and sterically modified 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene
gold(I) chloride complexes is described. Additionally, the
corresponding trans-[PdBr2(iPr2-bimy)(1,3,4-trisubstituted 1,2,3-
triazol-5-ylidene)] complexes are also generated and used to
examine the donor strength of the 1,3,4-trisubstituted 1,2,3-triazol-
1
5-ylidene ligands. All compounds have been characterized by H
and 13C NMR and IR spectroscopy, high-resolution electrospray
mass spectrometry (HR-ESI-MS), and elemental analysis. The
molecular structures of four of the gold(I) and four of the
palladium(II) complexes were determined using X-ray crystallography. Finally, it is demonstrated that these 1,2,3-triazol-5-
ylidene gold(I) chloride complexes (Au(trz)Cl) are able to catalyze the cycloisomerization of 1,6-enynes, in high yield and
regioselectivity, as well as the intermolecular direct etherification of allylic alcohols. Exploiting the Au(trz)Cl precatalysts allowed
the etherification of allylic alcohols to be carried out under milder conditions, with better yield and regioselectivity than selected
commercially available gold(I) catalysts.
been exploited as catalysts for a wide variety of organic
transformations (Figure 1b).
INTRODUCTION
■
In the past two decades N-heterocyclic carbenes (NHCs) have
become the ligands of choice for new catalyst development.1
Initially, Arduengo-type imidazol-2-ylidene complexes2 (A)
dominated the area, because this class of NHC is relatively
easily synthesized and handled. However, while these imidazol-
2-ylidenes (A) and the related 1,2,4-triazol-5-ylidenes (B) can
be readily sterically modified, the electron-donating ability of
these carbenes can only be tuned over a narrow range. To
overcome this limitation, a vast array of NHCs have been
generated in the past 15 years, including ring-expanded
carbenes (C and D),3 cyclic alkylaminocarbenes (CAAC, E),4
pyrid-2-ylidenes (F),5 pyrid-4-ylidenes (G),5 pyrazol-4-ylidenes
(H),6 and imidazol-4-ylidenes (I) (Figure 1a). The 1,3,4-
trisubstituted 1,2,3-triazol-5-ylidenes (trz, J) are some of the
most recent additions to the NHC family.7 These NHCs have
been termed abnormal NHCs (aNHC)/mesoionic carbenes
(MICs) because no sensible uncharged resonance structures
can be generated for these systems.8 They have attracted
considerable attention since their discovery in 20089 because
the 1,4-disubstituted 1,2,3-triazole units, from which the
aNHC/MICs are derived, are readily synthesized and function-
alized using the modular and functional group tolerant
copper(I)-catalyzed cycloaddition of azides and alkynes
(CuAAC) “click” reaction.10 Copper,11 palladium,12 ruthe-
nium,13 and iridium14 complexes containing trz ligands have
Recently, homogeneous gold catalysis has become an
extremely popular area of research because the soft, carbophilic
Lewis acidic nature of Au(I) ions enables the mild activation of
unsaturated C−C bonds.15 While much of the early work
exploited phosphine-containing gold(I) complexes, the now
ubiquitous N-heterocyclic carbene ligands (NHCs, A−G;
Figure 1) have become increasingly popular for the generation
of these types of catalysts.16 Au(I)−NHC complexes often
display enhanced stability and catalytic activity in comparison
to the phosphine analogues due to the greater σ-donor strength
of the carbene ligands. Because of our interest in “click”
coordination chemistry,17 we recently reported the synthesis of
the Au(trz)Cl complex 7a (Scheme 1) and showed that it was
catalytically active.18 Herein we build on this initial result and
exploit the modularity of the CuAAC “click” reaction to
generate a small family of sterically and electronically tuned
Au(trz)Cl catalysts. Additionally, an analogous series of Pd(II)
bis-carbene complexes were synthesized to enable the variation
of the 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene’s σ-donor
strength to be directly probed. Finally, the effect of this
systematic modification of the Au(trz)Cl complexes is explored
in two different gold(I)-catalyzed reactions: (1) the cyclo-
Received: August 3, 2013
Published: November 21, 2013
© 2013 American Chemical Society
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a
Scheme 1
Figure 1. (a) Selected examples of generic NHC metal complexes. (b)
Selected 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene metal catalysts,
1,11d 2,12i 3,13d and 4.14c
isomerization of 1,6-enynes and (2) the intermolecular direct
etherification of allylic alcohols.
RESULTS AND DISCUSSION
■
Ligand Synthesis and Characterization. The 1,4-
disubstituted-1,2,3-triazoles 5a−f10b,11d,19 were synthesized
using previously reported methods and converted into the
corresponding triazolium salts 6a−f in good yields (57−82%)
using Meerwein’s reagent ([Me3O]BF4) (Scheme S1, Support-
ing Information.).11d,18,20 The infrared spectra (IR) of 6a−f
confirm the presence of both the triazole unit (νC−H 3150−
2965 cm−1) and the BF4 anions (νB−F 1027−1065 cm−1) in
−
the isolated colorless materials. High-resolution electrospray
mass spectra (HR-ESI-MS) of 6a−f display signals correspond-
ing to [(6a−f) − (BF4 )]+ and [2(6a−f) − (BF4 )]+ ions, and
the proposed formulations were further supported by elemental
analysis. NMR spectroscopy provided additional evidence for
the formation of the triazolium salts 6a−f, with signals due to
the triazolium N-bound methyl group observed in the 1H NMR
(δ 4.0−4.5 ppm) and the 13C NMR (δ 35−40 ppm) spectra,
consistent with what has been previously reported.11d,18,20b
Gold(I) and Palladium(II) Complex Synthesis and
Characterization. The gold(I) and palladium(II) trz com-
plexes were synthesized using slight modifications of the
previously reported methods, as outlined in Scheme 1. The
1,2,3-triazolium salts 6a−f were dissolved in CH2Cl2/CH3CN
and treated with Ag2O in the presence of Me4NCl to generate
the silver(I) 1,2,3-triazolylidene complexes in situ. Addition of
Au(Me2S)Cl to the in situ generated silver(I) 1,2,3-
triazolylidenes resulted in transmetalation and provided, after
chromatography, the desired Au(trz)Cl complexes (7a−f;
Scheme 1) as colorless or yellow (7c) solids in excellent yields
(72−92%). The palladium complexes were also synthesized
−
−
a
Legend: (i) (a) 6a−f, Ag2O, Me4NCl, CH2Cl2/CH3CN (1/1), room
temperature, 6−14 h, (b) Au(SMe2)Cl, room temperature, 2 h; (ii)
(a) [PdBr2(iPr2-bimy)]2, nBu4NBr, CHCl3, reflux, 3 h, (b) 6a−f,
Ag2O, CH2Cl2, room temperature, 18 h.
using a silver(I) transmetalation protocol, as described by
Huynh and co-workers,21 and were isolated as yellow
complexes (8a−f; Scheme 1) with the formula trans-
[PdBr2(iPr2-bimy)(trz)] (where iPr2-bimy = 1,3-diisopropyl-
benzimidazolin-2-ylidene) in good to excellent yields (64−
86%). These gold(I) and palladium(II) compounds were
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1
characterized by elemental analysis, HR-ESI-MS, IR, and H
and 13C NMR spectroscopy. The elemental analyses of the
complexes were consistent with the proposed formulations, and
this was further supported by HR-ESI-MS. The gold complexes
7a−f display major signals due to [Au(trz)Cl + Na]+, [Au(trz)
− Cl − Au(trz)]+ and [Au(trz)2]+ ions, while the palladium
complexes show major peaks consistent with [PdBr(iPr2-
bimy)(trz)]+, [PdCl(iPr2-bimy)(trz)]+, and [PdBr2(iPr2-bimy)-
1
(trz) + Na]+ (Supporting Information). The H NMR spectra
of 7a−f and 8a−f were also consistent with complex formation.
1
The H NMR spectra of the triazolium salts 6a−f contain a
Ctrz−H proton signal between 7.5 and 9.0 ppm which is absent
in the spectra of the metal complexes (7a−f and 8a−f),
indicative of deprotonation and carbene formation.7,18,21
Additionally, the signals due to the N-methyl protons of the
triazole units experience an upfield shift on complex formation.
In the complexes with phenyl substituents the o-phenyl proton
signals undergo a downfield shift, due to the proximity of the
deshielding metal centers. The 13C NMR spectra of the
complexes display 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene
carbon signals at ∼160 ppm, consistent with previous
studies.7,18,21
The molecular structures of four of the gold complexes (7b−
d,f; Figure 2, Table 1, and the Supporting Information) were
determined using X-ray crystallography.22 The crystals were
grown via vapor diffusion of either diethyl ether or petroleum
ether into a dichloromethane solution of one of the complexes.
The Au(I) ions of the complexes are bound to a 1,3,4-
trisubstituted 1,2,3-triazol-5-ylidene and a chloride ligand in the
expected linear fashion (C−Au−Cl bond angles range from
176.3 to 178.8°). The Au−Ctrz (1.972−2.001 Å) and Au−Cl
(2.272−2.298 Å) bond lengths are similar to those observed in
other gold(I) triazolylidene complexes.7a,e,18,22,23
The extended solid-state structure of complex 7b contains
dimers (Supporting Information) that are held together by
weak hydrogen-bonding interactions between the methoxy
oxygen atom and the aryl proton situated ortho to the methoxy
group of the adjacent molecule (C−H···O = 2.690(3) Å, C···O
= 3.592(6) Å). Additionally, π−π interactions are observed
between the triazole ring and the six-membered methoxyphenyl
ring (centroid···centroid = 3.748 Å).
In the extended structures of 7c,d the complexes form
antiparallel dimers (Supporting Information). The 7c dimer is
stabilized by Cl···π interactions (Cl···centroid = 3.377, 3.668
Å). In the case of 7d, the dimer motif extends throughout the
crystal lattice as a double-stranded, one-dimensional supra-
molecular polymeric chain. The strands of the chain are
connected via weak24 C−H···Cl hydrogen bonding of the
methyl group of the triazole ring to the chloride of the adjacent
molecule (H···Cl = 2.686(2) Å, C···Cl = 3.537(1) Å), as well as
C−H···π interactions between the acidic benzylic protons and
the six-membered ring (centroid···H = 2.737 Å, centroid···C =
3.558 Å). The adjacent strands of the chains are connected by
Cl···π bonding (7d, 3.469 Å) between the chloride and the
electron-poor triazole ring.25
Figure 2. ORTEP27 diagrams of the gold(I) complexes (a) 7b, (b) 7c,
(c) 7d, and (d) 7f. The thermal ellipsoids are shown at the 50%
probability level. The benzyl group of 7b is disordered, but for clarity,
only one orientation is shown.
Table 1. Selected Bond Distances (Å) and Angles (deg) of
the Gold(I) Complexes 7a−d,f
7a18
7b
7c
7d
7f
Au1−C1
Au1−Cl1
1.982(4)
2.294(1)
103.1(3)
1.979(5)
2.292(2)
102.9(5)
1.986(5)
2.272(1)
103.2(5)
2.001(9)
2.298(2)
104.4(8)
1.974(8)
2.284(2)
103.2(7)
C2−C1−
N1
C1−Au1−
177.2(1)
176.3(2)
178.8(2)
178.6(3)
177.1(2)
Cl1
Surprisingly, no aurophilic interactions are observed in any of
the structures (7a18 or 7b−d,f); the shortest gold···gold
distance (7d, Au−Au = 3.612 Å)26 is greater than the sum of
the van der Waals radii of two Au(I) centers (3.60 Å).
Four of the palladium complexes, 8a−d, were also
characterized by X-ray crystallography (Figure 3, Table 2, and
the Supporting Information). All complexes crystallize in a trans
square-planar geometry and are essentially isostructural (Figure
The structure of 7f also displays dimers that are assembled
through weak hydrogen-bonding interactions between the
unsubstituted N2 nitrogen and the N-methyl proton of the
adjacent molecule (C−H···N = 2.673(6) Å, C···N = 3.52(1)
Å). The chloride ligands of these molecules also hydrogen bond
with the N-methyl proton of another molecule in the lattice
(H···Cl = 2.743(2) Å, C···Cl = 3.556(9) Å).
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Table 2. Selected Bond Distances (Å) and Angles (deg) of
Palladium(II) Complexes 8a−d
8a
8b
8c
8d
Pd1−C1
Pd1−C21
2.040(5)
2.015(5)
179.5(2)
102.3(4)
107.4(4)
176.50(2)
7.6(6)
2.050(2)
2.001(2)
177.86(9)
102.6(2)
107.6(2)
173.54(1)
2.0(3)
2.046(5)
2.007(4)
175.9(2)
102.2(4)
107.4(4)
173.49(2)
1.2(6)
2.04(2)
2.015(5)
179.3(6)
101.0(2)
107.1(4)
177.1(2)
40.0(2)
C1−Pd1−C21
C2−C1−N1
N4−C21−N5
Br1−Pd1−Br2
C2−C1−C21−N4
Å; C−Br(VDW) = 3.55 Å).21,28 Additionally, this is supported
by the shorter C21−Pd1 bond length in comparison to the
triazolylidyl C1−Pd1 bond (Table 2). The “unusual”
interaction of the tertiary isopropyl hydrogens and the
palladium center, previously discussed by Huynh,25,32 is also
present in the structures (Pd···H−C = 2.6592(2)−2.7738(4)
Å).
Complexes 8a−c form 1-D supramolecular polymeric tapes
(Supporting Information) that are assembled through offset
face-to-face π−π interactions between the 1,3,4-trisubstituted
1,2,3-triazol-5-ylidene ring and the benzene ring of the
benzimidazolylidene ligand of an adjacent molecule (cent-
roid···centroid distances range from 3.709 to 3.896 Å). The
complex 8d also extends into 1-D supramolecular ribbons
(Supporting Information). These ribbons are supported by C−
H···π interactions between the benzylic protons of the C
substituent and the N-bound benzyl group of an adjacent
molecule (C−H3···C17 = 2.82(1) Å, C3···C17 = 3.75(2); C−
H3···C16 = 2.90(1) Å, C3···C16 = 3.94(1) Å), as well as C−
H···Br interactions between the C-bound benzyl group and a
bromide ligand of an adjacent molecule (C−H8···Br1 =
2.873(8) Å, C8···Br1 = 3.78(1) Å).
Ligand Donor Properties. The mild CuAAC “click”
methodology used to generate the 1,3,4-trisubstituted 1,2,3-
triazol-5-ylidene ligands potentially provides a facile way to
tune both steric and electronic properties of the resulting
carbene complexes. The M−Ctrz bond lengths (Tables 1 and 2)
were examined to see if there is a correlation between the
electronic nature of the trz ligand and the metal−carbene bond
length in the solid-state upon side-arm (wingtip) substitution of
the compounds 7a−f and 8a−f. The Au−Ctrz bond lengths of
the gold(I) complexes 7b (Aryl-OMe) < 7a (Aryl-H) < 7c
(Aryl-NO2) follow the expected trend with the more electron
rich methoxy-substituted complex 7b displaying a shorter Au−
Ctrz bond than the parent complex 7a. Similarly, the electron-
poor nitro-substituted complex 7c has a longer Au−Ctrz bond
than the parent complex 7a. However, the observed differences
are of a similar magnitude to the experimental uncertainty
(Table 1). The correlation breaks down with complexes 7d,f,
where the observed Au−Ctrz bond lengths are longer (7d) and
shorter (7f) than would be predicted on the basis of inductive
arguments. Furthermore, the Pd−Ctrz bond lengths for
complexes 8a−d are all essentially identical within the
experimental uncertainty (Table 2). Therefore, there is no
obvious correlation between the observed M−Ctrz bond lengths
in the solid-state and the electronic nature of the trz ligand.
However, it is noted that the M−C distance can be affected by
other parameters such as crystal-packing effects.
Figure 3. ORTEP27 diagrams of the palladium(II) complexes (a) 8a,
(b) 8b, (c) 8c, and (d) 8d. The thermal ellipsoids are shown at the
50% probability level. The trz ligand of 8d is disordered, but for clarity,
only one orientation is shown.
3) with those previously reported.21 The bromide ligands sit
orthogonal to the plane of the heterocycles to minimize steric
interactions. The complexes 8a−d have acute C2−C1−N1
angles of 101−103° (Table 2) which are similar to those of 7a−
d,f (Table 1) and consistent with what has been observed
previously for 1,2,3-triazolylidenes.7,10 The bromide ligands of
the complexes angle toward the benzimidazolylidyl carbene
carbon, due to back-donation from the bromide ligand to the
empty p orbital of the carbon (Cbimy−Br = 3.048(3)−3.147(5)
As the solid-state data provided no useful information on the
donor strengths of the various trz ligands, the 13C NMR spectra
of palladium complexes 8a−f were used to provide insight into
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Figure 4. Superimposed 13C NMR spectra (CDCl3, 298 K) showing the reporter benzimidazol-2-ylidene carbon signals of 8a−f. The data are
referenced to 77.16 ppm,29 not 77.7 ppm as reported in the original Huynh papers.21,27
the ligand’s donor properties. Huynh and co-workers previously
showed that these benzimidazol-2-ylidene−dibromopalladium-
(II) complexes can be used to probe the σ-donor strength of
the ligands trans to the benzimidazol-2-ylidene.21,28 They have
found that there is a direct relationship between σ-donor
strength of the trans ligand and the chemical shift of the
benzimidazole carbene carbon in the 13C NMR spectra of the
dibromopalladium(II) complexes.21,28 Additionally, this system
has previously been used to show that the mesoionic trz ligands
are stronger donors than imidazol-2-ylidenes.21
to lead to an increase in the electron-donating properties of the
trz-d ligand due to the presence of a second inductively
donating methylene group. Likewise, the presence of the three
methyl groups on the mesityl substituents of complex 8f should
make this trz-f ligand more electron-donating than the
structurally similar diphenyl-trz-e. The observed 13C shifts of
8a,e suggest that the trz ligands in complexes 8d,f are weaker σ
donors than trz-a and trz-e. As the substituents on the trz ligand
of 8d,f are larger (bulkier) than those on the other examples, it
is postulated that steric effects lead to this observed weakening
1
The 13C NMR spectra of palladium complexes 8a−f were
obtained in CDCl3 solution at 298 K. Consistent with what
Huynh and co-workers previously reported,21 the benzimida-
zol-2-ylidene reporter peaks were observed at approximately
180 ppm (Figure 4).
of the σ-donor properties. H NMR spectroscopic and X-ray
crystallographic data provide some support for this theory. The
1H NMR spectra of all palladium complexes show characteristic
septet signals representing the two tertiary proton signals of the
benzimidazolylidene isopropyl groups. In most cases we see no
separation of these signals, indicative of freely rotating ligands
in solution. The exception is 8d, which displays two distinct
signals for the isopropyl groups indicative of hindered rotation
about the Pd−trz bond, presumably due to steric factors. In
addition to this, the solid-state structures of 8a−c have a
coplanar arrangement of the heterocyclic ligands, whereas in 8d
the aforementioned ligands twist out of this plane (C2−C1−
C21−N4 = 40.0(2)°). Although the solid-state structure of 8f
was not obtained, molecular models (Supporting Information)
show the presence of steric clashes that could weaken
interaction of trz-f with the Pd(II) ion. While not completely
as expected, these results indicate that electronic alteration of
the side-arm substituents (wingtip groups) does affect the
donor properties of the trz ligands and suggests that CuAAC
“click” chemistry could be exploited to modulate these
properties in a facile fashion.
The parent palladium(II) complex 8a displays the peak for
the benzimidazolylidene carbon at 180.26 ppm. Consistent with
expectations, the benzimidazolylidene carbon signal of the
more electron-rich methoxy-substituted complex 8b has shifted
downfield (δ 180.44 ppm), relative to the ligand in 8a,
suggesting that the 4-MeOC6H5-trz ligand is more electron
donating that the parent Ph-trz ligand. Similarly, the reporter
carbon of the nitro-substituted complex 8c is observed upfield
(δ 178.95 ppm) relative to 8a, indicating that the presence of
the electron-withdrawing functionality reduces the trz ligand’s
donor properties, consistent with expectation. Replacing the
benzyl substituent of the parent with a phenyl ring generating
the diphenyl-substituted complex 8e also leads to a reduction of
the trz ligand σ-donor strength (δ 179.87 ppm), as is expected
upon the removal of the electron-donating methylene linker.
The observed positioning of the benzimidazolylidene
reporter carbon signals in the dibenzyl-substituted complex
8d (δ 179.70 ppm) and the dimesityl-substituted complex 8f (δ
178.99 ppm) was unexpected. The data suggest that these
ligands are weaker σ donors than would be expected on the
basis of electronic arguments. Changing the phenyl substituent
of the parent complex (8a) to a benzyl in 8d would be expected
Catalysis with Gold(I) 1,3,4-Trisubstituted 1,2,3-
Triazol-5-ylidene Complexes. With the family of new
Au(trz)Cl complexes 7a−f in hand, we were keen to investigate
their application in catalysis. In particular, we wished to observe
what effect, if any, changing the substituents on the
triazolylidene ligand would have on catalysis. Thus, the enyne
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9 was subjected to skeletal rearrangement30 catalyzed by
various Au(trz)Cl precatalystsa typical test reaction for
catalytic activity of new gold complexes31 (Table 3). Our initial
AgSbF6, prior to introduction of 9 in order to ensure that the
silver is not playing a crucial role in the reaction.32 The reaction
behaves in exactly the same manner regardless of the presence
or absence of AgCl in the reaction, confirming that silver is not
playing a significant role in this reaction. A mercury drop test33
was also carried out on the reaction shown in entry 10, resulting
in full conversion, suggesting that the catalytic activity is not
due to the formation of heterogeneous nanoparticles. Finally,
reducing the catalyst loading to 2 mol % still produces an
excellent 93% yield within 1 min (entry 11) and shows that it
compares favorably with results from commonly used gold
catalysts (entries 12 and 13).
Table 3. Screen of 7a−f as Precatalysts in the Skeletal
Rearrangement of 9
Next, we were keen to demonstrate the utility of the
Au(trz)Cl complexes as precatalysts in a reaction developed
within one of our laboratories. We have previously shown that
direct allylic etherification using unactivated allylic alcohols and
alcohol nucleophiles is possible using gold catalysis (e.g.,
Scheme 2).34 The original method requires excess (5 equiv) of
Scheme 2. Example of Previous Conditions for Direct Allylic
Etherification
the alcohol nucleophile (e.g., 13) for best results, using
Au(PPh3)NTf2 as the catalyst.15e,35 An excess of 13 is to ensure
that the allylic alcohol 12 does not react with itself and also to
improve selectivity under these conditions. To our delight and
surprise, using the new Au(trz)Cl complexes 7a−f as
precatalysts allows not only for a significant reduction in the
amount of alcohol nucleophile to 1.1 equiv but also for the
reaction to be carried out at a much milder room temperature
(vs. 50 °C, Table 4), thus greatly improving on the original
conditions shown in Scheme 2.
a
b
c
1
Determined by H NMR analysis. Isolated yields. Mixture of 10
d
e
and 11 as indicated. 10 only. Same result if AgCl precipitate is
filtered out prior to reaction with 9. Full conversion even with
mercury drop test. 2 mol % catalyst. Ratio not reported. AgSbF6 not
added.
f
g
h
i
few results were rather disappointing, as they showed poor
selectivities, poor yields, and (within error) fairly similar results
(entries 1−4). However, suspecting that 11 may form from 10
over time, the reactions were repeated with a much shorter
reaction time (1 min vs 15 min before), and to our delight, the
selectivities and yields improved significantly (entries 5−10).
Electronic tuning seems to do little to the catalytic activity: the
parent Bn,Ph-substituted Au(trz)Cl 7a reacts with almost the
same excellent selectivity and yields (entry 5) as the electron-
rich (7b, entry 6) and electron-poor (7c, entry 7) versions.
Next, the effect of sterics around the trz was probed. Changing
from the parent Bn,Ph-substituted trz 7a to the more flexible
dibenzyl-substituted 7d (entry 8) causes a drop in selectivity
(13:1 vs >20:1) but not yield (93% vs 92%). Having diphenyl
substitution (7e, entry 9) retains the excellent >20:1 selectivity
but causes a drop in yield (72%). Finally, the more hindered
dimesityl-substituted 7f provides the best result in this series,
with an excellent 98% yield and >20:1 selectivity of 10:11.
Therefore, it seems that for the skeletal rearrangement 9 → 10,
steric tuning on the trz ligand has more influence than
electronic tuning. Increased steric protection around the Au
center provided by the Mes substituents in 7f appears to be
beneficial for the performance of the catalyst in this test
reaction. As a control, the reaction in entry 5 was also repeated
with the AgCl filtered out of the mixture of Au(trz)Cl 7a and
As shown in Table 4, all the Au(trz)Cl precatalysts 7a−f
screened (entries 1−6) provide good yields of the desired
product 14 in excellent regioselectivities (>20:1 of 14:15 vs
12:1 using the original conditions in Scheme 2) using only 1.1
equiv of the alcohol nucleophile 13 and a mild 25 °C. Unlike
the enyne skeletal rearrangement reaction shown in Table 3,
the allylic etherification reaction is sensitive to electronic tuning
on the trz ligand (entries 1−3). Changing from the parent
precatalyst 7a (entry 1) to the more electron rich 7b (entry 2)
gives a slightly improved yield and E:Z selectivity, while the
more electron-withdrawing 7c shows a noticeably lower yield of
14 (entry 3). Tuning the sterics around the trz ligand (entries
4−6) does not really seem to affect the yield of 14 or the
regioselectivity. Next, we were keen to see how Au(trz)Cl
precatalysts 7a−f compare with other commonly used gold(I)
catalysts (entries 7−10). Using the original catalyst Au(PPh3)-
36
NTf2 under these conditions results in incomplete con-
versions and poor selectivities, including at least 9% of self-
reaction of 12 (entry 7). The commercially available NHC
precatalysts Au(IPr)Cl and Au(IMes)Cl were also investigated
for comparison purposes (entries 8 and 9). Au(IPr)SbF6, like
Au(PPh3)NTf2, results in poor selectivities and conversions
(entry 8). Au(IMes)SbF6, on the other hand, provides a good
yield of 14 although the E:Z selectivity is poorer than with
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melting points were determined using a Mettler-Toledo FP62
apparatus and are uncorrected. 1H and 13C NMR spectra were
recorded on either a Varian 400 MR or a Varian 500 VNMRS
spectrometer at 298 K. Chemical shifts are reported in parts per
Table 4. Direct Allylic Etherifications using Au(trz)Cl (7a−f)
as Precatalysts
1
million (ppm) and referenced to residual solvent peaks (CDCl3, H δ
7.26 ppm, 13C δ 77.16 ppm; CD3CN, 1H 1.94, 13C 1.32, 118.26 ppm).
Coupling constants (J) are reported in hertz (Hz). Standard
abbreviations indicating multiplicity were used as follows: m =
multiplet, spt = septet, quint = quintet, q = quartet, t = triplet, d =
doublet, s = singlet, br = broad. IR spectra were recorded on a Bruker
ALPHA FT-IR spectrometer with an attached ALPHA-P measurement
module. Microanalyses were performed at the Campbell Micro-
analytical Laboratory at the University of Otago. High-resolution
electrospray mass spectrograms (HR-ESI-MS) were collected on a
Bruker micro-TOF-Q spectrometer.
a
b
a
entry
LAuCl
14:15
yield of 14, %
E:Z
1
2
3
4
5
6
7
7a
7b
7c
7d
7e
7f
>20:1 74
>20:1 76
>20:1 64
>20:1 67
>20:1 67
>20:1 66
6:1
8:1
8:1
9:1
The 1,2,3-triazoles 1-benzyl-4-phenyl-1H-1,2,3-triazole (5a),17e 1-
benzyl-4-(4-methoxy)phenyl-1H-1,2,3-triazole (5b),19c 1-benzyl-4-(4-
nitro)phenyl-1H-1,2,3-triazole (5c),37 1,4-dibenzyl-1H-1,2,3-triazole
(5d),19b 1,4-diphenyl-1H-1,2,3-triazole (5e),19a and 1,4-dimesityl-1H-
1,2,3-triazole (5f),11d the triazolium compounds 6a,18 6b,20b and
6e,f,11d and the complexes 7a18 and 8a21 were synthesized using
slightly modified literature procedures.
12:1
6:1
cd
,
Au(PPh3)
NTf2
4:1
incomplete reaction: 3:4:1
12:14:15 9% self-reaction of 12
5:1
c
8
Au(IPr)Cl
2.5:1
incomplete reaction: 3:2.5:1
4:1
5:1
4:1
12:14:15
9
Au(IMes)
Cl
>20:1 70
17:1 61
The gold(I)-catalyzed reactions were carried out without the need
for dry solvents or inert atmosphere. However, AgSbF6 was stored and
weighed out in a glovebox as a precaution to avoid hydrolysis to the
corresponding Brønsted acid in our screening experiments.
10
Au(PPh3)
Cl
a
b
c
Determined by 1H NMR analysis. Isolated yields of 14. Using
2,3,5,6-tetrachloronitrobenzene as internal standard. No AgSbF6
Synthesis of Triazolium 6c. 1-Benzyl-4-(4-nitrophenyl)-1H-
1,2,3-triazole (5c; 174 mg, 0.619 mmol, 1.0 equiv) was added to a
Schlenk flask, which was then evacuated and back-filled with argon
three times. Dry dichloromethane (CH2Cl2, 50 mL) was added, and
the solution was bubbled with argon for 5 min before [Me3O]BF4
(164 mg, 1.11 mol, 1.8 equiv) was added. The suspension was stirred
under argon for 2 days. MeOH (2 mL) was added, and the solvent was
removed under reduced pressure, resulting in the formation of a brown
oil which was stirred in petroleum ether (50 mL) for 2 days. The
product was isolated as a yellow powder by filtration (189 mg, 80%).
Mp: 176 °C dec. IR: ν (cm−1) 3157, 3137, 3109, 3081, 3015, 2967,
1602, 1576, 1528, 1490, 1457, 1445, 1430, 1357, 1318, 1290, 1195,
1162, 1065 (br), 1054 (br), 1023, 853, 750, 718, 706, 683, 623, 578,
560, 520, 474, 460. 1H NMR (400 MHz, CD3CN): δ 8.59 (s, 1H, Hc),
8.41 (d, J = 8 Hz, 2H, Ha), 7.84 (d, J = 8 Hz, 2H, Hb), 7.53 (m, 2H,
He), 7.49 (m, 3H, Hf/g), 5.80 (s, 2H, Hd), 4.19 (s, 3H, Hh). 13C NMR
(100 MHz, CD3CN): δ 150.79, 142.64, 132.85, 132.10, 130.80,
130.34, 130.30, 130.26, 129.34, 125.37, 58.30, 39.88. HR-ESI-MS: m/z
d
added.
Au(trz)SbF6 (entry 9). Finally, the phosphine counterpart
Au(PPh3)SbF6 provides poorer yields as well as selectivities
(entry 10) than the Au(trz)SbF6 catalysts.
CONCLUSION
■
In summary, the gold(I) complexes 7a−f and palladium(II)
complexes 8a−f have been synthesized and characterized
through a combination of 1H and 13C NMR and IR
spectroscopy, HR-ESI-MS, and elemental analysis. The
molecular structures of four of the gold(I) and four of the
palladium(II) complexes were determined using X-ray
crystallography. The σ-donor strength of the trz ligands a−f
has been assessed using the palladium(II) probe complexes
8a−f. These measurements confirm that that electronic and
steric alteration of the side-arm substituents (wingtip groups)
does effect the donor properties of the trz ligands and suggests
that CuAAC “click” chemistry could be exploited to modulate
these properties in a facile fashion. The gold(I) complexes 7a−f
have been used as efficient precatalysts for both the enyne
skeletal rearrangement reaction (Table 3) and the direct allylic
etherification reaction (Table 4). In the former, steric tuning on
the trz ligand seemed to improve yields, whereas the latter is
more sensitive to electronic tuning. Therefore, it is useful to
have a facile and modular method (via the CuAAC “click”
reaction) toward these Au(trz)Cl complexes in order to have
access to a range of these complexes for catalyst screening.
Pleasingly, the Au(trz)Cl complex 7b also outperforms a range
of commonly used commercially available gold(I) precatalysts
in the allylic etherification reaction (Table 4) and allows for the
procedure to be greatly improved (Table 4 vs Scheme 2).
−
677.2443 [2(6c) − BF4 ]+ (calcd for C32H30BF4N8O4 677.2419),
−
295.1194 [6c − BF4 ]+ (calcd for C16H15N4O2 295.1195). Anal. Calcd
for C16H15BF4N4O2: C, 50.29; H, 3.96; N, 14.66. Found: C, 50.55; H,
4.06; N, 14.47.
Synthesis of Triazolium 6d. 1-Benzyl-4-benzyl-1H-1,2,3-triazole
(5d; 358 mg, 1.44 mmol, 1.0 equiv) was added to a dry Schlenk flask,
which was then evacuated and back-filled three times. Dry CH2Cl2 (50
mL) was added, and the solution was bubbled with argon for 5 min.
[Me3O]BF4 (451 mg, 3.05 mmol, 2.1 equiv) was added, and the
suspension was stirred under argon for 3 days. The reaction was
quenched with MeOH (3 mL), and the solvent was removed under
reduced pressure to give a brown oil. Diethyl ether (50 mL) was added
to the oil and consequently stirred for 2 h, resulting in the formation of
a white precipitate. The product was isolated by filtration as a white
solid (415 mg, 82%). Mp: 125 °C dec. IR: ν (cm−1) 3124, 3070, 3038,
2991, 1607, 1585, 1499, 1454, 1384, 1361, 1315, 1210, 1175, 1157,
1
1027 (br), 760, 730, 704, 692, 645, 522, 479, 456. H NMR (500
MHz, CDCl3): δ 8.17 (s, 1H, He), 7.45−7.43 (m, 2H, Hg), 7.40−7.39
(m, 3H, Hh/i), 7.32 (t, J = 10 Hz, 2H, Hb), 7.27 (t, J = 10 Hz, 1H, Ha)
7.23 (d, J = 10 Hz, 2H, Hc), 5.63 (s, 2H, Hd) 4.19 (s, 2H, Hf), 4.09 (s,
3H, Hj). 13C NMR (125 MHz, CDCl3): δ 144.18, 132.97, 131.20,
130.12, 129.66, 129.62, 129.51, 129.04, 128.92, 128.24, 57.65, 38.05,
EXPERIMENTAL SECTION
■
−
29.44. HR-ESI-MS: m/z 615.2033 [2(6d) − BF4 ]+ (calcd for
General Considerations. Unless otherwise stated, all reagents
were purchased from commercial sources and used without further
purification. Petroleum ether is the fraction boiling in the range 40−60
°C, CH3CN refers to acetonitrile, and CH2Cl2 is dichloromethane. All
−
C34H36BF4N6 615.3031), 264.1533 [6d − BF4 ]+ (calcd for C17H18N3
264.1495). Anal. Calcd for C17H18BF4N3: C, 58.15; H, 5.17; N, 11.97.
Found: C, 58.16; H, 5.17; N, 12.00.
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Synthesis of Au(I) Complex 7b. To a solvent mixture of CH2Cl2
and CH3CN (1/1, 10 mL) were added 6b (0.142 mg, 0.387 mmol, 1.0
equiv), tetramethylammonium chloride (43 mg, 0.392 mmol, 1.0
equiv), and Ag2O (45 mg, 0.194 mmol, 0.5 equiv), and the contents of
the foil-covered reaction flask were stirred for 5 h. Au(SMe2)Cl (0.115
g, 0.391 mmol, 1.0 equiv) was added, and the resulting mixture was
stirred for 2 h. The mixture was filtered through a Celite plug
(CH2Cl2) and the solvent removed under reduced pressure to give a
brown oil. The product was purified by vapor diffusion of a
concentrated CH2Cl2 solution with diethyl ether to obtain a brown
oil, which was added dropwise to stirred petroleum ether to give a
white solid (185 mg, 92%), which was isolated by filtration. Mp: 155
°C dec. IR ν (cm−1) 3064, 3026, 2968, 2937, 2908, 2833, 1613, 1577,
1546, 1488, 1457, 1438, 1395, 1363, 1296, 1256, 1178, 1115, 1088,
1072, 1020, 845, 836, 820, 793, 748, 734, 702, 656, 617, 599, 571, 513,
459. 1H NMR (400 MHz, CDCl3): δ 7.63−7.61 (m, 2H, He), 7.55 (d,
J = 8 Hz, 2H, Hc), 7.42−7.38 (m, 3H, Hf/g), 6.99 (d, J = 8 Hz, 2H,
Hb), 5.66 (s, 2H, Hd), 4.05 (s, 3H, Hh), 3.85 (s, 3H, Ha). 13C NMR
(100 MHz, CDCl3): 161.18 (s, Ccarbene), 157.54, 147.16, 13.79, 131.09,
129.28, 129.21, 129.12, 118.23, 114.73, 59.13, 55.58, 37.86. HR-ESI-
MS: m/z 987.1759 [Au(trz-b)-Cl-Au(trz-b)]+ (calcd for
C34H34Au2ClN6O2 987.1763), 755.2396 [Au(trz-b)2]+ (calcd for
C34H34Au2N6O2 755.2409), 534.0626 [7b + Na]+ (calcd for
C17H17AuClN3NaO 534.0623), 280.1438 [6b − BF4]+ (calcd for
C17H18N3O 280.1450). Anal. Calcd for C17H17AuClN3O: C, 39.90; H,
3.35; N, 8.21. Found: C, 39.96; H, 3.25; N, 8.19.
31.24. HR-ESI-MS: m/z 955.19 [(Au(trz-d)-Cl-(Au(trz-d)]+ (calcd for
C34H34Au2ClN6 955.19), 723.25 [Au(trz-d)2]+ (calcd for C34H34AuN6
723.25), 518.07 [7d + Na]+ (calcd for C17H17AuClN3Na 518.07),
501.13 [Au(trz-d)(CH3CN)]+ (calcd for C19H20AuN4 501.14), 478.12
[Au(trz-d)(H2O)]+ (calcd for C17H19AuN3O 478.12), 460.11 [7d −
−
Cl−]+ (calcd for C17H17AuN3 460.11), 264.15 [6d − BF4 ]+ (calcd for
C17H18N3 264.15). Anal. Calcd for C17H17AuClN3: C, 41.19; H, 3.46;
N, 8.48. Found: C, 41.66; H, 3.62; N, 8.48.
Synthesis of Au(I) Complex 7e. To a solvent mixture of CH2Cl2
and CH3CN (1/1, 10 mL) were added 6e (203 mg, 0.627 mmol, 1.1
equiv), tetramethylammonium chloride (72 mg, 0.659 mmol, 1.1
equiv), and Ag2O (73 mg, 0.321 mmol, 0.5 equiv), and the contents of
the foil-covered reaction flask were stirred for 7 h. Au(SMe2)Cl (167 g,
0.570 mmol, 1.0 equiv) was added, and the resulting mixture was
stirred for an additional 3 h. The mixture was filtered through a Celite
plug (CH2Cl2), and the solvent was removed under reduced pressure
to give a brown oil. The product was purified by silica chromatography
(9/1 CH2Cl2/acetone), providing a colorless soild. This material was
redissolved in CH2Cl2 (2 mL) and added dropwise into stirred
petroleum ether, generating a white solid (267 mg, 89%), which was
isolated by filtration. Mp: 227 °C dec. IR: ν (cm−1) 3051, 2956, 1592,
1578, 1490, 1479, 1456, 1394, 1363, 1338, 1322, 1269, 1195, 1159,
1071, 1005, 920, 784, 769, 763, 699, 689, 673, 573, 510, 483. 1H NMR
(500 MHz, CDCl3): δ 8.09−8.06 (m, 2H, Hd), 7.71−7.69 (m, 2H,
Hc), 7.56−7.53 (m, 6H, Ha/b/e/f), 4.20 (s, 3H, Hg). 13C NMR (125
MHz, CDCl3): δ 157.30 (s, Ccarbene), 147.75, 139.12, 130.61, 130.58,
129.78, 129.65, 129.35, 126.28, 124.23, 38.17. HR-ESI-MS: m/z
957.1088 [2(7e) + Na]+ (calcd for C30H26Au2Cl2N6Na 957.0826),
899.1209 [(Au(trz-e)-Cl-(Au(trz-e)]+ (calcd for C30H26Au2ClN6
899.1238), 667.1904 [Au(trz-e)2]+ (calcd for C30H26AuN6
667.1884), 490.0368 [7e + Na]+ (calcd for C15H13AuClN3Na
490.0361), 270.0800 [(trz-e) + Cl]+ (calcd for C15H13ClN3
270.0798). Anal. Calcd for C15H13AuClN3: C, 38.52; H, 2.80; N,
8.98. Found: C, 38.55; H, 2.83; N, 9.01.
Synthesis of Au(I) Complex 7f. To a solvent mixture of CH2Cl2
and CH3CN (1/1, 10 mL) were added 6f (184 mg, 0.452 mmol, 1.2
equiv), tetramethylammonium chloride (63 mg, 0.537 mmol, 1.4
equiv), and Ag2O (53 mg, 0.229 mmol, 0.6 equiv), and the contents of
the foil-covered reaction flask were stirred for 14 h. Au(SMe2)Cl (110
mg, 0.373 mmol, 1.0 equiv) was added, and the reaction mixture was
stirred for an additional 2 h. The mixture was filtered through a Celite
plug (CH2Cl2), and solvent was removed under reduced pressure to
give a colorless oil. The product was purified by silica chromatography
(gradient CH2Cl2 → 9/1 CH2Cl2/acetone), providing a colorless
solid. This material was redissolved in CH2Cl2 (2 mL) and added
dropwise into stirred petroleum ether, generating a white solid.
Removal of the solvent mixture in vacuo provided a white
microcrystalline solid (170 mg, 83%). Mp: >230 °C. IR: ν (cm−1)
3022, 2952, 2922, 2856, 1757, 1726, 1612, 1534, 1458 (br), 1372,
1325, 1281, 1195, 1121, 1072, 1033, 845, 772, 738, 625, 610, 589, 564.
1H NMR (500 MHz, CDCl3): δ 7.00 (s, 4H, Hb,f), 3.88 (s, 3H, Hd),
2.36 (s, 3H, Ha/g), 2.35 (s, 3H, Ha/g), 2.10 (s, 6H, Hc), 2.07 (s, 6H,
He). 13C NMR (125 MHz, CDCl3): δ 161.87 (s, Ccarbene), 145.96,
141.03, 140.87, 137.99, 135.63, 134.21, 129.56, 129.21, 122.23, 36.82,
21.43, 21.38, 20.20, 17.54. HR-ESI-MS: m/z 1067.3024 [(Au(trz-f)-
Cl-(Au(trz-f)]+ (calcd for C42H50Au2ClN6 1067.3116), 835.3698
[Au(trz-f)2]+ (calcd for C42H50AuN6 835.3762), 574.1270 [7f +
Na]+ (calcd for C21H25AuClN3Na 574.1300). Anal. Calcd for
C21H25AuClN3: C, 45.70; H, 4.57; N, 7.61. Found: C, 45.99; H,
4.61; N, 7.65.
Synthesis of Au(I) Complex 7c. To a solvent mixture of CH2Cl2
and CH3CN (1/1, 10 mL) were added 6c (217 mg, 0.568 mmol, 1.2
equiv), tetramethylammonium chloride (63 mg, 0.575 mmol, 1.2
equiv), and Ag2O (73 mg, 0.315 mmol, 0.7 equiv), and the contents of
the foil-covered reaction flask were stirred for 7 h. Au(SMe2)Cl (0.138
g, 0.469 mmol, 1.0 equiv) was added, and the resulting mixture was
stirred for 3 h. The mixture was filtered through a Celite plug
(CH2Cl2), and the solvent was removed under reduced pressure to
give a yellow oil, which was purified by column chromatography (9/1
CH2Cl2/acetone). A concentrated CH2Cl2 solution of the product was
crystallized via vapor diffusion of diethyl ether to produce bright
yellow crystals (172 mg, 72%). Mp: 170 °C dec. IR: ν (cm−1) 3084,
2925, 2858, 1601, 1520, 1516, 1498, 1479, 1454, 1435, 1341(br),
1287, 1165, 1106, 1091, 1076, 1044, 1013, 863, 855, 765, 758, 744,
1
705, 661, 649, 587, 572, 496, 457. H NMR (400 MHz, CDCl3): δ
8.33 (d, J = 8 Hz, 2H, Ha), 7.88 (d, J = 8 Hz, 2H, Hb), 7.61−7.59 (m,
2H, Hd), 7.41−7.38 (m, 3H, He/f), 5.66 (s, 2H, Hc), 4.13 (s, 3H, Hg).
13C NMR (125 MHz, CDCl3): δ 159.32 (s, Ccarbene), 148.81, 145.10,
133.24, 132.49, 130.73, 129.60, 129.29, 129.24, 124.44, 59.50, 38.36.
HR-ESI-MS: m/z 1017.15 [Au(trz-c)-Cl-Au(trz-c)]+ (calcd for
C32H38Au2ClN8O8 1017.13), 785.20 [Au(trz-c)2]+ (calcd for
C32H28AuN8O4 785.19), 569.1 [Au(trz-c)(OS(CH3)2)]+ (calcd for
C18H20AuClN4O3S 569.1), 549.0 [7c + Na]+ (calcd for
C16H14AuClN4NaO2 549.04), 491.1 [7c − Cl−]+ (calcd for
C16H14AuN4O2 491.08). Anal. Calcd for C16H14AuClN4O2: C,
36.48; H, 2.68; N, 10.64. Found: C, 36.71; H, 2.63; N, 10.60.
Synthesis of Au(I) Complex 7d. To a solvent mixture of CH2Cl2
and CH3CN (1/1, 10 mL) were added 6d (138.0 mg, 0.392 mmol, 1.0
equiv), tetramethylammonium chloride (44.1 mg, 0.402 mmol, 1.0
equiv), and Ag2O (49.5 mg, 0.214 mmol, 0.5 equiv). and the contents
of the foil-covered reaction flask were stirred for 6 h. Au(SMe2)Cl
(114 mg, 0.387 mmol, 1.0 equiv) was added, and the resulting mixture
was stirred for 2 h. The reaction mixture was filtered through a Celite
plug (CH2Cl2) and the solvent removed under reduced pressure to
produce a colorless film. The colorless film was redissolved in CH2Cl2
(2 mL) and added dropwise into petroleum ether, resulting in a
colorless precipitate. The precipitate was isolated by filtration to give a
fine white powder (173 mg, 87%). Mp: 150 °C dec. IR: ν (cm−1)
3032, 2950, 1601, 1583, 1527, 1492, 1455, 1420, 1343, 1321, 1229,
1181, 1150, 1079, 1029, 848, 773, 726, 713, 697, 670, 573, 468, 450.
1H NMR (500 MHz, CDCl3): δ 7.57−7.55 (m, 2H, Hf), 7.44−7.22
(m, 8H, Ha/b/c/g/h), 5.64 (s, 2H, He), 4.18 (s, 2H, Hd), 3.84 (s, 3H, Hi).
13C NMR (125 MHz, CDCl3): δ 158.64 (s, Ccarbene), 145.94, 134.99,
133.73, 129.32, 129.29, 129.17, 129.04, 128.55, 127.69, 58.94, 37.14,
Synthesis of Pd(II) Complex 8b. Bis(μ-bromo)bis(1,3-diisopro-
pylbenzimidazolin-2-ylidene)dibromopalladium(II) (94 mg, 0.100
mmol, 1.0 equiv) and tetra-n-butylammonium bromide (TBAB) (66
mg, 0.205 mmol, 2.0 equiv) were heated at reflux in CHCl3 (5 mL) for
3 h. The solvent was removed in vacuo, the orange powder was
redissolved in CH2Cl2 (15 mL), and then Ag2O (28 mg, 0.121 mmol,
1.2 equiv) and 6b (78 mg, 0.212 mmol, 2.1 equiv) were added. The
resulting reaction mixture was stirred at room temperature for 24 h
and then filtered through Celite. The filtrate was washed with water (5
× 50 mL) and dried (MgSO4) and then the solvent removed in vacuo.
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The crude mixture was purified via column chromatography (silica,
CH2Cl2), to give the product as a light yellow solid (104 mg, 69%).
Mp: >230 °C. IR: ν (cm−1) 3035, 2976, 2935, 2838, 1610, 1574, 1474,
1438, 1420, 1395, 1386, 1313, 1292, 1255, 1178, 1143, 1093, 1076,
1022, 888, 827, 806, 749, 694, 653, 614, 594, 549, 518. 1H NMR (500
MHz, CDCl3): δ 7.93 (d, J = 10 Hz, 2H, Hi), 7.76 (d, J = 10 Hz, 2H,
Hk), 7.50 (m, 2H, Hb), 7.43 (t, J = 10 Hz, 2H, Hl), 7.38 (t, J = 5 Hz,
1H, Hm), 7.15 (m, 2H, Ha), 7.09 (d, J = 10 Hz, 2H, Hh), 6.09 (s, 2H,
Hn), 6.06 (spt, J = 7 Hz, 1H, Hd), 6.05 (spt, J = 7 Hz, 1H, Hf), 3.96 (s,
3H, Hj), 3.91 (s, 3H, Hg), 1.71 (d, J = 7 Hz, 6H, He), 1.64 (d, J = 7 Hz,
6H, Hc). 13C NMR (125 MHz, CDCl3): δ 180.44 (s, Ccarbene(bimy)),
160.62, 159.00 (s, Ccarbene(trz)), 145.17, 134.80, 133.76, 132.25, 129.44,
128.89, 128.60, 121.84, 121.83, 120.64, 113.94, 112.56, 58.48, 55.55,
53.79, 53.59, 36.99, 21.20, 21.06. HR-ESI-MS: m/z 770.0095 [8b +
Na]+ (calcd for C30H35Br2N5NaOPd 770.0128), 726.0625 [PdBr(Cl)-
(iPr2-bimy)(trz-b) + Na]+ (calcd for C30H35BrClN5NaOPd 726.0623),
668.1040 [8b-Br−]+ (calcd for C30H35BrN5OPd 668.1038), 624.1547
[PdCl(iPr2-bimy)(trz-b)]+ (calcd for C30H35ClN5OPd 624.1563),
586.1810 [Pd(iPr2-bimy)(trz-b) − H]+ (calcd for C30H34N5OPd
586.1804), 358.0563 [(trz-b) + Br]+ (calcd for C17H17BrN3O
358.0555). Anal. Calcd for C30H35Br2N5OPd: C, 48.18; H, 4.72; N,
9.36. Found: C, 48.39; H, 4.70; N, 9.38.
7.37 (t, J = 8 Hz, 3H, Hi,j), 7.28 (t, J = 8 Hz, 1H, Hn), 7.16 (m, 2H,
Ha), 6.13 (s, 2H, Ho), 6.05 (spt, J = 7 Hz, 1H, Hd), 5.93 (spt, J = 7 Hz,
1H, Hf), 4.62 (s, 2H, Hg), 3.78 (s, 3H, Hk), 1.66 (d, J = 7 Hz, 6H, Hc),
1.64 (d, J = 7 Hz, 6H, He). 13C NMR (125 MHz, CDCl3): δ 179.70 (s,
Ccarbene(bimy)), 158.99 (s, Ccarbene(trz)), 144.23, 136.83, 135.34, 133.84,
133.76, 129.04, 129.02, 128.85, 128.65, 128.36, 127.07, 121.86, 112.59,
58.07, 53.74, 36.31, 31.85, 21.05. HR-ESI-MS: m/z 754.0122 [8d +
Na]+ (calcd for C30H35Br2N5NaPd 754.0179), 710.0593 [PdBr(Cl)-
(iPr2-bimy)(trz-d) + Na]+ (calcd for C30H35BrClN5NaPd 710.0674),
652.1047 [8d − Br−]+ (calcd for C30H35BrN5Pd 652.1088), 344.0554
[(trz-d) + Br]+ (calcd for C17H17BrN3 344.0580). Anal. Calcd for
C30H35Br2N5Pd: C, 49.23; H, 4.82; N, 9.57. Found: C, 49.48; H, 5.03;
N, 9.52.
Synthesis of Pd(II) Complex 8e. Bis(μ-bromo)bis(1,3-diisopro-
pylbenzimidazolin-2-ylidene)dibromopalladium(II) (94 mg, 0.100
mmol, 1.0 equiv) and TBAB (68 mg, 0.211 mmol, 2.1 equiv) were
heated at reflux in CHCl3 (5 mL) for 3 h. The solvent was removed in
vacuo, the orange powder was redissolved in CH2Cl2 (15 mL), and
then Ag2O (28 mg, 0.121 mmol, 1.2 equiv) and 6e (68 mg, 0.210
mmol, 2.1 equiv) were added. The reaction mixture was stirred at
room temperature for 24 h and then filtered through Celite. The
filtrate was washed with H2O (4 × 50 mL) and the aqueous layer
discarded. The organic layer was removed in vacuo, and the yellow
residue was purified via column chromatography (silica, 9/1 CH2Cl2/
acetone), to give the product as a yellow solid (91 mg, 64%). Mp:
>230 °C. IR: ν (cm−1) 3058, 3006, 2979, 2935, 2880, 1594, 1497,
1474, 1423, 1401, 1388, 1369, 1315, 1261, 1175, 1143, 1093, 1074,
1020, 924, 778, 765, 749, 703, 692, 681, 604, 547, 522, 484. 1H NMR
(500 MHz, CDCl3): δ 8.49 (d, J = 10 Hz, 2H, Hk), 8.14 (d, J = 10 Hz,
2H, Hi), 7.62 (m, 6H, Hl/m/h/g), 7.47 (m, 2H, Hb), 7.13 (dd, J = 8 Hz,
2H, Ha), 5.87 (spt, J = 7 Hz, 1H, Hd/f), 5.86 (spt, J = 7 Hz, 1H, Hd/f),
4.15 (s, 3H, Hj), 1.62 (d, J = 7 Hz, 6H, Hc/e), 1.58 (d, J = 7 Hz, 6H,
Hc/e). 13C NMR (125 MHz, CDCl3): δ 179.87 (s, Ccarbene(bimy)),
160.08 (s, Ccarbene(trz)), 145.25, 140.11, 133.80, 133.67, 131.00, 129.82,
129.75, 128.88, 128.64, 128.20, 125.75, 121.82, 112.49 53.68, 53.58,
37.35, 21.03, 21.02. HR-ESI-MS: m/z 1430.9643 [2(8e) + Na]+ (calcd
for C56H62Br4N10NaPd2 1430.9823), 1327.0628 [8e+(8e-Br−)]+
(calcd for C56H62Br3N10Pd 1327.0762), 725.9799 [8e + Na]+ (calcd
for C28H32Br2N5NaPd 725.9858), 624.0727 [8e − Br−]+ (calcd for
C28H31BrN5Pd 624.0777), 578.1231 [PdCl(iPr2-bimy)(trz-e)]+ (calcd
for C28H31ClN5Pd 578.1303). Anal. Calcd for C28H31Br2N5Pd: C,
47.78; H, 4.44; N, 9.95. Found: C, 47.74; H, 4.37; N, 9.96.
Synthesis of Pd(II) Complex 8c. Bis(μ-bromo)bis(1,3-diisopro-
pylbenzimidazolin-2-ylidene)dibromopalladium(II) (94 mg, 0.100
mmol, 1.0 equiv) and TBAB (66 mg, 0.204 mmol, 2.0 equiv) were
heated at reflux in CHCl3 (5 mL) for 4 h. The solvent was removed in
vacuo, the orange powder was redissolved in CH2Cl2 (15 mL), and
then Ag2O (28 mg, 0.121 mmol, 1.2 equiv) and 6c (98 mg, 0.202
mmol, 2.0 equiv) were added. The reaction mixture was stirred at
room temperature for 36 h and then filtered through Celite. The
filtrate was washed with H2O (5 × 50 mL) and the aqueous layer
discarded. The organic layer was removed in vacuo, and the resulting
yellow residue was purified via column chromatography (silica,
CH2Cl2), to give the product as a yellow solid (104 mg, 68%). Mp:
>230 °C. IR: ν (cm−1) 3093, 3063, 3029, 2971, 2937, 2879, 1601,
1518, 1456, 1420, 1398, 1386, 1358, 1343, 1312, 1142, 1092, 1078,
1023, 865, 856, 757, 704, 647, 598, 547, 496, 458, 424. 1H NMR (500
MHz, CDCl3): δ 8.43 (d, J = 10 Hz, 2H, Hg), 8.28 (d, J = 10 Hz, 2H,
Hh), 7.74 (d, J = 8 Hz, 2H, Hj), 7.51 (dd, J = 7, 3 Hz, 2H, Hb), 7.45 (t,
J = 8 Hz, 2H, Hk), 7.40 (m, J = 8 Hz, 1H, Hl), 7.17 (dd, J = 7, 3 Hz,
2H, Ha), 6.15 (s, 2H, Hm), 5.97 (m, J = 7 Hz, 2H, Hd/f), 4.06 (s, 3H,
Hi), 1.70 (d, J = 7 Hz, 6H, Hc), 1.65 (d, J = 7 Hz, 6H, He). 13C NMR
(125 MHz, CDCl3): δ 178.95 (s, Ccarbene(bimy)), 161.91 (s, Ccarbene(trz)),
148.50, 143.44, 134.89, 134.36, 133.67, 131.81, 129.30, 129.00, 128.83,
123.65, 122.05, 112.66, 58.72, 53.92, 53.75, 37.53, 21.11, 21.02. HR-
Synthesis of Pd(II) Complex 8f. Bis(μ-bromo)bis(1,3-diisopro-
pylbenzimidazolin-2-ylidene)dibromopalladium(II) (93 mg, 0.099
mmol, 1.0 equiv) and TBAB (68 mg, 0.211 mmol, 2.1 equiv) were
heated at reflux in CHCl3 (5 mL) for 3 h. The solvent was removed in
vacuo, the orange powder was redissolved in CH2Cl2 (15 mL), and
then Ag2O (36 mg, 0.155 mmol, 1.6 equiv) and 6f (89 mg, 0.219
mmol, 2.2 equiv) were added. The reaction mixture was stirred at
room temperature for 18 h. After this time the reaction mixture was
filtered through Celite, the solvent of the filtrate was removed in
vacuo, and the crude mixture was purified via column chromatography
(silica, 9/1 CH2Cl2/acetone) to give the product as a yellow solid (112
mg, 72%). Mp: >230 °C. IR: ν (cm−1) 3056, 2975, 2919, 1612, 1475,
1422, 1401, 1369, 1315, 1271, 1183, 1142, 1093, 1062, 1022, 846, 740,
ESI-MS: m/z 1548.9823 [2(8c)
+
Na]+ (calcd for
C58H64Br4N12NaO4Pd2 1548.9854), 1503.0305 [8c+PdBr(Cl)(iPr2-
bimy)(trz-c) + Na]+ (calcd for C58H64Br3ClN12NaO4Pd2 1503.0366),
784.9826 [8c + Na]+ (calcd for C29H32Br2N6NaO2Pd 784.9873),
741.0341 [PdBr(Cl)(iPr2-bimy)(trz-c) + Na]+ (calcd for
C29H32BrClN6NaO2Pd 741.0368), 683.0756 [8c − Br−]+ (calcd for
C29H32BrN6O2Pd 683.0783), 373.0254 [(trz-c) + Br]+ (calcd for
−
+
C16H14BrN4O4 373.0295), 295.1178 [6c − BF4
]
(calcd for
C16H15N4O2 295.1190). Anal. Calcd for C29H33Br2N6O2Pd: C,
45.66; H, 4.23; N, 11.02. Found: C, 45.96; H, 4.26; N, 11.02.
1
Synthesis of Pd(II) Complex 8d. Bis(μ-bromo)bis(1,3-diisopro-
pylbenzimidazolin-2-ylidene)dibromopalladium(II) (93 mg, 0.099
mmol, 1.0 equiv) and TBAB (67 mg, 0.208 mmol, 2.0 equiv) were
heated at reflux in CHCl3 (5 mL) for 4 h. The solvent was removed in
vacuo, the orange powder was redissolved in CH2Cl2 (15 mL), and
then Ag2O (30 mg, 0.129 mmol, 1.3 equiv) and 6d (98 mg, 0.198
mmol, 2.0 equiv) were added. The reaction mixture was stirred at
room temperature for 24 h and then filtered through Celite. The
filtrate was removed in vacuo and the crude mixture purified via
column chromatography (silica, CH2Cl2), to give the product as a light
yellow solid (101 mg, 70%). Mp: >230 °C. IR: ν (cm−1) 3088, 2976,
2935, 2873, 1602, 1584, 1495, 1474, 1454, 1420, 1396, 1387, 1312,
1231, 1175, 1141, 1093, 1074, 848, 807, 750, 736, 729, 713 695, 548,
456. 1H NMR (500 MHz, CDCl3): δ 7.66 (d, J = 7 Hz, 2H, Hl), 7.57
(d, J = 8 Hz, 2H, Hh), 7.50 (m, 2H, Hb), 7.43 (t, J = 7 Hz, 2H, Hm),
564. H NMR (500 MHz, CDCl3): δ 7.36 (dd, J = 6, 4 Hz, 2H, Hb),
7.07 (m, 6H, Ha/h/l), 5.56 (spt, J = 6 Hz, 2H, Hd/f), 3.87 (s, 3H, Hj),
2.43 (s, 3H, Hg/m), 2.41 (s, 3H, Hg/m), 2.37 (s, 6H, Hi/k), 2.36 (s, 6H,
Hi/k), 1.50 (d, J = 7 Hz, 12H, Hc/e). 13C NMR (125 MHz, CDCl3): δ
178.99 (s, Ccarbene(bimy)), 162.32 (s, Ccarbene(trz)), 144.94, 139.79, 139.62,
136.37, 136.10, 133.89, 128.91, 128.51, 124.18, 121.56, 112.08, 53.23,
36.04, 21.60, 21.47, 21.37, 21.02, 19.56. HR-ESI-MS: m/z 1599.1626
[2(8f) + Na]+ (calcd for C68H86Br4N10NaPd2 1559.1722), 1553.2211
[8f
+ PdBr(Cl)(iPr2 -bimy)(trz-f) +
Na]+ (calcd for
C68H86Br3ClN10NaPd2 1553.2233), 810.0739 [8f + Na]+ (calcd for
C34H43Br2N5NaPd 810.0806), 766.1233 [PdBr(Cl)(iPr2-bimy)(trz-f)
+ Na]+ (calcd for C34H43BrClN5NaPd 766.1300), 708.1665 [8f −
−
Br−]+ (calcd for C34H43BrN5NaPd 708.1715), 320.2082 [6f − BF4 ]+
(calcd for C21H26N3 320.2121). Anal. Calcd for C34H43Br2N5Pd: C,
51.83; H, 5.50; N, 8.89. Found: C, 51.81; H, 5.54; N, 8.87.
7073
dx.doi.org/10.1021/om400773n | Organometallics 2013, 32, 7065−7076

Organometallics
Article
Representative Procedure for the Skeletal Rearrangement
This material is available free of charge via the Internet at
http://pubs.acs.org. The crystallographic data are also available
from the Cambridge Crystallographic Database as file nos.
CCDC 932209−932216.
of 9. Au(trz)Cl (0.0063 mmol, 5 mol %) was added to a solution of
9
30a (30 mg, 0.126 mmol) in CH2Cl2 (1 mL). In a separate vial, silver
hexafluoroantimonate (2.2 mg, 0.0063 mmol, 5 mol %) was dissolved
in CH2Cl2 (0.1 mL), and then this solution was transferred to 9 and
washed in with CH2Cl2 (0.14 mL). The reaction mixture was stirred
for 1 min at 23 °C, followed by immediate purification by flash column
chromatography (silica, gradient neat petroleum ether → 7/1
petroleum ether/diethyl ether). Product 10 was obtained as a colorless
oil. Spectroscopic analyses were in agreement with those previously
reported in the literature.30 IR: ν (cm−1) 2954 w, 2917 w, 2853 w (C−
AUTHOR INFORMATION
■
Corresponding Authors
*A.-L.L.: e-mail, A.Lee@hw.ac.uk; fax, +44 (0)131 4513180;
tel, +44 (0)131 4518030.
1
*J.D.C.: e-mail, jcrowley@chemistry.otago.ac.nz; fax, +64 3 479
7906; tel, +64 3 479 7731.
H), 1733 s (CO), 1653 w (CC), 1249 s (C−O−C). H NMR
(300 MHz, CDCl3): δ 5.73 (br s, 1H, CCH), 5.38 (br s, 1H, C
CH), 3.73 (s, 6H, CO2CH3), 3.19 (br s, 2H, CH2), 3.04 (br s, 2H,
CH2), 1.82 (s, 3H, CH3), 1.78 (s, 3H, CH3). 13C NMR (75 MHz,
CDCl3): δ 172.8 (C), 138.9 (C), 135.8 (C), 124.6 (CH), 120.8 (CH),
59.5 (C), 53.0 (CH3), 43.4 (CH2), 40.4 (CH2), 27.4 (CH3), 20.0
(CH3).
Author Contributions
§These authors contributed equally to the experimental work.
Notes
The authors declare no competing financial interest.
Representative Procedure for Allylic Etherification Reaction.
The gold(I) catalyst (0.0058 mmol, 5 mol %) was added to a solution
of allylic alcohol 1234a (15 mg, 0.117 mmol) and 4-phenylbutanol (13;
19.6 μL, 19.3 mg, 0.129 mmol, 1.1 equiv) in CH2Cl2 (0.1 mL). In a
separate vial, silver hexafluoroantimonate (2.0 mg, 0.0058 mmol, 5 mol
%) was dissolved in CH2Cl2 (0.1 mL) and then transferred to the
reaction mixture and washed in with CH2Cl2 (0.1 mL). The reaction
was stirred for 18 h at 25 °C. The crude mixture was filtered through a
silica plug with diethyl ether and concentrated under reduced pressure.
The product 14 was obtained as a colorless oil after purification by
flash column chromatography (silica, gradient neat hexane → 50/1
hexane/diethyl ether). Spectroscopic analyses were in agreement with
those previously reported in the literature.34a IR: ν (cm−1) 3027 w,
2954 m, 2937 m, 2862 m (C−H), 1655 w (CC), 1604 w, 1496 m,
ACKNOWLEDGMENTS
■
J.R.W. thanks the Department of Chemistry, University of
Otago, for funding a Ph.D. scholarship. J.D.C. and A.-L.L. thank
the Royal Society for an International Exchanges Grant
(IE110833). A.-L.L. thanks the EPSRC (EP/K00736X/1 and
DTA for PCY) for funding and the EPSRC National Mass
Spectrometry Services (Swansea) for analytical services.
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1
1453 m (aromatic CC), 1107 s (C−O−C). H NMR (400 MHz,
CDCl3): δ 7.31−7.15 (m, 5H, Ar−H), 5.39 (tq, J = 6.2, 1.2 Hz, 1H,
OCH2CH), 4.00 (d, J = 6.2 Hz, 2H, OCH2CH), 3.45 (t, J = 6.4 Hz,
2H, OCH2CH2), 2.65 (t, J = 7.4 Hz, 2H, CH2CH2Ph), 1.77 − 1.58 (m,
7H, alkyl CH2 and C(CH3)), 1.06 (s, 9H, C(CH3)3). 13C NMR (75
MHz, CDCl3): δ 147.0 (C), 142.6 (C), 128.6 (CH), 128.4 (CH),
125.8 (CH), 118.4 (CH), 70.4 (CH2), 68.3 (CH2), 36.4 (C), 35.9
(CH2), 29.6 (CH2), 29.0 (CH3), 28.3 (CH2), 13.2 (CH3). HRMS
(EI): m/z calcd for C18H28O [M]+ 260.2135, found 260.2136.
X-ray Collection and Refinement. X-ray data were recorded
using a Bruker Kappa X8 APEX II CCD diffractometer using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). Semiempirical
absorption corrections (SCALE) were applied. The structures were
solved by SHELXS-97.38 Full-matrix least-squares refinement on F2
was carried out using SHELXL-97 via the X-Seed graphical interface.39
All non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were included in calculated positions and were refined as riding
atoms with individual (or group, if appropriate) isotropic displacement
parameters.
Special Conditions/Variations. 7b: the benzyl group (C9−C15)
appeared to be rotationally disordered about the C11/C14 phenyl ring
axis and was modeled over three positions, refining to 0.5:0.25:0.25
occupancies. The disordered fragment was refined with the use of
SAME, FLAT, EADP, and ISOR restraints/constraints. The largest
residual electron density peaks are ca. 1 Å from the gold atom.
8d: the whole dibenzylmethyltriazolylidene ligand (C1−C17, N1−
N3) appeared to be rotationally disordered about the Pd1−C1 bond
(by ca. 180° for the triazolyl ring). The triazolylidene ligand and both
bromide ligands were modeled over two positions, refining to
0.75:0.25 occupancies. The disordered triazolylidene ligand was
refined with the use of SAME restraints across the two orientations,
and a FLAT restraint and EADP constraints were applied to each ring.
̈
̈
̈
̈
̈
̈
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIFs giving spectroscopic, crystallo-
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