Lewis Acid Catalysis with Cationic Dinuclear Gold(II,II) and Gold(III,III) Phosphorus Ylide Complexes

Article abstract of DOI:10.1021/acs.organomet.6b00403

The dinuclear gold(II,II) and gold(III,III) complexes [Au₂(μ-PY)₂(MeCN)₂]OTf₂ (2-OTf₂) and [Au₂(μ-PY)₂(μ-CH₂)(MeCN)₂]OTf₂ (3-OTf₂) (PY = [(CH₂)₂PPh₂]^-) have been synthesized and evaluated as Lewis acid catalysts for Mukaiyama addition and alkyne hydroamination reactions. 2-OTf₂ and 3-OTf₂ provide similar or improved catalytic activity for these reactions compared to the commonly used gold(I) Lewis acids Ph₃PAuOTf and IPrAuOTf. The versatile Lewis acidity of 2-OTf₂ was further demonstrated by its superior performance in a cascade reaction involving intramolecular hydroamination followed by intermolecular conjugate addition to generate a 2,3-substituted indole.

Full text of DOI:10.1021/acs.organomet.6b00403

Article
pubs.acs.org/Organometallics
Lewis Acid Catalysis with Cationic Dinuclear Gold(II,II) and Gold(III,III)
Phosphorus Ylide Complexes
Benjamin R. Reiner, Mark W. Bezpalko, Bruce M. Foxman, and Casey R. Wade*
Department of Chemistry, Brandeis University, 415 South Street MS 015, Waltham, Massachusetts 02453, United States
S
* Supporting Information
ABSTRACT: The dinuclear gold(II,II) and gold(III,III) complexes [Au2(μ-
PY)2(MeCN)2]OTf2 (2-OTf2) and [Au2(μ-PY)2(μ-CH2)(MeCN)2]OTf2 (3-
OTf2) (PY = [(CH2)2PPh2]−) have been synthesized and evaluated as Lewis
acid catalysts for Mukaiyama addition and alkyne hydroamination reactions.
2-OTf2 and 3-OTf2 provide similar or improved catalytic activity for these
reactions compared to the commonly used gold(I) Lewis acids Ph3PAuOTf
and IPrAuOTf. The versatile Lewis acidity of 2-OTf2 was further
demonstrated by its superior performance in a cascade reaction involving
intramolecular hydroamination followed by intermolecular conjugate addition
to generate a 2,3-substituted indole.
considering that organogold(III) complexes are often unstable
toward reductive elimination.23−25 Consequently, we envisaged
the dinuclear gold PY framework as a platform for exploring the
catalytic activity of well-defined gold(II) and gold(III) centers.
While several examples of cationic gold(II,II) and gold(III,III)
PY complexes have been reported, few contain combinations of
weakly coordinating anions and labile ancillary ligands expected
to result in active Lewis acid catalysts.26,27 Herein we report the
synthesis, characterization, and Lewis acid catalytic activity of
dicationic gold(II,II) and gold(III,III) PY complexes containing
weakly coordinating OTf− counteranions and MeCN ancillary
ligands.
INTRODUCTION
■
Over the past decade, the use of homogeneous gold catalysts
for C−C and C−X bond-forming reactions has increased
remarkably.1−5 Interest in gold-based catalysis stems from its
tolerance to oxygen and water, low toxicity, and low proclivity
for β-hydride elimination. Moreover, much of the catalytic
efficacy of gold is grounded in its soft Lewis acidity, which
allows for the activation of unsaturated organic substrates.
Complexes containing gold(I) and gold(III) centers have been
shown to activate alkenes and alkynes for nucleophilic
addition.6−8 However, divergent reactivity between gold(I)
and gold(III) has been observed in some cases and attributed to
differences in hard/soft Lewis acidity.9−11 For example,
Gevorgyan et al. have reported that Et3PAuCl and AuCl3
both catalyze the cyclization of haloallenyl ketones, but
generate different isomers of the halofuran products.9 More
recently, Toste et al. demonstrated that the gold(III) complex
[IPrAu(biphenyl)]OTf (IPr = N,N′-(2,6-diisopropylphenyl)-
imidazol-2-ylidene) activates α,β-unsaturated carbonyl com-
pounds for conjugate addition.12 However, the gold(I)
analogue IPrAuOTf exhibited no catalytic activity for the
reaction, affirming the contrast in hard/soft Lewis acidity
between gold(I) and gold(III) centers.
RESULTS AND DISCUSSION
■
The dinuclear gold PY complexes 1 and 3-Br2 were synthesized
according to literature procedures.18,19,21 Reaction of 1 with
PhICl2 afforded the gold(II,II) complex 2-Cl2 (Scheme 1). The
dicationic complex 2-OTf2 was isolated as a bright yellow solid
after treatment of 2-Cl2 with AgOTf in acetonitrile solution.
Notably, the use of weakly coordinating solvents such as
CH2Cl2 or THF for this reaction resulted in a mixture of
Scheme 1. Synthesis of 2-OTf2 and 3-OTf2
While the catalytic properties of gold(I) and gold(III) have
been well-established, those of gold(II) have been relatively
unexplored.13 Owing to a d9 electron configuration, gold(II)
complexes are typically isolated as multinuclear frameworks
containing covalent Au−Au bonds. Several dinucleating ligand
platforms have been used to support gold(II) complexes.14−16
Among these, phosphorus ylide (PY) ligands have been shown
to support rich redox chemistry. Dinuclear gold PY complexes
with the metals in the (I,I), (II,II), (I,III), and (III,III) oxidation
states have been structurally characterized.17−22 The strong σ-
donating character of the bridging PY ligands confers excellent
stability across the range of oxidation states. This is notable
Received: May 18, 2016
© XXXX American Chemical Society
A
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decomposition products, suggesting that the dication is
stabilized by coordination of MeCN. The 31P{1H} NMR
spectrum of 2-OTf2 in CD2Cl2 displays a single resonance at
34.5 ppm, shifted slightly upfield from that of 2-Cl2 (36.0
1
ppm). The H NMR spectrum features aromatic resonances
consistent with equivalent Ph groups and a doublet at 1.95 ppm
assigned to the CH2 groups of the PY ligand. The presence of a
singlet resonance at 2.2 ppm is consistent with two molecules
of coordinated MeCN. 2-OTf2 is stable to air and moisture but
exhibits moderate light sensitivity.
Treatment of 3-Br2 with AgOTf in CH2Cl2/MeCN solution
generated the dicationic complex 3-OTf2 as a colorless solid.
The complex is stable to air, moisture, and light in both
solution and the solid state. The 31P{1H} NMR spectrum of 3-
OTf2 in CD3CN displays a single resonance at 35.1 ppm,
1
shifted slightly upfield from that of 3-Br2 (37.5 ppm). The H
NMR spectrum shows several diagnostic features that indicate
the complex retains a rigid, boat-like conformation in
solution.21 These features include two sets of aromatic
resonances consistent with inequivalent Ph groups. Two virtual
triplets, centered at 1.88 and 2.46 ppm, suggest a diastereotopic
relationship between the methylene protons of the PY ligand.
Singlet resonances at 2.68 and 1.97 ppm are assigned to the μ-
CH2 ligand and two molecules of coordinated MeCN,
respectively.
Figure 1. Solid-state structures of 2-OTf2 (a) and 3-OTf2 (b). All
hydrogen atoms, solvent molecules, and OTf− counterions have been
omitted for clarity. Only one of two independent molecules in the
asymmetric unit of 3-OTf2 is shown. Pertinent metrical parameters can
be found in the text and Supporting Information.
The solid-state structures of 2-OTf2 and 3-OTf2 were
determined by single-crystal X-ray diffraction. X-ray quality
crystals of both complexes were obtained by vapor diffusion of
Et2O into MeCN solutions. 2-OTf2 crystallizes in the space
group P1 with one-half molecule of [2]2+, one OTf− ion, and
̅
1). We also sought to compare the activity of the gold PY
complexes with gold(I) Lewis acids. Although the cationic
gold(I) PY complex [Au(CH2PPh3)]+ would seem suitable for
one interstitial MeCN molecule in the asymmetric unit. The
structure of 2-OTf2 shows that the eight-membered metalla-
cycle adopts a chair conformation with both gold(II) centers in
square planar geometries (Figure 1a). The observed equiv-
a
1
Table 1. Gold-Catalyzed Mukaiyama Additions
alence of the Ph groups by H NMR spectroscopy at room
temperature suggests rapid ring flipping of the metallacycle in
solution. The Au−CPY distances (2.106(2) and 2.111(2) Å) are
comparable to those found in related Au(II,II) complexes.28
Notably, the Au−Au intermetallic distance of 2.5580(2) Å is
among the shortest observed for dinuclear gold complexes
bearing PY ligands.29
b
3-OTf crystallizes in the space group P1 with two molecules
̅
2
% yield
of [3]2+ and four OTf− ions in the asymmetric unit (Figure 1b).
The complex adopts a boat conformation with the gold(III)
centers in square planar coordination environments. Each gold
coordination sphere contains the PY ligands in a trans
arrangement, the μ-CH2 ligand, and a coordinated MeCN
molecule. The Au−CPY (2.031(17)−2.148(16) Å) and Au−
C29 (1.988(14)−2.060(16) Å) distances are similar to those
observed in related structures.19,21 The Au−Au distances
(3.0673(10) and 3.065(18) Å) are within the sum of the van
der Waals radii (3.32 Å), but longer than the sum of the
covalent radii (2.88 Å). This may suggest the presence of weak
aurophilic interactions, but these are rare for gold(III) and most
prevalent with face-to-face pairing of square planar com-
plexes.30−32
With 2-OTf2 and 3-OTf2 in hand, we sought to investigate
their activity as Lewis acid catalysts. Cationic gold(I) and
gold(III) complexes have demonstrated contrasting reactivity
for the Lewis acid catalyzed addition of silyl enol ethers to α,β-
unsaturated carbonyl compounds.12 Thus, we decided to screen
2-OTf2 and 3-OTf2 as catalysts for the addition of silyl enol
ether 4 to crotonaldehyde (5a) and cyclohexenone (5b) (Table
entry
catalyst
2-OTf2
cat. loading
substrate
6
7
1
4%
4%
4%
4%
4%
4%
4%
4%
4%
4%
4%
2%
0.5%
0.5%
5a
5a
5a
5a
5a
5a
5a
5b
5b
5b
5b
5a
5a
5a
68
70
62
0
27
26
31
0
2
3-OTf2
c
3
Ph3PAuOTf
c
4
IPrAuOTf
5
2-Cl2
0
0
6
3-Br2
0
0
7
AgOTf
2-OTf2
3-OTf2
Ph3PAuOTf
0
0
8
0
88
45
20
0
9
0
c
c
10
11
12
13
14
0
c
IPrAuOTf
0
2-OTf2
67
65
0
27
26
0
2-OTf2
Ph3PAuOTf
a
Reaction conditions: 5a/b (0.1 mmol), 4 (0.1 mmol), CH2Cl2 (0.5
mL), C6D6 (0.1 mL), 25 °C, 12 h. Determined by H NMR with
respect to an internal standard (trimethoxybenzene) at 12 h.
Generated in situ from LAuCl/AgOTf. See Supporting Information
for details.
b
1
c
B
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this purpose, it has previously demonstrated poor catalytic
activity owing to decomposition.33 Therefore, the commonly
employed Lewis acid catalysts Ph3PAuOTf and IPrAuOTf were
chosen for comparison.1
The addition reactions were carried out at room temperature
in CH2Cl2 solution with 4 mol % of catalyst and were
monitored by 1H NMR spectroscopy (Table 1). 2-OTf2
provided nearly complete conversion of 5a after 1 h, while
the reaction with 3-OTf2 required over 10 h to reach complete
conversion (Figure 2). Analysis of the first-order rate plots
Since the adjacent gold centers in 2-OTf may both
participate in substrate activation and catalysis, we evaluated
its catalytic efficiency at 2 mol % (4 mol % Au) loading (entry
12). Analysis of the reaction profile revealed that 2 mol % 2-
OTf provides a similar rate to 4 mol % Ph3PAuOTf for the
addition reaction with crotonaldehyde (Figure 2). This suggests
that Ph3PAuOTf and 2-OTf have similar catalytic efficiencies
on a per gold basis. However, when the catalyst loadings were
lowered to 0.5 mol %, 2-OTf provided complete conversion of
crotonaldehyde after 12 h, while no reaction was observed with
Ph3PAuOTf (entries 13 and 14). This indicates that 2-OTf is
less prone to deactivation by trace impurities than
Ph3PAuOTf.34
Recently, the Gutmann−Beckett method has been used to
gauge the relative Lewis acidity of gold complexes.12,35,36 We
decided to employ this method in order to gain insight into the
origin of the different catalytic activities of the gold complexes
in this study. Solutions of 2-OTf2 and 3-OTf2 in CH2Cl2 were
treated with two equivalents of Et3PO, and the difference in 31
P
NMR chemical shift (Δδ) between an Et3PO internal standard
(δ = 50) and gold-coordinated Et3PO species was calculated as
a measure of relative Lewis acid strengths. The 31P NMR
spectrum of 3-OTf2/Et3PO shows three distinct ylide
resonances that correspond to an equilibrium mixture of the
parent complex and the 1:1 and 2:1 Et3PO adducts (Figure
S36). A sharp resonance at 73 ppm (Δδ = 25) was assigned to
coordinated Et3PO in the 2:1 adduct. The room-temperature
31P NMR spectrum of 2-OTf2/Et3PO exhibits a sharp ylide
resonance at 32 ppm and a very broad signal around 53 ppm
that suggests the rate of Et3PO exchange is close to the time
scale of the NMR experiment (Figure S37). The broad signal
decoalesced upon cooling the sample to −45 °C, and
resonances corresponding to coordinated Et3PO appeared at
75 and 76 ppm (Δδ = 25−26). Additional ylide resonances also
began to emerge around 32 ppm, but were too poorly resolved
to determine the speciation at this temperature. When a
solution of IPrAuOTf in CH2Cl2 was treated with one
equivalent of Et3PO, a sharp 31P NMR signal corresponding
to the adduct appeared at 80 ppm (Δδ = 30) (Figure S38). At
room temperature, an equimolar mixture of Ph3PAuOTf and
Et3PO gives rise to a single PPh3 resonance at 26 ppm and a
broad signal around 71 ppm (Figure S39). The broad signal did
not fully decoalesce upon cooling to −45 °C. The Δδ values
determined from this study suggest that 2-OTf2 and 3-OTf2
exhibit comparable Lewis acidities (Δδ = 25−26) and
IPrAuOTf is a slightly stronger Lewis acid (Δδ = 30). No
meaningful comparison can be made with Ph3PAuOTf since
peak decoalescence was not observed. Interestingly, the ligand
exchange equilibria and dynamics of these complexes are very
different. At room temperature, Et3PO ligand exchange occurs
more rapidly for 2-OTf2 and Ph3PAuOTf than for 3-OTf2 and
IPrAuOTf. While catalytic efficiency for the enone additions
seems to better correlate with ligand exchange rates than
apparent Lewis acidity, further studies will be necessary to fully
elucidate the steric and electronic factors that influence catalytic
activity.
Figure 2. Reaction profile for the addition of 4 to crotonaldehyde (5a)
in the presence of 2-OTf2 (circles), 3-OTf2 (triangles), or Ph3PAuOTf
(squares).
revealed initial observed rate constants (kobs) of 1.1 × 10−3 and
8.4 × 10−5 s−1 with 2-OTf2 and 3-OTf2, respectively (Figures
S9 and S10). Furthermore, a mixture of aldol (6a) and
conjugate addition (7a) products was observed in a ca. 2.5:1
ratio for both catalysts (entries 1 and 2).
The reaction of 4 and 5a was carried out under the same
conditions using in situ generated Ph3PAuOTf and IPrAuOTf
as catalysts (entries 3 and 4). While Ph3PAuOTf provided
nearly complete conversion of 5a within 1.5 h, no reaction was
observed with IPrAuOTf after 12 h. The product selectivity
with Ph3PAuOTf (6a:7a = 2:1) was similar to that observed for
2-OTf2. Complexes 3-Br2 and 2-Cl2 showed no catalytic
activity for the reaction, confirming that halide exchange for the
more weakly coordinating OTf− anion is necessary to activate
the gold centers for catalysis (entries 5 and 6). In addition,
AgOTf exhibited no catalytic activity under the reaction
conditions (entry 7).
With cyclohexenone (5b) as a substrate, only the conjugate
addition product (7b) was observed. 2-OTf2 provided 90%
conversion of cyclohexenone after 12 h, while the reaction with
3-OTf2 reached only 50% conversion in 12 h (entries 8 and 9).
Analysis of the first-order rate plots revealed initial observed
rate constants (kobs) of 1.5 × 10−4 and 1.1 × 10−5 s−1 with 2-
OTf2 and 3-OTf2, respectively (Figures S11 and S12). While
Ph3PAuOTf provided 25% conversion after 12 h, no reaction
was observed with IPrAuOTf after 12 h (entries 10 and 11).
These results follow the same trend observed with
crotonaldehyde, but the overall reaction rates are slowed by
roughly a factor of 10 (Figure S13).
Next, we decided to probe the ability of 2-OTf2 and 3-OTf2
to activate C−C π bonds by screening their activity for the
hydroamination of phenylacetylene (8) with aryl amines 9a−c.
These reactions were carried out in the presence of 4 mol % of
1
each catalyst and monitored by H NMR spectroscopy (Table
2). 3-OTf2 and IPrAuOTf proved to be the most efficient
catalysts when toluidine (9a) was used as the nucleophile
C
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a
a
Table 2. Gold-Catalyzed Hydroamination
Table 3. Gold-Catalyzed Cascade Reaction
c
d
entry
catalyst
2-OTf2
ArNH2
% conv
% yield
1
9a
9b
9c
9a
9b
9c
9a
9b
9c
9a
9b
9c
57
83
83
76
38
35
38
63
67
72
40
83
54
77
81
63
34
32
25
54
64
69
39
79
b
b
entry
catalyst
none
% yield 12
% yield 13
2
2-OTf2
1
2
3
4
5
6
0
4
0
3
2-OTf2
HOTf
4
4
3-OTf2
c
2-OTf2
25
78
37
75 (68)
5
3-OTf2
3-OTf2
14
8
6
3-OTf2
d
e
e
e
Ph3PAuOTf
7
Ph3PAuOTf
Ph3PAuOTf
Ph3PAuOTf
d
c
IPrAuOTf
95 (90)
2
8
a
Reaction conditions: cyclohexenone (0.1 mmol), aniline (0.1 mmol),
9
e
catalyst (0.004 mmol), CH2Cl2 (1.0 mL), THF (1.0 mL), 40 °C, 12 h.
10
11
12
IPrAuOTf
b
e
Determined by GC-MS with respect to an internal standard
IPrAuOTf
c
(trimethoxybenzene) at 12 h. Isolated yield in parentheses.
e
IPrAuOTf
d
Generated in situ from LAuCl/AgOTf. See Supporting Information
a
Reaction conditions: phenylacetylene (0.1 mmol), aryl amine (0.1
mmol), catalyst (0.004 mmol), CH2Cl2 (0.5 mL), C6D6 (0.1 mL), 40
for details.
b
c
°C, 12 h. dipp = 2,6-diisopropylphenyl. Based on consumption of
aryl amine at 12 h. Determined by H NMR with respect to an
internal standard (1,3,5-trimethoxybenzene) at 12 h. Generated in situ
d
1
e
Ph3PAuOTf as catalysts afforded low yields of the cascade
product (entries 3−5). Only a trace amount of 13 was observed
in the reaction with IPrAuOTf after 12 h, but indole 12 was
generated in near-quantitative yield (entry 6). These results are
in agreement with previous experiments. While 2-OTf2 and 3-
OTf2 showed comparable activity as hydroamination catalysts,
2-OTf2 is a better catalyst for enone additions. This trend is
reflected in the higher yield of 13 delivered by 2-OTf2.
Moreover, while IPrAuOTf proves to be a good hydro-
amination catalyst, it is not competent for the conjugate
addition reaction necessary to furnish indole 13. Notably,
conducting the reaction in the presence of HOTf did not afford
either indole 12 or 13 in good yield, suggesting that
adventitious acid formed during the reaction is not responsible
for the observed catalysis (entry 2).
from LAuCl/AgOTf. See Supporting Information for details.
(entries 4 and 10). The catalytic activity of 3-OTf2 decreased
with the more sterically hindered amines 9b and 9c (entries 5
and 6), while 2-OTf2 and Ph3PAuOTf exhibited improved
activity with the larger aryl amine substrates. IPrAuOTf showed
decreased conversion with substrate 9b, but good activity for
the larger aniline 9c. This peculiar trend was reproducible over
several runs and with different batches of catalyst and substrate.
Amine coordination to Lewis acid catalyst sites is competitive
with alkyne activation in hydroamination reactions.37−42 The
rigid boat conformation of the metallacycle in 3-OTf2 results in
increased steric crowding around the gold coordination sites
compared to 2-OTf2. This steric crowding may suppress strong
binding of the relatively unhindered amine 9a to the gold
centers in 3-OTf2, resulting in higher catalytic activity. In the
reactions employing sterically bulky 9b and 9c, amine
coordination should be less competitive with alkyne activation
for all catalysts. Indeed, this would explain the increased
catalytic activity of 2-OTf2 and Ph3PAuOTf with these
substrates. However, 3-OTf2 exhibits the opposite trend,
suggesting that increased steric hindrance may block
nucleophile access to the activated alkyne.
From the results above, 2-OTf2 and 3-OTf2 exhibit greater
versatility as catalysts for Mukaiyama additions and hydro-
amination of alkynes than Ph3PAuOTf and IPrAuOTf. We
considered that this versatility might be leveraged for use in
cascade reactions. In order to test this hypothesis, we screened
the activity of the gold catalysts for the cascade reaction of
ortho-substituted aniline 11 and cyclohexenone (5b). The
reaction involves intramolecular hydroamination to afford 2-
phenylindole (12) and subsequent intermolecular conjugate
addition to cyclohexenone to generate 2,3-substituted indole 13
(Table 3).43−45
CONCLUSIONS
■
In summary, we report the synthesis, characterization, and
catalytic activity of cationic dinuclear gold(II,II) and gold-
(III,III) complexes supported by phosphorus ylide ligands. To
the best of our knowledge, this is the first reported example of
Lewis acid catalysis at gold(II) centers. 2-OTf2 and 3-OTf2
catalyze both Mukaiyama addition and alkyne hydroamination
reactions, offering similar or improved catalytic activity
compared to the gold(I) complexes Ph3PAuOTf and
IPrAuOTf. The ability of 2-OTf2 and 3-OTf2 to activate
alkynes and enones suggests both soft carbophilic and hard
oxophilic Lewis acid character. This hybrid Lewis acidity was
further demonstrated in a cascade reaction sequence involving
intramolecular hydroamination followed by intermolecular
enone addition to generate a 2,3-substituted indole. 2-OTf2
proved to be a good catalyst for this cascade sequence, while 3-
OTf2 and the commonly used gold(I) catalysts Ph3PAuOTf
and IPrAuOTf were ineffective. Ongoing work is focused on
further elucidating the steric and electronic factors that
influence the catalytic activity of Lewis acidic gold complexes
as well as exploring the utility of dinuclear gold PY complexes
in other types of catalysis.
The reaction was carried out at 40 °C in a 1:1 mixture of
THF and CH2Cl2 solution with 4 mol % of catalyst, and the
1
product distributions were determined by H NMR spectros-
copy and GC-MS. While the reaction with 2-OTf2 delivered
indole 13 in good yield, the reactions with 3-OTf2 and
D
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through a 0.45 μm PTFE syringe filter and then concentrated in vacuo
to afford the product as an off-white powder. X-ray quality crystals
were obtained by vapor diffusion of diethyl ether into a concentrated
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
a nitrogen-filled glovebox or standard Schlenk techniques unless
otherwise noted. All glassware was oven-dried in a 150 °C oven before
use. Solvents were degassed by sparging with ultra-high-purity argon
and dried via passage through columns of drying agents using a solvent
purification system from Pure Process Technologies.
Au2[PPh2(CH2)2] (1), Au2Br2[PPh2(CH2)2](μ-CH2) (3-Br2),
PPh3AuCl, IPrAuCl, and 1-phenyl-1-trimethylsiloxyethylene (4) were
prepared according to literature procedures.18,19,21,46−48 Toluidine and
mesitylamine were purified by sublimation and vacuum distillation,
respectively. All other chemicals were purchased from commercial
vendors and used without further purification. NMR spectra were
recorded at ambient temperature on a Varian Inova 400 MHz
1
solution of 3-OTf2 in acetonitrile. Yield: 31 mg, 94%. H NMR (400
3
MHz, CD3CN): δ 7.64−7.48 (m, 16H, Ar-H), 7.46 (t, 4H, JPH = 7.4
Hz, Ar-H), 2.68 (s, 2H, μ-CH2), 2.46 (dd, 4H, 2JPH = 12.9 Hz, 3JHH
=
12.9 Hz, PY-CH2), 1.97 (s, 6H, MeCN), 1.88 (dd, 4H, 2JPH = 12.1 Hz,
2JHH = 12.1 Hz, PY-CH2). 31P{1H} NMR (161.8 MHz, CD3CN): δ
35.1 (s). 19F NMR (376.4 MHz, CD3CN): δ −77.8 (s). 13C{1H}
NMR (100.5 MHz, CD3CN): δ 138.8 (s), 137.9 (s), 137.7 (d, JPC
=
70.2 Hz), 135.8 (t, 2JPC = 4.5 Hz), 135.4 (t, 2JPC = 4.5 Hz), 134.7 (m),
129.4 (d, JPC = 90.8 Hz), 19.5 (d, JPC = 48.8 Hz), 18.4 (s). Anal. Calcd
for C31H30Au2F6O6P2S2 + 1.33 C2H3N: C, 34.05; H, 2.89; N, 1.57.
Found: C, 33.75; H, 3.17; N, 1.58 (approximately 0.66 equiv of
acetonitrile was lost in drying).
1
instrument. H and 13C NMR chemical shifts were referenced to the
Generation of PPh3AuOTf and IPrAuOTf. Catalyst stock
solutions (13.3 mM) of PPh3AuOTf and IPrAuOTf were prepared
by addition of AgOTf (0.040 mmol) to CH2Cl2 solutions (3.0 mL) of
PPh3AuCl and IPrAuCl (0.040 mmol), respectively. The mixtures
were allowed to stir for 30 min, and the precipitated AgCl was
removed by filtration using a 0.45 μm PTFE syringe filter.34,52−54 The
resulting catalyst solutions were used immediately.
General Procedure for Mukaiyama Addition Reactions. In a
N2-filled glovebox, a vial was charged with 4 (0.1 mmol), 5a/b (0.1
mmol), CH2Cl2 (0.2 mL), and C6D6 (0.1 mL). The appropriate
amount of catalyst or catalyst stock solution was added (see Table 1),
and CH2Cl2 was added to reach a total reaction volume of 0.6 mL. For
runs that involved monitoring reaction progress, hexamethylbenzene
(0.01−0.05 mmol) was added as an internal standard and the reaction
mixtures were transferred to NMR tubes. For all other runs, the
reactions were quenched after 12 h with 10:1 MeOH/1 M HCl (0.1
mL), filtered over a plug of silica, and concentrated to dryness. The
crude residues were then extracted with CDCl3, and a known amount
of trimethoxybenzene (0.01−0.05 mmol) was added as an internal
standard. The product yields were determined by integration of the 1H
NMR spectra.
residual solvent chemical shifts. 31P NMR chemical shifts were
referenced to 85% H3PO4, and 19F NMR chemical shifts were
1
referenced to 99% F3CCO2H. Solvent-suppressed H NMR spectra
were collected using the WET1D sequence with default parameters.49
Briefly, spectra were collected using selective pulses of 86 ms with the
SEDUCE pulse shape. Gradient pulses were 2 ms in duration, had
amplitude ratios of 8:4:2:1, and were each followed by an additional 2
ms delay prior to the next RF pulse. The recycle delay between scans
was 30 s, 16K points were collected, and the acquisition time was 2.5 s.
GC-MS analysis was performed using an Agilent 7890A GC equipped
with a HP-5 capillary column (30 m, 0.25 mm i.d., 0.25 μm film
thickness) and a 5975C mass spectrometer as detector. The carrier gas
was helium, at a flow rate of 1 mL/min. For MS detection an electron
ionization system was used with an ionization energy of 70 eV.
Synthesis of [AuPY]2Cl2 (2-Cl2). This compound was prepared
using a modified literature procedure.50 A solution of PhICl2 (17 mg,
0.062 mmol) in CH2Cl2 (1.0 mL) was added dropwise to a stirring
solution of Au2[PPh2(CH2)2] (1) (51 mg, 0.062 mmol) in CH2Cl2
(1.0 mL). The reaction mixture initially turned bright green, but the
color faded to a pale yellow by the end of the addition. The reaction
was stirred for an additional 15 min. The reaction mixture was
concentrated to half of the original volume, and diethyl ether (5 mL)
was added, resulting in formation of a bright yellow precipitate. The
precipitate was collected by centrifugation and washed with additional
diethyl ether (3 × 5 mL). The precipitate was dried in vacuo to afford a
crystalline, bright yellow powder. Yield: 50 mg, 90% yield. The 1H and
31P NMR spectral data matched those reported in the literature.18,51
Synthesis of [Au2(μ-PY)2(MeCN)2]OTf2 (2-OTf2). The following
reaction was carried out in a 20 mL scintillation vial wrapped in black
electrical tape to rigorously exclude light. A solution of 2-Cl2 (20 mg,
0.022 mmol) in CH2Cl2 (1.0 mL) was added dropwise to a stirring
suspension of AgOTf (12 mg, 0.047 mmol) in MeCN (1.0 mL). The
reaction was stirred for an additional 30 min and then concentrated in
vacuo. The resulting residue was extracted with CH2Cl2 (5 mL), and
the mixture was filtered through a 0.45 μm PTFE syringe filter to give
a clear, orange solution. Acetonitrile (0.1 mL) was added to the filtrate,
furnishing a yellow solution, and the reaction was concentrated in
vacuo, resulting in precipitation of the product as a bright yellow
crystalline solid. X-ray quality crystals were obtained by vapor diffusion
of diethyl ether into a concentrated solution of 2-OTf2 in acetonitrile.
General Procedure for Hydroamination Reactions. In a N2-
filled glovebox, a vial was charged with 8 (0.1 mmol), 9a/b/c (0.1
mmol), CH2Cl2 (0.2 mL), and C6D6 (0.1 mL). The appropriate
amount of catalyst or catalyst stock solution was added (see Table 2),
and CH2Cl2 was added to reach a total reaction volume of 0.6 mL. A
known amount of trimethoxybenzene (0.01−0.05 mmol) was then
added as an internal standard. The reaction mixtures were transferred
to an NMR tube and heated at 40 °C for 12 h. The yields were
1
determined by H NMR spectroscopy.
General Procedure for Cascade Reaction of 11 and 5b. In a
N2-filled glovebox, a vial was charged with 11 (0.1 mmol), 5b (0.1
mmol), catalyst (0.004 mmol), CH2Cl2 (1.0 mL), and THF (1.0 mL).
The reaction mixture was heated at 40 °C for 12 h in a sealed vial. The
reaction was filtered over a plug of silica and concentrated to dryness.
The residue was extracted with CDCl3, and a known amount of
trimethoxybenzene (0.01−0.05 mmol) was added as an internal
standard. The product yield was determined by 1H NMR spectroscopy
and GC-MS.
ASSOCIATED CONTENT
* Supporting Information
■
1
Yield: 21 mg, 84%. H NMR (400 MHz, CD2Cl2): δ 7.61−7.53 (m,
S
12H, Ar-H), 7.50−7.46 (m, 8H, Ar-H), 2.20 (s, 6H, MeCN), 1.96 (d,
8H, 2JPH = 10.2 Hz, PY-CH2). 31P{1H} NMR (161.8 MHz, CD2Cl2): δ
34.5 (s). 19F NMR (376.4 MHz, CD2Cl2): δ −77.3 (s). 13C{1H} NMR
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00403.
2
(100.5 MHz, CD2Cl2): δ 133.32 (s), 131.2 (d, JPC = 9.1 Hz), 131.1
2
(s), 129.9 (d, JPC = 11.4 Hz), 129.9 (s), 14.6 (d, JPC = 60.3 Hz), 2.8
Additional experimental procedures and NMR and GC-
MS characterization data (PDF)
Crystallographic data for 2-OTf2 and 3-OTf2 (CIF)
(s). Anal. Calcd for C30H28Au2F6O6P2S2 + 1.5 C2H3N: C, 33.58; H,
2.78; N, 1.78. Found: C, 33.33; H, 2.82; N, 1.81 (approximately 0.5
equiv. of acetonitrile was lost in drying).
Synthesis of [Au2(μ-PY)2(μ-CH2)(MeCN)2]OTf2 (3-OTf2). A
solution of 3-Br2 (30 mg, 0.03 mmol) in CH2Cl2 (1.0 mL) was
added dropwise to a stirring suspension of AgOTf (16.0 mg, 0.062
mmol) in CH2Cl2 (1.0 mL). Following completion of the addition, the
reaction was stirred for an additional 30 min. The reaction was filtered
AUTHOR INFORMATION
Corresponding Author
*E-mail: cwade@brandeis.edu.
■
E
DOI: 10.1021/acs.organomet.6b00403
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(33) Cordon
metallics 2016, 35, 732.
́
, J.; Lop
́
ez-de-Luzuriaga, J. M.; Monge, M. Organo-
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
C.R.W. acknowledges Brandeis University for start-up funding.
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DOI: 10.1021/acs.organomet.6b00403
Organometallics XXXX, XXX, XXX−XXX