Synthesis, Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes

Article abstract of DOI:10.1002/anie.201713209

Methoxide abstraction from gold acetylide complexes of the form (L)Au[η¹-C≡CC(OMe)ArAr′] (L=IPr, P(^tBu)₂(ortho-biphenyl); Ar/Ar′=C₆H₄X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at ?78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr′]⁺ OTf^? in ≥85±5 % yield according to ¹H NMR analysis. ¹³C NMR and IR spectroscopic analysis of these complexes established the arene-dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh₂]⁺ OTf^? reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.

Full text of DOI:10.1002/anie.201713209

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Accepted Article
Title: Synthesis, Characterization, and Reactivity of Cationic Gold
Diarylallenylidene Complexes
Authors: Ross Widenhoefer and nana kim
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201713209
Angew. Chem. 10.1002/ange.201713209
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http://dx.doi.org/10.1002/ange.201713209

10.1002/anie.201713209
Angewandte Chemie International Edition
COMMUNICATION
Synthesis, Characterization, and Reactivity of Cationic Gold
Diarylallenylidene Complexes
Nana Kim and Ross A. Widenhoefer*
Abstract: Methoxide abstraction from gold acetylide complexes of
the form (L)Au[η1-C≡CC(OMe)ArAr'] [L = IPr, P(tBu)2o-biphenyl;
likely accumulation of positive charge at both the C1 and C3
positions of the unsaturated carbene ligand and the potential for
reactivity at both these positions. Although a number of cationic
gold vinyl carbene,[11] and allenylidene[12] complexes have been
reported, the electronic structure of these complexes is
dominated by the presence of one or more C1/C3 heteroatom π-
donor substituent(s) (Chart 2).[13-15] Here we report the synthesis,
spectroscopy, and reactivity of a cationic, two-coordinate gold
diphenylallenylidene complex and related diarylallenylidene
complexes.
Ar/Ar'
= C6H4X (X = H, Cl, Me, OMe)] with trimethylsilyl
trifluoromethanesulfonate (TMSOTf) at –78 °C formed the
corresponding
cationic
gold
diarylallenylidene
complexes
[(L)Au=C=C=CArAr']+ OTf– (2) in ≥85 ± 5% yield by 1H NMR. 13C
NMR and IR analysis of complexes 2 established the arene-
dependent delocalization of positive charge on both the C1 and C3
allenylidene carbon atoms.
The diphenylallenylidene complex
[(IPr)Au=C=C=CPh2]+ OTf– reacted with heteroatom nucleophiles at
the allenylidene C1 and/or C3 carbon atom.
Dipp
Me
Cationic, two-coordinate gold carbene complexes have been
OMe
N
OMe
N
Me
OMe
OMe
(L)Au
(L)Au
C C C
3
1
postulated as intermediates in
a
range of gold-catalyzed
(L)Au
(L)Au
C C C
1
2
3
transformations, most notably enyne cycloaddition[1] and alkene
cyclopropanation,[2] and reactive gold carbene complexes have
been detected and analyzed in the gas phase.[3,4] Nevertheless,
there remains considerable debate regarding the electronic
structure of the Au–C bond of gold carbene complexes,[5-9] and
little is known regarding fundamental reactivity of these
complexes. These deficiencies can be traced in large part to the
lack of suitable model complexes for spectroscopic analysis and
reactivity studies. Although a small number of cationic gold
carbene complexes lacking a C1 heteroatom π-donor substituent
2
Et
Ph
Et
Chart 2. Examples of β,γ-unsaturated gold carbene complexes (L
=
N-
heterocyclic carbene or tertiary phosphine) possessing one or more C1/C3
heteroatom π-donor substituent(s).[11,12]
Our approach to the synthesis of unstabilized gold β,γ-
unsaturated carbene complexes involved the Lewis acid
abstraction of a γ-methoxy group from a neutral gold precursor,
which was inspired by Brookhart's synthesis of unstabilized iron
carbene complexes via ionization of (α-methoxyalkyl)iron
complexes.[16,17] Toward this objective, we initially targeted gold
allenylidene complexes for synthesis owing to the ready
availability of the γ-functionalized gold acetylide precursors.[18,19]
Indeed, treatment of the gold γ,γ-diphenylacetylide complex
have been reported (A
bis(mesityl)carbene complex
-
D, Chart 1),[6-10] only the
possesses no remote
A
heteroatom or aromatic stabilization. However, complex A is
unreactive toward nucleophiles such as water owing to the
extreme steric shielding of the carbene center by both the octa-
tert-butyl NHC ligand and mesityl groups.[6]
OMe
(IPr)AuC≡CC(OMe)Ph2
(1a)
with
trimethylsilyl
trifluoromethanesulfonate (TMSOTf) in CD2Cl2 at –78 °C led to
immediate (<1 min) formation of the bright red, cationic gold
diphenylallenylidene complex [(IPr)Au=C=C=CPh2]+ OTf– (2a) in
85% ± 5% yield (1H NMR) along with trimethylsilyl methyl ether
(TMSOMe) as the sole byproduct in 92 ± 5% yield (1H NMR)
(Scheme 1). In a similar manner, diarylallenylidene complexes
2b-2f were synthesized in >90% yield via methoxide abstraction
from gold acetylide complexes 1b-1f.
Me
Me
Si
(L)Au
C
(L)Au
C
(L)Au
C
(L)Au
C
C
Si
Me
Me
C
A
B
D
OMe
Chart 1. Recent examples of gold carbene complexes (L = N-heterocyclic
carbene or tertiary phosphine) lacking a stabilizing C1 π-donor substituent.[6-10]
OMe
(L)Au
C
C
TMSOTf (1.3 equiv)
CD2Cl2, –78 °C
Of particular interest among the cationic gold carbene
R2
– TMSOMe
complexes are those possessing β,γ-unsaturation owing to the
1
R1
R1
R1
L = IPr, R1 = R2 = H (a)
L = IPr, R1 = R2 = Cl (b)
L = IPr, R1 = R2 = Me (c)
L = IPr, R1 = H, R2 = OMe (d)
L = IPr, R1 = R2 = OMe (e)
L = P(tBu)2o-biphenyl,
R1 = R2 = OMe (f)
[*]
Nana Kim, Prof. R. A. Widenhoefer
French Family Science Center, Duke University
Durham, NC 27708–0346 (USA)
(L)Au
C
C
C
(L)Au
C
C
C
1
2
3
E-mail: rwidenho@chem.duke.edu
[**]
We acknowledge the NSF (CHE-1213957) for support of this
research. We thank Dr. Roger Sommer (NC State University) for
solving the X-ray diffraction analysis.
R2
R2
2
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Synthesis of gold diarylallenylidene complexes 2.
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10.1002/anie.201713209
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COMMUNICATION
Complex 2a decomposed rapidly in solution at –63 °C (t1/2
=
cations and ΔG‡ for the decomposition of complexes 2 (Figure
1).[21-25] Similarly, the chemical shifts of the allenylidene C1 and
C3 carbon atoms decreased with the increasing electron donor
ability of the allenylidene aryl groups (Table 2), and strong
correlations were observed between Ef and the δ(C1) and δ(C3)
of complexes 2a and 2c-2e (Figure 2),[23] although the bis(4-
chlorophenyl)allenylidene complex 2b deviated from these
27 min; Table 1), but was sufficiently stable at –85 °C to permit
spectroscopic characterization (~5 h). Most notably, the 13C
NMR spectrum of 2a displayed resonances at δ = 248.4 and
191.3 assigned to the allenylidene C1 and C3 carbon atoms,
respectively, that were deshielded by ≥110 ppm relative to the
corresponding atoms of the neutral acetylide precursor 1a
(Table 2).[20]
The IR spectrum of 2a displayed a sharp
correlations.[26]
Also
worth
noting,
the
gold
absorbance at 2051 cm–1 assigned to the C1–C2 stretch, which
was red shifted by 69 cm–1 relative to the C≡C absorbance of 1a
(Table 2).
bis(anisyl)allenylidene complex 2f containing the P(tBu)o-
biphenyl ligand was significantly less stable than was IPr
complex 2e and 2f likewise displayed greater deshielding of the
C1 allenylidene resonance than did 2e (Tables 1 and 2).
Table 1. First-order rate constants and energy barriers for decomposition of
gold allenylidene complexes 2 in CD2Cl2 (~24 mM).
ΔG‡ (kcal/mol)
(104)k (s–1 [a]
)
compd
temp (°C)
250
230
210
190
170
2a
2a
2b
2c
2d
2e
2f
15.4
15.9
20.4
21.1
22.9
20.0
–63
–63
1
4.3 ± 0.2
2b
1.30 ± 0.03
3.2 ± 0.1
2d
2d
2c
11
22
1
3.41 ± 0.07
0.70 ± 0.02
6.2 ± 0.4
2e
2a
[a] First-order rate constants were determined from the linear slope of plots of
2e
ln[2] versus time to ≥2 half-lives.
2b
2c
Table 2. Selected absolute and relative 13C NMR and IR spectral data for gold
allenylidene complexes 2.
-8 -7 -6 -5 -4 -3 -2 -1
0
1
Ef
νC2C3 / ΔνC2C3 (cm–1 [a]
)
δ/Δδ (C1)[a]
δ/Δδ (C3)[a]
compd
Figure 2. Plot of Ef versus δ(C1) (¡) and δ(C3) (¨) for gold allenylidene
complexes 2a and 2c-2e where δ (C1) = (–6.8 ± 0.4)Ef + (208.0 ± 1.5), R =
0.99 and δ (C3) = (–2.7 ± 0.3)Ef + (175.8 ± 0.9), R = 0.99; Complex 2b was
not included in these correlations.
2a
2b
2c
2d
2e
2f
248.4 / +119.7
247.7 / +117.9
231.9 / +103.7
219.8 / +91.0
209.2 / +81.1
218.1[b] / +90.5
191.3 / +109.6
186.6 / +105.9
186.3 / +104.9
180.3 / +98.8
176.2 / +95.2
176.7 / +95.7
2051 / –69
2051 / –77
2056 / –56
2060 / –59
2066 / –47
2062 / –69
[a] Δδ and Δν correspond to the chemical shift and stretching frequency,
respectively, of 2 minus the corresponding value for 1. [b] JCP = 122 Hz.
24
23
2e
22
21
2d
2c
20
19
18
17
16
15
2b
2a
Figure 3. ORTEP diagram of 2e•2CH2Cl2. Ellipsoids are shown at 50%
probability with solvent, counterion, and hydrogen atoms omitted for clarity.
-8 -7 -6 -5 -4 -3 -2 -1
0
1
Ef
The enhanced thermal stability of the bis(anisyl)allenylidene
complex 2e allowed for single crystal X-ray diffraction analysis
(Figure 3). In the solid state, 2e adopts a rod-like configuration
with <3° deviation from linearity along the CIPr-Au-C1-C2-C3 axis.
Within the allenylidene fragment, the C1-C2 (1.210 Å) and C2-
C3 (1.412 Å) bond lengths are not significantly perturbed relative
to neutral (IPr)Au acetylide complexes.[19] The allenylidene
anisyl groups are twisted by ~24° out of the C2-C3-C4/C5 plane,
with relatively short C3–C4/C5 bonds (1.436 Å), which point to
significant electron donation from the anisyl groups to the
Figure 1. Plot of Ef versus the energy barrier for decomposition of gold
allenylidene complexes 2a-2e in CD2Cl2 (~24 mM) where ΔG‡ = (1.1 ± 0.1)Ef +
(23.1 ± 0.6), R = 0.97.
The solution-phase thermal stability of the gold allenylidene
complexes 2a - 2e increased significantly with the increasing
electron donor ability of the allenylidene aryl groups (Table 1),
and
a good correlation was observed between Mayer's
electrofugality parameter (Ef) for substituted benzhydrylium
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10.1002/anie.201713209
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COMMUNICATION
electron-deficient C3 atom.[7] Taken together, the spectroscopic,
structural, and thermal stability data for allenylidene complexes
2 point to the delocalization of positive charge on both the C1
and C3 allenylidene carbon atoms and the dominant role of the
C3 aryl groups in the stabilization of this charge. At the same
time, comparison of bis(anisyl)allenylidene complexes 2e and 2f
reveals that the (L)Au fragment also participates in the
stabilization of positive charge.
by X-ray crystallography (Figure 4). When the conversion of 5 to
6 was performed in the presence of excess pyridine-d5 (40 eq),
complete scrambling of pyridine-d5 into the γ-position of 5 was
observed before the onset of the conversion of 5 to 6. Allenyl
pyridinium complex 6 was likewise generated quantitatively by
treatment of the α-sulfonium gold allenyl complex 4 with pyridine
(1.5 equiv.) at 0 °C for 20 min. Taken together, the reactions of
2a with THT and pyridine point to rapid and reversible addition of
the nucleophile to the allenylidene C3 position followed by
slower, exergonic addition to the allenylidene C1 position.[31]
Diphenylallenylidene complex 2a reacted rapidly at –80 °C
with a number of heteroatom nucleophiles to form products
resulting from addition to the allenylidene C1 and/or C3 carbon
atom (Scheme 2). For example, treatment of 2a with methanol
at –78 °C led to rapid (< 1 min) disappearance of the deep red
color of 2a with formation of propargylic ether 3 in 85 ± 5% yield
(1H NMR), presumably generated via attack of methanol at the
C3 allenylidene carbon atom of 2a followed by
protodeauration.[27,28]
In comparison, treatment of 2a with
tetrahydrothiophene (THT; 1.5 equiv.) at –78 °C in CD2Cl2 led to
immediate color change to form the α-sulfonium gold allenyl
complex 4 in 100 ± 5% yield (1H NMR).[29] Selective attack of
THT at the C1 allenylidene carbon atom of 2a was established
by the resonance at δ = 207.4 in the 13C NMR spectrum of 4
assigned to the allenyl C(sp) carbon atom.
OMe
Figure 4. ORTEP diagram of 6. Ellipsoids are shown at 50% probability with
H
C C
counterion and hydrogen atoms omitted for clarity.
3
MeOH
–78 °C
In summary, we have synthesized the cationic, two-
coordinate diphenylallenylidene complex 2a and related
diarylallenylidene complexes 2b-2f via low-temperature
methoxide abstraction from the corresponding γ-methoxide gold
S
S
(IPr)Au
C
C
2a
C
C
C
C
acetylide
complexes.
Complexes
2a
and
the
–78 °C
(IPr)Au
bis(tolyl)allenylidene complex 2c represent both the first
examples of two-coordinate, β,γ-unsaturated gold carbene
complex lacking π-conjugated heteroatoms, and rare examples
of two-coordinate gold carbene complexes lacking such
stabilizing groups.[6] Spectroscopic analysis of complexes 2
established the arene- and (L)Au-dependent delocalization of
positive charge on both the C1 and C3 allenylidene carbon
atoms. Allenylidene complex 2a reacts rapidly at –78 °C with
methanol, THT, and pyridine via addition to the C1 and/or C3
allenylidene carbon atom.
4
pyridine
pyridine
0 °C, 20 min
–78 °C
N
N
–30 °C
15 min
(IPr)Au
C
C
C
C
C
(IPr)Au
6
5
Scheme 2. Reactions of heteroatom nucleophiles with 2a in CD2Cl2.
Keywords: gold • allenylidenes • carbenoids • carbene ligands •
bonding
Additional insight into the mechanism of the conversion of 2a
to 4 was provided by the reactions of pyridine with both 2a and 4.
For example, treatment of 2a with pyridine (1.4 equiv.) at –78 °C
led to immediate color change with formation of the γ-pyridinium
gold acetylide complex 5 in 94 ± 5% yield (1H NMR) (Scheme 2).
Attack of pyridine at the allenylidene C3 atom of 2a was
established by the presence of a nitrogen-coupled doublet at δ =
80.8 (1JCN = 5.5 Hz) in the 13C NMR spectrum of the doubly
labelled isotopomer 5-3-13C1-15N assigned to the propargylic
carbon atom.[30] When a solution of 5 was warmed at –30 °C for
15 min, the initially colorless solution turned yellow with
formation of the thermally-stable α-pyridinium gold allenyl
complex 6 in 99 ± 5% yield (1H NMR), which was characterized
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COMMUNICATION
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[30] More detailed assignments of the chemical shifts and carbon-carbon
and carbon-nitrogen coupling constants for complexes 5 and 6 and
their isotopomers is provided in the Supporting Information.
[31] In comparison, 2a failed to react with anisole, THF, or acetone below
the threshold for thermal decomposition of 2a.
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10.1002/anie.201713209
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Nana Kim and Ross A. Widenhoefer*
Page No. – Page No.
Synthesis, Characterization, and
Reactivity of Cationic Gold
Diarylallenylidene Complexes
Cationic gold allenylidene complexes of the form [(L)Au=C=C=CArAr']+ OTf– [Ar/Ar'
= C6H4X (X = H, Cl, Me, OMe)] were generated via methoxide abstraction from the
corresponding gold γ-methoxide acetylide precursors. 13C NMR and IR analysis of
gold allenylidene complexes established the arene-dependent delocalization of
positive charge on both the C1 and C3 allenylidene carbon atoms. The
diphenylallenylidene complex [(IPr)Au=C=C=CPh2]+ reacted with heteroatom
nucleophiles at the allenylidene C1 and/or C3 carbon atom.
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