Synthesis of Different Classes of Six-Membered Gold(I) NHC Complexes by the Isonitrile Route

Article abstract of DOI:10.1021/acs.organomet.6b00023

Different types of six-membered N-heterocyclic gold carbene complexes were prepared by using gold(I) isonitrile complexes as precursors in combination with suitably functionalized amines. The simple procedures provide an easy access to not only unsymmetrically substituted saturated carbene complexes but rarely reported types of carbene complexes such as six-membered N-heterocyclic oxo-carbene complexes, and six-membered partly unsaturated ligands can be obtained following the strategy.

Full text of DOI:10.1021/acs.organomet.6b00023

Article
pubs.acs.org/Organometallics
Synthesis of Different Classes of Six-Membered Gold(I) NHC
Complexes by the Isonitrile Route
Thomas Wurm, Florian Mulks, Constantin R. N. Bohling, Dominic Riedel, Poorya Zargaran,
̈
†
⊥,
Matthias Rudolph, Frank Rominger, and A. Stephen K. Hashmi *
Organisch-Chemisches Institut, Ruprecht-Karls-Universitat
̈
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Chemistry Department, King Abdulaziz University (KAU), Jeddah 21589, Saudi Arabia
S Supporting Information
⊥
*
ABSTRACT: Different types of six-membered N-heterocyclic gold carbene
complexes were prepared by using gold(I) isonitrile complexes as precursors in
combination with suitably functionalized amines. The simple procedures provide an
easy access to not only unsymmetrically substituted saturated carbene complexes
but rarely reported types of carbene complexes such as six-membered N-
heterocyclic oxo-carbene complexes, and six-membered partly unsaturated ligands
can be obtained following the strategy.
1
. INTRODUCTION
complexes can be obtained either by suitable leaving groups
10
that are attached to an intermoleculary offered nucleophile or
The major advantages of N-heterocyclic carbenes (NHCs), in
contrast to ligands such as phosphanes, are their therma1l
stability, their low toxicity, and their stability against oxidation.
This enables the application of NHC−transition metal
by the use of functionalized isonitriles that can undergo ring
11
closure in an intramolecular fashion.
Attracted by the simplicity of this strategy (especially for the
synthesis of unsymmetrically substituted NHC complexes), we
joined this field and have published the straightforward
2
complexes in a manifold of different reactions.
While most of the research is focusing on five-membered-
12
synthesis of five-membered saturated NHC−, unsaturated
ring carbenes, recently publications also dealt with the concept
of six-, seven-, and eight-membered NHC complexes. Due to
their higher basicity
contrast to their five-membered analogues, these expanded ring
13
3
NHC−, and N-heterocyclic oxo-carbene (NHOC)−metal
14
3c,4
3c
complexes. Unusual (with respect to the connectivity, not
and larger N−C −N angles, in
NHC
with respect to the electronic structure) saturated NHC−metal
15
5
complexes could also be obtained following a related strategy.
systems are promising scaffolds for transition metal catalysis.
Reports on the use of metal-coordinated isonitriles for the
synthesis of six-membered NHC complexes are rare and yet
The most common protocol for the synthesis of saturated
five-, six-, seven-, and eight-membered carbene systems starts
with the formation of a diamine (or diimine) and its
subsequent transformation into the corresponding amidinium
salt by reaction with a formyl (C1) unit. Nevertheless, this
approach is practical only for a limited class of large ring
16
17
limited to Pt and Pd complexes.
Herein we report our efforts to evaluate the utilization of the
template strategy for the synthesis of six-membered gold
carbene complexes by the isonitrile route. By variation of the
electrophilic functionality attached to the amine building block,
various classes of six-membered gold NHC complexes
including completely new types of ligands were accessible.
3
c,6
systems. A different protocol, starting from a 1,3-diazaally7l
anion fragment, was developed by Bertrand and co-workers.
The ring closure in this approach is enabled by a dielectrophile
such as a dibromoalkane. A drawback of this methodology is
the requirement for moisture-free conditions, which are
necessary because of the use of strong organolithium bases
for the anion formation. A revised protocol by Fallis and Cavell
circumvents these inert conditions by using potassium
2
. RESULTS AND DISCUSSION
We first focused on the synthesis of six-membered saturated
NHC gold chlorides. The 3-chloropropylamine hydrochlorides
needed as a 3C/1N building block for the saturated carbene
backbones were synthesized via a simple two-step procedure
starting from commercially available 3-bromopropanol (1)
8
carbonate as base, but the methodology is still limited to
certain substituents for the formation of unsymmetrically
substituted expanded ring NHCs. The formation of NHC−
metal complexes with expanded ring systems either by ligand
transfers from silver(I) complexes to other metals
deprotonation of amidinium salts is still a challenge in that
field of research.
(
Table 1). After substitution of the bromine under elevated
temperature, the resulting amino alcohols 3 were directly
transformed to the corresponding chloro compounds without
further purification. The final products were obtained as
hydrochlorides (4), which were purified via crystallization from
9
a−c
or by
9d
An alternative approach toward NHC complexes is the
template synthesis of metal-coordinated isonitriles with suitable
amines as nucleophiles. Following this route, cyclic carbene
Received: January 14, 2016
©
XXXX American Chemical Society
A
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Table 1. Synthesis of the Amine Hydrochlorides 4
a
entry
amine 2/amino alcohol 3
4 [yield (%)]
1
2
3
4
5
cyclohexylamine
4a (59)
4b (76)
cyclooctylamine
5b
cyclododecylamine
cyclopentadecylamine
2-(2-hydroxyethyl)piperidine
4c (89)
4d (77)
4e (89)
b
a
Reaction conditions: 2 M solution of amine in benzonitrile; 1.25 equiv 3-bromopropan-1-ol, 12 h, 95 °C. After workup redissolved in DCM and
b
addition of 3 equiv SOCl /reflux (yield over two steps). Starting from commercially available amino alcohol.
2
a
Table 2. Synthesis of Saturated Six-Membered Gold NHC Complexes
a
Reaction conditions: 0.1 M solution of isonitrile−gold(I) precursor in DCM, 1−2 equiv amine, 5 equiv NEt , rt, 1−3 d.
3
ethanol/petroleum ether. This two-step protocol led to high
overall yields irrespective if open chained (entries 1−4) or
cyclic secondary amines (entry 5) were applied.
visible, and besides the isonitriles with a biaryl substituent (6a,
6b), an electron-withdrawing ortho-CF group (6c) or the
3
sterically demanding Dipp-isonitrile (6d−g) could be con-
verted in high efficiency as well. A series with differently sized
cyclic amines in combination with the same isonitrile precursor
showed no effect of the ring size on the reaction yields (6d−f).
The reaction with cyclic amino precursor 4e delivered the
bicyclic annelated six-membered carbene complex 6g in good
yield, too. For compound 6f crystals suitable for X-ray crystal
structure analysis could be obtained. The solid-state molecular
With the 3C/1N building blocks in hand, we subjected them
to gold isonitrile complexes 5, which can simply be obtained
18
from tetrahydrothiophene gold(I) chloride or dimethylsulfide
19
gold(I) chloride by a ligand exchange reaction. In analogy to
our recently published procedure for five-membered NHC
12,13
complexes from metal-bound isonitriles,
the reactions were
performed in the presence of triethylamine in dichloromethane
at room temperature. The results of the complex syntheses are
summarized in Table 2. To our delight, our strategy toward the
six-membered carbene complexes turned out to be highly
versatile. All of the applied isonitriles precursors delivered good
to excellent yields of the corresponding complexes (6a−g) and
were obtained with all of the applied nucleophiles. No strong
effect of the substitution pattern at the arene systems was
20
structure is depicted in Figure 1.
A comparison of some key structural parameters of 6f with
20
13
20
its five-membered unsaturated (7) and saturated (8)
homologues is depicted in Table 3. The most significant
difference between the different carbene classes is the angle
between the two nitrogen atoms and the central carbene
carbon. The smallest angle of 104.5° is observed for the
B
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backbone is noteworthy, as it delivers the possibility for further
derivatizations.
As we have already reported, palladium bis-isonitrile
precursors 9 are also suitable substrates for the synthesis of
5b
six-membered NHC complexes. In addition to the presented
results, we were curious if cyclic amine 4e as nucleophile would
enable an entry toward annelated six-membered Pd-NHCs as
well. As shown in Scheme 2, like in the case of the gold
precursor, bicyclic NHC 10 could be obtained in high yield. In
complete agreement with our previous findings, only one of the
two isonitrile ligands reacted, while the other is preserved in the
product. The structure of the obtained complex 10 could be
verified by the results of an X-ray crystal structure investigation,
too (Figure 2).
Inspired by these results on the synthesis of saturated six-
membered NHC complexes, we wondered whether our
recently reported strategies on the isonitrile-based synthesis
1
3
of five-membered saturated NHC complexes and the
synthesis of five-membered NHOC complexes could be
Figure 1. Solid-state molecular structure of complex 6f.
14
expanded toward the synthesis of their corresponding six-
membered-ring homologues, too.
unsaturated five-membered carbene that possesses significant
aromatic character. Shifting to its saturated derivative leads to a
widening of the angle of about 4.9°. Not unexpectedly, the
additional methylene unit in the six-membered carbene
backbone induced a further widening of 9.6° compared to
carbene complex 8. The differences in other structural
parameters are less pronounced. While the unsaturated five-
membered system shows the shortest Au−carbene distance
First we focused on the synthesis of six-membered NHOC
complexes, a class of carbene complexes that was only once
22,23
mentioned in the literature.
In analogy to our previous
reports, an ester containing C3/N1 building block 11a was
used as a nucleophile in combination with the Dipp-Au
isonitrile precursor 5f (Scheme 3). Indeed a clean reaction took
place, but unfortunately only the open-chain NAC (nitrogen
acyclic carbene) complex 12 was formed, and even after
prolonged reaction times of 14 days or with different
postreaction treatments we were not able to convert 12 into
the desired cyclic carbene 13a.
24
(
197.4 pm), this distance is more or less the same for the other
two carbene classes (199.2 and 199.8 pm). No significant
differences are obtained for the distances of the adjacent
nitrogen atoms to the central carbene atom.
An explanation for this lack of reactivity was obtained by
quantum chemical calculations. The thermochemistry of the
intramolecular ring closure of the NAC complexes toward the
cyclic NHOC complexes was calculated on model systems for
the six-membered complexes as well as for its five-membered
counterparts. The results summarized in Table 4 indicate that
for both systems the formation of the NHOC complexes is an
Amines with additional functional groups in the tether were
suitable substrates as well. This was demonstrated by the
reaction of various aromatic isonitrile precursors with hydroxyl-
functionalized amine 4f (Scheme 1). All of the corresponding
carbene complexes 6h−j bearing a β-hydroxy moiety in the
carbene backbone could be isolated in good yields. The
incorporation of the hydroxyl functionality in the carbene
21
Table 3. Comparison of Key Structural Parameters of Complexes 6f, 7, and 8
C
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Scheme 1. Synthesis of Functionalized Six-Membered Carbenes
Scheme 2. Synthesis of Annelated Six-Membered Pd-NHC
Table 4. Thermochemistry of Model Systems for the
Transformation of 12 to 13
1
0
B3LYP-d3 ,
ai
GP
f
solv
g
n=0
solv
h
n=0
GP
f
solv
g
n=1
solv
n=1
h
n=0
n=1
c
ΔRE
6.32
5.23
3.10
5.16
2.83
12.5
10.4
0.90
10.7
8.52
10.7
8.40
d
Δ H
4.18
R
e
Δ G
−5.83
−7.17
−8.05
−1.06
−1.19
h
R
b
,
i
f
g
h
f
g
M06
GP
solv
solv
GP
solv
solv
n=1
n=0
n=0
n=0
n=1
n=1
c
ΔRE
7.96
5.78
6.96
5.01
6.94
4.67
−6.05
13.6
11.7
9.59
11.9
d
Δ H
Δ G
12.3
0.66
9.45
R
e
−4.20
−5.08
b
−1.07
−1.18
R
a
c
d
B3LYP-d3, cc-pVTZ. M06, cc-pVTZ. Electronic energy. Sum of
electronic and thermal enthalpies. Sum of electronic and thermal free
energies. GP = gas phase. IEFPCM calculation. SMD calculation.
All energies given in kcal/mol.
e
f
g
h
i
systems can explain the observed lack of reactivity of NAC
complex 12.
As a consequence, we then focused on the synthesis of, to the
best of our knowledge, yet unknown six-membered unfunction-
25
alized unsaturated NHC-gold chlorides 17 (Scheme 4). The
synthesis of the starting materials and applied reaction
conditions were adopted from our recently reported synthesis
of unsaturated five-membered NHC-gold chlorides based on
13
the isonitrile strategy. To our surprise, instead of the expected
unsaturated complexes 17, acidic treatment of NAC complexes
Figure 2. Solid-state molecular structure of carbene complex 10.
1
5 delivered an entirely new class of NHC complexes bearing a
hemiaminal functionality attached to the N−C−N unit of the
carbene. Most unfortunately the initial yields of this process
were low (<10%).
endothermic process. However, the observed trend is more
pronounced in the case of the six-membered system, and in the
case of the five-membered system the endothermicity can be
overcompensated by the positive reaction entropy, leading to
an overall exergonic process. In the case of the six-membered
system this compensation leads only to a slightly negative free
reaction enthalpy (calculations were performed for 25 °C
After optimization of the reaction conditions, an increased
yield was obtained by using technical grade dichloromethane as
solvent (see Table 5). This led us to the assumption that the
hydroxy functionality of complexes 16 was derived from traces
of water present in the reaction mixture.
Under the optimized conditions four derivates of complexes
16 could be obtained in good to moderate yields (Table 5).
(
298.15 K) and 1 atm). These results combined with the
increased steric limitations in the experimentally applied
Scheme 3. Attempted Synthesis of Six-Membered NHOC Complex 13a
D
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Scheme 4. Unexpected Synthesis of Six-Membered Hemiaminal NHC-Gold(I) Chlorides 16
Table 5. Overview of the Synthesized Hydroxy-
Functionalized Six-Membered NHC-Au(I) Chlorides
0.45°), which implies a limited influence of the hydroxy
functionality on the carbene moiety of the six-membered NHC
complexes.
With complexes 16 in hand, we were curious if it was
possible to oxidize the hydroxyl group in these compounds and
thus bypass the above-discussed thermodynamic problems for
the synthesis of six-membered NHOC complexes.
16d as a representative example could be oxidized by using
Dess-Martin periodinane, and the corresponding six-membered
NHOC complex 18 could be isolated in 58% yield (Scheme
26
5
). The assignment of this new type of carbene ligand could
be verified by its solid-state molecular structure (see Figure
20
4
).
Scheme 5. Oxidation of 16d to the Corresponding Six-
Membered NHOC Complex 18
Furthermore, it was possible to obtain crystals of compound
20
1
6c that were suitable for X-ray analysis. The corresponding
solid-state molecular structure is shown in Figure 3 together
with selected bond lengths and angles. A comparison of the
obtained structural data of 16c with those of compound 6f
without the hydroxy functionality reveals only slight deviations
in the measured structural parameters (Δd < 1.50 pm, ΔΦ <
Figure 4. Molecular structure of 18 in the solid state; dO5−C5: 122.1(4)
pm, dC5−N6: 140.6(4)pm, dN6−C1: 138.1(4) pm, dC1−N2: 132.3(4) pm,
dC1−Au1: 199.3(3), ΦN6−C1−N2: 117.0(3)°.
A comparison of the structural parameters of 16c and 18
reveals that by the oxidation the C−O bond length in 18 is
significantly reduced by 20.7 pm to 122.1 pm (a bond length
slightly shorter than the typical C−O bond length observed in
carboxamides). The N6−C5 bond with a bond length of 140.6
pm is elongated compared to that of typical carboxamides. The
changes in the bond lengths of the N−C−N unit are less
pronounced. The bond length of the N6−C1 bond is elongated
Figure 3. Molecular structure of 16c in the solid state; dC5−O5
:
:
142.8(7) pm, dN6−C5: 147.0(9) pm, dN6−C1: 135.4(3) pm, dN2−C1
133.1(3) pm, dC1−Au1: 199.7(2) pm, ΦN6−C1−N2: 118.51(19)°.
E
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by 2.7 pm, whereas the bond length of the C1−N2 bond is
slightly contracted. Similar to the delocalization of the nitrogen
lone-pair in carboxamides, the carbonyl group next to the N−
C−N unit reduces the electron-donating abilities of N6. As a
consequence, this effect is then compensated by a slightly
stronger donation of N2.
Table 7. Thermochemistry of Model Systems for the
Formation of 19 Starting from 15
After having accomplished the synthesis of a six-membered
NHOC complex, we refocused on the realization of the
synthesis of six-membered unsaturated gold chlorides. To get
an insight into the synthetic difficulties, we again conducted
quantum chemical calculations on the thermochemistry for the
formation of complex 17 and compared the results to those of
the five-membered system. The obtained results are presented
in Table 6. They predict that the formation of both the
ai
B3LYP-d3 ,
f
g
h
f
g
h
GP
solv
solv
GP
solv
solv
n=1
n=0
n=0
n=0
n=1
n=1
c
ΔRE
2.86
0.72
1.15
3.44
1.09
2.71
0.98
3.29
1.59
d
Δ H
Δ G
1.06
−1.06
−11.7
−0.72
−11.9
R
e
−9.57
−8.01
−9.78
−8.57
h
R
b
,
i
f
g
h
f
g
M06
GP
solv
solv
GP
solv
solv
n=1
n=0
n=0
n=0
n=1
n=1
c
ΔRE
4.89
2.66
0.89
−10.5
b
2.82
3.57
2.43
0.87
3.18
1.29
−10.03
d
Table 6. Thermochemistry of Model Systems for the
Formation of 17 Starting from 15
d
Δ H
Δ G
3.22
1.06
1.97
R
e
−7.59
−10.3
−9.25
−9.99
R
a
c
B3LYP-d3, cc-pVTZ. M06, cc-pVTZ. Electronic energy. Sum of
electronic and thermal enthalpies. Sum of electronic and thermal free
energies. GP = gas phase. IEFPCM calculation. SMD calculation.
All energies given in kcal/mol.
e
f
g
h
i
Table 8. Thermochemistry of Model Systems for the
Formation of 17 from 19
a
,
i
f
g
h
n=0
f
g
n=1
h
B3LYP-d3
GP
solv
solv
GP
solv
solv
n=1
n=0
n=0
n=1
c
ΔRE
5.81
2.06
1.82
21.6
17.5
19.7
15.4
19.3
15.0
d
Δ H
Δ G
2.02
−2.34
−2.17
R
e
−21.8
−24.2
−26.0
−6.15
−8.53
−8.18
h
R
b
,
i
f
g
n=0
h
n=0
f
g
n=1
M06
GP
solv
solv
GP
solv
solv
n=1
n=0
n=1
B3LYP-d3 ,
ai
GP
f
solv
g
solv
h
GP
f
solv
g
solv
n=1
h
c
ΔRE
11.1
7.22
3.00
6.83
3.26
25.9
23.6
19.5
23.5
19.0
n=0
n=0
n=0
n=1
n=1
d
c
Δ H
Δ G
7.48
21.9
Δ
Δ
R
R
E
2.95
1.34
0.68
18.2
15.6
1.86
17.0
14.4
1.25
16.0
13.4
0.39
R
e
d
−16.8
−20.3
b
−20.4
−2.47
−4.55
−5.53
H
0.96
−1.28
−12.5
solv n=0
−1.45
−14.1
solv n=0
R
e
a
c
d
Δ G
−12.2
B3LYP-d3, cc-pVTZ. M06, cc-pVTZ. Electronic energy. Sum of
electronic and thermal enthalpies. Sum of electronic and thermal free
energies. GP = gas phase. IEFPCM calculation. SMD calculation.
All energies given in kcal/mol.
R
e
b
,
i
f
g
h
f
g
h
M06
GP n=0
GP n=1
solv n=1
solv n=1
f
g
h
c
ΔRE
6.18
4.56
2.11
4.02
22.4
19.9
6.77
21.2
18.6
5.44
20.3
15.6
1.86
i
d
Δ H
4.26
2.20
R
e
Δ G
−9.18
−9.74
−10.1
R
a
b
c
d
B3LYP-d3, cc-pVTZ. M06, cc-pVTZ. electronic energy. Sum of
electronic and thermal Enthalpies. Sum of electronic and thermal Free
Energies. GP = gas phase. IEFPCM calculation. SMD calculation.
All energies given in kcal/mol.
unsaturated six- and five-membered carbene gold chlorides are
exergonic processes. However, the trend is more pronounced in
the case of the five-membered system.
Since these calculations delivered no explanation for the
obtained experimental results, we included the formation of the
possible hemiaminal ether intermediates 19 (which can be
formed upon elimination of one equivalent of alcohol) into our
thermochemical considerations.
e
f
g
h
i
Scheme 6. Mechanistic Hypothesis for the Formation of
Products 16
The thermochemistry of these species relative to acetal 15
Table 7) reveals that for both systems a similar negative free
(
energy is obtained. However, while the final elimination toward
the desired unsaturated product for the unsaturated five-
membered system is exergonic as well, the corresponding
process for the six-membered system is endergonic (see Table
8). Nevertheless, instead of the hemiaminal ether complexes 19
the hemiaminal complexes 16 are experimentally observed.
We postulate that these species are formed by the attack of
external water (present in the reaction solvent) at an
intermediate iminium species 20 that should easily be formed
under acidic conditions. The overall preference of 16 over 20
results from water being present in excess compared to the
ethanol concentration (Scheme 6).
To underline this hypothesis, we repeated the synthesis of 16
under as strict exclusion of water as experimentally possible
with these reagents (Scheme 7).
F
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Scheme 7. Synthesis of 19a under Exclusion of Water and Conversion of 19a to 16d
Indeed, under these conditions we were able to obtain the
predicted hemiaminal ether complex 19a together with the
corresponding hemiaminal complex 16d, which can be
attributed to unavoidable traces of water that stem from the
HCl in dioxane solution. A full spectroscopic characterization
was possible for the hemiaminal ether species. The control
experiment with the isolated hemiaminal ether complex 19a in
the presence of water cleanly delivered the hemiaminal complex
Compound 17b gave crystals suitable for an X-ray single-
crystal structure analysis. The resulting molecular structure in
the solid state of one of two independent molecules present in
the unit cell is depicted in Figure 5 together with selected bond
lengths and angles.
16d, which further confirms our mechanistic proposal.
Our quantum chemical calculations and the obtained
experimental results suggest that complexes like 19 and 16
are the thermodynamic sink of the investigated systems. In
order to finally obtain the desired six-membered unsaturated
gold chloride, we envisioned performing the reaction under
strictly water-free conditions. In addition by removal of the low-
boiling ethanol under reduced pressure, simple elimination of a
proton from iminium intermediate 20 or direct elimination of
ethanol from hemiaminal ether complex 19 should deliver the
target NHC complex containing an enamine-like substructure.
In order to realize this strategy, the higher boiling toluene was
needed as the solvent. Furthermore, it was necessary to use a
nonvolatile and completely water-free acid, for which the acidic
anion exchanger Amberlyst 15 was selected. Indeed these
reaction conditions in the presence of a reduced pressure (200
mbar) enabled us to isolate three differently substituted six-
membered unsaturated NHC-gold chlorides 17 in good to
moderate yields (Table 9).
Figure 5. Molecular structure of 17b in the solid state; dC5−N6
141.7(6) pm/142.6(6) pm, dN6−C1: 134.6(7) pm/135.2(7) pm,
C1−N2: 133.4(7) pm/132.7(6), dC1−Au1: 199.0(5) pm/200.3(5) pm,
:
d
dC4−C5: 130.6(8) pm/130.5(8) pm, ΦN6−C1−N2: 117.8(4)°/118.0(5)°.
The analysis of the structural parameters of the N−C−N unit
of 17b revealed no striking peculiarities compared to the
saturated derivatives; therefore it can be assumed that the
influence of the double bond on this unit is limited. Only the
rather short bond length of the C4−C5 bond is noteworthy.
The reactivity of these new complexes was investigated, too.
To complete our mechanistic picture, we used complex 17a and
exposed it to the reaction conditions that usually delivered the
hemiaminal complexes. Indeed 17a was cleanly converted to
the hydrolyzed complex 16a (Scheme 8). Furthermore, it is
important to note that, even against steric limitations, only 16a
was observed and no formation of a species of type 21 could be
detected. Thus, we assume that the reactivity of the double
bonds is comparable to the usual reactivity of an enamine
moiety. Further studies on the reactivity that can be explained
by the lack of aromatic stabilization compared to the five-
membered unsaturated complexes are currently ongoing in our
group.
Table 9. Overview of the Synthesized Unsaturated Six-
Membered NHC-Au(I) Chlorides 17
4
. CONCLUSION
In summary we have shown that our previously reported
isonitrile-based synthesis of unsymmetrically substituted six-
G
DOI: 10.1021/acs.organomet.6b00023
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 8. Conversion of 17a to 16a
membered saturated NHC-Pd(II) complexes can be transferred
to the synthesis of analogous Au(I) complexes. The use of
prefunctionalized nucleophiles even enabled the synthesis of
hydroxy-functionalized carbene backbones with great potential
for further derivatization. During our search for the synthesis of
so far rarely described six-membered NHOC complexes as well
as six-membered unsaturated NHC-Au(I) chlorides we
discovered a new class of NHC-Au(I) complexes bearing a
hemiaminal functionality within the backbone of the NHC
ligand. By careful quantum chemical studies we were able to
explore the thermochemistry of these species and deduced a
synthetic procedure for the synthesis of the six-membered
NHOC-Au(I) chlorides as well as the six-membered unsatu-
rated complexes. In contrast to the C−C double bond of the
related five-membered systems, the C−C double bond of the
six-membered homologues bears significant reactivity, and
further studies exploring the potential for postmodifications of
these species are under way.
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ASSOCIATED CONTENT
Supporting Information
■
Khrustalev, V. N.; Nechaev, M. S. J. Organomet. Chem. 2009, 694,
2454−2462. (d) Iglesias, M.; Beetstra, D. J.; Stasch, A.; Horton, P. N.;
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*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00023.
1H and 13C{ H} NMR, HRMS, and IR data of all new
1
(4) (a) Alder, R.; Blake, M. E.; Oliva, J. M. J. Phys. Chem. A 1999,
compounds (PDF)
Computational details (PDF)
103, 11200−11211. (b) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am.
Chem. Soc. 2004, 126, 8717−8724.
Crystallographic data (CIF)
Coordinate files of the model structures determined by
quantum chemistry (ZIP)
(5) For selected examples, see: (a) Dunsford, J. J.; Cavell, K. J. Dalton
Trans. 2011, 40, 9131−9135. (b) Spallek, M. J.; Riedel, D.; Hashmi, A.
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J. J.; Cavell, K. J.; Kariuki, B. M. Organometallics 2012, 31, 4118−4121.
(d) Kolychev, E. L.; Asachenko, A. F.; Dzhevakov, P. B.; Bush, A. A.;
AUTHOR INFORMATION
Corresponding Author
E-mail: hashmi@hashmi.de.
Author Contributions
Shuntikov, V. V.; Khrustalev, V. N.; Nechaev, M. S. Dalton Trans.
2013, 42, 6859−6866. (e) Phillips, N.; Tang, C. Y.; Tirfoin, R.; Kelly,
M. J.; Thompson, A. L.; Gutmann, M. J.; Aldridge, S. Dalton Trans.
■
*
2
014, 3, 12288−12298. (f) Morozov, O. S.; Lunchev, A. V.; Bush, A.
A.; Tukov, A. A.; Asachenko, A. F.; Khrustalev, V. N.; Zalesskiy, S. S.;
Ananikov, V. P.; Nechaev, M. S. Chem. - Eur. J. 2014, 20, 6162−6170.
(g) Dunsford, J. J.; Cavell, K. J. Organometallics 2014, 33, 2902−2905.
(h) Schwartsburd, L.; Mahon, M. F.; Poulten, R. C.; Warren, M. R.;
Whittlesey, M. K. Organometallics 2014, 33, 6165−6170. (i) Delvos, L.
B.; Hensel, A.; Oestreich, M. Synthesis 2014, 46, 2957−2964.
†
Crystallographic investigation (F. Rominger).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
P.Z. is generously supported by a Ph.D. fellowship of the
DAAD. Gold salts were donated by Umicore AG & Co. KG.
■
(
j) Brown, R. M.; Borau Garcia, J.; Valjus, J.; Roberts, C. J.;
Tuononen, H. M.; Parvez, M.; Roesler, R. Angew. Chem. 2015, 127,
372−6375; Angew. Chem., Int. Ed. 2015, 54, 6274−6277.
6) For selected examples, see: (a) Dunsford, J. J.; Cavell, K. J. Dalton
6
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DOI: 10.1021/acs.organomet.6b00023
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