From isonitriles to unsaturated NHC complexes of gold, palladium and platinum

Article abstract of DOI:10.1002/adsc.201401131

In a modular template synthesis, unsaturated NHC complexes of gold, palladium and platinum were synthesized from simple metal salts, isonitriles and amines with acetal or ketal groups. Upon the addition of amines with tethered acetal or ketal moieties to the metal-activated isonitrile, first nitrogen acyclic carbene (NAC) complexes are formed. These undergo ring closure and elimination to the unsaturated NHC complexes upon addition of acid. This simple strategy opens an attractive and fast approach to NHC complexes of gold, palladium and platinum. The modular approach allows a fast modification and is well-suited for the synthesis of unsymetrically and symmetrically substituted unsaturated NHC complexes.

Full text of DOI:10.1002/adsc.201401131

FULL PAPERS
DOI: 10.1002/adsc.201401131
From Isonitriles to Unsaturated NHC Complexes of Gold, Palla-
dium and Platinum
Dominic Riedel,a Thomas Wurm,a Katharina Graf,a Matthias Rudolph,a
Frank Rominger,a and A. Stephen K. Hashmia,b,*
a
Organisch-Chemisches Institut, Universitꢀt Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)-6221-544205; e-mail: hashmi@hashmi.de
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
b
Received: December 8, 2014; Revised: January 21, 2015; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201401131.
Abstract: In a modular template synthesis, unsaturat- simple strategy opens an attractive and fast approach
ed NHC complexes of gold, palladium and platinum to NHC complexes of gold, palladium and platinum.
were synthesized from simple metal salts, isonitriles The modular approach allows a fast modification
and amines with acetal or ketal groups. Upon the ad- and is well-suited for the synthesis of unsymetrically
dition of amines with tethered acetal or ketal moiet- and symmetrically substituted unsaturated NHC
ies to the metal-activated isonitrile, first nitrogen complexes.
acyclic carbene (NAC) complexes are formed. These
undergo ring closure and elimination to the unsatu- Keywords: carbene complexes; gold; isonitriles;
rated NHC complexes upon addition of acid. This NHC complexes; palladium; platinum
Introduction
most cases.[6] A template synthesis offers an attractive
alternative approach towards metal NHC complexes;
After the isolation of the first stable N-heterocyclic in this template synthesis a nitrogen nucleophile at-
carbene (NHC) in 1991[1] this new class of organic tacks an in situ formed metal-coordinated (and there-
compounds opened up a completely new field of re- fore activated) isonitrile. If a suitable electrophile is
search with tremendous impact on the whole chemical attached to the attacking amine, the former isonitrile
community.[2] Especially the use of NHCs as ligands nitrogen atom can undergo a subsequent nucleophilic
for homogeneous transition metals has contributed attack and the resulting ring closure gives access to
immensely to this continuing rise.[3]
saturated NHC complexes (Scheme 1, left).[7] In addi-
By far the most often applied strategies for the syn- tion, isonitriles with attached nucleophiles can follow
thesis of NHC complexes are based on heterocyclic a related concept via an intramolecular approach, in
azolium salts as carbene precursors. These can be con- these cases the heterocycle is closed by an existing
verted to the metal complexes via deprotonation and tether and protic carbenes are formed as products.[8]
coordination of the free NHC to a metal precursor.
Inspired by the pioneering work mentioned above,
This can either be done by isolation of the free car- our group was also attracted by the simplicity of this
bene and the reaction with a metal precursor or by an alternative approach. With the focus on the synthesis
in situ formation of the free carbene and its coordina- of highly active catalysts, we first entered the field by
tion in the presence of a suitable metal complex.[4] In the synthesis of NAC (nitrogen acyclic carbene) gold
addition, often protocols involve the transmetalation
of NHC ligands from silver NHC complexes.[5] The
drawbacks of these strategies are the necessity of
a strong base, the need for an inert atmosphere and
the formation of undesired silver containing byprod-
ucts. Furthermore, for some NHCs the necessary azo-
lium precursors are not easy accessible. This is espe-
cially the case for the synthesis of unsymmetrically
substituted NHCs that needs multi-step synthesis in left: intermolecular process, right: intramolecular process.
Scheme 1. NHC-Synthesis via metal isonitrile precursors;
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Dominic Riedel et al.
Scheme 2. Retrosynthetic analysis for the isonitrile based synthesis of unsymetrically substituted unsaturated NHC com-
plexes.
complexes that turned out to be potent catalysts for with 2,2-dimethoxyethanamine (Table 1). In analogy
various transformations.[9] In addition, catalytically to a procedure of Abdel-Magid et al., NaHB(OAc)3
active saturated unsymetrically substituted cyclic was used as reducing agent, which enabled the synthe-
NHCs of gold[10] and platinum[9,10a,11] were synthesized sis of various substituted 2,2-dimethoxyethlyamine de-
following the intermolecular strategy. Complexes with rivatives 3 in high yield.[14] The redox condensation
NHOC (N-heterocyclic oxo-carbene)[12] and even ab- was also possible if the acetal moiety was introduced
normal saturated carbene ligands[13] were also accessi- via the ketone part, which was used for the synthesis
ble by following related strategies. Herein we want to of ketal 3 f. A slightly different approach was used for
report our efforts on the synthesis of unsymetrically the synthesis of bornyl and adamantyl amines 3g and
substituted unsaturated NHC complexes. The feasibili- 3h (Scheme 3). As in this case the aldehyde com-
ty of the general concept was reported as early as pound is only available as an aqueous solution,
1975,[7f,g] but then only primary amine precursors a step-wise procedure including imine formation and
were used and only protic carbene derivatives had subsequent borohydride reduction was necessary.
been addressed. Interestingly, despite the enormous
These amino acetal precursors were then tested in
importance of these complexes in the field of cataly- the reactions with various metal isonitrile precursors.
sis, no reports on the synthesis of non-protic NHCs Due to the simple geometry of linear gold(I) com-
following this simple principle can be found in litera- plexes, these were applied in our first test reactions
ture. Our strategy is depicted in Scheme 2, it is based
on the use of aminoacetal precursors bearing a secon-
dary amine in order to install an additional substitu-
tent at the NHC nitrogen. After ring closure and
elimination of water the desired unsymmetrical unsa-
turated NHC complexes might be accessible via a re-
markably efficient and easy strategy.
Results and Discussion
A key point of our study was the synthesis of suitable
aminoacetal precursors. These were easily accessible
by reductive amination of ketone/aldehyde precursors Scheme 3. Synthesis of amine precursors 3g and 3h.
Table 1. Synthesis of N-(2,2-dialkoxyethyl)alkylamines 3
Entry
Ketone/Aldehyde 1
Amine 2
3 (Yield [%])
1
2
3
4
5
Heptadecan-9-one
Cyclohexanone
Cyclooctanone
Cyclododecanone
Cyclopentadecanone
2,2-Dimethoxyethylamine
2,2-Dimethoxyethylamine
2,2-Dimethoxyethylamine
2,2-Dimethoxyethylamine
2,2-Dimethoxyethylamine
3a (98)
3b (91)
3c (95)
3d (95)
3e (97)
6
Cyclohexylamine
3 f (92)
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Table 2. Syntheses of unsymmetrically substituted AuI-NHC complexes
Entry
1
Amine 3/R2/R3
3b/cyclohexyl/H
Isonitrile 4/R1
tert-Butyl
6 (Yield [%])[a]
6a (95)[c]
Time to form 6
7 (Yield [%])[b]
7a (>99)
48 h
2
3b/cyclohexyl/H
6b (95)[c]
24 h
24 h
7b (>99)[c]
3
3b/cyclohexyl/H
6c (95)[c]
7c (>99)[c]
4
5
3b/cyclohexyl/H
3h/adamantyl/H
6d (93)
48 h
72 h
7d (>99)
2-biphenyl
6e (96)[c]
7e (>99)[d]
6
3d/cyclododecyl/H
6 f (90)
72 h
7 f (>99)
7
8
3d/cyclododecyl/H
3g/(R)-bornyl/H
2-biphenyl
6g (98)[c]
6h (92)[e]
72 h
14 d
7g (>99)
7h (>99)[c]
9
3b/cyclohexyl/H
3c/cyclooctyl/H
3d/cyclododecyl/H
6i (96)[c]
6j (92)[c]
6k (92)[c]
24 h
24 h
24 h
7i (>99)
10
11
7j (>99)[c]
7k (>99)[c]
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Table 2. (Continued)
Entry
Amine 3/R2/R3
Isonitrile 4/R1
6 (Yield [%])[a]
Time to form 6
7 (Yield [%])[b]
7l (>99)[c]
12
3e/cyclopentadecyl/H
3 f/cyclohexyl/Me
6l (98)
24 h
13
14
6m (92)
72 h
7m (85)
3a/9-heptadecyl
3d/cylododecyl
6n (98)
6o (95)
24 h
24 h
7n (>99)[c]
7o (>99)[c]
15
cyclododecyl
[a]
General conditions: 0.1m solution of an isocyanogold(I) complex in dichloromethane; 1–2 equivalents of amine, room
temperature
General conditions: 0.1m solution of the acyclic Au-NAC complex in dichloromethane; 10 equivalents of HCl (4 N in di-
oxane); room temperature, 12 h.
X-ray crystal structure was determined, see supporting information.
Only soluble in DMSO and large excess of methanol.
[b]
[c]
[d]
(Table 2). Thioether gold(I) chloride complexes like electron-withdrawing substituents (entry 4) had no
THT-gold(I) chloride or DMS-gold(I) chloride were significant effect on the reactivity. Even a more steri-
used as precursors for all of the gold complexes. The cally demanding adamantyl substitutent at the amine
isonitrile gold complexes 5 were obtained by simple precursor 3h was tolerated with only a slight increase
addition of the isonitrile compound 4 to the gold pre- in reaction time (entry 5). Both, the sterically bulky
cursor in a CH2Cl2 solution. After evaporation of the isonitrile 4 f and 2-biphenyl substituted isonitrile 4e
solvent, crystaline compounds were obtained for vola- could be easily converted with the cycloalkyl amine
tile isonitriles, while impurities of high boiling isoni- precursor 3d (entries 6 and 7). Next, inspired by
triles could be efficiently separated by washing with Nolanꢂs IPrAuCl-catalyst,[15] we synthesized a series
diethyl ether. All of the isonitrile complexes were ob- of NAC complexes all bearing a diisopropylphenyl
tained, regardless of the used gold precursor, in quan- subunit that was introduced via the isonitrile precur-
titative yield without the need for further purification sor 5h (entries 8–14). All of the applied amines react-
steps; as previously demonstrated in the synthesis of ed smoothly except for bornyl amine 3g, which
the saturated NHC gold(I) complexes the isonitrile needed a reaction time of 2 weeks for its complete
complexes can also conveniently be formed in situ.[10a] transformation to NAC 6h (entry 8). A ketal precur-
Then the isonitrile gold(I) complexes 5 were react- sor could also be converted without any problem
ed with 1–2 equivalents of the amino acetal com- (entry 13). The preparation of an alkyl/alkyl-substitut-
pounds in CH2Cl2 at room temperature. Within 1–3 ed NAC complex was also possible, which was dem-
days (the reactions were monitored by thin layer onstrated by the efficient synthesis of complex 6o.
chromatography) nearly all of the open-chained NAC The conversion of the NAC complexes to the corre-
complexes 6 could be obtained in high yield. In order sponding unsaturated NHC complexes was unproble-
to check the dependency on the isonitrile, various iso- matic. Simple addition of 10 equivalents of HCl (4 N
nitriles were reacted with the same cyclohexane sub- in dioxane) in most of the cases delivered the desired
stituted amine 3b. In this series of reactions (en- products 7 in quantitative yields. Most of the inter-
tries 1–4) almost no dependency on the isonitrile com- mediate NAC complexes, as well as the final products,
ponent was observed and all of the reactions proceed- delivered crystals suitable for X-ray measurements
ed smoothly no matter if sterically bulky aliphatic (16 structures, see Table 2).[16] Four representative ex-
(entry 1) or aromatic substituents were applied amples are depicted in Figure 1. In all of the obtained
(entry 2). Furthermore, electron-donating (entry 3) or solid state structures of the NAC complexes the rota-
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From Isonitriles to Unsaturated NHC Complexes
of the process as shown above, we were curious if it
was possible to conduct the whole synthesis as a one
pot reaction. Therefore we repeated the synthesis of
7l starting from DMS gold(I) chloride, and 2,6-diiso-
propylphenylisonitrile according to Scheme 4.
Every step of the shown one pot sequence was
monitored by thin layer chromatography until com-
plete conversion of the relevant species was observed.
Finally, we were able to isolate 83% of the desired
complex 7l. With regard to the 3 consecutive steps
done in one flask, the loss of 14% yield compared to
the overall yield for the step-wise process with isola-
tion of the synthetic intermediates is quite acceptable
and demonstrates the efficiency of the developed
method.
As a next step palladium and platinum bis-isonitri-
les[10d,11] 8 and 11 were converted in the presence of
acetal 3b (Scheme 5). In the case of palladium no
clean formation of the NAC complex 9 was observed.
In this case the NAC was obtained as a mixture of ro-
Figure 1. Selected solid state molecular structures of the
gold(I) carbene complexes 6b and 7b (top left and right),
and 6k and 7k (bottom left and right). See SI for the X-ray
crystal structures of 12 additional examples of type 6 and 7, tamers and cis/trans isomers. Hence a full characteri-
two examples of 5 and N-cyclododecyl formamide.[16]
zation was not undertaken at this stage and instead
the complex obtained was directly converted to the
final NHC complex 10. Nevertheless, crystals suitable
mer with the acetal pointing away from the gold for an X-ray single crystal structure determination of
center was detected. This is also the favored rotamer the mixture were obtained and a solid state molecular
for the ring closing process which might explain the structure of the palladium NAC complex is depicted
high efficiency of the last reaction step. No large dif- in Figure 2. After ring closure, selective formation of
ferences for the gold-carbene carbon distance are ob- NHC complex 10 could be achieved in 88% yield.
served for both complex classes. For the NAC com- This indicates that even at room temperature a slow
plexes the distance ranges between 194 pm and rotation around the disubstituted nitrogen is possible,
200 pm. for the closed NHC complexes the range is which places the electrophile in the right position for
between 196 pm and 200 pm. As expected with angles the ring closure. In the case of the platinum NAC
between 113.78 and 119.38 the N-C-N angle for the complex, 12 was exclusively formed, which might be
NAC complexes is larger. The range for the NHCs is rationalized by slower ligand exchange processes at
from 102.88 to 108.38. The solid state structures for the platinum center. As in the case of the other
complexes with larger flexible cyclic rings show the metals, the overall sequence turned out to be highly
tendency of the cycloalkyl chain to point towards the efficient. In addition, all of the obtained species could
metal centre (which might have a positive influence be verified by X-ray single crystal structure analysis
on catalyst stability). This is not only the case for the (Figure 2).
depicted 12-membered cycle but also for the 15-mem-
In conclusion, we could demonstrate that unsymet-
bered ring (see SI). Interestingly this effect cannot be rically substituted unsaturated metal NHC complexes
observed for the corresponding sytem with a 9-hepta- can be obtained via the isonitrile route with high effi-
decyl substituent at the carbene nitrogen atom.
ciency. As in the case of the synthesis of the saturated
The reactions were performed in a two step se- analogues, a modular template synthesis was possible.
quence in order to isolate the intermediate NAC com- If one considers the simple synthesis of the starting
plexes, but due to the high yields obtained by all steps materials and the possibility to perform the whole re-
Scheme 4. One Pot synthesis of 7l.
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Scheme 5. Synthesis of unsaturated unsymmetrical palladium and platinum NHC complexes.
Experimental Section
General procedure for the preparation of (2,2-
dimethoxyethyl)amino nitrogen acyclic carbene
gold(I) chlorides (GP 5)
To a 0.1m solution of an isocyanogold(I) complex in di-
chloromethane 1–2 equivalents of an N-(2,2-dimethoxye-
thyl)amine were added. The mixture was stirred at room
temperature for 1–3 days. The solvent was removed under
reduced pressure. If necessary, the crude product was puri-
fied by column chromatography on silica gel (dichlorome-
thane as eluent). The synthesis of the desired acyclic gold(I)
carbenes can also be done in a one-pot procedure, starting
from (tht)AuCl in dichloromethane. After addition of the
isonitrile, the mixture is stirred at room temperature for 1 h
and the N-(2,2-dimethoxyethyl)amine is added subsequently.
All the complexes are air and moisture stable and can be
stored at room temperature without decomposition. Note
that in the case of aromatic N-(2,2-dimethoxyethyl)amines
no nucleophilic attack of the amine nitrogen to the isonitrile
carbon center could be observed!
Figure 2. Solid state molecular structures of complexes 9
and 10 (top left and right), and 12 and 13 (bottom left and
right).
action sequence in one pot, this strategy offers an at-
tractive alternative to existing strategies that are
based on imidazolium precursors and could be the
foundation for the synthesis of libraries of NHC com-
plexes: ten different isonitriles and ten different
amine building blocks in combination with one specif-
ic metal would allow the fast synthesis of a matrix of
100 different NHC complexes. The protocol also rep-
resents a significant improvement for the case of un-
symetrically disubstituted complexes, as in this case
existing protocols are based on multi-step sequences.
Investigations concerning the catalytic activity of such
libraries of catalysts are ongoing in our laboratory
and will be published in due course.
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(2,2-Dimethoxyethyl)amino nitrogen acyclic carbene
gold(I) chloride complex 6a was prepared according to GP
5; 119 mg (313 mmol) tert-butylisocyanogold(I) chloride and
25.9 mg (313 mmol) 3b in 5 mL dichloromethane were em-
ployed. The reaction mixture was stirred at room tempera-
ture for 48 h. Purification by column chromatography af-
forded a colorless crystalline solid; yield: 168 mg (295 mmol,
1
95%); H NMR (300 MHz, CD2Cl2): d=1.31–1.52 (m, 4H),
1.56 (s, 9H), 1.61–1.69 (m, 2H), 1.79–1.91 (m, 2H), 1.92 (m,
2H), 3.32 (d, J=4.8 Hz, 2H), 3.40 (s, 6H), 4.41 (t, J=
4.8 Hz, 1H), 4.80–4.87 (m, 1H), 7.78 (bs, 1H); 13C NMR
(75 MHz, CD2Cl2): d=25.54 (t), 26.01 (t, 2C), 31.54 (t, 2C),
31.83 (q), 31.87 (q, 2C), 48.87 (t), 54.09 (s), 55.49 (q, 2C),
70.57 (d), 105.89 (d), 194.01 (s); IR (KBr): n=3440, 3319,
2960, 2932, 2855, 1569, 1479, 1450, 1414, 1364, 1302, 1287,
1248, 1236, 1205, 1154, 1121, 1081, 1058, 985 cmÀ1; HR-MS
(FAB+): found: m/z=502.1624, calcd. for C15H30AuClN2O2
[M]+: 502.1611, m/z=467.1952, calcd. for C15H30AuN2O2 [M-
ClÀ]+: 467.1973.
mmol) N-(2,2-dimethoxyethyl)cyclohexanamine 3b in 5 mL
dry dichloromethane. Complete consumption of the starting
material was monitored by the shift of the IR stretching fre-
quencies of the isonitrile ligands after 4 days. After that the
solvent was removed under reduced pressure and the crude
product was dissolved in a minimum amount of dichlorome-
thane and covered with a layer diethyl ether, inducing crys-
tallization of the acyclic PtII carbene complex as colorless
crystals. The crystals were filtered off, washed with n-pen-
tane and dried under reduced pressure. 117 mg (162 mmol,
94%) of the title compound were gained as colorless solid;
1H NMR (300 MHz, CD2Cl2): d=1.43–1.64 (m, J=4H,
CH2), 1.72–1.76 (m, 2H, CH2), 1.77–1.99 (m, 2H, CH2), 2.01
(s, 3H, CH3), 2.27 (s, 6H, CH3), 2.49 (s, 3H, CH3), 3.47 (3H,
OCH3), 3.54 (3H, OCH3), 3.69 (d, J=5.4 Hz, 2H, CH2N),
4.53 (t, J=5.4 Hz, 1H, CH(OMe)2), 6.88–6.90 (m, 1H,
ArH), 7.10–7.14 (m, 2H, ArH), 7.20–7.27 (m, 3H, ArH),
8.70 (s, bs, 1H, NH); 13C NMR (75 MHz, CD2Cl2): d=17.86
(s, 2C), 18.71 (s, 2C), 24.85 (t, 2C), 24.63 (t, 2C), 24.15 (t,
1C), 30.51 (t, 1C), 54.20 (s, 1C), 55.82 (s, 1C), 66.62 (d, 1C),
104.58 (d, 1C), 126.92 (d, 1C), 127.35 (d, 2C), 127.68 (d, 1C),
128.66 (q, 1C), 128.38 (q, 1C), 134.45 (q, 2C), 134.61 (d, 1C),
137.05 (d, 1C), 146.54 (q, 1C), 175.54 (q, 1C); IR (KBr): n=
3431, 2932, 2855, 2182, 1628, 1537, 1473, 1449, 1415, 1379,
(2,2-Dimethoxyethyl)amino nitrogen acyclic carbene
gold(I) chloride complex 6h was prepared according to GP
5; 60.0 mg (143 mmol) 2,6-diisopropylphenylisocyanogold(I)
chloride and 34.5 mg (143 mmol) 3g in 5 mL dichlorome-
thane were employed. The reaction mixture was stirred at
room temperature for 2 weeks! Purification by column chro-
matography afforded a colorless crystalline solid; yield:
1357, 1309, 1262, 1222, 1190, 1119, 1077, 803, 719, 523 cmÀ1
;
HR-MS (FAB+): m/z=679.2381, calcd. for C28H39ClN3O2Pt
[M-Cl]+: 679.2379, m/z=644.2624, calcd. for C28H39N3O2Pt
[M-2Cl]+: 644.2690.
1
86.9 mg (132 mmol, 92%); H NMR (300 MHz, CD2Cl2): d=
General Procedure for the Preparation NHC
Complexes with PdII, PtII, and AuI (GP 6)
0.91 (s, 3H), 1.01 (s, 3H), 1.11 (s, 3H), 1.93 (d, J=6.9 Hz,
3H), 1.95 (d, J=6.9 Hz, 3H), 1.35 (d, J=6.9 Hz, 6H), 1.40–
1.47 (m, 2H), 1.52–1.57 (m, 2H), 1.80–1.83 (m, 1H), 1.94–
1.96 (m, 1H), 2.29–2.35 (m, 1H), 3.06 (sept, J=6.9 Hz, 1H),
3.15 (sept. J=6.9 Hz 1H), 3.45 (s, 3H), 3.48 (s, 3H), 3.82 (d,
J=4.5 Hz, 2H), 4.61 (t, J=4.5 Hz, 1H), 5.50–5.58 (m, 1H),
7.22 (d, J=7.7 Hz, 2H), 7.39 (t, J=7.7 Hz, 1H), 8.49 (bs,
1H); 13C NMR (CD2Cl2, 75 MHz): d=15.38 (q), 18.54 (q),
19.43 (q), 23.71 (q, 2C), 24.17 (q), 24.36 (q), 28.52 (t), 28.69
(d, 2C), 29.49 (t), 32.09 (t), 44.80 (d), 49.02 (s), 51.32 (s),
51.37 (t), 54.99 (q), 55.43 (q), 73.59 (t), 104.58 (d), 124.22 (d,
2C), 129.34 (d), 135.81 (s), 146.87 (s), 147.32 (s), 199.14 (s);
IR (KBr): n=3449, 3252, 2958, 2868, 2837, 1592, 1520, 1493,
1457, 1389, 1362, 1305, 1279, 1200, 1166, 1119, 1060, 801,
758 cmÀ1; HR-MS (FAB+): found: m/z=660.2691, calcd. for
C27H44AuClN2O2 [M]+: 660.2757, m/z=625.3053, calcd. for
C27H44AuN2O2 [M-ClÀ]+: 625.3068.
(2,2-Dimethoxyethyl)amino nitrogen acyclic carbene
platinum(II) dichloride 12
To a 0.1m solution of the acyclic AuI-, PdII-, and PtII-NAC
complexes in dichloromethane were added 10–20 equiva-
lents of HCl (4N in dioxane) and the mixture was stirred at
room temperature for 12 h. The reaction mixture was dilut-
ed with dichloromethane and washed 2 times with saturated
Complex 12 was prepared from 92.2 mg (173 mmol) cis-
[PtCl2(2,6-dimethylphenylisonitrile)2] and 32.1 mg (173
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NaHCO3 solution. The organic phase was dried over anhy-
drous Na2SO4 and the solvent was removed under reduced
pressure. If necessary, the crude product was purified by
column chromatography on silica gel (dichloromethane as
eluent) to afford the NHC complexes as crystalline solids in
quantitative yields. All the complexes are air and moisture
stable and can be stored at room temperature without de-
composition.
(s), 51.06 (s), 67.28 (d), 119.79 (d), 123.25 (d), 124.47 (d),
124.61 (d), 130.83 (d), 134.94 (s), 146.22 (s), 146.43 (s),
174.36 (s); IR (KBr): n=3429, 3266, 2961, 2870, 1628, 1472,
1458, 1425, 1411, 1391, 1364, 1346, 1313, 1213, 1191, 1124,
1059, 806, 762 cmÀ1; HR-MS (FAB+): found: m/z=596.2192,
calcd. for C25H36AuClN2 [M]+: 596.2233, m/z=561.2544,
calcd. for C25H36AuN2 [M-ClÀ]+: 561.2571.
NHC palladium(II) complex 10 was prepared from
170 mg (311 mmol) cis-[PdCl2(2,6-diisopropylphenyl isoni-
trile)2] and 58.1 mg (311 mmol) N-(2,2-dimethoxyethyl)cyclo-
hexanamine 3b in 8 mL dry THF. Complete consumption of
the starting material was monitored by the shift of the IR
stretching frequencies of the isonitrile ligands after 4 days.
After that the solvent was removed under reduced pressure
and the crude product was dissolved in a minimum amount
of dichloromethane and covered with a layer diethyl ether,
inducing crystallization of the acyclic PdII carbene com-
plexes as colorless crystals. The crystals were filtered off,
washed with n-pentane or diethyl ether and dried under re-
duced pressure. The obtained complex was gained as a mix-
ture of trans and cis complex and as a mixture of rotamers.
For this reason no full characterization was undertaken in-
stead the complex was directly converted to the correspond-
ing NHC complex according to GP 6; 204 mg (282 mmol) of
the NAC and 0.25 mL HCl (4N in dioxane) in 5 mL di-
chloromethane were employed. Evaporation of aprox. 90%
of the solvent and addition of pentane afforded the NHC
complex as a colorless crystalline solid; yield: 184 mg (274
NHC gold(I) complex 7a was prepared according to GP
6; 100 mg (198 mmol) 6a and 0.25 mL HCl (4N in dioxane)
in 5 mL dichloromethane were employed. Evaporation of
the solvent afforded the NHC complex as a colorless crystal-
line solid; yield: 87.1 mg (197 mmol,>99%); 1H NMR
(300 MHz, CD2Cl2): d=1.41–1.68 (m, 5H), 1.72–1.90 (m,
3H), 1.82 (s, 9H), 2.08–2.13 (m, 2H), 4.68–4.78 (m, 1H),
6.98 (d, J=1.9 Hz, 1H), 7.11 (d, J=1.9 Hz); 13C NMR
(75 MHz, CD2Cl2): d=25.56 (t), 25.83 (t, 2C), 31.80 (q, 3C),
34.37 (t, 2C), 58.94 (s), 62.66 (d), 115.73 (d), 118.95 (d),
168.73 (s); IR (KBr): n=3443, 3319, 2953, 2931, 2855, 1569,
1479, 1450, 1364, 1302, 1287, 1248, 1236, 1205, 1152, 1119,
1079, 1062, 985, 867 cmÀ1; HR-MS (FAB+): found: m/z=
439.1173, calcd. for C13H23AuClN2 [M+H]+: 439.1165, m/z=
403.1477, calcd. for C13H22AuN2 [M-ClÀ]+: 403.1449.
1
mmol, 88% over 2 steps); H NMR (300 MHz, CDCl3): d=
0.86 (d, J=6.9 Hz, 3H), 0.93 (d, J=6.9 Hz, 3H), 0.97 (d, J=
6.9 Hz, 3H, CH3), 1.15 (d, J=6.9 Hz, 3H, CH3), 1.24–1.39
(m, 2H), 1.37 (d, J=6.9 Hz, 6H), 1.59–1.62 (m, 4H), 1.78–
1.82 (m, 2H, CH2), 1.90–1.95 (m, 2H), 2.34 (sept, J=6.9 Hz,
1H), 2.60–2.68 (m, 1H, CH), 2.89 (sept, J=6.9 Hz, 1H),
3.11 (sept, J=6.9 Hz, 2H), 6.99–7.01 (m, 1H), 7.09–7.12 (m,
1H), 7.14 (d, J=7.7 Hz, 2H), 7.27–7.28 (m, 1H), 7.34–7.37
(m, 2H), 7.46 (t, J=7.7 Hz, 1H); 13C NMR (75 MHz,
CDCl3): d=22.14 (q), 22.87 (q), 23.05 (q, 2C), 23.36 (q, 2C),
25.27 (t), 25.43 (t), 25.66 (t), 26.57 (q), 26.64 (q), 28.63 (d),
28.99 (d), 29.87 (d, 2C), 33.84 (t), 34.55 (t), 61.02 (d), 118.65
(d), 123.55 (d), 123.98 (d, 2C), 125.58 (d), 126.73 (d), 131.01
(d), 131.12 (d), 144.15 (s, 2C), 146.12 (s, 4C), 148.15 (s),
154.93 (s); IR (KBr): n=3447, 3262, 2964, 2932, 2863, 2185,
1665, 1630, 1591, 1543, 1463, 1418, 1386, 1360, 1331, 1142,
1119, 1060, 800, 750 cmÀ1; HR-MS (FAB+): found: m/z=
638.2559, calcd. for C34H47ClN3O2Pd [M-ClÀ]+: 638.2493,
found: m/z=603.2808, calcd. for C34H47N3O2Pd [M-2ClÀ]+:
603.2805.
NHC gold(I) complex 7h was prepared according to GP
6; 60.0 mg (90.8 mmol) 6h and 0.25 mL HCl (4N in dioxane)
in 4 mL dichloromethane were employed. Evaporation of
the solvent afforded the NHC complex as a colorless crystal-
line solid;[4] yield: 54.2 mg (90.8 mmol,>99%); 1H NMR
(300 MHz, CD2Cl2): d=0.99 (s, 3H), 1.04 (s, 3H), 1.08 (s,
3H), 1.18 (d, J=6.9 Hz, 3H), 1.20 (d, J=6.9 Hz, 3H), 1.29
(d, J=6.9 Hz, 6H), 1.44–1.49 (m, 3H), 1.60–1.66 (m, 1H),
2.32 (sept, J=6.9 Hz, 1H), 2.47 (sept, J=6.9 Hz, 2H), 2.55–
2.65 (m, 1H), 5.24–5.30 (m, 1H), 7.01 (d, J=1.8 Hz, 1H),
7.28–7.34 (m, 2H), 7.33 (d, J=1.8 Hz, 1H), 7.52 (t, J=
7.9 Hz, 1H); 13C NMR (75 MHz, CD2Cl2): d=15.12 (q),
19.72 (q), 19.95 (q), 24.19 (q), 24.37 (q), 24.59 (q, 2C), 28.30
(t), 28.75 (d), 28.85 (t), 28.95 (d), 35.32 (t), 45.27 (d), 49.88
NHC platinum(II) complex 13 was prepared according to
GP 6; 100 mg (140 mmol) 12 and 0.25 mL HCl (4N in diox-
ane) in 5 mL dichloromethane were employed. Evaporation
of the solvent afforded the NHC complex as a colorless
8
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From Isonitriles to Unsaturated NHC Complexes
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crystalline solid; yield: 89.3 mg (139 mmol,>99%); H NMR
(300 MHz, CD2Cl2): d=1.60–1.67 (m, 5H, CH2), 1.76 (s, 3H,
CH3), 1.77–1.82 (m, 2H, CH2), 1.92–1.95 (m, 2H, CH2),
2.13–2.14 (m, 1H, CH2), 2.19 (s, 6H, CH3), 2.29 (s, 3H,
CH3), 2.44–2.45 (m, 1H CH2), 6.93 (d, J=7.4 Hz, 1H, ArH),
6.99 (d, J=1.9 Hz, 1H, =CH), 7.09 (d, J=7.4 Hz, 2H, ArH),
7.23 (t, J=7.4 Hz, 1H, ArH), 7.27 (d, J=7.4 Hz, 1H, ArH),
(d, J=1.9 Hz, 1H, =CH), 7.33(t, J=7.4 Hz, 1H, ArH);
13C NMR (75 MHz, CD2Cl2): d=17.85 (t), 18.77 (t, 2C),
19.11 (q), 25.62 (q, 2C), 25.87 (q), 33.64 (t), 34.88 (t), 60.65
(d), 119.08 (d), 123.69 (d), 128.35 (d), 128.46 (d, 2C), 129.73
(d, 2C), 130.19 (s), 134.57 (s), 135.79 (s, 2C), 137.53 (s, 2C),
137.76 (s), 143.66 (s), 153.52 (s); IR (KBr): n=3427, 2936,
2858, 2184, 1628, 1475, 1449, 1420, 1261, 1205, 1078, 956,
780, 705, 523; HR-MS (FAB+): m/z=615.1832, calcd. for
C26H31ClN3O2Pt [M-ClÀ]+: 615.1854, m/z=580.2164, calcd.
for C26H31N3O2Pt [M-2ClÀ]+: 580.2166.
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ÞÞ
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10 From Isonitriles to Unsaturated NHC Complexes of Gold,
Palladium and Platinum
Adv. Synth. Catal. 2015, 357, 1 – 10
Dominic Riedel, Thomas Wurm, Katharina Graf,
Matthias Rudolph, Frank Rominger,
A. Stephen K. Hashmi*
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