Synthesis and Properties of Hydrazino Amino Acyclic Carbenes of Gold(I), Platinum(II), Palladium(II) and Rhodium(III)

Article abstract of DOI:10.1002/adsc.201600615

The nucleophilic addition of protected and substituted hydrazine derivatives to isonitrile complexes of gold(I), platinum(II), palladium(II) and rhodium(III) provides the corresponding hydrazino amino acyclic carbene complexes. These are characterized by

Full text of DOI:10.1002/adsc.201600615

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DOI: 10.1002/adsc.201600615
Synthesis and Properties of Hydrazino Amino Acyclic Carbenes
of Gold(I), Platinum(II), Palladium(II) and Rhodium(III)
a
a
a
ˇ
Svetlana Tsupova, Matthias Rudolph, Frank Rominger,
and A. Stephen K. Hashmia,b,*
Organisch-Chemisches Institut, Universitꢀt Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax : (+49)-(0)6221-54-4205; phone: (+49)-(0)6221-54-8413; e-mail: hashmi@hashmi.de
a
b
Chemistry Department, Faculty of Science, King Abdulaziz University (KAU), 21589 Jeddah, Saudi Arabia
Received: June 14, 2016; Revised: October 16, 2016; Published online: && &&, 0000
Dedicated to Assoc. Prof. Uno Mꢀeorg.
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600689.
Abstract: The nucleophilic addition of protected and lytic activity in the gold(I)-catalyzed cycloisomeriza-
substituted hydrazine derivatives to isonitrile com- tion of N-propargylcarboxamides to alkylideneoxa-
plexes of gold(I), platinum(II), palladium(II) and zolines is investigated.
rhodium(III) provides the corresponding hydrazino
amino acyclic carbene complexes. These are charac-
terized by their spectroscopic data, four different Keywords: carbene ligands; cyclization; gold; homo-
X-ray single crystal structure analyses and their cata- geneous catalysis; hydrazines
Introduction
metals and therefore deactivate the catalysts and its
potential to reduce metals.[8] Even more rarely, have
Numerous reports on N-heterocyclic carbenes (NHC) hydrazines been used as part of the ligands.[9]
have attracted significant attention, as these com-
pounds perform exceedingly well as ligands in organ-
ometallic chemistry.[1] Besides the remarkable stability
of the obtained NHC complexes, the easy tunability
of their properties has contributed to the success of
this class of ligands. More recently, nitrogen acyclic
carbenes (NAC, also known as acyclic diamino car-
benes, ADC) and hydrogen-bond supported heterocy-
clic carbenes (HBHC) have been used in catalysis,
too.[2] These carbene ligands open up the possibility
to further vary the electronic and steric properties of
the carbene ligands as they do not possess the rigid
cyclic backbone of the NHC ligands.[3] Based on pio-
neering work of Crociani, Bonatti and Minghetti,[4] an
easy and high-yielding synthesis of transition metal-
NAC and transition metal-NHC complexes, which
makes use of the reactivity of coordinated isonitriles
was recently developed (Scheme 1).[5,6]
In contrast to the approaches mentioned above, hy-
drazines have received much less attention as ligands
than they have received as target molecules in synthe-
sis, especially if compared to amines.[7]
Two possible reasons for this trend stand out: the
hydrazineꢁs ability to act as a ligand with transition metal-bound isocyanides.
Scheme 1. Different carbene complexes obtained from
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In spite of the reports of Chugaev, who already in tive and significantly longer reaction times were ob-
1915 published the formation of a bridged carbene served. As shown in Figure 1, the typical conditions
from the reaction of hydrazine and a bisisocyanide used for amines, 0.1 M solution of the isonitrile com-
complex of platinum,[10] only few reports on the use plex in DCM in combination with 4 equiv. of
of hydrazines as part of a ligand can be found. Re- BocNHNH2 reached only 80% conversion after 11
cently, the synthesis of a chiral hydrazino-NHC from days at room temperature.
imidazolium derivatives was reported, however this
When the temperature was raised to 458C, the re-
multistep synthesis with unstable intermediates is not action proceeded more readily, 90% conversion was
very suitable for a systematic synthesis of the metal achieved after 5 days, but the isolated yield was only
complexes.[11] Furthermore, hydrazones, p-nitrophe- 54%. However, in more concentrated solution (0.5
nylhydrazine and acylhydrazine were also reported to M) full conversion and 80% yield of HAAC was ob-
add to palladium isonitrile complexes to yield acyclic tained in only 2.5 days at room temperature.
carbene complexes.[12]
An investigation of the scope by varying the isoni-
While a free persistent amino hydrazino carbene trile precursors in combination with BocNHNH2 re-
has been reported by the Bertrand group,[13] we here vealed that the reaction proceeds equally well with ar-
report on the synthesis of Hydrazino Amino Acyclic omatic and aliphatic isonitrile complexes. Sterically
Carbene (HAAC) complexes of gold, palladium, plat- bulky isonitrile substituents were also tolerated well
inum and rhodium, via the nucleophilic addition of (1a, 1b and 1f vs. 1c, Table 1). All of the obtained
a selection of substituted hydrazines to metal-coordi- HAAC complexes are stable in non-acidic solution,
nated isonitriles. This constitutes the first report of allowing an easy purification by silica gel chromatog-
such a ligand being employed on gold, platinum or raphy. However, they decompose rather rapidly in the
rhodium.
presence of traces of acid (e.g., in CDCl3 solution,
thus the NMR characterization was done in CD2Cl2
or DMSO-d6).
Results and Discussion
Table 1. The scope of isonitrile complexes of gold in the re-
action with BocNHNH2.
Synthesis
As a first test reaction a tert-butylisonitrilegold(I)
complex was reacted with BocNHNH2. In comparison
to aliphatic amines, hydrazines proved to be less reac-
Figure 1. Conversions of tert-butylisonitrilegold(I) chloride
in the reaction with BocNHNH2 in CD2Cl2; mesitylene was
1
used as internal standard (integration in the H NMR spec-
trum).
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Next, we explored the scope with regard to differ- yield was obtained (2d). As a consequence other ß-
ent hydrazine components (Table 2). The reaction of branched substrates were reacted with this isonitrile
1-butyl-2-Boc-hydrazine with tert-butylisonitrilegol- as well (2e and 2f). In the case of benzyl-substituted
d(I) chloride generated the corresponding HAAC hydrazine (entry 5), the low solubility of the obtained
complex 2a in almost quantitative yield. In addition, HAAC resulted in a slight drop in yield. A biphenyl
based on the increased nucleophilicity, the reaction moiety in hydrazine was well tolerated and the result-
was complete within only 2 days with only 2 equiva- ing HAAC was obtained in excellent yield (entry 6).
lents of the hydrazine derivative. A more sterically In conclusion, substituted and carbamate-protected
demanding isopropyl-substituted hydrazine reacted in hydrazines proved to be good staring materials for
a very good yield as well (2b). However, a b-branched the formation of the new HAAC gold(I) complexes,
substrate resulted in an incomplete conversion after 3 by employing their good nucleophilicity without the
days at room temperature (2c, 54% of the starting reduction of the noble metal precursor.
material were recovered, this corresponds to a yield
Recently, it was reported that gold iodides are su-
of 87% based on recovered starting material), imply- perior to chlorides and bromides in cycloaddition re-
ing an undesirable steric interaction of the isonitrile actions of gold isonitrile complexes.[14] Therefore, we
and the hydrazine. The reactivity could be restored by decided to test if the use of tert-butylisonitrilegold(I)
the use of an aryl-isonitrile complex which due to the iodide would accelerate the reaction. However in our
planar sp2-centre next to the isonitrile nitrogen is ster- reactions, the influence of iodide as ligand on the re-
ically less demanding. With this substrate an excellent action outcome was different. For tert-butylisonitrile-
gold iodide, the reaction with BocNHNH2 resulted in
the formation of a golden mirror on the wall of the
Table 2. The scope of hydrazines in the reaction with isoni-
trilegold halides.
reaction vessel, indicating a reduction of the gold
under these conditions. Reaction with isopropyl-sub-
stituted hydrazine gave low conversion and only
traces of HAAC complex and the reaction with n-
butyl-substituted hydrazine gave 67% of the desired
carbene complex (entry 10, Table 2). On the other
hand, when 1a was treated with NaI in DCM, the
slow formation of metallic gold was observed, which
points to an intrinsically lower stability of HAACAuI
complexes in comparison to the corresponding chlo-
ride complexes, which contributes to the less success-
ful reaction outcome.
Subsequently, HAAC complexes of other transition
metal were prepared by taking advantage of the de-
veloped method (Scheme 2). The reactions using di-
chloro-bis-isonitrilepalladium(II) complexes with
BocNHNH2 and CbzNHNH2 resulted in quantitative
yields of the new carbene complexes. Like in the case
of amines as nucleophile,[6a] only one of the isonitrile
ligands reacted with the hydrazine and no further cyc-
lization leading to Chugaev-type biscarbenes was ob-
served. In the case of palladium only 1 equiv. of hy-
drazine was necessary for a quantitative conversion
and the reactions were completed within hours. The
reaction did not tolerate a substitution on the reacting
nitrogen of the hydrazine, probably due to a higher
steric shielding of the metal, and only a minor conver-
sion was detected (in the crude NMR). Palladium
complexes featuring hydrazine- or p-nitrophenylhy-
drazine-based acyclic carbenes as ligands have been
shown to have catalytic activity in Sonogashira cross-
couplings.[15]
The tested platinum(II) complex reacted equally
well, providing its corresponding HAAC-Pt complex
in quantitative yield after 18 hours. The rhodium iso-
nitrile complex Cp*(Ph2CHNC)RhCl2 also reacted
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Scheme 2. Different carbene complexes obtained from
metal-bound isocyanides.
and the corresponding HAAC complex (mixture of 2
rotamers) was obtained in 53% yield after recrystalli-
zation.
Structure
Some of the obtained HAAC complexes could be
studied by X-ray crystallography.[16] The structures of
gold and rhodium complexes are shown in Figure 2
and bond lengths and angles are summarized in
Table 3. Overall, carbene structural parameters are
similar to those reported for NAC gold compounds.[2c]
The carbene ligands are planar (the sum of angles is
359.8–3608) and the C–Au–Cl angle is very close to
linearity. The bulky N-Boc unit is always pointing
away from the carbene part to minimize steric inter-
actions. The gold-carbene bond shows little variation,
Figure 2. Solid state molecular structures of 1a, 1f, 2f and 5.
Table 3. Selected bond lengths and angles from 1a, 1f, 2f
and 5. Averaged values are given for 1a and 1f.
HAAC
1a
1f
2f
5
M–C1
M–Cl
C1–N3
C1–N2
N3–N4
1.983(9)
2.261(3)
1.341(10) 1.34(2)
1.322(11) 1.30(2)
1.98(2)
2.281(7)
1.990(7)
2.2800(18)
1.329(8)
1.327(8)
1.408(7)
2.052(4)
–
1.344(5)
1.315(4)
1.394(4)
114.0(3)
114.1(2)
131.8(3)
–
1.394(9)
1.38(2)
remaining
between
1.98–1.99 ꢃ
(for
known
N2–C1–N3 116.0(8)
117.3(19) 117.3(6)
118.7(16) 121.8(5)
124.0(16) 120.8(5)
NACAuCl: 1.995–2.015 ꢃ). The Au–Cl bond length is
between 2.261–2.281 ꢃ. The N2–C1–N3 angles are
very similar to those reported in gold NAC com-
plexes, N3–C1–Au and N2–C1–Au angles depend on
the steric bulkiness of the substituents on the corre-
sponding nitrogen atoms of the carbene. For instance,
N3–C1–M
N2–C1–M
C1–M–Cl
117.2(6)
126.7(7)
175.7(3)
176.1(7)
177.26(19)
in the case of 2f, the sterics are roughly similar, so are tween BocNHNH and Ph2CHNH groups is even
the angles. With the larger difference in the case of 1a larger, hence the big difference in the bond angles
(BocNHNH is pointing away from the metal and (178). No intramolecular hydrogen bonding between
therefore has less sterical impact than t-BuNH), the the carbonyl group of Boc and carbene NH is detect-
bond angles also differ significantly (7–148). In the ed in the solid state. In the case of 1f, and partially
case of the rhodium HAAC 5, the steric bulkiness be- 1a, it would appear that intermolecular hydrogen
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Table 4. Gold-catalyzed oxazole synthesis. NMR yields,
using mesitylene as internal standard are shown. The
HAAC catalysts were activated with AgPF6 in the presence
of the substrate.
bonding exists in crystals, in particular 1f seems to
have crystallized as hydrogen bonded dimer from
CH2Cl2. 1a has three independent molecules in the
crystal and only two of them exhibit an aurophilic in-
teraction [Au–Au bond 3.2536(5) ꢃ].
In addition, the buried volumes, based on X-ray
structures, were calculated for some complexes.[17]
They are 41.6% and 37.7% for 1a and 1g, and 36.2%
for 2f. The maps for the steric hindrance are shown in
the Supporting Information. All the buried volumes
are very similar. Interestingly, while 2f has the small-
est value, the hindrance provided by the ligand is
more distributed along the perimeter of the coordina-
tion sphere.
Entry
1
Catalyst (amount)
Time
Yield
1a (1 mol%)
3 h
18%
50%
76%
10%
56%
83%
14%
39%
71%
22%
68%
98%
27%
72%
100%
50%
95%
100%
21%
61%
93%
24%
92%
18%
88%
95%
91%
18 h
2 days
3 h
18 h
2 days
3 h
18 h
2 days
3 h
18 h
2 days
3 h
18 h
2 days
3 h
18 h
2 days
3 h
18 h
2 days
3 h
2 days
3 h
2
3
4
5
6
7
1b (1 mol%)
1d (1 mol%)
1e (1 mol%)
2d (1 mol%)
2f (1 mol%)
2a (1 mol%)
Catalysis
As a next step we tested the catalytic properties of
the synthesized gold(I) HAAC complexes (Table 4).
The gold-catalyzed oxazoline synthesis was chosen as
a test reaction, due to its reliability and well-under-
stood mechanism.[18] All tested HAAC catalysts gave
good to excellent yields, and perfect selectivity.
Among the tested catalysts, HAAC 1 without any ad-
ditional substitution at the nitrogen atom, led to
lower reaction rates (only 10–22% of product after 3
hours). But even after longer reaction times no signif-
icant drop in catalytic activity was observed and good
yields could be obtained after 2 days (entries 1–4).
The more bulky catalysts 2d and 2f delivered higher
conversions after 3 hours (entries 5 and 6) and a good
stability was maintained after longer reaction times.[19]
Reducing the catalytic loading of 2a from 1 mol% to
0.5 mol% or even to 0.1 mol% resulted in slight drop
in catalytic activity, although the general conversion
profile looked very similar (entries 7–9). However, it
should be noted that when we tried to run the reac-
tion with the gold catalyst 2a, that was activated with
silver salt prior to the catalysis reaction, only sluggish
8
9
2a (0.5 mol%)
2a (0.1 mol%)
2 days
12 h
24 h
10[a]
11[b]
AuCl3 (5%)
PPh3AuNTf2 (2%)
[a]
From ref.[21]
[b]
From ref.[22]
conversions were obtained. This is probably the con- PPh3AuNTf2 gave 91% yield (entry 11).[22] A more
sequence of reduced stability of cationic gold HAAC exotic system reported previously by our group fea-
complex. When activation with silver is performed in turing a boronated alkyne-gold complex yielded 98%
the absence of substrate, rapid decomposition occurs after 12 hours with a 2 mol% loading.[23] Finally, 10%
and no catalytic activity is observed. Activation in the of a copper salt gave only 73% of product after an
presence of substrate however leads to conversion. overnight reaction.[24] As can be seen from Table 4,
That means that the substrate “stabilizes” the catalyst. the best catalyst reported here with only 1 mol%
With 0.1% loading there is a higher ratio between loading gave 95% NMR yield overnight (Table 4,
substrate and catalyst and therefore stabilization is entry 6). As such, our system is at least as efficient as
more efficient. In the case of a copper salt instead of those reported before.
a silver salt for the activation,[20] no catalytic activity
at all was observed.
As this is a known reaction, the results of our cata- Conclusions
lytic screening can be compared to previous catalytic
systems. The same reaction with 5% AuCl3 gave 95% We have developed an efficient modular synthesis of
yield after 12 hours (entry 10),[21] another publication new HAAC complexes of gold(I), palladium(II), plat-
emanating from our group reported that 2 mol% inum(II) and rhodium(III). These simple and easy re-
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minger, Adv. Synth. Catal. 2010, 352, 1315–1337.
actions between hydrazines and metal-isonitrile com-
plexes proceed under mild conditions which allow the
synthesis of the target compounds without reduction
of the noble metal centres. The developed method
serves an easy access to a novel type of ligand. The
catalytic ability of the obtained complexes was dem-
onstrated by the gold-catalyzed oxazoline synthesis
that could be accomplished in remarkable yields even
at low catalytic loadings.
[3] M. S. Collins, E. L. Rosen, V. M. Lynch, C. W. Bielaw-
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Experimental Section
General Procedure for the Synthesis of HAAC
Complexes of Gold (GP)
Isonitrilegold chloride (0.1 mmol) was dissolved in DCM
(0.2 mL) and hydrazine (0.4 mmol of monosubstituted hy-
drazine or 0.2 mmol of disubstituted hydrazine) was added.
The obtained slurry/solution was stirred until the starting
gold compound was consumed (TLC). Then, the reaction
mixture was directly transferred onto the column and the
HAAC complex was isolated by column chromatography on
silica gel using mixtures of dichloromethane and ethyl ace-
tate as an eluent.
{[2-(tert-Butoxycarbonyl)hydrazinyl](tert-butylamino)me-
thylene}gold(I) chloride (1a): According to the GP, the reac-
tion was run using 33.5 mg (0.1 mmol) of t-butylisonitrile-
gold chloride and 52.8 mg (0.4 mmol) of BocNHNH2 for 4
days and compound 1a was purified using DCM:ethyl ace-
tate 20:1 as eluent; yield: 39 mg (82%); white solid; mp
109.88C (dec.); Rf (DCM:ethyl acetate 20:1)=0.18.
1H NMR (400 MHz, CD2Cl2): d=7.55 (s, 1H, BocNH), 7.36
(br, 1H, NH), 6.06 (br, 1H, NH), 1.59 [s, 9H, C(CH3)3], 1.49
(s, 9H, Boc); 13C {1H} NMR (101 MHz, CD2Cl2): d=189.97
(s), 154.30 (s), 84.12 (s), 54.70 (s), 31.40 (q), 28.36 (q); FT-
IR (ATR): n=3166, 3139, 2966, 1873, 1596, 1470, 1400,
1386, 1190, 1135, 1058, 966, 840, 804, 757, 610 cmÀ1; HR-MS
(FAB+): m/z=412.2374, calcd. for [MÀCl]: 412.2592.
[7] a) U. Ragnarsson, Chem. Soc. Rev. 2001, 3, 205–220;
ˇ
b) S. Tsupova, U. Mꢀeorg, Heterocycles 2014, 88, 129–
173.
Acknowledgements
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Comments Inorg. Chem. 2008, 29, 46–72; c) L. M.
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arskiy, K. V. Luzyanin, V. Yu. Kukushkin, Coord.
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ˇ
S. Tsupova is grateful for a fellowship of the Fonds der
Chemischen Industrie. Noble metal salts were donated by
Umicore AG & Co. KG.
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Adv. Synth. Catal. 0000, 000, 0 – 0
7
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!

FULL PAPERS
8 Synthesis and Properties of Hydrazino Amino Acyclic
Carbenes of Gold(I), Platinum(II), Palladium(II) and
Rhodium(III)
Adv. Synth. Catal. 2016, 358, 1 – 8
ˇ
Svetlana Tsupova, Matthias Rudolph, Frank Rominger,
A. Stephen K. Hashmi*
Adv. Synth. Catal. 0000, 000, 0 – 0
8
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!