Room-temperature hydrohydrazination of terminal alkynes catalyzed by saturated abnormal n-heterocyclic carbene-gold(I) complexes

Article abstract of DOI:10.1002/chem.201402560

A number of saturated abnormal N-heterocyclic carbene (NHC) complexes of gold, in combination with KBAr^F₄ as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at r

Full text of DOI:10.1002/chem.201402560

DOI: 10.1002/chem.201402560
Communication
&
Homogeneous Catalysis
Room-Temperature Hydrohydrazination of Terminal Alkynes
Catalyzed by Saturated Abnormal N-Heterocyclic Carbene–Gold(I)
Complexes
Rubꢀn Manzano,[a] Thomas Wurm,[b] Frank Rominger,[b] and A. Stephen K. Hashmi*[a, b, c]
Chem. Eur. J. 2014, 20, 6844 – 6848
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Communication
um-, zinc-, and palladium-catalyzed synthesis of indoles from
arylhydrazines and alkynes. The rhodium-, iridium-, and titani-
um-catalyzed hydroamination of alkynes with substituted hy-
drazines was shown to give hydrazones with Markovnikov re-
gioselectivity.[10] An interesting study by Mountford and co-
workers reports the titanium-catalyzed anti-Markovnikov addi-
tion of diphenylhydrazine to terminal alkynes.[11] Similarly, the
gold-catalyzed reaction of alkynes[12] or allenes[13] with substi-
tuted hydrazines has been reported. Recently, a metal-free hy-
drohydrazination of alkynes has been developed for the syn-
thesis of azomethine imines.[14]
Abstract: A number of saturated abnormal N-heterocyclic
carbene (NHC) complexes of gold, in combination with
KBArF as activator, were successfully applied in the che-
4
moselective addition of hydrazine to alkynes. The reaction
proceeds even at room temperature, which was not possi-
ble to date with gold catalysts. The reaction can be ap-
plied to a number of substituted arylalkynes. With alkylal-
kynes the yields are low. The saturated abnormal NHC lig-
ands are resistant to isomerization to the saturated
normal NHC coordination mode under basic reaction con-
ditions. Under acidic conditions, a simple protonation at
the nitrogen atom not neighboring the carbene center
was observed and unambiguously characterized by an
X-ray crystal-structure analysis. Computational studies con-
firm that such an isomerization would be highly exother-
mic, the observed kinetic stability probably results from
the need to shift two protons in such a process.
In contrast, the use of unsubstituted hydrazine has remained
relatively unexplored. One of the few examples is the palladi-
um-catalyzed direct CÀN cross-coupling with hydrazine report-
ed by Lundgren and Stradiotto[15] in 2010 and by Buchwald
and co-workers[16] in 2013 for a continuous flow system. Chen
et al. reported the analogous copper-catalyzed cross-coupling
of aryl halides with hydrazine.[17] A palladium-catalyzed multi-
component synthesis of aryl hydrazines from hydrazine hy-
drate has also been developed.[18]
Hydrazine (N2H4) is currently manufactured on large scale for
important industrial applications, such as the production of
polymerization initiators, blowing agents for foamed plastics,
pesticides and pharmaceuticals, as well as a rocket fuel and
propellant for satellites and aircraft in its anhydrous form.[1]
However, the use of hydrazine as building block for the forma-
tion of carbon–nitrogen bonds in homogeneous metal cataly-
sis is uncommon, probably due to 1) its strong reducing char-
acter, which can lead to reduction of either the metal complex
to inactive metal(0) species (especially, in the case of noble,
easily reducible transition metals) or reduction of the organic
compounds; and 2) its ability to deactivate catalysts by forma-
tion of inert hydrazine–metal complexes.[2]
Bertrand and co-workers have reported a cyclic alkyl amino
carbenes (CAAC)–gold(I) complex (Figure 1, I) and an anti-
Bredt NHC–gold(I) complex (Figure 1, II) for the catalyzed reac-
Figure 1. Gold(I) catalysts for hydrazination of alkynes.
Substituted hydrazines have been extensively employed as
nucleophiles in catalytic processes, such as the Pd- and Cu-cat-
alyzed cross-coupling reaction of aryl halides with substituted
hydrazines,[3] as well as the hydrohydrazination of alkenes[4]
and carbodiimides.[5] Many efforts have also been devoted to
the use of alkynes as electrophilic component in the addition
of hydrazines. In this context, Fukumoto et al. reported a ruthe-
nium-catalyzed reaction between alkynes and 1,1-disubstituted
hydrazines, which leads to isocyanides in an anti-Markovnikov
addition.[6] Ackermann and co-workers,[7] Beller and co-work-
ers,[8] and Linderschmidt and co-workers[9] reported the titani-
tion between alkynes and anhydrous hydrazine.[19] The former
(I) was able to catalyze the addition of hydrazine to a few ter-
minal and internal alkynes as well as diynes at high tempera-
tures (1008C),[19a] the latter (II) catalyzed the hydrazination of
terminal alkylalkynes at room temperature, although terminal
arylalkynes or internal alkynes still required high temperatur-
es.[19b] The anti-Bredt NHC complex (II) is more similar to
CAACs than to classical NHCs in terms of its electronic proper-
ties, because one nitrogen adjacent to the carbene atom is
placed in a strained bridgehead position, thus being restricted
from donating its lone pair.
[a] Dr. R. Manzano, Prof. Dr. A. S. K. Hashmi
CaRLa - Catalysis Research Laboratory
In the context of providing easy access to a library of NHC–
gold(I) complexes starting from isocyanides,[20] we recently
published the synthesis of saturated abnormal NHC (saNHC)
gold(I) complexes (Figure 1, III) which can contain a variety of
substituents at both nitrogen atoms and at the a position of
the carbene atom.[21] With regard to the electronic situation,
these complexes very much resemble the successful CAAC
complexes. Both types of species, which possess only one ni-
trogen atom close to the carbene atom, are regarded as NHCs
with reduced heteroatom stabilization. Considering that this
type of complexes behave as good catalysts in the Hashmi
Im Neuenheimer Feld 584, 69120 Heidelberg (Germany)
[b] T. Wurm, F. Rominger, Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-54-4205
E-mail: hashmi@hashmi.de
[c] Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University, Jeddah 21589 (Saudi Arabia)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402560.
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Communication
phenol synthesis[21a] and taking
Table 1. Optimization of the reaction conditions.
into account the success of Ber-
trand’s catalysts in the gold(I)-
catalyzed hydrazination of al-
kynes,[19] we decided to explore
the catalytic activity of the
saNHC–AuI complexes for the
challenging addition of hydra-
zine to alkynes.
To determine the optimal re-
action conditions, phenylacety-
Entry[a]
Activator
[Au]
Solvent
T [8C]
Conversion [%]
Yield[b] [%]
lene (1a) was chosen as model
substrate. The stabilized hydra-
1
2
3
4
5
6
7
8c
9[c]
10
11
12
13
14[d]
15
16
AgNTf2
AgBF4
AgPF6
AgSbF6
4a
4a
4a
4a
4a
4a
4b
4c
4d
4e
4a
4a
4a
4a
4a
4a
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
[D8]toluene
CD2Cl2
THF
C6D6
C6D6
C6D6
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
5
n.r.
n.r.
n.r.
n.r.
75
95
25
92
63
12
89
76
n.r.
90
79
50
–
–
–
–
64
92
22
65
55
8
70
63
–
zine monohydrate (2) was em-
ployed, instead of anhydrous hy-
NaBArF
drazine, due to commercial avail-
4
ability and safety reasons. The
KBArF
4
KBArF
4
generation of the catalytically
KBArF
4
active cationic gold(I) species
KBArF
4
KBArF
was performed by in situ treat-
4
ment of halo–gold(I) complexes
KBArF
4
KBArF
4
with appropriate halide abstrac-
KBArF
4
tors in an appropriate solvent.
KBArF
74
72
35
4
KBArF
50
80
Our preliminary experiments
4
showed that significantly better
KBArF
4
yields were obtained in properly
[a] Reaction conditions: 0.2 mmol alkyne 1, 0.2 mmol hydrazine hydrate, 5 mol% gold(I) complex 4, 5 mol%
stirred reaction mixtures than in
activator, 0.6 mL solvent, 4 h. [b] Yield, determined by NMR spectroscopy, was determined with reference
to internal standard 1,4-di-tert-butylbenzene. [c] Reaction time was 18 h. [d] Reaction time was 24 h (BArF
tetrakis-(3,5-bis(trifluoromethyl)phenyl)borate). n.r.=no reaction.
:
4
NMR tubes, so all experiments
were run with magnetic stirring.
A series of experiments was car-
ried out and the results obtained
are shown in Table 1.
tions: after low conversions, a grey metal mirror was observed
(entry 10).
Silver salts with weakly or non-coordinating anions are
broadly used for the in situ generation of gold(I) catalytic spe-
cies. However, this strategy was incompatible with hydrazine,
because no conversion of alkyne 1a was observed after several
hours (Table 1, entries 1–4). Independent of the counterion,
black metal particles appeared in the reaction mixtures. Grati-
fyingly, appreciable reactivity was observed when tetrakis-(3,5-
The use of toluene and dichloromethane as solvents was tol-
erated (Table 1, entries 11–13), but yields and conversions were
significantly lower than those obtained in benzene. Tetrahydro-
furan was not compatible, because no conversion of alkyne
was observed (entry 13).
A short study on the reaction temperature demonstrated
that ambient temperature was optimal. When the reaction was
performed at 58C, a high conversion was achieved (90%,
Table 1, entry 14), but lower yield was obtained due to the for-
mation of the undesired azine. Higher temperatures gave the
product in lower yields, and especially detrimental was a tem-
perature of 808C, which drastically reduced the conversion and
the yield (entries 15 and 16). A reduction of the catalyst load-
ing to 1 mol% led to very low conversion rate, therefore,
5 mol% was chosen as optimal.
bis(trifluoromethyl)phenyl)borate (BArF ) salts were employed
4
for the halide abstraction from gold(I) complex 4a (entries 5–
6). Especially effective was the potassium salt (entry 6), which
allowed 95% conversion of phenylacetylene 1a and 92% yield
of the corresponding hydrazone (note that this transformation
is not catalyzed by Bertrand’s anti-Bredt-NHC complex[19b] at
room temperature, although 87% yield was obtained at 908C).
For the determination of the optimal catalyst, gold(I) com-
plexes 4b–d were also tested (Table 1, entries 7–9). The impact
of the substituent close to the carbene atom in the abnormal
NHC–gold(I) complexes (4a and b) on reactivity was clear. In
contrast to the methyl-substituted gold(I) complex 4a, the
nonsubstituted complex 4b led to very low conversion (com-
pare entries 6 and 7). NHC–gold(I) complexes 4c and d were
also able to promote the transformation (Table 1, entries 8 and
9), although at lower rates, which facilitated the formation of
the undesired azine (acetophenone azine).[22] The simple, stan-
dard Ph3P–gold(I) complex 4e was unstable under these condi-
Thus, our standard conditions involve the use of 5 mol% of
gold(I) complex 4a, 5 mol% of KBArF , benzene as solvent, and
4
room temperature (Table 1, entry 6). With these conditions in
hand, we decided to explore the scope of the reaction. To this
aim, a series of differently substituted aryl- and alkylalkynes
were employed under our standard conditions, and the results
are summarized in Table 2.
Phenylacetylene 1a reacted with hydrazine hydrate in the
presence of our catalytic system under optimal reaction condi-
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Table 2. Scope of the hydrazination reaction.
Entry[a]
R
t [h]
Yield[b] [%]
1
2
3
4
5
6
7
8
9
C6H5 (1a)
4
4
4
7
7
7
24
4
4
24
90 (3a)
91 (3b)
82 (3c)
63 (3d)
71 (3e)
81 (3 f)
31 (3g)
73[c] (3h)
51[c] (3i)
n.r.[d]
4-MeO-C6H4 (1b)
4-Me-C6H4 (1c)
4-Br-C6H4 (1d)
4-CF3-C6H4 (1e)
2-Me-C6H4 (1 f)
2-CF3-C6H4 (1g)
Bn (1h)
nBu (1i)
tBu (1j)
10
[a] Reaction conditions: 0.5 mmol alkyne 1, 0.5 mmol hydrazine hydrate,
Figure 2. Solid-state molecular structure of 5.
5 mol% gold(I) complex 4a, 5 mol% KBArF , 1.0 mL benzene, 208C.
4
[b] Isolated yield after purification through silica gel. [c] Yield, determined
by NMR spectroscopy, with reference to internal standard 1,4-di-tert-bu-
tylbenzene. [d] n.r.=no reaction.
two BF4 anions and a molecule of water. Efforts to alkylate this
nitrogen atom with iodomethane failed.
Despite these observations there should be a strong ther-
modynamic driving force for an isomerization of a saNHC to
a NHC ligand. Therefore we conducted a computational study
on the model isomerization reaction: due to the small substitu-
ents on the nitrogen atoms, mainly the electronic factors were
considered, and the results are listed in Table 3.
tions in 4 h, and acetophenone hydrazone 3a was isolated in
90% yield, obtained as an average of two runs (Table 2,
entry 1). Our catalytic system was also active for electron-rich
phenylacetylenes 1b and c, 91 and 82% isolated yields of the
corresponding hydrazones were obtained (Table 2, entries 2
and 3).[23] Slightly longer reaction times (7 h) were required for
electron-poor phenylacetylenes 1d and e, the corresponding
hydrazones were isolated in 63 and 71% yield, respectively
(Table 2, entries 4 and 5). Ortho-substituted phenylacetylene 1 f
was tolerated, and no appreciable difference in reactivity was
observed in comparison with the para-substituted analog (81
vs. 82% yield, compare entries 3 and 6 in Table 2), although
a slightly longer reaction time was required. Even the ortho-
substituted electron-poor phenylacetylene 1g showed some
reactivity, and 31% yield of the corresponding hydrazone was
isolated (entry 7). Then we diverted our attention to the hydra-
zination of alkylalkynes. Although the very bulky 3,3-dimethyl-
but-1-yne (1j) did not react under standard conditions after
24 h (Table 2, entry 10), the alkylalkynes 1h and i gave the cor-
responding hydrazones in 73 and 51% yield (Table 2, entries 8
and 9). The internal alkynes diphenylacetylene (1k) and 1-phe-
nylpropyne (1l) remained untransformed after 48 h under our
standard conditions.
Table 3. Thermochemical results [kJmolÀ1] of the isomerization reactions
abnormal NHCs and abnormal NHC–AuI complexes.
Computational method
DE[a]
DAuClE[a]
DAuBrE[a]
DAuIE[a]
HF, cc-pVTZ-pp
À83.0
À113
À112
À112
À78.5
À80.5
À81.2
À81.1
À78.9
À81.0
À81.7
À81.5
À79.4
À81.7
À82.3
À82.0
B3LYP, cc-pVTZ-pp
B2PLYP, cc-pVTZ-pp
MP2, cc-pVTZ-pp
[a] Energy in kJmolÀ1
.
All of these computational results clearly indicate that the
synthesized abnormal NHC–AuI complexes are, if steric effects
are neglected, thermodynamically instable (by about
À80 kJmolÀ1) towards the isomerization to the corresponding
normal NHC complexes. Therefore, we assume that in our
system, in contrast to literature known systems,[25] it is required
to overcome a high energetic barrier, which probably is
a result of the necessity to formally shift two hydrogen atoms
to perform this isomerization. Furthermore, the theoretical re-
sults, especially, the results obtained by theoretical methods,
which include electron-correlation effects (B3LYP, B2PLYP, MP2),
showed that the isomerization process for the free carbenes
would be more endothermic (ꢀ30 kJmolÀ1) than the reaction
In the context of the basic conditions of the reaction mix-
ture, the question whether the saNHC ligand in situ isomerizes
to the saturated normal NHC becomes relevant. Efforts to
induce such a rearrangement of 4a by a number of different
conditions (10 mol% KOtBu up to 758C in benzene, 1 equiv
HBF4·Et2O gave the N-protonated saNHC 5) were unsuccessful.
For compound 5, in the 1H NMR a downfield shift was ob-
served for all protons close to the remote nitrogen, which is in
accord with a protonation on that nitrogen atom. Compound
5 finally could be characterized by an X-ray crystal structure
analysis and with triethylamine sets free the saNHC again
(Figure 2).[24] The unit cell contains two cationic gold entities,
Chem. Eur. J. 2014, 20, 6844 – 6848
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for the corresponding AuI complexes. This implies that the in-
teractions between a gold(I)–halogen fragment and an saNHC
are stronger than between the same gold fragment and a satu-
rated normal NHC. Therefore, one can say that the electronic
properties of saNHCs better fit the electronic demands of
a gold(I) halogen fragment than the electronic properties of sa-
turated normal NHCs. In addition, the trend is nearly independ-
ent of the halide ligand bound on the other side of the gold
center, by looking at the results obtained with the same quan-
tum chemical method one observes only a small gain in
energy (ꢀ0.5 kJmolÀ1) by changing to the heavier halogens.
These findings, combined with the observed kinetic stability of
the saNHC ligands, certainly will give new impulses to the de-
velopment of new gold catalysts in the future.
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saNHC-gold(I) complexes allow to lower the temperature for
the addition of hydrazine to aryl- and alkylalkynes from 1008C
to room temperature. These mild conditions are beneficial for
applications in synthesis.
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Acknowledgements
R.M. worked at CaRLa of Heidelberg University, co-financed by
University of Heidelberg, the State of Baden-Wꢂrttemberg, and
BASF SE.
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[23] The formation of undesired azine in the case of electron-rich phenyla-
cetylenes is minimized by short reaction times and temperatures below
208C.
Keywords: alkynes · carbene ligands · density functional
calculations · gold · hydrazones
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Received: March 14, 2014
Published online on April 25, 2014
Chem. Eur. J. 2014, 20, 6844 – 6848
www.chemeurj.org
6848
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