Hydrohydrazination of Arylalkynes Catalyzed by an Expanded Ring N-Heterocyclic Carbene (er-NHC) Gold Complex under Solvent-Free Conditions

Article abstract of DOI:10.1002/adsc.201500658

[(THD-Dipp)AuOTf], supported by the strongly electron donating, sterically bulky THD-Dipp (1,3-bis(2,6-diisopropylphenyl)hexahydro-2H-1,3-diazepine-2-ylidene) seven-membered N-heterocyclic carbene ligand, efficiently promotes intermolecular addition of Ts- and Boc-hydrazine to arylalkynes under solvent-free conditions.

Full text of DOI:10.1002/adsc.201500658

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DOI: 10.1002/adsc.201500658
Hydrohydrazination of Arylalkynes Catalyzed by an Expanded
Ring N-Heterocyclic Carbene (er-NHC) Gold Complex Under
Solvent-Free Conditions
Oleg S. Morozov,a Pavel S. Gribanov,a Andrey F. Asachenko,a,b
Pavel V. Dorovatovskii,c Victor N. Khrustalev,d,e Viktor B. Rybakov,f
and Mikhail S. Nechaeva,b,f,
A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Prospect 29,
Moscow,119991, Russia
*
a
E-mail: m.nechaev@ips.ac.ru or m.s.nechaev@org.chem.msu.ru
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow,119991,
Russia
NRC Kurchatov Institute, Acad. Kurchatov Square 1, Moscow,123182, Russia
Department of Inorganic Chemistry, Peoplesꢀ Friendship University of Russia, Miklukho–Maklay Street 6,
Moscow,117198, Russia
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,Vavilov Street 28, Moscow
119991, Russia
M. V. Lomonosov Moscow State University, Leninskie Gory 1 (3), Moscow,119991, Russia
b
c
d
e
f
Received: July 8, 2015; Revised: February 10, 2016; Published online: March 1, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500658.
Abstract: [(THD-Dipp)AuOTf], supported by the lecular addition of Ts- and Boc-hydrazine to arylal-
strongly electron donating, sterically bulky THD- kynes under solvent-free conditions.
Dipp (1,3-bis(2,6-diisopropylphenyl)hexahydro-2H-
1,3-diazepine-2-ylidene) seven-membered N-hetero- Keywords: alkynes; gold; hydrazine; hydroamina-
cyclic carbene ligand, efficiently promotes intermo- tion; N-heterocyclic carbenes
À
Introduction
cross-coupling reactions via selective C H bond acti-
vation.[9]
Hydrazone derivatives are of great interest for syn-
The most general method for the synthesis of hy-
thetic organic chemistry as important building drazones is the reaction of hydrazines with carbonyl
blocks.[1] Hydrazone moieties can be found in many compounds.[10] This method is inapplicable for synthe-
compounds of biological interest.[2]
sis of hydrazines bearing carbonyl or other electro-
Aryl-, N-tosyl-, and N-Boc-hydrazones have found philic groups due to high nucleophilicity of both nitro-
numerous applications in various organic transforma- gen atoms in hydrazine. Therefore, it is highly desira-
tions. Thus, aryl hydrazones are precursors for indoles ble to develop other atom-economical protocols that
obtained via hydrazone rearrangement in Fischer involves fewer synthetic steps and readily available
indole synthesis.[3] N-Tosylhydrazones are useful pre- starting materials for the preparation of hydrazones.
cursors for synthesis of alkenes via base-induced
The alternative route for the synthesis of hydra-
Bamford-Stevens[4] and Shapiro[5] reactions. Recent zones is catalytic hydroamination of alkynes by hydra-
reports are devoted to the use of N-tosylhydrazones zine derivatives.[11] Gold(I) species are known to be
as safe precursors for diazo compounds in transition- efficient catalysts of inter- and intramolecular hydroa-
metal-catalyzed cross-coupling reactions.[6] A metal- mination of alkynes.[12] Bertrand et al.[13] have shown
À
free C C bond-forming reaction between tosylhydra- that gold(I) complexes featuring bulky cyclic (alkyl)-
zones and boronic acids was recently reported.[7] N- (amino)carbene (CAAC)[14] readily catalyze the addi-
Boc-hydrazones have found applications as building tion of H2NNH2 to a variety of non-activated alkynes
blocks in heterocyclic synthesis[8] and as a directing and allenes to afford a diverse array of acyclic and
groups in transition-metal-catalyzed dehydrogenative cyclic nitrogen-containing compounds. Recently, Ber-
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Oleg S. Morozov et al.
trandꢀs[15] and Hashmiꢀs[16] groups have shown that
anti-Bredt NHC and saturated abnormal N-heterocy-
clic carbene (saNHC) gold(I) complexes promote the
addition of hydrazine to terminal alkynes under mild
conditions. Mechanisms of catalytic reactions were ra-
tionalized using DFT calculations.[17]
To date only a few examples of gold catalyzed hy-
drohydrazination of non-activated alkynes by phenyl-
hydrazine (two examples),[18] and tosylhydrazide[19]
have been reported. No examples of hydrohydrazina-
tion with tert-butyl carbazate (Boc-hydrazide) have
been reported. Herein, we present a simple and versa-
tile method for highly selective addition of nucleo-
philically deactivated hydrazine derivatives to non-ac-
tivated arylalkynes. The catalytic reaction is mediated
by a NHC supported gold complex under solvent-free
conditions.
Figure 1. Molecular structure of (7-Dipp)Au(OTf) (1) ob-
tained using a synchrotron (left) and a diffractometer (right)
experiments. Anisotropic displacement ellipsoids are drawn
at the 30% and 20%, respectively, for the structures left
and right. An alternative position of the disordered triflate
group with the lesser occupancy is depicted by dashed lines.
Hydrogen atoms are omitted for clarity.
Results and Discussion
Er-NHCs surpass five-membered ring counterparts in
donor properties and steric stabilization of metal
complexes.[20] We have previously shown[21] that uti-
lization of the seven-membered-ring carbene bearing
bulky Dipp groups in combination with weakly coor-
In the preliminary set of experiments we have
dinating anions such as XÀ =OTfÀ or BF4 in [(THD- found that [(THD-Dipp)AuCl] activated by AgOTf
Dipp)AuX] (THD-Dipp=1,3-bis(2,6-diisopropylphe- efficiently promotes addition of various hydrazines to
nyl)hexahydro-2H-1,3-diazepine-2-ylidene) results in phenylacetylene (Table 1, entries 1–5). It is worth
extremely active catalytic systems for intramolecular noting that in the case of hydrazine hydrate only
hydroamination of 2-ethynylanilines leading to in- azine was formed, no hydrazone formation was ob-
doles under unprecedentedly mild conditions. This served. In all remaining cases, (E)-N-substituted hy-
finding prompted us to investigate catalytic activity of drazones were the major products. The reaction
such complexes in intermolecular hydrohydrazination proved to be highly sensitive to the choice of a solvent
À
of alkynes.
(entries 5–11) and the nature of weakly coordinating
We were able to obtain X-ray quality single crystals anion (entries 11–14). Non-activated complex exhibit
of [(THD-Dipp)Au(OTf)] (1) (Figure 1).[22] Two ex- virtually no activity (entry 15). The highest yield in
periments were made using a synchrotron and a more the reaction of p-toluenesulfonylhydrazide with phe-
conventional single crystal diffractometer (for details nylacetylene was obtained when preformed complex
see SI). Interestingly, the experiment using a diffrac- [(THD-Dipp)AuOTf] was used as
a
catalyst
tometer gave a more ordered structure (R1 =0.0639 vs (entry 16).
0.0755) than the synchrotron experiment. We suppose
To determine the scope of the developed catalytic
that it was due to different quality of crystals obtained protocol we studied the addition of challenging sub-
under different conditions. Further, we will discuss strates such as p-toluenesulfonyl hydrazide and tert-
the structure with a better R-factor. As expected, butyl carbazate to various aryl acetylenes (Table 2).
1 adopts a linear geometry with an O1-Au1-C1 bond Alkynes bearing electron-donating groups on benzene
angle of 176.2(3)8. The Au1-O1 bond length of ring show moderately high activity with both hydra-
2.112(9) Š shows a covalent bonding character be- zine derivatives (entries 1, 2, 3, 5, 7). In sharp con-
tween the gold and oxygen atoms. The Au1-C1 bond trast, alkynes bearing electron-withdrawing groups,
length is 1.936(8) Š. Au1-O1 and Au1-C2 distances even as weak as Cl, give products in moderate yields
lie in a range determined for related NHC gold com- (entries 6, 9). Sterically hindered alkynes give prod-
plexes containing oxygen-coordinated ligands (the ucts in low yields (entries 4, 8). In the case of 2-ethyn-
ranges of 2.019(7)–2.078(6) and 1.935(6)–2.006(17) Š, yl-1,3,5-trimethylbenzene 2j, no formation of the
respectively).[23] The angle about the oxygen O1 atom products was observed (entry 10). It should be noted
(131.7(4)8) is significantly distorted from the idealized that in all cases hydrazones were obtained as (E)-iso-
value of 109.58, likely due to the combined steric rea- mers, and no formation of anti-Markovnikov addition
sons of the NHC and triflate ligands.
products was observed.
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Hydrohydrazination of Arylalkynes
Table 1. Hydroamination of phenylacetylene.
Optimization of solvent-free reaction conditions have
been performed for hydrohydrazination of phenylace-
tylene with p-toluenesulfonyl hydrazide as a model
system (for details see SI). Optimized conditions are
the following: melting together the equimolar amount
of an alkyne and a hydrazine derivative in presence
of 0.25 mol% of [(THD-Dipp)AuOTf] at 1008C
under stirring. The reaction mixture appeared as
a clear melt that slowly solidified. Solidification indi-
cates the formation of the desired hydrazone.
Addition of p-toluenesulfonylhydrazide and tert-
butyl carbazate to various alkynes 1a–n was studied
(Table 3). In all cases, high or near quantitative yields
were obtained. Both TsNHNH2 and BocNHNH2
readily undergo addition to arylalkynes bearing
donor, acceptor, as well as sterically bulky substitu-
ents. Products of addition of hydrazines to diphenyla-
cetylene (2m, 3m) and octyne-1 (2n, 3n) were ob-
tained in high yields as well. Thus, the solvent-free
protocol is far more efficient than the solvent-mediat-
ed reaction. The advantages are: i. lower catalyst
loading (0.25 mol% vs 2.0 mol%), ii. higher yields, iii.
shorter reaction time, and iv. wider scope of arylal-
kyne substrates.
Entry RNHNH2
Solvent
Additive Yield[a]
1
2
3
N2H4*H2O
PhNHNH2
2,4-di-NO2-
PhNHNH2
BocNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
EtOH
EtOH
EtOH
AgOTf 97%[b]
AgOTf 98%
AgOTf 90%
4
5
6
7
8
9
EtOH
EtOH
Toluene
MeOH
THF
AgOTf 79%
AgOTf 83%
AgOTf 67%
AgOTf 70%
AgOTf 18%
AgOTf 12%
1,4-diox-
ane
10
11
12
13
14
15
16
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
TsNHNH2
MeCN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
AgOTf 57%
AgOTf 85%
AgBF4
80%
AgNTf2 35%
NaBArF 60%
4
–
–
<2%
87%[c]
[a]
[b]
Isolated yield.
We studied the influence of catalyst loading on the
Formation of (1E,2E)-1,2-bis(1-phenylethylidene)hydra-
zine was observed.
[(THD–Dipp)AuOTf] used as a catalyst.
ꢀ
yield of the reaction of 4-MeO-C6H4-C CH with R-
NH-NH2 (R=Ph, Ts, Boc; for details see SI). Experi-
ments were set at catalyst concentrations 0.25, 0.1,
and 0.01 mol%. At 0.01 mol% catalyst loading the
obtained yields were very low. Although values of
TONs are high (>1000), these numbers are meaning-
less since the reactions were far from completion. At
0.1 mol% catalyst loading the obtained yields were
high: 98%, 88%, and 93%, for R=Ph, Ts, and Boc,
respectively. The respective calculated TONs were
980, 880, and 930. Such high values far exceed TONs
reported previously for hydrohydrazination of aryl
acetylenes.[18a,19]
[c]
Table 2. Catalytic hydrohydrazination of terminal alkynes.[a]
Entry
Aryl
Isolated yield [%]
TsNHNH2
BocNHNH2
We have also found that for some substrates, reac-
tions catalyzed by [(THD-Dipp)AuOTf] under sol-
vent-free conditions do not stop at the formation of
hydrazones. Thus, interaction of phenylacetylene with
ethyl hydrazinecarboxylate leads to the formation of
(E,E)-bis-(1-phenylethylidene)-hydrazine in 90%
yield (Scheme 1a). In the case where a carboethoxy
moiety is attached to an alkyne, 3H-pyrazol-3-ones
are formed in high yields (Scheme 1b).
1
2
3
4
5
6
7
8
9
10
C6H5
87 (2a)
76 (2b)
80 (2c)
46 (2d)
62 (2e)
52 (2 f)
70 (2g)
47 (2h)
41 (2i)
0 (2j)
91 (3a)
90 (3b)
80 (3c)
23 (3d)
76 (3e)
55 (3 f)
68 (3g)
27 (3h)
58 (3i)
0 (3j)
4-Me-C6H4
4-MeO-C6H4
2-MeO-C6H4
4-PhO-C6H4
4-Cl-C6H4
4-Me2N-C6H4
naphthyl
4-MeOOC-C6H4
2,4,6-triMe-C6H2
[a]
Reaction conditions: 3 mmol of alkyne, 3 mmol of hydra-
zine derivative in 5 mL of CHCl3, 2 mol% [(THD-Dip-
p)AuOTf], 1–6 h.
Conclusions
In conclusion, [(THD-Dipp)AuOTf] promotes the ad-
dition of hydrazines to non-activated alkynes leading
Development of solvent-free synthetic protocols is to the formation of hydrazones with perfect Markov-
of great interest. Utilization of solvent-free technolo- nikov regioselectivity in high yields. We have devel-
gies lead to more simple, productive, energy- and oped a simple and practical protocol for the prepara-
cost-effective, clean and safe chemical processes.[24] tion of tosyl- and Boc-protected hydrazones under
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Oleg S. Morozov et al.
Table 3. Catalytic hydrohydrazination of alkynes under solvent-free conditions.[a]
[a]
Isolated yields
Mixture of (E)- and (Z)-isomers formed
[b]
Experimental Section
General procedure for Table 1
Under aerobic condition 26.0 mg (0.04 mmol) of [(THD-
Dipp)AuCl] was dissolved in 5 mL of appropriate solvent
(see Table 1). Then 0.04 mmol of additive was added and
the mixture was stirred for 5 min. To this solution phenyl-
acetylene (204 mg, 2 mmol) and the hydrazine derivative
(2 mmol) were added and the mixture was refluxed for 2 h.
After cooling to room temperature, the solution was passed
though thin pad of Silica gel to remove gold species, eluted
with dichloromethane and evaporated under reduced pres-
sure. The crude product was purified by column chromatog-
raphy (eluent EtOAc/hexane=10:1, Silica gel 43–60 mm).
Scheme 1. Reactions promoted by [(THD-Dipp)AuOTf].
solvent-free conditions without the need for an inert
atmosphere. In addition, our method is in close agree-
ment with the concept of “green” chemistry: (i) reac-
tion proceeds with high atom economy (E factor=
0.02-0.27),[24] (ii) non-toxic derivatives of hydrazine
are used, (iii) no solvent is used. We believe that our
findings might lead to broader use of gold-mediated
reactions in laboratory practice and in the develop-
ment of “green” industrial technologies.
General procedure for hydroamination of alkynes in
chloroform (Method A)
Under aerobic conditions to a solution of 3 mmol of alkyne,
3 mmol of hydrazine derivative in 5 mL of CHCl3 45.9 mg
(2 mol%) of [(THD-Dipp)AuOTf] was added. The solution
was then heated to reflux and stirred until reaction was
complete by TLC (eluent EtOAc/hexane=7:3, 1–6 h). After
cooling to room temperature, the solution was passed
though thin pad of Silica gel to remove gold species, eluted
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Hydrohydrazination of Arylalkynes
with dichloromethane (10 mL) and evaporated under re-
duced pressure. The crude product was purified by column
chromatography (eluent EtOAc/hexane=10:1, Silica gel 43–
60 mm).
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10:1, Silica gel 43–60 mm).
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(RSF grant 14–50–00126).
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Adv. Synth. Catal. 2016, 358, 1463 – 1468