Efficient [(NHC)Au(NTf2)]-catalyzed hydrohydrazidation of terminal and internal alkynes

Article abstract of DOI:10.3762/BJOC.16.175

The efficient hydrohydrazidation of terminal (6a–r, 18 examples, 0.1–0.2 mol % [(NHC)Au(NTf₂)], T = 60 °C) and internal alkynes (7a–j, 10 examples, 0.2–0.5 mol % [(NHC)Au(NTf₂)], T = 60–80 °C) utilizing a complex with a sterically demanding bispentiptycenyl-substituted NHC ligand and the benign reaction solvent anisole, is reported.

Full text of DOI:10.3762/BJOC.16.175

Efficient [(NHC)Au(NTf2)]-catalyzed hydrohydrazidation of
terminal and internal alkynes
Maximillian Heidrich and Herbert Plenio*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2080–2086.
Organometallic Chemistry, Technische Universität Darmstadt,
Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
https://doi.org/10.3762/bjoc.16.175
Received: 24 June 2020
Accepted: 11 August 2020
Published: 26 August 2020
Email:
Herbert Plenio* - Herbert.Plenio@tu-darmstadt.de
* Corresponding author
Associate Editor: K. Itami
Keywords:
© 2020 Heidrich and Plenio; licensee Beilstein-Institut.
License and terms: see end of document.
alkyne; gold; homogeneous catalysis; hydrohydrazidation; NHC ligand
Abstract
The efficient hydrohydrazidation of terminal (6a–r, 18 examples, 0.1–0.2 mol % [(NHC)Au(NTf2)], T = 60 °C) and internal
alkynes (7a–j, 10 examples, 0.2–0.5 mol % [(NHC)Au(NTf2)], T = 60–80 °C) utilizing a complex with a sterically demanding
bispentiptycenyl-substituted NHC ligand and the benign reaction solvent anisole, is reported.
Introduction
Cationic gold complexes with weakly coordinating counterions protodeauration and is leading to the release of the desired
(which are often considered to be solvent-separated ions) [1-3], organic product [41]. The activity of gold complexes in such
render powerful catalysts for the transformation of organic sub- reactions tends to be higher, when using sterically demanding
strates [4-6], specifically the reactions of alkynes with a variety ligands (NHC, phosphine) [41].
of heteroatom nucleophiles (Scheme 1) [7] such as water or
alcohol [8-22], primary or secondary amines [23-28], or Recently, the hydrohydrazidation of substituted phenyl-
hydrazine [29-31]. These reactions are synthetically useful since acetylenes using [(Ph3P)Au(NTf2)] as the catalyst was reported
gold catalysis is characterized by a good functional group toler- by Rassadin, Kukushkin et al. [42]. Catalyst loadings of
ance for oxygen and nitrogen-containing molecules, which tend 6 mol % at 60 °C were required to obtain the respective addi-
to be more difficult for catalytic transformations utilizing other tion product in yields of 66–93%. Internal alkynes were much
transition metals [32-36]. The [LAu(NTf2)]-catalyzed reaction less reactive and even at 12 mol % catalyst loading the reaction
can be described by a general mechanism (Scheme 1), in which of 3-hexyne and PhCONHNH2 yielded the respective addition
the coordination of LAu+ by the alkyne [37,38] leads to the acti- product in only 32% yield, with diphenylacetylene only 12% of
vation of the triple bond for a nucleophilic attack, generating a the addition product were obtained. We recently reported the
vinylgold complex [39,40]. This is followed by the substitution excellent activity of the very bulky bispentiptycenyl-substituted
of LAu+ with an electrophile E, corresponding (for E = H+) to a (NHC)Au complexes in the hydration of terminal and internal
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strate conversion. Nonetheless, all of the tested complexes per-
formed much better in hydrohydrazidation reactions than the
previously reported [(Ph3P)Au(NTf2)] [42].
Solvent screening: Among the solvents tested in the previous
study by Rassadin, Kukushkin et al., chlorobenzene was found
to be the best solvent concerning the catalytic activity [42].
However, according to the CHEM21 consortium chloroben-
zene is a problematic solvent, whose use should be avoided
[47]. We therefore performed an extensive solvent screening
focusing on “greener” solvents for the reactions of benzohy-
drazide with either phenylacetylene or 4-methoxyphenylacety-
lene (Table 1). It was found that the catalytic performance in
anisole was comparable to that in chlorobenzene. According to
Scheme 1: Simplified mechanism of the hydrohydrazidation (NuH=
ArCONHNH2) of alkynes.
alkynes [43] as well as in other transition-metal-catalyzed trans- both the CHEM21 consortium and the GSK solvent sustain-
formations [44,45]. We were now interested, whether catalysts ability guide the use of anisole is highly favorable [48]. To
with such ligands also display high activities in other gold-cata- clarify the role of chlorobenzene and anisole, most substrate
lyzed reactions.
screening reactions were performed in both solvents (see also
Scheme 3 and Scheme 4) and based on this the use of anisole as
a reaction solvent could be recommended.
Results and Discussion
Catalyst screening: Five different [(NHC)Au(NTf2)] com-
plexes 1, 2, 3, 4, and 5 with bulky NHC ligands were tested in Reaction time vs catalyst loading: In order to optimize cata-
the hydrohydrazidation of phenylacetylene with benzohy- lyst loading and reaction times the hydrohydrazidation of
drazide (Scheme 2; T = 60 °C, solvent chlorobenzene, reaction phenylacetylene was carried out at successively lower catalyst
time 24 h, catalyst loading 0.2 mol %). The following conver- loading (Table 2). The obvious consequence being a decrease in
sions were observed: complex 1 >99%, complex 2 16%, com- the substrate conversion.
plex 3 38%, complex 4 31%, and complex 5 60%. Complex 1
having a bispentiptycene-NHC ligand was the clear best among This, however, could be compensated to some extent by longer
the five NHC-gold complexes. In this group of gold complexes reaction times. Based on this observation catalyst decomposi-
with sterically demanding NHC ligands, the NHC ligands with tion appeared to be negligible and it seemed, that the role of
intermediate bulk provided the best catalytic performance. The gold is primarily that of a Lewis acid activating the alkyne.
use of the bulkiest complex 2 [46], resulted only in modest sub- Consequently, even at 0.05 mol % catalyst loading virtually
Scheme 2: [(NHC)Au(NTf2)] complexes tested in hydrohydrazidation reactions of phenylacetylene.
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Beilstein J. Org. Chem. 2020, 16, 2080–2086.
Table 1: Solvent screening for two hydrohydrazidation reactions using complex 1.
product
solvent
time (h)
conversion (%)a
PhCl
24b, 6c
24b, 6c
24b
81b
80b
46b
45b
31b
>99c
>99c
anisole
xylene
MeNO2
dioxane
MeCN
THF
24b, 6c
24b, 6c
6c
82c
67c
64c
53c
68c
6c
glyme
6c
aGC-based conversion (internal standard mesitylene); bsubstrate a; csubstrate b.
Table 2: Catalyst loading vs. reaction time in the hydrohydrazidation.
entry
complex 1 (mol %)
time (h)
conversion (%)a
1
2
3
4
5
6
0.2
24
>99
0.1
24/48
24/48/96
24/48
24/48
24
79/93
51/70/>99
22/29
2/4
0.05
0.02
0.01
zero
0
aGC-based conversion (internal standard mesitylene).
quantitative substrate conversion was observed after extending lene), and arylalkynes (phenylacetylene, 2,4,6-trimethylphenyl-
the reaction time to 96 h (Table 2, entry 3). In the absence of the acetylene, 2,4,6-triisopropylphenylacetylene, and para-substi-
Au catalyst, no product was formed.
tuted 4-R-phenylacetylenes (R = NMe2, OMe, CF3)) with elec-
tronically variable benzohydrazides (4-R = NMe2, H, NO2)
Hydrohydrazidation reactions of terminal alkynes: were tested, to cover a range of steric and electronic effects.
Following the initial optimization, a variety of hydrohydrazida-
tion reactions of terminal and internal alkynes with benzohy- Most hydrohydrazidation reactions showed nearly quantitative
drazides was studied (Scheme 3). Various combinations of product conversion to 6 and provided isolated yields in excess
alkylalkynes (1-octyne, cyclohexylacetylene, tert-butylacety- of 90% using as little as 0.1–0.2 mol % of complex 1. Just like
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Beilstein J. Org. Chem. 2020, 16, 2080–2086.
Scheme 3: Hydrohydrazidation of terminal alkynes in chlorobenzene and anisole using complex 1 (first line solvent chlorobenzene, second line sol-
vent anisole: catalyst loading, reaction time, conversion % (isolated yield %), reaction temperature 60 °C). aPartial decomposition during column chro-
matography observed.
in related reactions, the nucleophile exclusively added to the Hydrohydrazidation of internal alkynes: Previously, internal
β-carbon of the triple bond. A single substituent in the ortho- alkynes could not be converted efficiently into the respective
position of the alkyne did not exert a strong steric effect. Even hydrohydrazidation products via gold catalysis [42], while prod-
the hydrohydrazidation of the sterically highly demanding uct formation was claimed in a thermal reaction [49]. However,
2,4,6-triisopropylphenylacetylene provided 82% isolated yield in the presence of complex 1 internal alkynes provided the
based on 90% substrate conversion (Scheme 3, compound 6i). respective addition products 7 in excellent yields using catalyst
There was a pronounced electronic effect and electron-rich loadings in the 0.2–0.5 mol % range at slightly elevated temper-
arylalkynes reacted faster than those with electron-withdrawing ature (T = 60–80 °C, Scheme 4). A range of dialkyl-, alkyl-,
groups. The products derived from benzohydrazides with elec- aryl-, and diarylacetylenes was reacted with electronically vari-
tron-rich aromatic groups tended to be somewhat unstable able benzohydrazides (4-R = NMe2, H, NO2), to cover a range
during chromatographic purification (6j, 6q). In such cases it is of steric and electronic substituents. In case shorter reaction
advisable to facilitate product purification by choosing modi- times were desired, this could be compensated by increasing the
fied reaction conditions (e.g., higher catalyst loading or longer amount of catalyst or the reaction temperature. The order of the
reaction times), to ensure virtually quantitative product forma- alkyne reactivity in the present study was as follows: RCCH >
tion.
alkyl-CC-alkyl > aryl-CC-alkyl ≥ aryl-CC-aryl.
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Beilstein J. Org. Chem. 2020, 16, 2080–2086.
Scheme 4: Hydrohydrazidation of internal alkynes in chlorobenzene and anisole using complex 1. Reaction temperature was 60 °C unless otherwise
noted (first line solvent chlorobenzene, second line solvent anisole: catalyst loading, reaction time, conversion % (isolated yield %). a80 °C reaction
temperature; bisolated yield after 1 w; cisomeric ratio determined via NMR spectroscopy.
Disubstituted acetylenes with different substituents tended to Conclusion
produce isomers in the hydrohydrazidation reaction (Scheme 4, In conclusion, we have demonstrated the efficient hydrohy-
products 7b, c, d, i, and j). No preference for either isomer was drazidation of various terminal and internal alkynes utilizing the
observed – not even when using aryl-, alkylacetylenes, or elec- bispentiptycenyl-substituted [(NHC)Au(NTf2)] complex 1 using
tronically different aryl groups in tolanes. However, the steric catalyst loadings between 0.1–0.5 mol % at temperatures be-
bulk of a single methyl group in 2-methyltolane led to the selec- tween 60–80 °C in chlorobenzene, or anisole as a green reac-
tive formation of the single addition product 7h, but even such tion solvent.
sterically hindered tolanes (synthesized via Sonogashira cou-
pling) [50] show reasonable substrate conversion. To assess the Experimental
electronic effect of substituents on the hydrazide-substituted All reagents were obtained from commercial providers (Sigma-
RC6H4-CONHNH2 relatives (R= NMe2, NO2) were tested Aldrich, Alfa Aesar, TCI Europe) and were used without further
leading to 7f and 7g. With a view to the larger distance of the purification. All solvents were dried over CaH2 and distilled.
functional group from the reactive center, the electronic effect All screening reactions were performed in Schlenk tubes under
was weaker, but nonetheless the conversion of the electron-defi- normal atmosphere unless otherwise noted. 1H and 13C NMR
cient hydrazides (R = NO2) was slightly more efficient than spectra were recorded at the NMR department of the Techincal
with the electron-rich hydrazides (R = NMe2).
University Darmstadt at 300 MHz (1H) and 75 MHz (13C) or at
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Beilstein J. Org. Chem. 2020, 16, 2080–2086.
500 MHz (1H) and 126 MHz (13C), respectively. Chemical (15.5 mg, 0.01 mmol, 0.2 mol %) was added and the reaction
shifts are given in parts per million (ppm) on the delta scale (δ) mixture was stirred at 60 °C for 24 h. The mixture was cooled
and are referred either to tetramethylsilane (1H; 13C NMR = to 0 °C and the formed precipitate was collected and washed
0 ppm) or the residual solvent peak. Thin-layer chromatogra- with a small amount of pentane to afford N'-(1-phenylethyli-
phy (TLC) was performed using silica gel 60 F254 (0.2 mm) on dene)benzohydrazide (6d, 1.15 g, 4.83 mmol, 97% yield) as
alumina plates. For preparative chromatography silica gel 60 colorless needles.
(70–200 µm) was used. Measurements of the high resolution
mass spectra (HRMS) were performed on a Bruker Daltonics
Autoflex Speed TOF. Gas chromatographic data was obtained
on a Clarus 500 gas chromatography system.
Supporting Information
Supporting Information File 1
Characterization data and copies of NMR spectra and mass
spectrometric data.
Synthesis of gold complex 1. To a solution of the (NHC)AuCl
complex 1a (60 mg, 0.043 mmol, 1 equiv) in DCM (5 mL)
silver bis(trifluoromethanesulfonyl)amide (18 mg, 0.043 mmol,
1 equiv) was added and the mixture was stirred at rt for 30 min.
The formed suspension was filtered through a plug of celite and
the celite washed with a small amount of DCM. The volatiles of
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-175-S1.pdf]
the filtrate were evaporated under reduced pressure and the Funding
residue dried in vacuo, to yield complex 1 (62 mg, 0.039 mmol, Support of this work by the Deutsche Forschungsgemeinschaft
92% yield) as white solid. 1H NMR (500 MHz, CD2Cl2) δ via DFG 178/20-1 is gratefully acknowledged.
7.54–7.48 (m, 8H), 7.46–7.39 (m, 8H), 7.08–6.93 (m, 16H),
5.90 (s, 4H), 5.19 (s, 4H), 4.12 (t, J = 6.8 Hz, 4H), 2.11–2.05
ORCID® iDs
Maximillian Heidrich - https://orcid.org/0000-0002-0516-8270
(m, 4H), 2.03 (s, 6H), 1.83–1.74 (m, 4H), 1.19 (td, J = 7.4,
1.7 Hz, 6H); 13C NMR (126 MHz, CD2Cl2) δ 166.41, 151.25,
146.21, 145.15, 144.71, 144.08, 142.03, 138.12, 128.93, 126.38,
125.98, 125.48, 124.67, 124.04, 123.98, 123.94, 76.83, 51.06,
48.99, 48.92, 33.13, 20.23, 14.44, 10.38; HRESIMS (m/z):
[NHC(Au) + CH3CN]+ calcd for C83H67AuN3O2, 1334.48935;
found, 1334.48923.
Herbert Plenio - https://orcid.org/0000-0002-2257-983X
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