Mesoionic Carbene-Gold(I) Catalyzed Bis-Hydrohydrazination of Alkynes with Parent Hydrazine

Article abstract of DOI:10.1002/asia.201403408

A novel synthetic route gives access to mesoionic carbene and cyclopropenylidene supported gold chloride complexes. The corresponding cationic MIC-gold complex obtained by chloride abstraction allows for the first transition metal-catalyzed functionalization of both nitrogens of parent hydrazine.

Full text of DOI:10.1002/asia.201403408

CHEMISTRY
AN ASIAN JOURNAL
Accepted Article
Title: Mesoionic Carbene-Gold(I) Catalyzed Bis-Hydrohydrazination of Alkynes with
Parent Hydrazine
Authors: Daniel R Tolentino; Liqun Jin; Mohand Melaimi; Guy Bertrand
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To be cited as: Chem. Asian J. 10.1002/asia.201403408
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Chemistry - An Asian Journal
10.1002/asia.201403408
COMMUNICATION
Mesoionic Carbene-Gold(I) Catalyzed Bis-Hydrohydrazination of
Alkynes with Parent Hydrazine
Daniel R. Tolentino, Liqun Jin, Mohand Melaimi, and Guy Bertrand*[a]
Dedication ((optional))
Preparation of (CAAC)AuCl complexes is readily achieved by
the direct addition of free CAAC to (THT)AuCl (THT =
tetrahydrothiophene).[18] Attempts to generalize this synthetic
route to 1 and 2 resulted in the formation of [bis(carbene)Au]+Cl-
complexes 3 and 4, probably because of the high basicity of the
carbenes (Scheme 1). Such bis(carbene) complexes are known
to be catalytically inactive,[19] and therefore we looked for an
alternative synthetic route to access the desired mono(carbene)
gold chloride complexes. We found that BAC 1 and MIC 2 could
substitute triphenylphosphine in the Ph3PAuPh complex,
affording (L)AuPh 5 and 6. Subsequent addition of HCl affords
(L)AuCl complexes 7 and 8 in 74 and 91% yield, respectively.
These highly air-stable complexes were characterized by NMR
spectroscopy, and a crystal structure was obtained for the MIC
complex 8 (Figure 2).[20] The carbene-Au bond length [1.991(6)
Å] is comparable to those of the (CAAC)AuCl [1.973(4) Å][18] and
(pyrNHC)AuCl [1.983(3) Å].[10]
Abstract: A novel synthetic route gives access to mesoionic
carbene and cyclopropenylidene supported gold chloride
complexes. The corresponding cationic MIC-gold complex
obtained by chloride abstraction allows for the first transition
metal-catalyzed functionalization of both nitrogens of parent
hydrazine.
The hydroamination reaction is an atom-efficient route to
carbon-nitrogen bond containing products starting from readily
accessible materials.[1,2] Compounds featuring nitrogen-nitrogen
bonds constitute an important family of molecules, many of
which are natural products that display biological activity.[3] Thus,
the hydroamination reaction, using parent hydrazine as a
building block, is quite appealing, since it is by far more atom-
efficient than known methods.[4] However, like ammonia,
NH2NH2 readily forms Werner complexes, which are usually
inert.[5] Hydrazine is also a strong reducing agent, which can
induce the formation of inactive metal(0) particles[6] or lead to the
formation of undesired side-products.[7] Despite these
difficulties,[8] we have already shown that parent hydrazine can
be used in the hydrohydrazination of unactivated alkynes and
allenes using gold(I) chloride complexes supported by a cyclic
(alkyl)(amino)carbene (CAAC)[9] and an anti-Bredt N-heterocyclic
carbene (pyrNHC) (Figure 1).[10] Importantly, Hashmi et al.
reported that the gold-catalyzed hydrohydrazination of terminal
L
L
Au
L
THF
0 oC
Cl
+
rt
rt
(THT)AuCl
3,4
L
L
L
THF
0 oC
HCl/Et2O
DCM, rt
Au
Au
Cl
alkynes is even easier when
a saturated abnormal N-
+
heterocyclic carbene (saNHC) is used as ancillary ligand.[11]
CAACs,[12] pyrNHCs[13] and saNHCs are recognized as strong σ-
donating ligands,[14] but the π-accepting properties of the latter
are by far weaker than the other two.[15] This analysis prompted
us to consider the bis(diisopropylamino)cyclopropenylidene
(Ph3P)AuPh
3, 5 and 7: L = BAC 1
4, 6 and 8: L = MIC 2
5, 6
7, 8
(BAC) 1[15] and the 1,2,3-triazol-5-ylidene
2
[also named
Scheme 1. Synthesis of bis(carbene)Au+Cl- 3 and 4, and mono(carbene) gold
chloride complexes 7 and 8.
mesoionic carbene (MIC)],[17] as ligands.
R'
Dipp
N
N N
N
N(iPr)2
(iPr)2
N
N
N
N
N
Dipp
R
R'
R
R"
saNHC
Ph
R
CAAC
pyrNHC
BAC
1
MIC 2
Figure 1. CAAC, pyrNHC and saNHC ligands previously used in gold-
catalyzed hydrohydrazination, and BAC 1 and MIC 2 considered in this study
(Dipp: 2,6-diisopropylphenyl).
[a] D. R. Tolentino, L. Jin, Dr. M. Melaimi, Prof. G. Bertrand
UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
Figure 2. X-ray structure of 8. Hydrogen atoms are omitted for clarity;
Selected bond lengths [Å] and angles [°]: C-Au 1.991(6), Au-Cl 2.2761(17);
C-Au-Cl 174.20(17).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.

Chemistry - An Asian Journal
10.1002/asia.201403408
COMMUNICATION
Table 2. Bis-hydrohydrazination of terminal alkynes.
Initial catalytic studies were conducted at 100 °C, using 1
mol% of a stoichiometric mixture of (L)AuCl (7 and 8) with
KB(C6F5)4 as chloride scavenger, and a 1/1 mixture of parent
hydrazine and phenylacetylene as a model substrate. The
reactions were monitored by 1H NMR spectroscopy using benzyl
methyl ether as internal standard. The BAC-gold complex 7
showed minimal activity, only a small amount of hydrazone
(≈12%) was obtained after 61 hours (Table 1, entry 1). In
1 mol% 8
R
1 mol% KB(C6F5)4
R
H
N N
0.45 equiv NH2NH2
C6H6, 100 oC, 13h
R
Entry
R
% Yield[a]
95 (21)
marked contrast, when MIC complex
8 was used, total
conversion of the acetylene was observed after 12 hours.
Surprisingly, in addition to the mono-hydrohydrazinated
derivative, we also observed the formation of the corresponding
azine (Table 1, entry 2). Since such a bis-hydrohydrazination
reaction has no precedent, we explored this new avenue
towards the formation of molecules with N-N bonds. By using
0.45 equivalent of hydrazine with respect to phenylacetylene, full
conversion of NH2NH2 was observed, and the azine was
obtained in 84% yield after 12 hours (Table 1, Entry 2). Complex
8 displays excellent activity, even at catalyst loadings as low as
0.5 and 0.1 mol% (Table 1, Entries 3 and 4). It is even able to
facilitate the bis-hydrohydrazination of phenylacetylene in good
yield at 60 °C with 1 mol% loading (Table 1, Entry 5).
1
2
3
4
4-Methoxyphenyl
4-trifluoromethylphenyl
Cyclohexyl
92 (67)
88 (50)
1-Cyclohexenyl
100 (94)
[a] Determined by 1H NMR with benzyl methyl ether as an internal standard;
isolated yield in parentheses.
To illustrate the synthetic potential of this bis-
hydrohydrazination reaction, two different alkynes were
sequentially reacted to give an unsymmetrical azine (Scheme 2).
First, 4-trifluoromethylphenyl acetylene was converted to
hydrazone 9 using an excess of hydrazine; then phenyl
acetylene was added to give 10, which was isolated as a 50/50
mixture of stereoisomers in 61% yield.
Previous
reports
on
the
hydroamination
and
hydrohydrazination with (carbene)AuCl precatalysts have led to
the conclusion that the active species is likely a cationic gold
coordinated by one carbene,[18b] while the resting state is the
[(carbene)Au(NH3)]+X- and [(carbene)Au(N2H4)]+X- complex,
respectively.[9,10,21] Similarly, we found that the gold chloride
complex 8 is inactive in the absence of chloride scavenger, and
1 mol% 8
N
NH2
1 mol% KB(C6F5)4
F3C
H
F3C
2 equiv H2NNH2
C6H6, 100 o
9
C
-
that the air-stable and storable [(2)Au(N2H4)]+B(C6F5)4 complex
1 mol% 8
performed similarly to 8/KB(C6F5)4 and is certainly the resting
state of the catalyst (Table 1, entry 6). Lastly, to rule out the
formation of active gold nanoparticles, the reaction was carried
out in the presence of elemental mercury (Table 1, entry 7) and
catalysis still proceeded, which is consistent with homogeneous
catalysis.[22]
H
1 mol% KB(C6F5)4
C6H6, 100 oC, 6 h
N
N
F3C
10
Table 1. Optimization of reaction conditions with phenylacetylene.
Scheme 2. Step-wise preparation of unsymmetrical azine.
In summary,
a novel synthetic route allows for the
preparation of a gold chloride complex bearing a mesoionic
carbene as ancillary ligand. In the presence of a chloride
scavenger, this complex promotes the catalytic functionalization
of both nitrogens of parent hydrazine with terminal alkynes.
Current investigations are directed towards understanding ligand
effects leading to the bis-functionalization.
Temp.
(°C)
Time
(h)
Yield
(%)[a]
Entry
Pre-catalyst (mol%)
1
2
3
4
5
7 (1.0)
8 (1.0)
8 (0.5)
8 (0.1)
8 (1.0)
100
100
100
100
60
61
12
13
13
13
12[b]
84
82
78
81
-
[(2)AuN2H4]+B(C6F5)4
6
7
100
100
13
13
81
84
(1.0)[c]
8 (1.0) + Hg
Experimental Section
[a] Determined by 1H NMR with benzyl methyl ether as an internal standard.
[b] Percent yield refers to hydrazone product; no azine was observed. [c] No
addition of KB(C6F5)4.
All reactions were performed under an atmosphere of argon using
standard Schlenk or dry box techniques; solvents were dried over sodium
or CaH2. 1H, 13C, and 19F NMR spectra were obtained with Bruker
Advance 300 and Varian VX 500 spectrometers at 298K. Chemical shifts
(δ) are reported in parts per million (ppm) and were referenced to the
residual solvent peak. NMR multiplicities are abbreviated as follows: s =
singlet, d = doublet, t = triplet, m = multiplet, br = broad signal.
We then briefly explored the substrate scope of the gold-
catalyzed bis-hydrohydrazination (Table 2). Terminal alkynes
were converted efficiently. The 1H NMR spectra of the crude
products were very clean after evaporation of volatiles, and the
azines were isolated in reasonable yields. However, no clean
reactions were observed with internal alkynes.
Preparation of L2Au+Cl 3[23] and 4: (THT)AuCl (0.43 g, 1.33 mmol) and
MIC 2 (1.24 g, 2.66 mmol) were added to a flame-dried Schlenk flask and
cooled to 0 °C. Anhydrous THF (20 mL) was added, and the mixture was
slowly warmed to room temperature and stirred for 16 hours. Volatiles
were removed in vacuo. Washing the solid residue with anhydrous
benzene afforded 4 as an off-white solid in 93% yield. 1H NMR (300 MHz,

Chemistry - An Asian Journal
10.1002/asia.201403408
COMMUNICATION
CDCl3): δ = 7.52-7.45 (m, 4H, Ar), 7.28 (m, 2H, Ar), 7.18 (m, 9H, Ar),
7.01 (m, 7H, Ar), 2.18 (septet, J = 6.0 Hz, 4H), 2.08 (septet, J = 6.0 Hz,
4H), 1.02 (d, J = 6.0 Hz, 12H), 0.99 (d, J = 6.0 Hz, 12H), 0.96 (d, J = 6.0
Hz, 12H), 0.86 (d, J = 6.0 Hz, 12H). 13C NMR (125 MHz, CDCl3): δ =
173.3, 149.8, 144.8, 134.6, 132.5, 131.8, 130.2, 130.0, 128.9, 128.6,
128.3, 125.4, 124.8, 124.3, 29.3, 28.9, 25.3, 24.2, 24.0, 22.4. HR-MS
(m/z): [M]+ calcd. for C64H78AuN6+, 1127.5949; [M-Cl]+ found, 1127.5948.
m.p. = 315 °C (decomposition).
6H). 13C{1H} NMR (125 MHz, CDCl3): δ 160.7, 157.8, 131.2, 128.0,
113.6,
55.3,
14.8.
1,2-bis(1-(4-
trifluoromethyl)phenyl)ethylidene)hydrazine: 1H NMR (300 MHz,
C6D6): δ = 7.71 (d, J = 9.0 Hz, 4H), 7.41 (d, J = 9.0 Hz, 4H), 1.98 (s, 6H).
13C{1H} NMR (125 MHz, C6D6): δ 157.1, 141.6, 127.2, 125.4, 14.6.
19F{1H} NMR (282 MHz, C6D6): δ -62.5. HR-MS (m/z): [M+H]+ calcd. for
C18H15F6N2,
373.1134;
[M+H]+
found,
373.1136.
1,2-bis(1-
cyclohexylethylidene)hydrazine: 1H NMR (300 MHz, C6D6): δ = 2.17-
1.14 (br, m, cyclohexyl, 22H), 1.75 (s, 6H). 13C{1H} NMR (125 MHz,
C6D6): δ 164.4, 47.1, 30.7, 26.6, 26.5, 15.1. HR-MS (m/z): [M+H]+ calcd.
for C16H29N2, 249.2325; [M+H]+ found, 249.2327. 1,2-bis(1-(cyclohex-1-
en-1-yl)ethylidene)hydrazine: 1H NMR (300 MHz, C6D6): δ = 6.09 (m,
2H), 2.68 (m, 2H), 2.00 (s, 6H), 1.60 (m, 2H), 1.49 (m, 2H). 13C{1H} NMR
(125 MHz, C6D6): δ = 158.1, 138.5, 26.3, 25.1, 23.1, 22.7, 12.9. HR-MS
(m/z): [M+H]+ calcd. for C16H2hN2, 245.2012; [M+H]+ found, 245.2014.
Unsymmetrical azine: 8 (0.007 mmol) and KB(C6F5)4 (0.007 mmol)
were introduced in a flame-dried Schlenk tube, and anhydrous benzene
(0.4 mL) was added. 1-ethynyl-4-(trifluoromethyl)benzene (0.72 mmol)
and anhydrous hydrazine (1.43 mmol) were added, and the mixture was
stirred for 13 hours at 100 °C under argon. The reaction was then cooled
to room temperature and volatiles removed in vacuo. The crude product
was dissolved in benzene and passed through dry florisil silica gel to
afford 9 as a solid in 76% yield.[11b] 1H NMR (300 MHz, C6D6): δ = 7.54 (d,
J = 9 Hz, 2H), 7.37 (d, J = 9 Hz, 2H), 4.92 (s, 2H, br), 1.44 (s, 3H).
13C{1H} NMR (125 MHz, C6D6): δ 143.5, 126.1, 125.7, 125.6, 125.5, 10.6.
19F{1H} NMR (282 MHz, C6D6): δ -62.1. Phenylacetylene (0.82 mmol), 9
(0.54 mmol), 8 (0.006 mmol), KB(C6F5)4 (0.006 mmol), and anhydrous
benzene (1 mL) were added to a dry Schlenk tube. The tube was sealed
and heated to 100 °C for 6 hours, cooled to room temperature, and the
volatiles removed in vacuo. The crude product was dissolved in benzene
and passed through dry florisil silica gel to afford 10 as a white solid in
61% yield. 1H NMR (300 MHz, C6D6): δ = 7.89 (m, 2H), 7.71 (m, 2H),
7.40 (m, 2H), 7.19 (m, 3H), 2.13 (d, 12 Hz, 3H), 2.02 (d, 12 Hz, 3H).
13C{1H} NMR (125 MHz, C6D6): δ 158.6, 158.2, 157.1, 156.9, 142.0,
141.6, 138.9, 138.6, 130.0, 128.5, 127.1, 125.4, 14.6. 19F{1H} NMR (282
MHz, C6D6): δ -62.4. HR-MS (m/z): [M+H]+ calcd. for C17H16F3N2,
305.1263; [M+H]+ found, 305.1260.
Preparation of (L)AuPh 5 and 6: Free carbene 1 or 2 (1.28 mmol) and
PPh3AuPh (0.690 g, 1.28 mmol) were added to a flame-dried Schlenk
flask and cooled to 0°C. Anhydrous THF (15 mL) was added, and the
mixture was allowed to warm to room temperature while stirring for 1
hour. Volatiles were removed in vacuo, and the residue was washed with
anhydrous hexane to afford L-AuPh as solids in 74-91% yield. 5: 1H NMR
(300 MHz, CDCl3): δ = 7.54 (m, 2H, Ar), 7.20 (m, 2H, Ar), 6.98 (m, 1H,
Ar), 3.82 (br, 4H), 1.57 (br, 12H), 1.29 (br, 12H); 13C{1H} NMR (125 MHz,
CDCl3): δ = 170..2, 159.6, 146.5, 140.8, 127.3, 124.6, 54.2, 48.9, 21.9,
1
21.5. 6: H NMR (300 MHz, CDCl3): δ = 7.83 (m, 2H), 7.57 (m, 2H), 7.34
(m, 9H), 7.12 (m, 2H), 6.94 (m, 1H), 2.61 (septet, J = 6.0 Hz, 2H), 2.40
(septet, J = 6.0 Hz, 2H), 1.48 (d, J = 6.0 Hz, 6H), 1.19 (d, J = 6.0 Hz, 6H),
1.15 (d, J = 6.0 Hz, 6H), 0.98 (d, J = 6.0 Hz, 6H); 13C{1H} NMR (125 MHz,
CDCl3): δ = 182.5, 169.9, 148.8, 145.1, 144.9, 140.6, 135.5, 134.0, 133.8,
131.9, 131.3, 131.0, 129.5, 128.9, 128.4, 126.8, 124.6, 123.9, 29.0, 28.9,
25.1, 24.3, 24.2, 22.4.
Preparation of (L)AuCl 7 and 8: A 2.0 M solution of HCl in diethyl ether
(4 mL, excess) was added to a stirred dichloromethane solution (10 mL)
of (carbene)AuPh complex (1.28 mmol). The reaction was stirred for 1
hour at room temperature, then dried in vacuo to afford (carbene)AuCl as
a solid in quantitative yield. 7: 1H NMR (300 MHz, CDCl3): δ = 3.82 (br,
4H), 1.51 (br, 12H), 1.29 (br, 12H); 13C{1H} NMR (125 MHz, CDCl3): δ =
144.1, 133.9, 54.4, 48.8, 21.8, 21.3. m.p. = 198 °C (decomposition). 8: 1H
NMR (300 MHz, CDCl3): δ = 7.64-7.29 (m, 11H, Ar), 2.48 (septet, J = 6.0
Hz, 2H), 2.34 (septet, J = 6.0 Hz, 2H), 1.42 (d, J = 6.0 Hz, 6H), 1.17 (d, J
= 6.0 Hz, 6H), 1.13 (d, J = 6.0 Hz, 6H), 0.98 (d, J = 6.0 Hz, 6H); 13C{1H}
NMR (125 MHz, CDCl3): δ = 161.1, 148.0, 145.2, 144.9, 135.0, 132.2,
131.5, 130.8, 130.1, 128.9, 128.7, 125.8, 124.7, 124.3, 29.2, 29.0, 25.3,
24.3, 24.2, 22.4. m.p. = 235 °C (decomposition).
-
Preparation of [(2)AuN2H4]+B(C6F5)4 : 8 (60 mg, 0.086 mmol) and
KB(C6F5)4 (62 mg, 0.086 mmol) were added to a flame-dried Schlenk
flask and anhydrous CHCl3 (5 mL) was added. Anhydrous hydrazine (22
μL, 0.70 mmol) was added and the reaction was stirred at room
temperature for 1 hour. The mixture was then filtered and the filtrate dried
in vacuo to yield the hydrazine complex as an air-stable solid in 85%
yield. 1H NMR (300 MHz, CDCl3): δ = 7.59 (m, 2H, Ar), 7.49 (m, 2H, Ar),
7.35 (m, 7H, Ar), 5.00 (s, 2H, br), 3.64 (s, 2H, br), 2.42 (septet, J = 6.0 Hz,
2H), 2.30 (septet, J = 6.0 Hz, 2H), 1.37 (d, J = 6.0 Hz, 6H), 1.21 (d, J =
6.0 Hz, 6H), 1.16 (d, J = 6.0 Hz, 6H), 1.01 (d, J = 6.0 Hz, 6H). 13C{1H}
(125 MHz, CDCl3): 154.3, 149.1, 144.9, 134.4, 132.7, 132.1, 131.0, 129.0,
128.8, 124.9, 124.5, 29.4, 29.1, 25.3, 24.3, 22.2.
Acknowledgements
The authors gratefully acknowledge financial support from the
DOE (DE-FG02-13ER16370).
Keywords: gold • hydroamination • hydrazine • mesoionic
carbene • cyclopropenylidene
Optimization
of
Bis-hydrohydrazination:
C6D6
(0.4
mL),
phenylacetylene (100 μL, 0.91 mmol), anhydrous hydrazine (13 μL, 0.41
mmol), and the internal standard benzyl methyl ether (15 μL, 0.12 mmol)
were added to a mixture of pre-catalyst (1 mol% with respect to
phenylacetylene) and KB(C6F5)4 (1 mol% with respect to
phenylacetylene) in a dry J-Young NMR tube. For experiments with a
low catalyst loading (0.5 mol%, 0.1 mol%), a stock solution was prepared
by dissolving (2)AuCl (100 mg) in methylene chloride (5 mL). The
appropriate amount added to the NMR tube, and the solvent removed
under vaccum. Then, the other reagents and the solvent were added.
The tube was sealed, and placed in an oil bath and heated at the
[1]
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specified temperature.
spectroscopy.
The reaction was monitored by 1H NMR
General Procedure for Bis-Hydrohydrazination of Alkynes: 8 (5.0 mg,
0.007 mmol, 1 mol% with respect to. alkyne) and KB(C6F5)4 (5.0 mg,
0.007 mmol, 1 mol% with respect to alkyne) were introduced in a Teflon
sealed Schlenk tube. Anhydrous benzene (0.4 mL), the desired alkyne
(0.7 mmol), and anhydrous hydrazine (10 μL, 0.319 mmol) were added in
this order. The solution was then heated to 100 °C for 13 hours, after
which the reaction was cooled to room temperature and the volatiles
removed in vacuo. The crude product was then passed through a short
column of dry florisil silica gel (benzene or CH2Cl2 eluent). 1,2-bis(1-
phenylethylidene)hydrazine: 1H NMR (300 MHz, C6D6): δ = 7.94 (m,
2H), 7.20 (m, 8H), 2.15 (s, 6H). 13C{1H} NMR (125 MHz, C6D6): δ 158.3,
139.0, 129.7, 128.5, 127.0, 14.7. HR-MS (m/z): [M+H]+ calcd. for
[3]
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C16H17N2,
237.1384;
[M+H]+
found,
237.1384.
1,2-bis(1-(4-
methoxyphenyl)ethylidene)hydrazine:[24] 1H NMR (300 MHz, CDCl3): δ
= 7.89 (d, J = 9.0 Hz, 4H), 6.95 (d, J = 9.0 Hz, 4H), 3.86 (s, 6H), 2.33 (s,

Chemistry - An Asian Journal
10.1002/asia.201403408
COMMUNICATION
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Chemistry - An Asian Journal
10.1002/asia.201403408
COMMUNICATION
Layout 2:
COMMUNICATION
Daniel R. Tolentino, Liqun Jin, Mohand
Melaimi, and Guy Bertrand*
Page No. – Page No.
Mesoionic Carbene-Gold(I) Catalyzed
Bis-Hydrohydrazination of Alkynes
with Parent Hydrazine
Twice is better than once! An original synthetic route gives access to a mesoionic
carbene supported gold(I) complex, which is not only able to promote the
hydrohydrazination of terminal alkynes, but also the bis(hydrohydrazination), giving
access to azines.