Highly convenient regioselective intermolecular hydroamination of alkynes yielding ketimines catalyzed by gold(I) complexes of 1,2,4-triazole based N-heterocyclic carbenes

Article abstract of DOI:10.1021/ic100087d

A series of highly efficient gold(I) precatalysts of 1,2,4-triazole based N-heterocyclic carbenes, [1-R-4-R′-1,2,4-triazol-5-ylidene]AuCl [R = CH₂CO^tBu, R′ = CH₂ Ph (1c); R = CH ₂ CONH^tBu, R′ = CH₂ Ph (2c); R = CH ₂ CO^tBu, R′ = CH₂CO^tBu (3c), and R = C₆H₁₀OH, R′ = CH₂Ph (4c)] are reported for the hydroamination of terminal alkynes with a variety of sterically demanding o/p-substituted aryl amines yielding the corresponding ketimines in air. The gold 1c-4c complexes exhibited extremely high activity in comparison to the silver analogues 1b-4b, thereby highlighting the role of gold as a metal in the catalysis of the hydroamination reaction. Additionally, the 1,2,4-triazole based 1c-4c precatalysts showed significantly superior activity in comparison to the two representative imidazole analogues, namely, [1-(benzyl)-3-(N-t- butylacetamido)imidazol-2-ylidene]AuCl and [1-(2-hydroxy-cyclohexyl)-3-(benzyl) imidazol-2-ylidene]AuCl, thereby underscoring the importance of the 1,2,4-triazole based N-heterocyclic carbenes over the imidazole based ones in designing the gold(I) precatalysts for the hydroamination reaction. The gold(I) complexes (1c-4c) were synthesized by transmetalation reaction of the silver analogues 1b-4b with (SMe₂)AuCl in 60-76%yield while the silver 1b-4b complexes in turn were synthesized from the respective 1,2,4-triazolium halide salts by treatment with Ag₂O in 43-64% yield.

Full text of DOI:10.1021/ic100087d

4972 Inorg. Chem. 2010, 49, 4972–4983
DOI: 10.1021/ic100087d
Highly Convenient Regioselective Intermolecular Hydroamination of Alkynes
Yielding Ketimines Catalyzed by Gold(I) Complexes of 1,2,4-triazole Based
N-heterocyclic Carbenes
Chandrakanta Dash,† Mobin M. Shaikh,‡ Ray J. Butcher,§ and Prasenjit Ghosh*,†
†Department of Chemistry, and ‡National Single Crystal X-ray Diffraction Facility, Indian Institute of
§
Technology Bombay, Powai, Mumbai 400 076, India, and Department of Chemistry, Howard University,
Washington, D.C. 20059
Received January 15, 2010
A series of highly efficient gold(I) precatalysts of 1,2,4-triazole based N-heterocyclic carbenes, [1-R-4-R0-1,2,4-triazol-
5-ylidene]AuCl [R = CH2COtBu, R0 = CH2Ph (1c); R = CH2CONHtBu, R0 = CH2Ph (2c); R = CH2COtBu, R0 =
CH2COtBu (3c), and R = C6H10OH, R0 = CH2Ph (4c)] are reported for the hydroamination of terminal alkynes with a
variety of sterically demanding o/p-substituted aryl amines yielding the corresponding ketimines in air. The gold 1c-4c
complexes exhibited extremely high activity in comparison to the silver analogues 1b-4b, thereby highlighting the role
of gold as a metal in the catalysis of the hydroamination reaction. Additionally, the 1,2,4-triazole based 1c-4c
precatalysts showed significantly superior activity in comparison to the two representative imidazole analogues,
namely, [1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]AuCl and [1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-
2-ylidene]AuCl, thereby underscoring the importance of the 1,2,4-triazole based N-heterocyclic carbenes over the
imidazole based ones in designing the gold(I) precatalysts for the hydroamination reaction. The gold(I) complexes
(1c-4c) weresynthesized by transmetalationreactionofthesilver analogues1b-4bwith (SMe2)AuClin60-76% yield
while the silver 1b-4b complexes in turn were synthesized from the respective 1,2,4-triazolium halide salts by treatment
with Ag2O in 43-64% yield.
Introduction
methods like imination of ketones3 or the aminomercura-
tion/demercuration of alkynes4 that involve the stoichio-
metric use of toxic mercury reagents. The hydroamination
of alkynes have been successfully employed as key steps in a
variety of total syntheses of target molecules.5 The amines
and the alkynes being both inexpensive and readily available,
at least in the laboratory scale, have largely contributed to the
rising popularity of the reaction in recent times. Further-
more, the alkyne hydroamination is more facile compared to
the alkene hydroamination for reasons of the alkynes being
sterically less demanding and also for its π-bond being about
70 kJ/mol weaker than that in alkenes.6
The hydroamination of unsaturated substrates like the
alkynes and alkenes provide a fundamentally simple, atom
economic approach for the efficient construction of the C-N
bonds, which together with C-C bond formation lie at the
heart of organic synthesis and undoubtedly are among the
core challenges of the contemporary organic synthesis
today.1 Thus, finding novel and convenient ways of form-
ing these bonds are in obvious demand. In this regard the
catalytic hydroamination of alkynes yields reactive but
synthetically useful ketimines and enamines that are often
the key intermediates to numerous nitrogen containing
compounds of medicinal, agrochemical, and industrial
interest.2 It is distinctly superior to the other available
The hydroamination reaction, with all its advantages
though provide an enticing option, possesses an inherent
barrier for the reaction to occur owing to electrostatic reasons
as both of the substrates, namely, the amine and the alkyne or
*To whom correspondence should be addressed. E-mail: pghosh@
chem.iitb.ac.in. Fax: þ91-22-2572-3480.
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r
pubs.acs.org/IC
Published on Web 04/29/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4973
the alkene, are considered electron rich and, thus, calls for the
participation of metals in catalyzing the reaction by employ-
ing various activation pathways.1a,7 The metals on their part
facilitate the catalysis by activating either the amine or the
C-C multiple bonded substrate and upon subsequent pro-
tonolysis yields the desired product alongside regenerating
back the catalyst. Depending upon the choice of the metal
though, the catalytic hydroamination reaction can often be
air and moisture sensitive and thus be difficult to handle as is
mostly the case with the lanthanides, the alkali metals and the
early transition metals that are generally known to function
by the amine activation pathway.7a,8 In this regard, notable
are the late transition metals, which being less oxophilic
are less sensitive to air and moisture, exhibit desirable traits
for utility in the hydroamination catalysis, and have been
reported to function by the alkyne activation pathway.9 Gold
being one such metal, long known as an inert coinage metal
with little catalytic properties and whose catalytic exploits are
of recent interest,10 we became interested in exploring its
utility in the hydroamination reaction. We rationalized that
gold, in both of its (I) and (III) oxidation states, is known for
its high carbophilicity and diminished oxophilicity and is thus
well suited for catalyzing the hydroamination reaction most
likely by an alkyne activation pathway involving the nucleo-
philic attack by an amine.11
Figure 1
Our long-standing interest remains in exploring the utility
of N-heterocyclic carbenes12 in biomedical applications13 as
well as in chemical catalysis particularly that range from
producing biodegradable polymers by the ring-opening poly-
merization of
-lactide,10f,h,14 to a variety of C-C bond
L
forming reactions like the Pd mediated Suzuki,15 Sonogashira,16
and the Hiyama16c couplings and in Ni mediated base-free
Michael addition reactions.17 Furthermore, having success-
fully employed imidazole derived N-heterocyclic carbene
precatalysts in C-C bond forming reactions, we decided to
extend our study to C-N bond forming reactions and
particularly do so by employing the much less explored
1,2,4-triazole derived N-heterocyclic carbenes18 that provide
new steric and electronic environments compared to the
imidazole counterparts. Our specific aim was in gauging
the potential of the 1,2,4-triazole derived N-heterocyclic
carbenes in catalysis by performing a comparative study with
respect to the extensively studied imidazole derived N-hetero-
cyclic carbenes. Specifically, we decided to explore the poten-
tials of gold complexes of 1,2,4-triazole based N-heterocyclic
carbenes in the alkyne hydroamination reactions.
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Here in this contribution, we report a series of highly efficient
gold(I) precatalysts, 1c-4c, of 1,2,4-triazole based N-hetero-
cyclic carbenes for the hydroamination of terminal alkynes
with aryl amines yielding the ketimines in air (Figure 1, eq 1).
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Furthermore, we performed a comparison of the catalytic
activity of the gold(I) precatalysts, 1c-4c, with their silver
analogues 1b-4b as well as with the gold(I) imidazole-based
N-heterocyclic carbene counterparts of 2c and 4c, namely, the
[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]AuCl12c
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4974 Inorganic Chemistry, Vol. 49, No. 11, 2010
Dash et al.
Scheme 1
Scheme 2
and [1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-yli-
dene]AuCl10f complexes to highlight the superiority of
the gold(I) precatalysts (1c-4c) of 1,2,4-triazole derived
N-heterocyclic carbenes in the alkyne hydroamination
reaction. Additionally, the density functional theory
(DFT) studies performedon the 1c-4c complexes revealed
the 1,2,4-triazole based N-heterocyclic carbenes to be
strongly σ-donating ligands withverylittleπ-back bonding
tendencies similar to the much studied imidazole based
N-heterocyclic carbene counterparts.
Finally, the gold(I) 1c-4c precatalysts were synthesized by
the transmetalation reaction of silvercomplexes(1b-4b) with
(SMe2)AuCl in 60-76% yield and was accompanied by the
precipitation of an off-white silver halide salt. The observed
1,2,4-triazolium Ccarbene-Au peak at 175.8-177.2 ppm in the
1c-4c complexes compared well with the other reported
gold(I) 1,2,4-triazole based N-heterocyclic carbene com-
plexes, namely, [1,4-bis(methyl)-1,2,4-triazol-5-ylidene]AuCl
[173.0 ppm]22 and [2-phenyl-6,7-dihydro-5H-pyrrolo-[2,1-c]-
[1,2,3]triazol-2-ylidene]AuCl [167.8 ppm].23 Quite interestingly,
the 1,2,4-triazole Ccarbene-Au (NCN) resonances at 175.8-
177.2 ppm for the 1c-4c complexes were significantly upfield
shifted compared to its immediate silver(I) counterparts 1b-4b
at 184.4-187.9 ppm. A similar trend is also observed for the
imidazole based N-heterocyclic carbene complexes of gold and
silver, for example, the Ccarbene-Au (NCN) resonance for
[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]AuCl
appeared at 170.1 ppm while the Ccarbene-Ag (NCN) peak for
{[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]2-
Ag}þCl- appeared at 179.7 ppm.10f
The molecular structure determination by X-ray crystallo-
graphy revealed these gold(I) precatalysts, 1c-4c, to be
discrete monomers in keeping with their intended use as
single-site catalysts for the study of intermolecular hydro-
amination of terminal alkynes. For comparison, when the
molecular structures of two representative silver(I) analo-
gues, 1b and 2b, were determined, the 1b complex was found
to be dimeric while the other 2b complex was monomeric
quite similar to the gold(I) analogues, 1c-4c, (Figures 2-5
and Supporting Information, Figures S1-S2). All of the
monomeric gold(I) (1c-4c) and silver(I) (2b) complexes
displayed a linear geometry [— Ccarbene-M-X; M = Au,
X = Cl, 176.0(2)-178.3(3)o, M = Ag, X = Br, 170.35(8)o] at
the metal consistent with a two coordinated d10 metal center,
while the 1b complex showed a slightly deviated geometry
[—Ccarbene-Ag-Br = 157.93(13)o] as a result of coordination
Results and Discussion
A series of gold(I) precatalysts, 1c-4c, supported over new
N/O-functionalized 1,2,4-triazolium N-heterocyclic carbene
ligands, namely, [1-R-4-R0-1,2,4-triazol-5-ylidene]AuCl [R =
CH2COtBu, R0 = CH2Ph (1c); R = CH2CONHtBu, R0 =
CH2Ph (2c);R=CH2COtBu, R0 =CH2COtBu (3c), andR=
C6H10OH, R0 = CH2Ph (4c)] were designed for studying the
intermolecular hydroamination of alkynes. In particular, the
N/O-functionalized 1,2,4-triazolium halide salts (1a-4a) were
synthesized by the alkylation reaction of the appropriately
substituted 1,2,4-triazoles in 44-70% yield (Schemes 1
and 2). The formation of 1a-4a was very much evident from
the respective 1H NMR spectrum in which the characteristic
1,2,4-triazolium NC(5)HN resonance appeared at 10.6-
11.1 ppm, at a significantly downfield shifted region com-
pared to the imidazole based counterparts, NC(2)HN, at
about 9.70-10.1 ppm.10f,12b,c
The treatment of 1a-4a with Ag2O resulted in the silver
complexes 1b-4b in 43-64% yield (Schemes 1 and 2) by
following the convenient methodology developed by Lin
and co-workers19 and, as expected, the 13C{1H} NMR
spectrum showed a new peak at about 184.4-187.9 ppm
attributable to the metal bound 1,2,4-triazolium carbene
moiety (Ccarbene-Ag). In this regard it is noteworthy that
similar chemical shifts have been reported for other 1,2,4-
triazole based N-heterocyclic carbene complexes, namely,
[{(1,2-di-methyl-4-(i-propyl)-1,2,4-triazol-3,5-diylidene)-
Ag}þTfO-]n [182.9 ppm]20 and [{bis(4-benzyl-1,2,4-triazol-
5-ylidene)}dihydroborate]2Ag2 [189.3 ppm].21
(21) Papini, G.; Bandoli, G.; Dolmella, A.; Lobbia, G. G.; Pellei, M.;
Santini, C. Inorg. Chem. Commun. 2008, 11, 1103–1106.
(22) Wang, H. M. J.; Vasam, C. S.; Tsai, T. Y. R.; Chen, S.-H.; Chang,
A. H. H.; Lin, I. J. B. Organometallics 2005, 24, 486–493.
(19) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.
ꢀ
ꢀ
(20) Guerret, O.; Sole, S.; Gornitzka, H.; Teichert, M.; Trinquier, G.;
(23) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Organo-
Bertrand, G. J. Am. Chem. Soc. 1997, 119, 6668–6669.
metallics 2005, 24, 2411–2418.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4975
Figure 4. ORTEP drawing of 2b with thermal ellipsoids shown at the
˚
50% probability level. Selected bond lengths (A) and angles (deg): Ag1-
C1 2.092(7), Ag1-Br1 2.4614(9), N1-C1 1.360(8), N2-C1 1.337(9),
C1-Ag1-Br1 170.35(8), N1-C1-N2 101.5(6).
Figure 2. ORTEP drawing of 1b with thermal ellipsoids shown at
˚
the 50% probability level. Selected bond lengths (A) and angles (deg):
Ag1-C1 2.092(4), Ag1-Br1 2.4629(9), N1-C1 1.349(5), N2-C1 1.327(6),
C1-Ag1-Br1 157.93(13), N1-C1-N2 103.0(4).
Figure 5. ORTEP drawing of 2c with thermal ellipsoids shown at the
˚
50% probability level. Selected bond lengths (A) and angles (deg): Au1-
C1 1.950(10), Au1-Cl1 2.319(3), N1-C1 1.391(12), N2-C1 1.358(12),
C1-Au1-Cl1 178.3(3), N1-C1-N2 101.1(8).
Figure 3. ORTEP drawing of 1c with thermal ellipsoids shown at the
˚
50% probability level. Selected bond lengths (A) and angles (deg): Au1-
C1 1.972(12), Au1-Cl1 2.269(3), N1-C1 1.355(15), N2-C1 1.325(14),
C1-Au1-Cl1 177.7(4), N1-C1-N2 104.0(10).
in 1c-4c are indeed shorter than the Ccarbene-Ag distances
˚
[2.092(4)-2.092(7) A] in the 1b and 2b complexes.
The nature of M-NHC (M = Ag, Au) interaction in the
gold(I) 1c-4c and silver(I) 1b and 2b complexes were
investigated using a DFT study. Specifically, the geometry
optimized structures of the 1b, 2b, and 1c-4c were obtained
from X-ray coordinates followed by single point calculations
at B3LYP/SDD, 6-31G(d) level of theory (Supporting Infor-
mation, Tables S2-S6). The Natural Bond Orbital (NBO)
calculations were performed on these computed structures to
gain understanding on the electronic structure of these
complexes. Quite interestingly, both the Mulliken and nat-
ural charge analyses showed that binding of the free 1,2,4-
triazole based N-heterocyclic carbene fragment to the metal
center in these complexes, led to significant increase in its
electron density with concomitant decrease in the electron
density on the carbene carbon with respect to the free 1,2,4-
triazole NHC fragment (Supporting Information Tables
S7-S12). Further Natural Bond Orbital (NBO) analysis
revealed that the electron donation occurred on to the 5s
from an additional bridging bromine atom. The Ccarbene-M
˚
bond distances [M = Au; 1.950(10)-1.984(9) A and Ag;
˚
2.092(4)-2.092(7) A] are slightly shorter than the sum of
individual covalent radii of the bonding atoms [C-Au =
2.108 A and C-Ag = 2.111 A]24 but compare well with the
other reported gold(I) and silver(I) 1,2,4-triazole N-hetero-
cyclic carbene complexes, namely, [1,4-bis(methyl)-1,2,4-tria-
˚
˚
22
˚
zol-5-ylidene]AuCl [1.71(8) A], [2-phenyl-6,7-dihydro-5H-
pyrrolo-[2,1-c][1,2,3]triazol-2-ylidene]AuCl [1.979(5) A],
23
˚
andþ[{(1,2-di-methyl-4-(i-propyl)-1,2,4-triazol-3,5-diylidene)-
-
20
˚
Ag} TfO ]n [2.086(4) A], and so forth. Consistent with
˚
the smaller covalent radii of Au [1.37 A] compared to Ag
[1.46 A],25 the Ccarbene-Au distances [1.950(10)-1.984(9) A]
˚
˚
(24) Pauling, L. The Nature of Chemical Bond, 3rd ed.; Cornell University
Press: Ithaca, NY, 1960; pp 256-257.
(25) Tripathi, U. M.; Bauer, A.; Schmidbaur, H. Dalton Trans. 1997,
2865–2868.

4976 Inorganic Chemistry, Vol. 49, No. 11, 2010
Dash et al.
orbital of silver in the 1b and 2b complexes while to the 6s
orbital of gold in the 1c-4c complexes (Supporting Informa-
tion, Tables S13-S14). A careful scrutiny of the NBO results
indicated that the Au-Ccarbene bond in 1c-4c is composed of
an interaction of a Csp orbital of the 1,2,4-triazole based
NHC ligand with an2 sd orbital of the metal center
(Supporting Information, Table S15).
Au) bond dissociation energy (De) at B3LYP/SDD, 6-31G(d)
level of the theory for the gold 1c [79.7 kcal/mol], 2c [81.0
kcal/mol], 3c [80.7 kcal/mol], and 4c [81.2 kcal/mol] and the
silver 1b [56.7 kcal/mol] and 2b [58.5 kcal/mol] complexes
(Supporting Information, Table S17) were found to be
weaker than that estimated for gold(I) and silver(I) com-
plexes of imidazole based N-heterocyclic carbenes, namely,
[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]AuCl
[De: 82.7 kcal/mol]12c and [1-(benzyl)-3-(N-t-butylacetami-
do)imidazol-2-ylidene]AgCl [De: 62.2 kcal/mol].12c
A glimpse of the σ-donating character of the 1,2,4-triazole
based NHC ligand could be obtained by the Charge Decom-
position Analysis (CDA) studies carried out on the silver
1b-2b and the gold 1c-4c complexes using the AOMix-CDA
software, that estimated the extent of the σ-donation (d)
Significantly enough, the gold(I) 1c-4c complexes dis-
played good to high activity for the intermolecular hydro-
amination reaction of o/p-substituted aryl amines with
terminal alkynes in air. Specifically, when terminal alkynes
like, phenylacetylene, 4-ethynyl toluene were treated with
o/p-substituted aryl amines, namely, 2-methylaniline, 2,6-
dimethylaniline, mesitylaniline, 2,6-diethylaniline, and 2,6-
di-i-propylaniline at 2 mol % of the precatalyst loading in
presence of 1 equiv of AgBF4 in air, good to excellent con-
version (42-93%) were obtained in 12 h at 90 ꢀC. In this
context it is worth noting that there have been prior reports
of the use of gold(I) N-heterocyclic carbene complexes for
the hydroamination reactions at the temperature range 40-
200 ꢀC.11a,27 Quite interestingly, the similar catalysis runs
performed with the silver(I) 1b-4b complexes gave extremely
subdued conversions (0-29%), thereby highlighting the role
of gold in catalysis. Further comparison with the control
experiments performed with (SMe2)AuCl in the presence of
AgBF4 and also with the corresponding blank experiment
carried out with only AgBF4 showed significant amplifica-
tion of up to 85% under analogous conditions in case of the
gold 1c-4c precatalysts (Supporting Information, Table S18),
thus underscoring the significance of 1,2,4-triazole based gold
1c-4c precatalysts in the hydroamination reaction. Moreover,
another control experiment performed with a representative
1,2,4-triazole based N-heterocyclic carbene generated in situ
by treating the 1,2,4-triazolium bromide salt (1a) with KOtBu
or Et3N showed no conversions for two representative sub-
strates (Supporting Information, Table S19) thereby ruling
out the possibility of the presence of the decomposed ligand
behind the observed catalysis.
To investigate the regiochemistry of the hydroamination
reaction, the isolation and subsequent characterization of
the final products were carried out for a representative
precatalyst 1c. The hydroaminated products of the addition
reaction showed Markovnikov type N-H addition across
the alkyne substrates. Lastly, to verify the homogeneous
nature of catalysis, the mercury drop test28 showed no
significant decrease in the product yields under the same
conditions.
Important is the comparison of these gold(I) 1,2,4-triazole
based (1c-4c) precatalysts with their imidazole-based counter-
parts. Indeed, the 1,2,4-triazole based gold(I) (1c-4c) com-
plexes showed significantly higher activity compared to the two
representative imidazole derived analogues, namely, [1-(benzyl)-
3-(N-t-butylacetamido)imidazol-2-ylidene]AuCl12c and [1-(2-
hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]AuCl,10f
thereby highlighting the influence of 1,2,4-triazol-5-ylidene
σ
NHC f MXðM ¼ Ag, AuÞ
and π-back-donation (b)
π
NHC r MXðM ¼ Ag, AuÞ
occurring in these complexes. The high d/b ratio of 5.14-4.95
in gold 1c-4c and of 7.66-6.96 in silver 1b and 2b complexes
indicate strong σ-donating nature of the 1,2,4-triazole based
N-heterocyclic carbene ligands in parallel to that seen for
imidazole based N-heterocyclic carbenes (Supporting Informa-
tion, Table S16) in the gold(I) complexes, namely, [1-(benzyl)-
3-(N-t-butylacetamido)imidazol-2-ylidene]AuCl [4.49],12c
[1-(i-propyl)-3-{N-(t-butylacetamido)imidazol-2-ylidene}]2-
AuþCl- [5.23],12b and in the silver(I) complexes, namely,
[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]AgCl
[8.18],12c [1-(i-propyl)-3-{N-(t-butylacetamido)imidazol-2-
ylidene}]2AgþCl- [11.44].12b
Further insight into the nature of NHC-M (M = Ag and
Au) σ-interaction could be obtained by constructing the
molecular orbital (MO) correlation diagram from the indi-
vidual fragment molecular orbitals (FMOs) of the 1,2,4-tri-
azole based NHC ligand and the MX (M = Ag and Au)
fragments. Quite interestingly, the molecular orbital (MO)
depicting the σ-interaction in the gold(I) 1c [HOMO-8,
HOMO-9, and HOMO-28], 2c [HOMO-8, HOMO-10,
and HOMO-32], 3c [HOMO-6, HOMO-7, and HOMO-
28], and 4c [HOMO-7, HOMO-8, and HOMO-29] com-
plexes and in the silver(I) 1b [HOMO-6 and HOMO-24]
and 2b [HOMO-7 and HOMO-25] complexes were found
to be deeply buried thereby indicating the significantly stable
nature of the NHC-M (M = Ag, Au) interaction analogous
to the imidazole based NHC-M σ-interacting molecular
orbital (MO) of gold(I) and silver(I) complexes12b,c (Figure 6
and Supporting Information, Figures S3-S13). In this re-
gard it is worth noting that the well-known Schrock and
Fischer carbenes are very much susceptible to the electro-
philic and nucleophilic attacks unlike the N-heterocyclic
carbene complexes.26
Lastly, the strength of the NHC-M (M = Ag, Au)
interaction, estimated by computing NHC-M (M = Ag,
(26) (a) Valyaev, D. A.; Peterleitner, M. G.; Leont’eva, L. I.; Novikova,
L. N.; Semeikin, O. V.; Khrustalev, V. N.; Antipin, M. Y.; Ustynyuk, N. A.;
Skelton, B. W.; White, A. H. Organometallics 2003, 22, 5491–5497. (b) Esteruelas,
ꢀ
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M. A.; Gonzalez, A. I.; Lopez, A. M.; Onate, E. Organometallics 2003, 22, 414–425.
(c) Raubenheimer, H. G.; Esterhuysen, M. W.; Timoshkin, A.; Chen, Y.; Frenking, G.
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Organometallics 2002, 21, 3173–3181. (d) Barluenga, J.; Florez, J.; Fananas, F. J.
(27) Zeng, X.; Frey, G. D.; Kousar, S.; Bertrand, G. Chem.—Eur. J. 2009,
15, 3056–3060.
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J. Organomet. Chem. 2001, 624, 5–17. (e) Barluenga, J.; Trabanco, A. A.; Florez, J.;
García-Granda, S.; Martín, E. J. Am. Chem. Soc. 1996, 118, 13099–13100.
(f) Padolik, L. L.; Gallucci, J. C.; Wojcicki, A. J. Am. Chem. Soc. 1993, 115,
9986–9996.
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2003, 125, 10301–10310. (b) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A:
Chem. 2003, 198, 317–341.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4977
Figure 6. Simplified orbital interaction diagram showing the major contributions of the NHC-gold bond in 1c.
ligands in stabilizing these highly efficient gold(I) 1c-4c
precatalysts. A 2-4 fold improvement in the product yield
were observed for the 1,2,4-triazole based 1c-4c precata-
lysts. Significantly enough, apart from our present study of
the use of 1,2,4-triazole derived N-heterocyclic carbene
precatalysts, 1c-4c, for the intermolecular hydroamination
of terminal alkynes, we are aware of only two reports of the
use of N-heterocyclic carbene ligands in the terminal alkyne
hydroamination reaction. For example, a cationic gold(I)
complex of cyclic (alkyl)(amino)carbene (CAAC) displayed
good to excellent conversions (70-95%) for the hydroami-
nation of terminal acetylenes, namely, phenylacetylene,
t-butylacetylene, and cyclohexylacetylene, with a series of
primary and secondary amines at 5 mol % of the precatalyst
loading at 40-100 ꢀC in 7-24 h of the reaction time.27
Another example involved the N-heterocyclic carbene ligands,
namely, 1,3-dimethyl-imidazol-2-ylidene and [1,3-di-(2,4,6-
trimethylphenyl)-imidazol-2-ylidene], in conjunction with Ti-
much lower catalyst loading of 2 mol % compared to the
aforementioned reports where higher loadings of 5-10 mol %
were used.
Besides the N-heterocyclic carbene based precatalysts,
various other transition metal based initiators like AuCl3,30
Ti(NMe2)4,31 V(NMe2)4,32 lanthanides and actinides,33 and
so forth have been reported for the intermolecular hydro-
amination of terminalalkynesdisplaying comparable activity
to the gold(I) (1c-4c) precatalysts. Of the gold based pre-
catalysts for the intermolecular hydroamination of terminal
alkynes that exist, notable is the [Au(TPP)]Cl (TPP =
tetraphenylporphyrin) complex that carried out hydroami-
nation reaction of p-methoxyaniline to phenylacetylene at
5 mol % of the precatalyst loading at 80 ꢀC in 12 h of the
reaction time.34 Another Au(III) precatalyst is AuCl3, which
in presence of AgOTf exhibited good activity for intermole-
cular hydroamination of terminal alkynes with aniline deri-
vatives at room temperature at 5 mol % of the precatalyst
loading.30 The 1c-4c complexes were, however, less active
29
(NMe2)4 carried out the alkyne hydroamination reactions
under “Ligand Assisted Catalysis (LAC)” conditions at a
much higher 10 mol % of the catalyst loading. In this regard
it is worth pointing out that the hydroamination reaction
employing the gold 1c-4c precatalysts were performed at a
(30) Luo, Y.; Li, Z.; Li, C.-J. Org. Lett. 2005, 7, 2675–2678.
(31) Shi, Y.; Ciszewski, J. T.; Odom, A. L. Organometallics 2001, 20,
3967–3969.
(32) Lorber, C.; Choukroun, R.; Vendier, L. Organometallics 2004, 23,
1845–1850.
(33) Haskel, A.; Straub, T.; Eisen, M. S. Organometallics 1996, 15, 3773–
3775.
(34) Zhou, C.-Y.; Chan, P. W. H.; Che, C.-M. Org. Lett. 2006, 8, 325–328.
(29) Takaki, K.; Koizumi, S.; Yamamoto, Y.; Komeyama, K. Tetra-
hedron Lett. 2006, 47, 7335–7337.

4978 Inorganic Chemistry, Vol. 49, No. 11, 2010
Dash et al.
Figure 7. Overlay of experimental and simulated LCMS-ESImassspectruminCH3CN showing isotopic distribution of acetonitrile coordinated species of 3c.
than a gold(I) based precatalyst,35 (Ph3P)AuCH3, which at
0.01-0.5 mol % catalyst loading in presence of an acidic
promoter H3PW12O40, exhibited extremely high turnover
numbers (TONs) up to 9000 for hydroamination of a neat
mixture of a variety of alkynes with several aniline derivatives
at 70 ꢀC in 0.25-24 h.
Of particular interest is the mechanism of the intermole-
cular hydroamination of terminal alkynes by a late-transition
metal that generally proceeds via an alkyne activation path-
way, of which mainly two views persist. One involving a
(35) Mizushima, E.; Hayashi, T.; Tanaka, M. Org. Lett. 2003, 5, 3349–3352.

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4979
direct intermolecular nucleophilic attack by the amine on the
alkyne bound to the gold center; however, this route is less
likely as anilines with more electron-withdrawing substitu-
ents were reported to react faster.11c,35 Consequently, the
other more probable route involves coordination of the
aniline to the gold prior to the C-N bond formation. As
the catalysis was performed at 90 ꢀC, the thermal stability of
the 1c-4c precatalysts at this temperature was examined both
in the solid state by thermogravimetric analysis (See Support-
thereby highlighting the importance of gold as a metal in the
catalysis. Comparison of the gold(I) 1c-4c precatalysts with
the gold complexes of imidazole based N-heterocyclic car-
bene analogues, namely, [1-(benzyl)-3-(N-t-butylacetamido)-
imidazol-2-ylidene]AuCl and [1-(2-hydroxy-cyclohexyl)-
3-(benzyl)imidazol-2-ylidene]AuCl, further established the
importance of 1,2,4-triazole based N-heterocyclic carbene
in stabilizing the highly active gold precatalysts 1c-4c for the
alkyne hydroamination reaction.
1
ing Information, Figure S14) and in solution by H NMR
Experimental Section
analysis (See Supporting Information, Figures S15-S18).
1
Indeed, both the thermogravimetric analysis and H NMR
General Procedures. All manipulations were carried out using
standard Schlenk techniques. Solvents were purified and de-
gassed by standard procedures. 3,3-Dimethyl-2-oxobutyl-1,2,4-
triazole,38 2-hydroxy-cyclohexyl-1,2,4-triazole39 and (SMe2)-
AuCl40 were synthesized according to modified literatures
procedures. The synthesis of 1-(N-t-butylacetamido)-4-(benzyl)-
1,2,4-triazolium bromide (2a) was reported earlier.16c 1H and
13C{1H} NMR spectra were recorded in CDCl3 on a Varian
results suggested the 1c-4c precatalysts to be stable at the
hydroamination catalysis temperature of 90 ꢀC.
On the basis of the above observations a proposed me-
chanism (Scheme 3) involve the generation of the solvent
coordinated active species [(NHC)Au(CH3CN)]þBF4 (A)
-
from the precatalyst 1c-4c by treatment with AgBF4 in
acetonitrile accompanied by the precipitation of AgCl. The
formation of the solvent coordinated active species [(NHC)-
Au(CH3CN)]þBF4- (A), for a representative precatalyst 3c,
was corroborated by mass spectrometric studies (Figure 7
and Supporting Information, Figure S19).36 The acetonitrile
adduct species (A), from the representative precatalyst 3c,
was found to be stable in solution at hydroamination reaction
temperature of 90 ꢀC as observed in the LCMS-ESI experi-
ment (See Supporting Information, Figure S20). The reaction
of active species A with terminal alkyne (RCtCH) would
result in the formation of alkyne coordinated species
[(NHC)Au(alkyne)]þBF4- (B) which on reaction with an aryl
1
400 MHz NMR spectrometer. H NMR peaks are labeled as
singlet (s), doublet (d), triplet (t), multiplet (m), and septet (sept).
Infrared spectra were recorded on a PerkinElmer Spectrum One
FT-IR spectrometer. Mass spectrometry measurements were
done on a Micromass Q-Tof spectrometer. GC spectra were
obtained on a PerkinElmer Clarus 600 equipped with a FID.
GC-MS spectra were obtained on a PerkinElmer Clarus 600 T
equipped with EI source. ESI-MS experiments were performed
with a Varian Proctar 500-LCMS instrument, with upper mass
limit of m/z 2000, through direct infusion via a syringe pump.
Standard experimental conditions in the LCMS was at the given
capillary voltage 80 V, needle voltage 5000 V, syringe pressure
10 psi, temperature 320 ꢀC, flow rate 10 μL.min-1, RF loading
80%, nebulizing gas (helium) pressure 50 psi. Elemental Analysis
was carriedout onThermoQuest FLASH 1112 SERIES (CHNS)
Elemental Analyzer. The thermogravimetric analysis of the
1c-4c precatalysts were carried out using a Perkin-Elmer Pyris
thermal analyzer in the temperature range 30-600 ꢀC at a rate of
10 ꢀC/min under nitrogen flow (flow rate: 10 mL/min). X-ray
diffraction data for compounds 1b, 2b, and 1c-4c were collected
on an Oxford Diffraction XCALIBUR-S diffractometer, and
crystal data collection and refinement parameters are summar-
izedinthe Supporting Information, Table S1. The structures were
solved using direct methods and standard difference map techni-
ques, and were refined by full-matrix least-squares procedures on
F2 with SHELXTL (Version 6.10).41
amine would result in [(NHC)Au(amine)(alkyne)]þBF4
-
intermediate (C). The species C then would undergo hydro-
amination of the coordinated alkyne to give alkene coordi-
nated species [(NHC)Au(alkene)]þBF4 (D). Indeed, the
-
DFT calculations performed on the hydroamination of term-
inal alkynes with aryl amines showed the Markovnikov
product to be more stable than the anti-Markovnikov addi-
tion product (See Supporting Information, Figure S21 and
Tables S20-S60). Similar Markovnikov type addition for
hydroamination of terminal alkynes with arylamines has
been reported earlier.11a,27,37 Also the stereospecificity of the
hydroamination reaction, established by the isolation and
subsequent characterization of hydroaminated products for a
representative precatalyst 1c, showed Markovnikov type
addition occurring on the activated terminal alkyne. The
species D would finally yield the desired ketimine product
through tautomerism along with the regeneration of active
species A.
Synthesis of 1-(3,3-Dimethyl-2-oxobutyl)-4-(benzyl)-1,2,4-
triazolium Bromide (1a). A mixture of 3,3-dimethyl-2-oxobutyl-
1,2,4-triazole (0.680 g, 4.07 mmol) and benzyl bromide (0.696 g,
4.07 mmol) in acetonitrile (ca. 5 mL) was refluxed for 6 h, after
which the solvent was removed under vacuum. The reaction
mixture was washed in hot petroleum ether (ca. 3 ꢀ 15 mL) and
dried under vacuum to give the product 1a as a brown viscous
liquid (0.967 g, 70%). 1H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 11.1
(s, 1H, N-C(5)H-N), 9.18 (s, 1H, N-C(3)H-N), 7.57 (br, 2H,
C6H5), 7.36 (br, 3H, C6H5), 5.88 (s, 2H, CH2Ph), 5.81 (s, 2H,
CH2), 1.24 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz,
25 ꢀC): δ 204.8 (CO), 144.2 (N-C(5)H-N), 143.8 (N-C(3)H-N),
132.1 (ipso-C6H5), 129.8 (o-C6H5), 129.6 (m-C6H5), 129.2
(p-C6H5), 57.4 (CH2Ph), 52.1 (CH2), 43.5 (C(CH3)3), 26.1
Conclusions
In summary, a series of new highly convenient gold 1c-4c
precatalysts supported over N/O-functionalized 1,2,4-tria-
zole based N-heterocyclic carbenes for intermolecular
hydroamination of terminal alkynes with o/p-substituted
arylamines yielding ketimines in air have been successfully
designed. These gold(I) precatalysts 1c-4c were found to be
significantly more active than their silver analogues 1b-4b
(38) Xu, L. Z.; Jiao, K.; Zhang, S. S.; Kuang, S. P. Bull. Korean Chem.
Soc. 2002, 23, 1699–1701.
(39) Murabayashi, A.; Makisumi, Y. Heterocycles 1990, 31, 537–548.
(40) Brandys, M.-C.; Jennings, M. C.; Puddephatt, R. J. J. Chem. Soc.,
Dalton Trans. 2000, 4601–4606.
(41) (a) Sheldrick, G. M. SHELXL-97, Program for refinement of crystal
€ €
structures; University of Gottingen: Gottingen, Germany, 1997. (b) Sheldrick,
ꢀ
(36) de Fremont, P.; Marion, N.; Nolan, S. P. J. Organomet. Chem. 2009,
694, 551–560.
(37) (a) Leyva, A.; Corma, A. Adv. Synth. Catal. 2009, 351, 2876–2886.
ꢀ
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(b) Corma, A.; Concepcion, P.; Domínguez, I.; Fornes, V.; Sabater, M. J. J. Catal.
2007, 251, 39–47.
G. M. SHELXS-97, Structure solving program; University of Gottingen:
€
Gottingen, Germany, 1997.

4980 Inorganic Chemistry, Vol. 49, No. 11, 2010
Dash et al.
(C(CH3)3). IR data (cm-1) KBr pellet: 1732 (s) (νCdO). HRMS
(ES): m/z 258.1606 [(NHC)þH]þ, Calcd. 258.1606. Anal.
Calcd. for C15H20N3OBr: C, 53.26; H, 5.96; N, 12.42. Found:
C, 53.05; H, 5.92; N, 12.17%.
slow evaporation technique. 1H NMR (CDCl3, 400 MHz,
25 ꢀC): δ 7.97 (s, 1H, N-C(3)H-N), 7.42-7.35 (m, 5H, C6H5),
5.86 (br, 1H, NH), 5.40 (s, 2H, CH2Ph), 4.96 (s, 2H, CH2), 1.39
(s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 25 ꢀC): δ
175.8 (Au-NCN), 163.7 (CONH), 142.4 (N-C(3)H-N), 133.3
(ipso-C6H5), 129.7 (o-C6H5), 129.6 (m-C6H5), 128.6 (p-C6H5),
56.0 (CH2Ph), 53.1 (CH2), 52.6 (C(CH3)3), 28.9 (C(CH3)3). IR
data (cm-1) KBr pellet: 1681 (s) (νCONH). HRMS (ES): m/z
469.1290 [(NHC)Au]þ, Calcd. 469.1303. Anal. Calcd. for
C15H20N4OAuCl: C, 35.69; H, 3.99; N, 11.10. Found: C,
35.22; H, 3.91; N, 11.18%.
Synthesis of [1-(3,3-Dimethyl-2-oxobutyl)-4-(benzyl)-1,2,4-
triazol-5-ylidene]AgBr (1b). A mixture of 1-(3,3-dimethyl-2-
oxobutyl)-4-(benzyl)-1,2,4-triazolium bromide (1a) (0.436 g,
1.29 mmol) and Ag2O (0.150 g, 0.645 mmol) in acetonitrile
(ca. 30 mL) was stirred at room temperature for 4 h. The reaction
mixture was filtered, and the solvent was removed under vacuum
to give the product 1b as an off white solid (0.289 g, 50%). Single
crystals for X-ray diffraction studies were grown from the
mixture of chloroform and acetonitrile employing a slow eva-
poration technique. 1H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 7.88
(s, 1H, N-C(3)H-N), 7.25 (br, 5H, C6H5), 5.72 (s, 2H, CH2Ph),
5.37 (s, 2H, CH2), 1.22 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3,
100 MHz, 25 ꢀC): δ 207.8 (CO), 187.9 (Ag-NCN), 141.9 (N-
C(3)H-N), 135.0 (ipso-C6H5), 129.1 (o-C6H5), 128.5 (m-C6H5),
128.3 (p-C6H5), 58.1 (CH2Ph), 52.7 (CH2), 43.3 (C(CH3)3), 26.3
(C(CH3)3). IR data (cm-1) KBr pellet: 1723 (s) (νCdO). Anal.
Calcd. for C15H19N3OAgBr: C, 40.48;H, 4.30;N, 9.44. Found: C,
39.60; H, 4.17; N, 9.82%.
Synthesis of 1,4-Bis(3,3-dimethyl-2-oxobutyl)-1,2,4-triazo-
lium chloride (3a). A mixture of 3,3-dimethyl-2-oxobutyl-1,2,4-
triazole (1.66 g, 9.92 mmol) and R-chloropinacolone (1.34 g,
9.92 mmol) in acetonitrile (ca. 10 mL) was refluxed for 8 h. A
white precipitate was formed, washed in hot hexane (ca. 2 ꢀ 20
mL) and dried under vacuum to give product 3a as a white solid
(1.33 g, 44%). 1H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 10.6 (s, 1H,
N-C(5)H-N), 9.49 (s, 1H, N-C(3)H-N), 6.14 (s, 2H, CH2), 5.88
(s, 2H, CH2), 1.25 (s, 18H, C(CH3)3). 13C{1H} NMR (CDCl3,
100 MHz, 25 ꢀC): δ 206.3 (CO), 205.2 (CO), 145.2 (N-C-
(5)H-N), 145.0 (N-C(3)H-N), 56.9 (CH2), 53.3 (CH2), 43.3
(C(CH3)3), 43.2 (C(CH3)3), 26.1 (C(CH3)3), 25.9 (C(CH3)3). IR
data (cm-1) KBr pellet: 1722 (s) (νCdO). HRMS (ES): m/z
266.1873 [(NHC)þH]þ, Calcd. 266.1869.
Synthesis of [1-(3,3-Dimethyl-2-oxobutyl)-4-(benzyl)-1,2,4-
triazol-5-ylidene]AuCl (1c). A mixture of [1-(3,3-dimethyl-2-
oxobutyl)-4-(benzyl)-1,2,4-triazol-5-ylidene]AgBr (1b) (0.173 g,
0.390 mmol) and (SMe2)AuCl (0.115 g, 0.390 mmol) in acetoni-
trile (ca. 30 mL) was stirred at room temperature for 4 h when the
formation of an off-white AgBr precipitate was observed. The
reaction mixture was filtered, and the solvent was removed under
vacuumtogive the product 1c asanoff white solid(0.145g, 76%).
Single crystals for X-ray diffraction studies were grown from the
mixture of chloroform and acetonitrile employing a slow eva-
poration technique. 1H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 8.00
(s, 1H, N-C(3)H-N), 7.39 (br, 3H, C6H5), 7.36 (br, 2H, C6H5),
5.41 (s, 2H, CH2Ph), 5.36 (s, 2H, CH2), 1.29 (s, 9H, C(CH3)3).
13C{1H} NMR (CDCl3, 100 MHz, 25 ꢀC): δ 206.0 (CO), 176.1
(Au-NCN), 142.2 (N-C(3)H-N), 133.5 (ipso-C6H5), 129.6
(o-C6H5), 129.5 (m-C6H5), 128.4 (p-C6H5), 57.0 (CH2Ph), 52.9
(CH2), 43.7 (C(CH3)3), 26.2 (C(CH3)3). IR data (cm-1) KBr
pellet: 1723 (s) (νCdO). HRMS (ES): m/z 454.1180 [(NHC)Au]þ,
Calcd. 454.1194. Anal. Calcd. for C15H19N3OAuCl: C, 36.79; H,
3.91; N, 8.58. Found: C, 36.65; H, 3.99; N, 8.32%.
Synthesis of [1, 4-Bis(3,3-dimethyl-2-oxobutyl)-1,2,4-triazol-
5-ylidene]AgCl (3b). A mixture of 1,4-bis(3,3-dimethyl-2-oxo-
butyl)-1,2,4-triazolium chloride (3a) (0.619 g, 2.05 mmol) and
Ag2O (0.239 g, 1.03 mmol) in acetonitrile (ca. 30 mL) was stirred
for 4 h. The reaction mixture was filtered and the solvent was
dried under vacuum to give the product 3b as an off white solid
(0.461 g, 55%). 1H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 8.11 (s,
1H, N-C(3)H-N), 5.50 (s, 2H, CH2), 5.48 (s, 2H, CH2), 1.26 (s,
9H, C(CH3)3), 1.25 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3,
100 MHz, 25 ꢀC): δ 207.7 (CO), 207.4 (CO), 186.7 (Ag-NCN),
143.7 (N-C(3)H-N), 57.5 (CH2), 53.3 (CH2), 43.5 (C(CH3)3),
43.4 (C(CH3)3), 26.3 (C(CH3)3), 26.2 (C(CH3)3). IR data (cm-1
)
KBr pellet: 1722 (s) (νCdO). Anal. Calcd. for C14H23N3O2AgCl:
C, 41.15; H, 5.67; N, 10.28. Found: C, 41.46; H, 5.73; N, 11.01%.
Synthesis of [1, 4-Bis(3,3-dimethyl-2-oxobutyl)-1,2,4-triazol-
5-ylidene]AuCl (3c). A mixture of [1, 4-bis(3,3-dimethyl-2-oxo-
butyl)-1,2,4-triazol-5-ylidene]AgCl (3b) (0.187 g, 0.458 mmol)
and (SMe2)AuCl (0.135 g, 0.458 mmol) in acetonitrile (ca.
30 mL), was stirred at room temperature for 4 h when the
formation of an off-white AgCl precipitate was observed. The
reaction mixture was filtered, and the solvent was removed under
vacuum togivethe product3c as anoff white solid (0.139 g, 61%).
Single crystals for X-ray diffraction studies were grown from the
mixture of chloroform and acetonitrile employing slow evapora-
tion technique. 1H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 8.09(s, 1H,
N-C(3)H-N), 5.35 (s, 2H, CH2), 5.27 (s, 2H, CH2), 1.31 (s,
9H, C(CH3)3), 1.29 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3,
100 MHz, 25 ꢀC): δ 206.0 (CO), 205.7 (CO), 177.2 (Au-NCN),
143.7 (N-C(3)H-N), 57.0 (CH2), 52.8 (CH2), 43.9 (C(CH3)3),
Synthesis of [1-(N-t-butylacetamido)-4-(benzyl)-1,2,4-triazol-
5-ylidene]AgBr (2b). A mixture of 1-(N-t-butylacetamido)-
4-(benzyl)-1,2,4-triazolium bromide (2a) (0.544 g, 1.54 mmol)
and Ag2O (0.178 g, 0.769 mmol) in acetonitrile (ca. 40 mL) was
stirred at room temperature for 4 h. The reaction mixture was
filtered, and the solvent was dried under vacuum to give the
product 2b as an off white solid (0.304 g, 43%). Single crystals
for X-ray diffraction studies were grown from the mixture of
chloroform and acetonitrile employing a slow evaporation
1
technique. H NMR (CDCl3, 400 MHz, 25 ꢀC): δ 7.91 (s, 1H,
N-C(3)H-N), 7.31-7.30 (m, 3H, C6H5), 7.24 (br, 2H, C6H5),
5.31 (s, 2H, CH2Ph), 5.21 (s, 2H, CH2), 1.33 (s, 9H, C(CH3)3).
13C{1H} NMR (CDCl3, 100 MHz, 25 ꢀC): δ 185.9 (Ag-NCN),
165.2 (CONH), 142.2 (N-C(3)H-N), 134.6 (ipso-C6H5), 129.2
(o-C6H5), 128.8 (m-C6H5), 128.2 (p-C6H5), 56.7 (CH2Ph), 52.8
(CH2), 51.7 (C(CH3)3), 28.7 (C(CH3)3). IR data (cm-1) KBr
pellet: 1678 (s) (νCONH). Anal. Calcd. for C15H20N4OAgBr: C,
39.16; H, 4.38; N, 12.18. Found: C, 38.83; H, 3.95; N, 13.07%.
Synthesis of [1-(N-t-butylacetamido)-4-(benzyl)-1,2,4-triazol-
5-ylidene]AuCl (2c). A mixture of [1-(N-t-butylacetamido)-
4-(benzyl)-1,2,4-triazol-5-ylidene]AgBr (2b) (0.157 g, 0.342
mmol) and (SMe2)AuCl (0.101 g, 0.342 mmol) in acetonitrile
(ca. 30 mL) was stirred at room temperature for 4 h. The
reaction mixture was filtered and the solvent was removed under
vacuum to give the product 2c as an off white solid (0.126 g,
73%). Single crystals for X-ray diffraction studies were grown
from the mixture of chloroform and acetonitrile employing a
43.8 (C(CH3)3), 26.4 (C(CH3)3), 26.3 (C(CH3)3). IR data (cm-1
)
KBr pellet: 1724 (s) (νCdO). HRMS (ES): m/z 462.1442 [(NHC)-
Au]þ, Calcd. 462.1456. Anal. Calcd. for C14H23N3O2AuCl: C,
33.78; H, 4.66; N, 8.44. Found: C, 33.44; H, 4.67; N, 8.62%.
Synthesis of 1-(2-Hydroxy-cyclohexyl)-4-(benzyl)-1,2,4-tria-
zolium bromide (4a). A mixture of 2-hydroxy-cyclohexyl-1,2,4-
triazole (6.55 g, 39.2 mmol) and benzyl bromide (6.70 g, 39.2
mmol) in dichloromethane (ca. 15 mL) was refluxed for 10 h.
The mixture was cooled to room temperature, and the solvent
was removed under vacuum. The mixture was washed with
acetone (ca. 2 ꢀ 30 mL) and dried under vacuum to give the
1
product 4a as a white solid (6.80 g, 51%). H NMR (CDCl3,
400 MHz, 25 ꢀC): δ 10.9 (s, 1H, N-C(5)H-N), 8.48 (s, 1H, N-
C(3)H-N), 7.61-7.58 (m, 2H, C6H5), 7.45-7.40 (m, 3H, C6H5),
5.81 (d, 1H, 2JHH = 14 Hz, CH2), 5.69 (d, 1H, 2JHH = 14 Hz,

Article
Inorganic Chemistry, Vol. 49, No. 11, 2010 4981
CH2), 4.60-4.54 (m, 1H, C6H10), 3.86-3.79 (m, 1H, C6H10),
2.16-2.13 (m, 2H, C6H10), 2.04-1.93 (m, 1H, C6H10),
1.85-1.78 (m, 2H, C6H10), 1.56-1.33 (m, 3H, C6H10). 13C{1H}
NMR (CDCl3, 100 MHz, 25 ꢀC): δ 143.2 (N-C(5)H-N),
142.8 (N-C(3)H-N), 132.0 (ipso-C6H5), 129.9 (o-C6H5), 129.8
(m-C6H5), 129.7 (p-C6H5), 71.3 (C6H10), 69.0 (C6H10), 52.4
(CH2), 34.0 (C6H10), 29.8 (C6H10), 24.4 (C6H10), 24.1 (C6H10).
IR data (cm-1) KBr pellet: 3305(s), 3101(m), 3044(s), 2987(m),
2940(s), 2863(m), 1579(m), 1519(w), 1497(s), 1450(m), 1424(w),
1407(m), 1367(w), 1280(m), 1261(m), 1234(w), 1150(m), 1136(m),
1068(s), 957(m), 872(w), 785(w), 753(m), 709(s), 616(m). HRMS
(ES): m/z 258.1601 [(NHC)þH]þ, Calcd. 258.1606. Anal. Calcd.
for C15H20N3OBr: C, 53.26; H, 5.96; N, 12.42. Found: C, 53.47;
H, 5.92; N, 12.12%.
GAUSSIAN 0342 suite of quantum chemical programs. The
Becke three parameter exchange functional in conjunction
with Lee-Yang-Parr correlation functional (B3LYP) has
been employedin this study.43,44 Stuttgart-Dresden effective
core potential (ECP), representing 19 core electrons, along
with valence basis set; SDD is used for silver45 and gold46 and
all other atoms are treated with 6-31G(d) basis set.47 All
stationary points are characterized as minima by evaluating
Hessian indices on the respective potential energy surfaces.
Tight SCF convergence (10-8 a.u.) was used for all calcula-
tions. Natural bond orbital (NBO) analysis48 was performed
using the NBO 3.1 program implemented in the GAUSSIAN
03 package.
Inspection of the metal-ligand donor-acceptor inter-
actions was carried out using CDA.49 CDA is a valuable tool
in analyzing the interactions between molecular fragments
on a quantitative basis, with an emphasis on the electron
donation.50 The orbital contributions in the geometry opti-
mized silver(I) and gold(I) (NHC)MX (M = Ag, Au, X =
Br, Cl) type species, 1b, 2b, and 1c-4c can be divided into
three parts:
Synthesis of {[1-(2-Hydroxy-cyclohexyl)-4-(benzyl)-1,2,4-tri-
azol-5-ylidene]2Ag}þBr- (4b). A mixture of 1-(2-hydroxy-cyclo-
hexyl)-4-(benzyl)-1,2,4-triazolium bromide (4a) (0.919 g, 2.72
mmol) and Ag2O (0.316 g, 1.36 mmol) in acetonitrile (ca. 30 mL)
was stirred at room temperature for 4 h. The reaction mixture
was filtered, and the solvent was removed under vacuum to give
1
the product 4b as an off white solid (0.614 g, 64%). H NMR
(CDCl3, 400 MHz, 25 ꢀC): δ 7.91 (s, 2H, N-C(3)H-N),
(i) σ—donation from the [NHCfMX], M = Ag, Au, X =
Br, Cl fragment
(ii) π—back-donation from [NHCrMX], M = Ag, Au,
X = Br, Cl fragment
(iii) repulsive polarization (r)
2
7.31-7.29 (m, 10H, C6H5), 5.48 (d, 2H, JHH = 14 Hz, CH2),
2
5.26 (d, 2H, JHH = 14 Hz, CH2), 4.84-4.77 (m, 2H, C6H10),
3.82-3.76 (m, 2H, C6H10), 2.14-2.11 (m, 2H, C6H10),
1.78-1.75 (m, 6H, C6H10), 1.63-1.54 (m, 2H, C6H10),
1.48-1.33 (m, 6H, C6H10). 13C{1H} NMR (CDCl3, 100 MHz,
25 ꢀC): δ 184.4 (Ag-NCN), 142.2 (N-C(3)H-N), 135.2 (ipso-
C6H5), 129.2 (o-C6H5), 128.7 (m-C6H5), 128.3 (p-C6H5), 72.5
(C6H10), 68.7 (C6H10), 52.8 (CH2), 34.5 (C6H10), 31.6 (C6H10),
24.8 (C6H10), 24.6 (C6H10). IR data (cm-1) KBr pellet: 3392(s),
3109(w), 3032(w), 2935(s), 2858(s), 1650(m), 1537(m), 1496(w),
1448(s), 1348(s), 1223(m), 1163(w), 1072(s), 959(m), 871(w),
808(w), 759(w), 716(s), 655(w), 559(w), 505(w). Anal. Calcd.
for C30H38N6O2AgBr: C, 51.30; H, 5.45; N, 11.96. Found: C,
51.26; H, 5.52; N, 12.00%.
The CDA calculations are performed using the program
AOMix,51 and using the B3LYP/SDD, 6-31G(d) wave func-
tion for 1b, 2b, and 1c-4c. Molecular orbital (MO) composi-
tions and the overlap populations were calculated using the
AOMix Program. The analysis of the MO compositions in
terms of occupied and unoccupied fragment orbitals (OFOs
and UFOs, respectively), construction of orbital interaction
diagrams, the CDA was performedusing the AOMix-CDA.52
Further DFT calculations were also performed on the
Markovnikov and anti-Markovnikov products of the hydro-
amination of terminal alkynes with aryl amines at B3LYP/
6-31G* level of theory.
Synthesis of [1-(2-hydroxy-cyclohexyl)-4-(benzyl)-1,2,4-tri-
azol-5-ylidene]AuCl (4c). A mixture of {[1-(2-hydroxy-cyclo-
hexyl)-4-(benzyl)-1,2,4-triazol-5-ylidene]2Ag}þBr- (4b) (0.204 g,
0.291 mmol) and (SMe2)AuCl (0.172 g, 0.583 mmol) in acetoni-
trile (ca. 30 mL) was stirred at room temperature for 4 h when the
formation of an off-white AgBr precipitate was observed. The
reaction mixture was filtered, and the solvent was dried under
vacuum togive theproduct4c asanoffwhite solid (0.171 g, 60%).
Single crystals for X-ray diffraction studies were grown from the
acetonitrile employing a slow evaporation technique. 1H NMR
(CDCl3, 400 MHz, 25 ꢀC): δ 7.90 (s, 1H, N-C(3)H-N),
General Procedure for the Hydroamination of Alkynes. In a
typical run, a Schlenk flask was charged with a mixture of com-
plexes 1b or 2b or 3b or 4b or 1c or 2c or 3c or 4c (2 mol %, 0.01
mmol), AgBF4 (2 mol %, 0.01 mmol), and acetonitrile (ca. 5 mL)
and to this was added the mixture of arylamines, alkynes, and
diethyleneglycol-di-n-butyl ether (internal standard) in a molar
ratio of 1:1.5:1 (See Table 1). The reaction mixture was heated at
90 ꢀC for 12 h in air, after which it was filtered and the product
was analyzed by gas chromatography using diethyleneglycol-di-
n-butyl ether as an internal standard.
2
7.42-7.36 (br, 5H, C6H5), 5.47 (d, 1H, JHH = 15 Hz, CH2),
2
5.36 (d, 1H, JHH = 15 Hz, CH2), 4.62-4.55 (m, 1H, C6H10),
4.08-4.02 (m, 1H, C6H10), 2.19-2.15 (m, 1H, C6H10), 2.08-2.05
(m, 1H, C6H10), 1.85-1.83 (m, 3H, C6H10), 1.45-1.37 (m, 3H,
C6H10). 13C{1H} NMR (DMSO-d6, 100 MHz, 25 ꢀC): δ 171.8
(Au-NCN), 143.4 (N-C(3)H-N), 135.2 (ipso-C6H5), 128.5
(o-C6H5), 128.0 (m-C6H5), 127.6 (p-C6H5), 70.3 (C6H10), 68.4
(C6H10), 51.3 (CH2), 34.0 (C6H10), 31.2 (C6H10), 24.0 (C6H10),
23.5 (C6H10). IR data (cm-1) KBr pellet: 3472(s), 3108(m),
3044(w), 2939(s), 2864(m), 2225(w), 1538(m), 1447(s), 1235(m),
1068(s), 1003(w), 958(m), 900(w), 808(w), 754(w), 724(m),
599(w). HRMS (ES): m/z 454.1183 [(NHC)Au]þ, Calcd.
454.1194. Anal. Calcd. for C15H19N3OAuCl: C, 36.79; H, 3.91;
N, 8.58. Found: C, 37.14; H, 4.08; N, 8.24%.
General Procedure of Control Experiments for the Hydroami-
nation of Alkynes. In a typical run, a Schlenk flask was charged
(43) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100.
(44) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(45) (a) Alkauskas, A.; Baratoff, A.; Bruder, C. J. Phys. Chem. A 2004,
€
108, 6863–6868. (b) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123–141.
(46) (a) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2001, 123, 12899–
ꢀ
ꢀ
12900. (b) Faza, O. N.; Lopez, C. S.; Alvarez, R.; de Lera, A. R. J. Am. Chem.
Soc. 2006, 128, 2434–2437.
(47) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257–2261.
(48) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899–926.
(49) Dapprich, S.; Frenking, G. J. Phys. Chem. 1995, 99, 9352–9362.
(50) (a) Vyboishchikov, S. F.; Frenking, G. Chem.—Eur. J. 1998, 4, 1439–
1448. (b) Frenking, G.; Pidun, U. J. Chem. Soc., Dalton Trans. 1997, 1653–
1662.
(51) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis;
York University: Toronto, Canada, 1997; http://www.sg-chem.net/ (accessed June
27, 2009).
Computational Methods
The density functional theory calculations were performed
on the following silver(I) and gold(I) (NHC)MX (M = Ag,
Au, X = Br, Cl) type species, 1b, 2b, and 1c-4c using the
(42) Frisch, M. J. et al. GAUSSIAN 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(52) Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. J. Am. Chem. Soc. 2006,
128, 278–290.

4982 Inorganic Chemistry, Vol. 49, No. 11, 2010
Dash et al.
Table 1. Selected Results of Hydroamination of Alkynes Catalyzed by 1b-4b, 1c-4c, and Gold Complexes of Imidazole Based N-heterocyclic Carbenesa
a See refs in footnotes c and d. Reaction conditions: 0.50 mmol of aryl/alkyl amine, 0.75 mmol of alkyne, 2 mol % of catalyst 1b-4b or 1c-4c or gold
complexes of imidazole based N-heterocyclic carbenes, 2 mol % AgBF4 and 5 mL of CH3CN at 90 ꢀC (12 h) in air. b The yields (%) were determined by
GC using diethylene glycol di-n-butyl ether as an internal standard. c Ray, L.; Shaikh, M. M.; Ghosh, P. Inorg. Chem. 2008, 47, 230-240. d Ray, L.;
Katiyar, V.; Barman, S.; Raihan, M. J.; Nanavati, H.; Shaikh, M. M.; Ghosh, P. J. Organomet. Chem. 2007, 692, 4259-4269. .
Scheme 3
with a mixture of (SMe2)AuCl/AgBF4 (2 mol %) or [1-(benzyl)-
ether (internal standard) in a molar ratio of 1:1.5:1 (See Table 1 and
Supporting Information, Table S18). The reaction mixture was
heated at 90 ꢀC for 12 h in air, after which it was filtered and the
product was analyzed by gas chromatography using diethylenegly-
col-di-n-butyl ether as an internal standard.
3-N-t-butylacetamido)imidazol-2-ylidene]AuCl12c/AgBF4 (2mol%)
or [1-(2-hydroxy-cyclohexyl)-3-(benzyl)-imidazol-2-ylidene]AuCl10f
AgBF4 (2 mol %) and acetonitrile (ca. 5 mL) and to this was added
the mixture of arylamines, alkynes, and diethyleneglycol-di-n-butyl
/

Article
General Procedure of Blank Experiments. In a typical run, a
Inorganic Chemistry, Vol. 49, No. 11, 2010 4983
ESI-MS Experiment on the Stability of the Representative
Acetonitrile Adduct Species, [{1, 4-bis(3,3-dimethyl-2-oxobutyl)-
Schlenk flask was charged with the mixture of arylamines,
alkynes, and diethyleneglycol-di-n-butyl ether (internal standard)
in a molar ratio of 1:1.5:1 and AgBF4 (2 mol %) and acetonitrile
(ca. 5 mL) (See SupportingInformation, Table S18). The reaction
mixture was heated at 90 ꢀC for 12 h in air, after which it was
filtered and the product was analyzed by gas chromatography
using diethyleneglycol-di-n-butyl ether as an internal standard.
Mercury-Poisoning Experiment. A Schlenk flask was charged
with a mixture of a representative complex 1c (2 mol %, 0.01
mmol), AgBF4 (2 mol %, 0.01 mmol), excess Hg(0) (∼ 100 times
with respect to the catalyst loading), and acetonitrile (ca. 5 mL)
and to this was added the mixture of aryl amines, alkynes, and
diethyleneglycol-di-n-butyl ether (internal standard) in a molar
ratio of 1:1.5:1 (See Supporting Information, Table S18). The
reaction mixture was heated at 90 ꢀC for 12 h in air, after which it
was filtered and the product was analyzed by gas chromatography
using diethyleneglycol-di-n-butyl ether as an internal standard.
1H NMR Experiment on the Stability the 1c-4c Complexes at
90 ꢀC. A solution of 1c-4c in DMSO-d6 (0.6 mL) in an NMR tube
was heated at 90 ꢀC and was monitored by 1H NMR spectroscopy
at different intervals of time (0, 1, and 12 h). No significant change
-
1,2,4-triazol-5-ylidene}Au(CH3CN)]þBF4 (A). An acetonitrile
adduct species, [{1, 4-bis(3,3-dimethyl-2-oxobutyl)-1,2,4-tria-
zol-5-ylidene}Au(CH3CN)]þBF4 (A), proposed to be the
-
initiating species of the hydroamination catalytic cycle (Scheme
3), was generated by the reaction of a representative precatalyst 3c
with AgBF4 in CH3CN. The stability of the acetonitrile adduct
species A was examined by heating the reaction mixture at 90 ꢀC
for 12 h, after which it was filtered and injected in a LCMS-ESI
instrument. The ESI-MSspectrum was recordedina positive ion
mode. A peak at m/z 503 corresponded to the desired acetonitrile
adduct species, [{1, 4-bis(3,3-dimethyl-2-oxobutyl)-1,2,4-triazol-
5-ylidene}Au(CH3CN)]þBF4- (A), and was exactly analogous to
the one observed by the treatment of 3c with AgBF4 at room
temperature (Supporting Information, Figure S20).
Acknowledgment. We thank Department of Science and
Technology (DST), New Delhi, for financial support of this
research. We are grateful to the National Single Crystal X-ray
Diffraction Facility and Sophisticated Analytical Instrument
Facility at IIT Bombay, India, for the crystallographic and other
characterization data. Computational facilities from the IIT
Bombay Computer Center are gratefully acknowledged. C.K.D.
thanks CSIR-UGC, New Delhi, for a research fellowship.
1
in the H NMR resonances was observed as a function of time
thereby attesting to the stable nature of these 1c-4c complexes at
90 ꢀC (See Supporting Information, Figures S15-S18).
ESI-MS Experiment for Detection of the Acetonitrile Coordi-
nated Species. The electrospray ionization mass spectrometry
(ESI-MS) experiment was performed in a positive ion mode to
detect the proposed acetonitrile coordinated gold species of a
representative precatalyst 3c formed upon treatment with AgBF4
during catalysis of the alkyne hydroamination reaction. In a
typical experiment, a mixture of the precatalyst 3c (1 equiv) and
AgBF4 (1 equiv) in acetonitrile was injected in the LCMS-ESI
instrument, and spectrum was collected in positive ion mode.
Specifically, the positive ion mode spectrum showed a peak at m/z
503 corresponding to the desired acetonitrile coordinated gold
species (see Figure 7 and Supporting Information, Figure S19).
Supporting Information Available: Complete reference 42, the
catalysis data of control, blank and mercury-poisoning experi-
ment, TGA plot, X-ray metrical data comparison table, 1H and
13C{1H} NMR of silver(I) 1b-4b and gold(I) 1c-4c complexes.
Ortep plots of 3c and 4c, the B3LYP coordinates of the
optimized geometries for 1b, 2b, 1c-4c, NBO tables, CDA table
along with orbital interaction diagrams of 1b, 2b, 1c-4c and the
B3LYP coordinates of the Markovnikov and anti-Markovnikov
products of the hydroamination reaction of terminal alkynes and
aryl amines. This material is available free of charge via the
Internet at http://pubs.acs.org.