Gold(III) N-heterocyclic carbene complexes mediated synthesis of β-enaminones from 1,3-dicarbonyl compounds and aliphatic amines

Article abstract of DOI:10.1021/ic102268n

A series of gold(III) N-heterocyclic carbene complexes [1-(R ₁)-3-(R₂)imidazol-2-ylidene]AuBr₃ [R ₁ = i-Pr, R₂ = CH₂Ph (1c); R₁ = mesityl, R₂ = CH₂Ph (2c);

Full text of DOI:10.1021/ic102268n

1840 Inorg. Chem. 2011, 50, 1840–1848
DOI: 10.1021/ic102268n
Gold(III) N-Heterocyclic Carbene Complexes Mediated Synthesis of β-Enaminones
From 1,3-Dicarbonyl Compounds and Aliphatic Amines
Manoja K. Samantaray,† Chandrakanta Dash,† Mobin M. Shaikh,‡ Keliang Pang,§ 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, §Department of Chemistry, Columbia University,
||
3000 Broadway, New York, New York 10027, United States , and Department of Chemistry,
Howard University,Washington, DC, 20059, United States
Received November 11, 2010
A series of gold(III) N-heterocyclic carbene complexes [1-(R1)-3-(R2)imidazol-2-ylidene]AuBr3 [R1 = i-Pr, R2 = CH2Ph
(1c); R1 = mesityl, R2 = CH2Ph (2c); R1 = i-Pr, R2 = CH2COt-Bu (3c), and R1 = t-Bu, R2 = CH2COt-Bu (4c)] act as
effective precatalysts in the synthesis of β-enaminones from 1,3-dicarbonyl compounds and primary amines under ambient
conditions. Specifically the 1c-4c complexes efficiently catalyzed the condensation of a variety of cyclic as well as acyclic
1,3-dicarbonyl compounds, namely, acetyl acetone, benzoylacetone, 2-acetylcyclopentanone, and ethyl-2-oxocyclopenta-
necarboxylate with primary aliphatic amines, viz., methylamine, ethylamine, n-propylamine, i-propylamine, and n-butylamine,
yielding β-enamines at room temperature. Interestingly enough, the more electrophilic gold(III) 1c-4c complexes exhibited
superior activity in comparison to the gold(I) counterparts 1b-4b. A comparison along a representative 4a-c series further
underscored the importance of gold in the reaction as both the gold(I) 4b and gold(III) 4c complexes were more effective
than the silver analogue 4a. The density functional theory (DFT) study revealed that the strong σ-donating nature of the
N-heterocyclic carbene ligand results in a strong Ccarbene-Au(III) interaction in the 1c-4c complexes.
Introduction
philicity of the enone functionalities. Thus, because of their
strategic importance to organic synthesis, the development of
convenient high yielding methods remains a key challenge in
the area. A commonly used synthetic protocol employs the
condensation of 1,3-dicarbonyl compounds with amines that
proceeds via an addition-elimination pathway in which the
addition of amine to the carbonyl moiety results in a tetra-
hedral intermediate that subsequently undergoes elimination
of a water molecule to give the desired product.5 Not surpris-
ingly so, the azeotropic separation of water from the reaction
performed in aromatic solvent has thus been routinely used
over the years in the synthesis of these compounds.6 The other
approaches led to the development of methods that use
molecular sieves,7 dehydrating solvents,8 or the Lewis acids,9
which not only remove the water but also facilitate the
β-enaminones and β-enamino esters are important class of
synthetic intermediates for a variety of biologically active
compounds1 of pharmaceutical interests particularly for
their anticonvulsant,2 anti-inflammatory,3 and anticancer
properties.4 As valuable synthons, these β-functionalized
enamines display interesting properties that simultaneously
combine the nucleophilicity of enamine as well as the electro-
*To whom correspondence should be addressed. E-mail: pghosh@
chem.iitb.ac.in. Fax: þ91-22-2572-3480.
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Published on Web 01/21/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1841
nucleophilic attack of the amine on the 1,3-dicarbonyl sub-
strate. Consequently, many improved catalysis with inorgan-
the gold N-heterocyclic carbene complexes have thus become
a topic of current interest.24 However, quite surprisingly so,
of the gold(I) and gold(III) N-heterocyclic carbene com-
plexes known until date, the later has been relatively less
studied despite many gold(III) compounds exhibiting cata-
lytic properties recently. We are aware of only a handful of
examples of the utility of gold(III) N-heterocyclic carbene
complexes in catalytic transformations, namely, in the addi-
tion of water to alkynes,25 styrene polymerization,26 and in
phenol synthesis.27 Given the paucity of the examples of
applications of gold(III) N-heterocyclic carbene complexes in
catalysis and also partly because of our interest in the utility
of the N-heterocyclic carbenes28 in biomedical applications29
and in homogeneous catalysis,30 we became interested in
exploring the catalytic potentials of gold(III) N-heterocyclic
carbene complexes. In this regard we have recently reported
the utility of gold(I) N-heterocyclic carbene complexes for the
regioselective intermolecular hydroamination of alkynes31 and
also in the synthesis of biodegradable polylactide polymer by
the ring-opening polymerization (ROP) of L-lactide.32 Con-
tinuing further along the line and extending to the higher (III)
oxidation state, we chose to employ the strongly Lewis acidic
gold(III) N-heterocyclic carbene complexes for a convenient
synthesis of β-enaminones from the 1,3-dicarbonyl com-
pounds and the aliphatic amines rationalizing that the elec-
trophilic gold(III) center would facilitate the condensation
reaction.
ic salts like Zn(ClO4)2 6H2O, CoCl2 6H2O,11 Zn(OAc)2
10
3
3
3
2H2O,12 ZrOCl2 8H2O,13 Sc(OTf)3,14 etc. have been reported
3
lately. However, despite the advent, the new methods mostly
suffer from various limitations like, the need for high reac-
tion temperatures, prolonged reaction time, requirement of
special apparatuses and the involvement of tedious product
separation procedures, etc. The gold, being a late-transition
metal with reduced oxophilicity, provides a single and
green alternative to the synthesis of β-enaminones from
1,3-dicarbonyls and amines in terms of eliminating the
need for the use of corrosive acid catalysts and also by
avoiding the need for cumbersome azeotropic separation
procedures.15
Notably, the catalytic exploits of gold, otherwise known as
an inert coinage metal, have aroused intense interest lately,16
with the simple inorganic salts or the well-defined complexes of
the noble metal, in its þI or þIII oxidation states, rapidly
finding use in numerous catalytic transformations.17 In parti-
cular, the more electrophilic Au(III) precatalysts are increas-
ingly becoming popular in Lewis acid mediated diverse
processeslike the addition of nucleophiles to alkynes,18 cyclo-
isomerization reactions,19 phenol synthesis,20 the 1,3-dipolar
cycloaddition to nitrones,21 the addition of 2-methylfuran or
electron-rich arenes to methyl vinyl ketone,22 etc. With the
N-heterocyclic carbenes (NHCs) being phenomenally suc-
cessful in homogeneous catalysis,23 the catalytic potentials of
Here in this contribution, we report a series of gold(III)
N-heterocyclic carbene complexes, namely, [1-(R1)-3-(R2)-
imidazol-2-ylidene]AuBr3 [R1 = i-Pr, R2 = CH2Ph (1c); R1 =
mesityl, R2=CH2Ph (2c); R1=i-Pr, R2=CH2COt-Bu (3c),
and R1=t-Bu, R2 = CH2COt-Bu (4c)] that conveniently carry
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1842 Inorganic Chemistry, Vol. 50, No. 5, 2011
Samantaray et al.
Figure 1
Scheme 1
Figure 2. ORTEPof1cwiththermal ellipsoidsdrawn atthe 50% proba-
bility level. Selected bond lengths (angstroms) and angles (degrees): N1-
C1 1.33(3), N2-C1 1.345(19), Au1-C1 2.002(15), Au1-Br1 2.4154(19),
Au1-Br2 2.4101(18), Au1-Br3 2.366(3), N1-C1-N2 107.5(13),
C1-Au1-Br3 178.0(4).
gold(III) bromo N-heterocyclic carbene complexes, namely,
[1,3-bis(tert-butyl)imidazol-2-ylidene]AuBr3 [134.2 ppm],25
[1,3-bis(adamantyl)imidazol-2-ylidene]AuBr3 [132.9 ppm],25
and [1,3-bis(cyclohexyl)imidazol-2-ylidene]AuBr3 [136.8
ppm].25 Quite interestingly, the Au(III)-Ccarbene resonance
of a chloro derivative, namely, [4,5-dimethyl-N,N0-bis(2,6-di-
i-propylphenyl)imidazol-2-ylidene]AuCl3 [145.7 ppm],33 ap-
peared slightly downfield shifted compared to the 1c-4c
bromo derivatives [136.6-140.9 ppm].
The molecular structures of 1c-4c as determined by X-ray
diffraction studies revealed these complexes to be discrete
monomers displaying square planar geometry at the gold(III)
center in agreement with the d8 configuration of the central
metal atom while that of the corresponding gold(I) 1b, 3b,
and silver(I) 2a complexes exhibited two-coordinated linear
geometry at metal center in concurrence with the d10 config-
uration of the gold(I) and silver(I) centers. The Ccarbene-Au-
(III) bond distances in 1c-4c, [2.002(15)-2.047(11) A] were
in good agreement with not only the other reported Au(III)
bromo derivatives, namely, [1,3-bis(tert-butyl)imidazol-2-yli-
dene]AuBr3 [2.015(5) A],25 [1,3-bis(adamantyl)imidazol-2-
ylidene]AuBr3 [2.052(6) A],25 and [1,3-bis(cyclohexyl)-
imidazol-2-ylidene]AuBr3 [2.04(2) A]25 but also with the
gold(III) chloro derivatives, namely, [4,5-dimethyl-N,N0-bis-
(2,6-di-i-propylphenyl)imidazol-2-ylidene]AuCl3 [2.018 A]33
and [4-methyl-5-chloromethyl-N,N0-bis(2,6-di-i-propyl-
phenyl)imidazol-2-ylidene]AuCl3 [2.018 A].33 Lastly, consis-
tent with a shorter covalent radii of Au(I) [1.37 A] than Ag(I)
[1.46 A],34 both the Ccarbene-Au(I) distances [1.974(4)-
1.975(4) A] in 1b and 3b as well as the Ccarbene-Au(III) bond
distances [2.002(15)-2.047(11) A] in 1c-4c, were found to be
shorter than the Ccarbene-Ag(I) bond distances [2.077(2)-
2.087(3) A] in 1a-3a (see Figure 2).
out the condensation reaction of 1,3-dicarbonyl compounds
with amines giving β-enaminones under ambient conditions
(see Figure 1). Quite interestingly, the more electrophilic
gold(III) N-heterocyclic carbene complexes, 1c-4c, were
found to be more efficient than the gold(I) counterparts,
1b-4b.
Results and Discussion
A series of nonfunctionalized and O-functionalized steri-
cally demanding N-heterocyclic carbene ligands were em-
ployed for stabilizing several gold(III) complexes, [1-(R1)-
3-(R2)imidazol-2-ylidene]AuBr3 [R1 = i-Pr, R2 = CH2Ph (1c);
R1 = mesityl, R2 = CH2Ph (2c); R1 = i-Pr, R2 = CH2CO
t-Bu (3c) and R1=t-Bu, R2 = CH2CO t-Bu (4c), see Scheme 1]
for studying their utility as catalyst in the synthesis of
β-enaminones from 1,3-dicarbonyl compounds with alipha-
tic amines. Specifically, the gold(III) 1c-4c complexes were
synthesized by the oxidation of the gold(I) chloro derivatives
1b-4b with molecular bromine at room temperature in
47-72% yield. The gold(I) 1b-4b complexes were obtained
from the silver analogues, 1a-4a, by treatment with Ag2O in
52-71% yield. The formation of the gold(III) 1c-4c com-
plexes were very much evident by appearance of the relatively
upfield shifted Au(III)-Ccarbene resonance at 136.6-140.9
ppm in the 13C{1H} NMR spectrum not only from the
Au(I)-Ccarbene resonance in the gold(I) 1b-4b [168.9-
172.1 ppm] complexes but also from the Ag(I)-Ccarbene
resonance in the corresponding silver(I) [178.7-181.0] 1a-
4a analogues. The Au(III)-Ccarbene resonances in 1c-4c
however appeared in the range observed for other reported
Of particular interest is the nature of the Ccarbene-Au(III)
interaction in the gold(III) 1c-4c complexes particularly for
drawing a parallel with the more studied Ccarbene-Au(I) in-
teraction in the gold(I) N-heterocyclic carbene complexes28c,d
(33) Gaillard, S.; Bantreil, X.; Slawin, A. M. Z.; Nolan, S. P. Dalton
Trans. 2009, 6967–6971.
(34) Tripathi, U. M.; Bauer, A.; Schmidbaur, H. J. Chem. Soc., Dalton
Trans. 1997, 2865–2868.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1843
Figure 3. Simplified orbital interaction diagram showing the major contributions of the NHC-gold bond in 1c.
as well as in other related Au(I)/(III) complexes.35 In this
regard, the detailed density functional theory (DFT) study
was performed at the B3LYP/SDD, 6-31G(d) level of theory
using atomic coordinates obtained from their respective
X-ray diffraction data. Specifically, the geometry optimiza-
tion followed by single point calculations were performed
on the gold(I) 1b, 3b and gold(III) 1c-4c complexes (Sup-
porting Information , Tables S2-S7). The natural bond
orbital (NBO) calculations were then performed at the same
level of theory for obtaining additional insights about the
electronic structure of these complexes. The strong σ-donat-
ing ability of the imidazole based N-heterocyclic carbene
ligand to the metal center was very much evident in the Mulli-
ken and the natural charge analyses that showed, concomi-
tant to the ligand binding, loss of the electron density at the
carbene center of the NHC ligand fragment in gold(I) 1b and
3b complexes as well as in the gold(III) 1c-4c complexes
as opposed to the free NHC ligand (Supporting Informa-
tion, Tables S8-S13). Also quite expectedly, both natural
and Mulliken charge analysis showed that the metal center
is more electron deficient in gold(III) 1c-4c complexes as
compared to the gold(I) 1b and 3b complexes. The natural
bond orbital (NBO) analysis also showed that the electron
donation from the NHC ligand fragment occurred on to
the 6s orbital of the metal center in the gold(I) 1b, 3b, and
gold(III) 1c-4c complexes (Supporting Information, Tables
S14 and S15).
Additional insights on the NHC-Au interaction were ob-
tained from the charge decomposition analysis (CDA) studies
that examined the extent of σ-donation from the NHC ligand to
σ
σ
the metal center (NHC
s AuCl) or (NHC s AuBr ),
f f
3
as designated by d, compared to the back-donation of elec-
π
r
s
trons from the metal to NHC fragment (NHC
AuCl) or
π
s
r
(NHC
AuBr3), represented by b, in the gold(I) 1b, 3b
and gold(III) 1c-4c complexes. A high d/b ratio of 3.97-25.7
in gold(III) 1c-4c complexes and of 4.72-4.78 in gold(I) 1b
and 3b complexes indicate strong σ-donating nature of the
imidazole based N-heterocyclic carbene ligands (Supporting
Information, Table S16). A further glimpse of the NHC-Au
σ interaction could be obtained from the molecular orbital
(MO) correlation diagram constructed from the contribution
of the individual fragment molecular orbital (FMO) of the
NHC ligand and the AuXn (X = Cl, Br; n = 1, 3) fragments
in the gold(III) 1c-4c complexes and the gold(I) 1b and 3b
complexes. Quite interestingly, the molecular orbital (MO)
depicting the σ interaction in the gold(III) 1c [HOMO-29],
2c [HOMO-37], 3c [HOMO-32], and 4c [HOMO-34]
complexes and in the gold(I) 1b [HOMO-21] and 3b
[HOMO-23] complexes were found to be deeply buried
thereby indicating a stable nature of the NHC-Au interac-
tion (Figure 3 and Supporting Information, Figures S7-
S17). Indeed, the NHC-Au bond dissociation energy values
as computed using the B3LYP/SDD, 6-31G(d) level of
(35) (a) Krauter, C. M.; Hashmi, A. S. K.; Pernpointner, M. ChemCatCh-
em 2010, 2, 1226–1230. (b) Lein, M.; Rudolph, M.; Hashmi, S. K.; Schwerdtfeger,
P. Organometallics 2010, 29, 2206–2210. (c) Pernpointner, M.; Hashmi, A. S. K.
J. Chem. Theory Comput. 2009, 5, 2717–2725.

1844 Inorganic Chemistry, Vol. 50, No. 5, 2011
Samantaray et al.
Table 1. Selected Results Synthesis of β-Enaminones Catalyzed by 1b-4b and 1c-4ca
a Reaction conditions: 1.00 mmol of amine, 0.25 mmol of ketone, 5 mol % of catalyst, 15 mol % of AgBF4, 5 mL of CH3CN at room temperature for
6 h. b The yields (%) were determined by GC using diethylene glycol di-n-butyl ether as an internal standard. c The isolated yields for a representative
precatalyst 4c are given in parentheses. d Only the major product is shown.
theory for the gold(III) 1c [82.9 kcal/mol], 2c [82.8 kcal/mol],
3c [83.0 kcal/mol], and 4c [81.3 kcal/mol] and the gold(I) 1b
[82.5 kcal/mol] and 3b [83.7 kcal/mol] complexes are compar-
able to each other as well as to the other reported gold(I)
N-heterocyclic carbene complexes, namely, [1-(benzyl)-3-(N-
tert-butylacetamido)imidazol-2-ylidene]AuCl [82.7 kcal/
mol],28d [1-(i-propyl)-3-{N-(2,6-di-i-propylphenylacetamido)-
imidazol-2-ylidene}]2Au2 [81.0 kcal/mol],28c and [1-(tert-
butyl)-3-{N-(2,6-di-i-propylphenylacetamido)imidazol-2-ylide-
ne}]2Au2 [79.0 kcal/mol]28c (Supporting Information,
Table S17).
Significantly all of the gold(III) 1c-4c complexes carried
out efficient synthesis of β-enaminones fromthe reaction of 1,
3-dicarbonyl compounds with primary aliphatic amines in
good to excellent yields (24 to >99%) (eq 1; Table 1). Spe-
cifically, when the 1,3-dicarbonyls, namely, acetyl acetone,
benzoylacetone, 2-acetylcyclopentanone, and ethyl-2-oxocy-
clopentanecarboxylate, were treated with a variety of pri-
mary aliphatic amines, namely, methylamine, ethylamine,
n-propylamine, i-propylamine, and n-butylamine, at 5 mol %
of the 1c-4c precatalyst loading in the presence of 3 equiv of
AgBF4, the formation of the corresponding β-enaminones
were observed at room temperature after 6 h. Quite interest-
ingly, the gold(III) 1c-4c complexes were found to be more
active (24 to >99%) than the corresponding Au(I) 1b-4b
counterparts that displayed subdued conversions (1-49%)
under analogous conditions (Table 1), thereby underscoring
the importance of the electrophilicity of a gold(III) center in
the catalysis. Further support in favor of the greater electro-
philicity of the metal center in gold(III) 1c-4c complexes
than the gold(I) 1b and 3b complexes also came from the
density functional theory (DFT) studies based on the natural
and Mulliken charge analysis (Supporting Information,
Tables S8-S13). More interestingly so, the importance of
gold as a metal in the catalysis is conspicuous in the fact that
for a representative 4a-c series, both the gold(III) 4c (29 to
>99%) and the gold(I) 4b (1-40%) precatalysts showed
higher conversions that its silver(I) analogue 4a (3-26%)
(Table 1 and Supporting Information, Table S18). The pre-
sence of considerable ligand influence in the catalysis runs of
1c-4c become very much evident when compared to the
blank runs, performed in the absence of both AgBF4 and the
gold catalyst that showed a significantly subdued conversion
1-22%, thus highlighting the important difference made by
the N-heterocyclic carbene ligands in the catalysis (Table 1
and Supporting Information, Table S18). Similarly, a com-
parison with the control runs performed with only AgBF4
(1-39%) showed significant amplification in the case of the
gold(III) 1c-4c precatalysts (24 to >99%) (Table 1 and
Supporting Information, Table S18). Lastly, with the
intent of checking the possibility of free N-heterocyclic
carbene ligand, obtained from the decomposition of 1c-4c
precatalysts, being responsible for the observed catalysis,
experiments were performed with the in situ generated
carbene, prepared from the treatment of a representative
imidazolium chloride salt (4) with KOtBu, which showed
subdued conversion (13-36%) for three substrates as
compared to 4c (37 to >99%) under identical reaction
conditions (Table 1 and Supporting Information, Table
S19). The homogeneous nature of the catalysis reaction
was tested by performing the classical mercury drop
experiments that showed no reduction in gold(III) 1c-4c

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1845
activities in the presence (22 to >99%) and absence of
mercury (24 to >99%).
the 1c-4c complexes represent another new application of
N-heterocyclic carbene based gold(III) precatalysts for the
synthesis of β-enaminones from the reaction of 1,3-dicarbo-
nyls and primary aliphatic amines.
Experimental Section
General Procedures. All manipulations were carried out using
a combination of a glovebox and standard Schlenk techniques.
Solvents were purified and degassed by standard procedures.
Ag2O was purchased from S. D. Fine Chemicals (India) and
used without any further purification. 1-tert-Butylimidazole,40
[1-(i-propyl)-3-(benzyl)imidazol-2-ylidene]AgCl (1a),32a 1-
(benzyl)-3-(2,4,6-trimethylphenyl)imidazolium chloride (2),41
[1-(benzyl)-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene]AgCl
(2a),42 and [1-(i-propyl)-3-(3,3-dimethyl-2-oxobutyl)imidazol-2-
ylidene]AgCl (3a)28e were synthesized according to modified lite-
rature procedures. 1H and 13C {1H} NMR spectra were recorded
on a Varian and Bruker 400 MHz NMR spectrometer. 1H NMR
peaks are labeled as singlet (s), doublet (d), triplet (t), and septet
(sept). Infrared spectra were recorded on a Perkin-Elmer Spec-
trum One FT-IR spectrometer. Mass spectrometry measurements
were done on a Micromass Q-TOF spectrometer. Elemental ana-
lysis was carried out on Thermo Quest FLASH 1112 series (CHNS)
elemental analyzer. GC spectra were obtained on a PerkinElmer
Clarus 600 equipped with a flame ionization detector (FID). Gas
chromatography-mass spectrometry (GC-MS) spectra were ob-
tained on a PerkinElmer Clarus 600 T equipped with an electron
impact (EI) source. X-ray diffraction data for compounds 1b, 2a, 3b,
1c-2c, and 4c were collected on an Oxford Diffraction XCALI-
BUR-S diffractometer, and the data for compound 3c were collected
with a Bruker P4 diffractometer equipped with a SMART CCD
detector and crystal data collection and refinement parameters are
summarized in Table S1 in the Supporting Information. The
structures were solved using direct methods and standard differ-
ence map techniques and were refined by full-matrix least-squares
procedures on F2 with SHELXTL (version 6.10).43
In order to study the chemoselectivity of amination reaction
for the unsymmetrical 1,3-dicarbonyl compounds, namely,
benzoylacetone, 2-acetylcyclopentanone, and ethyl-2-oxocy-
clopentanecarboxylate, subsequent characterization employ-
ing 1H NMR, 13C{1H} NMR, and IR studies of the product,
formed for a representative precatalyst 4c, for these unsymme-
trical substrates were undertaken. In the case of the benzoy-
lacetone substrate (Table 1, entries 6 and 7), only one product
was observed consistent with the imination occurring at the
acetyl position.36 For the cyclic substrate, 2-acetylcyclopenta-
none (Table 1, entry 8), the imination occurred exclusively at
the ring carbonyl moiety for the reaction with ethyl amine37
whereas a mixture of isomers, consisting of the ring carbonyl
imination being the major product (41-60%) for the 1c-4c
precatalysts along with the presence of a minor product arising
from the imination of the acetyl moiety (11-17%) were ob-
served for the reactions with the n-propyl and n-butyl amines
(Table 1, entries 9 and 10). Lastly, for a representative cyclic
β-ketoester substrate, namely, ethyl-2-oxocyclopentanecarbox-
ylate (Table 1, entries 11-13), the imination occurred solely on
the ketone carbonyl moiety yielding cyclic β-aminoester.10,38
Important is the comparison of the performances of gold-
(III) 1c-4c precatalysts with other reported initiators for the
synthesis of β-enaminones from 1,3-dicarbonyl compounds
and primary aliphatic amines. Though several d-10,11,14 and
f-block9b,39 metals have been reported, we are aware of only
one prior report of the use of a gold(III) initiator, namely,
NaAuCl4,15 for the reaction of 1,3-dicarbonyl compounds
with primary and secondary amine substrates yielding β-ena-
minones in good to excellent yields at a relatively low catalyst
loading of 2.5 mol % at room temperature in 7 h. Hence
againstthis backdrop, the well-defined1c-4c precatalysts for
the condensation of 1,3-dicarbonyls with amines under am-
bient conditions is relevant as it represent yet another new
application of N-heterocyclic carbenes in gold catalysis.
Synthesis of [1-(i-Propyl)-3-(benzyl)imidazol-2-ylidene]AuCl
(1b). A mixture of [1-(i-propyl)-3-(benzyl)imidazol-2-ylidene]-
AgCl (1a) (0.556 g, 1.62 mmol) and (SMe2)AuCl (0.476 g, 1.62
mmol) in dichloromethane (∼50 mL) was stirred for 4 h at room
temperature. The reaction mixture was filtered, and the solvent
was dried under vacuum to give the product 1b as a white solid
(0.497 g, 71%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.33 (br,
5H, C6H5), 7.09 (s, 1H, NCHCHN), 7.00 (s, 1H, NCHCHN),
5.33 (s, 2H, CH2), 4.99 (sept, 1H, 3JHH = 6 Hz, CH(CH3)2), 1.46
(d, 6H, 3JHH = 6 Hz, CH(CH3)2). 13C{1H} NMR (CDCl3, 100
MHz, 25 °C): δ 168.9 (Au-NCN), 135.1 (ipso-C6H5), 128.8 (m-
C6H5), 128.4 (p-C6H5), 128.0 (o-C6H5), 120.8 (NCHCHN),
117.4 (NCHCHN), 54.9 (CH2), 53.5 (CH(CH3)2), 23.2
(CH(CH3)2). IR data (KBr pellet) cm-1: 3145 (m), 3109 (s),
2971 (s), 1588 (m), 1449 (s), 1427 (s), 1233 (s), 1189 (m), 747 (s),
719 (s), 461 (w). Anal. Calcd for C13H16AuClN2: C, 36.08; H,
3.73; N, 6.47. Found: C, 36.21; H, 3.59; N, 6.34.
Conclusions
Synthesis of [1-(i-Propyl)-3-(benzyl)imidazol-2-ylidene]AuBr3
(1c). A mixture of [1-(i-propyl)-3-(benzyl)imidazol-2-ylidene]-
AuCl (1b) (0.386 g, 0.891 mmol) and liquid Br2 (0.427 g, 2.67
mmol) in dichloromethane (∼10 mL) was stirred for 1 h at room
temperature. The reaction mixture was dried under vacuum to
give the product 1c as a brown solid (0.409 g, 72%). 1H NMR
(CDCl3, 400 MHz, 25 °C): δ 7.42 (br, 5H, C6H5), 7.21 (s, 1H,
In summary, the gold(III) 1c-4c complexes carry out the
condensation of 1,3-dicarbonyl compounds with primary
amines under ambient conditions. The gold(III) 1c-4c com-
plexes were found to be more superior to the gold(I) 1b-4b
complexes, thereby highlighting the greater electrophilicity of
the gold(III) center is better for catalysis. More significantly,
(36) Karthikeyan, G.; Perumal, P. T. Can. J. Chem. 2005, 83, 1746–1751.
(37) Park, K.-H. Copper(I) Complexes and Processes for Deposition of
Copper Flims by Atomic Layer Deposition. PCT International Application
2008018861, February 14, 2008..
(40) Gridnev, A. A.; Mihaltseva, I. M. Synth. Commun. 1994, 24, 1547–1455.
(41) Flahaut, A.; Roland, S.; Mangeney, P. J. Organomet. Chem. 2007,
692, 5754–5762.
ꢀ
(38) Hebbache, H.; Hank, Z.; Boutamine, S.; Meklati, M.; Bruneau, C.;
(42) Maishal, T. K.; Basset, J.-M.; Boualleg, M.; Coperet, C.; Veyre, L.;
Renaud, J.-L. C. R. Chim. 2008, 11, 612–619.
Thieuleux, C. Dalton Trans. 2009, 6956–6959.
(39) (a) Epifano, F.; Genovese, S.; Curini, M. Tetrahedron Lett. 2007, 48,
2717–2720. (b) Dalpozzo, R.; De Nino, A.; Nardi, M.; Russo, B.; Procopio, A.
Synthesis 2006, 1127–1132.
(43) (a) . Sheldrick, G. M. SHELXL-97, Program for refinement of crystal
structures; University of Gottingen: Germany, 1997. (b) .Sheldrick, G. M. SHELXS-
97, Structure solving program; University of Gottingen: Germany, 1997.

1846 Inorganic Chemistry, Vol. 50, No. 5, 2011
Samantaray et al.
NCHCHN), 7.00 (s, 1H, NCHCHN), 5.35 (s, 2H, CH2), 5.03
(sept, 1H, 3JHH = 6 Hz, CH(CH3)2), 1.57 (d, 6H, 3JHH = 6 Hz,
CH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 139.0
(Au-NCN), 132.7 (ipso-C6H5), 129.7 (m-C6H5), 129.6 (p-C6H5),
129.5 (o-C6H5), 123.7 (NCHCHN), 119.5 (NCHCHN), 55.0
(CH2), 54.2 (CH(CH3)2), 22.9 (CH(CH3)2). IR data (KBr pellet)
cm-1: 3130 (m), 2979 (s), 1468 (s), 1440(s), 1427 (s), 1233 (s), 1188
(s), 1073 (w), 1028 (w), 726 (s). Anal. Calcd for C13H16AuBr3N2:
C, 24.51; H, 2.53; N, 4.40. Found: C, 24.84; H, 3.12; N, 4.54.
Synthesis of 1-(Benzyl)-3-(2,4,6-trimethylphenyl)imidazolium
Chloride (2). 2,4,6-Trimethylphenylimidazole (0.436 g, 2.34
mmol) and benzyl chloride (0.296 g, 2.34 mmol) were taken in
toluene (∼10 mL) and heated at 120 °C for 12 h during which a
white precipitate was formed. The precipitate was collected by
filtration and was washed with hot hexane (∼15 mL) and dried
under vacuum to give the product 2 as white crystalline solid
(0.517 g, 71%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 10.84 (s,
1H, NCHN), 8.13 (s, 1H, NCHCHN), 7.73 (br, 2H, C6H5), 7.31
(br, 3H, C6H5), 7.07 (s, 1H, NCHCHN), 6.92 (s, 2H, m-C6H2-
{2,4,6-Me3}), 5.98 (s, 2H, CH2), 2.31 (s, 3H, p-C6H2{2,4,6-Me3}),
2.02 (s, 6H, o-C6H2{2,4,6-Me3}). 13C{1H} NMR (CDCl3, 100
MHz, 25 °C), δ 140.7 (ipso-C6H5), 138.1 (NCHN), 134.0 (ipso-
C6H2{2,4,6-Me3}), 133.9 (o-C6H2{2,4,6-Me3}), 130.7 (p-C6H2-
{2,4,6-Me3}), 129.5 (m-C6H2{2,4,6-Me3}), 128.9 (o-C6H5), 128.8
(p-C6H5), 128.7 (m-C6H5), 127.9 (NCHCHN), 123.3 (NCH-
CHN), 52.9 (CH2), 20.6 (p-C6H2{2,4,6-Me3}), 17.1 (o-C6H2-
{2,4,6-Me3}). IR data (KBr pellet) cm-1: 3155 (m), 3065 (m),
2975 (s), 1542 (m), 1457 (m), 1376 (w), 1197 (m), 1161 (m), 877
(w), 767 (m), 719 (s). HRMS (ES): m/z 277.1704 [NHC þ H]þ,
calcd 277.1705. Anal. Calcd for C19H21N2Cl: C, 72.95; H, 6.77;
N, 8.95. Found: C, 72.57; H, 7.22; N, 8.26.
(s). Anal. Calcd for C19H20AuClN2: C, 44.85; H, 3.96; N, 5.51.
Found: C, 45.21; H, 3.81; N, 5.06.
Synthesis of [1-(Benzyl)-3-(2,4,6-trimethylphenyl)imidazol-2-
ylidene]AuBr3 (2c). A mixture of [1-(benzyl)-3-(2,4,6-trimethyl-
phenyl)imidazol-2-ylidene]AuCl (2b) (0.206 g, 0.404 mmol) and
liquid Br2 (0.194 g, 1.21 mmol) in dichloromethane (∼5 mL) was
stirred for 1 h at room temperature. The reaction mixture was
dried under vacuum to give the product 2c as a brown solid
(0.198 g, 69%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.47 (br,
5H, C6H5), 7.11 (s, 1H, NCHCHN), 7.06 (s, 1H, NCHCHN),
6.98 (s, 2H, m-C6H2{2,4,6-Me3}), 5.56 (s, 2H, CH2), 2.35 (s, 3H,
p-C6H2{2,4,6-Me3}), 2.22 (s, 6H, o-C6H2{2,4,6-Me3}). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): δ 140.9 (Au-NCN), 135.2 (ipso-
C6H5), 132.9 (ipso-C6H2{2,4,6-Me3}), 130.1 (o-C6H2{2,4,6-Me3}),
129.8 (p-C6H2{2,4,6-Me3}), 129.7 (m-C6H2{2,4,6-Me3}), 129.4 (o-
C6H5), 126.3 (p-C6H5), 126.0 (m-C6H5), 123.2 (NCHCHN), 123.0
(NCHCHN), 55.6 (CH2), 21.3 (p-C6H2{2,4,6-Me3}), 19.6 (o-
C6H2{2,4,6-Me3}). IR data (KBr pellet) cm-1: 3159 (m), 2920
(m), 1482 (s), 1456 (s), 1432 (s), 1229 (s), 1032 (m), 720 (s), 683 (m).
Anal. Calcd for C19H20AuBr3N2: C, 32.00; H, 2.83; N, 3.93.
Found: C, 32.47; H, 2.91; N, 4.18.
Synthesis of [1-(i-Propyl)-3-(3,3-dimethyl-2-oxobutyl)imidazol-
2-ylidene]AuCl (3b). A mixture of [1-(i-propyl)-3-(3,3-dimethyl-2-
oxobutyl)imidazol-2-ylidene]AgCl (3a) (0.517 g, 1.47 mmol) and
(SMe2)AuCl (0.433 g, 1.47 mmol) in dichloromethane (∼40 mL)
was stirred for 4 h at room temperature. The reaction mixture
was filtered, and the solvent was dried under vacuum to give the
product 3b as a white solid (0.339 g, 52%). 1H NMR (CDCl3, 400
MHz, 25 °C): δ 7.02 (s, 1H, NCHCHN), 6.97 (s, 1H, NCHC-
HN), 5.20 (s, 2H, CH2), 5.05 (sept, 1H, 3JHH = 7 Hz, CH(CH3)2),
1.51 (d, 3JHH = 7 Hz, CH(CH3)2), 1.30 (s, 9H, C(CH3)3). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): δ 207.5 (CO), 170.8 (Au-NCN),
122.7 (NCHCHN), 116.7 (NCHCHN), 55.2 (CH2), 53.7 (CH-
(CH3)2), 43.7 (C(CH3)3), 26.3 (C(CH3)3), 23.4 (CH(CH3)2). IR
data (KBr pellet) cm-1: 1721 (s) (νCdO). Anal. Calcd for C12-
H20AuClN2O: C, 32.70; H, 4.57; N, 6.36. Found: C, 33.60; H, 4.69;
N, 5.83.
Synthesis of [1-(Benzyl)-3-(2,4,6-trimethylphenyl)imidazol-2-
ylidene]AgCl (2a). A mixture of 1-(benzyl)-3-(2,4,6-trimethyl-
phenyl)imidazolium chloride (2) (0.569 g, 1.82 mmol) and Ag2O
(0.211 g, 0.911 mmol) in dichloromethane (∼40 mL) were stirred
at room temperature for 4 h. The reaction mixture was filtered
and the solvent was removed under vacuum to give the product
2a as a dark brown solid (0.317 g, 41%). 1H NMR (CDCl3, 400
MHz, 25 °C): δ 7.32 (br, 5H, C6H5), 7.00 (s, 1H, NCHCHN),
6.89 (s, 2H, m-C6H2{2,4,6-Me3}), 6.78 (s, 1H, NCHCHN), 5.44
(s, 2H, CH2), 2.28 (s, 3H, p-C6H2{2,4,6-Me3}), 1.98 (s, 6H,
o-C6H2{2,4,6-Me3}). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C):
δ 181.0 (Ag-NCN), 139.4 (ipso-C6H5), 135.7 (ipso-C6H2{2,4,6-
Me3}), 135.3 (o-C6H2{2,4,6-Me3}), 134.6 (p-C6H2{2,4,6-Me3}),
129.3 (m-C6H2{2,4,6-Me3}), 129.1 (o-C6H5), 128.6 (p-C6H5),
127.5 (m-C6H5), 123.2 (NCHCHN), 121.4 (NCHCHN), 55.4
(CH2), 20.8 (p-C6H2{2,4,6-Me3}), 17.4 (o-C6H2{2,4,6-Me3}). IR
data (KBr pellet) cm-1: 3166 (m), 3130 (m), 3030 (w), 2917 (m),
1496 (s), 1445 (m), 1445 (s), 1411 (m), 1031 (m), 857 (s), 586 (w).
Anal. Calcd for C19H20AgClN2: C, 54.37; H, 4.80; N, 6.67.
Found: C, 55.23; H, 4.57; N, 6.62.
Synthesis of [1-(i-Propyl)-3-(3,3-dimethyl-2-oxobutyl)imidazol-
2-ylidene]AuBr3 (3c). A mixture of [1-(i-propyl)-3-(3,3-dimethyl-
2-oxobutyl)imidazol-2-ylidene]AuCl (3b) (0.102 g, 0.232 mmol)
and liquid Br2 (0.111 g, 0.696 mmol) in dichloromethane (∼10 mL)
was stirred for 1 h at room temperature. The reaction mixture
was dried under vacuum to give the product 3c as a brown solid
(0.071 g, 47%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.33 (s,
1H, NCHCHN), 7.29 (s, 1H, NCHCHN), 5.24 (s, 2H, CH2), 5.03
(sept, 1H, 3JHH = 6 Hz, CH(CH3)2), 1.57 (d, 3JHH = 6 Hz, CH-
(CH3)2), 1.32 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz,
25 °C): δ 205.3 (CO), 136.6 (Au-NCN), 126.3 (NCHCHN), 119.1
(NCHCHN), 54.2 (CH2), 54.1 (CH(CH3)2), 44.0 (C(CH3)3), 26.5
(C(CH3)3), 22.9 (CH(CH3)2). IR data (KBr pellet) cm-1: 1724 (s)
(νCdO). Anal. Calcd for C12H20AuBr3N2O: C, 22.35; H, 3.13; N,
4.34. Found: C, 22.01; H, 2.94; N, 4.57%.
Synthesis of [1-(Benzyl)-3-(2,4,6-trimethylphenyl)imidazol-2-
ylidene]AuCl (2b). A mixture of [1-(benzyl)-3-(2,4,6-trimethyl-
phenyl)imidazol-2-ylidene]AgCl (2a) (0.218 g, 0.519 mmol) and
(SMe2)AuCl (0.153 g, 0.519 mmol) in dichloromethane (∼50
mL) was stirred for 4 h at room temperature. The reaction
mixture was filtered, and the solvent was dried under vacuum to
Synthesis of 1-(tert-Butyl)-3-(3,3-dimethyl-2-oxobutyl) Imida-
zolium Chloride (4). A mixture of 1-tert-butylimidazole (1.88 g,
15.2 mmol) and chloropinacolone (2.05 g, 15.2 mmol) was put in
toluene (∼15 mL) and refluxed for 24 h during which a white solid
precipitated was formed. The precipitate was collected by decanting
off the toluene and was washed with hot hexane (∼15 mL) and dried
under vacuum to give the product 4 as a white solid (2.24 g, 57%).
1H NMR (CDCl3, 400 MHz, 25 °C): δ 10.67 (s, 1H, NCHN), 7.32
(s, 2H, NCHCHN), 5.97 (s, 2H, CH2), 1.72 (s, 9H, C(CH3)3), 1.32
(s, 9H,C(CH3)3).13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ207.0
(CO), 136.0 (NCHN), 124.2 (NCHCHN), 118.6 (NCHCHN), 59.7
(CH2), 54.0 (C(CH3)3), 43.0 (C(CH3)3), 29.5 (C(CH3)3), 25.9
(C(CH3)3). IR data (KBr pellet) cm-1: 1716 (s) (νCdO). HRMS
(ES): m/z 223.1813 [NHC þ H]þ, calcd 223.1810.
1
give the product 2b as a white solid (0.159 g, 60%). H NMR
(CDCl3, 400 MHz, 25 °C): δ 7.37 (br, 5H, C6H5), 7.06 (s, 1H,
NCHCHN), 6.94 (s, 2H, m-C6H2{2,4,6-Me3}), 6.83 (s, 1H,
NCHCHN), 5.50 (s, 2H, CH2), 2.34 (s, 3H, p-C6H2{2,4,6-Me3}),
2.04 (s, 6H, o-C6H2{2,4,6-Me3}). 13C{1H} NMR (CDCl3, 100
MHz, 25 °C): δ 172.1 (Au-NCN), 139.6 (ipso-C6H5), 135.3
(ipso-C6H1{2,4,6-Me3}), 134.7 (o-C6H2{2,4,6-Me3}), 129.4 (p-
C6H2{2,4,6-Me3}), 129.1 (m-C6H2{2,4,6-Me3}), 129.0 (o-C6H5),
128.7 (p-C6H5), 127.8 (m-C6H5), 122.8 (NCHCHN), 120.7
(NCHCHN), 55.0 (CH2), 21.1 (p-C6H2{2,4,6-Me3}), 17.8 (o-
C6H2{2,4,6-Me3}). IR data (KBr pellet) cm-1: 3126 (m), 2919
(m), 1487 (s), 1449 (s), 1418 (m), 1242 (s), 1031 (m), 855 (m), 725
Synthesis of [1-(tert-Butyl)-3-(3,3-dimethyl-2-oxobutyl)imidazol-
2-ylidene]AgCl (4a). A mixture of 1-(tert-butyl)-3-(3,3-dimethyl-2-
oxobutyl) imidazolium chloride (4) (2.18 g, 8.41 mmol) and Ag2O

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1847
(0.977 g, 4.21 mmol) in dichloromethane (∼50 mL) was stirred for
4 h at room temperature. The reaction mixture was filtered, and the
solvent was dried under vacuum to give the product 4a as a dark
brown solid (1.39 g, 45%). 1H NMR (CDCl3, 400 MHz, 25 °C): δ
7.21 (s, 1H, NCHCHN), 6.92 (s, 1H, NCHCHN), 5.21 (s, 2H,
CH2), 1.74 (s, 9H, C(CH3)3), 1.29 (s, 9H, C(CH3)3). 13C{1H}
NMR (CDCl3, 100 MHz, 25 °C): δ 208.0 (CO), 178.7 (Ag-NCN),
121.3 (NCHCHN), 118.7 (NCHCHN), 57.6 (CH2), 56.7
(C(CH3)3), 43.2 (C(CH3)3), 31.6 (C(CH3)3), 26.1 (C(CH3)3). IR
data (KBr pellet) cm-1: 1718 (s) (νCdO). Anal. Calcd for
C13H22AgClN2O: C, 42.70; H, 6.06; N, 7.66. Found: C, 42.73;
H, 6.00; N, 7.98.
CDA is a valuable tool in analyzing the interactions between
molecular fragments on a quantitative basis, with an emphasis
on the electron donation.51 The orbital contributions in the
geometry optimized gold(I) (NHC)AuCl type species, 1b and
3b, and gold(III) (NHC)AuBr3 type species, 1c-4c, can be
divided into three parts: (i) σ-donation from the [NHC f
AuCl] in gold(I) and [NHC f AuBr3] in the gold(III) fragment;
(ii) π-back-donation from the [NHC r AuCl] in gold(I) and
[NHC r AuBr3] in the gold(III) fragment; (iii) repulsive polar-
ization (r).
The CDA calculations are performed using the program
AOMix,52 using the B3LYP/SDD, 6-31G(d) wave function for
1b, 3b, and 1c-4c. Molecular orbital (MO) compositions 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, and
the charge decomposition analysis were performed using the
AOMix-CDA.53
Synthesis of [1-(tert-Butyl)-3-(3,3-dimethyl-2-oxobutyl)imidazol-
2-ylidene]AuCl (4b). Amixtureof[1-(tert-butyl)-3-(3,3-dimethyl-2-
oxobutyl)imidazol-2-ylidene]AgCl (4a) (0.509 g, 1.39 mmol) and
(SMe2)AuCl (0.411 g, 1.39 mmol) in dichloromethane (∼40 mL)
was stirred for 4 h at room temperature. The reaction mixture was
filtered, and the solvent was dried under vacuum to give the
1
product 4b as a white solid (0.383 g, 61%). H NMR (CDCl3,
400 MHz, 25 °C): δ 7.13 (s, 1H, NCHCHN), 6.92 (s, 1H,
NCHCHN), 5.32 (s, 2H, CH2), 1.86 (s, 9H, C(CH3)3), 1.31 (s,
9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ 207.6
(CO), 170.9 (Au-NCN), 120.6 (NCHCHN), 118.5 (NCHCHN),
59.2 (CH2), 56.5 (C(CH3)3), 43.7 (C(CH3)3), 31.8 (C(CH3)3), 26.4
(C(CH3)3). IR data (KBr pellet) cm-1: 1720 (s) (νCdO). Anal.
Calcd for C13H22AuClN2O: C, 34.34; H, 4.88; N, 6.16. Found: C,
35.17; H, 5.10; N, 6.12.
General Procedure for the Synthesis of β-Enaminones. In a
typical run, a 25 mL vial was charged with a mixture of
complexes 1b or 2b or 3b or 4b or 1c or 2c or 3c or 4c (5 mol
%, 0.013 mmol), AgBF4 (15 mol %, 0.039 mmol), and acetoni-
trile (∼5 mL), and to this was added the mixture of 1,3-
dicarbonyl compounds, primary aliphatic amines, and diethyl-
eneglycol-di-n-butyl ether (internal standard) in a molar ratio of
1:4:1 (Table 1). The reaction mixture was stirred at room
temperature for 6 h, after which it was filtered and the product
was analyzed by gas chromatography using diethyleneglycol-di-
n-butyl ether as an internal standard
Synthesis of [1-(tert-Butyl)-3-(3,3-dimethyl-2-oxobutyl)imidazol-
2-ylidene]AuBr3 (4c). A mixture of [1-(tert-butyl)-3-(3,3-dimethyl-
2-oxobutyl)imidazol-2-ylidene]AuCl (4b) (0.215 g, 0.472 mmol)
and liquid Br2 (0.227 g, 1.42 mmol) in dichloromethane (∼10 mL)
was stirred for 1 h at room temperature. The reaction mixture was
dried under vacuum to give the product 4c as a brown solid (0.197
General Procedure of Control Experiments for the Synthesis of
β-Enaminones. In a typical run, a 25 mL vial was charged with a
mixture of AgBF4 (15 mol %, 0.039 mmol) and acetonitrile (∼5
mL) and to this was added the mixture of 1,3-dicarbonyl
compounds, primary aliphatic amines, and diethyleneglycol-
di-n-butyl ether (internal standard) in a molar ratio of 1:4:1 (see
Supporting Information, Table S18). The reaction mixture was
stirred at room temperature for 6 h, after which it was filtered
and the product was analyzed by gas chromatography using
diethyleneglycol-di-n-butyl ether as an internal standard.
General Procedure for the Hg(0) Drop Test. In a typical run, a
25 mL vial was charged with a mixture of complex 4c (5 mol %,
0.013 mmol), AgBF4 (15 mol %, 0.039 mmol), excess Hg(0)
(∼100 times with respect to the catalyst loading), and acetoni-
trile (∼5 mL) and to this was added the mixture of 1,3-dicarbo-
nyl compounds, primary aliphatic amines, and diethyleneglycol-
di-n-butyl ether (internal standard) in a molar ratio of 1:4:1
(see the Supporting Information, Table S18). The reaction
mixture was stirred at room temperature for 6 h, after which it
was filtered and the product was analyzed by gas chromatogra-
phy using diethyleneglycol-di-n-butyl ether as an internal stan-
dard.
1
g, 63%). H NMR (CDCl3, 400 MHz, 25 °C): δ 7.40 (s, 1H,
NCHCHN), 7.23 (s, 1H, NCHCHN), 5.34 (s, 2H, CH2), 1.89 (s,
9H, C(CH3)3), 1.32 (s, 9H, C(CH3)3). 13C{1H} NMR (CDCl3, 100
MHz, 25 °C): δ 205.2 (CO), 138.3 (Au-NCN), 125.6 (NC-
HCHN), 122.2 (NCHCHN), 61.9 (CH2), 55.1 (C(CH3)3), 40.4
(C(CH3)3), 31.7 (C(CH3)3), 26.6 (C(CH3)3). IR data (KBr pellet)
cm-1: 1724 (s) (νCdO). Anal. Calcd for C13H22AuBr3N2O: C,
23.69; H, 3.36; N, 4.25. Found: C, 23.27; H, 3.39; N, 4.46.
Computational Methods. The density functional theory cal-
culations were performed on the following gold(I) (NHC)AuCl
type species, 1b and 3b, and gold(III) (NHC)AuBr3 type species,
1c-4c using the Gaussian 0344 suite of quantum chemical
programs. The Becke three parameter exchange functional in
conjunction with the Lee-Yang-Parr correlation functional
(B3LYP) has been employed in this study.45,46 The Stuttgart-
Dresden effective core potential (ECP) along with the valence
basis set SDD is used for gold,47 and all other atoms are treated
with the 6-31G(d) basis set.48 All stationary points are char-
acterized as minima by evaluating Hessian indices on the
respective potential energy surfaces. Tight SCF convergence
(10-8 au) was used for all calculations. Natural bond orbital
(NBO) analysis49 was performed using the NBO 3.1 program
implemented in the Gaussian 03 package.
Acknowledgment. We thank BRNS, Mumbai, 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 crystallo-
graphic and other characterization data. Computational facilities
from the IIT Bombay Computer Center are gratefully acknowl-
edged. M.K.S. and C.D. thanks CSIR-UGC, New Delhi, for
research fellowships.
Inspection of the metal-ligand donor-acceptor interactions
was carried out using charge decomposition analysis (CDA).50
(44) Frisch, M. J. et al. . Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford CT, 2004.
(45) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100.
(46) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(47) (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.
(48) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257–2261.
(49) Reed, A. E.; Curtiss, L. A.; Wienhold, F. Chem. Rev. 1988, 88, 899–
926.
(50) Dapprich, S.; Frenking, G. J. Phys. Chem. 1995, 99, 9352–9362.
(51) (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.
(52) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis;
York University: Toronto, Canada, 1997; http://www.sg-chem.net/ (accessed
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1848 Inorganic Chemistry, Vol. 50, No. 5, 2011
Samantaray et al.
Supporting Information Available: Complete ref 44; the
catalysis data of control, blank, and Hg(0) drop test results; 1H
and 13C{1H} NMR of silver(I), gold(I), and gold(III) complexes;
X-ray metrical data comparison table; ORTEP plots of 1b, 2a, 2c,
3b, 3c, and 4c; the B3LYP coordinates of the optimized geome-
tries for 1b, 3b, and 1c-4c; NBO tables andCDA table along with
orbital interaction diagrams of 1b, 3b, and 1c-4c. This material is
available free of charge via the Internet at http://pubs.acs.org.