Fluoride-free Hiyama and copperand amine-free Sonogashira coupling in air in a mixed aqueous medium by a series of PEPPSI[?]-themed precatalysts

Article abstract of DOI:10.1002/ejic.200900115

A new series of robust, user-friendly, and highly active PEPPSI-themed (pyridine-enhanced precatalyst preparation, stabilization and initiation) (NHC)PdX₂(pyridine)-type (X = Cl, Br) precatalysts of C4-C5 saturated imidazole(1-4) and triazole-based (5 and 6) N-heterocyclic carbenes for the Hiyama and Sonogashira couplings under amenable conditions are reported. Specifically 1-6 efficiently catalyze the fluoride-free Hiyama coupling of aryl halides with PhSi(OMe)₃ and CH₂=CHSi(OMe)₃ in air in the presence of NaOH as a base in a mixed aqueous medium (dioxane/H ₂O, 2:1 v/v). Along the same lines, these 1-6 precatalysts also promote the Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with phenylacetylene in air and in a mixed aqueous medium (DMF/H ₂O, 3/1 v/v). The complexes 1-6 were synthesized by the direct reaction of the respective imidazolinium and triazolium halide salts with PdCl₂ in pyridine in the presence of K₂CO₃ as a base. DFT studies on the catalytically relevant palladium(0) (NHC)Pd(pyridine) precursors 1a-6a reveal significant donation from the N-heterocyclic carbene lone pair onto the unfilled σ^* orbital of the trans Pd-pyridine bond. This weakens the Pd-bound "throwaway" pyridine ligand, and its dissociation marks the initiation of the catalytic cycle. ( Wiley-VCH Verlag GmbH and Co. KGaA, 69451 Weinheim, Germany, 2009).

Full text of DOI:10.1002/ejic.200900115

FULL PAPER
DOI: 10.1002/ejic.200900115
Fluoride-Free Hiyama and Copper- and Amine-Free Sonogashira Coupling in
Air in a Mixed Aqueous Medium by a Series of PEPPSI[‡]-Themed
Precatalysts
Chandrakanta Dash,[a] Mobin M. Shaikh,[b] and Prasenjit Ghosh*[a]
Keywords: Carbenes / Palladium / Cross-coupling / Hiyama reaction / Sonogashira reaction
A new series of robust, user-friendly, and highly active
PEPPSI-themed (pyridine-enhanced precatalyst preparation,
stabilization and initiation) (NHC)PdX2(pyridine)-type (X =
Cl, Br) precatalysts of C4–C5 saturated imidazole- (1–4) and
triazole-based (5 and 6) N-heterocyclic carbenes for the Hi-
yama and Sonogashira couplings under amenable conditions
are reported. Specifically 1–6 efficiently catalyze the fluo-
ride-free Hiyama coupling of aryl halides with PhSi(OMe)3
and CH2=CHSi(OMe)3 in air in the presence of NaOH as a
base in a mixed aqueous medium (dioxane/H2O, 2:1 v/v).
Along the same lines, these 1–6 precatalysts also promote the
Cu-free and amine-free Sonogashira coupling of aryl bro-
mides and iodides with phenylacetylene in air and in a mixed
aqueous medium (DMF/H2O, 3/1 v/v). The complexes 1–6
were synthesized by the direct reaction of the respective
imidazolinium and triazolium halide salts with PdCl2 in pyr-
idine in the presence of K2CO3 as a base. DFT studies on the
catalytically relevant palladium(0) (NHC)Pd(pyridine) pre-
cursors 1a–6a reveal significant donation from the N-hetero-
cyclic carbene lone pair onto the unfilled σ* orbital of the
trans Pd–pyridine bond. This weakens the Pd-bound “throw-
away” pyridine ligand, and its dissociation marks the initia-
tion of the catalytic cycle.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
gents in the Suzuki–Miyaura reactions as well as the hand-
ling of the toxic byproducts associated with the organotin
reagents in the Migita–Kosugi–Stille reactions, the Hiyama
coupling thus holds greater promise than these two now-
popular coupling reactions.
Introduction
Of late, the Hiyama[1] coupling is becoming increasingly
popular for the construction of biaryl frameworks along
the lines of the highly preferred Suzuki–Miyaura[2] reaction,
involving organoboron reagents, and the Migita–Kosugi–
Stille[3] reaction, employing organotin reagents, which are
routinely used in the synthesis of biaryl targets. The recent
success of the Hiyama coupling originates from the new
found prospect of organosilicon compounds as organome-
tallic nucleophiles in palladium-mediated cross-coupling re-
actions.[4] With regard to this it is worth noting that the
organosilicon reagents, because of their low cost, easy avail-
ability, environmentally benign nature, reagent stability,
functional group tolerance, and amenable reaction condi-
tions, have aroused interest in recent times. Furthermore, as
the Hiyama coupling significantly eliminates the purifica-
tion difficulties associated with the use of organoboron rea-
An important challenge of Hiyama coupling lies in en-
hancing the nucleophilicity of the organosilicon reagents,
which are inherently inert nucleophiles and hence unreac-
tive owing to the small electronegativity difference between
Si and C.[4] However, in the presence of an anionic activator
like the most commonly used F– ion or others like, OH– or
OR– ions, the nucleophilicity of the organosilicon reagent
increases as a result of the coordination of the anion to Si
thereby making the Si center pentacoordinate and anionic.
An important objective therefore lies in developing fluo-
ride-free activation of the organosilicon reagent owing to
the environmental issues associated with fluorine. An inge-
nious method of activation that deserves special mention is
that of the “strain release Lewis acidity” encountered in
siletane, which provides the first successful alternative to
fluorosilanes.[5] With regard to this our aim is in designing
highly efficient and robust Pd precatalysts of N-heterocyclic
carbenes for the fluoride-free Hiyama couplings under ame-
nable conditions. It is worth noting that although phos-
phanes have been commonly used as catalysts for the
Hiyama coupling,[4] the utility of the N-heterocyclic carb-
enes in this area remain largely unexplored.
[‡] Pyridine-Enhanced Precatalyst Preparation, Stabilization and
Initiation
[a] Department of Chemistry, Indian Institute of Technology Bom-
bay
Powai, Mumbai 400076, India
Fax: +91-22-2572-3480
E-mail: pghosh@chem.iitb.ac.in
[b] National Single Crystal X-ray Diffraction Facility, Indian Insti-
tute of Technology Bombay,
Powai, Mumbai 400076, India
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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F-Free Hiyama and Cu- and Amine-Free Sonogashira Coupling
Besides the Hiyama coupling, our other interest is in
Sonogashira coupling as it provides a much needed pathway
for the construction of the “enyne” framework common in
many bioactive molecules[6] as well as in materials related
applications.[7] Despite its new-found popularity, the Sono-
gashira cross-coupling reactions suffer from many limita-
tions like requiring stringent anaerobic conditions and it
yields unwanted homocoupled side products alongside the
desired cross-coupled products. Thus, a worthwhile objec-
tive in this area lies in achieving more amenable conditions
for the coupling reaction. A careful scrutiny of the mechan-
istic pathway of the Sonogashira coupling reveals that the
coupling reactions proceed via the formation of a Cu–ace-
tylide species, which is extremely air and moisture sensitive,
potentially explosive and yields unwanted homocoupled
products in the presence of oxygen or under oxidizing con-
ditions.[7] Hence, it is not surprising that the Cu-free Sono-
gashira coupling, which holds the possibility of making the
reaction more tolerant towards the aerobic conditions, is
becoming increasingly popular these days. However, many
of the Cu-free Sonogashira couplings involve the use of
amines that are not considered environmentally friendly.[7b]
Thus, achieving Sonogashira coupling under both the
amine-free and Cu-free conditions remains an extremely
important objective today. In this context it is worth men-
tioning that we have recently reported a series of highly
active Pd precatalysts supported over N/O-functionalized
N-heterocyclic carbenes for the amine-free Sonogashira
couplings of aryl iodides with terminal alkynes in air in a
Results and Discussion
The complexes 1–4, with a C4–C5 saturated imidazole
ring (Scheme 1), and the complexes 5 and 6 (Scheme 2),
with a triazole ring, were synthesized from the respective
imidazolinium chloride and triazolium bromide salts by the
reaction with PdCl2 in pyridine in the presence of K2CO3
as a base. The formation of the metal complexes were evi-
dent from the distinctive metal-bound carbene (Pd–Ccarbene
)
peak, which appeared at ca. 184.6–186.3 ppm for the C4–
C5 saturated imidazole complexes 1–4 and at 153.6–
159.0 ppm for the triazole complexes 5 and 6 in the 13C
NMR spectrum at a significantly downfield shifted reso-
nance relative to that of the imidazolinium (160.0–
161.3 ppm)[13–15] and triazolium (143.7–143.8 ppm) NCHN
peaks of the starting 1igand precursors.
Scheme 1.
Of foremost importance to a PEPPSI-themed precatalyst
mixed aqueous solvent.[8] We also established that precata- like 1–6 is the loosely bound pyridine moiety that appears
lysts with electron-rich metal centers are better for the at a different region from that of the free pyridine in the 1H
cross-coupling reaction.
NMR spectrum.[16] However, the definitive proof came
Our research revolves around exploring the utility of from the X-ray diffraction study, which confirmed the pres-
N/O-functionalized N-heterocyclic carbenes[9] in biomedical ence of the metal-bound pyridine moiety in the 1–6 com-
applications[10] as well as in chemical catalysis,[11] with spe- plexes (see Figures 2–3 and Figures S1–S4 in the Support-
cial emphasis on developing robust, user-friendly, and ing Information). In particular, the Pd–Npyridine distance
highly efficient precatalysts for the Pd-mediated C–C cross- in 1 [2.106(6) Å], 2 [2.107(2) Å], 3 [2.0950(16) Å], 4
coupling reactions[8,12] and hence, here in this contribution, [2.096(3) Å], 5 [2.092(8) Å], and 6 [2.100(3) Å] was compar-
we report on the use of a series of new PEPPSI-themed able to that of its related unsaturated analogues namely, the
(NHC)PdX2(pyridine) (X = halide) complexes of the C4– non-functionalized aryl-substituted trans-[1,3-bis(2,6-di-
C5 saturated imidazole- (1–4) and triazole-based (5 and 6) isopropylphenyl)imidazol-2-ylidene]PdCl2(3-chloropyridine)
N-heterocyclic carbenes for the two highly important but [2.137(2) Å],[17b] or with the N/O-functionalized ones like,
relatively less explored C–C cross-coupling reactions the trans-[1-benzyl-3-(tert-butylaminocarbonylmethyl)imid-
namely the Hiyama and Sonogashira couplings (Figure 1).
azol-2-ylidene]PdCl2(pyridine) [2.089(3) Å],[12b] trans-[1-(2-
hydroxycyclohexyl)-3-benzylimidazol-2-ylidene]PdCl2(pyr-
idine) [2.096(3) Å],[12b] and trans-[1-(o-methoxybenzyl)-3-
(tert-butyl)imidazol-2-ylidene]PdBr2(pyridine) [2.100(8) Å].[12b]
It is worth noting that owing to the strong trans influence
of the N-heterocyclic carbene (NHC) ligand, the Pd bound
pyridine moiety is expected to be considerably weakly
bonded. Hence, quite expectedly, the Pd–Npyridine distance
[2.092(8)–2.107(2) Å] in the complexes 1–6 was found to be
longer than that in the complexes without the NHC ligand
e.g. 2.040(3) Å in [2,6-bis(2Ј-indolyl)pyridine]Pd(pyr-
idine),[18] 2.037(3) Å in [bis(bis(2-pyridylmethyl)amine-
–
N,NЈ,NЈЈ)]Pd(pyridine)](ClO4 )2,[19] and 2.038(4) Å in
Figure 1. PEPPSI–themed (NHC)PdX2(pyridine) (X = halide)
complexes of the C4–C5 saturated imidazole- (1–4) and triazole-
based (5 and 6) N-heterocyclic carbenes.
–
[(2,2Ј:6Ј,2ЈЈ-terpyridine)Pd(pyridine)](ClO4 )2.[20] It is note-
worthy that a weakly-bound pyridine moiety is a strategic
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Scheme 2.
hallmark of a PEPPSI-themed precatalyst. The pyridine
moiety is considered to be a “throwaway” ligand as it paves
way for the incoming substrate.[17]
Figure 3. ORTEP drawing of 5 with thermal ellipsoids shown at
the 50% probability level. Selected bond lengths [Å] and angles
[°]: N(1)–C(1) 1.341(12), N(2)–C(1) 1.375(12), Pd(1)–C(1) 1.950(9),
Pd(1)–Br(1) 2.3669(15), Pd(1)–N(4) 2.092(8), C(1)–Pd(1)–N(4)
178.6(3), C(1)–Pd(1)–Br(1) 88.5(3).
Figure 2. ORTEP drawing of 1 with thermal ellipsoids shown at
the 50% probability level. Selected bond lengths [Å] and angles
[°]: N(1)–C(1) 1.334(9), N(2)–C(1) 1.317(9), Pd(1)–C(1) 1.993(7),
Pd(1)–Cl(1) 2.294(2), Pd(1)–N(3) 2.106(6), C(1)–Pd(1)–N(3)
174.2(3), C(1)–Pd(1)–Cl(1) 93.7(2).
nature of the bonding in 1a–6a was obtained from the post-
wave function analysis using the natural bond orbital
In order to gain insight about the relative electronic in- (NBO) method.[21]
fluence of the C4–C5 saturated imidazole- and triazole-
It is quite interesting that both the C4–C5-saturated
based N-heterocyclic carbene ligands on the metal center, imidazole- and the triazole-based ligands were found to be
detailed density functional theory (DFT) studies were car- of comparable electron-donating abilities as was evident
ried out on the catalytically relevant palladium(0) (NHC)- from the natural and Mulliken charge analyses, which
Pd(pyridine)-type precursors 1a–6a. Specifically, the geome- showed that the metal and carbene centers were of compar-
try optimization followed by single-point calculations were able electron densities in 1a–6a (see Tables S8–S13, Sup-
performed on 1a–6a at the B3LYP/SDD, 6-31G(d) level of porting Information). In all of the cases, as a result of elec-
theory after applying suitable modifications on the struc- tron donation from the free NHC ligand fragment onto the
tures obtained from X-ray analysis. Further insight into the metal center in the Pd–pyridine moiety, there was a de-
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F-Free Hiyama and Cu- and Amine-Free Sonogashira Coupling
crease in electron density on the carbene carbon in 1a–6a NHC–Pd σ-bonding molecular orbitals [1a (HOMO-11,
with respect to that in the free NHC ligand fragment along HOMO-25), 2a (HOMO-11, HOMO-21), 3a (HOMO-11,
with a concomitant increase in the electron density on the HOMO-19), 4a (HOMO-11, HOMO-19), 5a (HOMO-9,
metal center in 1a–6a with respect to that in the Pd–pyr- HOMO-17), and 6a (HOMO-11, HOMO-22)] were found
idine fragment (see Tables S8–S13, Supporting Infor- to be deeply seated, thereby, implying the inert nature of
mation). Furthermore, a comparison of the electronic con- the NHC–Pd σ bond. This also explains the fact that it is
figuration of the metal center in 1a–6a relative to that of relatively less vulnerable towards electrophilic or nucleo-
the Pd–pyridine fragment revealed that the electron do- philic attacks. It is worth noting that contrary to the inert
nation from the carbene center of the free NHC ligand frag- nature of the NHC–Pd bond that arises from the low lying
ment occurred at the 5s orbital of palladium (see NHC–Pd σ-bonding molecular orbitals in 1a–6a, the other
Tables S14–S15, Supporting Information). The natural metal–carbene bonds in carbene complexes like the Fischer
bond order (NBO) analysis revealed that the NHC–Pd and Schrock carbene complexes are generally much more
bond is composed of the interaction between a Ccarbene (sp2) reactive and are attacked by electrophiles and nucleo-
orbital with an sd hybrid Pd orbital (see Table S16, Sup- philes.[24]
porting Information).
Of foremost importance is the NHC–Pd σ-bonding mo-
lecular orbital [1a (HOMO-11), 2a (HOMO-11), 3a
(HOMO-11), 4a (HOMO-11), 5a (HOMO-9), and 6a
(HOMO-11)], which showed an interaction of the carbene
lone pair of the free NHC-ligand fragment with the σ*-anti-
bonding orbital of the Pd–pyridine bond indicating a strong
trans influence of the NHC moiety resulting in the weaken-
ing of the Pd–pyridine bond in 1a–6a. A comparison of the
extent of interaction between the carbene lone pair with the
In order to further probe the nature of the Pd–carbene
interaction in 1a–6a, particularly with regard to the extent
σ
of the NHCǞPd(NC5H5) forward donation, designated by
π
d, and the NHCǟPd(NC5H5) backward donation, desig-
nated by b, occurring in these complexes, charge decompo-
sition analyses[22] were performed. It is worth noting that
both the C4–C5 saturated imidazole- (1–4) and the triazole-
based (5 and 6) species were found to be predominantly
σ
NHCǞPd(NC5H5) donating (d) with very little σ*-antibonding Pd–pyridine orbital in the C4–C5 saturated
π
NHCǟPd(NC5H5) backward bonding (b) ability as was imidazole- 1a–4a and the triazole-based 5a and 6a species,
duly attested by a high d/b ratio (2.60–2.94) observed in revealed that a slightly higher carbene lone pair donation to
these species (see Table S17, Supporting Information).
the σ*-antibonding Pd–pyridine orbital was observed for
The NHC–Pd interaction was further probed by con- the C4–C5-saturated imidazole-based 1a–4a systems (10%)
structing a molecular orbital correlation diagram from the as compared to the triazole-based 5a and 6a ones (7–8%),
individual fragment molecular orbitals (FMOs) of the free which suggests that the Pd–pyridine moiety is more weakly
NHC ligand fragment and the Pd–pyridine fragment in 1a– bound in 1a–4a than in 5a and 6a (see Table S18). Indeed,
6a using the AOMix software[23] (Figures 4–5 and Fig- the Pd–pyridine bond dissociation energy De (Pd–pyridine)
ures S5–S8, Supporting Information). In all the cases, the computed at the B3LYP/SDD, 6-31G(d) level of theory
Figure 4. Orbital interaction diagram showing the major contributions of the NHC–palladium bond in 1a.
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C. Dash, M. M. Shaikh, P. Ghosh
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Figure 5. Orbital interaction diagram showing the major contributions of the NHC–palladium bond in 5a.
showed a marginally weaker Pd–pyridine bond [27.0– presence of NaOH as a base in a mixed aqueous medium
27.5 kcal/mol] in 1a–4a than in 5a and 6a [28.1–29.4 kcal/ (dioxane/H2O, 2:1 v/v) in good to excellent yields at 80 °C
mol]. A similar trend was observed for the Ccarbene–Pd bond (Table 1). A significant enhancement of the product yield
dissociation energy De (Ccarbene–Pd) with a slightly weaker of up to 91% for the C4–C5 saturated imidazole complexes
Ccarbene–Pd bond [55.3–56.1 kcal/mol] in 1a–4a than in 5a 1–4 and of up to 84% for the triazole complexes 5 and 6
and 6a [56.3–58.1 kcal/mol].
was observed relative to the control experiment performed
Quite significantly, the 1–6 complexes were found to be with PdCl2 under the same conditions (Table S19, Support-
efficient precatalysts for two important but rather less ex- ing Information). Thus, high efficiency and robustness form
plored cross-coupling reactions namely, the Hiyama coup- the key attributes of these PEPPSI-themed 1–6 precatalysts,
ling under the highly preferred fluoride-free conditions performed in air in a mixed aqueous medium under fluo-
[Equation (1) and Table 1] and the Sonogashira coupling, ride-free Hiyama conditions.
also under much desired Cu-free and amine-free conditions
[see Equation (2) and Table 2].
A comparison of the performances of these 1–6 precata-
lysts with the other reported catalysts is important, particu-
larly the N-heterocyclic-based ones for the Hiyama cross-
coupling reactions. The utility of N-heterocyclic carbenes in
Hiyama coupling still remains largely unexplored and we
are aware of one other isolated report of the use of N-het-
erocyclic carbene where under “Ligand Assisted Catalysis
(LAC)” conditions for Hiyama coupling, promoted by a
fluoride coinitiator by Nolan and co-workers,[25] the coup-
lings of aryl bromides and chlorides were attained at 3 mol-
% of the precatalyst loading at 80 °C in 1–6 h. It is worth
noting that in our case, apart from achieving the fluoride-
free conditions, well-characterized precatalysts 1–6, instead
of the in situ generated ones under the “Ligand Assisted
Catalysis (LAC)” conditions, were employed for Hiyama
couplings in air and in a mixed aqueous medium.
(1)
There are only a handful of examples[26] of fluoride-free
Hiyama couplings in which a OH– anion was used as the
coinitiator instead of a fluoride source, along the lines of
(2)
In particular, the fluoride-free Hiyama couplings of aryl the precatalysts 1–6. With regard to this, it is worth noting
halides with PhSi(OMe)3 and CH2=CHSi(OMe)3 were car- that because of the small electronegativity difference be-
ried out at 2 mol-% of the precatalysts 1–6 in air in the tween Si and C the organosilicon compounds are inherently
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F-Free Hiyama and Cu- and Amine-Free Sonogashira Coupling
Table 1. Selected results of the Hiyama cross-coupling reaction of aryl halides (ArX, X = I, Br, Cl) catalyzed by 1–6.
[a] The yields (%) were determined by GC using diethylene glycol dibutyl ether as an internal standard. [b] Reaction conditions: 1.00 mmol
of aryl halides, 1.20 mmol of phenyltrimethoxysilane, 3.00 mmol of NaOH, 2 mol-% of catalyst 1, 2, 3, 4, 5, or 6 and 6 mL of dioxane/
H2O (2:1), at 80 °C. [c] Reaction conditions: 0.50 mmol of aryl halides, 1.00 mmol of vinyltrimethoxysilane, 2.00 mmol of NaOH, 2 mol-
% of catalyst 1, 2, 3, 4, 5, or 6 and 6 mL of dioxane/H2O (2:1), at 80 °C.
weak nucleophiles, which require activation from a fluoride wanted homocoupled products in the presence of an oxygen
ion or any other source of anion that significantly enhances or oxidizing environment. Additionally, being amine-free by
the nucleophilicity of the organosilicon compounds by co- virtue of using Cs2CO3 as a base, the Sonogashira coupling
ordination to the Si center thereby making it pentacoordi- by 1–6 is thus environmentally friendly.
nate and anionic.[4]
Despite the Sonogashira coupling being relatively more
Apart from the fluoride-free Hiyama coupling, these 1–6 explored than the Hiyama coupling, the examples of the
precatalysts were equally efficient in carrying out the Sonoga- utility of N-heterocyclic carbenes in the Sonogashira reac-
shira coupling under highly desirable Cu-free and amine-free tion are still few in number.[27] Though many of the reports
conditions. Specifically, all of the complexes 1–6, at 3 mol-% are under “Ligand Assisted Catalysis (LAC)” conditions,
precatalyst loading, carried out the Cu-free and amine-free we are aware of only three examples in which structurally
Sonogashira coupling of aryl bromides and aryl iodides with characterized catalyst precursors have been used along the
phenyl acetylene in the presence of Cs2CO3 as a base under lines of the 1–6 precatalysts. For example, trans-[(3-pyrro-
amenable conditions in air and in a mixed aqueous medium lidinocarbamoyl-1-methylimidazolin-2-ylidene)(1-methyl-
[28]
at 90 °C in good to excellent yields (Table 2). The influence imidazole)]PdI2
was used for aryl bromides and iodide
of the NHC ligand in 1–6 on catalysis is palpable after having substrates while [1,1Ј-dimethyl-3,3Ј-methylene-4-diimid-
performed a control experiment with PdCl2 and shows up to azolin-2,2Ј-diylidene]PdI2[29] was used under Cu-free condi-
77% enhancement of the product yield for the C4–C5 satu- tions for the aryl bromide substrates. In this regard, we have
rated imidazole complexes 1–4 and up to 70% for the triazole recently reported highly efficient Pd-based precatalysts
complexes 5 and 6 (see Table S20, Supporting Information). namely,
Apart from the Cu-free and amine-free conditions the other azol-2-ylidene]2PdBr2 and cis-[1-benzyl-3-(tert-butylami-
most remarkable aspect of the catalysis is the extremely short nocarbonylmethyl)imidazol-2-ylidene]2PdCl2 for the Son-
trans-[1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imid-
[8]
[8]
reaction time of 1 h in which these cross-couplings occurred. ogashira coupling under amine-free conditions for aryl io-
Being Cu-free the Sonogashira coupling by complexes 1– dide substrates. Hence, the complexes 1–6 represent the first
6 tolerates the aerobic environment unlike the much famil- examples of the use of well-defined N-heterocyclic carbene-
iar Cu-assisted Sonogashira coupling that is air- and moist- based precatalysts for the Sonogashira coupling under both
ure-sensitive and often yields significant amounts of un- Cu-free and amine-free conditions.
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Table 2. Selected results of the Sonogashira cross-coupling reaction of aryl halides (ArX, X = Br, I) catalyzed by 1–6.[a]
[a] Reaction conditions: 0.49 mmol of aryl halides (ArX, X = Br, I), 0.98 mmol of phenylacetylene, 2 mmol of Cs2CO3, 3 mol-% of
catalyst 1, 2, 3, 4, 5, or 6 and 10 mL of DMF/H2O (3:1), at 90 °C for 1 h. [b] The yields (%) were determined by GC using diethylene
glycol dibutyl ether as an internal standard.
standard procedures. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium
chloride,[13] 1,3-bis(2,6-diethylphenyl)imidazolinium chloride,[14]
1,3-bis(2,4,6-trimethylphenyl)imidazolinium chloride,[15] 1,3-bis-
(2,6-dimethylphenyl)imidazolinium chloride,[15] and 1-isopropyl-
1,2,4-triazole (5ЈЈ)[30] were synthesized according to literature pro-
cedures. 1H and 13C{1H} NMR spectra were recorded in CDCl3
with a Varian 400 MHz NMR spectrometer. 1H NMR peaks are
labeled as singlet (s), doublet (d), triplet (t), multiplet (m), and sep-
tet (sept). Infrared spectra were recorded with a Perkin–Elmer
Spectrum One FT-IR spectrometer. Mass spectrometry measure-
ments were performed with a Micromass Q-Tof spectrometer. GC
spectra were obtained on a Schimadzu gas chromatograph GC-15A
equipped with a FID. GC–MS spectra were obtained with a Hew-
lett Packard GCD-1800 A equipped with an EI source. Elemental
Analysis was carried out with a Thermo Quest FLASH 1112
SERIES (CHNS) Elemental Analyzer. X-ray diffraction data for
Lastly, it is worth noting that consistent with the weakly
bound nature of the pyridine moiety in these PEPPSI-
themed 1–6 precatalysts as observed in the X-ray crystallo-
graphic and DFT studies, the formation of free pyridine
was indeed confirmed by GC and GCMS analysis for a
representative precatalyst 4 under both the Hiyama and the
Sonogashira conditions.
Conclusion
In summary, a series of robust, user-friendly and highly
active PEPPSI themed (NHC)PdX2(pyridine)-type (X = ha-
lide) precatalysts of C4–C5 saturated imidazole- (1–4) and
triazole-based (5 and 6) N-heterocyclic carbenes are re-
ported for the fluoride-free Hiyama and Cu-free and amine- compounds 1–6 were collected with an Oxford Diffraction Excal-
iber-S diffractometer and crystal data collection and refinement pa-
rameters are summarized in Table S1 (see Supporting Information).
The structures were solved by direct methods and standard differ-
ence map techniques, and were refined by full-matrix least-squares
procedures on F2 with SHELXTL (Version 6.10).[31]
free Sonogashira couplings. These precatalysts carried out
the cross-coupling reactions under amenable conditions in
air in a mixed aqueous medium in good to excellent yields.
The DFT studies showed that owing to the strong σ do-
nation from the N-heterocyclic carbene ligand to the metal
in the 1–6 precatalysts, the Pd-bound “throwaway” pyridine
ligand is significantly weakened and its dissociation marks
the initiation of the catalytic cycle.
CCDC-626236 (for 1), CCDC-634320 (for 2), CCDC-634319 (for
3), CCDC-632564 (for 4), CCDC-666361 (for 5), and CCDC-
668971 (for 6) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data centre via www.ccdc.cam.ac.uk/
data_request/cif.
Experimental Section
Synthesis of trans-[1,3-Bis(2,6-diisopropylphenyl)imidazolin-2-ylid-
ene]PdCl2(NC5H5) (1): A mixture of 1,3-bis(2,6-diisopropylphenyl)-
imidazolinium chloride (0.574 g, 1.34 mmol), PdCl2 (0.238 g,
General Procedures: All manipulations were carried out using stan-
dard Schlenk techniques. Solvents were purified and degassed by
1614
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1608–1618

F-Free Hiyama and Cu- and Amine-Free Sonogashira Coupling
1.34 mmol), and K2CO3 (0.927 g, 6.72 mmol) was heated at 90 °C
(p-CH ), 19.5 (o-CH ) ppm. IR data (KBr pellet ): ν = 2919 (s),
˜
3 3
in pyridine (ca. 4 mL) for 16 h. The solvent was then removed un- 2863 (m), 1675 (m), 1605 (m), 1490 (s), 1450 (s), 1303 (m), 1268
der vacuum, after which the residue was washed with aqueous
CuSO4 solution (2.01 g in ca. 30 mL of H2O) and the organic com-
ponent was extracted with ethyl acetate (ca. 3ϫ10 mL). The or-
ganic layer was collected and solvent was removed under vacuum
(s), 1073 (m), 1034 (m), 856 (m), 801 (w), 757 (m), 693 (m), 639
(w), 576 (w) cm–1. C26H31Cl2N3Pd (562.87): calcd. C 55.48, H 5.55,
N 7.47; found C 54.52, H 5.88, N 6.90.
Synthesis of trans-[1,3-Bis(2,6-dimethylphenyl)imidazolin-2-ylidene]-
PdCl2(NC5H5) (4): A mixture of 1,3-bis(2,6-dimethylphenyl)imid-
azolinium chloride (0.202 g, 0.644 mmol), PdCl2 (0.114 g,
0.644 mmol), and K2CO3 (0.444 g, 3.22 mmol) was heated at 85 °C
in pyridine (ca. 4 mL) for 16 hours. The solvent was then removed
under vacuum, after which the residue was washed with aqueous
CuSO4 solution (2.01 g in ca. 30 mL of H2O) and the organic com-
ponent was extracted with ethyl acetate (ca. 3ϫ10 mL). The or-
ganic layer was collected and solvent was removed under vacuum
1
to obtain the product 1 as a yellow solid (0.588 g, 68%). H NMR
3
(CDCl3, 400 MHz, 25 °C): δ = 8.51 (d, JHH = 7 Hz, 2 H, o-
3
3
NC5H5), 7.52 (t, JHH = 7 Hz, 1 H, p-NC5H5), 7.41 (t, JHH
=
3
8 Hz, 2 H, p-C6H3), 7.30 (d, JHH = 8 Hz, 4 H, m-C6H3), 7.08 (t,
3JHH = 7 Hz, 2 H, m-NC5H5), 4.06 (s, 4 H, CH2), 3.59 [sept, JHH
3
3
= 7 Hz, 4 H, CH(CH3)2], 1.56 [d, JHH = 7 Hz, 12 H, CH(CH3)2],
1.26 [d, JHH = 7 Hz, 12 H, CH(CH3)2] ppm. 13C{1H} NMR
3
(CDCl3, 100 MHz, 25 °C): δ = 186.3 (NCN-Pd), 151.4 (o-NC5H5),
147.7 (ipso-C6H3), 137.4 (p-NC5H5), 135.6 (o-C6H3), 129.5 (m-
C6H3), 124.6 (p-C6H3), 124.1 (m-NC5H5), 54.0 (NCH2CH2N), 28.9
[CH(CH3)2], 26.9 [CH(CH3)2], 24.4 [CH(CH3)2] ppm. IR data (KBr
1
to obtain the product 4 as a yellow solid (0.234 g, 68%). H NMR
3
(CDCl3, 400 MHz, 25 °C): δ = 8.36 (d, JHH = 7 Hz, 2 H, o-
3
3
NC5H5), 7.51 (t, JHH = 7 Hz, 1 H, p-NC5H5), 7.27 (t, JHH
=
pellet ): ν = 3173 (m), 2960 (s), 2925 (m), 2866 (m), 1634 (m), 1482
˜
3
7 Hz, 2 H, p-C6H3), 7.21 (d, JHH = 7 Hz, 4 H, m-C6H3), 7.05 (t,
3JHH = 7 Hz, 2 H, m-NC5H5), 4.06 (s, 4 H, CH2), 2.63 (s, 12 H, o-
CH3) ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ = 184.7
(NCN-Pd), 151.4 (o-NC5H5), 137.8 (ipso-C6H3), 137.6 (p-NC5H5),
137.5 (o-C6H3), 129.0 (m-C6H3), 128.9 (p-C6H3), 124.1 (m-NC5H5),
(s), 1451 (s), 1401 (s), 1385 (m), 1362 (w), 1328 (w), 1299 (m), 1273
(s), 1222 (w), 1076 (w), 1054 (w), 802 (m), 757 (m), 698 (m), 636
(w) cm–1. C32H43Cl2N3Pd (647.03): calcd. C 59.40, H 6.70, N 6.49;
found C 60.05, H 7.25, N 6.19.
Synthesis of trans-[1,3-Bis(2,6-diethylphenyl)imidazolin-2-ylidene]-
PdCl2(NC5H5) (2): A mixture of 1,3-bis(2,6-diethylphenyl)imid-
azolinium chloride (0.573 g, 1.54 mmol), PdCl2 (0.272 g,
1.54 mmol), and K2CO3 (1.06 g, 7.71 mmol) was heated at 90 °C
in pyridine (ca. 7 mL) for 14 hours. The solvent was then removed
under vacuum, after which the residue was washed with aqueous
CuSO4 solution (2.01 g in ca. 30 mL of H2O) and the organic com-
ponent was extracted with ethyl acetate (ca. 3ϫ10 mL). The or-
ganic layer was collected and solvent was removed under vacuum
51.1 (NCH CH N), 19.6 (o-CH ) ppm. IR data (KBr pellet ): ν =
˜
2
2
3
3126 (m), 3017 (w), 2958 (w), 2923 (w), 2854 (w), 1722 (w), 1634
(m), 1605 (m), 1496 (m), 1447 (m), 1400 (s), 1307 (w), 1277 (m),
1257 (w), 1218 (w), 1097 (w), 1072 (w), 1018 (w), 988 (w), 787 (w),
757 (w), 738 (w), 696 (w), 636 (w), 575 (w) cm–1. C24H27Cl2N3Pd
(534.82): calcd. C 53.90, H 5.09, N 7.86; found C 53.10, H 4.43, N
7.73.
Synthesis of 4-Benzyl-1-isopropyl-1,2,4-triazolium Bromide (5Ј): A
mixture of 1-isopropyl-1,2,4-triazole (3.27 g, 29.5 mmol), and ben-
zyl bromide (5.04 g, 29.5 mmol) was heated at 70 °C for 2 h. The
reaction mixture was washed in hot hexane (ca. 2ϫ10 mL) and
dried under vacuum to obtain the product 5Ј as a white solid
1
to obtain the product 2 as a yellow solid (0.509 g, 56%). H NMR
3
(CDCl3, 400 MHz, 25 °C): δ = 8.34 (d, JHH = 7 Hz, 2 H, o-
3
3
NC5H5), 7.50 (t, JHH = 7 Hz, 1 H, p-NC5H5), 7.40 (t, JHH
=
3
7 Hz, 2 H, p-C6H3), 7.29 (d, JHH = 7 Hz, 4 H, m-C6H3), 7.05 (t,
1
3JHH = 7 Hz, 2 H, m-NC5H5), 4.08 (s, 4 H, CH2), 3.29–3.19 (m, 4
(5.91 g, 71%). H NMR (CDCl3, 400 MHz, 25 °C): δ = 11.3 [s, 1
H, N-C(5)H-N], 8.98 [s, 1 H, N-C(3)H-N], 7.69 (d, 3JHH = 7 Hz, 2
H, o-C6H5), 7.38 (br., 3 H, m & p-C6H5), 5.85 (s, 2 H, CH2), 4.92
3
H, CH2CH3), 2.87–2.78 (m, 4 H, CH2CH3), 1.35 (t, JHH = 8 Hz,
12 H, CH2CH3) ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ
= 185.4 (NCN-Pd), 151.4 (o-NC5H5), 143.1 (ipso-C6H3), 137.5 (p-
NC5H5), 136.4 (o-C6H3), 129.3 (m-C6H3), 126.4 (p-C6H3), 124.1
(m-NC5H5), 52.4 (NCH2CH2N), 24.9 (CH2CH3), 15.1
3
3
[sept, JHH = 7 Hz, 1 H, CH(CH3)2], 1.62 [d, JHH = 7 Hz, 6 H,
CH(CH3)2] ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ =
143.8 [N-C(5)H-N], 140.9 [N-C(3)H-N], 132.4 (ipso-C6H5), 129.4
(o-C6H5), 129.3 (m-C6H5), 129.2 (p-C6H5), 56.2 (CH2), 51.3
(CH CH ) ppm. IR data (KBr pellet): ν = 2962 (s), 2923 (s), 2874
˜
2
3
[CH(CH ) ], 21.5 [CH(CH ) ] ppm. IR data (KBr pellet ): ν = 3100
˜
3 2
3 2
(m), 1679 (w), 1490 (s), 1465 (s), 1306 (m), 1278 (s), 1043 (m), 867
(w), 803 (m), 779 (m), 755 (m), 689 (m), 637 (w), 552 (w) cm–1.
C28H35Cl2N3Pd (590.92): calcd. C 56.91, H 5.97, N 7.11; found C
57.23, H 5.88, N 7.66.
(m), 3036 (s), 2982 (s), 2805 (w), 1811 (w), 1637 (w), 1585 (m), 1515
(m), 1500 (m), 1457 (w), 1445 (m), 1425 (m), 1407 (s), 1390 (s),
1371 (w), 1356 (m), 1313 (w), 1296 (m), 1181 (m), 1154 (s), 1072
(w), 1037 (w), 998 (w), 939 (w), 912 (w), 791 (w), 719 (s), 696 (s),
617 (s), 590 (w), 506 (w), 458 (w) cm–1. HRMS (ES): m/z =
202.1337 [(NHC) + H]+, calcd. 202.1344.
Synthesis of trans-[1,3-Bis(2,4,6-trimethylphenyl)imidazolin-2-ylid-
ene]PdCl2(NC5H5) (3): A mixture of 1,3-bis(2,4,6-trimethylphenyl)-
imidazolinium chloride (0.331 g, 0.996 mmol), PdCl2 (0.171 g,
0.966 mmol), and K2CO3 (0.666 g, 4.83 mmol) was heated at 90 °C
Synthesis of trans-[4-Benzyl-1-isopropyl-1,2,4-triazol-5-ylidene]-
in pyridine (ca. 4 mL) for 16 h. The solvent was then removed un- PdBr2(NC5H5) (5): A mixture of 4-benzyl-1-isopropyl-1,2,4-triazol-
der vacuum, after which the residue was washed with aqueous
CuSO4 solution (2.01 g in ca. 30 mL of H2O) and the organic com-
ponent was extracted with ethyl acetate (ca. 3ϫ10 mL). The or-
ganic layer was collected and solvent was removed under vacuum
ium bromide (0.161 g, 0.571 mmol), PdCl2 (0.101 g, 0.571 mmol),
and K2CO3 (0.393 g, 2.85 mmol) was heated at 90 °C in pyridine
(ca. 4 mL) for 14 h. The solvent was then removed under vacuum,
after which the residue was washed with aqueous CuSO4 solution
(2.01 g in ca. 30 mL of H2O) and the organic component was ex-
tracted with ethyl acetate (ca. 3ϫ10 mL). The organic layer was
collected and solvent was removed under vacuum to obtain the
1
to obtain the product 3 as a yellow solid (0.294 g, 52%). H NMR
3
(CDCl3, 400 MHz, 25 °C): δ = 8.43 (d, JHH = 7 Hz, 2 H, o-
3
3
NC5H5), 7.52 (t, JHH = 7 Hz, 1 H, p-NC5H5), 7.08 (t, JHH
=
7 Hz, 2 H, m-NC5H5), 7.02 (s, 4 H, m-C6H2), 4.03 (s, 4 H, CH2), product 5 as a yellow solid (0.132 g, 48%). 1H NMR (CDCl3,
2.58 (s, 12 H, o-CH3), 2.33 (s, 6 H, p-CH3) ppm. 13C{1H} NMR 400 MHz, 25 °C): δ = 8.96 (d, JHH = 7 Hz, 2 H, o-NC5H5), 7.71
3
(CDCl3, 100 MHz, 25 °C): δ = 184.6 (NCN-Pd), 151.5 (o-NC5H5), (t, 3JHH = 7 Hz, 1 H, p-NC5H5), 7.64 [s, 1 H, N-C(3)H-N], 7.45 (d,
138.6 (ipso-C6H3), 137.5 (p-NC5H5), 137.4 (o-C6H3), 135.1 (m-
C6H3), 129.7 (p-C6H3), 124.0 (m-NC5H5), 51.2 (NCH2CH2N), 21.3 (t, JHH = 7 Hz, 2 H, m-NC5H5), 5.68 [br., 3 H, CH2, CH(CH3)2],
3JHH = 7 Hz, 2 H, o-C6H5), 7.35 (m, 3 H, m-C6H5, p-C6H5), 7.29
3
Eur. J. Inorg. Chem. 2009, 1608–1618
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1615

C. Dash, M. M. Shaikh, P. Ghosh
FULL PAPER
1.54 [d, JHH = 7 Hz, 6 H, CH(CH3)2] ppm. 13C{1H} NMR
3
Computational Methods: The density functional theory calculations
(CDCl3, 100 MHz, 25 °C): δ = 153.6 (NCN-Pd), 152.9 (o-NC5H5), were performed on the palladium (0) (NHC)Pd(pyridine)-type spe-
142.4 [N-C(3)H-N], 138.2 (p-NC5H5), 133.8 (ipso-C6H5), 129.6 (o-
cies, 1a–6a, using the GAUSSIAN 03[32] suite of quantum chemical
C6H5), 129.4 (m-C6H5), 129.3 (p-C6H5), 124.8 (m-NC5H5), 55.7 programs. The Becke three parameter exchange functional in con-
(CH2), 53.3 [CH(CH3)2], 22.0 [CH(CH3)2] ppm. IR data (KBr pel-
junction with the Lee–Yang–Parr correlation functional (B3LYP)
let ): ν = 3136 (w), 2981 (m), 1604 (w), 1540 (m), 1450 (s), 1367 has been employed in this study.[33,34] The Stuttgart-Dresden effec-
˜
(m), 1254 (m), 1226 (m), 1166 (m), 1050 (m), 860 (m), 803 (m), 758 tive core potential (ECP), representing 19 core electrons, along with
(m), 718 (s), 692 (s) cm–1. C17H20Br2N4Pd (546.60): calcd. C 37.36,
H 3.69, N 10.25; found C 37.85, H 3.52, N 10.38.
valence basis sets (SDD) is used for palladium.[35] All other atoms
are treated with the 6-31G(d) basis set.[36] All stationary points are
characterized as minima by evaluating Hessian indices on the re-
spective potential energy surfaces. Tight SCF convergence
(10–8 a.u.) was used for all calculations. Natural bond orbital
(NBO) analysis was performed using the NBO 3.1 program im-
plemented in the GAUSSIAN 03 package.
Synthesis of 1-(tert-Butylaminocarbonylmethyl)-1,2,4-triazole (6ЈЈ):
A mixture of triazole (4.52 g, 65.5 mmol), N-tert-butyl-2-chloro-
acetamide (9.82 g, 65.5 mmol), and K2CO3 (10.8 g, 78.6 mmol) in
CH3CN (ca. 50 mL) was refluxed for 14 h. The reaction mixture
was then filtered and the filtrate was removed under vacuum to
obtain a residue, which was dissolved in chloroform (ca. 30 mL)
and washed with H2O (50 mL). The organic layer was collected
and dried under vacuum to give the product 6ЈЈ as a brown viscous
Inspection of the metal–ligand donor–acceptor interactions was
carried out using the charge decomposition analysis (CDA).[22]
CDA is a valuable tool in analyzing the interactions between mo-
lecular fragments on a quantitative basis, with an emphasis on the
electron donation.[37] The orbital contributions in the geometry op-
timized palladium(0) (NHC)Pd(pyridine)-type species 1a–6a can be
divided into three parts:
1
liquid (5.36 g, 45%). H NMR (CDCl3, 400 MHz, 25 °C): δ = 8.09
[s, 1 H, N-C(5)H-N], 7.91 [s, 1 H, N-C(3)H-N], 6.21 (br., 1 H, NH),
4.66 (s, 2 H, CH2), 1.26 [s, 9 H, C(CH3)3] ppm. 13C{1H} NMR
(CDCl3, 100 MHz, 25 °C): δ = 164.4 (CO), 150.6 [N-C(5)H-N],
144.3 [N-C(3)H-N], 51.5 (CH2), 50.8 [C(CH3)3], 27.6 [C(CH3)3]
(i) σ-donation from the [NHCǞPd(pyridine)] fragment,
(ii) π-back donation from the [NHCǟPd(pyridine)] fragment, and
(iii) repulsive polarization (r).
ppm. IR data (nujol ): ν = 1679 (s) (νCO) cm–1. HRMS (ES): m/z
˜
= 183.1245 [M + 1]+, calcd. 183.1246.
Synthesis of 4-Benzyl-1-(tert-butylaminocarbonylmethyl)-1,2,4-tri-
azolium Bromide (6Ј): A mixture of 1-(tert-butylaminocarbon-
ylmethyl)-1,2,4-triazole (3.82 g, 21.0 mmol) and benzyl bromide
(3.59 g, 21.0 mmol) in CH2Cl2 (ca. 10 mL) was refluxed for 6 h,
after which a precipitate separated out. The precipitate was washed
with hot petroleum ether (ca. 2ϫ10 mL) and dried under vacuum
to obtain the product 6Ј as an off white solid (5.46 g, 74%). 1H
The CDA calculations were performed using the program
AOMix,[23] using the B3LYP/SDD, 6-31G(d) wave function. Molec-
ular 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 orbit-
als (OFOs and UFOs, respectively), the construction of orbital in-
teraction diagrams, and the charge decomposition analysis (CDA)
was performed using the AOMix-CDA.[38]
NMR (CDCl3, 400 MHz, 25 °C): δ = 10.6 [s, 1 H, N-C(5)H-N],
3
8.68 [s, 1 H, N-C(3)H-N], 7.94 (br., 1 H, NH), 7.53 (d, JHH
=
7 Hz, 2 H, o-C6H5), 7.45–7.43 (m, 3 H, m-C6H5, p-C6H5), 5.67 (s,
2 H, CH2Ph), 5.48 (s, 2 H, CH2), 1.37 [s, 9 H, C(CH3)3] ppm.
13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ = 162.6 (CO), 143.7
[N-C(5)H-N], 143.5 [N-C(3)H-N], 131.7 (ipso-C6H5), 130.1 (o-
C6H5), 129.8 (m-C6H5), 129.5 (p-C6H5), 55.4 (CH2Ph), 52.6 (CH2),
General Procedure for the Hiyama Coupling Reaction
Coupling with Phenyltrimethoxysilane: In a typical run, performed
in air, a 25 mL vial was charged with a mixture of aryl halides,
PhSi(OMe)3, NaOH, and diethylene glycol dibutyl ether (internal
standard) in a molar ratio of 1:1.2:3:1. Complex 1, 2, 3, 4, 5, or 6
(2 mol-%) was then added to the mixture (Table 1). Finally, a mixed
solvent (dioxane/H2O, 2:1 v/v, 6 mL) was added to the reaction
mixture and heated at 80 °C for an appropriate period of time,
after which it was filtered and the product was analyzed by gas
chromatography using diethylene glycol dibutyl ether as an internal
standard.
52.4 [C(CH ) ], 28.8 [C(CH ) ] ppm. IR data (KBr pellet ): ν =
˜
3 3
3 3
1674 (s) (νCO) cm–1. HRMS (ES): m/z = 273.1711 [(NHC) + H]+,
calcd. 273.1715.
Synthesis of trans-4-Benzyl-[1-(tert-butylaminocarbonylmethyl)-
1,2,4-triazol-5-ylidene]PdBr2(NC5H5) (6): A mixture of 4-benzyl-1-
(tert-butylaminocarbonylmethyl)-1,2,4-triazolium bromide (0.467 g,
1.32 mmol), PdCl2 (0.234 g, 1.32 mmol), and K2CO3 (0.912 g,
6.61 mmol) was heated at 90 °C in pyridine (ca. 4 mL) for 14 h.
The solvent was then removed under vacuum, after which the resi-
due was washed with aqueous CuSO4 solution (4.01 g in ca. 30 mL
of H2O) and the organic component was extracted with ethyl ace-
tate (ca. 3ϫ10 mL). The organic layer was collected and the sol-
vent removed under vacuum to obtain the product 6 as a yellow
Coupling with Vinyltrimethoxysilane: In a typical run, performed
in air, a 25 mL vial was charged with a mixture of aryl halides,
CH2=CHSi(OMe)3, NaOH, and diethylene glycol dibutyl ether (in-
ternal standard) in a molar ratio of 1:2:4:1. Complex 1, 2, 3, 4, 5,
or 6 (2 mol-%) was added to the mixture (Table 1). Finally, a mixed
solvent (dioxane/H2O, 2:1 v/v, 6 mL) was added to the reaction
mixture and heated at 80 °C for an appropriate period of time,
after which it was filtered and the product was analyzed by gas
chromatography using diethylene glycol dibutyl ether as an internal
standard.
1
solid (0.521 g, 64%). H NMR (CDCl3, 400 MHz, 25 °C): δ = 8.98
3
(d, JHH = 7 Hz, 2 H, o-NC5H5), 7.82 [m, 2 H, p-NC5H5 and N-
3
C(3)H-N], 7.53 (d, JHH = 8 Hz, 2 H, o-C6H5), 7.47–7.44 (m, 3 H,
3
m-C6H5, p-C6H5), 7.39 (t, JHH = 7 Hz, 2 H, m-NC5H5), 6.13 (br.,
1 H, NH), 5.80 (s, 2 H, CH2Ph), 5.34 (s, 2 H, CH2), 1.36 [s, 9 H,
C(CH3)3] ppm. 13C{1H} NMR (CDCl3, 100 MHz, 25 °C): δ =
General Procedure for the Sonogashira Coupling Reaction: In a typi-
164.5 (CO), 159.0 (NCN-Pd), 152.8 (o-NC5H5), 143.5 [N-C(3)H- cal run, performed in air, a 25 mL vial was charged with a mixture
N], 138.4 (p-NC5H5), 133.2 (ipso-C6H5), 129.6 (o-C6H5), 129.5 (m- of aryl halides, arylalkyne, Cs2CO3, and diethylene glycol dibutyl
C6H5), 129.4 (p-C6H5), 124.9 (m-NC5H5), 57.2 (CH2Ph), 53.5 ether (internal standard) in a molar ratio of 1:2:4:1. Complexes 1,
(CH2), 52.3 [C(CH3)3], 28.7 [C(CH3)3] ppm. IR data (KBr pellet ): 2, 3, 4, 5, or 6 (3 mol-%) was added to the mixture (Table 2). Fi-
ν = 1688 (s) (νCO) cm–1. C20H25Br2N5OPd·0.5NC5H5 (657.23):
nally, a mixed solvent (DMF/H2O, 3:1 v/v, 10 mL) was added to
˜
calcd. C 41.12, H 4.22, N 11.72; found C 40.93, H 3.98, N 11.88.
the reaction mixture and heated at 90 °C for 1 h, after which it was
1616
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Eur. J. Inorg. Chem. 2009, 1608–1618

F-Free Hiyama and Cu- and Amine-Free Sonogashira Coupling
[10]
[11]
S. Ray, R. Mohan, J. K. Singh, M. K. Samantaray, M. M.
Shaikh, D. Panda, P. Ghosh, J. Am. Chem. Soc. 2007, 129,
15042–15053.
filtered and the product was analyzed by gas chromatography using
diethylene glycol dibutyl ether as an internal standard.
Pyridine Dissociation under Hiyama Conditions: PhSi(OMe)3
(0.714 g, 3.60 mmol), NaOH (0.361 g, 9.00 mmol), diethylene
glycol dibutyl ether (internal standard) (0.015 g, 0.071 mmol), and
a representative catalyst, complex 4 (0.037 g, 0.071 mmol) in
CH3CN (6 mL), were placed in a 25 mL vial. The reaction mixture
was heated at 70 °C for 4 h, after which it was filtered and the
reaction mixture was analyzed by GC and GCMS, which showed
the formation of 45% pyridine under the reaction conditions.
a) M. K. Samantaray, M. M. Shaikh, P. Ghosh, Organometal-
lics, DOI: 10.1021/om801186f; b) S. Ray, M. M. Shaikh, P.
Ghosh, Eur. J. Inorg. Chem., DOI: 10.1002/ejic.200801060; c)
L. Ray, V. Katiyar, S. Barman, M. J. Raihan, H. Nanavati,
M. M. Shaikh, P. Ghosh, J. Organomet. Chem. 2007, 692,
4259–4269; d) M. K. Samantaray, V. Katiyar, K. Pang, H.
Nanavati, P. Ghosh, J. Organomet. Chem. 2007, 692, 1672–
1682; e) L. Ray, V. Katiyar, M. J. Raihan, H. Nanavati, M. M.
Shaikh, P. Ghosh, Eur. J. Inorg. Chem. 2006, 3724–3730; f)
M. K. Samantaray, V. Katiyar, D. Roy, K. Pang, H. Nanavati,
R. Stephen, R. B. Sunoj, P. Ghosh, Eur. J. Inorg. Chem. 2006,
2975–2984.
a) L. Ray, M. M. Shaikh, P. Ghosh, Organometallics 2007, 26,
958–964; b) L. Ray, M. M. Shaikh, P. Ghosh, Dalton Trans.
2007, 4546–4555.
A. J. Arduengo III, R. Krafczyk, R. Schmutzler, H. A. Craig,
J. R. Goerlich, W. J. Marshall, M. Unverzagt, Tetrahedron
1999, 55, 14523–l4534.
Pyridine Dissociation under Sonogashira Conditions: Phenylacetyl-
ene (0.300 g, 0.294 mmol), Cs2CO3 (1.30 g, 4.00 mmol), diethylene
glycol dibutyl ether (internal standard) (0.016 g, 0.073 mmol), and
a representative catalyst, complex 4 (0.039 g, 0.073 mmol) in
CH3CN (6 mL), were placed in a 25 mL vial. The reaction mixture
was heated at 70 °C for 1 h, after which it was filtered and the
reaction mixture was analyzed by GC and GCMS, which showed
the formation of 29% pyridine under the reaction conditions.
[12]
[13]
[14]
[15]
[16]
N. Hadei, E. A. B. Kantchev, C. J. O’Brien, M. G. Organ, J.
Org. Chem. 2005, 70, 8503–8507.
Supporting Information (see also the footnote on the first page of
this article): CIF file giving crystallographic data for 1–6, control
experiment tables, X-ray metrical data comparison table, Ortep
plots of 2, 3, 4, and 6, the B3LYP coordinates of the optimized
geometries for 1a–6a, NBO tables, CDA table and orbital interac-
tion diagrams of 2a, 3a, 4a and 6a.
H. Türkmen, B. Çetinkaya, J. Organomet. Chem. 2006, 691,
3749–3759.
The 1H NMR resonances of free pyridine in CDCl3 are
3
(400 MHz, 25 °C): δ = 8.56 (d, JHH = 8 Hz, 2 H, o-NC5H5),
3
3
7.63 (t, JHH = 8 Hz, 1 H, p-NC5H5), 7.23 (t, JHH = 8 Hz 2
H, m-NC5H5).
[17]
a) M. G. Organ, M. Abdel-Hadi, S. Avola, N. Hadei, J. Nasiel-
ski, C. J. O’Brien, C. Valente, Chem. Eur. J. 2007, 13, 150–157;
b) C. J. O’Brien, E. A. B. Kantchev, C. Valente, N. Hadei, G. A.
Chass, A. Lough, A. C. Hopkinson, M. G. Organ, Chem. Eur.
J. 2006, 12, 4743–4748; c) M. G. Organ, S. Avola, I. Dubovyk,
N. Hadei, E. A. B. Kantchev, C. J. O’Brien, C. Valente, Chem.
Eur. J. 2006, 12, 4749–4755.
Acknowledgments
We thank the 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 Sophisti-
cated Analytical Instrument Facility at IIT Bombay, India, for the
crystallographic and other characterization data. Computational
facilities from the IIT Bombay Computer Center are gratefully ac-
knowledged. C. K. D thanks CSIR-UGC, New Delhi, for a re-
search fellowship.
[18]
[19]
[20]
[21]
Q. Liu, L. Thorne, I. Kozin, D. Song, C. Seward, M. DЈIorio,
Y. Ta o, S. Wa n g , J. Chem. Soc., Dalton Trans. 2002, 3234–3240.
D. Jaganyi, F. Tiba, O. Q. Munro, B. Petrovic´, Z. D. Bugarcˇic´,
ˇ
Dalton Trans. 2006, 2943–2949.
ˇ
Z. D. Bugarcˇic´, B. Petrovic´, E. Zangrando, Inorg. Chim. Acta
2004, 357, 2650–2656.
A. E. Reed, L. A. Curtiss, F. Wienhold, Chem. Rev. 1988, 88,
899–926.
S. Dapprich, G. Frenking, J. Phys. Chem. 1995, 99, 9352–9362.
S. I. Gorelsky, AOMix: Program for Molecular Orbital Analy-
sis; York University: Toronto, Canada, 1997; http://www.sg-
chem.net/ (accessed August, 9, 2008).
[1] Y. Hatanaka, T. Hiyama, J. Org. Chem. 1988, 53, 918–920.
[2] N. Miyaura, K. Yamada, A. Suzuki, Tetrahedron Lett. 1979,
20, 3437–3440.
[22]
[23]
[3] D. Milstein, J. K. Stille, J. Am. Chem. Soc. 1978, 100, 3636–
3638.
[24]
a) D. A. Valyaev, M. G. Peterleitner, L. I. LeontЈeva, L. N. No-
vikova, O. V. Semeikin, V. N. Khrustalev, M. Y. Antipin, N. A.
Ustynyuk, B. W. Skelton, A. H. White, Organometallics 2003,
22, 5491–5497; b) M. A. Esteruelas, A. I. González, A. M.
López, E. Oñate, Organometallics 2003, 22, 414–425; c) H. G.
Raubenheimer, M. W. Esterhuysen, A. Timoshkin, Y. Chen, G.
Frenking, Organometallics 2002, 21, 3173–3181; d) J. Bar-
luenga, J. Flórez, F. J. Fañanás, J. Organomet. Chem. 2001, 624,
5–17; e) J. Barluenga, A. A. Trabanco, J. Flórez, S. García-
Granda, E. Martín, J. Am. Chem. Soc. 1996, 118, 13099–13100;
f) L. L. Padolik, J. C. Gallucci, A. Wojcicki, J. Am. Chem. Soc.
1993, 115, 9986–9996.
H. M. Lee, S. P. Nolan, Org. Lett. 2000, 2, 2053–2055.
a) B. Inés, R. SanMartin, F. Churruca, E. Domínguez, M. K.
Urtiaga, M. I. Arriortua, Organometallics 2008, 27, 2833–2839;
b) Á. Gordillo, E. de Jesús, C. López-Mardomingo, Chem.
Commun. 2007, 4056–4058; c) D. Srimani, S. Sawoo, A. Sarkar,
Org. Lett. 2007, 9, 3639–3642; d) S. Shi, Y. Zhang, J. Org.
Chem. 2007, 72, 5927–5930; e) E. Alacid, C. Nájera, Adv.
Synth. Catal. 2006, 348, 2085–2091; f) E. Alacid, C. Nájera,
Adv. Synth. Catal. 2006, 348, 945–952; g) Á. Gordillo, E.
de Jesús, C. López-Mardomingo, Org. Lett. 2006, 8, 3517–
[4] a) T. Hiyama, J. Organomet. Chem. 2002, 653, 58–61; b) K.
Itami, K. Mitsudo, T. Nokami, T. Kamei, T. Koike, J. Yoshida,
J. Organomet. Chem. 2002, 653, 105–113; c) S. E. Denmark,
R. F. Sweis, Chem. Pharm. Bull. 2002, 50, 1531–1541.
[5] a) S. E. Denmark, B. D. Griedel, D. M. Coe, M. E. Schnute, J.
Am. Chem. Soc. 1994, 116, 7026–7043; b) S. E. Denmark, B. D.
Griedel, J. Org. Chem. 1994, 59, 5136–5138.
[6] K. C. Nicolaou, P. G. Bulger, D. Sarlah, Angew. Chem. Int. Ed.
2005, 44, 4442–4489.
[7] a) H. Doucet, J.-C. Hierso, Angew. Chem. Int. Ed. 2007, 46,
834–871; b) R. Chinchilla, C. Nájera, Chem. Rev. 2007, 107,
874–922; c) E. Negishi, L. Anastasia, Chem. Rev. 2003, 103,
1979–2017.
[25]
[26]
[8] L. Ray, S. Barman, M. M. Shaikh, P. Ghosh, Chem. Eur. J.
2008, 14, 6646–6655.
[9] a) M. K. Samantaray, K. Pang, M. M. Shaikh, P. Ghosh, Dal-
ton Trans. 2008, 4893–4902; b) M. K. Samantaray, K. Pang,
M. M. Shaikh, P. Ghosh, Inorg. Chem. 2008, 47, 4153–4165; c)
L. Ray, M. M. Shaikh, P. Ghosh, Inorg. Chem. 2008, 47, 230–
240; d) M. K. Samantaray, D. Roy, A. Patra, R. Stephen, M.
Saikh, R. B. Sunoj, P. Ghosh, J. Organomet. Chem. 2006, 691,
3797–3805.
Eur. J. Inorg. Chem. 2009, 1608–1618
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1617

C. Dash, M. M. Shaikh, P. Ghosh
FULL PAPER
3520; h) C. Wolf, R. Lerebours, Org. Lett. 2004, 6, 1147–1150.
[27] a) J.-H. Kim, D.-H. Lee, B.-H. Jun, Y.-S. Lee, Tetrahedron Lett.
2007, 48, 7079–7084; b) W. J. Sommer, M. Weck, Adv. Synth.
Catal. 2006, 348, 2101–2113; c) M. Eckhardt, G. C. Fu, J. Am.
Chem. Soc. 2003, 125, 13642–13643; d) C. Yang, S. P. Nolan,
Organometallics 2002, 21, 1020–1022; e) S. Caddick, F. G. N.
Cloke, G. K. B. Clentsmith, P. B. Hitchcock, D. McKerrecher,
L. R. Titcomb, M. R. V. Williams, J. Organomet. Chem. 2001,
617–618, 635–639.
Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, R. Morok-
uma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrew-
ski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V.
Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B.
Stefanov, G. Liu, A. Liashenko, P.Piskorz, I. Komaromi, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzales, J. A. Pople, GAUSSIAN 03,
Revision C.02, Gaussian, Inc., Wallingford CT, 2004.
[28] R. A. Batey, M. Shen, A. J. Lough, Org. Lett. 2002, 4, 1411–
1414.
[33]
[34]
[35]
A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100.
C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[29] W. A. Herrmann, C.-P. Reisinger, M. Spiegler, J. Organomet.
Chem. 1998, 557, 93–96.
[30] F. Dallacker, K. Minn, Chem.-Ztg. 1986, 110, 101–108.
[31] a) G. M. Sheldrick, SHELXL-97, Program for refinement of
crystal structures, University of Göttingen, Germany, 1997; b)
G. M. Sheldrick, SHELXS-97, Structure solving program, Uni-
versity of Göttingen, Germany, 1997.
[32] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr., T.
Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Ivengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
a) K. Pang, S. M. Quan, G. Parkin, Chem. Commun. 2006,
5015–5017; b) G. Yang, C. Jin, J. Hong, Z. Guo, L. Zhu, Spec-
trochim. Acta Part A 2004, 60, 3187–3195; c) Y. Zhang, L.
Zhang, H. Tao, X. Sun, L. Zhu, Spectrochim. Acta Part A 2003,
59, 493–509.
W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56,
2257–2261.
a) S. F. Vyboishchikov, G. Frenking, Chem. Eur. J. 1998, 4,
1439–1448; b) G. Frenking, U. Pidun, J. Chem. Soc., Dalton
Trans. 1997, 1653–1662.
S. I. Gorelsky, S. Ghosh, E. I. Solomon, J. Am. Chem. Soc.
2006, 128, 278–290.
[36]
[37]
[38]
Received: February 1, 2009
Published Online: March 18, 2009
1618
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Eur. J. Inorg. Chem. 2009, 1608–1618