Cyclic (Alkyl)amino Carbene Based Iron Catalyst for Regioselective Dimerization of Terminal Arylalkynes

Article abstract of DOI:10.1021/acs.organomet.6b00703

[(cAAC)Fe(CO)₄] (1) catalyzed head-to-head dimerization of terminal arylalkynes toward conjugated enynes in very high yield and high E selectivity (up to 84:16 E:Z). The protocol can be performed under extremely low catalyst loading down to 0.01 mol %, resulting in a high TON of 6500. A mechanistic pathway for arylalkyne dimerization has been proposed on the basis of a well-defined catalyst, an isolable intermediate, and quantum chemical calculations.

Full text of DOI:10.1021/acs.organomet.6b00703

Article
pubs.acs.org/Organometallics
Cyclic (Alkyl)amino Carbene Based Iron Catalyst for Regioselective
Dimerization of Terminal Arylalkynes
Mrinal Bhunia,† Sumeet R. Sahoo,† Gonela Vijaykumar,† Debashis Adhikari,*,‡ and Swadhin K. Mandal*,†
†Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
‡Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, SAS Nagar 140306, India
S
* Supporting Information
ABSTRACT: [(cAAC)Fe(CO)4] (1) catalyzed head-to-head
dimerization of terminal arylalkynes toward conjugated enynes
in very high yield and high E selectivity (up to 84:16 E:Z). The
protocol can be performed under extremely low catalyst
loading down to 0.01 mol %, resulting in a high TON of 6500.
A mechanistic pathway for arylalkyne dimerization has been
proposed on the basis of a well-defined catalyst, an isolable
intermediate, and quantum chemical calculations.
onjugated 1,3-enynes are influential building blocks in
organic synthesis for the preparation of natural products,
polysubstituted aromatic rings, and the key structural unit in
numerous biologically active molecules, drug intermediates, and
organic electronic materials.1 Owing to the widespread use of
1,3-enynes, development of novel methods toward their
effective and economic preparation is a highly desirable goal
metallic alkynes with alkenes as well as terminal alkynes with
preactivated alkenes, Wittig olefination of conjugated alkynals,
Suzuki coupling of alkynyl halides with boronic acid, and
dehydration of propargylic alcohols.2 In comparison to these,
an operationally simple and atom-economical dimerization of
terminal alkynes or direct hydroalkynylation across the
carbon−carbon triple bond to form conjugated enynes is a
very practical method.3 The dimerization of terminal alkynes
has been scrutinized primarily with a large number of
transition-metal-based catalysts, mostly based on precious
metals4 and f-block elements.5 However, the most recent
trend deals with the development of catalysts using earth-
abundant, nontoxic, and inexpensive metals such as iron for
alkyne dimerization.
C
Scheme 1. Dimerization of Phenylacetylene Catalyzed by
Complex 1 in Toluene
A thorough literature survey reveals that there have been
only a few reports of iron catalysts employed for alkyne
dimerization.6 The first iron-catalyzed alkyne dimerization used
FeCl3 as the catalyst in semistoichiometric loading (30 mol
%).6a Very recently, Milstein and co-workers6b showcased that
1−3 mol % loading of iron catalyst can achieve chemo-, regio-,
and stereoselective (Z-selective) alkyne dimerization without
using base or other additives. This remarkable scarcity of iron-
based catalysts strongly motivated us to develop an efficient
iron-based catalyst for alkyne dimerization under very low
catalyst loading condition. As a part of our ongoing research
program to design organometallic catalysts using carbenes as
ligands,7 we herein used a strong σ-donor ligand such as cyclic
(alkyl)amino carbene (cAAC)8 to generate a robust catalytic
platform. In this report, we describe an efficacious E-selective
in catalysis research. Numerous methods for the synthesis of
1,3-enynes are well-known, including Sonogashira coupling of
terminal alkynes with vinyl halides, cross-coupling of organo-
Scheme 2. Synthesis of Complex 1
Received: September 4, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00703
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head-to-head dimerization of terminal arylalkynes (Scheme 1)
with extremely low catalyst loading (0.2−0.01 mol %) of the
iron(0) complex [(cAAC)Fe(CO)4] (1), resulting in a high
TON of 6500, which is impressive given the very few reports of
iron-catalyzed alkyne dimerization reactions.6
Table 1. Optimization Study for Iron-Catalyzed
Dimerization of Phenylacetylene
a
RESULTS AND DISCUSSION
■
b
base (2
equiv)
cat. 1
temp yield (%) 2,
Synthesis and Characterization. The synthesis of 1 was
accomplished by the treatment of free cAAC with commercially
available diiron nonacarbonyl [Fe2(CO)9] in a 2:1 stoichio-
metric ratio in toluene at room temperature (Scheme 2).
Analytically pure pale yellow crystals of 1 were obtained in 65%
isolated yield from a concentrated toluene solution at −20 °C.
Compound 1 readily dissolves in toluene, benzene, THF, and
DMSO, and it shows limited solubility in hexane and pentane.
The nature of the complex has been probed by an array of
spectroscopic methods (1H, 13C NMR, and IR), as well as by
elemental analysis and single-crystal X-ray diffraction studies.
The 13C NMR spectrum of 1 reveals a resonance at 215.8 ppm,
assignable to the C1 carbon bound to the iron center which is
significantly shifted upfield upon metal coordination from the
corresponding chemical shift of free cAAC (304.2 ppm).8b To
unambiguously ascertain the expected bond connectivities, a
crystal was analyzed via single-crystal X-ray diffraction (Figure
1).9 As anticipated, the X-ray crystal structure of 1 exhibits
entry
(mol %)
solvent
(°C)
(2a:2b:3)
TON
1
2
3
4
KOtBu (2)
KOtBu (2)
KOtBu (2)
KOtBu
(2)
KOtBu (2)
KOtBu (2)
KOtBu (2)
KOtBu (2)
K2CO3 (2)
0.2
0.2
0.2
0.2
DMSO
toluene
toluene
toluene
RT
RT
90
25, 66:34:0
36, 70:30:0
65, 81:19:0
98, 82:18:0
120
490
5
0.2
0.2
0.2
0.2
0.2
0.2
hexane
Et2O
80
83, 38:50:0
15, 15:9:0
6
40
7
THF
80
95, 79:21:0
90, 50:48:0
70, 65:35:0
55, 58:42:0
8
benzene
toluene
toluene
95
9
120
120
10
Na2CO3
(2)
11
Cs2CO3
(2)
0.2
toluene
120
65, 71:29:0
12
13
KOAc (2)
0.2
0.2
toluene
toluene
120
120
60, 60:40:0
65, 52:48:0
NaOAc
(2)
14
15
16
K3PO4 (2)
−
0.2
0.2
−
0.2
0.2
0.2
0.2
0.1
0.05
0.01
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
120
120
120
120
120
120
120
120
120
120
60, 64:36:0
−
KOtBu (2)
KOtBu (2)
KOtBu (1)
KOtBu (3)
KOtBu (4)
KOtBu (2)
KOtBu (2)
KOtBu (2)
5, 68:32:0
36, 78:22:0
67, 74:26:0
97, 65:30:5
96, 65:21:14
98, 62:38:0
84, 53:47:0
65, 49:51:0
c
17
18
19
20
21
22
23
980
1680
6500
a
Reaction conditions unless specified otherwise: phenylacetylene (1
mmol), KOtBu (2 mmol), complex 1, toluene (1 mL), 120 °C, 4 h.
b
c
Isolated yield. Fe(CO)5 instead of complex 1.
head dimerization product 2 (Table 1, entry 4) was achieved in
toluene at 120 °C. After optimization of different bases (Table
1, entries 4 and 9−14), quantitative yield and higher selectivity
were realized when 2 equiv of KOtBu was used in the presence
of toluene at 120 °C for 4 h (Table 1, entry 4). The reaction
does not proceed in absence of bases (Table 1, entry 15). A
control experiment without catalyst 1 yielded only 5% of 1,3-
enyne product with 2 equiv of KOtBu (Table 1, entry 16),
which clearly indicates that the presence of our catalyst is
required for the reaction. To further test the necessity of a
cAAC-bound Fe(0), the catalysis was executed in the presence
of [Fe(CO)5] and base instead of catalyst 1, giving a 36% yield
of the dimerized product with moderate selectivity (Table 1,
entry 17). This unequivocally establishes that cAAC plays an
important role during the catalysis. To our delight, the catalyst
can even perform the reaction well with extremely low catalyst
loading (0.01 mol %) at 120 °C, leading to a considerably high
TON value of 6500 with moderate yield (65%, Table 1, entry
23).
Figure 1. View of the molecular structure of 1. Ellipsoids are set at the
50% probability level; hydrogen atoms have been omitted for clarity.
Selected bond distances (Å) and angles (deg): Fe1−C1, 1.995(2);
Fe1−C21, 1.813(2); Fe1−C22, 1.779(2); Fe1−C23, 1.793(2); Fe1−
C24, 1.793(2); C21−Fe1−C1, 99.36(9); C22−Fe1−C1, 173.86(9);
C24−Fe1−C1, 86.65(9); C23−Fe1−C1, 92.72(9).
iron(0) trapped in a distorted-trigonal-bipyramidal environ-
ment and bonded to the cAAC and four carbonyl moieties. The
Fe−C(carbene) bond distance is 1.995(2) Å, which compares
well with that of the previously crystallographically analyzed
[(IMes)Fe(CO)4] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene).10
Having well understood the structure of the complex, we
used 1 as a precatalyst for dimerization of terminal alkynes to
1,3-enynes. When a solution of complex 1 (0.2 mol %) and
phenylacetylene in toluene was stirred in the presence of 2
equiv of potassium tert-butoxide (KOtBu) at room temperature
for 4 h, a 36% (E:Z 70:30) yield of 1,4-diphenyl-1-buten-3-yne
was realized (Table 1, entry 2). Upon further optimization of
solvent and temperature, a quantitative yield of the head-to-
A preliminary kinetic plot of yield as a function of time
revealed that the reaction goes to near completion within 3 h
(see the Supporting Information for details). Further, the scope
of the dimerization of several terminal arylalkynes to 1,3-enynes
(Table 2) was investigated. It is noteworthy that the reaction
B
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In the case of a meta derivative of methoxyphenylacetylene,
excellent yield and selectivity were retained (Table 2, entry 6).
The dimerization of 4-fluorophenylacetylene afforded 90%
yield with a 73:27 mixture of the E:Z dimers (Table 2, entry 7).
Under similar reaction conditions, 4-bromophenylacetylene
gave the corresponding product in 92% yield with comparable
selectivity (E:Z = 74:26, Table 2, entry 8). An electron-
withdrawing trifluoromethyl substituent at the ortho position of
phenylacetylene yielded 92% of the corresponding dimer with
good selectivity (E:Z = 73:27, Table 2, entry 9). However, the
presence of two electron-withdrawing substituents in the same
ring, such as trifluoromethyl at both the 3- and 5-positions of
phenylacetylene offered a moderate yield (60%) of the
corresponding dimer with inverted selectivity (E:Z = 17:83)
(Table 2, entry 10). We surmise that the presence of two
strongly electron withdrawing groups makes the arylalkynes
sufficiently electron poor that binding to Fe(0) is disfavored
(see Mechanistic Understanding). A terminal alkyne containing
a heterocyclic ring such as pyridine lowered the yield of the
dimerized product and favored the Z isomer of the
corresponding dimer (Table 2, entry 12).
Table 2. Dimerization of Terminal Arylalkynes Catalyzed by
1
a
Mechanistic Understanding. To explore the mechanistic
course of the alkyne dimerization reaction catalyzed by 1, we
ventured on a tandem experimental and computational
approach. As is intuitive, the generation of electronic
unsaturation to 1 requires driving one CO ligand out, which
is an energy-uphill process, given the strong binding of the CO
ligand. Indeed, upon heating a mixture of 1 and phenyl-
acetylene at 90 °C, unsaturation at the iron center is created,
promoting the alkyne binding. This fact is in strong accord with
either a thermal or photochemical reaction toward the
generation of unsaturation for similar catalytic systems.10 The
alkyne-bound intermediate 3 (Figure 2) was isolated from a
benzene solution and thoroughly characterized by NMR, IR,
and mass spectroscopic studies. Intermediate 3 displays its
characteristic 13C NMR resonances at δ 78.7 and 82.2 ppm,
representing alkyne carbons. Furthermore, the resonance at
82.2 ppm vanishes on the DEPT-135 pulse sequence,
suggesting this carbon to be the phenyl-substituted atom.
Computationally (M06/lanl2dz+6-31G* for C, H, N, and O)11
we were also able to optimize 3 (see Figure 2), where alkyne
binds the iron in an η2 fashion, displaying C−Fe bond distances
of 2.04 and 2.13 Å, respectively. Not surprisingly, the C−C
bond of the alkyne clearly elongates to 1.27 Å from 1.21 Å for
the parent alkyne. We probed a further sequence of events
computationally and inferred that the alkynide, generated from
deprotonation of the alkyne by KOtBu, binds to the metal upon
creating further unsaturation. Expelling the second CO requires
energy, but the amount is smaller than that of the first step
since the already bound phenylacetylene can mitigate the effect
of electron loss, forming a metallacyclopropene type
intermediate. Upon alkynide binding, intermediate 4 is
generated (Figure 2), where alkyne and alkynide are perfectly
oriented toward C−C bond formation, leading to the
dimerization of alkyne. The exclusive formation of a head-to-
head isomer in this dimerization reaction can be nicely
explained from an atomic polar tensor (APT) charge analysis
of the intermediate 4 (Figure 3a). As alkynide behaves as a
nucleophile, its attack will be facilitated at a center which
possesses more positive charge. In 4, the unsubstituted carbon
bears a charge of −0.178e, in comparison to −0.638e at the
phenyl-substituted carbon of the bound alkyne (Figure 3).
From the charge analysis, it is intuitive to surmise that the other
a
Reaction conditions unless specified otherwise: arylalkyne (1 mmol),
KOtBu (2 mmol), complex 1 (0.2 mol %), toluene (1 mL), 120 °C, 4
1
h. The conversion and ratio of E:Z isomers are based on H NMR
b
spectroscopic analysis of the crude reaction mixture. 1H NMR
c
d
spectroscopic conversion. Isolated yield. The reaction was carried
out with 0.15 mmol of the substrate instead of 1.0 mmol.
showcases exclusive regioselectivity favoring only head-to-head
product. Under the standardized conditions, the presence of
both an electron-donating and an electron-withdrawing group
furnished the dimerized product 2 in good to excellent yield
(56−98%) along with good stereoselectivity. The E:Z ratio of
the product is somewhat dependent on the nature of the alkyne
and varies from 60:40 to 84:16 in favor of the E isomer in most
cases (Table 2, except for entries 5, 10, and 12).
Substrate Scope of Arylalkynes. The presence of an
electron-donating methyl group at the para or meta position of
the phenylacetylene moiety afforded high yield and good
selectivity to an E:Z mixture of the dimerized product (Table 2,
entries 2 and 3). The presence of methoxy group at the ortho
position of phenylacetylene dramatically lowered both the yield
and selectivity in comparison to that for a para substituent,
presumably because of the steric congestion at the ortho
position of methoxyphenylacetylene (Table 2, entries 4 and 5).
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Figure 2. Free energy landscape of the alkyne dimerization reaction catalyzed by 1. Asterisks designate the transition states which were not exactly
located but are shown for the purposes of illustration only. In the inset is shown 4-TS, in which the newly formed C···C bond distance is 1.88 Å.
structures (Figure 3b).13 Interestingly, we observed E to Z
product interconversion as a time-dependent phenomenon,
since the E:Z ratio at 1 h (64:36) can be increased to 82:18
after 4 h (Table S2, entry 3, in the Supporting Information).
Our proposed mechanism has a very close resemblance to
previous proposals for Zr- and Ru-catalyzed dimerization of
arylalkynes to 1,3-enynes.14
Figure 3. (a) APT charge distribution in 4. (b) Plausible mechanism
for Z to E isomerization via a metallacarbene type intermediate
structure.
CONCLUSIONS
Herein we have reported an efficient base-metal catalyst, which
can convert terminal arylalkynes to the corresponding
■
dimerized enyne products in excellent yield and good E:Z
ratio. The small amount of catalyst loading for this dimerization
translates into a high TON of 6500. Isolation of a reaction
head-to-tail isomer of the enyne product will require
significantly higher energy due to the assembly of two
negatively charged centers. The free energy landscape is
intermediate, further catalysis with this intermediate, and high-
shown in Figure 2, accumulating all the computational
level DFT calculations have helped to build a convincing
information. The C−C bond formation via 4-TS overcomes a
small energy barrier of 6.43 kcal/mol, leading to the σ-enynyl
mechanistic sketch for this atom-economical reaction.
intermediate 5, which forms after alkyne binding to the vacant
coordination site at iron (Figure 2). The formation of 5 is
EXPERIMENTAL SECTION
■
Methods. All reactions and manipulations were carried out under a
dry and oxygen-free atmosphere (N2) using standard Schlenk
techniques or inside a glovebox maintained below a 0.1 ppm level of
O2 and H2O. All glassware were oven-dried (130 °C) and evacuated
while hot prior to use. All solvents were distilled from Na/
benzophenone prior to use. All other chemicals were purchased
from commercial sources and used as received. Elemental analyses
were carried out using a Perkin Elmer 2400 CHN analyzer, and
samples were prepared by keeping them under reduced pressure (10−2
mbar) overnight. The melting points were measured in a sealed glass
highly exergonic and justifies the strong thermodynamic drive
behind the dimerization reaction. The σ-enynyl intermediate
can easily be protonated by the bound alkyne to realize the 1,4-
disubstituted enyne. The protonation of the anionic form of the
enyne by an alkyne has been invoked in several previous
reports.12
Moreover, the potential energy surface of the alkyne
dimerization reaction suggests that the major amount of energy
required for the reaction is in fact expended to drive CO out to
create unsaturation from the stable 1. To prove this conjecture
further as well as to authenticate the competence of 3, we
performed catalysis with isolated 3. Indeed, catalysis starting
from 3 can happen at a far lower temperature, 50 °C,
converting the phenylacetylene completely to its dimerized
product in quantitative yield. Notably, the E isomer of the
dimerized product is slightly thermodynamically more stable
than the kinetically controlled Z isomer. The Z to E
isomerization can be facilitated via iron−carbenoid resonance
tube on a Buchi B-540 melting point apparatus and are uncorrected.
̈
Analytical TLC was performed on Merck 60F254 silica gel plates (0.25
mm thickness). Column chromatography was performed on Merck 60
silica gel (100−200 mesh). Deuterated benzene was purchased from
Sigma-Aldrich, dried by sodium/potassium alloy, and stored over 4 Å
molecular sieves prior to use. NMR spectra were recorded on a JEOL
ECS 400 MHz spectrometer or a Bruker Avance III 500 MHz
spectrometer. All chemical shifts are reported in ppm using
tetramethylsilane as a reference. Chemical shifts (δ) downfield from
the reference standard are assigned positive values. Crystallographic
D
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data for the structural analysis of 1 have been deposited at the
Cambridge Crystallographic Data Center (CCDC number 1497028).
These data can be obtained free of charge from the Cambridge
Crystallographic Data Center.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Synthesis of (cAAC)Fe(CO)4 (1). In a glovebox, an oven-dried 50
mL Schlenk flask was charged with Fe2(CO)9 (200 mg, 0.55 mmol)
and cyclic (alkyl)amino carbene (cAAC) (286 mg, 1 mmol). Then the
Schlenk flask was taken out from the glovebox, connected to the
Schlenk line and dry toluene (25 mL) was added via a cannula at 25
°C under an argon atmosphere to this reaction mixture. The reaction
mixture was stirred overnight, and during the course of the reaction,
the color changed to yellow. Then the reaction mixture was filtered
through a celite pad and concentrated to ca. 5 mL. Storage of this
reaction mixture at −20 °C for 3 days afforded yellow crystals (185
mg, 0.43 mmol, 43%). Anal. Calcd for C24H31FeNO4: C, 63.58; H,
6.89; N, 3.09. Found: C, 63.62; H, 6.82; N, 3.04. IR (film, γCO in
S.K.M. thanks the SERB of India (DST, No. SR/S1/IC-25/
2012) for financial support. M.B. and G.V. thank the UGC,
New Delhi, India, for research fellowships. D.A. acknowledges
support from the computational facility at Indiana University,
Bloomington, IN, USA.
REFERENCES
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1
cm−1): 2039, 1988, 1926. H NMR (C6D6, 400 MHz, 298 K): δ 7.60
(br s, 1H), 7.39 (br s, 1H), 7.11−6.78 (m, 6H), 3.37 (br s, 2H), 3.05
(br s, 1H), 2.16−1.75 (m, 2H), 1.47−1.16 (m, 6H), 1.16−0.96 (m,
12H), 0.87−0.77 (m, 6H) ppm. 13C NMR (C6D6, 125 MHz, 298 K): δ
222.8, 208.7, 171.9, 132.2, 130.9, 129.0, 128.8, 128.0, 127.4, 127.2,
122.7, 96.2, 82.2, 79.9, 78.7, 66.5, 59.4, 32.1, 29.2, 28.3, 26.4 ppm.
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127.2, 96.2, 78.7, 66.5, 59.4 (CH2), 32.1, 29.2, 28.3 ppm.
General Procedure for Catalytic Dimerization of Arylalkynes
to 1,3-Enynes by (cAAC)Fe(CO)4 (1). In an oven-dried 25 mL
sealed tube equipped with a magnetic stirrer inside the glovebox,
complex 1 (0.2 mol %) was dissolved in 1.0 mL of toluene followed by
addition of KOtBu (224.4 mg, 2.0 mmol, 2 equiv). Then arylacetylene
(1.0 mmol, 1 equiv) was added. The mixture was heated to 120 °C for
4 h. After cooling the reaction mixture, the solvent was removed and
the crude product was purified by silica gel column chromatography
using distilled n-hexanes as eluent to give (E+Z)-1,4-diarylbut-1,3-
enyne.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00703.
Experimental procedure, spectroscopic data, and scanned
spectra (PDF)
X-ray crystallographic data for 1 (CIF)
Calculations and Cartesian coordinates of calculated
structures (XYZ)
(8) (a) N-Heterocyclic Carbenes in Transition Metal Catalysis. In
Topics in Organometallic Chemistry; Glorius, F., Ed.; Springer: Berlin,
2007; Vol. 21. (b) Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.;
Bertrand, G. Angew. Chem., Int. Ed. 2005, 44, 5705−5709. (c) Martin,
D.; Canac, Y.; Lavallo, V.; Bertrand, G. J. Am. Chem. Soc. 2014, 136,
AUTHOR INFORMATION
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Corresponding Authors
*E-mail for D.A.: adhikari@iisermohali.ac.in.
*E-mail for S.K.M.: swadhin.mandal@iiserkol.ac.in.
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DOI: 10.1021/acs.organomet.6b00703
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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(9) Crystal data for 1: C24H31FeNO4, Mr = 453.35, monoclinic, space
group P121/c1, a = 15.7767(5) Å, b = 9.8101(2) Å, c = 16.6158(5) Å,
α = 90°, β = 116.756(4)°, γ = 90°, V = 2296.30(12) nm3, Z = 4, ρcalc
=
1.311 Mg m−3, μ (Mo Kα) = 0.685 mm−1, T = 100 K, θ range for data
collection 2.46−25.03°, R1 = 0.0340 (I > 2σ(I)), wR2 = 0.0868 (all
data).
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DOI: 10.1021/acs.organomet.6b00703
Organometallics XXXX, XXX, XXX−XXX