Efficient and regioselective nickel-catalyzed [2 + 2 + 2] cyclotrimerization of ynoates and related alkynes

Article abstract of DOI:10.1039/c3ob41872c

A nickel-based catalytic system has been developed for [2 + 2 + 2] cyclotrimerization of various alkynes, especially ynoates. This catalytic system enables facile construction of substituted aromatic compounds in excellent yields with high regioselectivity. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ob41872c

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Efficient and regioselective nickel-catalyzed [2 + 2 + 2]
cyclotrimerization of ynoates and related alkynes†
Cite this: Org. Biomol. Chem., 2013, 11,
7653
Sanjeewa K. Rodrigo,a Israel V. Powell,b Michael G. Coleman,*b Jeanette A. Krausea
and Hairong Guan*a
Received 12th September 2013,
Accepted 30th September 2013
DOI: 10.1039/c3ob41872c
www.rsc.org/obc
A nickel-based catalytic system has been developed for [2 + 2 + 2] with turnover numbers (TONs) up to 196 and isomeric ratios
cyclotrimerization of various alkynes, especially ynoates. This cata- greater than 90 : 10.
lytic system enables facile construction of substituted aromatic
compounds in excellent yields with high regioselectivity.
During the course of developing a nickel-based catalytic
system for regioselective reductive coupling of ynoates and
aldehydes (Scheme 1),11 we found that in order to achieve high
yields for the desired silyl-protected γ-hydroxy-α,β-enoates, the
ynoate substrate must be added very slowly to the reaction
mixture. Close inspection of the byproducts revealed that one
of the competing pathways was the cyclotrimerization of
ynoates to generate benzene-1,2,4-tricarboxylates as the major
isomers. Nickel-catalyzed oligomerization of ynoates is known
in the literature, but in most cases tetra-substituted cycloocta-
tetraenes are the cycloaddition products.12 A study done in the
early sixties using Ni(PPh3)2(CO)2 as the catalyst showed that
oligomerization of ethyl propiolate produced 1,2,4- and 1,3,5-
substituted aromatic compounds in 89% and 6% yields,
respectively.13 Given that these benzene derivatives could be
used as new branching or cross-linking agents for polymers14
and as precursors for synthesizing organic light-emitting
diode materials,15 we became interested in optimizing the con-
ditions for the cyclotrimerization of ynoates and related
alkynes. In this paper, we will report these results and illus-
trate that such a process can be readily catalyzed by Ni(COD)2/
PPh3 or Ni(COD)2/NHC (NHC is an N-heterocyclic carbene).
The advantages of using this methodology are several fold.
First of all, catalysts can be easily generated in situ by mixing
commercially available reagents. More importantly, the cata-
lytic process is quite efficient; TONs as high as 2000 can be
Transition-metal-catalyzed [2 + 2 + 2] cycloaddition of alkynes
represents a versatile method for the construction of benzene
rings.1 It is especially useful when the desired substitution
pattern is not easily built via conventional aromatic substi-
tution reactions. The challenge of implementing this cycliza-
tion strategy, however, arises in performing the reactions of
unsymmetrical alkynes due to the formation of different regio-
isomers. Typically, [2 + 2 + 2] cycloaddition of a diyne and an
alkyne2 or intramolecular cycloaddition of a triyne3 is more
regioselective, and therefore it has been more widely utilized
in organic synthesis.1 Nevertheless, over the past two decades,
a plethora of transition metal systems (e.g., Ti,4 Co,5 Ni,6 Ru,7
Rh,8 Pd9 and other metals10) have been developed specifically
to catalyze intermolecular [2 + 2 + 2] cycloaddition of three
individual alkyne molecules. Despite the progress, many chal-
lenges remain, particularly to increase catalytic efficiency and
improve regioselectivity. While the majority of known proto-
cols employ 5–10 mol% of metal catalysts (with respect to the
total alkynes), there are a few cases where a catalyst loading as
low as 1 mol% is sufficient to convert alkynes to substituted
benzenes in high yield with high regioselectivity.4c,7e,8b,10d One
notable example, and arguably the most active catalytic system
reported to date, involves cyclopentadienyl-ruthenium-based
complexes,7e which promote the cyclotrimerization of alkynes
aDepartment of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati,
Ohio 45221-0172, USA. E-mail: hairong.guan@uc.edu; Fax: (+1) 513-556-9239;
Tel: (+1) 513-556-6377
bSchool of Chemistry & Materials Science, Rochester Institute of Technology,
Rochester, NY 14623, USA. E-mail: mgcsch@rit.edu; Fax: (+1) 585-475-7800;
Tel: (+1) 585-475-5108
†Electronic supplementary information (ESI) available: Experimental procedures
and spectroscopic characterization data for compounds 2a–p, 3d, and 3p. CCDC
948437 (2f) and 948438 (2m). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3ob41872c
Scheme 1 Nickel-catalyzed reductive coupling reactions.
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achieved, allowing the synthesis to be carried out on the gram better medium to carry out the reaction than the more polar
scale. Moreover, the observed regioselectivity is typically high solvents such as THF and CH3CN (entries 9 and 10).
and in a number of cases only one regioisomer is formed.
It should be mentioned that there are literature precedents
We commenced our study with the identification of for Ni(COD)2/PPh3-catalyzed [2 + 2 + 2] cycloaddition of
optimal conditions for the cyclotrimerization reaction using alkynes. The Mori group has reported cocyclization of diynes
ethyl propiolate 1a as the substrate. The results are summari- and acetylene catalyzed by 30 or 40 mol% of Ni(COD)2/PPh3
zed in Table 1. In agreement with our previous procedures for (1 : 4).17 Saito, Yamamoto, and co-workers have used the same
reductive coupling of ynoates and aldehydes,11 the first set of catalytic mixture (10 mol%) for regioselective cyclotrimeri-
experiments (entries 1–3) was to test the catalytic performance zation of 1-perfluoroalkylenynes.6b Ikeda et al. have employed
of Ni(COD)2 (10 mol%) combined with an equimolar amount 5 mol% of Ni(COD)2/PPh3 (1 : 2) for cross-cyclotrimerization of
of a phosphine ligand. In the presence of Ni(COD)2/PnBu3, two different alkynes.6a A more recent study by the Baran
only a negligible amount of product formed after 1 h at room group has demonstrated that 10 mol% of Ni(COD)2/PPh3 (1 : 1)
temperature, as determined by GC-MS (entry 1). While repla- is an effective catalyst for co-oligomerization of 1,3-dienes and
cing PnBu3 with PCy3 increased the yield to 20% (entry 2), the alkynes, but for some alkyne substrates, benzene derivatives
use of PPh3 led to a more significant improvement with GC are the main products.18 One common feature of the above-
yield reaching 82% (entry 3). Encouraged by this result, we mentioned catalytic processes as well as other Ni(COD)2-cata-
were curious to see if the commercially available Ni(PPh3)4 lyzed reactions is the high catalyst loading, which is, in part,
would catalyze the same reaction. With 1 mol% of Ni(PPh3)4 due to the low thermal stability of the Ni(0) species.19 In this
(entry 4), a quantitative conversion of 1a to aromatic com- regard, the relatively low catalyst loading needed in our system
pounds 2a and 3a (with a ratio of 97 : 3) was observed within is quite unusual. As a matter of fact, even with 0.05 mol% of
1 h. Although Ni(PPh3)4 proved to be an effective catalyst, it is the nickel catalyst, cyclotrimerization of 1a was found to be
a relatively expensive reagent. We thus decided to replace it complete within 2 h (eqn (1)). The calculated TON of 2000, to
with the much cheaper Ni(COD)2 along with 4 equivalents of the best of our knowledge, is the highest for any transition-
PPh3.16 The in situ generated catalyst turned out to be as metal-catalyzed cyclotrimerization reaction.
efficient as pure Ni(PPh3)4 in promoting the cyclotrimerization
reaction without compromising the level of regioselectivity
(entry 5). Additional optimization studies suggested that the
amount of PPh3 could be reduced to 3 equivalents relative to
Ni(COD)2 (entry 6). However, reducing the PPh3/Ni(COD)2 ratio
ð1Þ
further resulted in diminished yields (entries 7 and 8). We also
tested different solvents for the cyclotrimerization reaction.
Among the solvents tested, toluene was found to be a much
To explore the scope of the cyclotrimerization reaction, we
applied the optimized conditions to a variety of ynoates. As
shown in Table 2, methyl, ethyl, or tert-butyl propiolate
(entries 1–3) underwent facile cyclotrimerization to afford the
corresponding benzene-1,2,4-tricarboxylate as the major
product. The observed regioselectivity (97 : 3) appeared to be
unaffected by the size of the alkyl groups. However, the reac-
tion of 2-naphthyl propiolate was less selective, resulting in an
88 : 12 mixture of 1,2,4- and 1,3,5-isomers (entry 4). Attempts
to improve the regioselectivity by replacing 3 equiv. of PPh3
with 1 equiv. of an NHC ligand (Fig. 1) were unsuccessful
(entries 5 and 6). Ynoates with an internal CuC bond also
proved to be viable substrates, although a longer reaction time
or a higher temperature was necessary. For example, cyclo-
trimerization of diethyl acetylenedicarboxylate 1e took 5 h to
complete, providing a hexa-substituted benzene in nearly
quantitative yield upon isolation (entry 7). In contrast, no reac-
tion was observed for 1f after 24 h at room temperature. Yang
and co-workers had encountered a similar challenge15 when
using Hilt’s cobalt diimine catalyst5c to effect the cyclotrimeri-
zation of 1f. Fortunately, at 80 °C, nickel-catalyzed cyclotrimeri-
zation proceeded smoothly to yield the unsymmetrical
cyclotrimer 2f as the major isomer, albeit with a modest
selectivity of 80 : 20 (entry 8). The regioselectivity was, however,
Table 1 Optimization of the reaction conditionsa
Cat.
Yieldb
(%)
Entry Catalyst
(mol%) Solvent
2a : 3ac
1
2
3
4
5
6
7
8
9
Ni(COD)2 : PnBu3 (1 : 1) 10
Toluene
Toluene
Toluene
Toluene 100
Toluene 100
Toluene 100
Toluene
Toluene
THF
2
20
82
n.d.
n.d.
n.d.
97 : 3
97 : 3
97 : 3
n.d.
n.d.
n.d.
n.d.
Ni(COD)2 : PCy3 (1 : 1)
Ni(COD)2 : PPh3 (1 : 1)
Ni(PPh3)4
Ni(COD)2 : PPh3 (1 : 4)
Ni(COD)2 : PPh3 (1 : 3)
Ni(COD)2 : PPh3 (1 : 2)
Ni(COD)2 : PPh3 (1 : 1)
Ni(COD)2 : PPh3 (1 : 3)
Ni(COD)2 : PPh3 (1 : 3)
10
10
1
1
1
1
1
1
57
36
42
72
10
1
CH3CN
a Conditions: 1a (0.25 mmol), nickel catalyst and n-decane (internal
standard, 0.04 mmol) in 2 mL of solvent. b Combined (2a + 3a) yield
determined by GC-MS. c Ratio determined by 1H NMR (n.d. = not
determined).
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Table 2 Cyclotrimerization of various ynoatesa
Entry
Ynoate
Cat.
A
Time (h)
Temp. (°C)
Yieldb (%)
2 : 3d
97 : 3
97 : 3
1
2
1
1
23
23
90
91
A
3
A
1
23
88
97 : 3
4
5
6
A
B
C
1
1
1
23
23
23
86c
n.d.
n.d.
88 : 12
75 : 25
75 : 25
7
A
5
23
96
—
8
9
A
B
12
24
80
23
94c
92
80 : 20
97 : 3
10
11
12
A
B
C
24
24
24
23
23
23
74
n.d.
84
80 : 20
85 : 15
90 : 10
13
14
A
B
24
24
23
23
71
94
80 : 20
>99 : 1
15
16
B
B
24
24
23
23
89
92
>99 : 1
>99 : 1
a Abbreviations: 2-Np = 2-naphthyl, nPr = n-propyl, nHex = n-hexyl; reaction conditions: 1.5 mmol of 1 in 2 mL of toluene (for cat. A) or THF (for
cat. B and C), cat. A = 1 mol% Ni(COD)2/PPh3 (1 : 3), cat. B = 1 mol% Ni(COD)2/SIPr (1 : 1), cat. C = 1 mol% Ni(COD)2/SIMes (1 : 1). b Isolated yield
for 2. c Combined isolated yield for 2 and 3. d Ratio determined by 1H NMR or GC-MS.
Fig. 1 NHCs employed in this study.
substantially higher when SIPr was used in place of PPh3
(entry 9). The substitution pattern of 2f was further established
by X-ray crystallography (Fig. 2, see ESI†).‡ Worthy of particular
note is that 2f has been previously sought after as a precursor
to isotruxene,15 a potential building block for synthesizing
Fig. 2 X-ray structure of compound 2f (50% probability level).
various π-conjugated systems.20 Thus, the method developed
here could be a convenient synthetic route to these types of
molecules. Interestingly, Ni(COD)2/PPh3-catalyzed cyclotrimeri-
1a or 1b. For related reactions including the cyclotrimerization
zation of 1g (entry 10) and 1h (entry 13) took place at room
temperature, but displayed modest regioselectivity (80 : 20). of alkynes, metallacyclopentadiene intermediates are usually
generated via reductive coupling of alkynes,1 and the regio-
These results were somewhat surprising; in our previous study
of reductive coupling of ynoates and aldehydes,11 the reaction
selectivity in metallacycle formation is often influenced by
of 1h was found to be much more regioselective than that of polarization of the triple bonds.21 Given the disparity of
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electronic properties between an ester group and an alkyl the cyclopropyl group in 1p may undergo a ring-opening reac-
group, we had anticipated a high regioselectivity for 1g and 1h. tion. We were pleased to find that neither process was operating
Perhaps in these cases, steric effects are also important. We under our catalytic conditions. Analogous to the reaction of
then decided to switch to NHC ligands with the aim to other terminal alkynes, 1,2,4-trisubstituted benzene derivatives
improve the regioselectivity. In the presence of a 1 : 1 catalytic formed as the major isomers (entries 5 and 6).
mixture of Ni(COD)2 and SIPr, the ratio of 2g to 3g was
To further demonstrate the synthetic utility of the cyclotri-
increased slightly to 85 : 15 (entry 11). The use of SIMes merization reaction, we conducted a gram-scale synthesis of
resulted in a further improvement of selectivity to 90 : 10, and 2b using 0.05 mol% of nickel catalyst (eqn (2)). The regioselec-
2g could be separated from the mixture in 84% yield (entry 12). tivity remained high (97 : 3) favoring the unsymmetrical
In the case of 1h, the employment of SIPr as an ancillary isomer 2b, which was isolated in high yield.
ligand led to the exclusive formation of 2h (entry 14). The steri-
cally more demanding ynoates 1i and 1j did not react at room
temperature or 80 °C when using Ni(COD)2/PPh3 as the cata-
lyst. On the other hand, cyclotrimerization of both substrates
was smoothly catalyzed by Ni(COD)2/SIPr, giving 2i and 2j as
the predominant products (entries 15 and 16).
ð2Þ
The success of ynoate cyclotrimerization prompted us to
examine the reactivity of alkynes without the ester functionality
(Table 3). At room temperature in the presence of 1 mol% of Ni
(COD)2 and 3 mol% of PPh3, phenylacetylene 1k was regioselec-
tively converted to 1,2,4-triphenyl benzene 2k in 90% yield
In conclusion, we have described an effective nickel system
for the catalytic cyclotrimerization of ynoates and related
alkynes. This methodology provides access to a diverse array of
tri- or hexa-substituted benzene derivatives in an efficient and
highly regioselective manner. The ester-containing products
2a–j may find many applications in materials chemistry. Pre-
(entry 1). Similarly high yields and regioselectivity were obtained
for the cyclotrimerization of 1l and 1m (entries 2 and 3), and
one of the products 2m was also crystallographically character-
liminary studies have also shown that compounds 2d/3d, 2l,
ized (see ESI†). The reaction of 1n, an internal alkyne, required
2m and 2o display interesting photoluminescent properties.
These results will be reported elsewhere.
an elevated temperature; no reactivity was observed at room
temperature (entry 4). Compounds 1o and 1p have been rarely
tested for cyclotrimerization reactions. The pyridyl group in 1o
may potentially poison the catalyst by binding to nickel, while
Acknowledgements
We thank the U.S. National Science Foundation (CHE-0952083)
for support of this research and Dr Allen G. Oliver (University of
Notre Dame, Department of Chemistry & Biochemistry) for the
data collection using the Bruker ApexII DUO.
Table 3 Cyclotrimerization of various alkynesa
Notes and references
‡Crystal data for 2f: C33H30O6, M = 522.57, orthorhombic, a = 11.6106(2), b =
12.5495(2), c = 18.7424(4) Å, V = 2730.9(9) Å3, T = 150 K, space group Pna21, Z = 4,
λ = 1.54178 Å, reflections measured 28 929, 5161 unique (Rint = 0.0323) which were
used in the calculations. The final R1 was 0.0539 and the final wR2 (F2) was 0.1470
(all data).
Entry Alkyne
Time (h) Temp. (°C) Yieldb (%) 2 : 3d
1
2
12
12
23
23
90
92
>99 : 1
>99 : 1
Crystal data for 2m: C30H24O6, M = 480.49, orthorhombic, a = 11.3776(2), b =
15.9590(3), c = 26.0724(4) Å, V = 4734.10(14) Å3, T = 150 K, space group Pbca, Z = 8,
λ = 1.54178 Å, reflections measured 68 457, 4563 unique (Rint = 0.0588) which were
used in the calculations. The final R1 was 0.0454 and the final wR2 (F2) was 0.0988
(all data).
3
12
23
90
>99 : 1
4
5
24
12
80
23
76
95
—
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