Solvent-free solid acid-catalyzed nucleophilic substitution of propargylic alcohols: A green approach for the synthesis of 1,4-diynes

Article abstract of DOI:10.1039/c0gc00117a

Acid-treated K10 montmorillonite (H-K10 mont) exhibited an outstanding catalytic activity in the nucleophilic substitution of propargylic alcohols with alkynylsilanes. The reactions were carried out under solvent-free conditions, and the solid catalyst wa

Full text of DOI:10.1039/c0gc00117a

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www.rsc.org/greenchem | Green Chemistry
Solvent-free solid acid-catalyzed nucleophilic substitution of propargylic
alcohols: a green approach for the synthesis of 1,4-diynes†
Tao Wang, Rui-da Ma, Liu Liu and Zhuang-ping Zhan*
Received 17th May 2010, Accepted 2nd July 2010
DOI: 10.1039/c0gc00117a
Acid-treated K10 montmorillonite (H-K10 mont) exhibited
an outstanding catalytic activity in the nucleophilic sub-
stitution of propargylic alcohols with alkynylsilanes. The
reactions were carried out under solvent-free conditions,
and the solid catalyst was reusable with retention of its high
activity. This method provides a green and rapid route to
1,4-diynes.
properties, such as their cation exchange ability in the interlayer,
expansible interlayer space and tunable acidity.10 Furthermore,
the application of K10 mont in liquid-phase organic synthesis
allows facile catalyst separation and reutilization, providing
environmentally benign or green processes compared with
homogeneous acids. Recently, Patti et al. reported that K10
mont, on treatment by acid before use, exhibits significant
enhancements in its catalytic activity.11 This discovery indicates
the promising application value of acid-treated K10 mont in
organic synthesis. However, to the best of our knowledge, no
report of further studies in this field currently exists.
Herein, as a result of developments in the propargylic
substitution reaction by our group,12 an acid-treated K10 mont-
catalyzed synthesis of 1,4-diynes from propargylic alcohols
and terminal TMS-alkynes under solvent-free conditions is
reported.13 Not only did the acid-treated K10 mont display a
higher catalytic activity than virgin K10 mont, but the catalyst
could be easily separated and reused. Also, this is the first
clay-catalyzed sp–sp3 C–C coupling reaction carried out under
solvent-free conditions.
In our initial study, propargylic alcohol 1a was treated with
trimethyl(phenylethynyl)silane (2a) using commercial virgin
K10 as the catalyst under solvent-free conditions (Table 1).
Unfortunately, the desired product, 3aa, was obtained in only
48% yield (Table 1, entry 1). On the basis of the structural
characteristics of montmorillonite, we reasoned that such an
unsatisfactory yield was probably due to the inadequacy of the
exchangeable protons in the catalyst. Thus, we hypothesized
that the catalytic efficiency of K10 mont could be enhanced
on the condition that the charge compensating cations in the
hydrate interlayer were further exchanged for H+. To test our
hypothesis, K10 mont was treated with different concentrations
of hydrochloric acid under various conditions, whose catalytic
efficacy was subsequently examined (Table 1). We were pleased
to find that K10 mont treated with 1 M HCl at 80 ◦C
exhibited an excellent catalytic performance and dramatically
increased the yield of the expected product (Table 1, entry 4).
However, increasing the concentration of HCl to 5 M for the
treatment of K10 mont failed to exhibit any further improvement
in yield (Table 1, entry 5). To test whether Brønsted acids
such as HCl, p-toluenesulfonic acid and TfOH also possessed
similar high catalytic activities to acid-treated K10 mont, this
transformation was also performed using these strong acids
as catalysts under the same reaction conditions. However, the
results shown in Table 1 indicate that their catalytic efficiency
was rather lower than that of acid-treated K10 mont, whose
structural characteristics probably played a crucial role in the
reaction. The optimization results show that the employment of
The nucleophilic substitution of propargylic alcohols has turned
into an important organic transformation, as it provides facile
and expeditious routes to a wide range of propargylic products.1,2
Although substantial attention has been paid to this field in the
past decade, many studies have been restricted to the devel-
opment of metal-catalyzed homogeneous transformations,2a–f
and at the same time, very few reports on the preparation of
synthetically valuable 1,4-diynes via sp–sp3 C–C bond formation
currently exist.3 Jung et al. reported a propargylic substitution
reaction between terminal TMS-alkynes and propargyl halides
in the presence of a fluoride source and a catalytic amount
of copper iodide to prepare 1,4-diynes in good yields under
mild conditions.4 Takai and co-workers recently demonstrated
a rhenium-catalyzed smooth substitution of propargyl alcohols
with an alkynylsilane. It was proposed that this reaction pro-
ceeded via propargylic cations as intermediates.5 Cohen carried
out an InCl3-catalyzed nucleophilic substitution of a propargylic
alcohol with an alkynylsilane to afford a 1,4-diyne, which was
subsequently transformed into bioactive Nyasol.6 Yadav et al.
described an efficient procedure for the alkynylation of aryl
propargyl alcohols with alkynylsilanes using molecular iodine as
the catalyst.7 However, more extensive use of these approaches is
confined to some extent by virtue of the high price of the complex
metallic catalysts and the harmfulness of halogen-containing
solvents to the environment. Thus, it is still highly attractive
to develop efficient, economical and environmentally benign
methodologies for the synthesis of 1,4-diynes via a propargylic
substitution protocol.
Heterogeneous catalysis is of great importance in a variety of
chemical processes,8,9 and the corresponding solid acid catalysts
based on commercially available K-10 montmorillonite (K10
mont) have recently received much attention due to their unique
Department of Chemistry, College of Chemistry and Chemical
Engineering, and State Key Laboratory for Physical Chemistry of Solid
Surfaces, Xiamen University, Xiamen, 361005, Fujian, P. R. China.
E-mail: zpzhan@xmu.edu.cn; Fax: +86 (592) 2180318; Tel: +86 (592)
2180318
† Electronic Supplementary Information (ESI) available: Experimental
and spectral data of compounds 3aa–3bc. Copies of 1H and 13C NMR
spectra of new compounds. See DOI: 10.1039/c0gc00117a
1576 | Green Chem., 2010, 12, 1576–1579
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Table 1 Nucleophilic substitution of propargylic alcohol 1a with
alkynylsilane 2a using various acid catalystsa
Table 2 H-K10 mont-catalyzed nucleophilic substitution of propar-
gylic alcohols with alkynylsilanesa
Acid amount/
Entry R1; R2
R3
Product Yield (%)b
Entry
Catalyst
mmol g-1b
Time
Yield (%)c
1
2
3
4
5
6
7
8
1a: Ph; Ph
2a: H
2a: H
2a: H
2a: H
2a: H
2a: H
2a: H
2a: H
2a: H
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
89
81
85
86
82
81
87
83
82
n. r.c
85
83
81
80
89
82
1
2
3
4
5
6
7
8
9
K10 mont
A
B
0.25
0.73
0.86
0.98
1.03
—
—
—
—
45 min
35 min
35 min
25 min
25 min
24 h
24 h
24 h
24 h
48
54
67
89
88
1b: Ph; n-Bu
1c: Ph; TMS
1d: Ph; 1-cyclohexenyl
1e: Ph; cyclopropyl
1f: Ph; H
C
D
HCld
Trace
Trace
37
p-TsOH·H2Od
TfOHd
(none)
1g: 4-MeO-Ph; TMS
1h: 4-Cl-Ph; TMS
1i: 4-Br-Ph; TMS
1j: 4-COOMe-Ph; n-Bu 2a: H
1k: 1-naphthyl; H
1a: Ph; Ph
1e: Ph; cyclopropyl
1f: Ph; H
1a: Ph; Ph
1b: Ph; n-Bu
0e
9
10
11
12
13
14
15
16
3ja
a Reaction conditions: 1a (1 mmol), 2a (1 mmol), catalyst (20 wt% of 1a),
solvent-free, 40 ◦C. A: H-K10 mont (K10 mont treated with 0.1 M HCl
at 20 ◦C). B: H-K10 mont (K10 mont treated with 1 M HCl at 20 ◦C). C:
H-K10 mont (K10 mont treated with 1 M HCl at 80 ◦C). D: H-K10 mont
(K10 mont treated with 5 M HCl at 80 ◦C). b Determined by NH3-TPD.
c Isolated yields. d 0.1 mmol. e Starting material was recovered.
2a: H
3ka
3ab
3eb
3fb
3ac
3bc
2b: Br
2b: Br
2b: Br
2c: MeO
2c: MeO
a Reaction conditions: 1 (1 mmol), 2 (1 mmol), H-K10 mont (20 wt% of
1), 40 ◦C, 25 min. b Isolated yields. c n. r. = No reaction. Reaction time
was 24 h.
K10 mont treated with 1M HCl at 80 ◦C is reasonably efficient
for this nucleophilic substitution of propargylic alcohols with
alkynylsilanes.
Next, with the optimized reaction conditions in hand, the
generality of this novel H-K10 mont-catalyzed nucleophilic
substitution was examined (Table 2).14 To our delight, we found
this transformation to be very general for a wide range of
propargylic alcohols and alkynylsilanes. The employment of
propargylic alcohol 1b, bearing an alkyl chain at the terminal
position of the acetylene moiety, smoothly afforded the desired
product, 3ba, under mild conditions (Table 2, entry 2). Also, as
expected, 1,4-diynes 3ea and 3eb were obtained from propargylic
alcohol 1e in 82 and 81% yields, respectively, and no ring-
opening of the cyclopropyl groups was observed (Table 2, entries
5 and 13). When R2 was replaced by an unsaturated alkyl group,
the reaction also led to the formation of the desired 1,4-diyne in
good yield (Table 2, entry 4).
Interestingly, propargylic alcohols 1c and 1g–1i, containing
an alkynylsilane moiety, all gave the corresponding products
retaining their TMS groups (Table 2, entries 3, 7–9). Among
them, 1g possessing an electron-donating group in the aryl ring
(R1 = 4-MeO-Ph), reacted smoothly, affording 3ga in good yield
(Table 2, entry 7). Substrates 1h and 1i, bearing an electron-
withdrawing group, were also successfully employed in the
substitution reaction to give the corresponding products 3ha and
3ia in good yields of 83 and 82%, respectively (Table 2, entries
8 and 9). Unfortunately, propargylic alcohol 1j, bearing a 4-
methoxycarbonylphenyl group, failed to give any of the desired
product, even after a prolonged reaction time (Table 2, entry
10), perhaps because of the strong electron-withdrawing effect
of the methoxycarbonyl group. The above results indicate that
the substitution may occur via an SN1 mechanism, in which
a propargylic cation intermediate is formed, whose instability
clearly makes the nucleophilic substitution less favorable.
The reaction was not limited to substrates bearing internal
alkyne groups. For example, propargylic alcohols 1f and 1k, with
terminal alkyne groups, afforded the corresponding 1,4-diynes
in high yields under the same conditions (Table 2, entries 6, 11
and 14). Additionally, fused aromatic propargylic alcohol 1k
readily underwent this nucleophilic substitution (Table 2, entry
11).
We next turned to investigating the scope of alkynylsilane
2. Gratifyingly, both electron-rich and electron-poor alkynyl-
silanes reacted smoothly with propargylic alcohols, affording
the desired 1,4-diynes in good yields (Table 2, entries 12–16). It
should be noted that halogenated aromatic substrates, which
may participate in transition metal-catalyzed cross-coupling
reactions, survive the reaction conditions, allowing subsequent
elaboration of the products.
Nucleophilic substitution of alkyl-substituted propargylic
alcohols (R1 = alkyl) with alkynylsilanes has generally been
considered difficult to achieve, perhaps because of the instability
of the propargylic cation intermediates.5–7 However, we were
delighted to find that alkyl-substituted propargylic alcohol 1l
participated in the H-K10 mont-catalyzed nucleophilic substi-
tution as well (Scheme 1). Treatment of 1l with alkynylsilane
2a in the presence of H-K10 mont led to the formation of 3la,
albeit in low yield. In contrast, replacing propargylic alcohol
with propargylic acetate increased the yield of 3la to 48%.
As a further demonstration of the advantages of this novel
propargylic substitution, we studied the catalytic efficacy of the
catalyst after the reaction. We found that H-K10 mont could
be easily separated from the reaction system and continued to
exhibit good catalytic efficiency, even in the seventh recycling
experiment (Table 3).
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Table 3 Re-use of the H-K10 mont catalyst for the substitution of 1a
with 2aa
Recycle number
Fresh 1st 2nd 3rd 4th 5th 6th 7th
Isolated yield (%) 89
89 87 88 87 88 87 87
a Reaction conditions: 1a (1 mmol), 2a (1 mmol), H-K10 mont (20 wt%
of 1a), 40 ◦C.
Scheme 1 Nucleophilic substitution of alkyl-substituted propargylic
alcohol and acetate.
As HCl, p-toluenesulfonic acid and TfOH showed much lower
activities than H-K10 mont (Table 1), we supposed that the
specific structure of H-K10 mont was likely to play a pivotal
role in this transformation. To further test whether the reactions
took place at the surface of the catalyst or were homogeneously
catalyzed by leached protons, we carried out a Sheldon test.15
Gratifyingly, the result was totally in accordance with our
hypothesis; the H-K10 mont-catalyzed nucleophilic substitution
was demonstrated to be a heterogeneous reaction.†
A possible mechanism for the H-K10 mont-catalyzed nucle-
ophilic substitution reaction is proposed in Scheme 2. According
to the catalytic path, propargylic alcohol 1 is first protonated by a
H+ site to give a propargylic cation intermediate. The silicate lay-
ers are capable of acting as delocalized counteranions to stabilize
cationic intermediates, on which subsequent nucleophilic attack
of alkynylsilane 2 generates an alkenyl cationic intermediate
stabilized by both the phenyl ring and the silyl group.16 The
trimethylsilyl group is then eliminated to give the desired 1,4-
diyne. Finally, the H+ active site is regenerated by hydration so
as to enter into the next catalytic cycle.
Scheme 2 A proposed mechanism of the H-K10 mont-catalyzed
nucleophilic substitution.
be detected by a silver nitrate test. The clay was then dried at
110 ◦C to afford acid-treated K10 mont as a white-gray powder.
General procedure for the alkynylation of propargylic alcohols
A mixture of propargylic alcohol (1 mmol), alkynylsilane
(1 mmol) and H-K10 mont (20 wt% of propargylic alcohol) was
◦
stirred at 40 C in a round-bottomed flask. When the reaction
was complete (monitored by TLC), the product was dissolved
in ethyl acetate and separated from the catalyst by filtration.
The solvent was removed under reduced pressure, and then the
residue further purified by silica gel column chromatography
(petroleum ether) to afford the 1,4-diyne. The recovered catalyst
could be reused after drying at 110 ◦C for 1 h.
In summary, we have developed the H-K10 mont-catalyzed
nucleophilic substitution of propargylic alcohols with alkynylsi-
lanes, providing a green and rather facile approach to 1,4-diynes.
Besides the mild and solvent-free reaction conditions, the solid
catalyst can be easily separated and reused at least seven times
without any appreciable loss in its catalytic efficiency, which
gives it great potential for large-scale applications. Furthermore,
this substitution reaction has good functionality tolerance,
and the scope of substrates is broad. Further studies on
the application of this methodology are in progress in our
laboratories.
Acknowledgements
This research was financially supported by the National Natural
Science Foundation of China (no. 20772098).
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A mixture of K10 mont (10 g) and hydrochloric acid (200 ml)
was stirred at a chosen temperature for 6 h. The slurry obtained
was filtered and washed with de-ionized water until no Cl- could
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15 This idea was proposed by one of the referees and we consider it an
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©