Platinum-catalyzed hydrative cyclization of 1,6-diynes for the synthesis of 3,5-substituted conjugated cyclohexenones

Article abstract of DOI:10.3390/molecules15075045

We have developed a Pt(COD)Cl₂-catalyzed hydrative cyclization of 1,6-diynes leading to the formation of functionalized cyclohexenones in good yields.

Full text of DOI:10.3390/molecules15075045

Molecules 2010, 15, 5045-5052; doi:10.3390/molecules15075045
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Platinum-Catalyzed Hydrative Cyclization of 1,6-Diynes for the
Synthesis of 3,5-Substituted Conjugated Cyclohexenones
Chen Zhang 1, Jian-Feng Qi 1, Dong-Mei Cui 2,*, Qian Wang 2 and Xiu-Li Wang 3
1
School of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, China
2
College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, China
3
College of Chemistry and Bioengineering, Guilin University of Technology, Guangxi, Guilin,
China
* Author to whom correspondence should be addressed; E-Mail: cuidongmei@zjut.edu.cn;
Tel.: 86-571-88320320; Fax: 86-571-88320320.
Received: 4 June 2010; in revised form: 24 June 2010 / Accepted: 5 July 2010 /
Published: 23 July 2010
Abstract: We have developed a Pt(COD)Cl2-catalyzed hydrative cyclization of 1,6-diynes
leading to the formation of functionalized cyclohexenones in good yields.
Keywords: diyne; catalysis; hydrative cyclization; cyclohexenones
1. Introduction
Cyclohexenone derivatives are not only key intermediates in organic synthesis, but they also exhibit
important pharmacological activities [1-4]. Extensive synthetic efforts for conjugate cyclohexenones
have been reported, in which an annulation approach from acyclic precursors constitutes a useful entry
[5-13]. Despite this advance, there is still a great need to develop more convenient catalytic systems
that can accommodate such attractive features as easily accessible starting materials, mild reaction
conditions, and absence of co-products. The newly developed metal-catalyzed hydrative cyclization
reaction is not only an especially attractive “green” procedure, but also an ideal synthetic method for
preparing cyclic enone compounds [14-20]. The reported examples include hydrative cyclization of
1,n-diynes [14-17], 1-yne-5-enones [18], 1-en-5-ynes [19] and diynnols [20]. Recently, Liu and co-
workers reported a PtCl2-catalyzed hydrative cyclization of internal triynes to yield bicyclic
spiroketones [21-23]. As part of our ongoing studies on metal-catalyzed atom-economical reactions,

Molecules 2010, 15
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we succeeded in synthesizing conjugate cyclohexenone ring systems using the hydrative cyclization of
1,6-diynes with (PPh3)AuMe as a catalyst [24,25]. Herein, we report our studies on the use of
Pt(COD)Cl2 as a catalyst in this cyclization.
2. Results and Discussion
Initial hydrative cyclization experiments of 1,6-diyne 1a (0.5 mmol) with H2O (0.5 mmol) at 70 ºC
for 4 h in a sealed-tube were performed to screen catalysts. Pt(COD)Cl2 (COD = cyclooctadiene)
combined with methanesulfonic acid (CH3SO3H) showed good catalytic activity in this reaction,
furnishing cyclohexenone 2a in 75% yield without the formation of the corresponding hydration or
methanol adducts (Table 1, entry 1), while the reaction conducted in the absence of CH3SO3H did not
yield the cyclic product (Table 1, entry 2). Trifluromethanesulfonic acid (CF3SO3H) can also serve as
an excellent co-catalyst. PtCl2 in combination with PPh3 gave 2a in lower yield (Table 1, entry 4).
There was no reaction with other homogeneous metal complex systems, such as Pd(PiPr3)2Cl2 and
Ru(COD)Cl2 (Table 1, entries 5 and 6). During further optimization of the reaction conditions, we
found that a lower catalyst loading (2 mol%) afforded the product with decreased yield (Table 1, entry 3).
Table 1. Pt(II)-Catalyzed synthesis of cyclohexenone from 1,6-diyne a.
O
MeO2C
MeO2C
cat., 70 ℃
MeO2C
MeO2C
H2O
+
MeOH
1a
2a
Catalyst (mol%)
Time
(h)
Yield b
(%)
CH3SO3H
(mol%)
Entry
1
2
3
4
5
6
Pt(COD)Cl2 (5)
Pt(COD)Cl2 (5)
Pt(COD)Cl2 (2)
Pt(PPh3)Cl2 (2)
Pd(PiPr3)Cl2 (2)
Pt(COD)Cl2 (2)
50
-
50
50
50
50
3
3
3
4
4
4
75
Trace
21
12
NR
Nr
a
The reactions were performed with 1a (0.5 mmol), H2O (0.5 mmol), CH3SO3H (1-50
mol%), and catalyst (2-5 mol%) in MeOH (2 mL) at 70 ºC. b Isolated yields.
In order to demonstrate the efficiency and scope of the present method, we applied the optimum
conditions of entry 1 in Table 1 to the hydrative cyclization of several 1,6-diyne substrates bearing a
variety of functionalities at their 4-positions. The results are summarized in Table 2. Terminal
malonate derivatives 1a and 1b were found to be good substrates (Table 2, entries 1 and 2). This is
quite similar to the results of Au (I)–catalyzed reactions [24,25]. To our delight, the presence of two
hydroxyl groups as in compound 1c was tolerated, thus providing cyclohexenone 2c bearing hydroxyl
groups with no intramolecular alcohol addition products (Table 2, entry 3) [26,27]. Protecting groups
such as the single methyl ether in 1d or the double methyl ether in 1e were also compatible with the
present method (Table 2, entries 4 and 5). Cyclic products with different substituent group pairs, such
as the diphenylphosphoryl and ethoxycarbonyl in 2f, or the phenyl and methoxycarbonyl in 2h, were
also obtained in good yields (Table 2, entries 6 and 8). The acetylacetone derivative 1i and its reduced

Molecules 2010, 15
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derivative 1j were transformed into cyclic products 2i and 2j (Table 2, entries 9 and 10). In our hands
the spirocyclic compound 2k bearing a fluorene moiety was successfully obtained from diyne 1k in
48% yield (Table 2, entry 11).
Table 2. Pt (II) catalyze hydrative cyclyzation reaction of 1, 6-heptadiynes a.
yield (%) b
entry
1
1,6-diynes
time (h)
product
O
1
2
MeO2C
3
MeO2C
MeO2C
1a
2a
75
MeO2C
O
EtO2C
EtO2C
2
1b
3
2b
88
EtO2C
EtO2C
O
O
HO
HO
3
4
HO
HO
1c
1d
4.5
6
2c
2d
39
61
MeO
HO
MeO
HO
O
O
MeO
MeO
MeO
MeO
1e
12
2e
65
5
6
O
Ph
P
O
P
Ph
Ph
6
3
3
1f
2f
78
75
69
Ph
EtO2C
EtO2C
O
O
1g
2g
7
8
EtO2C
MeO2C
Ph
1h
1i
2h
2i
Ph
MeO2C
MeO2C
O
O
O
O
O
5
5
62
47
9
MeO2C
MeO2C
OH
MeO2C
OH
MeO2C
1j
2j
10
11c
1k
13
2k
48
a
All reactions were performed with 0.5 mmol of substrate, 0.5 mmol of H2O, 0.25 mmol
b
of CH3SO3H, and 5 mol% of Pt(COD)Cl2 in 2.0 mL of MeOH at 70 ºC; Isolated yields;
c 1 mL of MeOH and 1 mL of CH2Cl2 were used as solvent.
Presumably, the mechanism in this reaction could be similar to that of the PtCl2-catalyzed hydrative
cyclization of trialkyne functionalities [7]. We thus propose a mechanism (Scheme 1) involving an
initial coordination of the diyne to Pt(II) to afford the intermediate A. The addition of H2O takes place

Molecules 2010, 15
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to form the -carbonyl platinum species C. After a second hydration at the remaining alkyne of species
C, the resulting diketone species D undergoes a subsequent aldol condensation to form a product 2.
Alternatively, cyclohexenone 2 could result from an alkyne insertion into intermediate E, followed by
hydrodemetalation of intermediate F. The reason behind the catalytic activity of acid as an additive is
unclear, although acid is proposed to exert a tuning effect on the activity of Pt catalysts.
Scheme 1. Proposed mechanism for The Pt-catalyzed hydrative cyclization of 1, 6-diynes.
O
HO
O
[Pt]
E
E
[Pt]
[Pt]
H2O
H2O
E
E
E
E
E
E
E
E
[Pt]
O
1
A
B
C
D
O
HO
O
E
[Pt]
E
[Pt]
E
E
E
E
E
F
2
3. Experimental
3.1. General
Under otherwise noted, materials were obtained from commercial suppliers and used without
further purification. Diynes were prepared by the procedures in the literature [29,30]. Thin layer
chromatography (TLC) was performed using silica gel 60 F254 and visualized using UV light. Column
1
13
chromatography was performed with silica gel (mesh 300-400). H-NMR and C-NMR spectra were
recorded on a Bruker Avance 400 MHz or 500 MHz spectrometer in CDCl3 with Me4Si as an internal
standard. Data are reported as follows: chemical shifts in ppm (δ), multiplicity (s = singlet, d = doublet,
t = triplet, q = quartet, br = broad and m = multiplet, coupling constant (Hz) and integration. Infrared
spectra (IR) were obtained on a 370 FT-IR spectrometer; absorptions are reported in cm-1. Mass
spectra (MS) and high resolution mass spectra (HRMS) were obtained at the Zhejiang University of
Technology Mass Spectrometry Facility.
3.2. General procedure for the hydrative cyclization of diynes
To a reactor containing diyne (0.5 mmol), methanol (2 mL), and H2O (10 μL) under nitrogen
Pt(COD)Cl2 (9.0 mg, 0.025 mmol, 5 mol%) and CH3SO3H (20 μL) were added. The resulting yellow
solution was then sealed and stirred at 70 ºC for 3-13 hours until the starting diyne was consumed, as
judged by TLC. The mixture was quenched with a saturated solution of NaHCO3 and then extracted
with ethyl acetate (20 mL × 3). The organic layer was washed with brine, dried over Na2SO4 and
concentrated in vacuo. The residue was purified by column chromatography (silica gel) (Eluent:
hexane/ethyl acetate) to yield the corresponding cyclized product in an analytically pure form.

Molecules 2010, 15
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1
Dimethyl 3-methyl-5-oxocyclohex-3-ene-1,1-dicarboxylate (2a) [24]. A pale yellow oil; H-NMR
(400 MHz, CDCl3) δ 5.88 (s, 1H), 3.75 (s, 6H), 2.90 (s, 2H), 2.87 (s, 2H), 2.01 (s, 3H); 13C-NMR (100
MHz, CDCl3) δ 194.5, 170.2, 158.7, 126.2, 55.5, 53.3, 41.7, 36.3, 24.3.
Diethyl 3-methyl-5-oxocyclohex-3-ene-1,1-dicarboxylate (2b) [24]. A colorless oil; 1H-NMR
(400 MHz, CDCl3) δ 5.88 (q, J = 1.2 Hz, 1H), 4.20 (q, J = 7.0 Hz, 4H), 2.89 (s, 2H), 2.86 (s, 2H), 2.01
13
(d, J = 1.2 Hz, 3H), 1.24 (t, J = 7.0 Hz, 6 H); C-NMR (100 MHz, CDCl3) δ 194.8, 169.8, 158.7,
126.2, 62.2, 55.5, 41.7, 36.2, 24.3, 13.9.
1
5,5-Bis(hydroxymethyl)-3-methylcyclohex-2-enone (2c) [25]. White solid, m.p.: 64–65 ºC. H-NMR
13
(500 MHz, CDCl3) δ 5.88 (s, 1H), 3.91 (br, 2H), 3.55 (s, 4H), 2.30 (s, 4H), 1.98 (s, 3H); C-NMR
(125 MHz, CDCl3) δ 199.8, 161.4, 125.6, 66.5, 42.4, 40.8, 35.0, 24.6.
1
5-(Hydroxymethyl)-5-(methoxymethyl)-3-methylcyclohex-2-enone (2d). A pale yellow oil; H-NMR
(400 MHz, CDCl3) δ 5.84-5.83 (m, 1H), 3.53-3.45 (m, 2H), 3.35-3.26 (m, 5H), 2.88 (br, 1H), 2.36 (s,
13
2H), 2.24 (s, 2H) , 1.92 (s, 3H); C-NMR (100 MHz, CDCl3) δ 198.6, 160.2, 125.6, 77.5, 67.2, 59.4,
41.8, 41.2, 34.9, 24.2; IR (KBr) υmax 3445, 2927, 1651, 1382, 1104 cm-1; HRMS (EI) for C10
H16
O3:
calcd. 184.1099. Found 184.1097.
1
5,5-Bis(methoxymethyl)-3-methylcyclohex-2-enone (2e) [24]. A pale yellow oil; H-NMR (400 MHz,
13
CDCl3) δ 5.86 (s, 1H), 3.31 (s, 6H), 3.23 (s, 4H), 2.34 (s, 2H), 2.32 (s, 2H), 1.94 (s, 3H); C-NMR
(100 MHz, CDCl3) δ 198.9, 159.8, 125.4, 75.4, 59.2, 41.6, 41.3, 34.8, 24.3.
Ethyl 1-(diphenylphosphoryl)-3-methyl-5-oxocyclohex-3-enecarboxylate (2f) [24]. White solid,
m.p. 122.3–125.5 ºC. 1H NMR (400 MHz, CDCl3) δ 8.06-8.02 (m, 2H), 7.90-7.85 (m, 2H), 7.69-7.47
(m, 6H), 5.84 (s, 1H), 3.93-3.79 (m, 2H), 3.03-2.83 (m, 4H), 1.92 (s, 3H), 0.91-0.86 (m, 3H); 13C-
NMR (100 MHz, CDCl3) δ 194.4 (d, J c-p = 11.3 Hz), 170.8 (d, J c-p = 20.2 Hz), 159.2 (d, J c-p =
12.4 Hz), 132.2 (q, J c-p = 2.8 Hz), 131.9 (d, J c-p = 8.9 Hz), 131.6 (d, J c-p = 8.9 Hz), 129.0
(d, J c-p = 10.6 Hz), 128.3 (d, J c-p = 2 Hz), 128.2 (d, J c-p = 2 Hz), 128.0 (d, J c-p = 11 Hz), 125.6,
61.7, 53.0 (d, J c-p = 57 Hz), 39.1, 33.9, 24.1, 20.6 (d, J c-p = 4.1 Hz), 13.1.
1
Methyl 3-methyl-5-oxocyclohex-3-enecarboxylate (2g) [24]. A pale yellow oil; H-NMR (400 MHz,
CDCl3) δ 5.91 (s, 1H), 3.72 (s, 3H), 3.10-3.04 (m, 1H), 2.67-2.51 (m, 4H), 2.00 (s, 3H); 13C-NMR
(100 MHz, CDCl3) δ 196.8, 173.5, 160.2, 126.5, 52.1, 39.6, 38.6, 33.0, 24.2.
Methyl 3-methyl-5-oxo-1-phenylcyclohex-3-enecarboxylate (2h) [24]. Colorless crystals;
m.p. 83.0-84.0. 1H-NMR (400 MHz, CDCl3) δ 7.37-7.28 (m, 5H), 5.94-5.93 (m, 1H), 3.64 (s, 3H),
3.29-3.21 (m, 2H), 2.81-2.73 (m, 2H), 2.05 (s, 3H); 13C-NMR (100 MHz, CDCl3) δ 196.6, 174.0,
160.4, 140.0, 128.9, 127.7, 126.5, 125.5, 52.8, 51.8, 45.1, 40.1, 24.6.
1
Methyl 1-acetyl-3-methyl-5-oxocyclohex-3-enecarboxylate (2i) [25]. A pale yellow oil; H NMR
(400 MHz, CDCl3) δ 5.87 (s, 1H), 3.76 (s, 3H), 2.93 (d, J = 16.4 Hz, 1H), 2.84 (d, J = 0.8 Hz, 2H),
2.72 (d, J = 16.4 Hz, 1H), 2.20 (s, 3H), 2.01 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 202.0, 194. 8,
170.9, 158.8, 126.1, 61.6, 53.2, 41.2, 35.4, 25.8, 24.3.

Molecules 2010, 15
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Methyl 1-(1-hydroxyethyl)-3-methyl-5-oxocyclohex-3-enecarboxylate (2j) [25]. A pale yellow oil;
1H NMR (400 MHz, CDCl3) δ 5.85 (s, 1H), 3.87-2.84 (m, 1H), 3.69 (s, 3H), 2.88-2.79 (m, 2H),
2.70-2.66 (br, 1H), 2.62-2.36 (m, 2H), 2.16-1.84 (m, 3H), 1.25-1.17 (m, 3H); 13C-NMR (100 MHz,
CDCl3) δ 197.5, 174.7, 160.4, 126.0, 71.5, 54.0, 52.4, 41.6, 35.5, 24.5, 18.7.
1
5-Fluorene-3-methylcyclohex-2-enone (2k) [25]. A white solid; m.p. 167-168 ºC; H-NMR (500 MHz,
CDCl3) δ 7.74 (d, J = 7.5 Hz, 2H), 7.48 (d, J = 7.5 Hz, 2H), 7.40-7.37 (m, 2H), 7.29-7.26 (m, 2H), 6.27
(d, J = 1 Hz, 1H), 2.68 (s, 4H) , 2.02 (s, 3H); 13C-NMR (125 MHz, CDCl3) δ 198.1, 160.4, 149.9,
139.3, 127.9, 127.6, 127.1, 123.0, 120.1, 51.0, 45.7, 41.2, 24.5.
4. Conclusions
In summary, various 3,5-substituted conjugated cyclohexenones were synthesized by
Pt(II)–catalyzed hydrative cyclization of 1,6-diynes. Advantages of the present method are the easily
accessible starting materials, mild conditions, lack of coproducts and the fact that several types of
functional groups were tolerated. Further studies are underway to expand the scope of the present
method and are directed toward further method development on these cyclohexenone scaffolds as well
as applications in natural product and the bioactive molecule synthesis.
Acknowledgements
This work was partially supported by the Natural Science Foundations of China (Nos. 20672099,
20572094).
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