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Chemistry Letters Vol.37, No.8 (2008)
Rhenium-catalyzed Coupling of 2-Propynyl Alcohols
and Several Nucleophiles via Dehydration
Yoichiro Kuninobu,Ã Hirokazu Ueda, and Kazuhiko TakaiÃ
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology,
Okayama University, Tsushima, Okayama 700-8530
(Received May 14, 2008; CL-080497; E-mail: kuninobu@cc.okayama-u.ac.jp, ktakai@cc.okayama-u.ac.jp)
Treatment of 2-propynyl alcohols with several nucleophiles
Table 1. Investigation of several catalystsa
in the presence of a catalytic amount of a rhenium complex,
[ReBr(CO)3(thf)]2, gave coupling products via dehydration. In
these reactions, C–C, C–O, and C–S bonds can be constructed
under mild conditions.
O
O
OH
O
O
catalyst (5.0 mol %)
+
Ph
CH2Cl2, 0 °C, 1 h
Ph
3aa
Ph
1a
2a
Ph
Entry
Catalyst
Yield/%b Entry
Catalyst
Yield/%b
c
1
2
3
4
5
6
Re2(CO)10
ReCl(CO)5
ReBr(CO)5
trace
0
7
8
ReOCl3(PPh3)2
c
0
31
Cross-coupling reactions between organometallic reagents
and organohalides or triflates are powerful and effective methods
to synthesize more complex compounds efficiently.1 However,
these reactions provide metal halides or triflates as side products.
One strategy that eliminates such side products is cross-coupling
reaction via dehydration, which is an atom economical and
environmentally friendly approach. Therefore, there have re-
cently been many reports on carbon–carbon bond formation
via dehydration. Some examples of these include reactions
between 2-propynyl alcohols and allylsilanes,2 alkynylsilanes,2d
active methylene compounds,3 ketones,4 and aromatic com-
pounds.2b,2c,5 Carbon–heteroatom bond–formation reactions
using 2-propynyl alcohols have also been reported.6,7 We report
herein dehydrative coupling reactions between 2-propynyl
alcohols and several carbon and heteroatom nucleophiles.
Treatment of 2-propynyl alcohol 1a with 2,4-pentanedione
(2a) in the presence of a catalytic amount of a rhenium complex,
Re2(CO)10, in dichloromethane at 0 ꢀC for 1 h, gave a trace
amount of the coupling product 3aa (Table 1, Entry 1). The
use of rhenium(I) complexes, ReCl(CO)5 and ReBr(CO)5, did
not provide 3aa (Table 1, Entries 2 and 3). The optimal catalyst
was the rhenium complex [ReBr(CO)3(thf)]2, which provided
the desired product 3aa in 93% yield (Table 1, Entry 4).8,9
Rhenium chlorides, ReCl3 and ReCl5, also afforded 3aa in good
yields (Table 1, Entries 5 and 6). However, high-valent rhenium
complexes, ReOCl3(PPh3)2 and Re2O7, did not work well
(Table 1, Entries 7 and 8). Although the manganese complex,
Mn2(CO)10, did not exhibit any catalytic activity (Table 1,
Entry 9), FeCl3, InCl3, and BiCl3 promoted the coupling
reaction in moderate yields (Table 1, Entries 10–12).
A 2-propynyl alcohol having an alkyl group at the terminal
position of the acetylene moiety, 1b, also afforded the coupling
product 3ba, quantitatively (Table 2, Entry 1). In contrast, when
using a secondary 2-propynyl alcohol bearing a terminal acety-
lene moiety 1c, and a tertiary 2-propynyl alcohol 1d, the yields
of the adducts, 3ca and 3da, were low (Table 2, Entries 2 and 3).
2-Propynyl alcohols 1c and 1d were consumed in both cases;
however, in Entry 3, a conjugate enyne, which would be formed
by dehydration of 1d, was not observed. 1-Phenyl-1-pentyn-3-ol
and 3-phenyl-2-propyn-1-ol did not produce a coupling product.
By using the rhenium catalyst, [ReBr(CO)3(thf)]2, several
substrates could be employed as nucleophiles under mild condi-
tions (Table 3). 1,3-Diketones 2b–2e worked well as nucleo-
Re2O7
Mn2(CO)10
c
0
9
trace
78
c
[ReBr(CO)3(thf)]2
ReCl3
ReCl5
93
82
72
10
11
12
FeCl3
InCl3
BiCl3
54
64
a1a (1.0 equiv), 2a (1.0 equiv). b1H NMR yield. c2.5 mol %.
Table 2. Reactions between several 2-propynyl alcohols 1 and
2,4-pentanedione (2a)a
O
O
OH
O
O
[ReBr(CO)3(thf)]2 (2.5 mol %)
solvent, 1 h
R3
+
R2
R1
R3
1
2a
R2
R1
3
Entry
R1
R2 R3
Solvent Temp/ꢀC
Yield/%b
1c
2e
3e
nC6H13 Ph
H
H
1b CH2Cl2
1c CH3NO2
40
80
0
3bad 96 (>99)
3ca 14 (14)
3da 10 (10)
H
Ph
Ph
Me Me 1d CH3NO2
a1 (1.0 equiv), 2a (1.0 equiv). bIsolated yield. Yield determined by 1H NMR
is reported in parentheses. c2a (3.0 equiv). dca. 3:1 mixture of keto and enol
forms. e2a (2.0 equiv).
philes, and the coupling products 3ab–3ae were obtained in
excellent to quantitative yields (Entries 1–4).10,11 ꢀ-Keto ester
2f gave the coupling product 3af in 84% yield (Entry 5).11
Phenol (2g), furan (2h), and indole (2i) substrates also acted as
carbon nucleophiles, and afforded the corresponding coupling
products 3ag–3ai (Entries 6–8). C–O and C–S bonds were
formed using ethanol (2j), allyl alcohol (2k), thiophenol (2l),
and 1-undecanethiol (2m), 2-propynyl ethers, 3aj and 3ak, and
2-propynyl thioethers, 3al and 3am, were obtained in 62%–
85% yields (Entries 9–12).
Although there has been a report on transition-metal-cata-
lyzed reactions between 2-propynyl acetates and silyl enol ethers
(Nicholas-type reaction),12,13 efficient reactions using 2-propyn-
yl alcohols have not been reported to date. By using a rhenium
complex, [ReBr(CO)3(thf)]2, as a catalyst, the reactions of
2-propynyl alcohols, 1e or 1f, with silyl enol ether, 2n, proceed-
ed, and ꢁ-alkynylketones 3en and 3fn were obtained in 96% and
72% yields, respectively (eq 1).14
Based on our previous report,2d the reaction mechanism is
proposed as follows: (1) formation of a 2-propynyl cation via
dehydroxylation; (2) nucleophilic attack of a nucleophile to
the propargyl cation; and (3) deprotonation.
Copyright Ó 2008 The Chemical Society of Japan
Kuninobu, Yoichiro
