Rhenium complex-catalyzed coupling reaction of enol acetates with alcohols

Article abstract of DOI:10.1016/j.tetlet.2014.09.054

The reaction of enol acetates with alcohols in the presence of a catalytic amount of a rhenium complex, such as ReBr(CO)₅, produced the corresponding ketones and aldehydes in moderate to good yields. It was suggested that the preparation of an ether, an intermolecular dehydrated product, was the first step of the reaction.

Full text of DOI:10.1016/j.tetlet.2014.09.054

Accepted Manuscript
Rhenium complex-catalyzed coupling reaction of enol acetates with alcohols
Rui Umeda, Yuuki Takahashi, Yutaka Nishiyama
PII:
S0040-4039(14)01555-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.09.054
TETL 45148
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
2 July 2014
3 September 2014
10 September 2014
Please cite this article as: Umeda, R., Takahashi, Y., Nishiyama, Y., Rhenium complex-catalyzed coupling reaction
of enol acetates with alcohols, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.09.054
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Rhenium complex-catalyzed coupling
reaction of enol acetates with alcohols
Rui Umeda, Yuuki Takahashi, and Yutaka Nishiyama*
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1
Tetrahedron Letters
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Rhenium complex-catalyzed coupling reaction of enol acetates with alcohols
Rui Umeda, Yuuki Takahashi and Yutaka Nishiyama∗
Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The reaction of enol acetates with alcohols in the presence of a catalytic amount of a rhenium
complex, such as ReBr(CO)5, produced the corresponding ketones and aldehydes in moderate to
good yields. It was suggested that the preparation of an ether, an intermolecular dehydrated
product, was the first step of the reaction.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Rhenium
Alcohols
Enol acetates
α-Alkylated carbonyl compounds
The development of the transition metal-catalyzed carbon-
carbon bond formation has significantly contributed to organic
synthesis and many such methods have been reported. Although
organic halides, alcohol derivatives having good leaving groups,
such as tosylates and mesylates, or organometallic compounds
were widely used as coupling partners for the carbon-carbon
bond formation, the direct use of alcohols for the formation is
limited, because of the poor leaving ability of the hydroxy unit
and the decomposition of the catalysts and active intermediates.
alkylated carbonyl compounds by the reaction of enol acetates
and alcohols. On these methods, there some disadvantages (i)
limitation of the substrates, (ii) the use of toxic and expensive
catalyst, (iii) unstable against moisture or air and (iv) the use of
small excess amounts of reagent.
We and some Japanese groups developed ReX(CO)5 (X = Cl
or Br) can be used as a catalyst for organic reaction instead of
various Lewis acid complexes.8,9 On the continuous studies on
the use of rhenium catalyst in organic synthesis, we now find that
the ReBr(CO)5 complex acts as the catalyst for the coupling
reaction of enol acetates and alcohols to afford the ketones and
aldehyde in moderate to good yields (Scheme 1). On the reaction,
the reaction pathway, including the intermolecular dehydration of
alcohol followed by the coupling reaction with enol acetates, was
clearly disclosed.
The α-alkylation of carbonyl compounds is one of the
fundamental transformations in organic synthesis, and various α-
alkylation methods of carbonyl compounds have been
developed.1 Although the reaction of enolates or enamines with
electrophiles was widely used for the alkylation of carbonyl
compounds, the direct use of an alcohol as an electrophile
remains a problem in organic synthesis. Recently, the direct α-
alkylation of carbonyl compounds with alcohols has been
developed.2,3 Furthermore, alternative approaches via the
catalytic transfer dehydrogenation of alcohols has been
reported.4,5 However, this is applicable only for the primary
alcohols and required strong basic conditions.
In order to determine the optimized reaction conditions, we
investigated the effects of the rhenium complex, reaction
temperature and solvent on the reaction of 1-phenylethanol (1a)
with isopropenyl acetate (2a) (Table 1). When 1-phenylethanol
(1a) was allowed to react with isopropenyl acetate (2a) in the
presence of a catalytic amount of ReBr(CO)5 (5 mol%) in a 1,2-
dichloroethane at 80 °C for 5 h, the coupling reaction of 1a and
2a smoothly proceeded to give 4-phenyl-2-pentanone (3a) in
88% yield (Entry 1). No reaction took place in the absence of the
rhenium complex (Entry 2). For the lower reaction temperature
(60 °C), the yield of 2a was slightly decreased (Entry 3). Even
when toluene and THF were used as the solvent, the reaction
smoothly proceeded (Entries 5 and 6). In the case of acetonitrile,
the reaction did not proceed (Entry 7). The use of CH2ClCH2Cl
as the solvent led to the highest yield of 3a (Entry 1). When
ReCl(CO)5 was used instead of ReBr(CO)5 as the catalyst, 1a
R2
OAc
R3
R2
O
cat.ReBr(CO)5
R1
OH
R1
R3
+
R1 = aryl or allyl
Scheme 1.
7
Recently, it was shown that InI3,6 GaBr3,6 La(OTf)3 and
7
Hf(OTf)3 catalyst are a novel catalyst for the preparation of α-
———
∗ Corresponding author. Tel.: +81 6 6368 0902; fax: +81 6 6339 4026; e-mail: nishiya@kansai-u.ac.jp

2
Tetrahedron
was coupled with 2a to give 3a in 53% yield (Entry 8). In the
13). For the reaction of propargylic alcohol, the corresponding
ketone was not formed due to the formation of various
complicated products. In the case of tertiary alkyl alcohol, such
as 5-butylnonan-5-ol, 5-butylnon-4-ene, dehydrated product, was
formed in 51% yield. When (R)-(+)-1-phenylethanol (1a), chiral
alcohol, was treated with 2a, 3a was formed with a racemic
isomers (Scheme 2).
case of other rhenium complexes, such as Re2(CO)10, Re2O7,
ReCl5 and CH3ReO3, no formation of 3a was observed (entries 9-
12).10
Table 1. Reaction of 1-phenylethanol (1a) with isopropenyl
acetate (2a)a
O
cat.Re complex
OAc
cat.ReBr(CO)5 (5 mol%)
OH
Me
OAc
OH
+
rac-3a
+
Solvent
CH2ClCH2Cl (2 mL)
80 °C, 5 h
1a
Entry
2a
3a
Yield (%)b
2a
(R)-1a
41%
Catalyst
Solvent
Temp (ºC)
(0.3 mmol)
(0.3 mmol)
1
ReBr(CO)5
none
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
toluene
80
80
60
100
80
80
80
80
80
80
80
80
88 (72)
Scheme 2.
2
0
Table 2. Reaction of alcohols 1 with isopropenyl acetate (2a)a
3
ReBr(CO)5
ReBr(CO)5
ReBr(CO)5
ReBr(CO)5
ReBr(CO)5
ReCl(CO)5
Re2(CO)10
Re2O7
52
86
58
56
0
R2
O
R2
OAc
cat.ReBr(CO)5
CH2ClCH2Cl
4
R1
R1
OH
+
5
6
THF
1
2a
3
7
CH3CN
Entry
Alcohol
Product
Yield (%)b
8
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
CH2ClCH2Cl
53
0
O
9c
10c
11
12
OH
X
X
0
ReCl5
0
1
2
3
4
5
6
X = 4-CH3 (1b)
X = 4-CH3O (1c)
X = 4-Cl (1d)
X = NO2 (1e)
X = 2-CH3 (1f)
X = 3-CH3 (1g)
R
84 (3b)
80 (3c)
38 (3d)
0c (3e)
57 (3f)
62 (3g)
CH3ReO3
0
aReaction conditions: 1a (0.3 mmol), 2a (0.3 mmol), Re
catalyst (5 mol%), solvent (2 mL), 5 h.
bGC yield. The number in parenthesis shows the isolated
yield.
cRe catalyst (2.5 mol%).
R
O
OH
The rhenium complex catalytic system is a quite efficient and
general synthetic method of α-alkylated ketones by the reaction
of alcohols and enol acetates. These results are shown in Table 2.
The 1-aryl substituted ethanol bearing electron-donating groups
on the aromatic ring, such as a 1-(4-methylphenyl)- and 1-(4-
methoxyphenyl)-1-ethanol, gave the corresponding 4-aryl
substituted 2-pentanones, 3b,c, in 84 and 80% yields (Entries 1
and 2). For the reaction of 1-(4-chlorophenyl)-1-ethanol, the
yield of ketone 3d was low (Entry 3). In the case of the 1-aryl-1-
ethanol substituted electron withdrawing group such as the nitro
group, 3e was not formed and 1-(4-nitrophenyl)ethyl acetate,
which was a transesterfied product, was obtained in 82% yield
(Entry 4). 1-(2-Methylphenyl)- and 1-(3-methylphenyl)ethanols
gave 4-(2-methylphenyl)- and 4-(3-methylphenyl)-2-pentanones,
3f,g, in 57 and 62% yields, respectively (Entries 5 and 6). 4-
Phenyl-2-hexanone, 4,4-diphenyl-2-butanone, and 4,4,4-
triphenyl-2-butanone, 3h-j, were also prepared by the coupling
reaction between isopropenyl acetate (2a) and 1-phenyl-
propanol, benzhydrol, or triphenylmethanol (Entries 7-9). The
preparation of the 4-naphthyl substituted 2-pentanones, 3k,l, was
successfully achieved using the rhenium catalytic system (Entries
10 and 11). Similarly, the coupling of 4-methoxybenzyl alcohol,
a primary alcohol, with isopropenyl acetate (2a) proceeded to
form 4-(4-methoxyphenyl-2-butanone (3m) in 56% yield
together with uncharacterized products (Entry 12). The δ,γ-
unsaturated ketones 3n was obtained by the reaction of allylic
alcohol such as 1-cyclohexene-3-ol with 2a in 45 yields (entry
7
8
9
R = C2H5
R = Ph
Ph
29 (3h)
83 (3i)
78 (3j)
Ph
Ph
O
OH
Ph
(1j)
O
10
11
63 (3k)
63 (3l)
OH
(1k)
(1l)
O
OH
O
12
13
56 (3m)
45 (3n)
OH
H3CO
H3CO
(1m)
O
OH

3
aReaction conditions: 1a (0.3 mmol), enol acetates (0.3
(1n)
mmol), ReBr(CO)5 (5 mol%), CH2ClCH2Cl (2 mL), at 80 ºC for
5 h.
aReaction conditions: alcohol (0.3 mmol), 2a (0.3 mmol),
ReBr(CO)5 (5 mol%), CH2ClCH2Cl (2 mL), at 80 ºC for 5 h.
bIsolated yield.
cSyn / anti = 1.0 / 1.5. dSyn / anti = 1.0 / 1.3.
bIsolated yield.
c1-(4-nitrophenyl)ethyl acetate was formed in 82% yield.
To obtain information about the reaction regarding the
coupling of 1a and 2a, the time-dependence of the products was
followed by GC measurement at the appropriate time intervals
(Fig. 1). The GC analysis of the reaction product showed that
bis(phenylethyl) ether (4a) was formed as the main product at an
early stage of the reaction.11 The yield of 4a then gradually
decreased and the yields of 3a and α-methylbenzyl acetate (5a)
increased. The acetate 5a was gradually converted into 3a. In
fact, when bis(phenylethyl)ether (4a) (0.3 mmol) was treated
with two equivalent amounts of 2a (0.6 mmol) in the presence of
the ReBr(CO)5 catalyst, 4-phenyl-2-pentanone (3a) was formed
in 77% yield (0.47 mmol) (Scheme 3). The conversion of 5a into
the 3a was also achieved by the rhenium catalyst. Based on these
results, it was suggested that both 4a and 5a were intermediates
during the coupling reaction.
Next, 1-phenylethanol (1a) was treated with various types of
enol acetates 2 and these results are shown in Table 3. The
rhenium-catalyzed reaction of the enol acetates 2, which were
derived from the acetophenones, and 1a gave the corresponding
coupling products 3o-s (entries 1-5). It is interested to note that
the yields of the products from the reaction with the electron
withdrawing group substituted enol acetate were higher than
those of the reaction with electron donating group substituted
ones (entries 1-5). 2-(1-Phenylethyl)cyclohexanone (3t) was also
prepared by the reaction of 1a and cyclohexenyl acetate 2g (entry
6). Similarly, the enol acetate 2h, which was prepared from an
aldehyde, gave the corresponding α-alkylated aldehyde 3u (entry
7). The treatment of the mixture of E- and Z-isomers 2i (E/Z =
1.5/1.0) and 1a gave a mixture of regioisomers 3v (E/Z =
1.5/1.0). It is interesting to note that when the mixture of E- and
Z-isomers 2i (E/Z = 1.0/7.2) was used as the enol acetate, the
distribution of products 3v (E/Z = 1.3/1.0) was almost same
(entries 8 and 9).
Table 3. Reaction of 1-phenylethanol (1a) with enol acetates
2a
cat.ReBr(CO)5
2
+
3
OH
CH2ClCH2Cl
1a
Enol acetates
Entry
Product
Yield (%)b
OAc
O
Figure 1. Time-dependence curves for the reaction of 1a (0.3 mmol) with
2a (0.3 mmol) in CH2ClCH2Cl (2 mL) at 80 ºC for 6 h
X
X
cat.ReBr(CO)5 (5 mol%)
1
2
3
4
5
6
X = H (2b)
X = 4-CH3 (2c)
57 (3o)
48 (3p)
9 (3q)
3a
2a
O
+
CH2ClCH2Cl (2 mL)
80 °C, 5 h
4a
X = 4-CH3O (2d)
X = 4-Cl (2e)
X = 4-NO2 (2f)
OAc
77%
(0.3 mmol)
(0.6 mmol)
81 (3r)
65 (3s)
58 (3t)
O
O
cat.ReBr(CO)5 (5 mol%)
O
3a
2a
+
CH2ClCH2Cl (2 mL)
5a
80 °C, 5 h
85%
(0.3 mmol)
(0.3 mmol)
(2g)
Scheme 3.
7
O
32 (3u)
OAc
H
(2h)
Based on the above observations, the plausible reaction
pathways for the rhenium-catalyzed reaction are shown in
Scheme 4. First, the decarbonylation of ReBr(CO)5 to form
ReBr(CO)4, which is the coordinatively unsaturated 16-electron
complex, is the first step of the catalytic reaction.12 The C-O
bond of bis(phenylethyl) ether (4a), which is in situ formed by
the intermolecular dehydration of alcohol 1a, is activated by the
coordination of the rhenium species to increase the positive
charge on the α-methylbenzyl group.13 The addition of the
benzilic cation generated from I to the C=C double bond of enol
acetate 2a followed by the deacetoxylation of II gave the
n-C10H21
O
OAc
H
(2i)
n-C10H21
8
9
E / Z = 1.5 / 1.0
E / Z = 1.0 / 7.2
43c (3v)
23d (3v)

4
Tetrahedron
R.; Tanigawa, N.; Tanaka, S.; Tamaru, Y. Tetrahedron 2003, 59,
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1555; (d) Guillena, G.; Ramón, D. J.; Yus, M. Angew. Chem. Int.
Ed. 2007, 46, 2358.
corresponding ketone 3a and α-methylbenzyl acetate (5a). The
reaction of 5a with 2a was also easily occurred in the presence of
the rhenium catalyst to afford 3a as shown Scheme 4.
O
O
ReBr(CO)5
- CO/∆
ReBr(CO)4
O
or
5.
(a) Buil, M. L.; Esteruelas, M. A.; Herrero, J.; Izquierdo, S.;
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(c) Liao, S.; Yu, K.; Li, Q.; Tian, H.; Zhang, Z.; Yu, X.; Xu, Q.
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Tetrahedron Lett. 2005, 46, 3683; (n) Kwon, M. S.: Kim, N.; Seo,
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O
O
+
Ph
O
O
5a
Ph
Ph OH
1a
3a
OReBr(CO)4
or
H2O
OReBr(CO)4
Ph
O
or
O
Ph
O
O
Ph
Ph
Ph
O
ReBr(CO)4
I'
(from 5a)
ReBr(CO)4
I
II
Ph
O
OReBr(CO)4
or
Ph
O
O
2a
OReBr(CO)4
Scheme 4. A plausible reaction pathway
6. Nishimoto, Y.; Onishi, Y.; Yasuda, M.; Baba, Y. Angew. Chem.
Int. Ed. 2009, 48, 9131.
7. Noji, M.; Ohno, T.; Fuji, K.; Futaba, N.; Tajima, H. Ishii, K. J.
Org. Chem. 2003, 68, 9340.
8. The use of rhenium complexes in organic synthesis has shown a
tremendous potential in the past few decades. For recent reviews,
see: (a) Kuninobu, Y. Synth. Org. Chem. Jpn. 2013, 71, 425; (b)
Kuninobu, Y.; Takai, K. Bull. Chem. Soc. Jpn. 2012, 85, 656; (c)
Kuninobu, Y.; Takai, K. Chem. Rev. 2011, 111, 1938; (d) Hua, R.;
Jiang, J.-L. Curr. Org. Synth. 2007, 4, 151; (e) Kusama, H.;
Narasaka, K. Synth. Org. Chem. Jpn. 1996, 54, 644.
We exhibited that the rhenium complex-catalyzed reaction of
enol acetates with alcohols gave the α-alkyl substituted carbonyl
compounds. The application of the reaction and determining the
reaction pathway are now in progress.
Acknowledgments
This work was financially supported by the MEXT-Supported
Program for the Strategic Research Foundation at Private
Universities, Grant-in-Aid for Scientific Research, and the
Kansai University Research Grants: Grant-in-Aid for
Encouragement of Scientists.
9. For recent examples, see: (a) Umeda, R.; Tabata, H.; Tobe, Y.;
Nishiyama, Y. Chem. Lett. 2014, 43, 883; (b) Umeda, R.; Nishi,
S.; Kojima, A.; Kaiba, K.; Nishiyama, Y. Tetrahedron Lett. 2013,
54, 179; (c) Nishino, Y.; Tatsuzaki, T.; Tsubakihara, A.;
Kuninobu, Y.; Takai, K.; Synlett 2011, 2585; (d) Umeda, R.;
Kaiba, K.; Morishita, S.; Nishiyama, Y. ChemCatChem 2011, 3,
1743; (e) Umeda, R.; Nishimura, T.; Kaiba, K.; Tanaka, T.;
Takahashi, Y.; Nishiyama, Y. Tetrahedron 2011, 67, 7217; (f)
Umeda, R.; Kaiba, K.; Tanaka, T.; Takahashi, Y.; Nishimura, T.;
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the etherification of alcohols, see: (a) Luzung, M. R.; Toste, F. D.
J. Am. Chem. Soc. 2003, 125, 15760; (b) Liu, Y.; Hua, R.; Sun,
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13. We have shown that C-O bond of ether was effectively cleavage
by rhenium complex. See ref 9e and 9f.

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