Rhenium(I)-catalyzed cyclization of silyl enol ethers containing a propargyl carboxylate moiety: Versatile access to highly substituted phenols

Article abstract of DOI:10.1002/chem.200903586

Selective migration: The rhenium-catalyzed reaction of 2-siloxy-1-en-5-ynes containing an acyloxy substituent at the propargylic position gave highly substituted phenols in good yields (see scheme). Depending on the structure of the silyl enol ether, either nucleophilic addition of this moiety or 1,2acyloxy migration occurs to give two kinds of synthetically useful, substituted phenols. (Chemical Equation Presented)

Full text of DOI:10.1002/chem.200903586

DOI: 10.1002/chem.200903586
Rhenium(I)-Catalyzed Cyclization of Silyl Enol Ethers Containing a
Propargyl Carboxylate Moiety: Versatile Access to Highly Substituted
Phenols
Kodai Saito, Yuji Onizawa, Hiroyuki Kusama, and Nobuharu Iwasawa*[a]
The use of propargyl carboxylates as substrates for the
electrophilic activation of alkynes has been studied exten-
sively and a variety of useful transformations have been de-
veloped utilizing intermediates generated by 1,2- or 1,3-acy-
loxy migration of the carboxylates.[1] As part of our ongoing
research into the [W(CO)5(L)]- and [ReCl(CO)4(L)]-cata-
lyzed cyclization of acetylenic silyl enol ethers through elec-
trophilic activation of the alkyne moiety,[2] we have devel-
oped an interest in the reactions of silyl enol ethers contain-
ing a propargyl carboxylate moiety. In this case, two possi-
bilities exist concerning the electrophilically activated
alkyne moiety; these are attack of the silyl enol ether and
1,2- or 1,3-acyloxy migration[3] and further reaction path-
ways of the produced intermediates are another focus of our
interest. In this communication, we report the regioselective
preparation of highly substituted phenols by using 2-siloxy-
1-en-5-ynes with an acyloxy substituent at the propargylic
position.
Scheme 1. The reaction of 1a with [W(CO)6].
When a silyl enol ether containing a propargyl benzoate
moiety 1a was treated with 20 mol% of [W(CO)6] in tolu-
ene in the presence of 4 ꢀ molecular sieves (MS) under
photoirradiation from a high-pressure Hg lamp, at ambient
temperature the starting material was consumed within 3 h
and 2,3-disubstituted phenol 2a was obtained in 46% yield
(Scheme 1). The proposed mechanism for the formation of
2a is shown in Scheme 2. A 6-endo nucleophilic attack of
the silyl enol ether moiety onto the alkyne, which is electro-
philically activated by [W(CO)5], occurs to give the zwitter-
ionic six-membered cyclic intermediate A in which the ben-
zoate moiety remains intact.[4] This is followed by deproto-
nation of the silyloxonium to give cyclohexadiene derivative
Scheme 2. The proposed reaction mechanism.
B, from which elimination of the benzoate occurs by elec-
tron donation from the silyl dienol ether moiety to give the
cationic pentadienyl intermediate C. Finally, migration of
the methyl substituent occurs along with protonation of the
À
C W bond to afford the final product 2a and regenerate the
catalyst. It should be noted that there are very few exam-
ples[5] of the alkyne part of the propargyl carboxylate being
electrophilically activated for attack by nucleophiles without
an accompanying acyloxy migration and the carboxylate
moiety then utilized for further manipulation. As the unique
transformation of silyl enol ether 1a into a synthetically
useful, substituted phenol was found to proceed catalytical-
ly,[6] we decided to optimize the reaction conditions.
[a] K. Saito, Dr. Y. Onizawa, Dr. H. Kusama, Prof. Dr. N. Iwasawa
Department of Chemistry, Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8551 (Japan)
Fax : (+81)3-5734-2931
Examination of the effect of other typical electrophilic
metal complexes, such as PtII, AuI, and ReI, on the reaction
revealed that, although the commonly employed PtII or AuI
E-mail: niwasawa@chem.titech.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903586.
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COMMUNICATION
catalysts did not give good results, the yield was improved
by using [Re(CO)5Br][7,8] under photoirradiation (Table 1,
entries 1–6).[9] As the deprotonation from the silyloxonium
of substrates containing different substituents at the tether
(R2 and R3) revealed that selective migration is mostly ach-
ieved in this reaction. Thus, Me- and Et- or Me- and Bn-sub-
stituted (Bn=benzyl) substrates 1e and 1 f were converted
to the corresponding phenols 2e and 2 f, respectively, in
which the Et or Bn group migrated selectively, in good
yield.[11] Me- and CO2Et-substituted 1g was also converted
to the corresponding phenol with selective migration of the
CO2Et substituent. Although internal alkyne substrates re-
quired a higher temperature (1508C in xylene) and an in-
creased amount of rhenium catalyst (20 mol%), these reac-
tions proceeded successfully with various substituents at the
alkyne terminus, such as methyl (1h), phenyl (1i), and
ethoxycarbonyl (1j), and the corresponding 2,3,5-trisubsti-
tuted phenols were obtained in reasonable yield. Finally, the
substituted silyl enol ether 1k was cleanly converted to the
corresponding 2,3,6-trisubstituted phenol using 10 mol%
[Re(CO)5Br] in toluene at 1108C. Thus, we have succeeded
in developing an efficient method for the synthesis of vari-
ous types of substituted phenols by a [Re(CO)5Br]-catalyzed
reaction of 2-siloxy-1-en-5-ynes that have an acyloxy sub-
stituent at the propargylic position.
To support the proposed mechanism of the reaction, sev-
eral attempts to trap the reactive intermediates were carried
out. When the rhenium-catalyzed reaction was carried out
in the presence of p-bromostyrene, a unique product (3) was
obtained. The photochemical reaction of substrate 1a with
20 mol% rhenium complex in the presence of p-bromostyr-
ene gave cyclopropane-containing polycyclic compound 3[12]
in 70% yield.[13] This product is thought to be produced as
shown in Scheme 3. Following the mechanism described in
Scheme 2, the Re-containing cationic pentadienyl intermedi-
ate D was generated. Then p-bromostyrene nucleophilically
Table 1. Examination of the reaction conditions for phenol synthesis.
Metal catalyst[a]
Additives
Conditions[b]
Yield [%] 2a
1
2
3
[W(CO)6]
PtCl2
AuCl
4 ꢀ MS
4 ꢀ MS
4 ꢀ MS
none
hv, RT, 3 h
1108C, 8 h
RT, 1 d
46
17
trace
3
4
AuCl, AgOTf
RT, 1 d
5
6
7
8
9
10
11
N
none
RT, 1 d
trace
76
84
4 ꢀ MS
NaHCO3
NEt3
none
NaHCO3
NaHCO3
hv, RT, 3 h
hv, RT, 3 h
hv, RT, 3 h
hv, RT, 3 h
hv, RT, 12 h[d]
1108C, 3 h[d]
N.R.[e]
66
83
80
[a] 20 mol% of metal catalysts were used. [b] The substrate concentration
was 0.05m. [c] 10 mol% of metal catalysts were used. [d] The substrate
concentration was 0.1m. [e] N.R.=no reaction.
intermediate A is thought to be a crucial step, the effect of
bases on this reaction was examined. Whereas addition of
Et3N completely suppressed the reaction, the yield of 2a
was improved to 84% by the addition of NaHCO3 as a base
(Table 1, entry 7). Furthermore, this reaction was found to
proceed with only 10 mol% [Re(CO)5Br] in toluene at
1108C without the need for photoirradiation(Table 1,
entry 11).[10] With optimal conditions in hand, the generality
of the reaction was examined.
As summarized in Table 2, the catalytic reaction at 1108C
in toluene proceeded smoothly with cyclic substrates 1b–d
to give the corresponding phenols fused with a ring-expand-
ed, six- or seven-membered ring in good yields. Examination
Table 2. Generality of the Re-catalyzed phenol synthesis using acyclic
substrates.
Starting
material
R1
R2
R3
R4
Yield [%]
1b
1c
1d
H
H
H
H
H
H
H
H
H
Me
-(CH2)4-
-(CH2)5-
-CH2-(1,2-C6H4)-CH2-
H
H
H
H
H
H
Me
Ph
CO2Et
H
90 (2b)
70 (2c)
88 (2d)
81 (2e)[a]
60 (2 f)[c]
74 (2g)
76 (2h)
51 (2i)
1e
Me
Me
Me
Me
Me
Me
Me
Et
Bn
1 f[b]
1g
CO2Et
Me
1h[d]
1i[d]
1j[d]
1k
Me
Me
Me
58 (2j)
78 (2k)
[a] It contained a small amount (ca. 5%) of an unidentified product,
which could be Et migrated product. [b] Photoirradiation conditions by
using high-pressure Hg lamp at room temperature. [c] The yield of desily-
lated product. [d] 20 mol% of [Re(CO)5Br] was used in xylene at 1508C.
Scheme 3. The reaction of 1a with rhenium catalysts in the presence of p-
bromostyrene.
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N. Iwasawa et al.
attacks the cation at the terminal position; this is followed
by attack, in a formal [4+2] cycloaddition, of the silyl enol
ether moiety on the benzylic cation produced, to give the
zwitterionic cycloadduct E. Protonation of the alkenyl rheni-
um moiety at the b-position to the metal generates rheni-
um–carbene complex F,[14] which undergoes insertion into
finally, eliminates the silyl ether to aromatize and afford the
product 5a.[17] Thus, this reaction gives another useful
method for the construction of phenols containing a fused
ring by using this rhenium-catalyzed tandem reaction.
In conclusion, we have developed a rhenium-catalyzed
synthesis of substituted phenols using 2-siloxy-1-en-5-ynes
that have an acyloxy substituent at the propargylic position.
The characteristic features of this reaction are: 1) depending
on the structure of the silyl enol ether moiety, either nucleo-
philic addition of the silyl enol ether moiety, or 1,2-acyloxy
migration occurs, 2) regioselective synthesis of substituted
phenols is achieved by the selective migration of the tether
substituents, and 3) a rhenium(I)–carbonyl complex, which
is rarely used in organic synthesis, gives specifically good re-
sults.
À
the neighboring C H bond to give the final product 3 and
regenerate the catalyst. This result supports the proposed
mechanism of the substituted phenol synthesis (Scheme 2)
and, in addition, suggests that the rhenium metal is retained
in the substrate even after the generation of the cationic
pentadienyl intermediate.
Next we examined the reaction using substrate 4a, which
contains a six-membered cyclic silyl enol ether moiety. In
addition, it was found that treatment of 4a under the opti-
mized conditions gave a different type of substituted phenol
5a, which has a fused five-membered ring, in 83% yield
(Scheme 4). Seven- and eight-membered cyclic silyl enol
ethers also gave the corresponding ring-contracted fused
phenols in good yield. Unfortunately, internal alkynes were
not susceptible to reaction with this type of silyl enol
ether.[15]
Experimental Section
Example procedure for the cyclization reaction: Compound 1a (40.0 mg,
0.1 mmol) was added to a mixture of [Re(CO)5Br] (4.0 mg, 0.01 mmol, 10
mol%) and NaHCO3 (8.4 mg, 0.1 mmol) in degassed toluene (1 mL).
After the mixture was heated at 1108C for 3 h, the solvent was removed
under reduced pressure to give the crude product, which was purified by
preparative thin layer chromatography (5% ethyl acetate in hexane) to
give 22.2 mg of 2a (0.08 mmol, 80%) as a colorless oil.
Acknowledgements
This research was partly supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan. Y.O. was granted a Research Fellowship of the
Japan Society for the Promotion of Science for Young Scientists.
Scheme 4. The reaction of 4a–c with [Re(CO)5Br].
The proposed mechanism for the formation of phenol 5a
is shown in Scheme 5. In this case, activation of the alkyne
part of substrate 4a occurs by rhenium-promoted 1,2-ac-
Keywords: alkynes · cycloaddition · elimination · phenols ·
rhenium
ACHTUNGTRENNUNG
yloxy migration of the propargyl carboxylate unit,[3] which
leads to the formation of rhenium–carbene complex inter-
mediate G. Then, intramolecular nucleophilic attack of the
silyl enol ether on the rhenium–carbene complex occurs,
which results in the formation of zwitterionic intermediate
H. Electron donation from the allyl–rhenium moiety pro-
motes alkyl migration to give the intermediate I,[16] which,
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Scheme 5. The proposed reaction mechanism.
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Silyl Enol Ethers
COMMUNICATION
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[7] Use of [Re(CO)5Br] as a Lewis acid catalyst in the Friedel–Crafts
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Soc. Jpn. 1995, 68, 2379–2383; therefore, in the reaction of 1a with
[Re(CO)5Br], [ReX(CO)4(L)] might have promoted elimination of
the BzO group.
[8] For examples of rhenium-catalyzed reactions, see: a) Y. Kuninobu,
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202–209; b) Y. Kuninobu, A. Kawata, K. Takai, J. Am. Chem. Soc.
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[ReBr(CO)4(L)] requires photoirradiation or heating to high tem-
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[11] In case of Me and Bn substituted substrate, 2,6-disubstituted phenol
was also obtained in 20% yield. 2,3- and 2,6-disubstituted phenols
were separated after desilylation (TBAF, THF, 08C). For similar mi-
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endo cyclization of the silyl enol ether to an electrophilically activat-
ed allene intermediate produced by [W(CO)5(L)]-catalyzed 1,3-acy-
loxy migration. However, it is known that propargyl esters with a
[12] The structure of 3 was determined by X-ray crystallography. CCDC-
752714 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[13] Compound 3 was also obtained without photoirradiation by using a
catalytic amount of [{Re(CO)3ACTHNUTRGNE(NUG thf)Br}2] in toluene at room tempera-
ture for 1 day in 63% yield. Additionally, the protonated product of
the cycloadduct E was obtained.
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was supported by the reac-
À
130, 6288–6297; for examples of cyclopropane formation by C H
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ence of MeOH (10 equiv),
which gave 6 (20% yield in
1H NMR) produced by direct
methanolysis of the inter-
mediate A.
[5] a) Y. Harrak, C. Blaszykowski, M. Bernard, K. Cariou, E. Mainetti,
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[15] If an internal alkyne substrate was employed under these reaction
conditions, a complex mixture of products was obtained.
[16] Phenol 5a could also be formed via cyclopropanation of the silyl
enol ether by the rhenium carbene complex G, followed by alkyl mi-
gration as shown below to give the intermediate I:
[6] For substituted phenol syntheses from alkyne substrates, see:
a) A. S. K. Hashmi, T. M. Frost, J. W. Bats, J. Am. Chem. Soc. 2000,
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´
J. P. Weyrauch, E. Kurpejovic, T. M. Frost, B. Miehlich, W. Frey,
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N. Iwasawa et al.
[17] In this case, direct addition of the silyl enol ether to the electrophili-
cally activated alkyne would give silyloxonium intermediate J, which
cannot undergo further deprotonation. An equilibrium probably
exists between the alkyne-p complex and the intermedate J and 1,2-
acyloxy migration occurs from the p complex as another possible re-
action pathway.
Received: December 31, 2009
Published online: March 18, 2010
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Chem. Eur. J. 2010, 16, 4716 – 4720