Convergent synthesis of silylated allylic alcohols by a stereoselective domino, sequential radical-coupling reaction

Article abstract of DOI:10.1002/1521-3773(20020415)41:8<1407::AID-ANIE1407>3.0.CO;2-Z

The domino, sequential coupling reaction provides a convergent and stereoselective synthetic route for allylic alcohols from carbonyl compounds, alkynes, and alkenes (see scheme). The reaction involves sequential radical coupling reactions initiated by th

Full text of DOI:10.1002/1521-3773(20020415)41:8<1407::AID-ANIE1407>3.0.CO;2-Z

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Convergent Synthesis of Silylated Allylic
Alcohols by a Stereoselective Domino,
Sequential Radical-Coupling Reaction**
Shigeru Yamago,* Masaki Miyoshi, Hiroshi Miyazoe,
and Junichi Yoshida*
The construction of multiple carbon carbon bonds by
tandem reactions represents an efficient approach to the
synthesis of complex molecular structures from simple
organic building blocks.[1] Although radical-mediated intra-
molecular tandem cyclization exemplifies such an approach,[2]
extensions to the intermolecular reaction (Scheme 1; I× III×)
have been severelylimited, and require careful choice of
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Scheme 1. A X: Radical precursor; B, C: radical acceptors.
coupling partners.[3] This limitation could be attributed to the
difficultyin the selective reaction of the transient radicals, for
example, I×, II×, and III×, with certain coupling partners in
radical chain reactions. The problem might be solved if we
could prepare the radical intermediates I×, II×, and III× as their
radical precursors I, II, and III, and subsequentlycouple them
with radical acceptors in an atom- or group-transfer manner
(Scheme 1).[4] The reaction would then be an iterative atom-
or group-transfer radical reaction.[5] However, there have
been no reports of such transformations, with the exception of
living radical polymerization, in which the same alkenes react
[6]
consecutively.
We have reported the group-transfer coupling of tri-
methylsilyl phenyl telluride (1), carbonyl compounds, and
isonitriles, which involves the selective carbon carbon bond
formation of the a-siloxyradical 2 with isonitriles.[7] It should
be noted that the reaction of silyl tellurides with carbonyl
compounds in the absence of isonitrile afforded the a-siloxy
telluride 3. This unique feature of the reaction prompted us to
investigate alkynes as a third coupling partner (Scheme 2).
Here we report a novel group-transfer coupling of 1, carbonyl
compounds, and alkynes to give the silylated allylic alcohol 5.
As the product also possesses a reactive carbon tellurium
bond, radical-mediated transformation via the vinyl radical 4
[20] G. Stork, K. Zhao, Tetrahedron Lett. 1989, 30, 2173 2174.
[21] M. Yamaguchi, J. Inanaga, K. Hirata, H. Saeki, T. Katsuki, Bull.
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[22] K. Horita, T. Yoshioka, T. Tanaka, Y. Oikawa, O. Yonemitsu,
Tetrahedron 1986, 42, 3021 3028.
[*] Prof. S. Yamago, Prof. J. Yoshida, M. Miyoshi, Dr. H. Miyazoe
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering
Kyoto University, Kyoto 606-8501 (Japan)
Fax : (‡81)75-753-5911
[23] R. W. Murray, R. Jeyaraman, J. Org. Chem. 1985, 50, 2847 2853.
E-mail: yamago@sbchem.kyoto-u.ac.jp
yoshida@sbchem.kyoto-u.ac.jp
[**] This work was partlysupported bya Grant-in-Aid for Scientific
Research from the Ministryof Education, Science, Sports, and
Culture, Japan. We thank Dr. K. Itami for the X-rayanalysis.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2002, 41, No. 8
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Table 1. Synthesis of silylated allylic alcohols 5.[a]
Me3SiO
R3
O
R5
Me3Si
R1
R3
R1
R2
+
+
O
R3
R5
R4
O
R1
R2
R3
Me3SiTePh
+
+
R4
6
100 o
no solvent
C
R1
R2
1
TePh
R2
Me3SiTePh (1)
[- TePh]
Bu3SnH
5
EntryKetone
Alnkey
(R3)
Time Yield
[h]
(
E:Z)[b]
R3
R5
Me3SiO
(R1, R2)
[%]
Me3SiO
Me3SiO
R3
3 R4
R
R1
R1
R2
2
R1
R2
R5
1
Ph, Ph
Ph, Ph
Ph, Ph
Ph, p-MeO-C6H4
Ph, p-Me2N-C6H4 Ph
Ph, Ph
Ph, Ph
Ph, iPr
-(CH2)3-
Ph, H
Ph
12
93
78
97
96:4
93:7
93:7
R2
R4
2[c]
3[c]
4[c]
5[c]
6[c]
7[c]
8
p-EtO2C-C6H4 12
p-Br-C6H4
Ph
4
[- Bu3SnTePh]
[+ Bu3Sn ]
12
12
12
12
[+ TePh]
Me3SiO
[+ TePh]
[- TePh]
77 (93)[d] 95:5
R3
Me3SiO
61
82
90:10
80:20
3-pyridyl
1-cyclohexenyl 12
Ph
Ph
R1
R2
3
R1
R2
TePh
TePh
64 (87)[d] 92:8
5
36
40
4
51
66
93:7
87:13
9
10
Scheme 2.
Ph
74 (82)[d] 74:26
11
12[c,e]
c-C6H11, H
Ph, Ph
Ph
CO2Et
36
2
82
69
67:33
85:15
would effect further carbon carbon bond formation, thus
providing a convergent and stereoselective synthetic route to
allylic alcohols with tri-substituted carbon carbon double
bonds.[8]
[a] A mixture of 1 (1.3 equiv), carbonyl compound (1.0 equiv) and alkyne
(2.0 equiv) was heated at 1008C in a sealed tube. Full experimental details and
characterization data can be found in the Supporting Information. [b] The
ratio was determined by 1H NMR of the crude reaction mixture. [c] The
reaction was carried out in propionitrile or CD3CN (about 0.6m concentration
of starting carbonyl compound). [d] Yield based on the converted carbonyl
compounds. [e] The reaction was carried out in a stepwise manner.
The sequential reaction to form 5 (method A) was initially
examined byusing 1, benzophenone, and phenyl acetylene as
substrates. Compound 1 and benzophenone were treated at
room temperature in MeCN to give 3a (R1, R2 ˆ Ph), which
was treated with phenyl acetylene at 1008C to give 5a (R1, R2,
R3 ˆ Ph) in 85% yield.[9] Alternatively, the reaction was
accomplished in one step byheating a mixture of 1 (1.2 equiv-
alents), benzophenone (1.0 equivalents), and phenyl acet-
ylene (1.2 equivalents) without solvent at 1008C for 12 h
(method B) to give 5a in 93% yield after recrystallization.
Owing to its simplicity, we routinely used method B. The
reaction does not require the use of a solvent, although
various solvents, such as EtCN, (CH2Cl)2, and supercritical
CO2, could be also used.
reaction gave the desired coupling products in good to
excellent yields (entries 1 3, 6 and 7; where R1 ˆ R2 ˆ Ph).
The reaction with electron-deficient alkynes such as acetylene
carboxylic esters, however, did not give the desired coupling
products because of the preferential reaction of 1 with the
alkynes. Method A was effective in such cases, and the
coupling of 3a with ethyl propiolate at 1008C for 2 h afforded
the desired coupling product 5b (R1, R2 ˆ Ph, R3 ˆ CO2Et) in
good yield (entry 12).
The high E stereoselectivityof the olefinic moietyof the
products is remarkable, when it is considered that these
reactions took place at 1008C. Because the isolated (E)-5b
and (Z)-5b did not show anysigns of stereochemical isomer-
ization even under the harsher reaction conditions of 100
1308C for 13 h, the stereochemistryis controlled kinetically,
although we have alreadyreported that structurallyrelated
vinyl tellurides undergo stereochemical isomerization under
photo-irradiation.[8e] The level of selectivityis more than 9:1
in the reaction with most of the ketones used, while in the
reaction with aldehydes and sterically less hindered ketones,
such as cyclobutanone, the level is slightly lower. The
observed decrease in selectivityis consistent with the reduced
steric bulk of the siloxyalkyl moiety in 3 in the group-transfer
step.
Because the coupling product 5 is a radical precursor of the
vinyl radical 4, we examined the radical generation from 5 and
subsequent C C coupling reactions next. However, while
alkyl tellurides are superior precursors for carbon-centered
radicals,[11] there are no reports on vinyl-radical generation
from vinyl tellurides. We found that the tributyltin radical
selectivelycleaved the vinylic carbon tellurium bond in 5 to
generate the vinyl radical 4, which could be used for further
carbon carbon bond formations (Scheme 3).[12] Thus, treat-
ment of (E)-5a with tributyltin hydride[11a] in the presence of
The reaction showed high E selectivity(96%), which was
[10]
confirmed byX-raystructural analysis.
The X-rayanalysis
also confirmed the regioselectivity; the addition of the a-
siloxyradical 2a, which is generated from 1 and benzophe-
none, took place at the less hindered side of the alkyne. The
subsequent group-transfer reaction of the resulting vinyl
radical 4 gave rise to the formation of 5 (Scheme 2). As the
stereochemistryis controlled kinetically(see below), the
group-transfer takes place from the less hindered side of 4, to
avoid steric interaction with the bulky siloxyalkyl moiety.
The present reaction is applicable to a varietyof carbonyl
compounds and alkynes, and its scope and efficiency are
1
summarized in Table 1. Since a varietyof R , R2, and R3
groups can be used, the present reaction would be useful for
the diversity-oriented synthesis of allylic alcohol derivatives.
Aromatic ketones and aldehydes (entries 1 8 and 10) are
especiallygood substrates. The reaction with electron-rich
alkynes was completed within 4 12h at 1008C in most cases
and gave the desired coupling product in good to excellent
yields. Aliphatic ketones and aldehydes were less reactive and
required longer reaction times, but the reaction eventually
afforded the desired coupling products in good yields
(entries 9 and 11). Aryl- and heteroaryl-substituted alkynes
and conjugated enynes were excellent acceptors, and the
1408
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COMMUNICATIONS
Angew. Chem. Int. Ed. 1999, 38, 2027; d) T. Toru, Y. Watanabe, M.
Tsusaka, R. Gautam, K. Tazawa, M. Bakouetila, T. Yonega, Y. Ueno,
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Me3SiO
Ph
Me3SiO
Ph
a)
b)
Ph
Ph
Ph
Ph
TePh
H
(E)-5a
7
[4] H. Fischer, Chem. Rev. 2001, 101, 3581; A. Studer, Chem. Eur. J. 2001,
7, 1159.
92% (94% Z)
c)
Me3SiO
Ph
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Matyjaszewski, J. Xia, Chem. Rev. 2001, 101, 2921; c) A. Gridnev,
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Me3SiO
Ph
CO2Et
Ph
Ph
CO2Me
CO2Me
Ph
Ph
9
8
78% (96% Z)
58% (97% Z)
Scheme 3. a) Bu3SnH
(1.2 equiv),
azobisisobutyronitrile
(AIBN)
(0.1 equiv), 808C, 1 h (no solvent); b) dimethyl fumarate (5 equiv), Bu3SnH
(2.4 equiv), AIBN (0.1 equiv), benzene, 808C, 1 h (syringe pump addition
of Bu3SnH solution over 1 h); c) ethyl 2-tributylstannylmethylacrylate
(3 equiv), AIBN (0.1 equiv), 808C, 3 h (no solvent).
[7] a) H. Miyazoe, S. Yamago, J. Yoshida, Angew. Chem. 2000, 112, 3815;
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Iida, J. Yoshida, Org. Lett. 2000, 2, 3671.
AIBN gave 7 in 92% yield. Because the stereochemical
isomerization of vinyl radicals takes place very rapidly,[13] the
observed high Z stereoselectivitycan be also ascribed to the
selective hydrogen atom abstraction from 4, which is stereo-
selective because of the bulky siloxyalkyl group. The vinyl
radical 4a also reacted with electron-deficient alkenes.[11]
Thus, treatment of (E)-5a with dimethyl fumarate in the
presence of tributyltin hydride afforded the corresponding
coupling product 8 (R3, R4 ˆ CO2Me; see Scheme 3) in good
yield. The coupling of (E)-5a with ethyl 2-tributylstannyl-
methylacrylate also resulted in the desired coupling product 9
in good yield. In both cases, the olefinic stereochemistry of 5a
was highlypreserved in the products.
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methyl-1-piperidinoxyl; radical chain inhibitor) had a marginal effect
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chain pathwayis negligible.
[10] CCDC 176516 contains the supplementarycrystallographic data for
this paper. These data can be obtained free of charge via www.ccdc.ca-
m.ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: (‡44)1223-336-033; or deposit@ccdc.cam.ac.uk).
In summary, we have developed a highly convergent and
stereocontrolled synthesis of silylated allyl alcohols by
domino, sequential radical-coupling reactions. Because the
coupling reaction can be carried out with a varietyof
combinations of carbonyl compounds, alkynes, and agents to
trap the vinyl radicals, this method provides a novel and
powerful strategy for the diversity-oriented synthesis of allylic
alcohols. Moreover, since the reaction does not require the
use of a solvent, it also has the advantage of being environ-
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Angew. Chem. Int. Ed. 2002, 41, No. 8
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