Synthesis of (arylalkenyl)silanes by palladium-catalyzed regiospecific and stereoselective allyl transfer from silyl-substituted homoallyl alcohols to aryl halides

Article abstract of DOI:10.1021/ja0755111

The reactions of silyl-substituted homoallyl alcohols with aryl halides under palladium catalysis result in regiospecific and stereoselective allyl transfer from the alcohols to aryl halides, offering modular synthesis of (E)-3-aryl-1-alkenyl-, (E)-1-aryl-2-alkenyl-, and optically active (E)-1-aryl-2-butenylsilanes. Copyright

Full text of DOI:10.1021/ja0755111

Published on Web 09/29/2007
Synthesis of (Arylalkenyl)silanes by Palladium-Catalyzed Regiospecific and
Stereoselective Allyl Transfer from Silyl-Substituted Homoallyl Alcohols to
Aryl Halides
Sayuri Hayashi, Koji Hirano, Hideki Yorimitsu,* and Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto-daigaku Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
Received July 24, 2007; E-mail: yori@orgrxn.mbox.media.kyoto-u.ac.jp; oshima@orgrxn.mbox.media.kyoto-u.ac.jp
Scheme 1
We have reported palladium-catalyzed allyl transfer from ho-
moallyl alcohols to aryl halides through carbon-carbon bond
cleavage.1,2 The allyl transfer proceeds in a regio- and stereospecific
manner, reflecting the structure of homoallyl alcohols used. Here
we report regiospecific and stereoselective allyl transfer reactions
for the synthesis of aryl-substituted (E)-1- or 2-alkenylsilanes from
silyl-substituted homoallyl alcohols. Alkenylsilanes are indispen-
sable reagents in modern organic synthesis.3 Developing new
methods for highly selective synthesis of vinyl- and allylsilanes,
including optically active ones, is thus still quite important.4
Treatment of 1-bromonaphthalene (2a) with 1a, containing an
allylic silane moiety, in the presence of K2CO3 under Pd(OAc)2/
P(cHex)3 catalysis provided vinylsilane 3a′ in good yield with
moderate E selectivity (Scheme 1). The reaction with 1b having a
t
bulkier BuMe2Si group proceeded to yield 3a with excellent
selectivity of E/Z ) 95:5. The improvement of the stereoselectivity
Table 1. Synthesis of (E)-3-Aryl-1-propenylsilanes 3a
would originate from the stronger preference of the tBuMe2Si group
being at the pseudoequatorial position in the transition state of the
retroallylation. It is worth noting that only one silyl group at the
allylic position can be a decisive factor in determining the
stereoselectivity, whereas tedious preparation of diastereomerically
pure and differently 1,1,2-trisubstituted homoallyl alcohols was
essential to attain high E selectivity in the previous report.1
The scope of aryl halides is wide enough to afford a variety of
(E)-3-aryl-1-propenylsilanes in excellent yields (Table 1).5,6 Steri-
cally demanding (entry 1), electron-deficient (entries 2-5), and
electron-rich (entry 6) aryl bromides participated in the reaction.
The use of P(cHex)3 as a ligand allowed us to use aryl chlorides as
substrates (entries 7 and 8).
entry
Ar
−
X
2
3
yield (%)
E/Z
1
2
3
4
5
6
7
8
2,6-Me2C6H3Br
4-CF3C6H4Br
2b
3b
3c
3d
3e
3f
3g
3f
3g
92
75
89
87
97
92
89
92
93:7
96:4
95:5
96:4
95:5
94:6
97:3
95:5
2c
4-CH3COC6H4Br
4-HCOC6H4Br
4-EtOCOC6H4Br
4-CH3OC6H4Br
4-EtOCOC6H4Cl
4-CH3OC6H4Cl
2d
2e
2f
2g
2f-Cl
2g-Cl
We then focused on homoallyl alcohol 4a, containing a (Z)-1-
alkenylsilane moiety. The reactions of 4a with aryl bromides in
the presence of Cs2CO3 under Pd(OAc)2/PAr3 catalysis provided
1-aryl-2-propenylsilanes in high yields (Table 2,5 entries 1-6). P(c-
Hex)Ph2 was exceptionally essential to attain high yield when
electron-rich aryl bromide 2g was used (entry 7).
a The reaction conditions are the same as shown in Scheme 1.
can be explained in a fashion similar to that in Scheme 1 (Scheme
2).
The reactions of optically active (S)-4d (96% ee) with 2-sub-
stituted aryl bromides resulted in excellent chirality transfer to (E)-
1-aryl-2-butenylsilanes 5 (Table 35). The enantiomeric excesses of
the products were indirectly determined after converting allylsilanes
5 to the corresponding 1-aryl-1-butanols 6. The conversion consisted
of hydrogenation with hydrazine, acid-mediated conversion of the
phenyl group on silicon to a trifluoroacetoxy group, and Tamao-
Fleming oxidation with retention of configuration of the chiral
carbon.7
The excellent chirality transfer is rationalized as follows (Scheme
3). Comparing 7a and 7b, two possible chairlike transition states
of the retroallylation step, 7a would be the more preferable because
the methyl group at the allylic position occupies the pseudoequa-
torial position. The palladium center would approach the Re face
of the alkene moiety, which leads to the formation of 8a having
Interestingly, silylated homoallyl alcohols 4b-d having one
methyl group at the allylic position were converted to (E)-1-aryl-
2-butenylsilanes stereoselectively (entries 8-16). Fortunately the
allyl transfer reaction to 1-bromonaphthalene 2a always provided
the E isomers exclusively (entries 8, 10, and 15). The exclusive
formation of the E isomers would result from the steric factor of
the 1-naphthyl group on palladium in the transition state of the
t
retroallylation. The Me3Si, BuMe2Si, and Me2PhSi groups were
compatible under the reaction conditions. The bulkiness of the silyl
groups had little influence on stereoselectivity (entries 9 vs 11 and
n
13 vs 16). On the other hand, when the larger substituent, Bu,
was introduced at the allylic position, the E selectivity of the
reaction was excellent (entry 13 vs 17). The E selective formation
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J. AM. CHEM. SOC. 2007, 129, 12650-12651
10.1021/ja0755111 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Synthesis of 1-Aryl-2-alkenylsilanes
Scheme 3
entry
2
Si
R
4
conditionsa
5
yield (%)b
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2a
2d
2a
2d
2e
2f
tBuMe2Si
H
4a
4a
4a
4a
4a
4a
4a
4b
4b
4c
4c
4c
4c
4c
4d
4d
4e
Ac
A
5aa
5ba
5ca
5da
5ea
5fa
5ga
5ab
5db
5ac
5dc
5ec
5fc
88
66
81
61
68
88
74
A
Ac,d
A
Ad
Ae
B
tBuMe2Si
Me3Si
Me
Me
92 (100:0)
83 (89:11)
92 (100:0)
68 (95:5)
46 (96:4)
91 (96:4)
46 (100:0)
93 (100:0)
84 (94:6)
92 (100:0)
9
B
B
B
B
Scheme 4
10
11
12
13
14
15
16
17
B
2g
2a
2f
Bf
B
5gc
5ad
5fd
5fe
Me2PhSi
Me3Si
Me
C
2f
nBu
Cg
of each isomer, the mixture was converted to 1-phenylbutanol
according to the procedure described in Table 3. The enantiomeric
excess of 6kd was 85% ee. The ee value of 6kd strongly supports
that complete chirality transfer to both (E)- and (Z)-5kd took place
according to the mechanism shown in Scheme 3.
The present method provides a new access to (arylalkenyl)silanes,
including optically pure allylic silanes, from silyl-substituted
homoallyl alcohol and aryl halide.
a Conditions A: 5 mol % Pd(OAc)2, 20 mol % P(p-tol)3, 1.44 equiv
Cs2CO3, reflux, 4-15 h. Conditions B: 5 mol % Pd(OAc)2, 20 mol %
PPh3, 1.20 equiv Cs2CO3, reflux, 4-7 h. Conditions C: 2.5 mol %
Pd(OAc)2, 10 mol % PPh3, 1.20 equiv Cs2CO3, reflux, 45 min. b E/Z Ratios
of 5 are in parentheses. c PPh3 was used instead of P(p-tol)3. d Reaction
run using 2.5 mol % of Pd(OAc)2 and 10 mol % of the ligand. e P(cHex)Ph2
(10 mol %) was used. f P(tBu)3 (5 mol %) was used instead. g The reaction
time was 5 h.
Scheme 2
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research from MEXT and JSPS. S.H. and K.H.
acknowledge JSPS for financial support. This paper is dedicated
to the memory of the late Professor Yoshihiko Ito.
Supporting Information Available: Experimental details and
characterization data for new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
Table 3. Chirality Transfer from Optically Active (S)-4d to
(E)-1-Aryl-2-butenylsilanes
References
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(5) The regioselectivity of each reaction was greater than 99:1. One exception
is the reaction in entry 6, Table 1, wherein the regioselectivity was 98:2.
We assume that the bulky silyl groups would accelerate the reductive
elimination steps and that the conceivable isomerization of the σ-allyl-
(aryl)palladium intermediates scarcely took place.
entry
2
yield of 5 (%)
ee of 6 (%)
1
2
3
4
2a
2b
5ad, 92
5bd, 97
5hd, 94
5id, 90
6ad, 96
6bd, 96
6hd, 94
6id, 96
2-PhC6H4Br (2h)
5
5jd, 87
6jd, 95
(6) When allylsilane having an aryl or an isopropoxy group on the silicon
was employed, decomposition of the allylsilane was observed and no
coupling products were obtained.
(7) Suginome, M.; Iwanami, T.; Ohmori, Y.; Matsumoto, A.; Ito, Y. Chem.
Eur. J. 2005, 11, 2954-2965.
E,R configuration. Immediate reductive elimination from 8a without
loss of the chirality provides (E,S)-5.
The reaction of optically active (S)-4d (96% ee) with bromoben-
zene provided a mixture of (E)- and (Z)-5kd in a ratio of 93:7
(Scheme 4). Since we could not determine the enantiomeric excess
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