Rhodium-catalyzed substitution of allylic carbonates with enoxysilanes

Article abstract of DOI:10.1016/S0040-4039(00)01448-9

Substitution at the allylic position proceeds smoothly in rhodium-catalyzed reactions of allyl carbonates with enoxysilanes under almost neutral conditions to give γ,δ-unsaturated ketones in good to excellent yields. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01448-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8807–8811
Rhodium-catalyzed substitution of allylic carbonates with
enoxysilanes
Takako Muraoka, Isamu Matsuda* and Kenji Itoh
Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa,
Nagoya 464-8603, Japan
Received 4 August 2000; accepted 25 August 2000
Abstract
Substitution at the allylic position proceeds smoothly in rhodium-catalyzed reactions of allyl carbonates
with enoxysilanes under almost neutral conditions to give g,d-unsaturated ketones in good to excellent
yields. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: rhodium; cation complex; catalyst; enoxysilane; allylation.
Transition-metal catalysts have been explored as a powerful tool for carbonꢀcarbon bond
formation under mild conditions. Although there have been many reports on the substitution of
allylic derivatives catalyzed by palladium,1 nickel,2 molybdenum,3 iron,4 tungsten,3a,5 or
ruthenium6 complexes, only a few examples of the rhodium-catalyzed allylic substitution have
been reported.2k,7 Moreover, there are exceptional reports showing the successful use of
enoxysilanes in place of soft carbanions as a nucleophile under neutral conditions in an allylic
substitution.3a,4b,8
We have already reported rhodium-catalyzed Mukaiyama-type aldol coupling of enoxysilanes
(1) with aldehydes or ketones under neutral conditions,9 in which it is pointed out that the
rhodium enolate complex, 2, plays an important role (Scheme 1). If this is the case, allyl
carbonate (4) would behave as an electrophile in the coupling with an enoxysilane (1). Thus, we
designed rhodium-catalyzed reactions of 1 with 4. Here we report successful substitution of
allylic carbonates with enoxysilanes in the presence of a cationic rhodium catalyst to give the
g,d-unsaturated ketones.
* Corresponding author. Tel: +81-52-789-5113; fax: +81-52-789-3205; e-mail: matsudai@apchem.nagoya-u.ac.jp
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01448-9

8808
Scheme 1.
Firstly, when a mixture of allylcarbonate 4a and 4 equivalents of 1a in CH2Cl2 was heated at
80°C for 19 h in the presence of 1 mol% [Rh(COD)(DPPB)]OTf [7a, COD=1,5-cyclooctadiene,
DPPB=1,4-bis(diphenylphosphino)butane] two products, 5a and 6a (5a:6a=53:47) appeared,
which were obtained as an unseparable mixture in 58% yield (67% conversion based on 4a, entry
1 in Table 1). After several trials, the yield of products was remarkably improved by using 1
Table 1
Substitution of 4a with 1a catalyzed by rhodium complexa
Entry Catalyst
Solvent
Conversion of 4a (%)
Product10
Ratio (5a:6a)c
Yield (%)b
1
2
3
4
5
6
7
8
[Rh(COD)(DPPB)]OTf (7a)
[Rh(COD)(MePh2P)2]OTf (7b)
Rh4(CO)12 (7c)
[Rh(COD)(Me2PhP)2]OTf (7d)
7b
7b
7b
7b
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
THF
67
100
48
100
100
100
100
58
86
42
76
100
66
63
0
53:47
52:48
55:45
55:45
55:45
51:49
58:42
–
C6H6
DMSO
d
–
a A mixture of 4a (0.7–1 mmol), 1a (4 equiv. for 4), and 7 (1 mol% for 4) in the solvent was heated in a sealed tube
for 19 h at 80°C.
b Isolated yield.
c Determined by GLC analyses.
d Not determined.

8809
mol% of [Rh(COD)(MePh2P)2]OTf (7b) as a catalyst under similar reaction conditions,
although the regioselectivity was not altered (86% isolated yield, entry 2 in Table 1). Other
rhodium complexes, such as Rh4(CO)12 (7c, entry 3 in Table 1) and [Rh(COD)(Me2PhP)2]OTf
(7d, entry 4 in Table 1) were less effective than 7b, and RhCl(PPh3)3, Rh(CO)2(acac),
Rh(C2H4)2(acac), RhH(PPh3)4, [Rh(COD)(DPPB)]PF6, [Rh(COD)(DPPB)]BF4, [Rh(COD)-
(PPh3)2]PF6 exhibited no catalytic activity for the coupling under similar conditions. The
efficacy of 7b was also affected by the solvent in the reaction of 1a with 4a. The reaction in
CH3CN (100% isolated yield, entry 5 in Table 1) progressed smoothly at a similar level to that
in CH2Cl2. The catalysts worked in THF (66% isolated yield, entry 6 in Table 1) and in
benzene (63% isolated yield, entry 7 in Table 1) albeit with less efficiency than that in CH2Cl2,
while the reaction did not proceed at all in DMSO.
Other types of allylic carbonates and enoxysilanes were subjected to the reaction for
exploring the scope of the present rhodium-catalyzed substitution of 4. The results are
summarized in Table 2. Linear allylic carbonates were uniformly converted into the corre-
sponding g,d-unsaturated ketones in high yield from the reactions with 1a (entries 1–6 in
Table 2). Almost identical results were obtained in the reaction of 1a with either a cinnamyl
derivative (4d) or its regioisomer (4e) to give the corresponding products 5d and 6d in 80%
yield (entries 3 and 4 in Table 2). Disubstitution at the allylic terminus distal to the leaving
group (4g) seems to improve the isomer ratio of the products (5g:6g=3:1, entry 6 in Table 2).
Cyclohexenyl derivatives (4h and 4i) were also available to the coupling with 1a under
similar conditions to give 5h and a mixture of 5i and 6i (5i:6i=69:31), respectively, in
moderate yield (entries 7 and 8 in Table 2). These results are in clear contrast to the fact that
simple allylic carbonate (4, R4=R5=H) did not interact with 1a under similar condi-
tions.
One of the most direct advantages to the operation under neutral conditions is that the
designed transformation is applicable to substrates bearing functional groups susceptible to an
acid or a base. In fact, the functional groups such as carbamoyl, carbonyl (ketone), and
alkoxy carbonyl groups remained intact during the reactions of 4j, 4k, and 4l with 1a to give
the corresponding products in high to moderate yields (entries 9–11 in Table 2).
High yields of products were also observed in the reactions of linear enoxysilane 1b and 1c
with 4a (entries 12 and 13 in Table 2). In contrast to these results, a coupling reaction of 4a
with 1-trimethylsilyloxycyclohexene 1d gave the corresponding products only in 30% yield
(8ad:9ad=54:46, entry 14 in Table 2). The low reactivity of cyclic enoxysilane (1d) might be
attributed to the steric hindrance and/or E configuration of the olefinic part.
Although a pathway to the products is also elucidated by the intermediacy of the h3-allyl
rhodium complex in addition to our working hypothesis, as shown in Scheme 1, any informa-
tion with respect to the mechanistic consideration is not obtained at present.
In summary, we have shown that enoxysilanes are endowed with sufficient nucleophilicity in
the substitution of 4 catalyzed by cationic rhodium complexes under neutral conditions. The
present operation provides a facile method for the chemoselective homologation of allylic
carbonates involving other functions such as amide, ketones, and ester, which remain intact
even in the reaction without a procedure for protection.

8810
Table 2
Substitution of 4 with 1 catalyzed by 7ba

8811
Acknowledgements
We gratefully acknowledge financial support (12450360) from the Ministry of Education,
Science, Sports, and Culture, Japan.
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