Rhodium-catalyzed allylation of benzyl acetates with allylsilanes

Article abstract of DOI:10.1002/adsc.201100202

Benzyl acetate reacted with allyltrimethylsilane to give an allylation product in the presence of a catalytic amount of the (cyclooctadiene)rhodium(I) chloride dimer {[Rh(cod)Cl]₂}, sodium tetrakis[3,5- bis(trifluoromethyl)phenyl]borate (NaBARF), and triphenyl phosphite [P(OPh) ₃] in refluxing 1,2-dichloroethane. Primary, secondary and tertiary benzyl acetates could be used for the reaction. Moreover, allylation of gem-benzyl acetate was possible with [Rh(cod)Cl]₂, NaBARF, and P(OPh)₃. Monoallylation and diallylation of gem-benzyl acetate could be controlled by altering the reaction conditions. Cationic rhodium species generated in situ act as a Lewis acid catalyst to give a benzyl carbocation by elimination of the acetoxy group from the benzylic carbon. Copyright

Full text of DOI:10.1002/adsc.201100202

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DOI: 10.1002/adsc.201100202
Rhodium-Catalyzed Allylation of Benzyl Acetates with
Allylsilanes
Gen Onodera,a Eriko Yamamoto,a Shota Tonegawa,a Makoto Iezumi,a
and Ryo Takeuchia,*
a
Department of Chemistry and Biological Science, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara,
Kanagawa 252-5258, Japan
Fax : (+81)-42-759-6493; phone: (+81)-42-759-6231; e-mail: takeuchi@chem.aoyama.ac.jp
Received: March 20, 2011; Revised: April 29, 2011; Published online: August 10, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100202.
Abstract: Benzyl acetate reacted with allyltrimethyl- tion and diallylation of gem-benzyl acetate could be
silane to give an allylation product in the presence of controlled by altering the reaction conditions. Cat-
a catalytic amount of the (cyclooctadiene)rhodium(I) ionic rhodium species generated in situ act as a
chloride dimer {[Rh
bis(trifluoromethyl)phenyl]borate (NaBARF), and elimination of the acetoxy group from the benzylic
triphenyl phosphite [P(OPh)3] in refluxing 1,2-di- carbon.
ACHUTGTNRENN(GU cod)Cl]2}, sodium tetrakisACHTUNGTRENN[UGN 3,5- Lewis acid catalyst to give a benzyl carbocation by
AHCTUNGTRENNUNG
chloroethane. Primary, secondary and tertiary benzyl
acetates could be used for the reaction. Moreover, Keywords: allylation; allylsilane; benzyl acetate; rho-
allylation of gem-benzyl acetate was possible with dium; sodium tetrakis
[Rh(cod)Cl]2, NaBARF, and P(OPh)3. Monoallyla- nyl]borate (NaBARF)
ACHTUNGTRNE[NUNG 3,5-bis(trifluoromethyl)phe-
N
ACHTUNGTRENNUNG
Introduction
complexes were new and efficient Lewis acid catalysts
for the Mukaiyama aldol reaction and Mannich reac-
Lewis acid catalysis is essential for efficient carbon- tion.[11m] It is clear that these cationic complexes are
carbon bond-forming reactions.[1] Typical Lewis acid- sufficiently electrophilic to act as Lewis acids. These
catalyzed or -mediated carbon-carbon bond-forming results prompted us to explore late transition metal
reactions include the Friedel–Crafts reaction,[2] Diels– Lewis acid-catalyzed carbon-carbon bond-forming re-
Alder reaction,[3] Mukaiyama aldol reaction[4] and Sa- actions.
kurai–Hosomi allylation.[5] These reactions are useful
The Sakurai–Hosomi reaction is one of the most
for constructing complex molecular structures from useful carbon-carbon bond-forming reactions.[5] Lewis
readily available compounds under mild conditions. acid-activated aldehydes, ketones, acetals, and ketals
Representative Lewis acids for these reactions are react with allylsilanes to give allylation products. In
boron,[6] aluminum,[7] tin[8] and titanium[9] compounds. contrast, the reactions of benzylic electrophiles such
To date, enantioselective Lewis acid catalysts for as benzyl halides, benzyl alcohols and their deriva-
these reactions have been reported. In contrast to tives with allylsilanes have been less developed than
early transition metals such as titanium and zirconi- those of aldehydes and ketones.[12–15] Recently, the
um, much less attention has been paid to late transi- Lewis acid-mediated or -catalyzed allylation of ben-
tion metals as Lewis acids because of their lower zylic electrophiles using allylsilanes has been report-
Lewis acidity.[10] Late transition metal complexes with ed. Boron,[12] indium,[13] and bismuth[14] compounds,
a non-coordinated counteranion have been expected montmorillonite,[15] and molecular iodine[16] have been
to act as potential Lewis acid catalysts because disso- used as Lewis acid catalysts for the reaction. No late
ciation of the non-coordinated counteranion from the transition metal catalysts have been reported to be
metal center generates a positive charge leading to a used in this reaction. New Lewis acid catalysts are
high electrophilicity. In the course of our study on iri- still needed to further expand the scope and selectivi-
dium-catalyzed carbon-carbon bond-forming reac- ty of the reaction. In this article, we report the full de-
tions,[11] we found that cationic iridium and rhodium
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Gen Onodera et al.
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tails of the rhodium-catalyzed allylation of benzyl ace- in quantitative yield. PACHTNUGTRENNUNG
tates using allylsilanes.
ligand. When 8 mol% of PACHTUNTGRENNUNG
used, the product 3aa was obtained in 93% yield
(entry 4). The electron-withdrawing property of
Results and Discussion
PACHTUNGTRENNUNG
1-Phenylethyl acetate (1a) reacted with 3 equivalents tron-donating property of PPh3 and DPPE would
of allyltrimethylsilane (2a) to give 4-phenyl-1-pentene reduce the Lewis acidity of cationic rhodium species
(3aa) in the presence of a catalytic amount of the rho- to inhibit the reaction. An increase in the amount of
dium(I) complex [Rh
diene), NaBARF {BARF=tetrakis
methyl)phenyl]borate}, and triphenyl phosphite excess amount of NaBARF relative to the Rh Cl
[P(OPh)3] in refluxing 1,2-dichloroethane. It is well bond is needed to generate a sufficient amount of cat-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
À
ACHTUNGTRENNUNG
known that the reaction of a transition metal-Cl bond ionic rhodium species. When 8 mol% of NaBARF
with NaBARF generates a cationic transition metal (NaBARF/Rh=2) was used, the reaction was com-
complex with the tetrakisACTHNUGTRNEUNG[3,5-bis(trifluoromethyl)phe- pleted in 1 h and gave 3aa in 99% yield (entry 6). The
nyl]borate anion. Catalyst screening was performed solvent affected the reaction. 1,2-Dichloroethane gave
by reacting 1-phenylethyl acetate (1a) with allyltrime- the best result (entry 6). Benzene, ethyl acetate and
thylsilane (2a). The results are summarized in Table 1. THF were less effective (entries 7–9). The concentra-
In the absence of any ligand, the reaction did not tion did not affect the reaction (entries 6,10 and 11).
occur (entry 1). The catalytic activity depended on [RhACTHNUGTRENUNG(cod)Cl]2 alone did not have any catalytic activity
the ligand used (entries 2–4). When PPh3 and DPPE for the reaction (entry 12). NaBARF alone also did
were used as ligands, the reaction did not occur (en- not have any catalytic activity (entry 13). The cationic
tries 2 and 3) and the starting material was recovered rhodium complex [RhACTHNUTRGNE(UNG cod)2]SbF6 had a lower catalyt-
Table 1. Reaction of benzyl acetate 1a with allylsilane 2a.[a]
Entry
Catalyst (mol%)
Additive (mol%)
Ligand (mol%)
Time [h]
Yield [%][b]
3aa/4aa[c]
1
2
3
4
5
6
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
[Rh
N
NaBARF (2)
NaBARF (4)
NaBARF (4)
NaBARF (4)
NaBARF (6)
NaBARF (8)
NaBARF (8)
NaBARF (8)
NaBARF (8)
NaBARF (8)
NaBARF (8)
none
none
PPh3 (8)
DPPE (4)
24
24
24
15
6
0
0
0
93
85
99
46
0
–
–
–
99/1
99/1
99/1
99/1
–
P
P
P
P
P
P
P
P
P
P
P
P
N
1
7[d]
8[e]
9[f]
10[g]
11[h]
12
13
14
15
24
24
24
1
0
–
99
87
0
0
54
83
99/1
99/1
–
1
24
24
24
15
none
[Rh(cod)2]SbF6 (4)
[Ir(cod)Cl]2 (2)
NaBARF (4)
none
NaBARF (4)
–
98/2
49/51
G
[a]
Reaction conditions: 1a (1 mmol) and 2a (3 mmol) in the presence of catalyst, additive, and ligand in 1,2-dichloroethane
(5 mL).
[b]
[c]
[d]
[e]
[f]
Isolated yield (combined yield of 3aa and 4aa).
Determined by H NMR.
Benzene was used as solvent.
Ethyl acetate was used as solvent.
THF was used as solvent.
10 mL of 1,2-dichloroethane were used as solvent.
2 mL of 1,2-dichloroethane were used as solvent.
1
[g]
[h]
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Rhodium-Catalyzed Allylation of Benzyl Acetates with Allylsilanes
Table 2. Reaction of various acetates 1 with allylsilane 2a.[a]
Entry 1
Product Time
[h]
Yield
[%][b]
1
2
3
3ba
3ca
3da
3
94
91
99
0.5
1
Scheme 1. Allylation of benzyl alcohol 5.
ic activity than the combination of [RhACTHNUTRGENUG(N cod)Cl]2 and
NaBARF, and the product 3aa was obtained in 54%
yield (entry 14). The reaction catalyzed by
[IrACHTUNGTRENNUNG(cod)Cl]2 instead of [RhACHTUGNTREN(NUNG cod)Cl]2 gave a mixture of
4
5
6
3ea
3fa
3ga
24
24
24
75
0
3aa and 4aa in 83% yield (entry 15). In the iridium-
catalyzed reaction, the allylated product 3aa was iso-
merized to the internal alkene 4aa. Isomerization
from 3aa to 4aa was not observed under rhodium cat-
alysis. This result showed that a rhodium catalyst was
more suitable for the reaction than an iridium cata-
lyst.
0
The direct substitution of a hydroxy group by a nu-
cleophile is an important process in organic synthesis.
The direct substitution of a hydroxy group with allyl-
silanes mediated by a catalytic amount of Lewis acid
has been reported.[12a,13a–d,14,15] The reaction is more
direct if benzyl alcohol can be used as a substrate. We
tried the catalytic substitution of a hydroxy group
with allylsilane. The reaction of 1-phenyl-1-ethanol
(5) with allylsilane 2a under the same optimized reac-
tion conditions as above gave the products in 50%
yield (Scheme 1). Unfortunately, direct substitution of
alcohol 5 did not give a satisfactory result. Acetate 1a
was more suitable for this transformation than alcohol
5.
To examine the scope and limitation of the reac-
tion, a series of acetates 1 was subjected to the reac-
tion with allylsilane 2a under the optimized condi-
tions. The results are summarized in Table 2. The
electronic property of the substituent on the phenyl
ring affected the reaction. Substrates bearing elec-
tron-donating substituents such as methyl, methoxy
and chloro groups (1b–d) smoothly reacted with allyl-
silane 2a to give the corresponding products in yields
of 91–99% (entries 1–3). Acetate 1e bearing a me-
thoxycarbonyl group needed a longer reaction time to
complete the reaction than 1b–d. Product 3ea was ob-
tained in lower yield than products 3aa–ca (entry 4).
Nitro and cyano groups on the phenyl ring strongly
disturbed the reaction to give no allylated products
(entries 5 and 6). The starting materials were recov-
ered in quantitative yield after refluxing in 1,2-di-
chloroethane for 24 h. 1-(1-Naphthyl)ethyl acetate 1h
and 1-(2-naphthyl)ethyl acetate 1i smoothly under-
7
3ha
1
98
8
9
3ia
3ja
1
1
97
98
10
11
3ka
3la
1
1
96
89
12
13
3ma
3na
1
6
97
39[c]
14
15
3oa
3pa
1
9
96
83[d]
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phenyl ring is necessary for the reaction of primary
benzyl acetate. The reaction of acetate 1f bearing a
ferrocenyl moiety gave 3ra in 96% yield (entry 17).
On the basis of these results, we turned our atten-
tion to the scope of allylsilanes. Methallyltrimethylsi-
lane 2b, cinnamyltrimethylsilane 2c and prenyltrime-
thylsilane 2d were chosen as allylsilane components
because they have different substitution structures of
the carbon-carbon double bond. They were allowed
to react with primary acetate 1q, secondary acetate 1a
or tertiary acetate 1o. The results are summarized in
Table 3. Primary acetate 1q was reacted with methal-
lyltrimethylsilane (2b), cinnamyltrimethylsilane (2c),
and prenyltrimethylsilane (2d) to give the respective
allylated products 3qb, 3qc, and 3qd in high yields
(entries 1–3). As in the Lewis acid-mediated or -cata-
lyzed reaction of aldehydes with allylsilanes 2c and
2d, the carbon-carbon bond-forming reaction was re-
giospecific to occur at the g-position of allylsilanes.
The reaction of secondary acetate 1a with allylsilanes
2b–d gave the corresponding products (3ab, 3ac, and
Table 2. (Continued)
Entry 1
Product Time
[h]
Yield
[%][b]
16
3qa
3ra
1
1
95
96
17
[a]
Reaction conditions: 1 (1 mmol) and 2a (3 mmol) in the
presence of [Rh(cod)Cl]2 (0.02 mmol), NaBARF
(0.08 mmol), and P(OPh)3 (0.08 mmol) in refluxing 1,2-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
dichloroethane (5 mL).
Isolated yield.
2% yield of 4na was obtained as an unseparable by-prod-
[b]
[c]
uct.
[d]
10% yield of 4pa was obtained as an unseparable by-
product.
went the reaction with 2a to give the corresponding 3ad) in yields of 83–98% (entries 4–6). The allylated
products (3ha and 3ia) in nearly quantitative yields product 3ac was obtained as a 54/46 mixture of diaste-
(entries 7 and 8). Acetates that are more hindered reomers. Contiguous tertiary and quaternary carbons
than 1a such as diphenylmethyl acetate (1j) and 1-fer- were constructed in high yield. With a tertiary acetate,
rocenylethyl acetate (1k) smoothly reacted with 2a to the yield depended on the allylsilane used. The terti-
give the allylated products in excellent yields (en- ary acetate 1o reacted with methallyltrimethylsilane
tries 9 and 10). Propargylic substitution with a carbon (2b) to give the allylated product 3ob in high yield,
nucleophile is an attractive approach for obtaining while the reaction of 1o with g-substituted allylsilanes
useful building blocks as product.[17,18] Previous studies 2c and 2d gave the products 3oc and 3od in lower
revealed that the substitution product was allene[19] or yields (entries 7–9). Two contiguous quaternary car-
alkyne.[17,18] The selectivity of the product depends on bons were constructed by the reaction.
the substrates and catalyst. Under our reaction condi-
An allyl group is introduced at a benzylic position
tions, propargylic acetate 1l reacted with allylsilane 2a by substitution of an acetoxy group. We can extend
to give the alkyne product 3la in 89% yield this chemistry to a gem-diacetate,[21] which is easily
(entry 11). The propargylic position was substituted prepared by the reaction of aldehydes with acetic an-
with an allyl moiety. The carbon-carbon triple bond in hydride.[22] The results are summarized in Table 4.
the substrate was inert during the reaction. No allene The molar ratio of allylsilane 2 to 6 was important.
product was obtained. The reaction of 1,2,3,4-tetrahy- gem-Diacetate 6 reacted with 1.5 equivalents of allyl-
dronaphthalen-1-yl acetate (1m) gave 3ma in 97% silane 2a in refluxing 1,2-dichloroethane for 24 h to
yield, while the reaction of 1-acenaphthenyl acetate give 7 in 23% yield (entry 1). Decomposition of 6 to
(1n) gave 3na in a lower yield of 39% (entries 12 and benzaldehyde occurred as a competitive reaction to
13). Tertiary acetate 1o smoothly reacted with 2a to allylation. To increase the yield of allylation product 7
give product 3oa in quantitative yield (entry 14). Ada- and 8a, the allylation must be faster than the decom-
mantyl acetate 1p was transformed to the allylated position of 6 to benzaldehyde. An increase in the
product in high yield, but it needed a longer reaction amount of allylsilane 2a may promote allylation. We
time to complete than 1o (entry 15). Products 3oa and tried the reaction with 3 equivalents of allylsilane 2a,
3pa have all-carbon-substituted quaternary carbon but the results were unsatisfactory: product 7 was ob-
centers.[20] Notably, our reaction represents a novel tained in 41% yield and diallylation product 8a was
catalytic method for the construction of an all-carbon- not obtained at all (entry 2). The reaction with
substituted quaternary carbon center. Our reaction 6 equivalents of allylsilane 2a exclusively gave dially-
could be applied to primary acetates. Acetate 1q sub- lation product 8a in 43% yield (entry 3). To obtain
stituted with a methoxy group smoothly reacted with monoallylation product 7 in high yield, the reaction
2a to give product 3qa in 95% yield. On the other temperature was important. The reaction with
hand, benzyl acetate did not react with 2a under the 6 equivalents of allylsilane 2a at room temperature
same reaction conditions, and the starting material gave monoallylation product 7 in 90% yield (entry 4).
was recovered. An electron-donating group on a A decrease in the amount of 2a from 6 equivalents to
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Rhodium-Catalyzed Allylation of Benzyl Acetates with Allylsilanes
Table 3. Reaction of benzylic acetate 1 with allylsilane 2.[a]
Entry
1
2
Product
Time [h]
5
Yield [%][b]
99
1
2
1q
1q
2
99
3
4
6
1
92
83
2b
2c
5
1a
1a
5
95[c]
6
7
8
2d
2b
2c
9
6
4
98
88
54
1o
1o
9
2d
3
79
[a]
Reaction conditions: 1 (1 mmol) and 2 (3 mmol) in the presence of [Rh
(OPh)3 (0.08 mmol) in refluxing 1,2-dichloroethane (5 mL).
Isolated yield.
Product 3ac was obtained as an unseparable diastereomers in 54/46 ratio determined by H NMR.
ACHTUNGTRNEUN(NG cod)Cl]2 (0.02 mmol), NaBARF (0.08 mmol), and
PACHTUNGTRENNUNG
[b]
[c]
1
Table 4. Reaction of gem-diacetate 6 with allylsilane 2a.[a]
Entry
Equiv. of 2a
Temperature
Time [h]
Yield of 7 [%][b]
Yield of 8a [%][b]
1
2
3
4
5
6
1.5
3
6
6
6
reflux
reflux
reflux
reflux
r.t.
24
24
5
24
0.5
0.5
23
41
0
0
90
72
0
0
43[c]
29[d]
0
5
r.t.
0
[a]
Reaction conditions: 6 (1 mmol) and 2a in the presence of [Rh
(0.08 mmol) in 1,2-dichloroethane (5 mL).
Isolated yield.
ACHUTGTNRNE(NUG cod)Cl]2 (0.02 mmol), NaBARF (0.08 mmol), and PACHTUNGTRENNUNG(OPh)3
[b]
[c]
A 12% yield of (E)-4-phenyl-1,5-heptadiene and a 7% yield of (E)-1-phenyl-1,3-butadiene were obtained as unseparable
by-products.
[d]
A 22% yield of (E)-4-phenyl-1,5-heptadiene and a 5% yield of (E)-1-phenyl-1,3-butadiene were obtained as unseparable
by-products.
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Scheme 2. Allylation of benzyl acetate 7.
5 equivalents decreased the yield. The reaction of 1a
with 5 equivalents of 2a gave 7 in 72% yield (entry 6).
We could obtain both monoallylation product 7 and
diallylation product 8a in moderate to high yield by
choosing suitable reaction conditions.
The reaction of gem-diacetates with allylsilanes will
be a new route to 1,6-heptadienes, which are good
substrates for cycloisomerization. Much attention has
been paid to the cycloisomerization of a,w-dienes cat-
alyzed by transition metal complexes,[23] since this can
be used to construct cyclic compounds in a single step
without any wasteful by-products. Our catalytic reac-
tion would be more valuable if two different allyl
groups could be introduced at the benzylic carbon.
Based on the results described above, we tried a se-
quential substitution of a gem-diacetate. The results
are shown in Scheme 2. The reaction of 7 with
3 equivalents of methallyltrimethylsilane 2b at room
temperature for 1 h gave 8b in 86% yield. Prenyltri-
methylsilane 2d could be used for the sequential sub-
Scheme 3. Plausible reaction pathway.
stitution. The reaction of 7 with 3 equivalents of 2d at was obtained in 80% yield as a racemic mixture.[25]
room temperature for 0.3 h gave 8c in 95% yield. The optical purity of the starting material was com-
Contiguous tertiary and quaternary carbons were con- pletely lost in the product. Elimination of an acetoxy
structed in high yields.
group gives a planar achiral benzyl carbocation inter-
We considered that this allylation reaction proceeds mediate. Nucleophilic attack of allylsilane 2 occurs on
via a carbocation intermediate. First, the cationic rho- both sides of the benzyl carbocation plane to give a
dium species is formed by the reaction of racemic mixture of 3aa. The results strongly support
[RhACHTUNGTRENNUNG
(cod)Cl]2 with NaBARF.[24] The coordination of the formation of a benzyl carbocation intermediate by
carbonyl oxygen atom to cationic rhodium species the elimination of an acetoxy group. The reaction of
promoted dissociation of the acetoxy group to gener- 2-octyl acetate with allyltrimethylsilane in refluxing
ate a benzyl carbocation intermediate. Nucleophilic 1,2-dichloroethane for 24 h gave no product, and the
attack of allylsilane 2 at the g-position of the silicon starting material was recovered quantitatively. Elimi-
atom to a benzyl carbocation intermediate gives inter- nation of an acetoxy group from an aliphatic carbon
mediate 9, where a positive charge is generated at the did not occur under the optimized conditions, since
b-position of the silicon atom. Nucleophilic attack of an aliphatic secondary carbocation is less stable than
an acetoxy anion to silicon atom induces the elimina- a benzyl carbocation.
tion of trimethylsilyl acetate to give the allylated
We tried the enantioselective allylation of 1a with
product 3 (Scheme 3). To obtain evidence of the for- 2a using chiral phosphoramidite ligands (aS,R,R)-L1
mation of a benzyl carbocation intermediate, we ex- and (aS,S,S)-L2 (Figure 1) which were expected to be
amined the reaction of (R)-1-phenylethyl acetate effective for the reaction due to an electronic-with-
[(R)-1a] with allyltrimethylsilane (2a). Product 3aa drawing property similar to that of PACTHNUTRGNEUNG
(OPh)3.[26] Un-
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Rhodium-Catalyzed Allylation of Benzyl Acetates with Allylsilanes
ed and filled with argon. To the flask were added 1,2-di-
chloroethane (5 mL) and P(OPh)3 (24.4 mg, 0.08 mmol).
AHCTUNGTRENNUNG
Acetate (R)-1a (164 mg, 1.0 mmol) and allylsilane 2a
(346 mg, 3.0 mmol) were added to the reaction mixture. The
mixture was stirred under reflux for 3 h. The progress of the
reaction was monitored by GLC. After the reaction was
complete, the solvent was evaporated under vacuum.
Column chromatography (n-hexane) of the residue gave
3aa; yield: 117 mg (0.80 mmol, 80%). The optical rotation
was measured: [a]2D7: +0.08 (c 0.75, CHCl3).
Figure 1. Chiral phosphoramidite ligands.
fortunately, the reaction of 1a with 2a under refluxing
1,2-dichloroethane for 24 h did not occur. The starting
material remained in quantitative yield.
Procedure for the Allylation of gem-Diacetate (6)
with Allyltrimethylsilane (2a).
A flask was charged with [RhACHTNUTRGENNG(U cod)Cl]2 (10.9 mg, 0.02 mmol)
and NaBARF (82.9 mg, 0.09 mmol). The flask was evacuat-
ed and filled with argon. To the flask were added 1,2-di-
Conclusions
chloroethane (5 mL) and P
ACHTUNGRTEN(NUNG OPh)3 (24.1 mg, 0.08 mmol). Al-
lylsilane 2a (667 mg, 5.8 mmol) and gem-diacetate
6
We have identified a novel Lewis acid catalysis of cat-
ionic rhodium complex generated in situ. Benzyl ace-
tates react with allylsilanes to give allylated products
in high yields. Primary, secondary and tertiary ace-
tates could be allylated efficiently. Benzyl gem-diace-
tate could be used for the allylation. 1,6-Heptadienes
were obtained as the product. Further development
of the Lewis acid catalysis of a cationic rhodium com-
plex is underway in our laboratory.
(208 mg, 1.0 mmol) were added to the reaction mixture. The
mixture was stirred at room temperature for 0.5 h. The prog-
ress of the reaction was monitored by GLC. After the reac-
tion was complete, the solvent was evaporated under
vacuum. Column chromatography (n-hexane/AcOEt=90/
10) of the residue gave 7; yield: 171 mg (0.90 mmol, 90%).
1-Phenyl-3-butenyl acetate (7): 1H NMR (500 MHz, in
CDCl3, TMS): d=2.06 (s, 3H), 2.55 (ddd, J=14.2, 6.9, and
6.0 Hz, 1H), 2.65 (ddd, J=14.2, 7.8, and 6.9 Hz, 1H), 5.05
(dm, J=10.1 Hz, 1H), 5.07 (dm, J=17.0 Hz, 1H), 5.70 (ddt,
J=17.0, 10.1, and 6.9 Hz, 1H), 5.81 (dd, J=7.8 and 6.0 Hz,
1H), 7.26–7.34 (m, 5H); 13C NMR (125 MHz, CDCl3): d=
21.2, 40.7, 75.1, 118.0, 126.5, 127.9, 128.4, 133.3, 140.0, 170.2;
HR-MS (FAB): m/z 191.1077, calcd. for C12H15O2 [M+H]+:
191.1072.
Experimental Section
Representative Procedure for the Allylation of
Acetates (1 and 7) with Allylsilanes (2)
A flask was charged with [RhACHTNUTRGENNG(U cod)Cl]2 (10.0 mg, 0.02 mmol)
and NaBARF (71.0 mg, 0.08 mmol). The flask was evacuat-
ed and filled with argon. To the flask were added 1,2-di-
Acknowledgements
This research was supported financially by a grant from the
Ministry of Education, Culture, Sports, Science, and Technol-
ogy of Japan.
chloroethane (5 mL) and PACHTNUTRGENNG(U OPh)3 (24.4 mg, 0.08 mmol).
Acetate 1a (175 mg, 1.1 mmol) and allylsilane 2a (343 mg,
3.0 mmol) were added to the reaction mixture. The mixture
was stirred under reflux for 1 h. The progress of the reaction
was monitored by GLC. After the reaction was complete,
the solvent was evaporated under vacuum. Column chroma-
tography (n-hexane) of the residue gave 3aa; yield: 155 mg
(1.1 mmol, 99%).
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Rhodium-Catalyzed Allylation of Benzyl Acetates with Allylsilanes
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asc.wiley-vch.de
2021