Allylation of Alcohols and Carboxylic Acids
TABLE 1. Rea ction of 2a w ith 1 Ca ta lyzed by
SCHEME 2
Ir (cod )2]+BF 4
- a
conv (%)
yield (%)
4a 5a
run
1 (equiv)
2a
84
87
96
>99
5
1
3a
23
56
86
99
trace
n.d.
7
6a
1
2
3
4
1
2
5
10
10
10
10
10
10
76
50
24
27
3
1
1
4
1
1
4
16
11
2
n.d.
n.d.
n.d.
n.d.
74
that [Rh(cod)2]+BF4- promotes the normal ester exchange
reaction between 1 and 2a rather than the allylation to
give 4a (11%) in preference to 3a (7%) (run 7). It was
found that 6a was obtained in good yield (74%) when a
small amount of Na2CO3 (3 mol %) was added to the
catalytic solution involving [Ir(cod)2]+BF4-. The reaction
at 90 °C resulted in a considerable decrease of allyl ether
3a (run 9). This shows that the present allylation calls
for higher temperature than 90 °C. The reaction seem to
proceed through the formation of π-allyl iridium complex,
followed by nucleophlic attack of alcohol (Scheme 2).
On the basis of these results, a variety of alcohols were
allowed to react with 1 under the same conditions as run
4 in Table 1. These results are shown in Table 2.
Like 2a , primary alcohol 1-butanol (2b ) reacted
smoothly with 1 under these conditions to form allyl butyl
ether (3b) in excellent yield (98%) (run 1). However,
secondary and tertiary alcohols such as 2-octanol (2c) and
1-adamantanol (2d ) are less reactive than primary alco-
hols. As a result, although the reaction must be prolonged
from 5 h for primary alcohols to 15 or 20 h for secondary
and tertiary alcohols, 2c and 2d afforded the correspond-
ing allyl ethers 3c and 3d in 85 and 62% yields along
with 2-octanone (8%) and 1-adamantyl acetate (9%),
respectively, as side products (runs 2 and 3). Benzyl
alcohol (2e) gave allyl benzyl ether (3e) in 82% yield (run
4).
We next tried the reaction of various phenols with 1
(runs 5-10). Phenol (2f) reacted with 1 at 100 °C to form
allyl phenyl ether (3f) in 79% yield, but the same reaction
at 120 °C produced 3f (80%) and Claisen rearrangement
product o-allylphenol (8%) (runs 5 and 6). p-Cresol (2g)
and p-methoxyphenol (2h ) were also allylated with 1 in
almost the same extent to form the corresponding allyl
ethers 3g and 3h (runs 7 and 8). Under these conditions,
hydroquinone (2i) afforded a mixture of mono- and
diallylated products 3i and 3i′ in fair yields (run 9). In
the reaction of p-aminophenol (2j), the amino group was
preferentially allylated rather than phenolic one to lead
to p-diallyaminophenol (3j) and allyl p-diallylamino-
phenyl ether (3j′) (run 10). In a similar manner as 2j,
p-hydroxybenzoic acid (2k ) was allylated with 1 to give
double-allylated product (3k ′) along with a small amount
of monoallylated products (3k ) (run 11). This shows that
the allylation of the carboxyl group in 2k takes place
more easily than that of the hydroxyl one.
n.d.
n.d.
1
11
13
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
5b
6c
7d
8e
9f
3
1
19
99
49
11
21
15
4
27
trace
n.d.
a
2a (1 mmol) was allowed to react with 1 in the presence of
[Ir(cod)2]+BF4 (0.01 mmol) in toluene (1 mL) at 100 °C for 5 h.
-
[IrCl(cod)]2 (0.01 mmol) was used as a catalyst. c [IrCl(CO)(P-
b
Ph3)2] (0.01 mmol) was used as a catalyst. [Rh(cod)2]+BF4- (0.01
d
mmol) was used as a catalyst. e Na2CO3 (0.03 mmol) was added.
f At 90 °C.
(1%), and octyl octanoate (6a ) (16%) which is thought to
be formed through the Tishchenko-type reaction of the
resulting aldehyde 5a (run 1). The reaction of a 1:2
mixture of 2a and 1 afforded 3a in 56% yield and
Tishchenko product 6a (11%), but the yields of 4a and
5a were the same as those of the stoichiometric reaction
(run 2). Previously, we reported that Ir complexes such
as [IrCl(cod)]2 catalyzes the hydrogen transfer reaction
of R,â-unsaturated ketones with alcohols to form satu-
rated ketones rather than allylic alcohols.6 This shows
that alkenes serve as a good hydrogen acceptor from
alcohols. In the present reaction, it is probable that allyl
acetate 1 was hydrogenated to propyl acetate by an Ir-
dihydride complex which is formed as a transient inter-
mediate in the course of the reaction. To depress the
formation of aldehyde 5a , the reaction must be carried
out in the presence of excess 1 toward alcohol 2a . The
reaction of 2a with 1 (5 equiv) under the influence of
[Ir(cod)2]+BF4 (1 mol %) in toluene at 100 °C for 5 h
-
afforded ally octyl ether (3a ) (86%) along with small
amounts of octyl acetate (4a ) (4%), octanal (5a ) (4%), and
octyl octanoate (6a ) (2%) (run 3). When 1 (10 equiv)
toward alcohol 2a was employed, 3a was obtained in
almost quantitative yield (>99%) (run 4). It is interesting
-
to note that [Ir(cod)2]+BF4 catalyst promotes the ally-
lation rather than the usual ester exchange reaction,
since esters react usually with alcohols to form ester
exchange products (Scheme 1).
Under the same conditions, however, [IrCl(cod)]2 did
not promote the present allylation at all (run 5). Simi-
larly, [IrClCO(PPh3)2] was inert for the allylation of 2a
with 1 (run 6). It is known that the cationic complex is
more active than the neutral one, since the coordination
of the cationic complex to the substrate smoothly takes
place compared with the neutral complex.7 It was found
It is interesting to note that 1-hexanethiol (2l) was
smoothly allylated by the use of [Ir(cod)2]+BF4 (1 mol
-
%) at 100 °C, since thiols easily coordinate to transition
metal complexes such as Pd to prevent their catalysis
(run 12). This is the first successful allylation catalyzed
by iridium complex.8
Table 3 shows the results for the preparation of allyl
carboxylates by the exchange reaction of carboxylic acids
with 1 by the same catalytic system. It was found that
the reaction of carboxylic acids with 1 takes place more
easily than that of alcohols allyl acetates in good yields.
The exchange reaction between aliphatic carboxylic
acids such as heptanoic acid (7a ) or 2-ethylhexanoic acid
(4) (a) Takeuchi, R. Synlett 2002, 1954-1965. (b) Takeuchi, R.; Ue,
N.; Tanabe, K.; Yamashita, K.; Shiga, N. J . Am. Chem. Soc. 2001, 123,
9525-9534. (c) Takeuchi, R.; Tanabe, K. Angew. Chem., Int. Ed. 2000,
39, 1975-1978. (d) Takeuchi, R.; Kashio, M. J . Am. Chem. Soc., 1998,
120, 8647-8655. (e) Takeuchi, R.; Kashio, M. Angew. Chem., Int. Ed.
1997, 36, 263-265.
(5) Okimoto, Y.; Sakaguchi, S.; Ishii, Y. J . Am. Chem. Soc. 2002,
124, 1590-1591.
(6) Sakaguchi, S.; Yamaga, T.; Ishii, Y. J . Org. Chem. 2001, 66,
4710-4712.
(7) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev. 2001,
101, 2067-2096.
J . Org. Chem, Vol. 69, No. 10, 2004 3475