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S. Tanaka et al. / Journal of Organometallic Chemistry 692 (2007) 295–298
2. Results and discussion
a decarboxylative allylation product (entry 3) [13]. The sub-
strate/catalyst (S/C) ratio could be increased by a factor of
20 without a significant decrease in the yield (entry 4). The
S/C could be further increased to 1000000 by the continu-
ous addition of methanol and removal of the volatile
coproduct, allyl methyl ether, at 70 °C (entry 5), with the
corresponding turnover number (TON) and turnover fre-
We recently found that various allyl ethers could be
cleaved by a combination of [CpRu(CH3CN)3]PF6 and 2-
quinolinecarboxylic acid in alcoholic solvents [9]. Our pre-
liminary mechanistic study [9,10], as well as other reports
on the CpRu complexes [11], have indicated that the p-allyl
complex 3 is involved both as a key intermediate and in the
resting state of the catalysis. This led us to try to isolate 3
and to remove an AOC group by catalysis using 3. Thus,
mixing [CpRu(CH3CN)3]PF6, 2-quinolinecarboxylic acid,
and 2-propen-1-ol in a 1:1:1 ratio in acetone at room tem-
perature gave, in 99% isolated yield, complex 3 as a pale
yellow solid, the stability of which in both air and moisture
was high enough for easy handling (vide infra) [12]. Table 1
lists representative results of the deprotection of alcohols
catalyzed by 3. The standard substrate was set to an
AOC-protected 2-phenylethan-1-ol (1a), and the condi-
quency (TOF) approaching 1000000 and 10000 hÀ1
,
respectively. Ethanol and 2-propanol were the solvents of
choice (entries 6 and 7). tert-Butyl alcohol could be used,
but it had less reactivity (entry 8), possibly due to the
low solubility of the catalyst. Methanol containing water,
DMF, THF, or dichloromethane could be used (entries
9–12), not only to facilitate the dissolution of a wide range
of substrates but also to provide a potential application to
solid-phase synthesis. Acetonitrile was found to be a poor
cosolvent that significantly lowered the reactivity (entry
13). The AOC groups of primary, secondary, and tertiary
alkanols, such as 1a, 1b, and 1c and phenol 1d, were quan-
titatively removed by the catalyst (entries 14–16).
tions were optimized as
a
starting point of
[1a] = 100 mM, [3] = 0.2 mM, CH3OH, and 30 °C. Under
these conditions, 1a was converted to 2-phenylethan-1-ol
(2a) in >99% yield within 0.5 h (entry 1). The substrate
concentration could be increased to 500 mM (entry 2)
but, with [1a] = 1 M, the yield tended to be lowered by
2–5% owing to the formation of allyl 2-phenylethyl ether,
PF6
N
Ru
O
O
3
Table 1
The high chemoselectivity of this method was further
demonstrated by using a series of mono-AOC-protected
diols, 1e–h, in which another hydroxy group was protected
as the benzyl (Bn) ether, benzoate (Bz), methoxymethyl
(MOM) ether, or tert-butyldiphenylsilyl (TBDPS) ether.
Under the conditions [1e–h] = 500 mM, [3] = 1 mM,
CH3OH, and 30 °C, 1e–h were completely converted after
3 h to give the corresponding monools in >99% yield with-
out modification of the Bn, Bz, MOM, and TBDPS groups
at all (entries 17–20).
Catalytic deallyloxycarbonylation of alcohols by the [CpRu(p-C3H5)(2-
quinolinecarboxylato)]PF6 complex (3)a
Entry Substrate S/Cb
Solvent
Time (h) Yield
(%)c
1
2
3
4g
5h
6
7
8
9
10
1a
500 CH3OH
500 CH3OH
1000 CH3OH
0.5
0.5
0.5
>99
>99
98f
1ad
1ae
1ad
1ad
1a
10000 CH3OH
1000000 CH3OH
500 C2H5OH
6
99
9 days
99
1
1
6
>99
>99
11
1a
1a
500 i-C3H7OH
500 t-C4H9OH
500 1:1 CH3OH–H2O
500 1:1 CH3OH–DMF
500 1:1 CH3OH–THF
500 1:1 CH3OH–CH2Cl2
500 1:1 CH3OH–CH3CN
500 CH3OH
500 CH3OH
500 CH3OH
500 CH3OH
500 CH3OH
Analogous to the removal of AOC, the present catalysis
was found to be applicable to other allyl esters 4. Thus, as
shown in Table 2, the allyl esters of primary alkyl, secondary
alkyl, tertiary alkyl, and aryl carboxylic acids in 4a–e, as well
as the phosphonic acid diallyl ester 4f, were converted to the
corresponding acids 5a–f in quantitative yields (entries 1–6).
In methanol, the primary alkanoates were found to undergo
allyl/methyl ester exchange. The side reaction could be
avoided by using 2-propanol instead of methanol. The reac-
tivity was found to be about 10 times as high as that of
[CpRu(P(C6H5)3)(CH3CN)2]PF6 [14], which is also known
to catalyze allyl ester cleavage in methanol. Whereas the
CpRu–P(C6H5)3 complex isomerized allyl 5-hexenoate (4e)
to the internal olefin, the complex 3 smoothly removed the
allyl group without any isomerization, giving 5e in >99%
yield. The chemoselective cleavage of allyl esters in multi-
functional molecules such as 6a and 7a was also found to
be possible. Each of the tert-butyl, Fmoc, and allyl groups
in a protected a-amino carboxylic acid or dipeptide was
1a
1a
2
1
1
1
>99
>99
>99
>99
99
11
12
13
14
15
16
17
18
19
20
a
1a
1a
1a
3
1bd
1cd
1dd
1ed
1fd
1gd
1hd
0.5
0.5
0.5
3
3
3
>99
>99
>99
>99
>99
>99
>99
500 CH3OH
500 CH3OH
3
The reactions were carried out at 30 °C with [1] = 100 mM unless
otherwise specified.
b
S/C = substrate/catalyst.
c
Determined by 1H NMR analysis.
d
[1] = 500 mM.
[1] = 1 M.
2% of allyl 2-phenylethyl ether was formed.
50 °C.
e
f
g
h
Reaction was operated under an argon stream at 70 °C.