Facile and selective deallylation of allyl ethers using diphosphinidenecyclobutene-coordinated palladium catalysts

Article abstract of DOI:10.1021/jo049732q

(π-Allyl)palladium triflate bearing a 1,2-bis(4-methoxyphenyl)-3,4- bis(2,4,6-tri-tert-butylphenylphosphinidene)cyclobutene ligand (DPCB-OMe), [Pd(η³-C₃H₅)(DPCB-OMe)]OTf, efficiently catalyzes deallylation of a variety of allyl ethers in aniline to give corresponding alcohols in high yields under mild conditions. The reactions can be performed in air without loss of a variety of functionalities including vinyl, alkynyl, hydroxy, acetoxy, silyloxy, and acetal groups. Allyl 2-allyloxybenzoate selectively undergoes deallylation of the allyloxy group to give allyl salicylate in quantitative yield.

Full text of DOI:10.1021/jo049732q

F a cile a n d Selective Dea llyla tion of Allyl Eth er s Usin g
Dip h osp h in id en ecyclobu ten e-Coor d in a ted P a lla d iu m Ca ta lysts
Hiromi Murakami,† Tatsuya Minami,‡ and Fumiyuki Ozawa*,†
Research Center for Elements Science, Institute for Chemical Research, Kyoto University, Uji,
Kyoto 611-0011, J apan, and Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University, Sumiyoshi-ku, Osaka 558-8585, J apan
ozawa@scl.kyoto-u.ac.jp
Received February 14, 2004
(π-Allyl)palladium triflate bearing a 1,2-bis(4-methoxyphenyl)-3,4-bis(2,4,6-tri-tert-butylphenylphos-
phinidene)cyclobutene ligand (DPCB-OMe), [Pd(η3-C3H5)(DPCB-OMe)]OTf, efficiently catalyzes
deallylation of a variety of allyl ethers in aniline to give corresponding alcohols in high yields under
mild conditions. The reactions can be performed in air without loss of a variety of functionalities
including vinyl, alkynyl, hydroxy, acetoxy, silyloxy, and acetal groups. Allyl 2-allyloxybenzoate
selectively undergoes deallylation of the allyloxy group to give allyl salicylate in quantitative yield.
SCHEME 1
In tr od u ction
The palladium-catalyzed allylation is a useful means
of constructing C-C, C-N, and C-O bonds in organic
synthesis.1 This reaction generally employs allylic esters
derived from allylic alcohols as allylation agents. On the
other hand, there have been considerable efforts to
develop the catalysts that enable direct conversion of
allylic alcohols into allylation products. This is mainly
because such a reaction forms water as the only coproduct
and possibly serves as an environmentally benign process
with high atom efficiency.2 However, due to the poor
leaving ability of the OH group, most of the catalysts so
far examined required rather severe conditions; other-
wise, the reactions were conducted with in situ activation
of allylic alcohols using considerable amounts of additives
such as Lewis acids.3-5
We recently found that (π-allyl)palladium complex 1
bearing 1,2-bis(4-methoxyphenyl)-3,4-bis(2,4,6-tri-tert-bu-
tylphenylphosphinidene)cyclobutene (DPCB-OMe) effi-
ciently catalyzes direct conversion of allylic alcohols into
N- and C-allylation products under mild conditions (eq
1).6 We proposed a novel catalytic process given in
Scheme 1. Thus, unlike common allylation reactions
involving oxidative addition of a C-O bond to a Pd(0)
species, it was considered that the C-O bond of allylic
alcohols is cleaved by the action of hydridopalladium
complex A. Proton-transfer from the Pd to the OH group
in C, followed by elimination of water from D, forms
π-allyl complex B. We reasoned that strong π-accepting
ability of DPCB-OMe ligand as an sp2-hybridized phos-
phorus compound7 efficiently stabilizes D as a Pd(0)
† Kyoto University.
‡ Osaka City University.
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10.1021/jo049732q CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/26/2004
4482
J . Org. Chem. 2004, 69, 4482-4486

Facile and Selective Deallylation of Allyl Ethers
species by π-back-donation to facilitate the proton-
transfer in C.
As described below, complex 1 exhibits high catalytic
performance for deallylation of a variety of allylic ethers
in aniline. The reactions can be conducted in air without
loss of various functionalities.
Resu lts a n d Discu ssion
First of all, the catalytic reactions were examined with
aliphatic ethers as substrates. The results are sum-
marized in Table 1. As a representative example, allyl
hexyl ether (2; 71 mg, 0.50 mmol) was treated with 2 mol
% of 1 in aniline (0.46 mL, 5.0 mmol) at 50 °C. The
reaction was completed in 2 h as confirmed by GLC. The
reaction mixture was poured into a 6 N HCl solution and
extracted with Et2O. Purification of the ethereal extract
by silica gel column chromatography gave a 95% yield of
n-hexanol. The deallylation could be performed also in
toluene (0.8 mL) as a solvent using 2 equiv of aniline
(1.0 mmol), while the reaction became somewhat slower
(6 h) at the same temperature.
A variety of aliphatic ethers (3-14) were similarly
deallylated. The present catalysis could be successfully
applied to the substrates having vinyl (3), alkynyl (5),
hydroxy (5), acetoxy (7, 8), silyloxy (9, 10), and acetal
(11-14) groups. In particular, TBDMS (9, 10), THP (11),
and MOM (12) as typical protecting groups for alcohols
remained unchanged in the catalytic reactions. Thus, the
allylic deprotection catalyzed by 1 is compatible with
other protection methods of alcohols. All reactions pro-
ceeded quantitatively as confirmed by GLC. 4-Pentenol
derived from 3 was significantly volatile, and its isolated
yield was somewhat lowered (84%). Since the products
having acetal units were unstable under acidic condi-
tions, their isolation was carried out directly by column
chromatography without acid extraction.
In this paper, we report application of this novel
catalysis. A particular interest is focused on catalytic
deallylation of allylic ethers (eq 2). Thus, the protection
and deprotection of alcohols are central subjects in
organic synthesis, especially in carbohydrate synthesis.8
The allyl group is utilized as a versatile protecting group,
owing to the easy introduction as well as the high
stability of the resulting ethers under a wide range of
reaction conditions. The deprotection was originally
achieved by two-steps procedures, consisting of transition
metal-catalyzed isomerization of allyl group and hydroly-
sis or oxidative cleavage of the resulting enol ethers.9
More recently, one-step procedures have been investi-
gated as more convenient methods (e.g., DDQ,10 NaBH4/
I2,11 TiCl3/Li/THF,12 Yb(OTf)3,13 electrochemically gener-
ated Ni,14 ZrCl4/NaBH4,15 TMSCl/NaI,16 CeCl3‚7H2O/
NaI,17 TiCl4/Bu4NI,18 t-BuLi,19 I2,20 p-TSA,21 and SmI222).
The palladium-catalyzed allylation has been examined
as a simple approach to catalytic deprotection.8a,23-28
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Kocienski, P. J . Chem. Soc., Perkin Trans. 1 2001, 57, 2109.
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Synthesis, 3rd ed.; J ohn Wiley and Sons: New York, 1999.
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Tetrahedron Lett. 1996, 37, 6603. (b) Vate`le, J . M. Tetrahedron Lett.
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64, 4211.
Deallylation of aromatic ethers could be performed
much more easily with 0.1 mol % of 1 at 30 °C (eq 3).
Table 2 lists the results for allyl phenyl ethers having a
variety of para-substituents. The reactions were com-
pleted within 30 min, except for 21 and 22 having the
acetyl and bromo substituents, giving almost quantitative
yields of phenols after chromatographic purification. The
reaction of 15 was accomplished in a few minutes, when
the amount of catalyst was increased to 0.5 mol %.
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We next examined deprotection of allyloxycarbonyl
(Alloc), which is known to be a useful protecting group
for the hydroxy groups in carbohydrates and amino and
imido groups in nucleoside bases and peptides. Very
recently, deprotection of this group from several carbam-
ates has been examined with Pd(PPh3)4 (1 mol %) and
K2CO3 (3 equiv) in MeOH, where the reactions take hours
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J . Org. Chem, Vol. 69, No. 13, 2004 4483

Murakami et al.
TABLE 1. Dea llyla tion of Allyl Alk yl Eth er s Ca ta lyzed by 1 in An ilin ea
a
All reactions were run at 50 °C using 0.5 mmol of allyl ethers and 2 mol % of 1 in aniline (0.46 mL, 10 equiv).
TABLE 2. Dea llyla tion of P a r a -Su bstitu ted Allyl P h en yl
Eth er s (eq 3)a
for completion at room temperature.27 On the other hand,
in the presence of 1 mol % of 1 in aniline, the deprotection
from cyclohexylcarbamate 23 was completed within 1 min
at 30 °C (eq 4).29
compd
Y
time (min)
yield (%)
15
16
17
18
19
20
21
22
H
CN
CHO
CO2Me
CH2OH
CH(OH)Me
COMe
Br
20
20
20
20
20
30
60
180
98
99
99
99
99
99
99
>99
a
All reactions were run at 30 °C using 0.5 mmol of allyl ethers
and 0.1 mol % of 1 in aniline (0.46 mL, 10 equiv).
Highly selective deallylation was observed for 2-, 3-,
and 4-hydroxybenzoic acid derivatives having allyl ether
and allyl ester moieties (24-26) (eqs 5-7). The reaction
site was dramatically changed with substitution patterns.
Thus, the ortho isomer 24 exclusively underwent deallyl-
ation from the allyl ether part, even in the presence of
an excess of aniline, to give a quantitative yield of allyl
salicylate 27 (eq 5).30 On the other hand, treatment of
the meta and para isomers 25 and 26 with 1 equiv of
aniline in toluene caused deallylation from the allyl ester
part in 93 and 92% selectivities, respectively (eqs 6 and
7), while both allyl groups were eliminated when the
reactions were conducted with more than 2 equiv of
aniline.
In contrast to the above reactions using catalyst 1, no
selective deallylation proceeded with Pd(PPh3)4 as a
catalyst (eq 8). For example, treatment of 24 with 1 equiv
of aniline in THF31 in the presence of 2 mol % of
Pd(PPh3)4 caused simultaneous elimination of the two
allyl groups to give a quantitative yield of salicylic acid,
(29) The reaction was finished in 10 min with 0.1 mol % of 1.
(30) Product 27 remained unchanged for 24 h at 30 °C in the reaction
solution, but converted to salicylic acid at 50 °C.
4484 J . Org. Chem., Vol. 69, No. 13, 2004

Facile and Selective Deallylation of Allyl Ethers
SCHEME 2
catalyzed systems recently reported.23-28 Furthermore,
it was noted that only one of the allyl groups in allyl
allyloxybenzoates 24-26 can be deprotected, whereas the
Pd(PPh3)4 catalyst shows no notable selectivity. Selective
deallylation from the allyl ether part in 24 is of particular
interest, because allyl ethers are generally less reactive
than allyl esters toward palladium-catalyzed allylations.
Since the meta and para isomers 25 and 26 underwent
deallylation from the allyl ester part, the unusual selec-
tivity observed for 24 should be related to a neighboring
group effect. Scheme 2 illustrates a possible reaction
process. First, the substrate 24 is coordinated to A.
Proton-transfer from the Pd to the ether oxygen in E,
followed by transfer of the Pd(DPCB-OMe) moiety from
the carbonyl group to the allyl group in F , forms G. It
was considered that the protonation takes place at the
ether oxygen with higher electron density, rather than
the ester oxygen. Elimination of 27 from G affords B,
which subsequently reacts with aniline to reproduce A.
Further applications of this highly selective catalysis will
be reported in due course.
along with byproduction of N,N-diallylaniline. This reac-
tion pattern was preserved when a smaller amount of
aniline was employed. Thus, the reaction with 0.5 equiv
of aniline afforded 50% yield of salicylic acid; i.e., 50% of
24 remained unreacted. Similarly, 25 and 26 reacted with
aniline (1 equiv) in THF in the presence of Pd(PPh3)4 (2
mol %) to give 3- and 4-hydroxybenzoic acids.
Exp er im en ta l Section
Allylic substrates were prepared by allylation of the corre-
sponding alcohols. Catalyst 1 was synthesized as previously
reported.6b,32
Ca ta lytic Dea llyla tion . Complex 1 (11.1 mg, 0.010 mmol)
and biphenyl (15.4 mg, 0.10 mmol, GLC standard) were placed
in a 10 mL Schlenk tube and dissolved in allyl alkyl ether (0.50
mmol) and aniline (0.46 mL, 5.0 mmol). The solution was
stirred at 50 °C until the starting ether disappeared in GLC.
The mixture was extracted with a 6 N HCl solution and Et2O
and subsequently subjected to silica gel column chromatog-
raphy to afford deallylation products as listed in Table 1.
Deallylation products derived from 11-14 were isolated
without acid extraction. The reactions listed in Table 2 and
eqs 5-7 were similarly conducted using 0.1 mol % of 1 (0.56
mg, 0.50 µmol) at 30 °C. The 1H NMR spectra of reaction
products (except for 27-29) were identical with those of the
starting alcohols.
Su m m a r y
We demonstrated that complex 1 bearing a DPCB-OMe
ligand serves as a highly efficient catalyst for deallylation
of allylic compounds. The reactions could be conducted
simply by mixing allylic substrates with aniline as the
scavenger of allyl group. The observed reactivity was
comparable or higher than that of the palladium-
Since 27-29 were known compounds, they were character-
1
ized by H NMR and IR spectroscopy. The site of deallylation
was clearly confirmed by 1H NMR spectroscopy. Thus, the
(31) THF was used as a solvent because the Pd(PPh3)4-catalyzed
reaction was significantly slow in toluene.
(32) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.;
Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40, 4501.
J . Org. Chem, Vol. 69, No. 13, 2004 4485

Murakami et al.
starting compounds 24-26 exhibited two sets of methylene
proton signals at δ 4.6 and 4.8, which were assignable to the
allyl ether and allyl ester groups, respectively. The former
signal disappeared in the spectrum of 27, whereas the latter
disappeared in the spectra of 28 and 29.
10.8, 1.5, 1.5 Hz, 1H), 5.43 (ddt, J ) 17.1, 1.5, 1.5 Hz, 1H),
6.06 (ddd, J ) 17.1, 10.8, 5.1 Hz, 1H), 6.96 (d, J ) 8.7 Hz,
2H), 8.06 (d, J ) 8.7 Hz, 2H). IR (KBr): 3080, 3025, 2982,
2942, 2875, 2253, 1677, 1605, 1428, 1253, 907, 733 cm-1
.
Dea llyla tion of 24-26 Ca ta lyzed by P d (P P h 3)4 (eq 8).
As an example, 25 (218.3 mg, 1.0 mmol), PhNH2 (94.6 mg, 1.01
mmol), and Pd(PPh3)4 (23.1 mg, 0.020 mmol) were placed in a
10 mL Schlenk tube and dissolved in THF (1 mL) at room
temperature. The solution was stirred for 30 min. GLC
analysis of the solution revealed complete conversion of aniline
to diallylaniline.6a The product was extracted with AcOEt and
a 6 N HCl solution. Organic extract was dried over MgSO4
and concentrated to dryness to give 3-hydroxybenzoic acid as
a white solid (139.0 mg, 99%).
Allyl Sa licyla te (27).33 White solid. 1H NMR (CDCl3, 20
°C): δ 4.81 (ddd, J ) 5.7, 1.2, 1.2 Hz, 2H), 5.45 (ddt, J ) 10.1,
1.2, 1.2 Hz, 1H), 5.50 (ddt, J ) 17.1, 1.2, 1.2 Hz, 1H), 6.10
(ddd, J ) 17.1, 10.1, 5.7 Hz, 1H), 7.05 (dd, J ) 8.4, 0.9 Hz,
1H), 7.15 (ddd, J ) 7.8, 7.2, 0.9 Hz, 1H), 7.56 (ddd, J ) 8.4,
7.2, 1.8 Hz, 1H), 8.20 (dd, J ) 7.8, 1.8 Hz, 1H). IR (KBr): 3239,
3063, 3011, 2925, 2856, 1660, 1612, 1249, 995 cm-1
.
3-Allyoxyben zoic Acid (28).34 White solid. 1H NMR (CDCl3,
20 °C): δ 4.61 (ddd, J ) 5.1, 1.5, 1.5 Hz, 2H), 5.32 (ddt, J )
10.8, 1.5, 1.5 Hz, 1H), 5.45 (ddt, J ) 17.4, 1.5, 1.5 Hz, 1H),
6.07 (ddd, J ) 17.4, 10.8, 5.1 Hz, 1H), 7.18 (ddd, J ) 7.8, 2.4,
0.6 Hz, 1H), 7.38 (dd, J ) 7.8, 7.7 Hz, 1H), 7.63 (dd, J ) 2.4,
1.5 Hz, 1H), 7.72 (ddd, J ) 7.7, 1.5, 0.6 Hz, 1H). IR (KBr):
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research (Nos. 14078222,
14044091, and 15350060) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, J apan.
3286, 3074, 2830, 1716, 1604, 1263, 999 cm-1
.
4-Allyoxyben zoic Acid (29).35 White solid. 1H NMR (CDCl3,
20 °C): δ 4.62 (ddd, J ) 5.1, 1.5, 1.5 Hz, 2H), 5.33 (ddt, J )
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails and 1H NMR spectra of all new compounds and 27-29.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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4486 J . Org. Chem., Vol. 69, No. 13, 2004