Palladium-catalyzed regiospecific tandem allylation of 2-aminophenols using 2-butene-1,4-diol

Article abstract of DOI:10.1016/j.tetlet.2004.01.128

The direct activation of C-O bonds in 2-butene-1,4-diol by palladium complexes has been accelerated by carrying out the reactions in the presence of a titanium reagent. Palladium-catalyzed regiospecific tandem allylation of 2-aminophenols with 2-butene-1,4-diol leads to 3,4-dihydro-2-vinyl-2H-1,4- benzoxazines.

Full text of DOI:10.1016/j.tetlet.2004.01.128

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2693–2697
Palladium-catalyzed regiospecific tandem allylation of
2-aminophenols using 2-butene-1,4-diol
Shyh-Chyun Yang,* Hwe-Chen Lai and Yan-Chiu Tsai
Graduate Institute of Pharmaceutical Sciences, Kaohsiung Medical University, Kaohsiung 80708, Taiwan, ROC
Received 12 August 2003; revised 24 November 2003; accepted 14 January 2004
Abstract—The direct activation of C–O bonds in 2-butene-1,4-diol by palladium complexes has been accelerated by carrying out the
reactions in the presence of a titanium reagent. Palladium-catalyzed regiospecific tandem allylation of 2-aminophenols with
2-butene-1,4-diol leads to 3,4-dihydro-2-vinyl-2H-1,4-benzoxazines.
Ó 2004 Elsevier Ltd. All rights reserved.
Morpholine derivatives have aroused increasing interest
due to their presence in a large number of therapeuti-
cally and biologically active compounds.1 Numerous
1,4-benzoxazine derivatives have been prepared to lead
to biologically active compounds. Research activities in
this area continue to generate new compounds having
unusual skeletons.2 A principal goal of organometallic
chemistry is the catalytic synthesis of organic com-
pounds by fine tuning the chemistry of organic ligands
covalently bound to transition metals. Most organo-
metallic chemistry has focused on complexes with
covalent metal–carbon or metal–hydrogen bonds.
Transition metal g3-allyl complexes, as well as transition
metal r-alkyl complexes, play important roles as active
species and key intermediates in many reactions cata-
lyzed by transition metal complexes.3 The platinum
group transition metals, in particular palladium and
rhodium, have become workhorse elements in many
commercialized catalytic processes that include hydro-
genations, hydroformylations, acetic acid production,
and other C–C and C–H bond forming processes.4
Although carbon–oxygen, carbon–nitrogen, or carbon–
sulfur bonds are found in the majority of important
organic molecules, catalytic organometallic reaction
chemistry that leads to the formation of carbon–het-
eroatom bonds is less common than that forming car-
bon–carbon and carbon–hydrogen bonds. Moreover,
the construction of C–N bonds in amines is particularly
rare.5 In large part, routes to the necessary reactive
intermediates for such catalysis and the fundamental
reactions required of such intermediates are poorly
developed. The palladium-catalyzed allylation of
nucleophiles is an established, efficient, and highly ste-
reo- and chemoselective method, which has been widely
applied to organic chemistry. The catalytic cycle
requires the formation of the cationic g3-allylpalla-
dium(II) complex, an intermediate that is generated by
oxidative addition of allylic compounds including allylic
halides,6 acetates,7 and carbonates8 to a Pd(0) complex
and which can be attacked by nucleophiles at both ter-
mini of the allylic system. However, there are few
reports on palladium(0)-catalyzed reaction of bifunc-
tional allylic diacetates and dicarbonates with nucleo-
philes featuring their bifunctionality.9 Recently, Lhoste
reported that (Z)-1,4-bis(methoxycarbonyloxy)but-2-
ene reacted with o-aminophenols in the presence of a
palladium catalyst, giving 3,4-dihydro-2-vinyl-2H-1,4-
benzoxazines.10 However, there have been only limited
and sporadic reports dealing with the direct cleavage of
the C–O bond of allylic alcohols on interaction with a
transition metal complex.11 Successful applications
using allylic alcohols directly in catalytic processes are
even more limited. This apparently stems from the poor
capability of a nonactivated hydroxyl to serve as a
leaving group.12 We have recently reported our attempts
and some successful applications of a process involving
C–O bond cleavage with direct use of allylic alcohols
catalyzed by palladium complexes.13 In this paper, we
wish to report the catalytic palladium complex, which
mediates regiospecific tandem allylation of 2-amino-
phenols with 2-butene-1,4-diol directly for the con-
struction of 3,4-dihydro-2-vinyl-2H-1,4-benzoxazines.
Initially, we treated a mixture of 2-aminophenol (1a,
1.5 mmol) and allyl acetate (2a, 1.2 mmol) in the
* Corresponding author. Fax: +886-(7)-3210683; e-mail: scyang@
kmu.edu.tw
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.128

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S.-C. Yang et al. / Tetrahedron Letters 45 (2004) 2693–2697
presence of Pd(OAc)2 (1 mol %), PPh3 (4 mol %), and
ꢀ
molecular sieves (MS 4 A) (200 mg) in refluxing benzene
reaction time increased the yields of 6a and 7a to 39%
and 47%, respectively (entry 12). The reaction is
ꢀ
accompanied by formation of water. Without MS 4 A
(3 mL) under nitrogen for 3 h. 2-Allylaminophenol (3)
and 2-diallylaminophenol (4) were formed in 68% and
23% yields, respectively (Scheme 1). The 1H and 13C
NMR of NCH2 appear at d 3.77 and 47.1 ppm for 3 and
at d 3.51 and 57.5 ppm for 4. This regiospecificity is in
agreement with the fact that the nitrogen nucleophile is
generally more reactive than the oxygen nucleophile
toward p-allylpalladium complexes.14 Similarly, direct
allylation of 1a with allyl alcohol (2b) in the presence of
25 mol % of Ti(OPri)4 gave 3 and 4 in 61% and 31%
yields, respectively.
for water removal, the yields of products were decreased
(entry 13). The reaction did not occur in the absence of
the phosphine ligand (entry 14). As expected, increasing
the relative amount of the 2-aminophenol favored the
formation of the desired cyclic compound 6a (entries 2,
15, and 16). It was known that several factors, such as
the solvent and nature of the nucleophile, can alter the
product pattern in metal-catalyzed allylation.15 At
50 °C, eight solvents were investigated, dioxane, HMPA,
and DMF gave worst (entries 1 and 17–23). In the
reaction under reflux, the yields of products 6a and 7a
were increased (entries 2 and 24–26). Benzene gave the
best results.
Then, we examined the extension of this reaction to the
bifunctional 2-butene-1,4-diol. The palladium-catalyzed
cyclization of 2-aminophenol with 2-butene-1,4-diol
directly was investigated under various conditions
(Scheme 2). When a mixture of 2-aminophenol (1a,
1.5 mmol) and 2-butene-1,4-diol (5, 1.2 mmol) was
heated in the presence of catalytic amounts of Pd(acac)2
(0.075 mmol), PPh3 (0.3 mmol), Ti(OPri)4 (0.75 mmol),
A comparative study of different catalysts in benzene
was reported (Table 2). As the catalyst precursor,
Pd(acac)2 (entry 1), Pd(OAc)2 (entry 2), Pd(OCOCF3)2
(entry 3), Pd2(dba)3 (entry 4), and PdCl2(MeCN)2 (entry
5) showed good catalytic activity. Other palladium
complexes such as PdCl2 (entry 6) and Pd(PPh3)4 (entry
7) were less active and gave lower yields. Many reports
have indicated16 that chloride ions can strongly influence
the catalytic activity of palladium catalysts, and it
seemed reasonable that this factor might be responsible
for the low reactivity of PdCl2 in the present system.
However, using Pd(PPh3)4 with extra PPh3 as catalyst
increased the yield of products (entry 8). Screening of
various monodentate ligands (entries 1 and 9–20)
showed that PPh3, (2-MePh)3P, (2-furyl)3P, and (3-
MePh)3P were the most effective ligands. The bidentate
ligand including dppm (entry 21), dppe (entry 22), dppp
(entry 23), dppb (entry 24), and dpph (entry 25)
decreased the yield of products. Dppm and dppe gave
only 6a.
ꢀ
and MS 4 A (200 mg) in benzene (5 mL) under nitrogen
at 50 °C for 6 h, 3,4-dihydro-2-vinyl-2H-1,4-benzoxazine
(6a) together with 7a were formed in 32% and 27%,
1
respectively (entry 1 in Table 1). The H and 13C NMR
of OCH appear at d 4.56 and 74.4 ppm for 6a and at d
4.60 and 74.4 ppm for 7a. In the reaction under reflux
for 6 h, the yields of products 6a and 7a were increased
to 45% and 45%, respectively (entry 2). Increasing the
reaction time favored the formation of the cyclic com-
pound 7a (entries 2–4). The absence of a titanium agent
gave only 40% yield of products (entry 5). Decreasing
the amount of Ti(OPri)4 decreased the yields of products
(entry 6). The effect of addition of Ti(OPri)4 to promote
the palladium-catalyzed allyl-OH bond cleavage
remarkably enhanced both the reaction rate and yield.
Titanium reagents such as Ti(OBu)4 (entry 7), Ti(OBui)4
(entry 8), and Ti[O(CH2)17CH3]4 (entry 9) were also
effective for the allylation. TiCl4 (entry 10) and Ti(OEt)4
(entry 11) did not promote the reaction to any great
extent. In the presence of Ti(OEt)4, increasing the
We also studied the influence of substituents on the
2-aminophenol on the reactivity of the amination of
2-butene-1,4-diol (5) using Pd(acac)2, PPh3, and
Ti(OPri)4. The results collected in Table 3 showed that
RO
OH
2a, R=COCH3
b, R=H
Pd catalyst
NH2
OH
OH
N
+
N
H
1a
4
3
Scheme 1.
OH
O
OH
Pd catalyst
O
N
+
+
N
H
NH2
N
O
OH
1a
6a
5
7a
Scheme 2.

S.-C. Yang et al. / Tetrahedron Letters 45 (2004) 2693–2697
2695
Table 1. Reaction of 2-aminophenol (1a) with 2-butene-1,4-diol (5)a
Table 2. Reaction of 2-aminophenol (1a) with 2-butene-1,4-diol (5):
palladium catalyst and phosphine ligand effectsa
Entry
Titanium
reagent
Solvent
Yield (%)b (6a:7a)
Entry
Palladium
Ligand
Yield (%)b
(6a:7a)
1
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
––
Benzenec
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
59 (55:45)
90 (50:50)
80 (62:38)
76 (39:61)
40 (68:32)
56 (58:42)
84 (41:59)
89 (45:55)
78 (49:51)
3 (100:0)
48 (73:27)
86 (45:55)
59 (60:40)
0
2
1
Pd(acac)2
Pd(OAc)2
PPh3
PPh3
PPh3
PPh3
90 (50:50)
89 (61:39)
85 (55:45)
82 (50:50)
76 (90:10)
20 (100:0)
42 (56:44)
77 (54:46)
63 (98:2)
3d
4e
2
3
Pd(OCOCF3)2
Pd2(dba)3
5
4
f
6
Ti(OPri)4
5
PdCl2(MeCN)2 PPh3
7
Ti(OBu)4
Ti(OBui)4
6
PdCl2
PPh3
––
8
7
Pd(PPh3)4
Pd(PPh3)4
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
Pd(acac)2
9
Ti[O(CH2)17CH3]4 Benzene
8
PPh3
Bu3P
(PhO)3P
10
11
12e
13g
14h
15i
16j
17
18
19
20
21
22
23
24
25
26
TiCl4
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Toluenec
THFc
9
Ti(OEt)4
Ti(OEt)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
Ti(OPri)4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
58 (97:3)
(2-MePh)3P
(2-Furyl)3P
(2-Pyridyl)Ph2P
(3-MePh)3P
(4-MePh)3P
(4-MeOPh)3P
(4-FPh)3P
(4-ClPh)3P
(2,6-Di-MeOPh)3P
(2,4,6-Tri-MeOPh)3P
dppm
81 (24:76)
85 (61:39)
37 (97:3)
76 (70:30)
86 (32:68)
58 (43:57)
59 (60:40)
56 (71:29)
58 (62:38)
18 (0:100)
9 (0:100)
5 (64:36)
78 (31:69)
71 (57:43)
83 (47:53)
86 (56:44)
68 (54:46)
65 (66:34)
44 (96:4)
MeCNc
49 (58:42)
13 (100:0)
2 (100:0)
c
CH2Cl2
Dioxanec
HMPAc
DMFc
21 (100:0)
22 (100:0)
48 (98:2)
dppe
dppp
Toluene
THF
MeCN
dppb
dpph
56 (85:15)
58 (67:33)
a Reaction conditions: 1a (1.5 mmol),
(0.075 mmol), PPh3 (0.3 mmol), titanium reagent (0.75 mmol), and
5
(1.2 mmol), Pd(acac)2
a Reaction conditions: 1a (1.5 mmol),
5
(1.2 mmol), Pd catalyst
(0.075 mmol), phosphine ligand (0.3 mmol), Ti(OPri)4 (0.75 mmol),
ꢀ
ꢀ
molecular sieves 4 A (200 mg) in a solvent were refluxed for 6 h.
and molecular sieves 4 A (200 mg) in benzene were refluxed for 6 h.
b Isolated yield was based on 5.
c Stirred at 50 °C.
b Isolated yield was based on 5.
d Reflux for 3 h.
e Reflux for 9 h.
butene-1,4-diol (5) worked well with 2-aminophenols
containing electron-donating groups, giving generally
good yields of the corresponding cyclic compounds
(entries 1 and 2). Allylation of 2-amino-4-chlorophenol
(1d) gave 6-chloro-3,4-dihydro-2-vinyl-2H-1,4-benzox-
azine (6d) and (E)-1,4-bis(6-chloro-3,4-dihydro-2-vinyl-
2H-1,4-benzoxazin-4-yl)but-2-ene (7d) in 41% and 19%
yields, respectively (entry 3). Conversely, 2-aminophe-
nols having strong electron-withdrawing groups, such as
f 0.25 mmol of Ti(OPri)4 was used.
g
ꢀ
Without MS 4 A.
h Without PPh3.
i 0.75 mmol of 5 was used.
j 1.8 mmol of 5 was used.
the nature of the substituent had an influence on the
reaction rate and the product yield. The amination of 2-
Table 3. Reaction of 2-aminophenols (1b–h) with 2-butene-1,4-diol (5)a
R
OH
O
OH
Pd(acac)2-PPh3
Ti(OPri)4, MS 4A
OH
O
N
R
R
+
+
N
H
NH2
N
O
1
6
R
5
7
Entry
1
R
Yields (%)b
1
2
3
4
5
6
7
1b
1c
1d
1e
1f
4-Me
5-Me
6b 59
6c 40
6d 41
6e 44
6f 32
6g 27
6h 37
7b 27
7c 49
7d 19
4-Cl
4-NO2
5-NO2
4-Cl, 5-NO2
4-SO2C2H5
1g
1h
a
i
ꢀ
Reaction conditions: 1 (1.5 mmol), 5 (1.2 mmol), Pd(acac)2 (0.075 mmol), PPh3 (0.3 mmol), Ti(OPr )4 (0.75 mmol), and molecular sieves 4 A (200 mg)
in benzene were refluxed for 6 h.
b Isolated yield was based on 5.

2696
OH
S.-C. Yang et al. / Tetrahedron Letters 45 (2004) 2693–2697
(b) DÕAmbra, T. E.; Estep, K. G.; Bell, M. R.; Eissenstat,
OH
OH
R
1
M. A.; Josef, K. A.; Ward, S. J.; Haycock, D. A.;
Baizman, E. R.; Casiano, F. M.; Beglin, N. C.; Chippari,
S. M.; Grego, J. D.; Kulling, R. K.; Daley, G. T. J. Med.
Chem. 1992, 35, 124–135; (c) Largeron, M.; Dupuy, H.;
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3. Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal
Chemistry; University Science Books: Mill Valley, CA,
1987.
OH
Pd0Ln
NH2
Pd
X
R
OH
[Pd]
- H2O or
HOTi(OR)3
N
H
L
OH
8
9
X = OH or OTi(OR)3
5
OH
O
5, Pd(0)
-PdHXLn
R
R
7
N
H
N
H
ꢁ
4. (a) Gagne, M. R.; Nolan, S. P.; Marks, T. J. Organomet-
Pd
X
allics 1990, 9, 1716–1718; (b) Walsh, P. J.; Baranger, A.
M.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 1708–
1719; (c) McGrane, P. L.; Jensen, M.; Livinghouse, T. J.
Am. Chem. Soc. 1992, 114, 5459–5460; (d) Baranger, A.
M.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1993,
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ker, D.; Philippot, K. J. Organomet. Chem. 1994, 469,
221–228.
L
6
10
Scheme 3.
the nitro group (entries 4–6), gave lower chemical yields.
These differences in reactivity are likely due to the
nucleophilicity of the corresponding 2-aminophenol.
2-Amino-4-chloro-5-nitrophenol (1g) gave only product
6g in 27% yield (entry 5); the lower yield observed may
arise from the nature of the nitro group. The more acidic
compound is probably less reactive in the attack on the
p-allyl complex (entries 4–7).
5. For instance: (a) Tsuji, J. In The Chemistry of the Metal–
Carbon Bond; Hartley, F. R., Patai, S., Eds.; John Wiley &
Sons: Chichester, UK, 1985; Vol. 3, Chapter 3.2; (b) Sato,
F. In The Chemistry of the Metal–Carbon Bond; Hartley,
F. R., Patai, S., Eds.; John Wiley & Sons: Chichester, UK,
€
1985; Vol. 3, Chapter 3.3; (c) Backvall, J. E. In Advances in
Metal-Organic Chemistry; Liebeskind, L. S., Ed.; JAI:
Greenwich, CT, 1989; Vol. 1.
ꢀ
6. (a) Connell, R. D.; Rein, T.; Akermark, B.; Helquist, P. J.
A plausible reaction pathway for this regiospecific for-
mation is shown in Scheme 3. Diol 5 or an allyl titanate,
formed by an alcohol exchange reaction between 5 and
titanium reagent, reacts with Pd(0) species generated in
situ to afford the p-allylpalladium intermediate 8.
Intermolecular nucleophilic substitution of the amino
group of 1 takes place at the less hindered terminus of
the p-allyl system to give an allylic amine 9. Intramo-
lecular nucleophilic attack on the second p-allylpalla-
dium intermediate 10 at the more substituted internal
allylic carbon atom produces 6. Compound 7 is
obtained by two successive nucleophilic substitutions of
compound 6 on the diol 5 in the presence of palladium.
Org. Chem. 1988, 53, 3845–3849; (b) Sakamoto, M.;
Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1996, 69,
1065–1078.
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In summary, we have shown that palladium(0)-cata-
lyzed regiospecific tandem allylation of 2-aminophenols
using 2-butene-1,4-diol directly is a simple and efficient
route for 3,4-dihydro-2-vinyl-2H-1,4-benzoxazines for-
mation. The addition of Ti(OPri)4 to promote the pal-
ladium-catalyzed allyl-OH bond cleavage remarkably
enhanced both the reaction rate and yield. Increasing
the relative amount of the 2-aminophenol favored the
formation of the desired cyclic compound 6a.
Acknowledgements
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We gratefully acknowledge the National Science
Council of the Republic of China for financial support.
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