Oxidation of benzyl chlorides and bromides to benzoic acids with 30% hydrogen peroxide in the presence of Na2WO4, Na2VO4, or Na2MoO4 under organic solvent-free conditions

Full text of DOI:10.1021/jo001796n

J . Org. Chem. 2001, 66, 3235-3237
3235
Oxid a tion of Ben zyl Ch lor id es a n d
Br om id es to Ben zoic Acid s w ith 30%
Hyd r ogen P er oxid e in th e P r esen ce of
Na WO , Na VO , or Na MoO u n d er
2 4 2 4 2 4
Or ga n ic Solven t-F r ee Con d ition s
environmental benign method will be a more attractive
and practical way from the point of view of synthetic
chemistry. In fact, Yadav has found that capsule mem-
brane supported phase-transfer-catalyzed (CM-PTC) oxi-
dation of benzyl chloride using hydrogen peroxide as the
oxidizing reagent. In that case, benzaldehyde was the
major product.8 Recently, Kumar also reported that
benzaldehyde could be successfully obtained using Ti
silicates having Bronsted acidity in the presence of
hydrogen peroxide and water through the oxidation of
benzyl chloride.9
Min Shi* and Yan-Shu Feng
Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Lu, Shanghai 200032, China
Herein, we wish to report that benzyl chloride 1a can
be directly oxidized to the corresponding benzoic acid 2a
in an eco-safer way using 30% hydrogen peroxide with
mshi@pub.sioc.ac.cn
+
-
Received December 27, 2000
Na
2
WO
4
2 3 8 17 3 4
‚2H O as a catalyst and [CH (n-C H ) N] HSO
as a phase-transfer agent (PTC) without any organic
solvents (Scheme 1). The oxidation of benzyl chloride
Due to deterioration of the environment, “green chem-
istry” has been receiving increasingly more attention
since the 1990s. In research of environment-benign
oxidants, many chemists try to use hydrogen peroxide
(
10.0 mmol) was first carried out by vigorously stirring
the biphasic reaction system (benzyl chloride and water)
at 90 °C with a large excess of 30% H (4.1 mL, 36
mmol), Na WO ‚2H O (66 mg, 0.2 mmol), and [CH (n-
(93 mg, 0.2 mmol). The yield of 2a was
only 15% (Table 1, entry 2). Using 30% H (3.0 mL, 26
O
2
1
2
to substitute for the commonly used inorganic oxidants
2
2
4
2
3
because water is the sole expected side product. Transi-
+
-
8 17 3 4
C H ) N] HSO
tion-metal-catalyzed oxidations with dilute hydrogen
peroxide under phase-transfer conditions are particularly
attractive because they are free of most drawbacks
2
O
2
mmol) under the same reaction conditions, 2a was
obtained in a lower yield (8%) (Table 1, entry 1). After
addition of 100 mg of 4 Å MS to the catalytic system, the
yield of 2a increased to 23% (Table 1, entry 3). These
results show that the 4 Å MS can promote this oxidation
encountered in homogeneous systems and allow use of
dilute aqueous solutions of the oxidant.3
,4,5b
A number of
chemists reported on the oxidation of primary and
secondary alcohols to ketones and acids with hydrogen
peroxide under aqueous/organic biphasic conditions, re-
2 2
reaction to some extent. Considering that H O may
decompose during the reaction, we employed a speed-
controlled addition system by a syringe pump to realize
continuous slow addition. The yields of 2a were signifi-
cantly increased using a syringe pump. The results are
shown in Table 1. The metal catalyst and phase-transfer
agent must coexist in this oxidation system (Table 1,
entries 5-7). The yield of 2a was raised to 83% in a
5
spectively. Later, it was found that some aliphatic and
aromatic aldehydes could also be oxidized to acids under
the same conditions.5f,6 However, it is well-known that
benzaldehydes or benzyl alcohols are basically prepared
7
from benzyl chlorides in industry or laboratory. Thus,
direct oxidation of benzyl chlorides to benzoic acids by
shorter period (10 h) if Na
2
WO
4
2
‚2H O, PTC, and 4 Å MS
(
1) (a) Sp a¨ th, E.; Pailer, M.; Schmid, M. Chem. Ber. 1941, 74, 1552.
were present in the reaction system (Table 1, entry 5).
The 4 Å MS was also crucial for achieving high yields of
(
b) Matsumoto, M.; Kobayashi, H.; Hotta, Y. J . Org. Chem. 1984, 49,
4
2
740. (c) R e` mi, L.; Pailer, L.; Xiao, L.; Catherine, H. Eur Pat. 0 424
42 A2, 1991.
(
2
a because the yield decreased to 60% in the absence of
2) Strukul, G., Ed. Catalytic Oxidations with Hydrogen Peroxide
as Oxidant; Kluwer: Dordrecht, The Netherlands, 1992.
3) (a) Payne, G. B.; Williams, P. H. J . Org. Chem. 1959, 24, 54. (b)
J acobson, S. E.; Mares F.; Zambri, P. M. J . Am. Chem. Soc. 1979, 101,
946.
4) For hydrogen peroxide epoxidation of olefins under aqueous/
4 Å MS (Table 1, entry 4). At lower temperatures such
as 50 or 70 °C, this oxidation process was sluggish. We
(
also confirmed that, using 1 mol % of Na
2
WO
4
2
‚2H O
6
(
catalyst, the yield of 2a decreased from 83% to 74% under
the same conditions. Thus, the best reaction conditions
were elucidated in Table 1, entry 5.
organic biphasic conditions, see: (a) Venturelo, C.; Alneri, E.; Ricci,
M. J . Org. Chem. 1983, 48, 3831. (b) Venturello, C.; D’Aloisio, R. J .
Org. Chem. 1988, 53, 1553. (c) Bortolini, O.; Di Furia, F.; Modena G.;
Seraglia, R. J . Org. Chem. 1985, 50, 2688. (d) Sato, K.; Aoki, M.;
Ogawa, M.; Hashimoto, T.; Noyori, R. J . Org. Chem. 1996, 61, 8310.
Besides Na
O and Na
2
WO
4
‚2H
4
2
O, we also tried to use Na
2 4
VO ‚
2
H
2
2
MoO
‚2H
2
O as catalyst in this catalytic
(
e) Sato, K.; Aoki, M.; Ogawa, M.; Hashimoto, T.; Panyella, D.; Noyori,
R. Bull. Chem. Soc. J pn. 1997, 70, 905.
5) (a) J acobson, S. E.; Muccigrosso, D. A.; Mares, F. J . Org. Chem.
979, 44, 921. (b) Bortolini, O.; Conte, V.; Di Furia, F.; Modena G. J .
oxidation system. However, the yields were merely 43%
and 72%, respectively (Scheme 2, Table 2). The best
(
1
oxidation catalyst is Na
2
WO
4
2
‚2H O.
Org. Chem. 1986, 51, 2661. (c) Barak, G.; Dakka, J .; Sasson, Y. J . Org.
Chem. 1988, 53, 3553. (d) Ishii, Y.; Yamawaki, K.; Ura, T.; Yamada,
H.; Yoshida, T.; Ogawa, M. J . Org. Chem. 1988, 53, 3587. (e) Zennaro,
R.; Pinna, F.; Strukul, G.; Arzoumanian, H. J . Mol. Catal. 1991, 70,
Various benzyl chlorides could also be converted to the
corresponding aromatic acids under the same reaction
conditions (Scheme 3, Table 3). The substrates bearing
electron-withdrawing groups could be more easily oxi-
2
69. (f) Venturello, C. Gambaro, M. J . Org. Chem. 1991, 56, 5924. (g)
Dengel A. C.; Griffith, W. P.; Parkin, B. C. J . Chem. Soc., Dalton Trans.
1
993, 2683. (h) Neumann, R.; Gara, M. J . Am. Chem. Soc. 1995, 117,
5
066. (i) Arends, I. W. C. E.; Sheldon, R. A.; Wallau, M.; Schuchardt,
U. Angew. Chem., Int. Ed. Engl. 1997, 36, 1144. (j) Sato, K.; Aoki, M.;
Takagi, J .; Noyori, R. J . Am. Chem. Soc. 1997, 119, 12386. (k) Sato,
K.; Takagi, J .; Aoki, M.; Noyori, R. Tetrahedron Lett. 1998, 39, 7549.
(7) (a) Mcewen, W. L. Org. Synth. 1955, 3, 788. (b) Angyal, S. J .;
Tetaz, J . R.; Wilson, J . G. Org. Synth. 1963, 4, 690. (c) Krohnke, F.
Angew. Chem. 1963, 75, 181. (d) Reid, W.; Bender, H. Chem. Ber. 1956,
89, 1574.
(l) Sato, K.; Aoki, M.; Noyori, R. Science. 1998, 281, 1646. (m) Sato,
K.; Aoki, M.; Takagi, J .; Zimmerann, K.; Noyori, R. Bull. Chem. Soc.
J pn. 1999, 72, 2287.
(8) Yadav, G. D.; Mistry, C. K. J . Mol. Catal. A: Chem. 1995, 102(2),
67.
(9) Kumar, R.; Bhaumik, A. Microporous Mesoporous Mater. 1998,
21(4-6), 497.
(6) Sato, K.; Hyodo, M.; Takagi, J .; Aoki, M.; Noyori, R. Tetrahedron
Lett. 2000, 41, 1439.
1
0.1021/jo001796n CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/11/2001

3
236 J . Org. Chem., Vol. 66, No. 9, 2001
Notes
Sch em e 1
Ta ble 3. Oxid a tion of Ben zyl Ch lor id e a n d Br om id es
10.0 m m ol) w ith 30% H2O2 (36.0 or 72.0 m m ol) in th e
P r esen ce of Na 2WO4‚2H2O (0.2 m m ol),
(
+
-
[
CH3(n -C8H17)3N] HSO4 (0.2 m m ol) a n d 4 Å MS (100 m g)
a t 90 °C
H2O2a (mL)
time (h)
yield (%)
b
product
Ta ble 1. Oxid a tion of Ben zyl Ch lor id e (10.0 m m ol) w ith
2b
2c
4.1
4.1
8.2
8.2
8.2
8.2
8.2
8.2
4.1
8.2
10
10
18
18
18
18
18
18
10
18
86
91
37
68
34
61
30
a
3
0% H2O2 a t 90 °C
2
2
2
2
2
2
d
e
f
H2O2 PTC Na2WO4‚2H2O 4 Å MS time yieldb
entry (mL) (mg)
(mg)
(mg)
(h)
(%)
g
h
i
1
2
3
4
5
6
7
3.0
4.1
4.1
4.1
4.1
4.1
4.1
93
93
93
93
93
66
66
66
66
66
24
24
24
24
10
10
10
8
15
23
60
83
28
47
100
c
c
c
c
c
2j
88
33
e
2h
100
100
100
a
Dropwised with a syringe pump at the speed of 0.5 mL per
93
hour. b Isolated yield. c Benzyl bromide was used. 32% of o-
d
e
a
CF PhCH OH was formed at the same time. Naphthylmethyl
bromide was used.
Benzyl chloride/H2O2/PTC/Na2WO4 ) 10:36:0.2:0.2 (mole ra-
3
2
tio). b Isolated yield. Added dropwise over 8 h with a syringe
c
pump.
Sch em e 4
Sch em e 2
Ta ble 2. Oxid a tion of Ben zyl Ch lor id e (10.0 m m ol) in
+
-
th e P r esen ce of [CH3(n -C8H17)3N] HSO4 (0.2 m m ol) a n d
1
00 m g 4 Å MS a t 90 °C w ith 30% H2O2 Dr op w ised w ith a
Syr in ge P u m p
under the same reaction conditions. The yield for oxida-
tion of benzyl bromide to benzoic acid was slightly higher
than that for benzyl chloride (Scheme 3, Table 3).
catalyst
time (h)
yielda (%)
Na2VO4‚2H2Ob
Na2MoO4‚2H2O
Na2WO4‚2H2Ob
18
18
10
43
72
83
b
The oxidation species in the present reaction system
is well documented in the literature.1
0,11
The resulting
a
Isolated yield. b 0.2 mmol of catalyst.
peroxy intermediates are transformed to the desired
5
a,f,k
benzoic acids by a Baeyer-Villiger mechanism.
Be-
Sch em e 3
cause the formation of o-trifluoromethylbenzyl alcohol 3d
was observed in the reaction, we believe that benzyl
chlorides or bromides were first hydrolyzed to the cor-
responding benzyl alcohols, before being oxidized to the
final products.1 To confirm this speculation, we carried
out the reaction of benzyl chloride 1a with 5 equiv of
water under our catalytic system at 90 °C and found that
1,5a
8
86 mg (82%) of benzyl alcohol 3a was indeed formed
(Scheme 4). On the basis of these results, we propose the
plausible mechanism of the oxidation process shown
above in Scheme 4.
In conclusion, we have developed a useful, practical,
and eco-safer protocol for the preparation of aromatic
acids from benzyl halides. J udging from the conditions
employed, this oxidation method showed have great
prospects in industrial applications. Efforts to elucidate
the scope and limitations of this oxidation system are
underway.
dized to the corresponding aromatic acids than those
having electron-donating ones. Thus, many arylaldehydes
having electron-donating groups need longer reaction
Exp er im en ta l Section
time and large excess of H
Table 3). For example, p-nitrobenzonic acid 2c was
obtained in 91% yield with 4.1 mL of H , but anisic
acid 2f was formed in only 34% yield with 8.1 mL of H
2 2
O to achieve higher yields
(
Gen er a l Meth od s a n d Ma ter ia ls. Melting points were
obtained with a Yanagimoto micro melting point apparatus and
are uncorrected. 1H NMR spectra were recorded on a Bruker
2
O
2
2
O
2
under the same reaction conditions. Steric effects may
also play an important role in this reaction because
oxidation of o-trifluoromethylbenzyl chloride 1d gave the
carboxylic acid 2d in only 37% yield, along with 32% yield
of the corresponding alcohol 3d . No aliphatic acid could
be produced through the oxidation of aliphatic halide
(
10) (a) Conor, J . A.; Ebsworth, E. A. V. Adv. Inorg. Chem. Radio-
chem. 1964, 6, 279. (b) Arcoria, A.; Ballistreri, F. P.; Tomaselli, G. A.;
Di Furia, F.; Modena, G. J . Mol. Catal. 1983, 18, 177. (c) Arcoria, A.;
Ballistreri, F. P.; Tomaselli, G. A.; Di Furia, F.; Modena, G. J . Mol.
Catal. 1984, 24, 189. (d) Raciszewski, Z. J . Am. Chem. Soc. 1960, 82,
267.
(11) J acobson, S. E.; Tang, R.; Mares, F. Inorg. Chem. 1978, 17, 3055.
1

Notes
J . Org. Chem., Vol. 66, No. 9, 2001 3237
AM-90 spectrometer or a Bruker AM-300 in CDCl
3
with tet-
were isolated by silica gel column chromatography (eluent:
ramethylsilane (TMS) as internal standard; J values are given
in Hz. Mass spectra were recorded on an HP-5989 instrument,
and HRMS was measured on a Finnigan MA+ mass spectrom-
eter. Organic solvents were dried by standard methods when
necessary. Some of the solid compounds reported in this paper
gave satisfactory CHN microanalyses with a Carlo-Erba 1106
analyzer. Commercially obtained reagents were used without
further purification. The preparation of methyltri-n-octylammo-
nium hydrogensulfate (PTC) is according to the literature.3
All reactions were monitored by TLC with Huanghai GF254 silica
gel coated plates. Flash column chromatography was carried out
on 300-400 mesh silica gel.
EtOAc/petroleum ether ) 1/10). 2d (a white solid, 730 mg, 37%).
Mp: 108-109 °C (lit.1 mp 108-109.5 °C). H NMR (CDCl
3d
1
, 300
3
MHz): δ 7.64 (1H, dd, J ) 7.8, 7.8 Hz, Ar), 7.89 (1H, d, J ) 7.8
Hz, Ar), 8.32 (1H, d, J ) 7.8 Hz, Ar), 8.40 (1H, s, Ar), 10.6 (1H,
br s, OH). 13C NMR (CDCl
, 75 MHz): δ 123.60 (q, J C-F ) 270.75
3
Hz), 127.21 (q, J C-F ) 3.75 Hz), 129.28, 130.20, 130.43 (q, J C-F
) 3.60 Hz), 131.37 (q, J C-F ) 32.92 Hz), 133.41, 171.04 (q, J C-F
) 2.25 Hz). Anal. Calcd: C, 50.54; H, 2.65. Found: C, 50.45; H,
a,12
2.73.
1
3
d (a colorless oil, 563 mg, 32%). Bp: 90 °C/20 mmHg. H NMR
(
7
CDCl
.43-7.79 (4H, m, Ar). C NMR (CDCl
123.54 (q, J ) 2.85 Hz), 124.14 (q, J C-F ) 202.95 Hz), 124.38
3
, 90 MHz): δ 1.72-2.16 (1H, br s, OH), 4.82 (2H, s, CH
2
),
13
3
, 75 MHz): δ 64.54,
P r ep a r a tion of Ben zoic Acid (2a ). To the reaction mixture
C-F
of benzyl chloride 1a (1.27 g, 10.0 mmol), Na
2
WO
4
‚2H
2
O (66 mg,
(q, J C-F ) 2.85 Hz), 128.98, 130.04, 130.93 (q, J
141.75. MS (EI) m/z (rel int): 176 (59.34, M ), 175 (30.22, M
C-F
) 24.08 Hz),
+
+
0
.2 mmol), PTC (93 mg, 0.2 mmol), and 4 Å MS (100 mg) was
-
-
+
+
+
added 30% hydrogen peroxide solution (4.1 mL, 36 mmol)
dropwise at a speed of 0.5 mL/h by means of a syringe pump
over 8 h at 90 °C with stirring vigorously. After being stirred
for another 2 h at 90 °C, the mixture was cooled to room
temperature. The aqueous layer was extracted with 10.0 mL of
EtOAc three times. The combined organic layer was washed with
1), 127 (100, M - 49), 107 (92.51, M - 69), 80 (20.52, M
+
+
96), 77 (37.22, M - 99). HRMS (EI): m/z 176.0443 (M ),
C H F O requires M 176.0449.
8
7
3
P r ep a r a tion of Tolu ic Acid (2e). This compound was
prepared in the same manner as that described for the prepara-
13e
tion of 2d (a white solid, 0.92 g, 68%). Mp: 180-181 °C (lit.
1
0 mL of saturated aqueous Na
MgSO . The solvent was removed under reduced pressure. After
recrystallization from hexane, 2a was obtained as a white solid
2
S
2
O
4
and dried over anhydrous
1
mp 182-183 °C). H NMR (CDCl
3
, 90 MHz): δ 2.45 (3H, s, CH
3
),
4
7
9
1
.28 (2H, d, J ) 8.1 Hz, Ar), 8.04 (2H, d, J ) 8.1 Hz, Ar), 8.30-
13
.96 (1H, br s, OH). C NMR (CDCl
3
, 75 MHz): δ 27.80, 126.64,
1
3a
1
(
1.07 g, 83%). Mp: 121-122 °C (lit. mp 121-123 °C). H NMR
29.23, 130.29, 144.66, 172.37. Anal. Calcd: C, 70.58; H, 5.92.
(
CDCl , 90 MHz): δ 7.34-7.61 (3H, m, Ar), 8.17 (2H, d, J ) 6.3
3
Found: C, 70.23; H, 5.97.
1
3
Hz, Ar), 8.90-9.83 (1H, br s, OH). C NMR (CDCl
δ 128.50, 129.42, 130.23, 133.79, 172.28. MS (EI) m/z (rel int):
3
, 75 MHz):
P r ep a r a tion of An isic Acid (2f). This compound was
prepared in the same manner as that described in the prepara-
tion of 2d (a white solid, 0.52 g, 34%). Mp: 184-185 °C
+
+
+
1
22 (89.94, M ), 105 (100, M - 17), 77 (81.50, M - 45), 74
+
+
+
(
9.89, M - 48), 51 (36.16, M - 71), 50 (25.52, M - 72). Anal.
13f
1
(
lit. mp 186-187 °C). H NMR (CDCl
3
, 90 MHz): δ 3.90 (3H,
Calcd: C, 68.84; H, 4.95. Found: C, 68.35; H, 5.01.
s, CH ), 6.94 (2H, d, J ) 8.1 Hz, Ar), 8.08 (2H, d, J ) 8.1 Hz,
3
Benzyl bromide was oxidized to 2a in the same manner as
that described above, but the yield of 2a can reach 88%.
P r ep a r a tion of 4-F lu or oben zoic Acid (2b). This compound
was prepared in the same manner as that described above as a
13
Ar). C NMR (CDCl
64.08, 171.56. Anal. Calcd: C, 63.15; H, 5.30. Found: C, 63.10;
H, 5.21.
3
, 75 MHz): δ 55.50, 113.78, 121.70, 132.38,
1
P r ep a r a tion of 4-Isop r op ylben zoic Acid (2g). This com-
pound was prepared in the same manner as that described in
1
3b
white solid (1.03 g, 86%). Mp: 184-186 °C (lit.
mp 185.4-
1
1
8
8
87.2 °C). H NMR (CDCl , 90 MHz): δ 7.16 (1H, dd, J ) 8.9,
3
the preparation of 2d (a white solid, 1.0 g, 61%). Mp: 118-120
.9 Hz, Ar), 7.36 (1H, dd, J ) 8.9, 8.9 Hz, Ar), 8.15 (1H, dd, J )
.9, 5.4 Hz, Ar), 8.24 (1H, dd, J ) 8.9, 5.4 Hz, Ar), 8.40-9.26
13g
mp 118-119 °C). 1H NMR (CDCl
3
, 300 MHz): δ
°
1
J
C (lit.
.28 (6H, d, J ) 8.0 Hz, CH
) 8.2 Hz, Ar), 8.05 (2H, d, J ) 8.2 Hz, Ar). C NMR
, 75 MHz): δ 23.65, 34.35, 126.61, 127.01, 130.44,
55.37, 172.64. Anal. Calcd: C, 73.15; H, 7.37. Found: C, 73.10;
3
), 2.99 (1H, m, Ar), 7.33 (2H, d,
1
3
(1H, br s, OH). C NMR (CDCl
3
, 75 MHz): δ 115.74 (d, J C-F
)
)
13
2
2
5
1.975 Hz), 125.65, 132.88 (d, J C-F ) 9.45 Hz), 166.4 (d, J C-F
(
CDCl
3
53.5 Hz), 170.91. Anal. Calcd: C, 60.01; H, 3.40. Found: C,
9.25; H, 3.61.
1
H, 7.34.
P r ep a r a tion of 4-Nitr oben zoic Acid (2c). This compound
was prepared in the same manner as that described above as a
P r ep a r a tion of 2-Na p h th oic Acid (2h ). This compound was
prepared in the same manner as that described in the prepara-
slightly yellow solid (1.52 g, 91%). Mp: 241-243 °C (recrystal-
tion of 2d (white solid, 516 mg, 30% yield). Mp: 185-187 °C
1
3c
mp 243.5-244.5 °C). 1H NMR
lization from EtOAc) (lit.
13h
mp 185-186 °C). 1H NMR (CDCl
, 90 MHz): δ 5.27-
(
6
lit.
3
(
(
(
D
3
CCOCD
2H, d, J ) 10.8 Hz, Ar), 3.65-3.96 (1H, br s, OH). C NMR
CCOCD , 75 MHz): δ 124.42, 131.72, 137.22, 151.50, 166.27.
3
, 90 MHz): δ 8.26 (2H, d, J ) 10.8 Hz, Ar), 8.39
.80 (1H, br s, OH), 7.39-7.78 (2H, m, Ar), 7.78-8.49 (5H, m,
Ar), 8.73 (1H, s, Ar). C NMR (CDCl
26.80, 127.84, 128.36, 128.69, 129.57, 130.91, 132.19, 132.48,
36.00, 171.93. Anal. Calcd for: C, 76.73; H, 4.68. Found: C,
6.64; H, 4.65.
13
13
3
, 75 MHz): δ 125.42,
D
3
3
1
1
7
Anal. Calcd: C, 50.31; H, 3.02, N, 8.38. Found: C, 50.23; H, 3.08;
N, 8.40.
P r ep a r a tion of 2-Tr iflu or om eth ylben zoic Acid (2d ). This
compound was prepared in the same manner as that described
Ack n ow led gm en t. We thank the State Key Project
above. However, 8.2 mL (72 mmol) of H
2 2
O was added dropwise
over 16 h into the substrate 1d , and the products 2d and 3d
of Basic Research (Project 973) (No. G2000048007) and
the National Natural Science Foundation of China for
financial support. We also thank the Inoue Photochi-
rogenesis Project (ERATO, J ST) for chemical reagents.
(
(
12) Dehmlow, E. V.; Vehre, B.; Broda. W. Synthesis 1985, 508.
13) (a) Hedges, E. S.; Myers, J . E. J . Chem. Soc. 1925, 127, 2432.
(b) Harton, W. J .; Robertson, D. E. J . Org. Chem. 1960, 25, 1016. (c)
Connors, K. A.; Bender, M. L. J . Org. Chem. 1961, 26, 2498. (d)
Benkeser, R. A.; Severson, R. G. J . Am. Chem. Soc. 1951, 73, 1353. (e)
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Su p p or tin g In for m a tion Ava ila ble: 13C NMR spectra of
2a -h . This material is available free of charge via the Internet
at http://pubs.acs.org.
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