Microwave-Promoted and Chelation-Controlled Double Arylations of Terminal Olefinic Carbon of Vinyl Ethers

Article abstract of DOI:10.1021/jo035815f

Herein we report a rapid, palladium-catalyzed terminal diarylation of the chelating olefin N,N-dimethyl(2-ethenyloxy)ethanamine under noninert conditions utilizing controlled microwave heating as a convenient energy source. Among the aryl bromides examine

Full text of DOI:10.1021/jo035815f

Micr ow a ve-P r om oted a n d Ch ela tion -Con tr olled Dou ble Ar yla tion s
of Ter m in a l Olefin ic Ca r bon of Vin yl Eth er s
Andreas Svennebring, Peter Nilsson, and Mats Larhed*
Organic Pharmaceutical Chemistry, Department of Medicinal Chemistry, Uppsala Biomedical Centre,
Uppsala University, Box 574, SE-751 23 Uppsala, Sweden
mats@orgfarm.uu.se
Received December 15, 2003
Herein we report a rapid, palladium-catalyzed terminal diarylation of the chelating olefin N,N-
dimethyl(2-ethenyloxy)ethanamine under noninert conditions utilizing controlled microwave heating
as a convenient energy source. Among the aryl bromides examined, both electron-rich and electron-
poor substrates were demonstrated to furnish useful yields after only 10-120 min of directed
microwave heating at 160-200 °C. The good terminal regioselectivity suggests that the precatalyst
(Herrmann’s palladacycle) serves as a source of weakly coordinated palladium(0) in the investigated
high-temperature Heck process.
The Heck arylation of carbon-carbon double bonds is
mediated by a variety of Pd(0) and Pd(II) catalyst
precursors, although the nature of the “true” catalyst is
still under debate.1,2 In general, the Heck coupling of aryl
halides with both electron-rich and electron-poor terminal
alkenes affords monoarylated products.3-6 With electron-
deficient olefins under harsh reaction conditions, such
as with excess of the aryl halide7 or under high pressure,8
a 2-fold terminal arylation to give 1,1-diarylalkene
derivatives may occur.5 Alternatively, the olefin may be
equipped with a catalyst-directing functionality that
increases the reactivity of the double bond.9-12 In a recent
article from our laboratory, the introduction of a specific,
palladium-coordinating dimethylamino group allowed for
regioselective, terminal â,â-diarylation of the acyclic vinyl
ether 1 with aryl iodides.13
First, these are precursors to synthetically valuable
diaryl substituted acetaldehydes, and second, pharma-
cophores consisting of a diarylmethine group linked to a
dimethylamine moiety through a spacer are very abun-
dant in the present flora of therapeutic agents. Thus, this
new class of compounds has potential for medicinal
applications.14 A series of GABA uptake inhibitors have
also been reported with similar structures.15
The combination of transition-metal catalysts and
microwave heating is not only a hot topic but also an area
that is likely to have an impact on several fields of
modern chemistry.16,17 The palladium-catalyzed coupling
reaction that with classical heating typically needs days
to reach completion can now with high reproducibility
be brought to full conversion in only minutes, consuming
only a fraction of the energy that normally is needed for
a standard, oil bath-heated reaction.18 To reach maximum
catalyst activity, it is desirable to go beyond the boiling
point of the employed solvent. These superheating condi-
tions can be easily obtained with modern single-mode
microwave equipment supplied with temperature and
pressure controlling devices. In addition, the preparative
potential of this methodology has increased with the
rapid development of catalytic systems that can with-
stand the sometimes extreme temperatures that are
induced under irradiation.19 Herrmann’s Pd(II)-phos-
phapalladacycle 320 is an excellent thermostable precata-
We were interested in further studies of regioselective
double terminal arylations of vinyl ethers for two reasons.
(1) Herrmann, W. A.; Bohm, V. P. W.; Reisinger, C.-P. J . Organomet.
Chem. 1999, 576, 23-41.
(2) Amatore, C.; J utand, A. Acc. Chem. Res. 2000, 33, 314-321.
(3) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009-
3066.
(4) Whitcombe, N. J .; Hii, K. K.; Gibson, S. E. Tetrahedron 2001,
57, 7449-7476.
(5) Braese, S.; De Meijere, A. Double and Multiple Heck Reactions.
In Handbook of Organopalladium Chemistry for Organic Synthesis;
Negishi, E.-i., Ed.; Wiley & Sons: New York, 2002; Vol. 1, pp 1179-
1208.
(6) Larhed, M.; Hallberg, A. Scope, Mechanism, and Other Funda-
mental Aspects of the Intermolecular Heck Reaction. In Handbook of
Organopalladium Chemistry for Organic Synthesis; Negishi, E.-i., Ed.;
Wiley & Sons: New York, 2002; Vol. 1, pp 1133-1178.
(7) Gurtler, C.; Buchwald, S. L. Chem.-Eur. J . 1999, 5, 3107-3112.
(8) Sugihara, T.; Takebayashi, M.; Kaneko, C. Tetrahedron Lett.
1995, 36, 5547-5550.
(14) Lednicer, D. Strategies for Organic Drug Synthesis and Design,
1st ed.; Wiley & Sons: New York, 1998.
(15) Andersen, K. E.; Sorensen, J . L.; Huusfeldt, P. O.; Knutsen, L.
J . S.; Lau, J .; Lundt, B. F.; Petersen, H.; Suzdak, P. D.; Swedberg, M.
D. B. J . Med. Chem. 1999, 42, 4281-4291.
(9) Larhed, M.; Andersson, C.-M.; Hallberg, A. Tetrahedron 1994,
50, 285-304.
(16) Larhed, M.; Moberg, C.; Hallberg, A. Acc. Chem. Res. 2002, 35,
717-727.
(10) Itami, K.; Nokami, T.; Yoshida, J . Org. Lett. 2000, 2, 1299-
(17) Olofsson, K.; Hallberg, A.; Larhed, M. In Microwaves in Organic
Synthesis; Loupy, A., Ed.; Wiley & Sons: Weinheim, Germany, 2002;
pp 379-403.
1302.
(11) Alonso, I.; Carretero, J . C. J . Org. Chem. 2001, 66, 4453-4456.
(12) Nilsson, P.; Larhed, M.; Hallberg, A. J . Am. Chem. Soc. 2003,
125, 3430-3431.
(13) Nilsson, P.; Larhed, M.; Hallberg, A. J . Am. Chem. Soc. 2001,
123, 8217-8225.
(18) Lidstro¨m, P.; Tierney, J .; Wathey, B.; Westman, J . Tetrahedron
2001, 57, 9225-9283.
(19) Kaiser, N. F. K.; Bremberg, U.; Larhed, M.; Moberg, C.;
Hallberg, A. Angew. Chem., Int. Ed. 2000, 39, 3596-3598.
10.1021/jo035815f CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/09/2004
J . Org. Chem. 2004, 69, 3345-3349
3345

Svennebring et al.
TABLE 1. Sym m etr ic On e-P ot Ch ela tion -Con tr olled
Micr ow a ve-P r om oted â,â-Dia r yla tion of 1
SCHEME 1
temp timeb
isolated
entry
1
Ara
(°C) (min) product R,â/4c yield (%)d
p-MeO-Ph- 160
10
10
10
55
55
20
10
10
55
30
10
360
55
4a
8/92
6/94
7/93
5/95
38
52
45
80
69
65
67
63
59g
55g
46g
9
2a
180
200
160
180
160
180
180f
160
180
200
2
3
o-Me-Ph-
4b
4c
2b
5/95
Ph-e
2c
10/90
10/90
10/90
12/88
12/88
14/86
23/77
29/71
4
p-Cl-Ph-
2d
4d
4e
lyst in Heck chemistry for both aryl triflates and aryl
bromides 2, although nonactivated aryl chlorides react
only reluctantly.1 Furthermore, no aryl migration or
ligand decomposition occurs at 150 °C, allowing for
recovery of unchanged dimeric palladacycles in yields up
to 70%.1 The catalyst can, as a consequence, be recycled
with little loss of activity.1 Despite these outstanding
characteristics, mechanistic details about this class of
catalysts still remain partly obscure. The fact that aryl
palladium(IV) complexes do exist stimulated the sugges-
tion that an alternative redox cycle involving Pd(II)/
Pd(IV) had to be taken into account.21-23 Recent inves-
tigations by Herrmann and Beller, however, also support
the classic Pd(0)/Pd(II) interconversion with phospha-
palladacycles as precatalysts.1,22
We have now developed a rapid protocol for microwave-
assisted regioselective double â-arylations of the chelat-
ing vinyl ether N,N-dimethyl(2-ethenyloxy)ethanamine
(1) using palladacycle 3 as the palladium source. We
demonstrate that by proper selection of experimental
parameters, it is possible to achieve symmetrical and
nonsymmetrical terminal â,â-diarylations with both elec-
tron-rich and electron-poor aryl bromides.
To achieve diarylation, we planned to take advantage
of the fact that intramolecular reactions, in general,
proceed faster than their intermolecular counterparts. In
a previous paper,13 we disclosed that the dimethylami-
noethyl group in 1 not only promoted high regioselectivity
in the Heck synthesis of the â,â-diarylation products from
aryl iodides, but also that chelating olefins reacted faster
than a nonchelating alkyl vinyl ether derivative. We
suggested that the improved reactivity was due to an
initial nitrogen coordination to the aryl palladium species
prior to π-intermediate formation.13 The oxidative addi-
tion complex is thus presented for the double bond in a
“pseudo-intramolecular” process, accelerating the π-com-
plex formation and the subsequent migratory insertion.
Sym m etr ic P r od u cts. One-pot microwave-heated
bisarylations of unsubstituted 1 were carried out in
sealed vessels under air employing an excess of the aryl
bromide (3.0-5.0 equiv) with a low amount of pallada-
cycle 3 (0.5 mol %) in 10% aqueous DMF (Scheme 1). To
increase the stability of the underligated catalytic Pd(0)
system, a highly ionic reaction cocktail was preferred
using lithium chloride (2.0 equiv) and sodium acetate
(1.35 equiv) as additive and potassium carbonate as base
(2.2 equiv). For each individual bisarylation, several
5
p-OCH-Ph- 160h
2e
180h
36
a
b
5 equiv of 2. >95% conversion of 1 and 5 by GC-MS.
c Determined by GC-MS and 1H NMR. Yield of the â,â-product
d
4 (no R,â-isomer), >95% purity by GC-MS. e Only 3 equiv of 2c.
f Conventional heating with an oil bath. 10-15% 4,4′-dichlo-
g
h
rostilbene detected by GC-MS. Remaining 5e.
reaction temperatures were explored. Irradiation times
from 5 min to 6 h were evaluated. Five different aryl
bromides (2a -e) were selected since earlier studies had
established that the electron density and the substitution
pattern of the aryl moiety had a critical effect on the
regioselectivity in monoarylations of vinyl ethers.9 Se-
lected preparative results obtained during these studies
are summarized in Table 1. The final diarylated products
4 were fully purified from the R,â-isomers in all reactions,
while the intermediate monoarylated 5 products were
never isolated.
In contrast to most Heck arylations with electron-poor
olefins, but in accordance with previous results with vinyl
ethers, the electron-rich and neutral 2a -c afforded good
two-step yields of terminally diarylated product 4a -c
(entries 1-3, Table 1). Even the orthosubstituted 2b,
whose vinylation might be sterically problematic, was
efficiently converted into the desired â,â-bisarylated
product 4b with high yield and regioselectivity. Classic
heating at 180 °C furnished almost identical reaction
results as microwave heating (entry 3). Successful
chemoselective activation of the bromosubstituent on 2d
produced good yields at all investigated temperatures.
Full conversion of the monoarylated intermediate 5 was
obtained in all cases except for p-bromobenzaldehyde 2e.
In fact, all formyl-functionalized compounds (2e, 5e, and
4e) underwent a competing reductive amination process
under the employed high-temperature conditions, form-
ing the corresponding N,N-dimethylbenzylamine byprod-
ucts. Since it is known that DMF may act as a combined
dimethylamine and carbon monoxide source, vinylation
of 2e was also investigated using DMAc as a more
thermostable alternative to DMF.24 Unfortunately, this
change of solvent consistently afforded lower isolated
yields. It is notable that in all entries the bisarylation
occurred with high regioselectivity at the terminal â-car-
bon (R,â/4 < 14:86) and that only minor amounts of biaryl
and stilbene derivatives were formed. The regioselectivity
was also found to be largely independent of the reaction
temperature permitting fast reactions (down to 10 min)
(20) trans-Di(µ-acetato)bis[o-(di-o-tolylphosphino)benzyl]dipalladi-
um (II). Registry number provided by the author: 172418-32-5.
(21) Shaw, B. L. New J . Chem. 1998, 22, 77-79.
(22) Beller, M.; Riermeier, T. H. Eur. J . Inorg. Chem. 1998, 29-35.
(23) Bedford, R. B. Chem. Commun. 2003, 1787-1796.
(24) Wan, Y. Q.; Alterman, M.; Larhed, M.; Hallberg, A. J . Org.
Chem. 2002, 67, 6232-6235.
3346 J . Org. Chem., Vol. 69, No. 10, 2004

Arylation of Terminal Olefinic Carbon of Vinyl Ethers
TABLE 2. Ch ela tion -Con tr olled Micr ow a ve-P r om oted â-Ar yla tion of Mon oa r yla ted Vin yl Eth er 5
a
b
Determined by GC-MS and 1H NMR. Determined for the â,â-product 6 by GC-MS and NOE experiments. c Combined yield of the
d
E- and Z-isomers (no R,â-product), >95% purity by GC-MS. Performed in DMAc.
SCHEME 2
at very high temperatures (160-200 °C) without reduced
regioselectivity. The exception concerns the very sluggish
and less selective vinylation of the electron-deficient 2e
(entry 5).
Un sym m etr ica l P r od u cts. A series of aryl bromides
(2) was reacted with â-arylated olefins 5a ,b under air
using almost identical reaction conditions as those em-
ployed in the symmetrical examples (Scheme 2). The
mono â-phenylated, pure 5a was selected as the standard
olefin for the sequential, unsymmetrical protocol to assess
the productivity and selectivity of the second arylation.
The preparative results are presented in Table 2.
The geometrical isomers of unsymmetric â,â-products
6, except 6a , were all readily separable on silica column
and characterized as the individual compounds. However,
the isolated yields are in all cases reported for the
diastereomeric E/Z-mixtures. Slightly electron-rich (2b)
or neutral (2h and 2c) delivered the best yields in the
study accompanied with good â,â-selectivity (entries 3-5,
Table 2). The electron-poor substrate 2i furnished the
best terminal selectivity, which is in accordance with the
preference for electron-deficient aryl groups to migrate
to the most electron-rich (â) vinyl carbon in the insertion
step.25 The selectivities were in all cases high (R,â/â,â <
9:91), except for the sterically very hindered 2,6-dimethyl-
J . Org. Chem, Vol. 69, No. 10, 2004 3347

Svennebring et al.
SCHEME 3
found that decreased catalyst loading afforded better
results.29 We believe that the high regiocontrol indicates
that 3 acts as a catalyst reservoir30 for Pd(0) and that
the release of the underligated metal(0) is accelerated
by water under our high-temperature conditions (Scheme
3). In addition, the aggregation rate of Pd(0) is probably
of higher order than the reaction rate, since lowering the
catalytic load prolongs the active catalyst lifetime.
We have shown that diverse aryl bromides can be used
in microwave-heated, chelation-controlled diarylations of
vinyl ethers. Thus, the metal presenting dimethylami-
noethyl group does not only provide high â,â-selectivity
at high reaction temperatures, but also enhances the
reactivity of the double bond. We strongly believe that
the concept of chelation-promoted multiarylations can be
successfully applied also to cyclic Heck substrates. Fi-
nally, with support from the obtained high regioselectiv-
ity we postulate a traditional Pd(0)/Pd(II) catalytic cycle
with Herrmann’s palladacycle serving as an efficient
palladium(0) source.
substituted 2g (entry 2). The somewhat lowered â-selec-
tivity and the poor yield encountered with 2g is probably
a consequence of steric congestions in the insertion step,
requiring high temperature and long reaction time. The
broad scope of aryl substrates usable in the Heck reaction
manifests itself in the reactions with electron-rich N,N-
dimethylamino-substituted 2f and heteroaromatic 2j,
both delivering usable yields (entries 1 and 7). As
experienced in the one-pot arylations (Table 1), the
selectivities and yields in Table 2 were rather insensitive
to the employed reaction temperatures allowing for high-
temperature reactions with reaction times down to only
10 min (entries 3, 4, and 8, Table 2). The last introduced
aryl substituent preferred the Z-position in all cases.
Thus, by switching the order of arylation, the opposite
geometrical isomer of 6d could be preferentially synthe-
sized (entries 4 and 5, Table 2). The introduction of a
second phenyl group to monophenylated 5a (entry 8,
Table 2) proceeded with a higher â,â-selectivity than the
one-pot double phenylation procedure starting directly
from 1 (entry 3, Table 1). The reduced performance of
the tandem arylation is possibly explained by a catalytic
aging process (e.g., Pd-black formation), rendering the
second phenylation in the one-pot protocol less regiose-
lective.
The potential for 3 to catalyze chelation-controlled
regioselective â,â-arylations of vinyl ethers was illus-
trated in the previous section. Whereas the preparative
advantages are clear, the oxidation state of the active
metal is not obvious. The square planar 16e- π-species
A is commonly accepted as key intermediate with the
Pd(0)/Pd(II) redox mechanism, directing the insertion via
chelation to afford terminal arylation (Scheme 3).6,13 With
a Pd(II)/Pd(IV)-based catalytic cycle, a possible nitrogen-
coordinated octahedral alkeneic Pd(IV) π-intermediate
must be postulated before the regiocontrolling migratory
insertion step. If the Pd(II)/Pd(IV) pathway was active,
and not the more widely recognized Pd(0)/Pd(II) route,
some difference in regioselectivity should be expected.
Performing the reactions with classic Pd(0)-generating
standard precatalysts such as Pd(OAc)2/PPh3, Pd(OAc)2/
P(o-tol)3, or Pd(PPh3)2Cl2 does not give any substantial
difference in stereo- or regioselectivity,26 but only in
conversion and yield. Furthermore, Herrmann has re-
ported a similar distribution of isomers in the Heck
arylation of nonchelating butyl vinyl ether with both
palladacycle 3 and with classic Pd(OAc)2.27
Exp er im en ta l Section
Gen er a l P r oced u r e for Sym m etr ic Dia r yla tion s (Ta ble
1). The following chemicals were added to a thick-walled
tube: 3 (3.2 µmol, 3.0 mg), LiCl (1.32 mmol, 56.0 mg), NaOAc
(0.880 mmol, 72.2 mg), K2CO3 (1.43 mmol, 198 mg), aryl
bromide 2 (quantity according to entry), olefin 1 (0.651 mmol,
75.0 mg), DMF (2 mL), and water (0.20 mL). The tube was
then sealed under air, and the contents were magnetically
stirred and microwave-heated at a specified temperature for
an appropriate time (see Table 1 for details). After cooling,
the reaction mixture was diluted with dietyl ether and washed
twice with 0.1 M NaOH. The combined aqueous phases were
additionally extracted three times with diethyl ether. The
etheral phases were combined and dried with K2CO3 (s). After
evaporation of the solvent, silica column chromatography was
performed using gradient elution (Et2O/isohexane) containing
triethylamine (1%).
N,N-Dim et h yl-2-[(2,2-d i-p -a n isyl)et h en yloxy]et h a n -
a m in e (4a ). Yellowish oil, 52% yield (0.111 g, reaction
performed at 180 °C, 10 min, >95% by GC-MS). 1H NMR (400
MHz, CDCl3): δ 7.38 (d, J ) 8.9 Hz, 2H), 7.17 (d, J ) 8.7 Hz,
2H), 6.86 (d, J ) 8.9 Hz, 2H), 6.85 (d, J ) 8.7 Hz, 2H), 6.40 (s,
1H), 4.01 (t, J ) 6.1 Hz, 2H), 3.81 (s, 6H), 2.65 (t, J ) 6.1 Hz,
2H), 2.31 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 158.6, 158.2,
143.8, 133.4, 131.1, 130.7, 129.7, 119.9, 113.9, 113.4, 71.8, 58.9,
55.44, 55.35, 46.3. MS m/z (relative intensity 70 eV): 327 (M+,
18), 72 (100), 58 (25). Anal. Calcd for C20H25NO3: C, 73.37; H,
7.70. Found: C, 73.3; H, 7.8.
Gen er a l P r oced u r e for Non sym m etr ic Dia r yla tion s
(Ta ble 2). The following chemicals were added to a thick-
walled tube: 3 (3.2 µmol, 3.0 mg), LiCl (1.32 mmol, 56.0 mg),
NaOAc (0.880 mmol, 72.2 mg), K2CO3 (0.782 mmol, 108 mg),
aryl bromide (quantity according to entry), olefin 5 (0.65
mmol), DMF (2.0 mL), and water (0.2 mL). The tube was then
closed under air, and the contents were magnetically stirred
and microwave-heated at the temperature and time specified
in Table 2. After cooling, the reaction mixture was diluted with
dietyl ether, and washed twice with 0.1 M NaOH. The
combined aqueous phases were additionally extracted three
times with diethyl ether. The etheral phases were combined
and dried with K2CO3 (s). After evaporation of the solvent,
Early attempts to develop a high-speed microwave
protocol for â,â-diarylation of 1 revealed that water
addition was crucial for catalyst activity.28 It was further
(25) Andersson, C.-M.; Hallberg, A.; Daves, G. D., J r. J . Org. Chem.
1987, 52, 3529-3536.
(28) Rosner, T.; Le Bars, J .; Pfaltz, A.; Blackmond, D. G. J . Am.
Chem. Soc. 2001, 123, 1848-1855.
(29) de Vries, A. H. M.; Mulders, J .; Mommers, J . H. M.; Henderickx,
H. J . W.; de Vries, J . G. Org. Lett. 2003, 5, 3285-3288.
(30) Consorti, C. S.; Zanini, M. L.; Leal, S.; Ebeling, G.; Dupont, J .
Org. Lett. 2003, 5, 983-986.
(26) For selectivity values with the different catalytic systems, see
the Supporting Information.
(27) Herrmann, W. A.; Brossmer, C.; Reisinger, C. P.; Riermeier,
T. H.; Ofele, K.; Beller, M. Chem.-Eur. J . 1997, 3, 1357-1364.
3348 J . Org. Chem., Vol. 69, No. 10, 2004

Arylation of Terminal Olefinic Carbon of Vinyl Ethers
silica column chromatography was performed using gradient
elution (Et2O/isohexane) containing triethylamine (1%).
N,N-Dim eth yl-2-[(2-p h en yl-2-(2-n a p h th yl))eth en yloxy]-
eth a n a m in e (6d ). Colorless oil, 62% yield (0.128 g, reaction
performed at 180 °C, 20 min, >95% by GC-MS). Major
isomer: (Z)- first GC-MS peak, first eluted on column
127.4, 126.9, 126.8, 126.4, 125.8, 121.0, 72.2, 59.0, 46.4. MS
m/z (relative intensity 70 eV): 317 (M+, 12), 215 (8), 72 (100),
58 (54). Anal. Calcd for C22H23NO: C, 83.24; H, 7.30. Found:
C, 82.9; H, 7.1.
Ack n ow led gm en t. We acknowledge the financial
support from the Swedish Research Council and from
Knut and Alice Wallenberg’s Foundation. We also thank
Personal Chemistry AB for providing us with the Smith
Synthesizer. Finally, we would like to thank Dr. Boris
Schmidt and Dr. Kristofer Olofsson for intellectual
contributions to this project.
1
chromatography. H NMR (400 MHz, CDCl3): δ 7.77 (s, 1H),
7.72-7.62 (m, 3H), 7.50-7.45 (m, 1H), 7.36-7.30 (m, 2H),
7.25-7.13 (m, 5H), 6.51 (s, 1H), 3.96 (t, J ) 5.9 Hz, 2H), 2.56
(t, J ) 5.9 Hz, 2H), 2.21 (s, 6H). 13C NMR (100 MHz, CDCl3):
δ 145.9, 140.9, 135.6, 133.7, 132.6, 130.3, 129.1, 128.7, 128.6,
128.3, 127.8, 127.5, 126.8, 126.0, 125.8, 120.9, 72.2, 59.0, 46.4.
MS m/z (relative intensity 70 eV): 317 (M+, 11), 215 (10), 72
(100), 58 (55). Minor isomer: (E)- second GC-MS peak, second
eluted on chromatography. 1H NMR (400 MHz, CDCl3):
δ
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectral data for all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
7.75-7.64 (m, 3H), 7.59 (s, 1H), 7.40-7.32 (m, 4H), 7.29-7.13
(m, 4H), 6.57 (s, 1H), 3.99 (t, J ) 6.1 Hz, 2H), 2.59 (t, J ) 6.1
Hz, 2H), 2.23 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 146.0,
138.4, 137.9, 133.9, 132.6, 130.3, 128.3, 128.1, 128.0, 127.9,
J O035815F
J . Org. Chem, Vol. 69, No. 10, 2004 3349