Nucleophilic reactivities of ketene acetals

Article abstract of DOI:10.1002/ejoc.200400134

The kinetics of the reactions of ketene acetals with benzhydrylium ions have been followed photometrically. The reactions proceed with rate-determining C-C bond formation that is considerably faster than expected for outer-sphere electron transfer process

Full text of DOI:10.1002/ejoc.200400134

FULL PAPER
Nucleophilic Reactivities of Ketene Acetals
Takahiro Tokuyasu[a] and Herbert Mayr*[b]
Dedicated to Professor Gottfried Märkl on the occasion of his 75th birthday
Keywords: Carbocations / CϪC coupling / Kinetics / Linear free energy relationship / Substituent effects
The kinetics of the reactions of ketene acetals with benzhyd-
ously reported electrophilicity parameters (E) of benzhydryl-
rylium ions have been followed photometrically. The reac- ium ions suggest that several of the compounds investigated
tions proceed with rate-determining C−C bond formation
that is considerably faster than expected for outer-sphere
electron transfer processes. Structure−reactivity relationships
are discussed. The excellent correlations between the ob-
served second-order rate constants (log k2) and the previ-
in this work can be used as reference nucleophiles for the
development of electrophilicity scales.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
the same reason, silylated enol ethers are more widely used
in organic synthesis than are alkyl enol ethers.
Ketene acetals have electron-rich CϭC double bonds,
and react readily with many electrophiles under moderate
conditions. Because of their high reactivity and ease of
preparation, these compounds are often employed as build-
ing blocks in organic synthesis.[1Ϫ4] Their reactions with
carbon electrophiles in aldol,[1] Michael,[2] and Mannich re-
actions[3] have been investigated extensively with respect to
diastereoselective and enantioselective synthesis.[4] While
1,1-dialkoxyalkenes 1 (R1, R2 ϭ alkyl) readily polymerize
when treated with electrophiles,[5] the corresponding siloxy
derivatives 1 [R1 and/or R2 ϭ Si(alkyl)3] usually give the
corresponding carboxylic acids or esters in high yield, be-
cause the electrophilic attack at the CϭC double bond is
followed by a rapid desilylation reaction (Scheme 1).[6] For
In this present study, we have determined the nucleophilic
reactivities of ketene acetals 1 from the kinetics of their re-
actions with benzhydrylium ions 2 (Table 1), which have re-
cently been recommended as reference electrophiles for the
construction of a comprehensive nucleophilicity scale.[7,8]
Table 1. Benzhydrylium ions 2 used as reference electrophiles in
this work
Scheme 1. General difference in the reactions of siloxy- and alkoxy-
alkenes with electrophiles
[a]
Department of Materials Chemistry & Frontier Research
Center, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan
[b]
Department Chemie der Ludwig-Maximilians-Universität
München,
Butenandtstr. 5Ϫ13 (Haus F), 81377 München, Germany
E-mail: herbert.mayr@cup.uni-muenchen.de
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2004, 2791Ϫ2796
DOI: 10.1002/ejoc.200400134
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. Tokuyasu, H. Mayr
FULL PAPER
The nucleophilicity parameter N, defined by Equation (1),
should provide a comparison of the activating effect of al-
koxy and siloxy groups on CϭC double bonds and give
information about potential electrophilic reaction partners
of ketene acetals.[8]
log k ϭ s (N ϩ E)
(1)
k in Ϫ1·sϪ1 at 20 °C, s ϭ nucleophile-specific slope pa-
rameter, N ϭ nucleophilicity parameter, E ϭ electrophilic-
ity parameter.
In view of a shortage of reference nucleophiles having N
Ͼ 9 in the previously published nucleophilicity scales,[7Ϫ9]
we hoped, furthermore, that some of the ketene acetals in-
vestigated in this work might prove to be suitable reference
nucleophiles for the characterization of new electrophiles.
Scheme 3. Reaction of the ketene acetal 1a with (dma)2CHϩ
Ϫ
BF4 (2d)
To avoid polymerization that arises from the reaction of
cation 3a with further units of ketene acetal 1a, the reaction
was conducted in EtOH/CH3CN in the presence of 2,6-di-
tert-butylpyridine. In agreement with the nucleophilicity
parameters of N ϭ 9.81 and s ϭ 0.81 determined for 1a in
this work (see below), and a solvent nucleophilicity of N1 ϭ
5.77 and s ϭ 0.92 obtained for a 20/80 (v/v) ethanol/aceto-
nitrile mixture,[10] benzhydrylium ions react faster with 1a
than with ethanol in a 0.36  solution of 1a in EtOH/
CH3CN (solvent nucleophilicities N1 refer to first-order rate
constants). On the other hand, a much smaller reactivity
ratio k1a/kEtOH was expected in reactions with the dialkoxy-
carbenium ion 3a. Because of the anomeric stabilization[11]
of the resulting ortho ester (or of acetals), alkoxy-substi-
tuted carbenium ions react much faster with O-nucleophiles
than predicted by the N parameters derived from reactions
with benzhydrylium ions. 2,6-Di-tert-butylpyridine was ad-
ded as a proton scavenger to prevent the acid-catalyzed re-
action of 1,1-diethoxyethene (1a) with ethanol.[12] Consist-
ent with the preceding analysis, the ester 4a, derived from
the reaction of the cation 3a with ethanol, could be trapped
in 21% yield under these conditions.
Results and Discussion
As illustrated in Scheme 2, we carried out product studies
with the most readily available benzhydrylium salt,
(dma)2CHϩBF4Ϫ. By analogy to previously described reac-
tions of benzhydrylium tetrafluoroborates with silylated
ketene acetals,[6,7] compounds 1b, 1c, and 1e reacted readily
Ϫ
with (dma)2CHϩBF4 in dichloromethane to yield the es-
ters 4b (ϭ 4c) and 4e in high yields. The silyl ester 4d, which
Ϫ
was formed from (dma)2CHϩBF4 and 1d, was not iso-
lated, but the initially obtained reaction product was hy-
drolyzed to give the carboxylic acid 5d.
The reactions of benzhydrylium ions 2 with the ketene
acetals 1 (Ͼ 10 equiv.) were followed by UVϪVis spec-
troscopy in CH2Cl2 using the instrumentation described
previously.[13] From the exponential decays of the carben-
ium absorbances, we derived the pseudo-first-order rate
constants, kobsd. (sϪ1). Since, in all cases, kobsd. was pro-
portional to the concentrations of the ketene acetals 1, we
calculated second-order rate constants k2 (Table 2) from
plots of kobsd. versus the concentrations of the nucleo-
philes (Figure 1).
The plots of the rate constants of the reactions of ketene
acetals 1 with benzhydrylium ions 2 against the correspond-
ing electrophilicity parameters E gave linear correlations
(Figure 2), from which parameters N and s of the ketene
acetals 1 were determined (Table 2) according to Equa-
tion 1.
Scheme 2. Products of the reactions of (dma)2CHϩBF4 (2d) with
Ϫ
the silylated ketene acetals 1bϪe
All of the ketene acetals 1 studied in this work have nu-
cleophilicity parameters N that range from 9.8 to 11.5,
which are approximately five orders of magnitude greater
In contrast, the reaction of 1,1-diethoxyethene (1a) with than those of structurally related enol ethers.[6Ϫ8] Compar-
Ϫ
the benzhydrylium salt (dma)2CHϩBF4 in CH2Cl2 pro- able results have been reported for acid-catalyzed hydrolyses
duced only polymeric material (Scheme 3).
of enol ethers and ketene acetals: the protonation of 1,1-
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Nucleophilic Reactivities of Ketene Acetals
FULL PAPER
Table 2. Second-order rate constants for the reactions of benzhyd-
rylium tetrafluoroborates 2 with ketene acetals 1 (CH2Cl2, 20 °C)
and calculated nucleophilicity (N) and slope (s) parameters for
ketene acetals 1
Figure 2. Correlation of the rate constants (log k2, at 20 °C, in
CH2Cl2) for the reactions of ketene acetals (1) with the benzhydryl-
ium ions (2) versus their electrophilicity parameters E (correlation
for 1c is not shown)
Table 3. Relative reactivities of vinyl ethers and ketene acetals to-
ward benzhydrylium ions and protons
[a] k[H2CϭC(OR)2]/k[H2CϭCH(OR)].[b] Calculated from Equation
(1) by using E ϭ Ϫ7.02 for 2d and N ϭ 3.92 and s ϭ 0.90 for
[c]
ethoxyethene (from ref.[8]). From ref.[14]
.
Consistent with previous observations in the enol ether
series,[6] compounds 1b and 1c have similar nucleophilic re-
activities (N ϭ 10.21 and 10.32, respectively), suggesting
that CϪC bond formation is the rate-determining step. If
the initial step of Scheme 2 would be reversible, i.e., if
desilylation would be rate-determining, compound 1c
should be considerably less reactive, because the desilylation
of the trimethylsiloxy-substituted carbocation 3b is much
faster than attack at the sterically hindered tert-butyldi-
methylsilyl group in the cation 3c (Scheme 2).
[a]
Calculated from rate constants obtained at lower temperatures
by using the Eyring activation parameters ∆H‡ ϭ 38.3 kJ·molϪ1
and ∆S‡ ϭ Ϫ70.9 J·molϪ1 KϪ1
Comparison of the nucleophilicities of compounds 1a
and 1b shows that the activating effect of a trimethylsiloxy
group is similar to that of an alkoxy group. A similar re-
lationship has previously been observed in the enol ether
series.[6] Possibly, the slightly more shielded C-2 atom in 1,1-
dialkoxyethenes (1a, δC ϭ 55.6 ppm), relative to the corre-
sponding siloxy compounds (1c, δC ϭ 60.4 ppm), indicates
a higher electron donating ability of the alkoxy unit, relative
Figure 1. Determination of the second-order rate constant
k2 (Ϫ1 Ϫ1, 20 °C, CH2Cl2) of the reaction of (thq)2CHϩ with 1c to the siloxy unit, in the ground states of the ketene acetals.
s
from the slope of the linear correlation of kobsd. with the nucleo-
phile concentration
This ground state effect may be compensated, however, by
the more effective hyperconjugative stabilization of the si-
loxy-substituted carbenium ions 3b,c relative to 3a, which,
dimethoxyethene is a million times faster than that of thus, gives rise to the comparable reactivities of 1,1-dialk-
methoxyethene (Table 3).[14]
oxyalkenes and the corresponding siloxy derivatives.
Eur. J. Org. Chem. 2004, 2791Ϫ2796
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T. Tokuyasu, H. Mayr
FULL PAPER
A comparison of the reactivity parameters of ketene acet- do not occur in any of the reactions so far investigated.[18]
als 1b and 1f[7,8] shows that a phenoxy group activates the To examine whether electron transfer takes place in the re-
CϭC double bond considerably less than does a butoxy actions examined in this work, the actually observed acti-
group (Scheme 4).
vation free energies, ∆G‡obsd., were compared with the free
energies of electron transfer, ∆G°ET
.
From the linear correlation between the one-electron re-
duction potentials of benzhydrylium ions and their empiri-
cal electrophilicity parameters, E, we derived Equation (2),
which relates the free energy of one-electron transfer
(kJ·molϪ1) with the oxidation potential of the correspond-
ing nucleophile (E°ox vs. SCE) and the empirical electrophil-
icity parameter E of the benzhydrylium ion.[18]
Scheme 4. Substituent effects on the nucleophilic reactivity of ke-
tene acetals
∆G°ET ϭ 96.5E°ox Ϫ 1.91 Ϫ 6.85E
(2)
It has been noticed repeatedly that the slope parameter s
for π-systems is largely dependent on the number of alkyl
groups at the nucleophilic center.[8] In line with previous
observations in the alkene and allylsilane series, compounds
1a, 1b, and 1c (terminal double bond) have similar slope
parameters (0.79Ϫ0.82; Table 2). The slope parameters in-
crease, however, from 1b to 1d to 1g as the number of
methyl groups at the position of electrophilic attack in-
creases (Scheme 5). Consistent with this trend, the s param-
eter of 1e is considerably larger than that of 1h.
When ∆G°ET, as calculated by Equation (2), is plotted
against the electrophilicity parameters E of benzhydrylium
ions (Figure 3)[19,20], we see that over the whole range in
which Equation (1) is applicable (10Ϫ5 Ͻ k Ͻ 108 Ϫ1 sϪ1),
∆G°ET is considerably larger than the experimentally ob-
served activation free energy for the electrophile nucleophile
combinations. Consequently, outer-sphere electron transfer
can be ruled out for the reactions studied in this work.
Scheme 5. Effect of methyl substituents at the position of electro-
philic attack (N/s). Nucleophilicity parameters for 1h and 1g were
taken from ref.[7]
Although a rationalization for the decrease in N and the
increase in s by the number of methyl groups at the position
of electrophilic attack (as shown in Scheme 5) has been at-
tempted,[15] a conclusive explanation for this phenomenon
is still missing.
Two possible pathways for the combinations of electro-
philes with nucleophiles, namely electron transfer and polar
reactions, have been discussed.[16Ϫ18] While Lewis acid-pro-
moted MukaiyamaϪMichael reactions of ketene acetals
have been proposed to proceed not only by the simple polar
reaction pathways but also by electron transfer,[17a] outer-
sphere electron transfer processes have been excluded for
the reactions of trityl cations with ketene acetals.[17b] Re-
cently, we have examined combination reactions of our ref-
erence electrophiles 2 with a large variety of nucleophiles,
Figure 3. Comparison of the calculated free energies for electron
transfer ∆G°ET [from Equation (2)] and experimental activation
free energies ∆G‡obsd.(20 °C) for the reactions of ketene acetals (1)
with benzhydrylium ions (2) in the range of 100 Ͼ ∆G‡obsd.
Ͼ
27 kJ·molϪ1 corresponding to Ϫ5 Ͻ log k2 Ͻ 8; E°ox(1a) ϭ ϩ1.14
V (vs SCE, from ref.[19]); E°ox(1b) ഠ E°ox [1-ethoxy-1-(trimethyl-
and have found that outer-sphere electron transfer processes silyloxy)ethene] ϭ 1.28 V (vs SCE, from ref.[20]
)
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Nucleophilic Reactivities of Ketene Acetals
FULL PAPER
Conclusion
Ethyl 3,3-Bis[4-(dimethylamino)phenyl]propionate (4a): Ethanol
(1.5 mL) was added to a solution of (dma)2CHϩBF4Ϫ (2d; 165 mg,
0.485 mmol) in CH3CN (7.5 mL) at Ϫ40 °C. Immediately after-
wards, a solution of 2,6-di-tert-butylpyridine (810 mg, 4.23 mmol)
and ketene acetal 1a (482 mg, 4.15 mmol) in CH3CN (1.0 mL) was
added. The reaction mixture was warmed to room temperature
over 20 min, and then poured into 0.1  KOH solution (20 mL).
After extraction with CH2Cl2 (2 ϫ 20 mL), the combined organic
phases were washed with brine and dried (MgSO4). The solvent
was evaporated under reduced pressure, and the components were
separated by column chromatography on silica gel (toluene/diethyl
ether, 97:3) to give 4a as an oil that contained small amounts of
inseparable impurities (35 mg, ca. 21%). 1H NMR: δ ϭ 1.12 (t, J ϭ
7.1 Hz, 3 H, OCH2CH3), 2.88 (s, 12 H, NMe2), 2.96 (d, J ϭ 8.1 Hz,
2 H, 2-H), 4.02 (q, J ϭ 7.1 Hz, 2 H, OCH2CH3), 4.37 (t, J ϭ
The substituent effects on the nucleophilic reactivities of
ketene acetals are in close analogy to those previously ob-
served for other CϭC double bond systems.[8] The good
accessibility of compounds 1bϪ1d, their ease of handling,
and their clean reactions with electrophiles suggest that
these compounds are suitable as reference nucleophiles, al-
though the rate constants of their reactions with benzhydry-
lium ions were not employed for the determination of the
parameters E, N, and s of the reference compounds. Actu-
ally, this proposal deviates from our previous strategy that
no further reference compounds in the range Ϫ6 Ͻ N Ͻ 12
should be defined.[7] It should be noted, however, that the
consideration of these reactions in the original correlation
analysis,[7] which included the full set of reference com-
pounds, would not have affected any of the other reactivity
parameters, because the excellent correlations of log k2 vs.
E observed in this work indicate a perfect match with the
previously determined electrophilicity parameters.
8.1 Hz, 1 H, Ar2CH), 6.65, 7.09 (2 d, AAЈBBЈ system with JAB
ϭ
8.7 Hz, 2 ϫ 4 H, ArH) ppm. 13C NMR: δ ϭ 14.1 (q, OCH2CH3),
40.7 (q, NMe2), 41.43 (t, C-2), 45.3 (d, Ar2CH), 60.2 (t,
OCH2CH3), 112.8, 128.2 (2 d, Ar), 132.5, 149.2 (2 s, Ar), 172.3
(s, C-1) ppm. HRMS (EI): calcd. m/z for C21H28N2O2, 340.2151;
found, 340.2145.
Butyl 3,3-Bis[4-(dimethylamino)phenyl]propionate (4b). A: 1-Bu-
toxy-1-(trimethylsiloxy)ethene (1b; 368 mg, 1.95 mmol) was added
to a solution of (dma)2CHϩBF4 (2d; 340 mg, 1.00 mmol) in
Ϫ
Experimental Section
CH2Cl2 (20 mL) under nitrogen. After stirring for 20 min, the reac-
tion mixture was poured into 1.5  aq. NH3 solution (20 mL) and
then the organic phase was separated. The aqueous phase was ex-
tracted with CH2Cl2 (20 mL). The combined organic phases were
washed with brine and dried (MgSO4). After evaporation of the
solvent under reduced pressure, the product was purified by column
chromatography on silica gel (toluene/diethyl ether, 97:3) to give 4b
(354 mg, 96%); m.p. 60Ϫ61 °C (hexane). 1H NMR: δ ϭ 0.85 (t,
J ϭ 7.3 Hz, 3 H, CH3), 1.18Ϫ1.28, 1.42Ϫ1.50 (2 m, 2 ϫ 2 H,
CH2CH2), 2.88 (s, 12 H, NMe2), 2.96 (d, J ϭ 8.1 Hz, 2 H, 2-H),
3.97 (t, J ϭ 6.7 Hz, 2 H, OCH2), 4.37 (t, J ϭ 8.0 Hz, 1 H, Ar2CH),
6.65, 7.09 (2 d, AAЈBBЈ system with JAB ϭ 8.7 Hz, 2 ϫ 4 H, ArH)
ppm. 13C NMR: δ ϭ 13.7 (q, CH3), 19.0 (t, CH2), 30.6 (t, CH2),
40.7 (q, NMe2), 41.4 (t, C-2), 45.3 (d, Ar2CH), 64.1 (t, OCH2),
112.8, 128.2 (2 d, Ar), 132.5, 149.2 (2 s, Ar), 172.4 (s, C-1) ppm.
C23H32N2O2 (368.51): calcd. C 74.96, H 8.75, N 7.60; found C
74.84, H 8.72, N 7.55.
General Remarks: The benzhydrylium cations used in this work
were prepared by the methods described in ref.[7]. 1,1-Diethoxy-
ethene (1a) was purchased (Fluka), and silyl ketene acetals 1b,[21]
1c,[21] 1d,[22] and 1e[23] were prepared by the reported methods. H
1
NMR (400 MHz) and 13C NMR (100 MHz) spectra were obtained
in CDCl3 solutions (25 °C) using tetramethylsilane as the internal
standard.
Kinetics: The kinetics of ketene acetals 1 with benzhydrylium ions
2 were followed by UVϪVis spectroscopy in CH2Cl2. Solutions of
Ar2CHϩBF4Ϫ were combined with an excess amount (Ͼ 10 equiva-
lents) of the ketene acetal solutions, and then the decay of the car-
benium absorbance at 590Ϫ640 nm was monitored as a function
of time to give pseudo-first-order kinetics.
Slow reactions (τ1/2 Ͼ 10 s) were measured using a J&M TIDAS
diode array spectrophotometer that was connected to a Hellma
661.502-QX quartz Suprasil immersion probe (5 mm light path)
through fiber optic cables using standard SMA connectors. All ki-
netic measurements were conducted in Schlenk flasks with the ex-
clusion of moisture, and the temperature of the solutions during the
kinetic measurements was maintained by circulating bath cryostats
within Ϯ 0.1 °C (water/glycol bath for T Ͼ Ϫ10 °C; ethanol bath
for T Ͻ Ϫ10 °C) and monitored using a thermocouple probe that
was inserted into the reaction mixture. When kinetic experiments
were conducted at variable temperatures, second-order rate con-
stants k2 were calculated at 20 °C from the Eyring equation.
B: 1-Butoxy-1-(tert-butyldimethylsiloxy)ethene (1c; 190 mg,
0.825 mmol) was added to a solution of (dma)2CHϩBF4 (2d;
Ϫ
177 mg, 0.520 mmol) in CH2Cl2 (10 mL) under nitrogen. After stir-
ring for 3 min, the reaction mixture was treated as described for
the reaction with 1b. Purification of the crude product by column
chromatography on silica gel (toluene/diethyl ether, 96:4) gave 4b
(185 mg, 97%).
3,3-Bis[4-(dimethylamino)phenyl]-2-methylpropionic
Acid
(5d):
Bis(silylated) ketene acetal 1d (162 mg, 0.742 mmol) was added to
a solution of (dma)2CHϩBF4Ϫ (2d; 114 mg, 0.335 mmol) in CH2Cl2
(20 mL) under a nitrogen atmosphere. After stirring for 3 min, the
reaction mixture was poured into water (20 mL) buffered at pH 5.
The organic phase was separated and the aqueous phase was ex-
tracted with CH2Cl2 (15 mL). The combined organic phases were
washed with brine and dried (MgSO4) and then the solvent was
evaporated under reduced pressure. The crude product (95 mg),
which contained colored contaminants, was dissolved in diethyl
ether and basified to pH 13 with aqueous KOH solution and then
the phases were separated. The aqueous phase was acidified to pH
5 by adding 2  HCl. After extraction with CH2Cl2, the organic
phase was dried (MgSO4). Evaporation of the solvent gave 5d
A stopped-flow spectrophotometer system (Hi-Tech SF-61DX2
controlled by Hi-Tech KinetAsyst2 software) was used to investi-
gate rapid reactions (τ1/2 Ͻ 10 s), and pseudo-first-order rate con-
stants kobsd. (sϪ1) were obtained by least-square fitting of the single
exponential At ϭ A0 exp(Ϫkobsd.t) ϩ C to the absorbance data
(averaged from at least four kinetic runs at each nucleophile con-
centration).
The detailed kinetic data (concentrations and rate constants of the
individual kinetic experiments) are given in the Supporting Infor-
mation (see also the footnote on the first page of this article).
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T. Tokuyasu, H. Mayr
FULL PAPER
[3] [3a]
M. Arend, B. Westermann, N. Risch, Angew. Chem. 1998,
(75 mg, 69%) as a colorless solid material; m.p. 199Ϫ200 °C
(CH2Cl2/cyclohexane). 1H NMR (CD2Cl2): δ ϭ 1.09 (d, J ϭ
6.8 Hz, 3 H, 2-Me), 2.84, 2.86 (2 s, 2 ϫ 6 H, NMe2), 3.22 (dq, J ϭ
11.7, 6.8 Hz, 1 H, 2-H), 3.84 (d, J ϭ 11.5 Hz, 1 H, Ar2CH), 6.62,
6.65, 7.07, 7.12 (4 d, 2 AAЈBBЈ systems with JAB ϭ 8.8 Hz, 4 ϫ 2
H, ArH) ppm. 13C NMR (CD2Cl2): δ ϭ 17.2 (q, 2-Me), 40.62,
40.63 (2 q, NMe2), 44.4 (d, C-2), 53.1 (d, Ar2CH), 113.0, 113.1,
128.0, 128.6 (4 d, Ar), 131.3, 132.4, 149.4, 149.5 (4 s, Ar), 180.4 (s,
CO2H) ppm. HRMS (FAB): calcd. m/z for C20H27N2O2 ([M ϩ H])
327.2073; found m/z, 327.2070.
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66Ϫ77.
[6]
[7]
1
(CH2Cl2/pentane). H NMR: δ ϭ 1.24 (s, 3 H, 3-Me), 2.11 (ddd,
J ϭ 13.0, 8.1, 4.9 Hz, 1 H), 2.73Ϫ2.80 (m, 1 H), 2.85, 2.91 (2 s, 2
ϫ 6 H, NMe2), 3.61 (td, J ϭ 8.9, 5.0 Hz, 1 H), 4.02Ϫ4.08 (m, 1
H), 4.32 (s, 1 H, Ar2CH), 6.59 (d, AAЈBBЈ system with JAB
[8]
[9]
ϭ
8.9 Hz, 2 H, Ar), 6.69 (d, AAЈBBЈ system with JAB ϭ 8.7 Hz, 2 H,
Ar), 7.13 (d, AAЈBBЈ system with JAB ϭ 8.9 Hz, 2 H, Ar), 7.17(d,
AAЈBBЈ system with JAB ϭ 8.7 Hz, 2 H, Ar) ppm. 13C NMR: δ ϭ
24.7 (q, 3-Me), 31.7 (t, C-4), 40.40, 40.44 (2 q, NMe2), 47.7 (s, C-
3), 55.1 (d, Ar2CH), 65.0 (t, C-5), 112.3, 112.5 (2 d, Ar), 128.5,
129.1 (2 s, Ar), 129.9, 130.0 (2 d, Ar), 149.0, 149.1 (2 s, Ar), 182.7
(s, C-2) ppm. C22H28N2O2 (352.47): calcd. C 74.97, H 8.01, N 7.95;
found C 74.87, H 8.02, N 7.86.
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2796
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 2791Ϫ2796