Nucleophilic reactivities of silyl ketene acetals and silyl enol ethers containing (C6F5)3SiO and (C6H 5)3SiO groups

Article abstract of DOI:10.1002/ejoc.200400706

The kinetics of the reactions of tris(pentafluorophenyl)silyl and triphenylsilyl ketene acetals and enol ethers with benzhydrylium cations have been measured photometrically. The second-order rate constants (log k ₂) have been used to determine the nucleophilicity parameters of these π systems, which provide a quantitative comparison of their reactivities with those of other types of nucleophiles. A change from, trimethylsilyloxy to triphenylsilyloxy reduces reactivity by only a factor of ten, while replacement of triphenylsilyloxy with tris(pentafluorophenyl)silyloxy decreases the nucleophilicity of the enolate C=C double bond by three to four orders of magnitude. The cation-stabilizing ability of the tris(pentafluorophenyl)silyloxy group is thus similar to that of a methyl group. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejoc.200400706

FULL PAPER
Nucleophilic Reactivities of Silyl Ketene Acetals and Silyl Enol Ethers
Containing (C6F5)3SiO and (C6H5)3SiO Groups
Alexander D. Dilman*[a] and Herbert Mayr[b]
Keywords: Silicon / Perfluorinated ligands / Silyl ketene acetals and enol ethers / Kinetics / Substituent effects
The kinetics of the reactions of tris(pentafluorophenyl)silyl
and triphenylsilyl ketene acetals and enol ethers with benz-
hydrylium cations have been measured photometrically. The
second-order rate constants (log k2) have been used to deter-
mine the nucleophilicity parameters of these π systems,
which provide a quantitative comparison of their reactivities
with those of other types of nucleophiles. A change from tri-
methylsilyloxy to triphenylsilyloxy reduces reactivity by only
a factor of ten, while replacement of triphenylsilyloxy with
tris(pentafluorophenyl)silyloxy decreases the nucleophilicity
of the enolate C=C double bond by three to four orders of
magnitude. The cation-stabilizing ability of the tris(pen-
tafluorophenyl)silyloxy group is thus similar to that of a
methyl group.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
tions.[7] This observation stands in sharp contrast with the
behavior of conventional trialkylsilyl enol ethers, which do
not react with aldehydes in the absence of additional acti-
vators.[1a,1c] For the efficient elaboration of methods for al-
dol and related reactions it is therefore important to under-
stand the magnitude of the electronic influence of substitu-
ents at silicon on the reactivity of the enolate C=C double
bond.
In this study we have quantitatively evaluated the effect
of silicon substitution on the nucleophilicity of silyl ketene
acetals 1a and 1b and the silyl enol ethers 2a, 2b, 3a, and
3b by studying the kinetics of their reactions with benzhy-
drylium ions (Table 1). The reactivities of (C6F5)3SiO- and
(C6H5)3SiO-substituted C=C double bonds were then com-
pared with those of Me3SiO-, Me-, and H-containing deriv-
atives.
Introduction
Silyl ketene acetals and silyl enol ethers have found wide-
spread use as nucleophilic reagents in aldol,[1] Michael,[2]
and Mannich[3] reactions. In these processes, electrophilic
attack at the carbon–carbon double bond is followed by
rapid desilylation (Scheme 1). In accord with this mecha-
nism, the reactivity of the double bond is primarily deter-
mined by the substituent R1 adjacent to the silyloxy group,
as has been demonstrated by extensive investigations of tri-
alkylsilyl derivatives.[4,5] The silyl group serves mainly as a
positively charged leaving group serving to quench the car-
bocationic center, thus preventing the polymerization of the
π system.[6]
Table 1. Nucleophilic π systems.
Scheme 1.
Recently, one of us introduced a new type of enol ethers
containing tris(pentafluorophenyl)silyl groups, and these
compounds smoothly undergo uncatalyzed aldol reac-
[a] N. D. Zelinsky Institute of Organic Chemistry,
119991 Moscow, Leninsky prosp. 47, Russian Federation
Fax: +7-095-135-5328
E-mail: adil25@mail.ru
[b] Department Chemie und Biochemie der Ludwig-Maximilians-
Universität,
München, Butenandtstr. 5–13 (Haus F), 81377 München, Ger-
many
Fax: +49-89-2180-77717
Results and Discussion
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.
Our approach is based on previous work by the
München group that demonstrates that the reactions of car-
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200400706
Eur. J. Org. Chem. 2005, 1760–1764

Silyl Ketene Acetals and Silyl Enol Ethers with (C6F5)3SiO and (C6H5)3SiO Groups
FULL PAPER
bocations with π nucleophiles can be analyzed by Equa-
tion (1)
Analogously with previously described reactions,[4,5] cat-
ions 4 reacted readily with compounds 1–3 in dichloro-
methane, affording the corresponding products 5. This reac-
tion course was investigated for the combinations listed in
Scheme 2.
log k = s (N + E)
(1)
where k is the second-order rate constant of a reaction at
20 °C, N is the nucleophilicity parameter, E is the electro-
philicity parameter, and s is a nucleophile-specific slope
parameter.[8] As reference electrophiles we employed the
benzhydrylium cations 4a–h,[9] either employed as stable tet-
rafluoroborates (4a–g·BF4) or generated in situ from the
[10]
precursor 4h-Cl and BCl3 (Table 2).
Table 2. Benzhydrylium ions 4 used as reference electrophiles.
Scheme 2.
The kinetics of the reactions of the benzhydrylium ions
4 with the nucleophiles 1–3 (Ͼ10 equiv.) in dichlorometh-
ane at 20 °C were monitored by UV/Vis spectroscopy with
the instrumentation described previously.[9,11] From the ex-
ponential decays of the carbenium ions’ absorbances we de-
rived the pseudo-first-order rate constants, which were plot-
ted against nucleophile concentrations to afford second-or-
der rate constants (Table 3). For some reactions of com-
pounds 1a, 1b, 2a, and 2b, activation parameters were deter-
mined from experiments at variable temperature. Plotting
of the logarithms of the second-order rate constants against
Table 3. Second-order rate constants and activation parameters for the reactions of benzhydrylium ions 1 with nucleophiles 1–3 (CH2Cl2)
and calculated nucleophilicity (N) and slope (s) parameters for 1–3.
[a]
Assumed slope as for analogous trimethylsilyl derivative.[4,8a]
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A. D. Dilman, H. Mayr
FULL PAPER
the electrophilicity parameters of benzhydrylium ions[9]
Figure 2 illustrates the influence of different silyloxy
(Figure 1) allows one to derive nucleophilicity (N) and slope groups on the reactivity of different π systems. Comparison
(s) parameters of nucleophiles 1–3 by using Equation (1).
of the pairs 1c/1b, 2c/2b, and 3c/3b shows that substitution
of Me3SiO by Ph3SiO leads to a decrease in double bond
reactivity by a factor of approximately ten. Perfluorination
of the phenyl rings diminishes the nucleophilicity by three
to four orders of magnitude, (cf. 1a/1b, 2a/2b, and 3a/3b),
due to the significant electron-withdrawing influence of
fluorine.[14] This inductive effect is so strong that it largely
compensates for the electron-donating ability of oxygen,
thus making the tris(pentafluorophenyl)silyloxy group be-
have more like an alkyl substituent (cf. 2a with 1-methylcy-
clopentene and 3a with isobutylene).
Figure 1. Correlation of the rate constants (log k2, at 20 °C, in
CH2Cl2) for the reactions of nucleophiles with the benzhydrylium
ions (4) in relation to their electrophilicity parameters E.
In order to compare the effects of R3SiO groups with
that of hydrogen we also studied the nucleophilicity of 2,3-
dihydrofuran 1d. However, alkyl enol ethers readily poly-
merize when treated with electrophiles,[6] and for that
reason trifluoroethanol and 2,6-lutidine were added to the
reaction mixtures when the kinetics of electrophilic ad-
ditions to 2,3-dihydrofuran were studied (Scheme 3).[12] Be-
cause of the stabilizing anomeric interaction of two alkoxy
groups, the intermediate carboxonium ion shows a higher
selectivity for alcohols (compared to π nucleophiles) than
the initial benzhydrylium ion. On a preparative scale the
product 5f was isolated as a 1.3:1 mixture of diastereoiso-
mers in 38% yield from the reaction of 1d with 4e-BF4.
The kinetics of the reaction of 1d with the bis(p-chloro)
benzhydrylium ion were studied previously, giving the rate
constant of 1.3×109 Lmol–1 s–1.[13] This point was not
taken for the calculation of N and s because of the proxim-
ity of the diffusion limit. Nevertheless, the rate constant of
the reaction of 1d with (p-ClC6H4)2CH+ as calculated by
Equation (1) with the parameters of 1d determined here af-
fords the value of 2.2×109 Lmol–1 s–1, which is very close
to that found experimentally.
Figure 2. Relative nucleophilicities of silyl ketene acetals, enol
ethers, and alkenes. N parameters of 1c,[9] 2c,[4] 3b, 3c,[4] and al-
kenes[8a,9] are taken from the literature.
In view of the low nucleophilicities of the tris(penta-
fluorophenyl)silyl enol ethers 2a and 3a, their uncatalyzed
coupling with aldehydes is remarkable.[7a] Obviously, the
Lewis acidity of the silyl moiety plays an important role in
the rate-determining step of the aldol reaction (Scheme 4).
Since the three pentafluorophenyl groups have been shown
in this work to reduce the nucleophilicity of the C=C
double bond significantly, it must be concluded that in reac-
tions with aldehydes this effect is overcompensated by the
increased Lewis acidity of the silicon atom.
Scheme 3.
Scheme 4.
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Silyl Ketene Acetals and Silyl Enol Ethers with (C6F5)3SiO and (C6H5)3SiO Groups
FULL PAPER
(CH2), 38.8 (CH2), 104.7 (CH), 128.3 (CHPh), 130.6 (CHPh), 134.4
(CPh), 135.9 (CHPh), 155.1 (CO) ppm. HRMS (EI): calcd. m/z for
C23H22OSi, 342.1440; found 322.1447.
In summary, the reactivity of silyl ketene acetals and silyl
enol ethers may be considerably affected by the nature of
the silyl group substituents. While replacement of methyl by
phenyl (that is, sp3 by sp2) has a weak effect, the reactivity
decreases dramatically with incorporation of fluorine atoms
in the phenyl ring.
3-{Bis[4-(dimethylamino)phenyl]methyl}tetrahydrofuran-2-one (5a):
A solution of 1a (420 mg, 0.68 mmol) in CH2Cl2 (5 mL) was added
to a solution of (dma)2CH·BF4 (194 mg, 0.57 mmol) in CH2Cl2
(5 mL) at room temperature. The mixture was stirred for 30 min,
quenched with a solution of NH4F (42 mg) and AcOH (102 mg) in
MeOH (2.5 mL)/H2O (0.5 mL), stirred for an additional 15 min,
filtered through celite (solution in CH2Cl2), dried (CaSO4), and
concentrated under vacuum. Flash column chromatography on sil-
ica gel (ether/pentane/NEt3 from 1:1:0.01 to 3:1:0.01) gave a sample
containing traces of butyrolactone, which was removed by heating
at 90–100 °C/0.03 mbar for 2 h to afford 5a (137 mg, 71%) as a
greenish oil. Rf (ether/pentane, 1:1) = 0.26. 1H NMR (CDCl3): δ =
2.08 (dq, J = 8.2, 12.9 Hz, 1 H), 2.27–2.39 (m, 1 H), 2.81 (s, 6 H,
NMe2), 2.84 (s, 6 H, NMe2), 3.23 (ddd, J = 4.4, 8.1, 9.4 Hz, 1 H),
3.69 (td, J = 5.1, 8.5 Hz, 1 H), 4.02 (dt, J = 7.6, 8.7 Hz, 1 H), 4.48
(d, J = 4.4 Hz, 1 H, CHAr2), 6.54 (d, J = 8.8 Hz, 2 H, Ar), 6.61
(d, J = 8.7 Hz, 2 H, Ar), 6.93 (d, J = 8.8 Hz, 2 H, Ar), 7.04 (d, J
= 8.7 Hz, 2 H, Ar) ppm. 13C NMR (CDCl3): δ = 26.2 (CH2CH),
40.96 (NMe2), 41.04 (NMe2), 44.3 and 49.0 (CHCH), 66.9 (CH2O),
112.9 and 113.0 (2 CHAr), 129.2 and 130.1 (2 CHAr), 129.9 and
131.1 (2 CAr), 149.6 and 149.8 (2 CAr), 179.4 (C=O) ppm. IR (film,
Experimental Section
General Remarks: The benzhydrylium cations[9] used in this work
and the tris(pentafluorophenyl)silyl ketene acetal 1a[7b] and enol
ethers 2a and 3a[7b] were prepared by literature procedures. NMR
spectra were recorded on a Bruker ARX 300 instrument in CDCl3
distilled from CaH2 and stored over MS (4A).
Kinetics: Measurements were performed with a J&M TIDAS diode
array spectrophotometer connected to a Hellma 661.502-QX
quartz Suprasil immersion probe (5 mm light path) through fiber
optic cables using standard SMA connectors. All kinetic measure-
ments were conducted in Schlenk flasks with the exclusion of
moisture, and the temperature of the solutions during the kinetic
measurements was maintained by circulating bath cryostats within
0.1 °C (water/glycol bath for T Ͼ –10 °C; ethanol bath for
T Ͻ –10 °C) and monitored with a thermocouple probe inserted
into the reaction mixture.
cm–1): ν = 2985, 2886, 2801, 1766, 1614, 1520, 1481, 1445, 1349,
˜
1210, 1161, 1061, 1026, 948, 810, 734, 573. MS (EI): m/z (%) = 338
(8) [M]+, 254 (19), 253 (100) [4e], 237 (12). HRMS (EI): calcd.
m/z for C21H26N2O2, 338.1994; found 338.1972.
5-(Triphenylsilyloxy)-2,3-dihydrofuran (1b):[15] Butyllithium (5.3 mL
of a 2.5 m solution in hexane, 13.2 mmol) was added to a solution
of iPr2NH (2.0 mL, 14.5 mmol) in THF (30 mL) at 0 °C. The mix-
ture was stirred for 15 min at 0 °C and cooled to –78 °C, followed
by dropwise addition of butyrolactone (0.91 mL, 11.9 mmol). After
the system had been stirred for 10 min, a solution of Ph3SiCl
(4.28 g, 14.5 mmol) in THF (7 mL) was rapidly injected. The mix-
ture was allowed to warm to room temperature over 3 h. The sol-
vent was removed under vacuum, and the residue was diluted with
hexane (20 mL) and filtered through celite under nitrogen. The fil-
ter cake was washed with dry dichloromethane, with the filtrate
being collected into a separate flask. Evaporation of dichlorometh-
ane afforded 2.28 g of 1b containing ca. 15% of 3-(triphenylsilyl)
tetrahydrofuran-2-one, ca. 56% yield (this sample was used for kin-
etic experiments). M.p. 83–101 °C. 1H NMR (CDCl3): δ = 2.38 (td,
J = 2.1, 8.7 Hz, 2 H, CH2CH), 3.51 (t, J = 2.1 Hz, 1 H, CH), 4.06
(t, J = 8.7 Hz, 2 H, CH2O), 7.21–7.31 (m) and 7.54–7.59 (m) (15
H, 3 Ph) ppm. 13C NMR (CDCl3): δ = 28.9 (CH2C), 68.5 (CH2O),
70.4 (CH), 128.4 (CHPh), 130.9 (CHPh), 133.2 (CPh), 135.9 (CHPh),
158.7 (CO2) ppm. Evaporation of the hexane filtrate gave 2.50 g
containing 1b, 10% of 3-(triphenylsilyl)tetrahydrofuran-2-one, and
10% of other unidentified impurity. Attempted distillation failed
due to decomposition of 1b.
1-(Triphenylsilyloxy)cyclopentene (2b):[16] Triethylamine (0.88 mL,
6.4 mmol), cyclopentanone (470 μL, 5.3 mmol), and a solution of
NaI (0.79 g, 5.3 mmol) in MeCN (7 mL) were successively added to
a solution of Ph3SiCl (1.56 g, 5.3 mmol) in MeCN/benzene (12 mL/
2 mL) at room temperature. The mixture was stirred for 1.5 h and
extracted with hexane (3×20 mL). The combined hexane solution
was concentrated under vacuum, diluted with hexane, filtered, con-
centrated, and the residue was distilled (bulb-to-bulb, 230–245 °C/
3.9×10–2 mbar), followed by recrystallization from hexane (ca.
5 mL, with cooling to –10 °C) to furnish 2b (1.15 g, 63% yield) as
white crystals. M.p. 73–79 °C. 1H NMR (CDCl3): δ = 1.61–1.72
3-[Bis(julolidin-9-yl)methyl]dihydrofuran-2-one (5b). A solution of
(jul)2CH·BF4 (163 mg, 0.37 mmol) in CH2Cl2 (5 mL) was added to
a solution of 1b (147 mg, 0.37 mmol) in CH2Cl2 (2 mL). After stir-
ring for 30 min, the mixture was quenched by addition of a solution
of KF (1 g) and AcOH (1 mL) in MeOH (6 mL)/H2O (1 mL),
stirred for 30 min, poured into ether (70 mL), washed with water
(2×30 mL), dried (CaSO4), and concentrated under vacuum with
heating at ca. 100 °C. Flash column chromatography on silica gel
(hexane/ethyl acetate/NEt3 4:1:0.01) afforded 5b (59 mg, 36%
yield).[9]
2-{Bis[p-(dimethylamino)phenyl]methyl}cyclopentanone (5d): A solu-
tion of (dma)2CH·BF4 (173 mg, 0.51 mmol) in CH2Cl2 (6 mL) was
added to a solution of 2b (183 mg, 0.54 mmol) in CH2Cl2 (3 mL).
After stirring for 1 h, the mixture was quenched by addition of
concentrated aq. ammonia (10 mL), poured into ether (50 mL),
washed with water (4×20 mL), dried (CaSO4), and concentrated
under vacuum. Flash column chromatography on silica gel (hex-
ane/ether/NEt3 5:2:0.01) afforded 5d (105 mg, 61% yield).[4]
Reactions of Bis(4-methoxyphenyl)methyl Chloride with Tris(pen-
tafluorophenyl)silyl Enol Ethers (Compounds 5c, 5e): Gaseous BCl3
(60 mL) was bubbled through a solution of (ani)2CHCl (160 mg,
0.61 mmol) in CH2Cl2 (5 mL), followed by addition of a solution
of silyl enol ether 2a or 3a (0.67 mmol) in CH2Cl2 (3 mL). After
stirring for 10 min the mixture was quenched with a solution of
NH4F (250 mg) and AcOH (560 mg) in MeOH (2.5 mL)/H2O
(0.5 mL), stirred for an additional 15 min, filtered through celite
(solution in CH2Cl2), dried (CaSO4), and concentrated under vac-
uum. The product was purified by flash column chromatography
on silica gel with elution with hexane/ethyl acetate.
2-Bis(4-methoxyphenyl)methylcyclopentanone (5c): 67% yield. Rf
(hexane/ethyl acetate, 3:1) = 0.25. 1H NMR (CDCl3): δ = 1.53–1.90
(m, 2 H, CH2), 2.01–2.09 (m, 2 H, CH2), 2.12–2.20 (m, 2 H, CH2), (m, 4 H, CH2CH2), 2.09–2.25 (m, 2 H, CH2CO), 2.74–2.83 (m, 1
4.47 (quintet, J = 2.1 Hz, 1 H, CH), 7.22–7.32 (m) and 7.54–7.60 H, CHCO), 3.67 (s, 3 H, OMe), 3.70 (s, 3 H, OMe), 4.47 (d, J =
(m) (15 H, 3 Ph) ppm. 13C NMR (CDCl3): δ = 21.8 (CH2), 29.1
4.8 Hz, 1 H, CHAr2), 6.68 (d, J = 8.7 Hz, 2 H, Ar), 6.75 (d, J =
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A. D. Dilman, H. Mayr
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8.7 Hz, 2 H, Ar), 6.90 (d, J = 8.7 Hz, 2 H, Ar), 7.10 (d, J = 8.7 Hz,
2 H, Ar) ppm. 13C NMR (CDCl3): δ = 20.9 (CH2), 27.6 (CH2),
38.8 (CH2), 48.8 (CH), 53.6 (CH), 55.5 (OMe), 55.6 (OMe), 114.0
and 114.1 (2 CHAr), 129.6 and 130.5 (2 CHAr), 135.4 and 136.3 (2
[1]
a) T. Mukaiyama, Org. React. 1982, 28, 203–331; b) E. M. Car-
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C
Ar), 158.3 and 158.4 (2 CAr), 224.1 (C=O) ppm. IR (film, cm–1):
ν = 2959, 2836, 1737, 1609, 1510, 1464, 1248, 1178, 1034, 837, 582.
˜
[2]
[3]
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HRMS (EI): calcd. m/z for C20H22O3, 310.1569; found 310.1542.
4-Bis(4-methoxyphenyl)-2-butanone (5e): 82% yield.[17]
3-{Bis[4-(dimethylamino)phenyl]methyl}-2-(2,2,2-trifluoroethoxy)tet-
rahydrofuran (5f):
A solution of (dma)2CH·BF4 (299.3 mg,
0.88 mmol) in CH2Cl2 (10 mL) was added to a solution of 2,3-
dihydrofuran (1.00 mL, 13.1 mmol), CF3CH2OH (360 μL,
5.28 mmol), and 2,6-lutidine (307 μL, 2.64 mmol) in CH2Cl2
(2 mL) at room temperature. After stirring for 1 h, the mixture was
poured into ether (50 mL), washed with water (2×25 mL), dried
(CaSO4), and concentrated under vacuum. The crude sample con-
tained a 1.3:1 mixture of isomers. Column chromatography on sil-
ica gel in hexane/ethyl acetate (6:1) afforded two fractions with a
38% combined yield: the upper fraction contained the major iso-
[4]
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1
mer (75.3 mg): Rf = 0.26. H NMR (CDCl3): δ = 1.45–1.57 (m, 1
H), 1.98 (qt, J = 5.2, 7.9 Hz, 1 H, CH2CH2O), 2.789 (s, 6 H,
NMe2), 2.794 (s, 6 H, NMe2), 3.03 (dddd, J = 0.9, 4.4, 7.9, 12.4 Hz,
1 H, CHCH2), 3.42 (d, J = 12.4 Hz, 1 H, CHO2), 3.55 (dq, JH,F
=
8.7, JH,H = 12.1 Hz, 1 H, CHAHBCF3), 3.77 (dq, JH,F = 9.0, JH,H
= 12.1 Hz, 1 H, CHAHBCF3), 3.81–3.97 (m, 2 H, CH2O), 4.73 (s,
1 H, CHAr2), 6.56 (d, J = 8.8 Hz, 2 H, Ar), 6.57 (d, J = 8.8 Hz, 2
H, Ar), 7.01 (d, J = 8.8 Hz, 2 H, Ar), 7.07 (d, J = 8.8 Hz, 2 H,
Ar) ppm. 13C NMR (CDCl3): δ = 29.5 (CH2CH2O), 47.07 (NMe2),
47.11 (NMe2), 50.3 and 52.5 (CHAr2 and CHCHO2), 64.4 (q, JC,F
= 34.3 Hz, CH2CF3), 67.8 (CH2O), 108.1 (CHO2), 113.3 and 113.4
(2 CHAr), 124.4 (q, JC,F = 278.2 Hz, CF3), 128.7 and 128.8 (2
CHAr), 132.8 and 132.9 (2 CAr), 149.48 and 149.52 (2 CAr) ppm.
[7]
[8]
IR (film, cm–1): ν = 2947, 2889, 1614, 1520, 1480, 1446, 1348, 1279,
˜
1223, 1163, 1118, 1058, 987, 949, 805, 571. MS (EI): m/z (%) = 422
(14) [M]+, 254 (19), 253 (100) [4e], 237 (13). HRMS (EI): calcd.
m/z for C23H29F3N2O2, 422.2181; found 422.2209. Lower fraction
contained the minor isomer along with 20% of the major isomer
(65.5 mg): Rf = 0.26. 1H NMR (CDCl3): δ = 1.63–1.84 (m, 2 H,
CH2CH2O), 2.79 (s, 6 H, NMe2), 2.80 (s, 6 H, NMe2), 2.77–2.89
(m, 1 H), 3.51 (dq, JH,F = 8.7, JH,H = 11.9 Hz, 1 H, CHAHBCF3),
3.79 (dq, JH,F = 9.0, JH,H = 11.9 Hz, 1 H, CHAHBCF3), 3.80–3.88
(m, 3 H), 4.58 (d, J = 4.0 Hz, 1 H, CHAr2), 6.56 (d, J = 8.8 Hz, 2
H, Ar), 6.58 (d, J = 8.8 Hz, 2 H, Ar), 7.09 (d, J = 8.8 Hz, 4 H,
Ar) ppm. 13C NMR (CDCl3): δ = 28.7 (CH2CH2O), 41.1 (NMe2),
41.2 (NMe2), 49.3 and 50.4 (CHAr2 and CHCHO2), 64.3 (q, JC,F
= 34.6 Hz, CH2CF3), 68.3 (CH2O), 104.1 (CHO2), 113.2 and 113.3
(2 CHAr), 124.5 (q, JC,F = 278.2 Hz, CF3), 128.6 and 128.7 (2
CHAr), 133.2 and 133.6 (2 CAr), 149.39 and 149.44 (2 CAr) ppm.
[9]
H. Mayr, T. Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker,
B. Kempf, R. Loos, A. R. Ofial, G. Remennikov, H. Schimmel,
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[12] p-Amino-substituted benzhydrylium ions are stable in trifluor-
oethanol, see: S. Minegishi, S. Kobayashi, H. Mayr, J. Am.
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[13] J. Bartl, S. Steenken, H. Mayr, J. Am. Chem. Soc. 1991, 113,
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[14] According to Taft’s constants the pentafluorophenyl group (σ*
= 1.50) is a much stronger acceptor than phenyl (σ* = 0.60)
and lies between 3,5-dinitrophenyl (σ* = 1.37) and difluo-
romethyl (σ* = 2.00) groups, see: T. Korenaga, K. Kadowaki,
T. Ema, T. Sakai, J. Org. Chem. 2004, 69, 7340–7343.
[15] Jpn. Kokai Tokkyo Koho, 1983, JP 58162585 (Chem. Abstr.
1984, 100:68153r). For 1b neither procedure nor spectroscopic
data were reported.
Supporting Information (see also footnote on the first page of this
article): Concentrations and rate constants of the individual kinetic
runs.
[16] a) M. Penso, D. Albanese, D. Landini, V. Lupi, J. Mol. Cat.
A: Chem. 2003, 204–205, 177–185; b) D. Albanese, D. Landini,
M. Penso, S. Petricci, Synlett 1999, 199–200.
[17] C. Jia, W. Lu, J. Oyamada, T. Kitamura, K. Matsuda, M. Irie,
Y. Fujiwara, J. Am. Chem. Soc. 2000, 122, 7252–7263.
Received: October 7, 2004
Acknowledgments
This work was supported by INTAS (fellowship to A.D., project #
2003-55-1185) and by the Ministry of Science of the Russian Feder-
ation (project MK-2352.2003.03).
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Eur. J. Org. Chem. 2005, 1760–1764