the conversions of 2a and 2b into 3a and 3b, respectively,
proceeded already at 0 °C, while elevated temperatures
were necessary for the preparation of the perfluorophenyl
substituted product 3c.8 Enol ethers 3b and 3c were
isolated in yields of 87% and 75%, respectively. We
attribute the lower yield of 66% for 3a to its higher
volatility.
with the silyl enol ethers 3aꢀc with conveniently measur-
able rates.
Table 1. Reference Electrophiles Utilized for Quantifying the
Nucleophilicities of 3aꢀc
In order to quantify the influence of the electron-with-
drawing substituents on the nucleophilic reactivities of the
corresponding trimethylsilyl enol ethers 3aꢀc, the benzhy-
drylium method9 was employed, which allowed us to
compare the data with those of previously investigated
silyl enol ethers.9a,b,10
Benzhydrylium ions with variable p- and m-substituents,
which cover a broad range of reactivity while the steric
shielding of the reaction center is kept constant, have
been used as reference electrophiles for the construc-
tion of a comprehensive nucleophilicity scale based
on eq 1, where electrophiles are characterized by one
parameter (E) and nucleophiles are characterized by
the solvent-dependent parameters sN (slope) and N
(nucleophilicity).9a
log k(20°C) ¼ sN(N þ E)
ð1Þ
a Empirical electrophilicities E from ref 9a. b The electrophilicities of
4a and 4b in ref 9a have been revised in ref 11.
The benzhydrylium ions 4aꢀf (Table 1) with electro-
philicity parameters E ranging from 0 to þ5.47 reacted
The fluorinated enol ethers 3aꢀc gave analogous reac-
tion products as previously investigated silylated enol
ethers,10 as demonstrated by the formation of 6a and 6b
from 4a-GaCl4 and 3a or 3b, respectively, and of 6c from
4f-GaCl4 and 3c (Scheme 2).
(5) For examples with special substitution patterns(in particular with
additional electron-withdrawing groups), see: (a) Jullien, J.; Pechine,
J. M.; Perez, F.; Piade, J. J. Tetrahedron 1982, 38, 1413–1416. (b) Seifert,
€
F. U.; Roschenthaler, G.-V. J. Fluorine Chem. 1994, 68, 169–174. (c)
Neumayer, D. A.; Belot, J. A.; Feezel, R. L.; Reedy, C.; Stern, C. L.;
Marks, T. J.; Liable-Sands, L. M.; Rheingold, A. L. Inorg. Chem. 1998,
37, 5625–5633. (d) Kruchok, I. S.; Gerus, I. I.; Kukhar, V. P. Tetra-
hedron 2000, 56, 6533–6539. (e) Dilman, A. D.; Belyakov, P. A.;
Korlyukov, A. A.; Tartakovsky, V. A. Tetrahedron Lett. 2004, 45,
3741–3744. (f) Chung, W. J.; Ngo, S. C.; Higashiya, S.; Welch, J. T.
Tetrahedron Lett. 2004, 45, 5403–5406. (g) Levin, V. V.; Dilman, A. D.;
Belyakov, P. A.; Korlyukov, A. A.; Struchkova, M. I.; Tartakovsky,
V. A. Eur. J. Org. Chem. 2004, 5141–5148. (h) Chizhov, D. L.;
As the intermediates 5 and the products 6 are colorless,
the nucleophilic attack at the electrophilic center was
followed spectrophotometrically. Addition of at least
8 equiv of the trimethylsilyl enol ethers 3 to solutions of
the colored benzhydrylium tetrachlorogallates 4-GaCl4 in
CH2Cl2 led to monoexponential decays of the absorbances
of the benzhydrylium ions (Figure 1). As previous studies
have shown that the rates of the reactions are only slightly
affected by the counterion and the desilylations to prod-
ucts 6 are fast,10 the measured reaction rates correspond
to the formation of the CꢀC bonds.
€
€
Roschenthaler, G.-V. J. Fluorine Chem. 2006, 127, 235–239. (i) Buttner,
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€
(k) Buttner, S.; Bendrath, F.; Langer, P. Tetrahedron Lett. 2010, 51,
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(6) Compound 2b was previously described, but its synthesis was not
specified: Nakazawa, N.; Kawamura, M.; Sekiya, A.; Ootake, K.;
Tamai, R.; Kurokawa, Y.; Murata, J. Nippon Reito Kucho Gakkai
Ronbunshu (Trans. of the JSRAE) 2001, 18, 427–434.
(7) (a) Kondo, A.; Iwatsuki, S. J. Fluorine Chem. 1984, 26, 59–67.
(b) Croxtall, B.; Fawcett, J.; Hope, E. G.; Stuart, A. M. J. Fluorine
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Scheme 2. Reactions of 3 with the Benzhydrylium Salts 4-GaCl4
(8) Gostevskii, B. A.; Vyazankina, O. A.; Vyazankin, N. S. Zh.
Obshch. Khim. 1984, 54, 2613–2617; J. Gen. Chem. USSR 1985, 54,
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(10) (a) Burfeindt, J.; Patz, M.; Muller, M.; Mayr, H. J. Am. Chem.
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The reactions with the highly electrophilic benzhy-
drylium ions 4aꢀd were studied between ꢀ70 and ꢀ10 °C.
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