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10 Other esters, such as 2-phenylethyl and (E)-3-phenyl-2-propen-1-yl ester,
also underwent the similar reaction to give the corresponding
b-reduction products in 64% and 50% yields, respectively.
Scheme 3 A possible reaction mechanism.
11 The c-product, which could be given by the reaction with crotyl bromide
at the c-position, was not observed at all.
A possible reaction mechanism is described in Scheme 3. The
starting ester 1 undergoes the vinylic fluorine–copper exchange
reaction with organocuprate to form the corresponding vinyl
copper species 2, which is stabilized due to the strong electron-
withdrawing effect of a CF3 group13 and is not susceptible to the
reductive elimination leading to 3 at 278 uC. By subsequent
treatment with various electrophiles including H2O, the vinyl
copper species 2 is converted to the corresponding tetrafluorocro-
tonates 4, along with the formation of homo-coupling product
(R–R). On the other hand, raising the reaction temperature from
278 to 220 uC promotes the elimination of Cu–F14 in 2, giving
rise to the corresponding alkynoate 6, which may undergo Michael
addition of an excess amount of cuprate to afford 3-substituted
-4,4,4-trifluorocrotonate 5.
12 T. Konno, T. Daitoh, A. Noiri, J. Chae, T. Ishihara and H. Yamanaka,
Tetrahedron, 2005, 61, 9391–9404; T. Konno, T. Daitoh, A. Noiri,
J. Chae, T. Ishihara and H. Yamanaka, Org. Lett., 2004, 6, 933–936.
13 T. Yamazaki, N. Shinohara, T. Kitazume and S. Sato, J. Fluorine
Chem., 1999, 97, 91–96; T. Yamazaki, H. Umetani and T. Kitazume,
Israel J. Chem., 1999, 39, 193–205; T. Yamazaki, H. Umetani and
T. Kitazume, Tetrahedron Lett., 1997, 38, 6705–6708.
14 For related reports on the interaction between metal and fluorine atom,
see: T. Yamazaki, J. Synth. Org. Chem., Jpn., 2004, 62, 911–918;
T. Ishihara, J. Synth. Org. Chem., Jpn., 1999, 57, 313–322; T. Ooi,
K. Furuta and K. Maruoka, Chem. Lett., 1998, 817–818; T. Ooi,
N. Kagoshima, D. Uraguchi and K. Maruoka, Tetrahedron Lett., 1998,
39, 7105–7108; T. Ooi, N. Kagoshima and K. Maruoka, J. Am. Chem.
Soc., 1997, 119, 5754–5755; T. Ooi, D. Uraguchi, N. Kagoshima and
K. Maruoka, Tetrahedron Lett., 1997, 38, 5679–5682; T. Yamazaki and
T. Kitazume, J. Synth. Org. Chem., Jpn., 1996, 54, 665–674;
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1990, 55, 4969–4971.
In summary, we have found that the fluorine–metal exchange
reaction of 2,3,4,4,4-pentafluorocrotonate
1 with Grignard
15 A typical procedure for the preparation of benzyl 2,4,4,4-tetrafluoro-3-
iodo-2-butenoate 4 (El 5 I) is as follows: A 30 mL two-necked round-
bottomed flask equipped with a magnetic stirrer bar, a rubber septum
and an inlet tube for argon was charged with a suspended solution of
CuCN (0.037 g, 0.418 mmol) in THF (1 mL). To this solution was
slowly added a solution of phenylmagnesium bromide (0.836 mmol) in
THF via a syringe at 278 uC. The whole was warmed up at 220 uC and
stirred for 15 min. To the resulting solution was added benzyl 2,3,4,4,4-
pentafluoro-2-butenoate (1, 0.050 g, 0.190 mmol) via a syringe at
278 uC. After being stirred at 278 uC for 1 h, the reaction mixture was
treated with iodine (0.241 g, 0.950 mmol) in THF at 278 uC for 1 h.
After stirring for 1 h, the reaction mixture was poured into ice-cooled
saturated aqueous NH4Cl (30 mL), followed by extraction with ether
(30 mL 6 5). The organic layers were dried over anhydrous sodium
sulfate, filtered and concentrated with a rotary evaporator. Column
chromatography of the residue using hexane/benzene (2 : 1) yielded pure
benzyl 2,4,4,4-tetrafluoro-3-iodo-2-butenoate (0.055 g, 78%). 4 (El 5 I):
1H NMR (CDCl3) d 5.35 (s, 2H), 7.35–7.50 (m, 5H); 19F NMR (CDCl3,
reagents in the presence of copper(I) salt takes place efficiently to
generate the b-metallated tetrafluorocrotonate intermediate, of
which hydrolysis gives the b-reduction product 4-H in good yield.
Treatment of the organocopper intermediate with various electro-
philes, such as iodine, allyl bromide, methallyl bromide and crotyl
bromide, gave the corresponding 3-substituted 2,4,4,4-tetrafluoro-
crotonates 4 in good yields (65–82%).15 Further study of the scope,
mechanistic implications, and synthetic applications of the
reactions is currently ongoing in our laboratory.
Notes and references
1 J. W. Dankwardt, J. Organomet. Chem., 2005, 690, 932–938;
N. Yoshikai, H. Mashima and E. Nakamura, J. Am. Chem. Soc.,
2005, 127, 17978–17979; K. Lamm, M. Stollenz, M. Meier, H. Go¨rls
and D. Walther, J. Organomet. Chem., 2003, 681, 24–36; F. Mongin,
L. Mojovic, B. Guillamet, F. Tre´court and G. Que´guiner, J. Org. Chem.,
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Commun., 2001, 2254–2255; V. P. W. Bo¨hm, C. W. K. Gsto¨ttmayr,
T. Weskamp and W. A. Herrmann, Angew. Chem., Int. Ed., 2001, 40,
3387–3389.
CFCl3) d 280.78 (q, J 5 24.4 Hz, 1F), 258.25 (d, J 5 24.4 Hz, 1F); 13
C
NMR (CDCl3) d 68.81, 74.01 (dq, J 5 38.1, 38.1 Hz), 120.87 (q,
J 5 274.9 Hz), 128.73, 128.74, 128.97, 133.89, 150.89 (dq, J 5 294.1,
2.8 Hz), 158.51 (d, J 5 33.6 Hz); IR (neat) 3069 (w), 2962 (w), 1743 (vs),
1627 (m), 1498 (m), 1312 (vs), 1237 (vs), 1187 (vs), 1147 (vs), 956 (s) cm21
;
HRMS (FAB) Calcd for (M+) C11H7IF4O2: 373.9427, Found 373.9438.
Anal. Calcd for C11H7F4IO2: C, 35.32: H, 1.89. Found: C, 35.50; H,
1.92.
2 Y. M. Kim and S. Yu, J. Am. Chem. Soc., 2003, 125, 1696–1697;
D. A. Widdowson and R. Wilhelm, Chem. Commun., 2003, 578–579;
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 3679–3681 | 3681