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each case. Those values are 1.6 (ꢁ0.2) ꢃ 10ꢂ5 sꢂ1, and 7.1
(ꢁ0.4) ꢃ 10ꢂ6 sꢂ1, respectively [10]. A measure of the
reliability of the current kinetic data can be obtained by
comparing the latter rate constant with that calculated from
our earlier gas phase kinetic study with the undeuterated
compound [1]. That calculated value for k1 is 5.9 ꢃ 10ꢂ6 sꢂ1
which is quite consistent with the obtained solution value.
4.3. 2,2-Difluorocyclopropanecarboxylic acid, 10 [12]
2,2-Difluorocyclopropanecarboxylic acid butyl ester, 9
(3.56 g, 20.0 mmol), was added to a solution of NaOH
(3.2 g, 80 mmol) in H2O (50 mL). The reaction mixture was
refluxed overnight. After removing all the solvents under
vacuum, the white residue obtained was dissolved in some
water. Then concentrated hydrochloric acid was added
slowly until the pH was below 7. The mixture was then
extracted with diethyl ether (30 mL ꢃ 5), and the organic
layer dried with MgSO4. After removing the solvent, 2,2-
difluorocyclopropanecarboxylic acid, 10, was obtained
(2.17 g, 89%) as light yellow prisms: mp 60–62 8C (Lit.
3. Conclusion
The two rate constants give a ratio of 2.3, which can be
compared with the predicted value of 3 published by Borden.
This effective confirmation by our current experiments of
the predicted ratio of rate constants for the two competing
processes serves to provide excellent credibility to the
kinetic model of the methylenecyclopropane energy surface
that was described by Borden in the 1999 paper [7]. Thus,
the final kinetic issue related to the effect of fluorine
substituents on the thermal rearrangement of 1,1-difluoro-2-
methylene-cyclopropane has seemingly been answered.
1
61–62 8C); [12] H NMR d 1.67–1.82 (m, 1H), 1.94–2.08
(m, 1H), 2.31–2.44 (m, 1H), 11.75 (br s, 1H); 13C NMR, d
17.1 (m), 25.4 (dd, J = 13.1 and 12.1 Hz), 110.6 (dd,
J = 283.5 and 289.1 Hz), 173.7; 19F NMR, d ꢂ125.7 (dm,
J = 152.6 Hz, 1F), ꢂ140.7 (dm, J = 152.6 Hz, 1F).
4.4. Dideuterio-(2,2-difluorocyclopropyl)-methanol, 11
Under N2 atmosphere, to a solution of LiAlD4 (18.0 mL,
1 M, 18.0 mmol) in ether, a solution of 2,2-difluorocyclo-
propanecarboxylic acid, 10 (2.44 g, 20.0 mmol) in fresh
distilled ether (35 mL) was added slowly while cooling the
flask with an ice-water bath. Then the reaction mixture was
allowed to be warmed to RTand to react overnight. After the
reaction was quenched slowly with water (40 mL), the
mixture was poured into a saturated solution of sodium
potassium tartrate (100 mL). The solution was extracted
three times with ether, and the combined ether layers then
washed with brine and dried with MgSO4. After removing
the ether, dideuterio-(2,2-difluorocyclopropyl)-methanol,
11, was obtained (1.58 g, 72%) as a colorless oil: 1H
NMR, d 0.98–1.14 (m, 1H), 1.30–1.44 (m, 1H), 1.71–1.84
(m, 1H); 13C NMR, d 14.1 (t, J = 11.1 Hz), 23.8 (t,
J = 11.1 Hz), 58.6 (m), 113.8 (t, J = 282.5 Hz); 19F NMR, d
ꢂ129.1 (dm, J = 164.8 Hz, 1F), ꢂ145.7 (dm, J = 164.8 Hz,
1F).
4. Experimental
4.1. General
1H, and 13C NMR spectra were determined at 300 MHz
(1H), 75 MHz (13C) using CDCl3 as solvent, unless
otherwise mentioned, and tetramethylsilane as an internal
standard; 19F NMR spectra were measured on a Varian XL at
282 MHz, referenced to external CFCl3 in CDCl3. Tetra-
hydrofuran (THF) was distilled under nitrogen immediately
before use from sodium/benzophenone. All other reagents
and solvents were obtained from commercial sources and
were used without additional purification.
4.2. 2,2-Difluorocyclopropanecarboxylic acid butyl
ester, 9 [11]
A three-necked flask is charged with 100 mL of dry
toluene, 0.20 g of sodium fluoride (0.06 eq.) and 10.0 g of n-
butyl acrylate (0.078 mol). The solution is heated to reflux
and slow N2 bubbling is initiated with stirring for 1 h.
Trimethylsilyl 2-fluorosulfonyl-2,2-difluoroacetate (32.0 g,
0.128 mol, 1.6 eq.) is then added dropwise. The reaction
mixture is refluxed for 8 h, and then cooled down to RT.
Toluene is removed by simple distillation at atmospheric
pressure, and the residue distilled at reduced pressure to
obtain product, 9 (9.1 g, 66%) as a colorless liquid: bp 95–
97 8C at 50 mm; 1H NMR, d 0.78–0.88 (m, 3H), 1.18–1.32
(m, 2H), 1.44–1.66 (m, 3H), 1.84–1.96 (m, 1H), 2.23–2.35
(m, 1H), 3.98–4.03 (m, 1H); 13C NMR, d 13.4 (m), 16.2 (m),
18.9, 25.5 (m), 30.4, 65.2, 110.6 (dd, J = 283.0 and
288.1 Hz), 166.5; 19F NMR, d ꢂ125.5 (dd, J = 152.6 and
12.2 Hz, 1H), ꢂ141.3 (dd, J = 152.6 and 12.2 Hz, 1F).
4.5. Dideuterio-(o-nitrophenylselenyl)methyl-2,
2-difluorocyclopropane, 12
Under N2 atmosphere, to a solution of dideutero-(2,2-
difluorocyclopropyl)-methanol, 11 (0.76 g, 6.9 mmol) and
2-nitrophenyl selenocyanate (1.92 g, 8.46 mmol) in dry THF
(16 mL), was added Bu3P (2.1 mL, 8.46 mmol) at RT. After
the reaction mixture was stirred for 4 h at RT, the solvent was
removed in vacuo. The residue was purified by column
chromatography on silica gel (hexanes/AcOEt: 14:1) to give
dideutero-(o-nitrophenylselenyl)methyl-2,2-difluorcyclo-
propane, 12 (1.83 g, 90%) as a yellow solid: mp 43–45 8C;
1H NMR, d 1.11–1.22 (m, 1H), 1.53–1.66 (m, 1H), 1.82–
1.95 (m, 1H), 7.31–7.38 (m, 1H), 7.47–7.58 (m, 2H), 8.29
(dd, J = 8.2 and 1.3 Hz, 1H); 13C NMR, d 17.6 (t,
J = 11.1 Hz), 20.7 (t, J = 11.1 Hz), 22.0 (m), 113.8 (t,