Synthesis, basicity, and dynamics of a perfluorocyclohexenyl anion

Article abstract of DOI:10.1021/jo040246c

Electron transfer to perfluoro-1,3-dimethylcyclohexane in moist THF has yielded two quite different products. Tetrabutylammonium iodide irradiated with ultraviolet light gives a tetrabutylammonium enolate, but potassium fluorenone ketyl affords a cyclohex

Full text of DOI:10.1021/jo040246c

Synthesis, Basicity, and Dynamics of a Perfluorocyclohexenyl
Anion
Xudong Chen*,† and David M. Lemal*
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755
xudong.chen@dartmouth.edu; david.m.lemal@dartmouth.edu
Received August 10, 2004
Electron transfer to perfluoro-1,3-dimethylcyclohexane in moist THF has yielded two quite different
products. Tetrabutylammonium iodide irradiated with ultraviolet light gives a tetrabutylammonium
enolate, but potassium fluorenone ketyl affords a cyclohexenyl anion. This allylic anion was isolated
as its conjugate acid, a rather strong carbon acid. Ring inversion in the anion, measured by 19F
NMR line shape analysis, is characterized by these activation parameter values: ∆Hq ) 8.84 (
0.14 kcal/mol and ∆Sq ) 0.81 ( 0.6 cal mol-1 K-1
.
Introduction
upon them generally result in wholesale destruction of
the molecule.8-10 Powerful electron donors induce loss of
a fluoride ion, and since the resulting intermediates are
more easily reduced than the starting material, a reaction
cascade ensues with hemorrhaging of fluoride ions.11 We
have developed methods for stopping the cascade at an
early stage by placing a negative charge on the molecule,
thus inhibiting further assault by an electron donor.12
For example, perfluoromethylcyclohexane (3) can be
transformed into the tetrabutylammonium enolate 4
either photochemically or “thermally” at room tempera-
ture. Irradiation of tetrabutylammonium iodide in THF
Perfluoroalkylcarbanions are ordinarily generated by
fluoride ion addition to perfluoroalkenes or deprotonation
of monohydroperfluoroalkanes. Although most of these
species are rather unstable, a variety of tertiary examples
have been isolated as tris(dimethylamino)sulfonium (TAS)
or cesium salts.1-3 They enjoy stabilization by negative
hyperconjugation with CR-F bonds, as well as by the
inductive and field effects of fluoro substituents.2,4 Their
stability depends importantly on the nature of the
counterion, for if the positive charge is neither diffuse
nor buried the cation assists fluoride ion loss to give
alkene(s).
Pentafluoroallyl anion (1) has been investigated com-
putationally5 and perhaps also generated in a mass
spectrometer,6 but the only perfluoroallylic anion to be
synthesized and studied in solution prior to the present
work is 2, isolated as a TAS salt.7 In the course of a study
of fluorocarbon functionalization, we have accidentally
discovered the first example of a perfluorocyclohexenyl
anion. Its mode of formation is novel.
excites a charge-transfer-to-solvent (CTTS) transition,13
and the solvated electrons reduce 3 to perfluoro-1-
methylcyclohexene (5) via initial loss of the most vulner-
able fluorine, the tertiary one (Scheme 1). Hydroxide ion
resulting from the presence of a small amount of water
intercepts the alkene in an addition-elimination reac-
tion, and deprotonation of the resulting enol yields 4. The
(5) Dixon, D. A.; Fukunaga, T.; Smart, B. E. J. Phys Org. Chem.
1988, 1, 153.
Functionalization of saturated perfluorocarbons is
challenging because they are highly inert and because
the vigorous conditions required for successful attack
(6) A C3F5 anion has been generated in the gas phase from
octafluorocyclobutane by dissociative electron capture, but its structure
is uncertain. Lifshitz, C.; Grajower, R. Int. J. Mass Spectrom. Ion Phys.
1972/73, 10, 25.
(7) Farnham, W. B.; Middleton, W. J.; Fultz, W. C.; Smart, B. E. J.
Am. Chem. Soc. 1986, 108, 3125.
† Walter H. Stockmayer Fellow, 2002-2003.
(1) Smart, B. E.; Middleton, W. J.; Farnham, W. B. J. Am. Chem.
Soc. 1986, 108, 4905.
(8) Richmond, T. G. Top. Organomet. Chem. 1999, 3, 243.
(9) Burdeniuc, J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber./Recueil
1997, 130, 145.
(2) Farnham, W. B.; Dixon, D. A.; Calabrese, J. C. J. Am. Chem.
Soc. 1988, 110, 2607.
(10) Saunders, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 2615.
(11) When six-membered rings are present, aromatization can halt
the loss of fluoride ion.
(3) Bayliff, A. E.; Bryce, M. R.; Chambers, R. D.; Matthews, R. S.
Chem. Commun. 1985, 1018. Chambers, R. D.; Taylor, G.; Powell, R.
L. Chem. Commun. 1978, 433.
(12) Stoyanov, N. S.; Ramchandani, N.; Lemal, D. M. Tetrahedron
Lett. 1999, 40, 6549.
(13) Fox, M. F.; Hayon, E. Trans. Faraday Soc. 1 1976, 72, 1990.
Blandamer, M. J.; Fox, M. F. Chem. Rev. 1970, 70, 59.
(4) Dixon, D. A.; Fukunaga, T.; Smart, B. E. J. Am. Chem. Soc. 1986,
108, 4027. Farnham, W. B.; Smart, B. E.; Middleton, W. J.; Calabrese,
J. C. Dixon, D. A. J. Am. Chem. Soc. 1985, 107, 4565.
10.1021/jo040246c CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/22/2004
J. Org. Chem. 2004, 69, 8205-8208
8205

Chen and Lemal
SCHEME 1
of the presence of a tertiary fluorine in an allylic position.
Attack by two successive electrons then gives the robust
anion 9 as its tetrabutylammonium salt.
The contrast in reaction course in moist THF between
the photochemical and thermal conditions probably
reflects a difference in hydroxide ion concentration during
reaction. As a better electron donor than base, the fluor-
enone radical anion may be quite selective in attacking
fluorocarbon in preference to water, whereas solvated
electrons readily reduce both. Just as the negative charge
on enolate 7 protects the molecule from further reduction,
the charge on anion 9 plays the same role.
SCHEME 2
Since hydrofluorocarbon 10, a potent electrophile, was
quickly destroyed by dissolution in methanol, attempts
to measure its acidity were carried out in nonhydroxylic
solvents. Addition of strong, hindered amine bases such
as triethylamine or Proton Sponge to 10 in acetonitrile
gave stable solutions of the anion 9. However, lithium
diisopropylamide (LDA) transformed 10 quite cleanly into
diene 13, reflecting the ability of lithium ion to facilitate
fluoride ion loss. Rapid, ill-defined decomposition was
observed with a variety of weak bases such as dichloro-
acetate ion and the enolates of dimedone and triacetyl-
methane.
same tetrabutylammonium enolate can be prepared
without light using tetrabutylammonium fluorenone
ketyl (TBAFK) as the source of electrons.
Results and Discussion
When the reductive functionalization reaction was
carried out photochemically with perfluoro-1,3-dimeth-
ylcyclohexane (6), enolate 7 was obtained, free of the
isomeric ion oxygenated at the 2-position of 6 (Scheme
2).12 The selectivity of this reaction may be attributable
to a steric effect, as the bulky tetrabutylammonium ion
may approach C5 more easily than C2 to assist loss of
fluoride ion. Enolate 7 was isolated as its methyl enol
ether 8.
Eventually, a hindered base was found that caused
minimal decomposition and was close enough in basicity
to 9 to allow for quantitative comparison: the 2,6-dini-
trocresolate ion (14). Measurements of the acidity of 10
were made in acetonitrile-d3 with variable excesses of
triethylamine and 2,6-dinitrocresol (15). They were car-
Under the thermal conditions, however, no 7 was
observed. Instead, the 19F NMR spectrum of the reaction
mixture showed that a species with 2-fold symmetry had
been formed. Its stability under the reaction conditions
suggested the presence of a negative charge, and it
became clear that the species was the allylic anion 9.
Protonation of 9 with sulfuric acid yielded hydroperfluo-
rocyclohexene 10, isolated in ∼87% overall yield. Now
the reduction was run photochemically in the absence of
water, and 9 was again the product. Apparently, if
hydroxide ion fails to capture the initially formed alkene
11, leading to enolate 7, the alkene undergoes fluoride
ion-catalyzed isomerization to 12. Calculations at the
B3LYP/6-31G* level of theory14 indicate that alkene
1215a is only slightly lower in energy than 1115 (∆E ) 0.60
kcal/mol16), but it is more susceptible to reduction because
(14) Gaussian 98 Revision A.5: Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratman, R. E.; Burant, J.
C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain,
M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.;
Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P.
M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian, Inc.; Pittsburgh,
PA, 1998.
(15) (a) Burdeniuc, J.; Crabtree, R. H. Organometallics 1998, 17,
1582. (b) Vaughan, J. F. S.; Pellatt, M. G.; Sherrington, J.; Hope, E.
G. WO Pat. Appl. 2001-EP6676 20010613.
(16) This value is for
difference.
0 K, uncorrected for zero-point energy
8206 J. Org. Chem., Vol. 69, No. 24, 2004

Synthesis of a Perfluorocyclohexenyl Anion
FIGURE 1. Structure of perfluoro-1,3-dimethylcyclohex-2-
enyl anion (9) calculated at the B3LYP/6-31G* level of theory.
FIGURE 3. Eyring plot for ring inversion of perfluoro-1,3-
dimethylcyclohex-2-enyl anion (9).
the broad NMR signals representing the methylene
fluorines decoalesced. When the spectrum finally re-
sharpened at -100 °C, the signal at δ -107.9 had
resolved into a pair of doublets (J ) 262 Hz) at δ -90.8
and -126.0, and the δ -138.2 peak had become a pair of
doublets (J ) 247 Hz) at δ -125.2 and -152.9. Line
shape analysis over the temperature range -100 to +21
°C gave a set of rate constants, and the resulting Eyring
plot (Figure 3) revealed these activation parameter
values: ∆Hq ) 8.84 ( 0.14 kcal/mol and ∆Sq ) 0.81 (
0.6 cal mol-1 K-1. The barrier lies between the ring
inversion barrier for perfluorocyclohexene (∆Hq ) 7.2 (
FIGURE 2. Ring inversion in perfluoro-1,3-dimethylcyclohex-
2-enyl anion (9).
ried out at -45°C to limit exchange broadening (see
below) and to slow decomposition. Ratios of anion 9 to
its conjugate acid 10 were determined by 19F NMR
integration. In this way, cyclohexene 10 was found to be
53 ( 4 times more acidic in acetonitrile than cresol 15,
the pKa of which is 4.23 in water.17 This acidity ratio was
calculated on the assumption that the decomposition of
9 does not consume base, and the acidity of 10 is
somewhat greater yet if it does.18 Clearly, cyclohexene
10 is a strikingly acidic fluorocarbon.
0.2 kcal/mol, ∆Sq ) 2.6 ( 1.6 cal mol-1 K-1 19
and that
)
for perfluorocyclohexane (∆Gq ) 10.9 kcal/mol at 25 °C).20
The only other barrier reported for ring inversion in a
cyclohexenyl system is that for 1,2,3,4,5,6-hexamethyl-
cyclohexenyl cation (Ea ) 8.9 kcal/mol, log A ) 10.8 s-1).21
Based on their activation parameter values, ring inver-
sion of 9 is calculated to be 160 times faster than that of
this cation at 25 °C.
The calculated geometry of anion 9 reflects the role of
negative hyperconjugation in stabilizing its charge, for
the length of the 10 methyl and methylene C-F bonds
flanking C1 and C3 correlate nicely with their degree of
alignment with the allylic π system (Figure 1). For
example, the quasiaxial C-F bonds at C4 and C6,
misaligned by only 7.5°, are 1.393 Å long, but the
quasiequatorial bonds, misaligned by 53°, are 1.379 Å
in length. For comparison, the C-F bonds at C5, where
hyperconjugation cannot occur, average 1.363 Å in length.
The 19F NMR spectrum of anion 9 in THF comprised
four signals: narrow multiplets at δ -49.6 (two CF3s)
and -106.7 (vinyl F) and very broad singlets at δ -107.9
(two CF2s) and -138.2 (CF2). The breadth of the latter
resonances indicated that a dynamic process was occur-
ring at an intermediate exchange rate on the NMR time
scale. B3LYP/6-31G* calculations ascribe Cs symmetry
to the anion, with five of the ring carbons coplanar and
the CF3 carbons just 4-5° out-of-plane (Figure 1). C5 is
bent strongly out-of-plane in the other direction (flap
angle 42°), and the dynamic process revealed by NMR is
ring inversion involving that carbon (Figure 2). Thus, the
anion exists in a “half-chair” conformation, but one that
differs from the cyclohexene half-chair in which two
carbons lie outside the plane of the other four.
Experimental Section
NMR spectra were performed on 300 and 500 MHz ma-
chines. 19F NMR spectra were recorded at 282.2 and 470.3
(17) Robinson, R. A. J. Res. Nat. Bur. Stand., Sec. A. 1967, 71, 385.
(18) Keq is calculated to be 69 ( 6 if decomposition of 9 consumes 1
equiv of base.
(19) Anderson, J. E.; Roberts, J. D. J. Am. Chem. Soc. 1970, 92, 97.
(20) Burdon, J.; Hotchkiss, J. C.; Jennings, W. B. Tetrahedron 1977,
33, 1745. For ring inversion of perfluorocyclohexane in the solid state,
see: Ellett, J. D.; Griffin, R. G.; Waugh, J. S. J. Am. Chem. Soc. 1974,
96, 345.
(21) Hogeveen, H.; Volger, H. C. Rec. Trav. Chim. Pays-Bas 1968,
87, 1047.
As the temperature was lowered, the methyl and
methine fluorine resonances of 9 changed very little, but
J. Org. Chem, Vol. 69, No. 24, 2004 8207

Chen and Lemal
MHz, with trichlorofluoromethane as internal standard. 13C
δ 4.16 (m, broad, 1H). 19F NMR (CDCl3): δ -58.67 (narrow
m, 3F, C1), -61.36 (s, 3F, C3), -88.98 (s, broad, 1F, C2),
-102.54 (d, 294 Hz, 1F), -120.35 (d, 273 Hz, 1F), -122.73 (d,
294 Hz, 1F), -125.15 (d, 267 Hz, 1F), -128.07 (d, 273 Hz, 1F),
-147.77 (d, 267 Hz, 1F). 13C NMR (19F decoupled, CDCl3): δ
157.70 (d, 10 Hz, C2), 120.76 (d, 10 Hz, CF3 on C3), 119.26 (s,
CF3 on C1), 110.77 (m, CF2, C4, 5, or 6), 110.23 (m, C1), 109.64
(m, CF2, C4, 5, or 6), 107.43 (s, CF2, C4, 5, or 6), 48.60 (d, 137
Hz, C3). IR (neat, cm-1): 1704, 1390, 1223, 1164, 1023, 898.
HRMS: calcd for C8HF13 343.9871, found 343.9871.
Photochemical Synthesis of Perfluoro-1,3-dimethyl-
cyclohex-2-enyl Anion (9). Perfluoro-1,3-dimethylcyclohex-
ane (6) (10 mg, 0.025 mmol), tetrabutylammonium iodide (46
mg, 0.125 mmol), and CD3CN (1 mL) were added to a quartz
NMR tube. It was sealed with a septum and attached to the
jacket surrounding the UV source. The whole assembly was
immersed in cooling water, as described above. The mixture
was irradiated for 20 h. Perfluoro-1,3-dimethylcyclohex-2-enyl
anion (9) was the major product, as shown by NMR.
Formation of Perfluoro-1,3-dimethylcyclohex-2-enyl
Anion (9) from 3H-Perfluoro-1,3-dimethylcyclohexene
(10). 3H-Perfluoro-1,3-dimethylcyclohexene (10) (15 mg, 0.044
mmol) and CD3CN (0.5 mL) were added to an oven-dried NMR
tube. Proton Sponge (46 mg, 0.044 mmol) dissolved in CD3CN
(0.5 mL) was added to the tube. Perfluoro-1,3-dimethylcyclo-
hex-2-enyl anion (9) was formed quantitatively, as indicated
by NMR.
Formation of Perfluoro-1,3-dimethylcyclohex-1,3-di-
ene (13) from 3H-Perfluoro-1,3-dimethylcyclohexene
(10). 3H-Perfluoro-1,3-dimethylcyclohexene (10) (20 mg, 0.058
mmol) and CD3CN (0.5 mL) were placed in an oven-dried NMR
tube. Lithium diisopropylamide (LDA) (29 µL, 2.0 M, 0.058
mmol) was added to the NMR tube. Perfluoro-1,3-dimethyl-
cyclohex-1,3-diene (65) was formed cleanly, as indicated by
NMR. 19F NMR (CD3CN): δ -62.08 (d, poorly resolved, 3F,
CF3), -63.83 (d, poorly resolved, 3F, CF3), -102.20 (m, broad,
1F, C2), -125.29 (s, 2F, C5 or 6), -130.45 (s, 2F, C5 or 6),
-160.31 (m, broad, 1F, C4). GC/MS (m/e): 324 (M+).
NMR spectra were recorded at 125.7 MHz. GC was performed
1
using a
/
in. × 10 ft column packed with 10% OV-101 on
4
Chromasorb-W-AW-DMCS 60/80. The UV source for photore-
action was a Vycor-filtered 450 W Hanovia medium-pressure
mercury arc contained in a quartz water jacket.
THF was distilled from potassium benzophenone ketyl, and
acetonitrile was distilled from calcium hydride. After distil-
lation, technical perfluoro-1,3-dimethylcyclohexane was fur-
ther purified by GC as a mixture of cis and trans isomers. The
mixture was 90% pure by GC. Caution should be taken in
handling methyl triflate, as it is very poisonous.
Photochemical Synthesis and Methylation of Per-
fluoro-2,4-dimethylcyclohex-1-enolate (7). Perfluoro-1,3-
dimethylcyclohexane (6) (0.60 g, 90% pure, 1.5 mmol), water
(18 mg, 1.0 mmol), tetrabutylammonium iodide (1.38 g, 3.74
mmol), and THF (10 mL) were placed in a 25 mL quartz tube.
It was sealed with a septum and attached to the jacket
surrounding the lamp. The whole assembly was immersed in
water contained in a large beaker. Cooling water was intro-
duced to the beaker continuously, and a pan with a draining
hose contained the overflow. The mixture was irradiated for
20 h while being stirred with a motor-driven horseshoe magnet
placed close to the quartz tube outside the beaker. Formation
of perfluoro-2,4-dimethylcyclohex-1-enolate (7) was observed
by NMR. Treatment with excess methyl triflate transformed
the enolate into its O-methyl ether (8), which was purified by
preparative GC. Yield: 0.28 g, 50% based on perfluoro-1,3-
dimethylcyclohexane. 1H NMR (CDCl3): δ 4.2 (t, poorly
resolved, CH3). 19F NMR (CDCl3): (The labels “cis” and “trans”
in the assignments locate fluorine atoms relative to the CF3
group at C4. Assignments were made with the help of NOE
measurements.) δ -59.08 (s, 3F, CF3 on C2); -70.45 (s, 3F,
CF3 on C4); -98.53, -105.38 (ABq, J ) 307 Hz, 2F; A, cis, C3;
B, trans, C3); -105.04 (dd, J ) 24, 301 Hz, 1F, trans, C6);
-119.51 (d, 289 Hz, 1F, cis, C5); -125.09 (d, 301 Hz, 1F, cis,
C6); -135.92 (d, 289 Hz, 1F, trans, C5); -183.87(s, 1F, C4).
IR (neat, cm-1): 2977, 1664, 1470, 1382, 1340, 1282, 1223,
1157, 1079, 1017, 940, 894, 671. HRMS: calcd for C9H3F13O
373.9976, found 373.9976.
Measurement of 3H-Perfluoro-1,3-dimethylcyclohex-
ene (10) Acidity. 2,6-Dinitro-p-cresol (60.7 mg, 0.306 mmol)
and triethylamine (∼15.4 µL, 0.113 mmol) were dissolved in
CD3CN (2.00 mL), total volume 2.06 mL. To an oven-dried
NMR tube was added 3H-perfluoro-1,3-dimethylcyclohexene
(10) (15.0 mg, 0.0436 mmol), dissolved in CD3CN (0.5 mL). It
was sealed with a septum and cooled to -15 °C in an ethylene
glycol slush bath. Amine/phenol mixture (1.00 mL) was added
to the NMR tube, and the contents were well mixed before
NMR measurements were taken at -45 °C. The molar ratio
of cresol/cresolate to triethylammonium ion was measured as
“Thermal” Synthesis of Perfluoro-1,3-dimethylcyclo-
hex-2-enyl Anion (9) and 3H-Perfluoro-1,3-dimethylcy-
clohexene (10). The following operations were done under
nitrogen protection, and glassware was dried in an oven at
120 °C overnight. Tetrabutylammonium bromide (3.87 g, 12
mmol), fluorenone (2.16 g, 12 mmol), potassium (0.48 g, 12
mmol), and THF (40 mL) were added to a 60 mL dropping
funnel with a fritted filter plate. The mixture was magnetically
stirred overnight, and then nitrogen pressure was applied to
force tetrabutylammonium fluorenone ketyl solution through
the filter plate into a 50 mL flask. Potassium bromide was
thus removed by filtration, and the deep red ketyl solution
was transferred to a 50 mL syringe. Perfluoro-1,3-dimethyl-
cyclohexane (6) (0.74 g, 90% pure, 1.9 mmol) was added to a
100 mL two-neck round-bottom flask. The ketyl solution was
slowly added to the above flask over the course of 8 h,
controlled by a syringe pump. Formation of perfluoro-1,3-
1
2.70:1.00 by H NMR. The ratio of 3H-perfluoro-1,3-dimeth-
ylcyclohexene (10) to perfluoro-1,3-dimethylcyclohex-2-enyl
anion (9) to decomposed fluorocarbon was 1.36:10.14:1.00, as
revealed by 19F NMR.
Another measurement was carried out, using 3H-perfluoro-
1,3-dimethylcyclohexene (10) (15.0 mg, 0.0436 mmol) and
1.00 mL of a 3.08 mL mixture of 2,6-dinitro-p-cresol (151.8
mg, 0.766 mmol), triethylamine (∼26.9 µL, 0.183 mmol),
and CD3CN (3.0 mL). The molar ratio of cresol to triethyl-
amine was measured as 4.10:1.00 by 1H NMR. The ratio of
3H-perfluoro-1,3-dimethylcyclohexene (10) to perfluoro-1,3-
dimethylcyclohex-2-enyl anion (9) to decomposed fluorocarbon
was 1.32:7.48:1.00, as revealed by 19F NMR.
dimethylcyclohex-2-enyl anion (9) was observed by NMR. 19
F
NMR (CD3CN): δ -48.53 (m, narrow, 6F, two CF3s), -105.39
(m, narrow, 1F, C2), -106.50 (s, broad, 4F, C4 and 6), -136.90
(s, broad, 2F, C5).
Triglyme (freshly distilled, 40 mL) was added to the above
mixture, and THF was removed under vacuum. Concentrated
sulfuric acid (10 mL) was added while the mixture was stirred
and cooled in a water bath. The resulting 3H-perfluoro-1,3-
dimethylcyclohexene (10) was vacuum transferred to a liquid
nitrogen-cooled U-trap. Yield: 0.54 g, 85%. 1H NMR (CDCl3):
Acknowledgment. We thank the National Science
Foundation for support of this work.
JO040246C
8208 J. Org. Chem., Vol. 69, No. 24, 2004