Fraenkel et al.
Experimental Section
2-Neopentyl-cis-bicyclo[3.3.0]octenyllithium, TMEDA 8. A
solution of 3-methylene-1,4-cyclooctadiene (232 mg, 1.93 mmol)
and TMEDA (237 mg, 2.04 mmol) in 4 mL of pentane was treated
dropwise at -90 °C with tert-butyllithium (1.13 mL, 1.7 M,
1.92 mmol). After being stirred for 2 h with warming to room
temperature, the reaction mixture was concentrated in vacuo and
washed with dry pentane, 3 × 4 mL, and then dried again in
vacuo. A 0.35 M solution was prepared of the title product in
diethyl ether-d10 for NMR studies. NMR data are summarized in
Figure 2.
2-Neopentyl-cis-bicyclo[3.3.0]-1-octene, 7 E ) H. A solution
of 3-methylene-1,4-cyclooctadiene 5 (0.2 g, 5.8 mmol) in diethyl
ether, 3 mL, was treated dropwise at -78 °C with tert-butyllithium
(3.44 mL, 1.7 M, 5.85 mmol). The mixture was stirred for 3 h at
-78 °C and then for 1 h at room temperature. The reaction mixture
was quenched with water and extracted into diethyl ether. The
combined ether extracts were dried with MgSO4 and concentrated
to give the title compounds as a colorless liquid.
FIGURE 5. 13C and (1H) NMR shifts, δ scale, of cyclopentenyllithium
in diethyl ether-d10, 280 K. Top: TMEDA complex. Bottom: PMDTA
complex.
Racemic trans-2-Neopentyl-3-trimethylstannyl-4,5-trimeth-
ylenecyclopentene, 10. A solution of triene 5 (0.7 g, 5.8 mmol) in
diethyl ether was treated dropwise at -78 °C with t-butyllithium
(3.44 mL, 5.85 mmol) in pentane. The mixture was stirred for 3 h
at -78 °C, then at room temperature for 1 h. The reaction mixture
was cooled again to -78 °C and reacted with a solution oftrim-
ethylstannyl chloride (1.04 g, 5.22 mmol) in 3 mL diethyl ether.
After warming to room temperature for over 1 h the reaction mixture
was quenched with aqueous NH4Cl-H2O and extracted into diethyl
ether and then concentrated. Distillation of the residue, bp 77-
broadening of the CH2N 13C resonance of complexed TMEDA.
At 180 K 13C NMR of 11 prepared in the presence of PMDTA
shows separate resonances for free and complexed ligand. These
two average by 250 K due to fast exchange of PMDTA between
the latter free and complexed states.
On standing, the solution of 11 with PMDTA develops new
13C resonances at 130.9, 33.71, and 23.61, all δ units close to
those reported for cyclopentene12 at 130.8, 32.8, and 23.3 δ
units, respectively. In addition, the latter solution of 11 develops
weak 13C resonances at 66 δ, 143.7 δ, and 78.7 δ which are
similar to those reported for C1, C2, and C3 of pentadienyllithium
14, by Ford13 at, respectively, 66 δ, 143.9 δ, and 86.9 δ and by
Bates2c at, respectively, 71.1 δ, 142.2 δ, and 142.2 δ.
78 °C/0.1 Torr, gave 1 g of the title compound in 56.2% yield. 13
C
NMR (CDCl3): -0.091 (SnCH3), 25.61 (CH2(CH2)2), 32.71, 37.14
(CH2(CH2)2), 44.85 (CH2C(CH3)3), 46.27 (H-CSn), 46.27 (CHCH-
Sn), 50.41 ((CH-CHdC-CH2), 126.72 (CHdC-CH2), 144.4
(CH2CdCH).
Cyclopentenyllithium•TMEDA, 11‚TMEDA. A 25 mL Schlenk
tube was charged with 6 mL of dry diethyl ether, TMEDA (348.6
mg, 3 mmol), and 3-trimethylstannyl-1-cyclopentene11 (692.7 mg,
3 mmol) under an argon flow. Methyllithium (2.0 mL, 1.5 M,
3.0 mmol) in diethyl ether was added by syringe at -78 °C. A
white solid soon formed. The mixture was stirred at room
temperature for 2 h, 1 mL of THF was added at -50 °C, and stirring
was continued for 1 h at room temperature. Solvent was removed
in vacuum, and the residue was washed with pentane three times
(3 × 6 mL). An NMR tube was flame dried under vacuum and
charged with product (35 mg) under argon before it was transferred
to a high vacuum line (10-6 Torr) trapped with liquid nitrogen.
Volatile impurities were pumped out into a liquid nitrogen trap.
After 3 h, THF-d8 (0.5 mL) was vacuum transferred to the NMR
tube cooled by a liquid nitrogen bath. Under high vacuum, the NMR
tube was sealed with a small hot flame.
Cyclopentenyllithium‚PMDTA, 11‚PMDTA. A 25 mL Schlenk
tube was charged with 6 mL of dry diethyl ether, PMDTA
(519.9 mg, 3 mmol), and 3-trimethylstannyl-1-cyclopentene
(692.7 mg, 3 mmol) under an argon flow. Methyllithium (2.0 mL,
1.5 M, 3.0 mmol) in diethyl ether was added by a syringe at
-78 °C. The mixture was stirred at room temperature for 2 h, and
then 1 mL of THF was added at -50 °C and stirring continued
1 h at -50 °C. Solvent was removed in vacuum, and the residue
was washed with 5 mL pentane one time at -78 °C. An NMR
tube was flame dried under vacuum and charged with title product
(49 mg) under argon before it was transferred to a high vacuum
line (10-6 Torr) trapped with liquid nitrogen. Volatile impurities
were pumped out in to a liquid nitrogen trap. After 3 h, diethyl
ether-d10 (0.5 mL) was vacuum transferred to the NMR tube cooled
by a liquid nitrogen bath. Under high vacuum, the NMR tube was
sealed with a small hot flame.
Thus, in summary, the fate of cyclopentadienyllithium is
to deprotonate the solvent and to undergo an allowed ring
opening.
In summary, we have shown that the bicyclic allylic anion
in 8 is a highly unsymmetric species with both rings being
nonplanar and the allylic bond lengths being different, which
indicates a partially localized allylic anion. The compound
undergoes a rapid inversion which involves allyl bond shifts as
well as transfer of the out of plane sites on the two rings.
While the thermodynamic fate of 8 is to remain closed that
of cyclopentyl anion as in 11•PMDTA is to deprotonate the
solvent and to open to pentadienyllithium.
(9) (a) Bartoli, G.; Markantoni, E.; Petrini, M. J. Chem. Soc. Chem.
Commun. 1991, 793. (b) Branner-Joergensen, S.; Berg, A. Acta Chemica
Scand. 1966, 20, 2192. (c) Zair, T.; Santelli-Rouvier, C.; Santelli, M. J.
Org. Chem. 1993, 58, 2686.
(10) (a) Zaidlewicz, M. J. Organomet. Chem. 1991, 409, 103. (b)
Benkeser, R. A.; Mozdzen, E. C.; Muench, W. C.; Roche, R. T.; Siklosi,
M. P. J. Org. Chem. 1979, 44, 1370.
(11) Jones, K.; Newton, R. F.; Yarnold, C. Synth. Commun. 1992, 22,
3089.
(12) Kalinowski, H.-O.; Berger, S.; Braun, S. 13C-NMR-Spektroskopie;
Georg Thieme Verlag: Stuttgardt, 1984; p 122.
(13) Ford, W. T.; Newcomb, M. J. Organomet. Chem. 1971, 26, C51.
4964 J. Org. Chem., Vol. 72, No. 13, 2007