1,5-Hydrogen Shift in 1,3(Z)-Pentadiene
A R T I C L E S
of anhydrous tetrahydrofuran (THF) in a 100-mL, round-bottomed flask
under argon, 8.5 mL of a solution of n-butyllithium (2.0 M in pentane)
were added at 0 °C. After the mixture was warmed to room temperature,
stirred for 30 min, and recooled to 0 °C, a solution of p-tolylmethyl
bromide (1.57 g, 8.5 mmol) in anhydrous THF (5 mL) was added by
syringe over a 10-min period. After the mixture was stirred for 2 h at
0 °C and overnight at 25 °C, absolute ethanol was added dropwise
until no further solid precipitated. The filtered precipitate was washed
3 times with THF, dispersed in 6 mL of water, filtered, washed with
1 mL of water, and dried over phosphorus pentoxide to give 2.1 g
(53%) of a colorless solid: 1H NMR (CDCl3) 2.23 (s, 3H), 2.95-3.02
(m, 2H), 3.96-4.02 (m. 2H), 7.00 (d, J ) 8.05 Hz, 2H), 7.12 (d, J )
8.05 Hz, 2H), 7.65-7.85 (m. 15H).
mercury lamp for 10 h. GC analysis (50 °C for 15 min, 15 °C per min
to 200 °C, and finally 15 min isothermal) showed two major peaks:
starting material at 23.08 min and a new peak at 21.36 min in a ratio
of 1 to 0.9.
(Z)- and (E)-2-Ethylidene-1-methylenecyclohexane (Z)-7 and (E)-
7. In a 500-mL, four-necked, round-bottomed flask fitted with a 150-
mL, pressure-equalizing addition funnel, methyltriphenylphosphonium
bromide (17.6 g, 0.05 mol) was suspended in tetrahydrofuran (THF)
(150 mL, freshly distilled from sodium and benzophenone) under an
argon atmosphere. n-Butyllithium (24.5 mL, 2.0 M solution in pentane)
was added at -78 °C (became red/orange). The resulting Wittig reagent
was warmed to room temperature, stirred for 30 min, then cooled to
-78 °C. A solution of the 1:0.9 mixture of enones (6.1 g, 0.05 mol) in
THF (60 mL) was added with stirring over 1 h. After an additional
hour of being stirred, the reaction mixture was diluted with water (200
mL) and separated into an organic layer and an aqueous layer, which
was extracted with pentane (2 × 40 mL). The combined organic
solutions were washed with water (4 × 200 mL), dried over Na2SO4,
vacuum filtered, and concentrated by distillation. Distillation at reduced
pressure gave 2.66 g of a clear oil consisting of 30% of (Z)-7 and 39%
of (E)-7 by analytical GC. Preparative GC at 72 °C provided pure
samples. (Z)-7: 1H NMR 1.46-1.54 (m, 4H), 1.72 (dt, J ) 0.98 Hz,
6.84 Hz, 3H), 2.12-2.17 (m, 4H), 4.77 (d, J ) 2.93 Hz, 1H), 4.99 (t,
J ) 0.98 Hz, 1H), 5.27 (q, J ) 6.84 Hz, 1H); 13C NMR 14.48, 28.04,
28.39, 37.13, 38.28, 110.68, 118.15, 141.03, 146.81. (E)-7: 1.41 (m,
2H), 1.47 (m, 2H). 1.52 (d, J ) 6.84 Hz, 3H), 2.18 (m, 4H), 4.66 (d,
J ) 0.98 Hz, 1H), 4.98 (d, J ) 1.95 Hz, 1H), 5.64 (q, J ) 6.84 Hz,
1H); 13C NMR 13.05, 26.45, 27.50, 28.38, 35.82, 107.39, 117.49,
141.08, 151.51 (the missing 13C(C-sp2) resonance is presumed to be
hidden under the solvent).
(E,Z)- and (E,E)-1-Phenyl-5-p-tolylpenta-1,3-diene [(E,Z)-5 and
(E,E)-5]. Triphenyl-â-p-tolylethylphosphonium bromide (1.82 g, 3.95
mmol) was converted to its Wittig reagent as above. A solution of trans-
cinnamaldehyde (0.52 g, 3.95 mmol) in anhydrous THF (5 mL) was
added at 0 °C by syringe over a period of 20 min. After concentration
in a vacuum, the mixture was treated with 20 mL of water and extracted
twice with pentane (30 mL each). The combined extracts were washed
with water (40 mL) and brine, dried over Na2SO4, filtered, concentrated,
and subjected to column chromatography (neutral alumina, hexane) to
give a mixture of (E,Z)-5 (67%) and (E,E)-5 (33%) as a colorless oil
1
(66%). H NMR (E,Z)-5: 2.12 (s, 3H), 3.47 (d, J ) 7.81 Hz, 2H),
5.60-5.65 (m, 1H), 6.19-6.24 (m, 1H), 6.49 (d, J ) 15.63 Hz, 1H),
1
6.97-7.27 (m, 10H). H NMR (E,E)-5: 2.14 (s, 3H), 3.29 (d, J1 )
6.84 Hz, 2H), 5.77-5.83 (m, 1H), 6.11-6.16 (m, 1H), 6.34 (d, J )
15.63 Hz, 1H), 6.70 (dd, J1 ) 15.63 Hz, J2 ) 10.74 Hz, 1H), 6.91-
7.27 (m, 9H).
(E,Z)- and (E,E)-5-Phenyl-1-p-tolylpenta-1,3-diene (E,Z)-6 and
(E,E)-6. The conversion of triphenyl-â-phenylethylphosphonium bro-
mide (1.00 g, 2.2 mmol) and 4′-methylcinnamaldehyde (0.32 g) to a
mixture (0.35 g) of (E,Z)-6 (70%) and (E,E)-6 (30%) was effected by
the procedure above: 1H NMR (E,Z)-6: 2.09 (s, 3H), 3.46 (d, J )
7.81 Hz, 2H), 5.55-5.60 (m, 1H), 6.22-6.26 (m, 1H), 6.53 (d, J )
15.63 Hz, 1H), 6.95-6.97 (m, 2H), 7.06-7.25 (m, 8H); (E,E)-6 2.09
(s, 3H), 3.27 (d, J1 ) 6.84 Hz, 2H), 5.72-5.78 (m, 1H), 6.11-6.16
(m, 1H), 6.38 (d, J ) 15.63 Hz, 1H), 6.67-6.72 (m, 1H), 6.95-6.97
(m, 2H), 7.04-7.22 (m. 7H).
Kinetics. The sample (2-3 mg) in 0.2 mL of (benzene-d6) in an
NMR tube was degassed through three freeze-thaw cycles and sealed
under vacuum. The sealed tube was then heated in the vapors of boiling
solvents for the specified periods of time (data given in the Supporting
Information, Table SI-2): anisole, bp 153.6-153.8; diethyl oxalate,
bp 185.1-185.4.
(E)-2-Ethylidenecyclohexan-1-one. In a 250-mL, two-necked,
round-bottomed flask fitted with a 30-mL, pressure-equalizing addition
funnel, sodium ethoxide (430 mg, 6.4 mmol) was added to cyclohex-
anone (13.2 mL, 128 mmol), cooled to 0 °C, and stirred for 15 min, at
which time a solution of acetaldehyde (14.3 mL, 255 mmol) and
cyclohexanone (13.2 mL, 128 mmol) was added from the addition
funnel dropwise over 2 h. After being stirred at 0 °C for 1 h, the reaction
mixture was quenched with water (3 mL), transferred to a separatory
funnel, and treated with oxalic acid (0.21 g, 1.6 mmol) dissolved in
water (3 mL). After being washed with brine (2 × 20 mL), the organic
solution was distilled under reduced pressure, first at 95 °C and 95
mmHg to remove residual water and cyclohexanone, then at 110 °C
and 5 mmHg to afford 3.3 g of (E)-2-ethylidenecyclohexan-1-one in
21.9% of theoretical yield: 1H NMR 1.24 (m, 2H); 1.32 (m, 5H); 2.03
(m, 2H); 2.19 (t, J ) 6.59 Hz, 2H); 6.86 (m, 1H); 13C NMR 13.07,
23.38, 23.60, 26.35, 40.16, 133.16, 137.17, 198.03.
Heating (Z)-2-Ethylidene-1-methylenecyclohexane, (Z)-7. Samples
of (Z)-7 (7 mg, 7.5 mmol) in 0.7 mL of benzene-d6 in a lead-potash
glass tube were degassed through three freeze-thaw cycles and sealed
under vacuum. The tubes were heated in the vapors of the following
boiling solvents: n-butyl alcohol (117.4-118.3 °C), tetrachloroethylene
(121.3-121.8 °C), chlorobenzene (131.3-132.0 °C), mesitylene (164.9-
165.2 °C), p-methylanisole (175.5-175.9 °C), and diethyloxalate
(185.5-186.0 °C). For analysis, the tubes were opened. The contents
were transferred to NMR tubes and analyzed quantitatively by the
absorption at 5.27 and 4.77 for (Z)-7 and those at 6.89 and 5.15 ppm
for 8. A sample of the product, 1-methyl-2-vinylcyclohexene (8) was
isolated from the combined reaction products of the various kinetic
runs by preparative GC using a 3-m, 20% Carbowax 20M on Anakrom
column at 72 °C: 1H NMR 1.41-1.45 (m, 2H), 1.48-1.51 (m, 2H),
1.61 (s, 3H), 1.83-1.87 (m, 2H), 2.09-2.12 (m, 2H), 4.99 (d, J )
10.99 Hz, 1H), 5.15 (d, J ) 17.34 Hz, 1H), 6.89 (dd, J ) 17.34, 10.99
Hz, 1H); 13C NMR 19.15, 23.05, 23.16, 25.07, 33.00, 110.10, 133.10,
135.53.
(Z)-2-Ethylidene-3,4-dihydro-2H-naphthalene-1-one. Following
the general procedure of Riahi et al.,35a R-tetralone (6.0 g, 0.04 mol)
and acetaldehyde (1.8 g, 0.04 mol) are condensed to yield 5.7 g of
crude product, which, after flash-chromatography (22 × 3.5 cm2 column
of silica gel, 14:1 petroleum ether/ethyl acetate), affords 2.8 g of (E)-
2-ethylidene-3,4-dihydro-2H-naphthalene-1-one (2.4 g after evaporative
distillation, mp 38 °C (lit. 38 °C35b)): 1H NMR35b,c (benzene-d6) 1.38
(d of t, J ) 6.84, 1.47 Hz, 3H), 2.29 (br t, J ) 6.35 Hz, 2H), 2.42 (t,
J ) 6.35 Hz, 2H), 6.82 (d, J ) 7.33 Hz, 1H), 7.01 (t of d, J ) 7.81,
0.98 Hz, 1H), 7.06 (t of d, J ) 7.32, 1.95 Hz, 1H), 8.42 (dd, J ) 7.81,
1.47 Hz, 1H) (one H is hidden under the H of the solvent benzene-d6);
1H NMR (CDCl3) 1.88 (d, J ) 7.32, Hz, 3H), 2.79 (br t, J ) 6.35 Hz,
2H), 2.95 (t, J ) 6.84 Hz, 2H), 7.03 (q of d, J ) 5.37, 1.47 Hz, 1H),
7.25 (t, J ) 6.35 Hz, 1H), 7.33 (t, J ) 7.33 Hz, 1H), 7.46 (t of d, J )
7.33, 1.47 Hz, 1H), 8.09 (dd, J ) 6.84, 0.98 Hz, 1H); 13C NMR
(benzene-d6) 14.10, 25.81, 29.54, 127.72, 128.89, 129.13, 133.30,
134.56, 134.93, 136.63, 144.21, 186.69.
Irradiation of (E)-2-Ethylidenecyclohexan-1-one. In a 500-mL,
single-necked, round-bottomed flask, a sample of ketone (7.24 g, 58
mmol) in benzene (450 mL) was degassed by passing through it a
stream of nitrogen for 20 min. The flask was then fitted with a reflux
condenser and irradiated with a Hanovia 450-W, medium-pressure
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J. AM. CHEM. SOC. VOL. 126, NO. 43, 2004 14215