Kinetics of 1-(E)-Propenyl-2-methylcyclobutanes
J . Org. Chem., Vol. 67, No. 10, 2002 3255
data from thermal reactions, a DB-1301 [J & W, 6% (cyano-
propylphenyl)methylsiloxane, 30 m × 0.25 mm × 0.50 µm film
thickness] column was used.
If one views the same experimental result from the
standpoint of orbital symmetry theory and the stereo-
chemical characteristics of “allowed” [1,3]-sigmatropic
shifts of carbon, one has 1 giving 3:4 through both
“allowed” and “forbidden” paths in ≈64:36 proportions,
while 2 gives 3:4 through “forbidden” and “allowed” paths
in a 71:29 ratio. The reactions seem best viewed as
diradical-mediated, nonconcerted, stepwise isomeriza-
tions. Some products do have stereochemistry consistent
with expectations for “allowed” reactions, but such paths
enjoy no substantial energy of concert. They are not
kinetically dominant. Other reactions of the same sort,
stereochemically classed as “forbidden”, occur at very
similar and sometimes faster rates. That 1 but not 2 gives
cis-3,6-dimethylcyclohexene (6) but not the trans isomer
5 is most suggestive. The trans starting material 1 seems
well-positioned for forming the cis product 6 by way of
12 efficiently in a suprafacial fashion. J ust why k(1f5),
k(2f5), and k(2f6) are too small to be of significance in
competitive kinetic circumstances remains unclear.
Both 1 and 2 have access to a conformational isomer
of 11 appropriate to a hydrogen transfer from the methyl
group to C3 to form 1,6-(E)-octadiene (10). Similar
internal disproportionation processes in 1-vinyl- and
1-alkenyl-2-methylcyclobutanes have been noted previ-
ously.5,22
Might 11 also lead to 1,5-(E)-octadiene (9)? Should not
11 prefer a six-centered activated complex to an eight-
centered transition structure for a hydrogen transfer
leading to an acyclic diene? Perhaps, but no evidence now
at hand may serve to answer the question. Substantial
amounts of 9 are formed indirectly from 2 or from 1 by
way of 2. A second less-significant reaction path leading
from 11 to 9 directly would not have been detected in
the present kinetic studies.
The very similar stereochemical balance between trans
and cis isomers of 3,4-dimethylcyclopentene formed under
kinetic control from trans-1-(E)-propenyl-2-methylcyclo-
propane, 73:27,4 and the stereochemical outcome for 1,3-
carbon shifts leading to 3 (63-64%) and 4 (37-36%)
starting from trans isomer 1 provides some grounds for
hope that the theoretical tactics that have been so
illuminating for vinylcyclopropane-to-cyclopentene isomer-
izations2 will similarly serve to elucidate in detail just
how a conformationally flexible diradical such as 11 or
12 may contort in the brief interval between its genesis
from 1 or 2 and its demise as it locates a product-defining
transition structure. The allowed-forbidden reversal of
1,3-shifts shown by 1 and 2 provides a strong stimulus
for such efforts. Full experimental and theoretically
detailed definitions of reaction stereochemistry and
mechanism for the parent vinylcyclobutane-to-cyclohex-
ene isomerization remain significant goals.
4-P en ten -1-a l.23 To a 100-mL flask were added PCC14 (1.9
g, 8.7 mmol) and dry CH2Cl2 (25 mL). 4-Penten-1-ol (0.5 g, 5.8
mmol) in CH2Cl2 (4 mL) was added dropwise to the PCC
solution over 5 h. The mixture was stirred for an additional 2
h at room temperature. At that time the black mixture was
diluted with ether (35 mL). The brown suspension was then
filtered through Florisil. The black tar that remained in the
flask was washed with ether (3 × 20 mL), the washings were
filtered through Florisil, and the Florisil was washed with
ether (35 mL). The ethereal solutions were combined and
concentrated by distillation to yield 4-penten-1-al (0.46 g, 94%)
as a 17% solution: MS m/z (rel intensity) 84 (8, M+), 83 (31),
69 (13), 56 (58), 55 (100), 41 (72), 39 (92), 29 (94).
1,5-(Z)-Octa d ien e (7).5,24 To a 50-mL flask were added
propyltriphenylphosphonium bromide (0.94 g, 2.5 mmol) and
THF (15 mL). The solution was cooled to -78 °C and 1.4 M
MeLi (1.8 mL, 2.5 mmol) in ether was added dropwise. The
mixture was warmed to 0 °C and stirred for 1 h. At that time,
the orange solution was cooled to -78 °C and 4-penten-1-al
(0.19 g, 2.3 mmol) was added dropwise in THF (5 mL). The
mixture was slowly warmed to room temperature and stirred
for 3 h under argon. The yellow reaction mixture was quenched
with water (25 mL) and the aqueous layer was extracted with
pentane (3 × 20 mL). The organic layers were combined,
washed with water (3 × 20 mL), dried (Na2SO4), filtered, and
concentrated to give 0.21 g (85%, 5Z:5E 6.3:1, Ultra 2) of 1,5-
octadienes 7 and 9 as a 3% solution. A sample of 7 was purified
by preparative GC (2.3 m, 20% ODPN on Chromosorb P-NAW,
58 °C): 1H NMR δ 5.92-5.74 (m, 1 H), 5.47-5.27 (m, 2 H),
5.10-4.92 (m, 2 H), 2.20-1.88 (m, 6 H) 0.96 (t, J ) 7.4 Hz, 3
H) (compare ref 5); 13C NMR δ 138.5, 132.1, 128.3, 114.5, 33.9,
26.6, 20.6, 14.3; MS m/z (rel intensity) 110 (2, M+), 95 (8), 81
(30), 69 (48), 53 (14), 41 (100), 39 (46).
3-Eth ylh exa -1,5-d ien e (8).15,24 Preparative GC (2.3-m, 20%
ODPN on Chromosorb P-NAW, 50 °C) provided a 41-mg
sample of dienes 7 and 9 (84:16), which was dissolved in 600
µL of pentane. The sample was injected into the kinetic bulb,
heated at 275.1 ( 0.1 °C for 2 h, and removed from the bulb.
The GC (DB-1301) trace showed peaks for 1,5-(Z)-octadiene
(7, 8.9 min, 49%), a new component (8, 6.0 min, 7%), and 1,5-
(E)-octadiene (9, 8.7 min, 44%). The new component was
collected by preparative GC (2.3 m, 20% ODPN on Chromosorb
P-NAW, 50 °C) and shown to be 3-ethylhexa-1,5-diene (8): 1H
NMR δ 5.88-5.68 (m, 1 H), 5.65-5.48 (m, 1 H), 5.10-4.88 (m,
4 H), 2.22-1.81 (m, 3 H), 1.52-1.37 (m, 1 H), 1.35-1.17 (m, 1
H), 0.86 (t, J ) 7.4 Hz, 3 H) (compare ref 15); 13C NMR δ 142.5,
137.2, 115.5, 114.3, 45.4, 39.1, 26.9, 11.5 (compare ref 15); MS
m/z (rel intensity) 95 (19), 81 (97), 79 (32), 69 (100), 68 (52),
67 (86), 53 (51), 41 (100), 39 (99), 27 (65).
1,5-(E)-Octa d ien e (9).5,24 To a 50-mL flask were added
propyltriphenylphosphonium bromide (1.29 g, 3.4 mmol) and
THF (20 mL). The solution was cooled to -78 °C and 1.4 M
MeLi (2.4 mL, 3.4 mmol) in ether was added dropwise. The
mixture was warmed to 0 °C and stirred for 1 h. At that time
the orange solution was cooled to -78 °C and 4-penten-1-al
(0.27 g, 3.2 mmol) was added dropwise in THF (3 mL). The
reaction mixture was stirred for 15 min at -78 °C and 1.4 M
MeLi (2.4 mL, 3.4 mmol) in ether was then added dropwise.16
The dark red solution was stirred for 40 min at -78 °C. At
that time tert-butyl alcohol (0.47 g, 6.3 mmol) was added and
the mixture was slowly warmed to room temperature. After 2
h at room temperature the mixture was quenched with 1 M
HCl (20 mL) and the aqueous layer was extracted with pentane
(3 × 20 mL). The organic layers were combined, dried (Na2-
SO4), filtered, and concentrated to give 0.28 g (80%) of 1,5-
Exp er im en ta l Section
Mass spectra were determined using a GC/MS system and
an Ultra 2 (HP, cross-linked 5% phenylmethyl siloxane, 25 m
× 0.2 mm × 0.33 µm film thickness) column. Preparative GC
purifications employed custom-made 6.35-mm o.d. stainless
steel or aluminum packed columns. Analytical GC work was
performed with 25 m Ultra 2 and 25 m × 0.2 mm × 0.1 µm
film thickness Carbowax 20M columns. For analyses of kinetic
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Macdonald, F., Exec. Eds.; Chapman & Hall, Cambridge University
Press: New York, 1996; Vol. 5, pp 5129-5130, P-0-00722. (b) Mont-
gomery, L. K.; Matt, J . W. J . Am. Chem. Soc. 1967, 89, 6556-6564.
(24) Prevost, C.; Miginiac, P.; Miginiac-Grozeleau, L. Bull. Soc.
Chim. Fr. 1964, 10, 2485-2492.
(22) (a) Chickos, J . S.; Frey, H. M. J . Chem. Soc., Perkin Trans. 2
1987, 365-370. (b) Chickos, J . S. J . Chem. Soc., Perkin Trans. 2 1987,
1109-1111.