Scheme 2
Scheme 3 a
preparation and rearrangement of divinylcyclopropanes via
this methodology.
Reaction of cation 3 with vinylmagnesium chloride, in
CH
diyl)iron complexes 4a (Scheme 3). Use of CH
is crucial for addition of Grignard reagents at C2; use of
,2-dichloroethane, toluene, THF, dioxane, or mixtures led
2
Cl
2
, gave the corresponding (2-alkenyl-3-pentene-1,5-
2
Cl as solvent
2
a
a, R ) H; b, R ) Me.
1
to diminished yields of 4a. The structure of pentenediyl
complex 4a was assigned on the basis of its NMR spectral
reagent, while the (S)-Mosher’s ester of (-)-9b was deter-
mined to be >95% de.
In a similar fashion, reaction of 3 with the Grignard
13
data. In particular, a C NMR signal at δ 11.4 ppm and a
1
H NMR signal at δ 0.24 (d) ppm are characteristic of a
reagents derived from 2-bromo-1-propene, R-bromostyrene,
carbon σ-bonded to iron and its attached proton.5
1
-bromo-2-methylpropene, and 1-bromocyclopentene gave
the corresponding (pentenediyl)iron complexes 4c-f (Table
). Oxidative decomplexation of 4c gave the divinylcyclo-
Oxidative decomplexation of 4a with excess CAN/
methanol gave cis-divinylcyclopropane 5a. This compound
rearranges at 40-60 °C to give the known (3-methoxycar-
1
propane 5c along with the rearranged cycloheptadiene
product (ca. 2.5:1, 88% yield). Reduction of this mixture
gave the (2,6-cycloheptadien-1-yl)methanol 9c (Cope rear-
rangement occurs at <23 °C). In comparison, oxidative
decomplexation of (pentenediyl)iron complexes 4d or 4e,
which contain an electron-rich alkenyl group, gave dimin-
ished yield of divinylcyclopropane. Further experimentation
indicated that this diminished yield was due to secondary
oxidation of the divinylcyclopropane product by CAN. For
this reason, we explored alternative oxidation conditions, the
most successful of which was the use of alkaline hydrogen
peroxide at low temperature (conditions B). While the
chemical yields under conditions B were good, the products
consisted of a mixture of cis- and trans-divinylcyclopropanes,
as evidenced by NMR spectroscopy. These mixtures could
be converted into a single cycloheptadiene product by the
standard reduction/Cope rearrangement conditions. Monitor-
ing of this reaction by VT NMR spectroscopy indicated that
the cis-divinylcyclopropane rearranges at temperatures lower
than those of the trans isomer; rearrangement of the trans
6
bonyl)-1,4-cycloheptadiene. Alternatively, reduction of the
cyclopropanecarboxylate (LAH/ether) gave the rearranged
(
2,6-cycloheptadien-1-yl)methanol 9a. Presumably, the in-
termediate divinylcyclopropane 8a rapidly rearranges at <23
C. It is known that the presence of an electron-withdrawing
°
group strengthens the distal cyclopropane ring bond, and this
should have an effect on the rate of the Cope rearrangement.
In a similar fashion, reaction of rac-3 with the Grignard
reagent prepared from cis-1-propenyl bromide gave rac-4b.
Oxidative decomplexation of 4b gave rac-5b, which upon
reduction gave the cyclopropylcarbinol rac-8b. In compari-
son to the parent divinylcyclopropane 8a, the cis-alkenyl
cyclopropane 8b is stable at ambient temperatures and only
rearranges at elevated temperature (125 °C) to give a single
7
cycloheptadiene rac-9b. This methodology can be extended
to the enantioselective preparation of cycloheptadienes. Thus
8
reaction of (1R)-3 with cis-1-propenyl Grignard reagent gave
(
+)-4b, which upon oxidative decomplexation gave the
optically active divinylcyclopropane (+)-5b. Reduction of
+)-5b gave (+)-8b which, upon rearrangement at elevated
(
isomer presumably occurs via isomerization to the cis isomer
temperature, gave (-)-9b. Both (+)-4b and (+)-5b were
via a diradical opening of the cyclopropane ring.1
1
determined to be >95% ee on the basis of H NMR
Generation of the mixture of cis- and trans-divinylcyclo-
propanes (5/5′) is rationalized due to the difference in the
oxidizing agent involved. For the (pentenediyl)iron complex
spectroscopy in the presence of a chiral lanthanide shift
(
6) Pikulik, I.; Childs, R. F. Can. J. Chem. 1977, 55, 251-258.
4
e (Scheme 3), treatment with CAN is presumed to involve
(7) The higher temperature for Cope rearrangement of divinylcyclopro-
-
single electron oxidation to afford a 17e intermediate, which
undergoes rapid reductive elimination to give the cis-
divinylcyclopropane 5e. Alternatively, treatment of 4e with
alkaline hydrogen peroxide proceeds via nucleophilic attack
panes possessing a cis-alkenyl group reflects the boatlike transition state
for this [3.3] sigmatropic rearrangement: Schneider, M. P.; Rau, A. J. Am.
Chem. Soc. 1979, 101, 4426-4427.
(8) For preparation of (1R)-3 see: Tao, C.; Donaldson, W. A. J. Org.
Chem. 1993, 58, 2134-2143.
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Org. Lett., Vol. 7, No. 10, 2005