Synthesis of cyclopropanes via organoiron methodology: Preparation and rearrangement of divinylcyclopropanes - Studies directed toward the synthesis of hydroazulenes

Article abstract of DOI:10.1055/s-2006-950312

Addition of alkenyl Grignard reagents to (1-methoxycarbonylpentadienyl) iron(¹⁺) cation generates the corresponding (2-alkenylpent-3-en-1,5- diyl)iron complexes. Oxidatively induced reductive elimination of these complexes gives divinylcyclopro

Full text of DOI:10.1055/s-2006-950312

PAPER
3639
Synthesis of Cyclopropanes via Organoiron Methodology: Preparation and
Rearrangement of Divinylcyclopropanes – Studies Directed toward the
Synthesis of Hydroazulenes
P
reparation and
R
a
e
arrangeme
t
nt of
D
h
ivinylcyclop
a
ropanes niel J. Wallock,a Dennis W. Bennett,b Tasneem Siddiquee,b Daniel T. Haworth,a William A. Donaldson*a
a
Department of Chemistry, Marquette University, P. O. Box 1881, Milwaukee, WI 53201-1881, USA
Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53201-0413, USA
Fax +1 (414)288 7066; E-mail: william.donaldson@marquette.edu
b
Received 25 August 2006; revised 4 September 2006
Dedicated to Professor Martin F. Semmelhack on the occasion of his 65th birthday
L(CO)2Fe+
Nu
Abstract: Addition of alkenyl Grignard reagents to (1-methoxycar-
Nu
"Nu:–"
[ox]
–
bonylpentadienyl)iron(1+) cation generates the corresponding (2-
PF6
alkenylpent-3-en-1,5-diyl)iron complexes. Oxidatively induced re-
ductive elimination of these complexes gives divinylcyclopropanes
which can undergo subsequent Cope rearrangement to give 1,4-cy-
cloheptadienes.
MeO2C
MeO2C
MeO2C
Fe(CO)2L
5, L = CO
6, L = PPh3
3, L = CO
4, L = PPh3
Key words: organometallic reagents, iron, rearrangements
Scheme 2
The Cope rearrangement of cis- or trans-divinylcyclopro-
panes (cis-1 or trans-1), is known to afford 1,4-cyclohep-
tadienes 2 (Scheme 1).1 Divinylcyclopropanes may be
prepared by olefination of cyclopropanecarboxalde-
hydes,2 reaction of 2-metalated vinylcyclopropanes,3 and
rhodium-catalyzed cyclopropanation of vinyldiazo-
methanes.4
preparation and rearrangement of divinylcyclopropanes
via this methodology.7
We have previously reported that the reaction of 3 with
alkenyl or alkynyl cuprates5b,e gives (2E,4Z-di-
enoate)Fe(CO)3 complexes, e.g. 7 (Equation 1, Table 1).
In contrast, reaction of 3 with ethynylmagnesium bromide
in the absence of CuBr gave the bis-iron tetraene complex
8 as a mixture of diasteromers. Reaction of cation 3, with
vinylmagnesium halides afforded mixtures of 5a, 7, and 8;
the ratios and isolated yields of these products depends on
the nature of the halide ion and reaction solvent mixture.
In a similar fashion, reaction of the (CO)2PPh3 ligated cat-
ion 4 with (vinyl)MgCl gave 6a.
N2
[M]
R
CHO
R
MeO2C
[M] = Li
or CuLn
"Rh(II)"
olefination
3 or 4
[3,3]
∆
R
R
or
10
H
MgBr
Fe(CO)2L
R
9
MgX
trans-1
cis-1
2
Fe(CO)2L
4
3
2
Scheme 1
E
11
E
E
E
Fe(CO)2L
We5 and others6 have shown that (1-methoxycarbonyl-
pentadienyl)iron(1+) cations 3/4 undergo nucleophilic ad-
dition at the C1/C5 terminal positions to afford
(diene)iron complexes or at the C2/C4 internal carbons to
afford stable (pentenediyl)iron complexes 5/6
10
5a, L = CO
6a, L = PPh3
7, L = CO
8, L = CO
9
Fe(CO)2L
Equation 1 (E = CO2Me)
(Scheme 2). We have further demonstrated that oxidative- The structures of pentenediyl complexes 5a/6a were as-
ly induced reductive elimination of complexes 5 give vi- signed on the basis of their NMR spectral data. In partic-
13
1
nylcyclopropanecarboxylates.5d,g,i We herein report on the ular, a C NMR signal at ca. d = 10–11 and a H NMR
signal at ca. d = 0.25 are characteristic of a carbon s-bond-
ed to iron and its attached proton.5c,g,i This structural as-
signment was corroborated by single-crystal X-ray
SYNTHESIS 2006, No. 21, pp 3639–3646
Advanced online publication: 09.10.2006
DOI: 10.1055/s-2006-950312; Art ID: Z17406SS
x
x.
x
x
.
2
0
0
6
diffraction analysis of 5a (Figure 1). The structure of 8
was assigned on the basis of its 1H NMR spectral data. In
© Georg Thieme Verlag Stuttgart · New York

3640
N. J. Wallock et al.
PAPER
Table 1 Addition of Alkenyl/Alkynyl Grignard to 3/4
Cation
[c]a (M)
0.02
0.1
Solvent
THF–Et2O
CH2Cl2
CH2Cl2
THF
RMgX (equiv)
9 X = Clb (1.1)
10c (1.5)
Product(s) (%)
7 (42)
3
3
3
3
3
3
4
4
4
8 (57)
0.1
9 X = Brd (1.1)
9 X = Brd (1.1)
9 X = Brd (1.5)
9 X = Cle (1.5)
9 X = Cle (1.1)
9 X = Cle (1.7)
9 X = Cle (1.5)
5a (41)
0.1
5a (16), 8 (54)
5a (28), 8 (32)
5a (49–56)
6a (37)
0.1
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
0.1
0.1
0.1
6a (29)
0.02
6a (49)
a Initial concentration of cation 3/4, –65 °C.
b CuBr·SMe2 (0.33 equiv) was added.5e
c 0.5 M in THF.
d 1.0 M in THF.
e 1.6 M in THF.
particular the signals at d = 2.12–2.20, 5.38–5.49, and The yield of 5a was greatest using the vinylmagnesium
6.02–6.16, assigned to H2/H11, H4/H10, and H3/H11, re- chloride and CH2Cl2 as solvent; use of vinylmagnesium
spectively, are characteristic of (2E,4Z-dienoate)iron bromide in THF or toluene led to diminished yields of 5a
complexes.5b,e,h
with concomitant increase in the formation of the dimeric
complex 8. Since the Grignard reagents were obtained as
solutions in THF the number of equivalents of this reagent
and the initial concentration of the cation (in CH2Cl2)
were important factors in maximizing the yield of the pen-
tenediyl products 5a/6a.
The formation of the dimeric complex 8 is rationalized by
initial single-electron transfer from the alkynyl/alkenyl
Grignard reagent to afford a (pentadienyl)iron radical 11
(Scheme 3). Kochi has previously reported that certain
nucleophilic additions to (pentadienyl)iron cations pro-
ceed via initial electron-transfer.8 Dimerization of 11 af-
fords the bimetallic tetraene complex 8. Since the
precursor 3 is racemic, the product 8 is formed as a mix-
ture of diastereomers with respect to the iron coordina-
tion. Birch and co-workers have previously reported that
CH2Cl2 suppresses radical dimerization in the reaction of
(cyclohexadienyl)iron complexes with alkyllithiums and
alkyl Grignard reagents.9
(CO)3Fe+
(CO)3Fe
–
RMgX
"SET"
11
PF6
8
MeO2C
MeO2C
3
11
Scheme 3
Figure 1 Ortep drawing of 5a with crystallographic numbering,
50% ellipsoids. Selected bond lengths (Å): Fe(1)–C(4), 2.133(2);
Fe(1)–C(5), 2.087(2); Fe(1)–C(6), 2.174(2); Fe(1)–C(8), 2.1139(18);
C(4)–C(5), 1.395(3); C(5)–C(6), 1.397(3); C(6)–C(7), 1.517(3);
C(7)–C(8), 1.526(3).
In a similar fashion, reaction of 3 or 4, in CH2Cl2, with the
Grignard reagents derived from (Z)-1-bromopropene
(12b), 2-bromoprop-1-ene (12c), a-bromostyrene (12d),
and 1-bromo-2-methylpropene (12e) (in THF) gave the
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York

PAPER
Preparation and Rearrangement of Divinylcyclopropanes
3641
corresponding (pentenediyl)iron complexes 5b–e or 6c–e, and an additional methyl signal (d = 1.32, ddd, 3 H) is
respectively (Equation 2, Table 2). The structures of these present. It was not possible to assign the stereochemistry
complexes were assigned by comparison of their NMR of the isomerized C=C. It is not clear why complex 14 is
spectral data with that of 5a/6a. Reaction of the Grignard labile to this isomerization (compared to the structural sta-
reagent prepared from (E)-1-bromopropene gave an in- bility of 5a–f). Similarly, the mechanism by which this
separable mixture of trans- and cis- isomers (5f and 5b, isomerization occurs is as yet unknown.
1.5:1). The partial isomerization (ca. 30%) of (E)-1-bro-
Me
mopropene upon Grignard formation has previously been
H
noted.10
MgCl
slow
1
5
5
1
CH2Cl2
–70 °C
E
Me
E
Me
E
Me
RE
R1
RZ
RE
RZ
Fe+
(CO)3
Fe
Fe
H
H
12
(CO)3
(CO)3
R1
MgBr
5b–f, L = CO
13
14 (47%)
15
3 or 4
6c–e, L = PPh3
CH2Cl2, –70 °C
MeO2C
Scheme 4 (E = CO2Me)
Fe(CO)2L
Equation 2
Oxidative decomplexation of 5a or 6a with excess CAN/
methanol gave cis-divinylcyclopropane 16a (Scheme 5).
This compound partially rearranges upon chromatogra-
phy or completely at 40–60 °C to give the known12 (3-
methoxycarbonyl)cyclohepta-1,4-diene (17a).
Table 2 Addition of Substituted Alkenyl Grignard to 3 or 4a
Cation
RMgBr R1
RE
H
RZ
Me
H
Product(s) (%)
5b (73)
3
3
4
3
4
3
4
3
12bb
12cc
12cc
12dd
12dd
12ec
12ec
12fe
H
Me
Me
Ph
Ph
H
H
5c (52)
CAN
40–60 °C
H
H
6c (39)
5a or 6a
MeOH
MeO2C
MeO2C
16a
H
H
5d (49)
17a
H
H
6d (51)
(58–69%, 2 steps)
Me
Me
Me
Me
Me
H
5e (71)
Scheme 5
H
6e (67)
In order to avoid olefin isomerization, an alternative
decomplexation/reduction/rearrangement protocol was
developed. To this end, reduction of the cyclopropanecar-
boxylate 16a (LiAlH4, Et2O, 23 °C) directly gave the
cyclohepta-2,6-dien-1-ylmethanol (18a) (Scheme 6,
Table 3). Since this procedure gave superior overall
yields, it was utilized for the other (pentenediyl)iron com-
plexes 5b–d and 14 (Scheme 6, Table 3). The presumed
divinylcyclopropylmethanol intermediates 19c or 19d
were not observed, but rearranged at or below ambient
temperature (< 23 °C) to the corresponding cyclohepta-
dienes 18c or 18d. In comparison, the divinylcyclopro-
panemethanol 19b, bearing a RZ substituent, was stable at
23 °C and only rearranged to the cycloheptadiene 18b at
elevated temperature.
H
5f + 5b (1.5:1)
(56)
a Reactions were performed with 1.5 equiv of alkenyl Grignard re-
agent at –70 °C; initial concentration of cation, 0.1 M.
b Prepared from (Z)-1-bromopropene and Mg in THF (1.1 M).
c 0.5 M in THF.
d Prepared from a-bromostyrene and Mg in THF (1.0 M).
e Prepared from (E)-1-bromopropene and Mg in THF (1.1 M).
Addition of vinyl Grignard to (1-methoxycarbonylhexadi-
enyl)iron(1+) cation (13)11 proceeds via C2 attack to afford
the (2-ethenylhexenediyl)iron complex 14 as a semi-sta-
ble oil (Scheme 4). Upon standing, 14 slowly isomerizes
to a separable mixture of 14, 15 and a third unidentified
compound. Due to this isomerization, 14 was always
freshly purified prior to further use. The structural assign-
ments of 14 and 15 as (hexenediyl)Fe(CO)3 complexes are
based on their NMR spectral data. The 1H NMR spectral
data for 14 are relatively similar to that for 5a, except for
the absence of the H5exo signal (for 5a: d = 3.55, ddd) and
the presence of the C5-CH3 signal (for 14: d = 1.88, d, 3
H). For complex 15, the H1 and H4 signals (d = 1.08 and
4.81, respectively) are shifted downfield due to their allyl-
ic nature as compared to 14 (d = 0.33 and 4.22 respective-
ly)], signals for the vinyl methylene protons are absent,
RE
RZ
RE
R5
RZ
R1
R1
R5
i) oxidation
ii) LiAlH4
∆
5a–d
or 14
HO
HO
19a–d,f
18a–d,f
Scheme 6
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York

3642
N. J. Wallock et al.
PAPER
Table 3 Decomplexation/Reduction/Rearrangement of (Pentenediyl)iron Complexes 5
Complex
R1
H
RE
H
H
H
H
H
RZ
H
R5
H
Rearrangement temp (°C)
Product (%)
5a
5b
5c
5d
14
<23
125
<23
<23
50
18a (83)
18b (61)
18c (82)
18d (39)
18f (51)
H
Me
H
H
Me
Ph
H
H
H
H
H
Me
In comparison to the above results, oxidative decomplex- tack on coordinated CO, and decarbonylation, to generate
ation of (pentenediyl)iron complex 5e with CAN/metha- a 16e– intermediate. Reductive elimination from the 16e–
nol gave only a complex mixture of unidentified products; intermediate is slower, and a competitive reaction is a p–
switching to MeCN as solvent gave the cis-divinylcyclo- s–p rearrangement that migrates the iron from one face to
propane (cis-16e) in dismally low yield (Scheme 7). Fur- the opposite face of the pentenediyl ligand. This isomer-
ther experimentation indicated that this diminished yield ization leads to an inversion of configuration at C3, which
was due to secondary reaction of the divinylcyclopropane is reflected in the product after reductive elimination. No-
product with CAN/methanol. For this reason, we explored tably, the ratio of cis-16e:trans-16e produced from de-
alternative oxidation conditions. The most successful complexation with H2O2/HO– varies depending on the
proved to be alkaline hydrogen peroxide at low tempera- reaction temperature.
ture. While the chemical yields under these conditions
were good, the products consisted of a mixture of cis- and
trans-divinylcyclopropanes (cis-16e and trans-16e), as
Me
evidenced by NMR spectroscopy. This mixture could be
Me
reductive
elimination
H
converted into a single cycloheptadiene product by the
standard reduction/Cope rearrangement conditions. Mon-
itoring of this reaction by VT NMR spectroscopy indicat-
ed that the cis-divinylcyclopropane rearranges at
temperatures lower than the trans-isomer; rearrangement
of the trans-isomer presumably occurs via isomerization
to the cis-isomer via a diradical opening of the cyclopro-
pane ring.1
H
3
CAN
5e
cis-16e
H
MeO2C
Fe
17 e–
(CO)3
H2O2, HO–
– "CO2"
Me
Me
(CO)2
Fe
3
H
H
π–σ–π
H
3
H
H
MeO2C
MeO2C
Me
Me
H
Fe
16 e–
Me
Me
Me
(CO)2
Me
CAN
MeCN
H2O2
NaOH
reductive
elimination
reductive
elimination
5e
cis-16e +
MeOH
–45 °C
Me
MeO2C
MeO2C
(1:2.2)
Me
cis-16e
trans-16e
Me
H
(11%)
Me
Me
H
H
Me
MeO2C
MeO2C
i) LiAlH4
ii) 195 °C
H
H
1:2 at –78 °C
1:1 at 23 °C
cis-16e
trans-16e
HO
18e (80%, 3 steps)
Scheme 8
Scheme 7
Finally, application of this methodology to the preparation
of the hydroazulene skeleton was explored. To this end,
reaction of 3 with the Grignard reagent derived from 1-
bromocyclopentene gave 5g (Scheme 9). Oxidative de-
complexation using H2O2/HO– gave a mixture of cis- and
trans-divinylcyclopropanes 16g (1:2.5). Reduction fol-
lowed by heating the product to 220 °C in mesitylene
(closed tube) gave the hexahydroazulene 18g.
Generation of the mixture of cis- and trans-divinylcyclo-
propanes (cis-16e/trans-16e) is rationalized due to the dif-
ference in the oxidized species involved. For the
(pentenediyl)iron complex 5e (Scheme 8), treatment with
CAN is presumed to involve single electron oxidation to
afford a 17e– intermediate, which undergoes rapid reduc-
tive elimination with retention of configuration at C1 and
C3 to give cis-16e. Alternatively, treatment of 5e with al-
kaline hydrogen peroxide proceeds via nucleophilic at-
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York

PAPER
Preparation and Rearrangement of Divinylcyclopropanes
3643
MoKa radiation (l = 0.71073 Å) in the Q/2Q bisecting mode. For-
mula: C12H12FeO5, monoclinic, space group P21/c, a = 14.162(2),
b = 6.377(2), c = 15.355(5) Å, b = 112.140(10)°, V = 1284.5(5) Å3,
Z = 4, dcalcd = 1.510 Mg/m3, u = 1.183 mm–1, 2078 unique reflec-
tions, R = 0.0338, Rw = 0.0830.13–16
Mg, THF,
H2O2, HO–
Br
3
MeO2C
CH2Cl2, –78 °C
inverse addition
Fe
(CO)3
5g (49–51%)
Bis(tricarbonyl iron)[dimethyldodeca-(2E,4Z,8Z,10E)-tetra-
enedioate] (8)
H
To a stirred mixture of 3 (820 mg, 2.0 mmol) in anhyd CH2Cl2 (20
mL) under N2 at –70 °C, was added a solution of ethynylmagnesium
bromide in THF (6.0 mL, 0.5 M, 3.0 mmol). The mixture was stirred
for 1 h, quenched with sat. aq NH4Cl (5 mL), and warmed to r.t. The
mixture was poured onto H2O (25 mL), the layers were separated,
and the aqueous phase was extracted with CH2Cl2 (20 mL). The
combined organic phases were dried (MgSO4), filtered through a
short bed of SiO2 and concentrated. Immediate column chromatog-
raphy (SiO2, hexanes–EtOAc, 3:1) afforded 8 as an orange oil (300
1) LiAlH4, THF
2) mesitylene, ∆
MeO2C
HO
cis-16g/trans-16g
18g (76%, 3 steps)
(1:2.5)
Scheme 9
1
mg, 57%). Analysis by H NMR spectroscopy indicated that this
was a mixture of two diastereomers.
In summary, a synthesis of divinylcyclopropanes from
(pentadienyl)iron(1+) cations has been developed. The di-
vinylcyclopropane products undergo Cope rearrangement
to afford cycloheptadienes. Applications of this method-
ology to the synthesis of hydroazulene containing natural
products will be reported in due course.
1H NMR (CDCl3): d = 1.80 (dd, J = 9.3, 13.5 Hz, 2 H), 2.00 (dd,
J = 9.6, 13.8 Hz, 2 H), 2.12–2.20 (m, 4H), 2.44–2.62 (m, 4 H), 2.73,
2.78 (2 dd, J = 3.9, 13.8 Hz, 4 H total), 3.69, 3.70 (2 s, 12 H total),
5.38–5.49 (m, 4 H), 6.02–6.16 (m, 4 H).
Reaction of 4 with Vinyl Grignard Reagent
The reaction of 4 (0.322 g, 0.5 mmol) in anhyd CH2Cl2 (25 mL) at
–70 °C with vinylmagnesium chloride (1.6 M in THF) was carried
out in a fashion similar to the preparation of 5a. Immediate column
chromatography (SiO2, hexanes–EtOAc, gradient 10:1 → 5:1) af-
forded 6a (0.130 g, 0.247 mmol, 49%) as a microcrystalline orange
solid; mp 174–175 °C (dec.).
1H and 13C NMR spectra were recorded at 300 MHz and 75.5 MHz
respectively. High-resolution mass spectra were obtained from the
Washington University Resource for Biomedical and Bioorganic
mass spectrometry. THF was distilled from sodium benzophenone
ketyl prior to use. Anhyd CH2Cl2 and DMF were purchased from
Aldrich Chemical Company. Reactions were performed in flame-
dried glassware under N2 unless otherwise noted. Compounds 3,5b
4,5c and 1311 were prepared by literature procedures. Experimental
procedures and characterization data for 5b–g and 18b–d, g can be
found in the supporting information of ref. 7.
IR (KBr): 3022, 2955, 2069, 2010, 1976, 1687, 1439, 1162 cm–1.
1H NMR (CDCl3): d = 0.25 (dd, JH,P = 3.6, JH,H = 8.4 Hz, 1 H),
2.13–2.23 (m, 1 H), 2.48–2.57 (m, 1 H), 3.75 (s, 3 H), 3.78–3.98 (m,
2 H), 4.25 (br t, J = 7.0 Hz, 1 H), 4.65–4.78 (m, 2 H), 5.34 (ddd,
J = 5.3, 10.3, 17.3 Hz, 1 H), 7.29–7.45 (m, 15 H).
13C NMR (CDCl3): d = 10.2 (d, JC,P = 15.6 Hz), 43.1 (d, JC,P = 2.3
Hz), 51.5, 56.9, 61.8 (d, JC,P = 1.1 Hz), 97.7 (d, JC,P = 2.3 Hz),
111.4, 128.6 (d, J = 9.5 Hz), 130.3 (d, J = 2.3 Hz), 132.9 (d,
JC,P = 9.8 Hz), 133.2 (d, JC,P = 38.1 Hz), 142.0 (d, JC,P = 5.8 Hz),
182.0 (d, JC,P = 2.0 Hz), 217.8 (d, JC,P = 10.4 Hz), 218.0 (d,
JC,P = 16.5 Hz).
Reaction of 3 with Vinyl Grignard Reagent
To a stirred mixture of 3 (1.64 g, 4.0 mmol) in anhyd CH2Cl2 (40
mL) under N2 at –70 °C, was added a solution of vinylmagnesium
chloride in THF (3.75 mL, 1.6 M, 6.0 mmol). The dark reaction
mixture was stirred for 1 h, quenched with sat. aq NH4Cl (5 mL),
and warmed to r.t. The mixture was poured onto H2O (25 mL), the
layers were separated, and the aqueous phase was extracted with
CH2Cl2 (2 × 20 mL). The combined organic phases were dried
(MgSO4), filtered through a short bed of SiO2 and concentrated. Im-
mediate column chromatography (SiO2, hexanes–EtOAc, 40:1) af-
forded 5a as an orange oil that crystallized upon cooling (0.659 g,
56%); mp 36–38 °C.
31P NMR (CDCl3): d = 62.9.
Anal. Calcd for C18H16FeO4P: C, 58.72; H, 4.38. Found: C, 59.06;
H, 4.67.
Reaction of 4 with Isopropenyl Grignard Reagent
The reaction of 4 (322 mg, 0.5 mmol) in anhyd CH2Cl2 (20 mL) at
–70 °C with a solution of isopropenylmagnesium bromide (0.5 M in
THF, 1.5 mL, 0.75 mmol) was carried out in a fashion similar to the
preparation of 5a. Immediate column chromatography (SiO2, hex-
anes–EtOAc, 10:1) afforded 6c as a microcrystalline orange solid
(0.104 g, 39%); mp 188–190 °C (dec.).
IR (KBr): 3090, 2066, 1992, 1694, 1165 cm–1.
1H NMR (CDCl3): d = 0.24 (d, J = 8.5 Hz, 1 H), 2.39 (ddd, J = 0.9,
2.3, 12.2 Hz, 1 H), 3.55 (ddd, J = 1.5, 2.3, 8.4 Hz, 1 H), 3.69 (s, 3
H), 3.71–3.82 (m, 1 H), 4.47 (dddd, J = 1.2, 1.2, 7.0, 7.6 Hz, 1 H),
4.67 (dddd, J = 0.5, 6.7, 8.5, 12.2 Hz, 1 H), 4.77–4.81 (m, 1 H),
4.82–4.85 (m, 1 H), 5.38 (ddd, J = 5.4, 10.1, 17.5 Hz, 1 H).
13C NMR (CDCl3): d = 11.4, 41.9, 51.7, 54.4, 63.5, 98.2, 112.7,
141.2, 180.5, 203.8, 210.3, 210.7.
IR (KBr): 3070, 2979, 2940, 1992, 1939, 1688, 1435, 1148, 697
cm–1.
1H NMR (CDCl3): d = 0.30 (dd, JH,P = 3.4, JH,H = 8.8 Hz, 1 H), 1.49
(s, 3 H), 2.00–2.11 (m, 1 H), 2.41–2.51 (m, 1 H), 3.64–3.74 (m, 1
H), 3.77 (s, 3 H), 3.92–4.06 (m, 1 H), 4.20–4.29 (m, 1 H), 4.48 (s, 2
H), 7.29–7.46 (m, 15 H).
Anal. Calcd for C12H12FeO5: C, 49.35; H, 4.14. Found: C, 49.55; H,
4.19.
X-ray Structural Analysis of 5a
13C NMR (CDCl3): d = 11.5 (br d, JC,P = ~15 Hz), 19.7, 46.3 (d,
JC,P = 2.3 Hz), 51.5, 57.0 (d, JC,P = 2.3 Hz), 62.3 (d, JC,P = 1.2 Hz),
97.8 (d, JC,P = 2.0Hz), 107.9, 128.6 (d, JC,P = 9.4 Hz), 130.3 (d,
JC,P = 2.1 Hz), 132.9 (d, JC,P = 10.4 Hz), 133.2 (d, JC,P = 38.0 Hz),
A crystal suitable for X-ray diffraction analysis was grown from
hexanes–EtOAc. A crystal (0.3 × 0.2 × 0.2 mm) was attached to a
glass fiber and mounted on a Bruker P4 diffractometer under a cold
N2 stream. Intensity data were collected at 152 K using graphite
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York

3644
N. J. Wallock et al.
PAPER
148.4 (d, JC,P = 5.2 Hz), 182.3 (d, JC,P = 2.5 Hz), 217.8 (d, JC,P = 2.0
Hz), 218.0 (d, JC,P = 4.1 Hz).
identified by comparison to its NMR spectral data previously ob-
tained.7
31P NMR (CDCl3): d = 63.0.
1H NMR (CDCl3): d (partial) = 0.19 (d, J = 8.4 Hz, 1 H), 1.54 (dd,
J = 6.3 Hz, 3 H), 2.42 (dd, J = 1.6, 12.1 Hz, 1 H), 3.55 (td, J = 1.8,
8.4 Hz, 1 H), 3.68 (s, 3 H), 4.98 (ddd, J = 1.5, 6.3, 15.3 Hz, 1 H),
5.24 (dqd, J = 1.2, 6.3, 15.3 Hz, 1 H).
Anal. Calcd for C30H29FeO4P: C, 66.68; H, 5.41. Found: C, 66.42;
H, 5.56.
Reaction of 4 with (a-Styryl) Grignard Reagent
13C NMR (CDCl3): d = 12.7, 17.8, 41.5, 51.6, 54.3, 64.9, 97.8,
The reaction of 4 (322 mg, 0.5 mmol) in anhyd CH2Cl2 (20 mL) at
–70 °C with a solution of (a-styryl)magnesium bromide [prepared
from a-bromostyrene (2.0 mmol) and Mg ribbon ( 2.0 mmol) in
THF (1 mL)] was carried out in a fashion similar to the preparation
of 5a. Immediate column chromatography (SiO2, hexanes–EtOAc
gradient, 10:1 → 5:1) afforded 6d as a yellow foam (0.153 g, 51%);
mp 161–163 °C (dec.).
123.8, 134.9, 180.6, 203.9, 210.5, 210.8.
Reaction of 13 with Vinyl Grignard Reagent
The reaction of 13 (1.70 g, 4.01 mmol) in anhyd CH2Cl2 (40 mL)
with a solution of vinylmagnesium chloride in THF (3.8 mL, 1.6 M,
6.1 mmol) was carried out in a fashion similar to the preparation of
5a. Immediate column chromatography (SiO2, hexanes–EtOAc
gradient, 30:1 → 20:1) gave 14 (0.575 g, 47%) as an unstable red
oil. On standing, 14 (yellow band) slowly isomerized to 15 (orange
band) along with another unidentified compound (red band), both of
which could be removed by additional chromatographic purifica-
tion.
IR (KBr): 3056, 3023, 2945, 1997, 1941, 1686, 1481, 1434, 1159,
1091, 746, 696, 588 cm–1.
1H NMR (CDCl3): d = 0.29–0.37 (m, 1 H), 2.01–2.12 (m, 1 H),
2.35–2.44 (m 1 H), 3.68 (s, 3 H), 3.88–4.03 (m, 1 H), 4.27–4.43 (m,
2 H), 4.85 (s, 1 H), 4.94 (s, 1 H), 7.19–7.27 (m, 5 H), 7.28–7.44 (m,
15 H).
13C NMR (CDCl3): d = 10.5 (br d, JC,P = ~15 Hz), 43.3 (d, JC,P = 2.2
Hz), 51.4, 56.7 (d, JC,P = 2.7 Hz), 61.7, 97.1 (d, JC,P = 2.1 Hz),
111.2, 127.29, 127.34, 128.1, 128.7 (d, JC,P = 9.5 Hz), 130.4 (d,
JC,P = 2.0 Hz), 133.0 (d, JC,P = 9.8 Hz), 133.2 (d, JC,P = 38.5 Hz),
140.2, 152.9 (d, JC,P = 5.2 Hz), 182.1 (d, JC,P = 2.5 Hz), 217.7 (d,
JC,P = 1.7 Hz), 217.9 (d, JC,P = 5.1 Hz).
(2-Vinylhexenediyl)iron Complex 14
IR (neat): 2950, 2060, 1992, 1698, 1435, 1381, 1171, 921 cm–1.
1H NMR (CDCl3): d = 0.33 (d, J = 8.5 Hz, 1 H), 1.88 (d, J = 6.2 Hz,
3 H), 3.31–3.42 (m, 1 H), 3.64–3.77 (overlapped m, 1 H), 3.68 (s, 3
H), 4.22 (ddd, J = 0.9, 6.8, 7.8 Hz, 1 H), 4.53 (dd, J = 6.9, 11.4 Hz,
1 H), 4.75–4.83 (m, 2 H), 5.38 (ddd, J = 5.3, 9.4, 18.1 Hz, 1 H).
13C NMR (CDCl3): d = 10.7, 21.6, 42.2, 51.7, 57.5, 73.7, 101.0,
112.6, 141.3, 180.6, 204.6, 210.4, 211.2.
31P NMR (CDCl3): d = 63.2.
Anal. Calcd for C13H14FeO5: C, 51.01; H, 4.61. Found: C, 51.31; H,
4.88.
Reaction of 4 with (2-Methylprop-1-enyl) Grignard Reagent
The reaction of 4 (1.932 g, 2.999 mmol) in anhyd CH2Cl2 (120 mL)
with a solution of (2-methyl-1-propenyl)magnesium bromide (0.5
M in THF, 9.0 mL, 4.5 mmol) was carried out in a fashion similar
to the preparation of 5a. Immediate column chromatography (SiO2,
hexanes–EtOAc gradient, 15:1 → 5:1) afforded 6e as a microcrys-
talline yellow-orange solid (1.115 g, 67%); mp 160–161 °C (dec.).
(Hexenediyl)iron Complex 15
1H NMR (CDCl3): d = 1.08 (q, J = 1.7 Hz, 1 H), 1.32 (ddd, J = 1.4,
1.4, 7.0 Hz, 3 H), 1.86 (d, J = 6.2 Hz, 3 H), 2.97 (qd, J = 6.2, 12.0
Hz, 1 H), 3.68 (s, 3 H), 4.40 (dd, J = 7.0, 11.0 Hz, 1 H), 4.81 (qd,
J = 0.6, 7.0 Hz, 1 H), 5.28 (ddq, J = 1.2, 2.0, 7.0 Hz, 1 H).
IR (KBr): 3075, 3030, 2950, 1997, 1942, 1684, 1480, 1435, 1240,
1157, 746, 697 cm–1.
13C NMR (CDCl3): d = 13.4, 13.8, 21.2, 30.0, 51.4, 66.9, 71.4, 97.6,
126.1, 132.3, 204.6, 209.8, 211.5.
1H NMR (CDCl3): d = 0.10 (dd, JH,P = 3.7, JH,H = 8.0 Hz, 1 H), 1.49
(d, J = 1.2 Hz, 3 H), 1.56 (d, J = 1.2 Hz, 3,H), 2.24–2.33 (m, 1 H),
2.52–2.60 (m, 1 H), 3.70–3.83 (m, 1 H), 3.74 (s, 3 H), 3.94 (br q,
J = 8.2 Hz, 1 H), 4.29 (br t, J = 7.0 Hz, 1 H), 4.58 (br d, J = 8.9 Hz,
1 H), 7.29–7.43 (m, 15 H).
13C NMR (CDCl3): d = ~13 (br), 18.5, 25.7, 39.1 (d, JC,P = 2.8 Hz),
51.4, 57.1, 64.4 (d, JC,P = 1.7 Hz), 96.5, 128.6 (d, JC,P = 9.2 Hz),
129.4, 130.3 (d, JC,P = 2.3 Hz), 131.2 (d, JC,P = 5.5 Hz), 132.9 (d,
JC,P = 9.8 Hz), 133.3 (d, JC,P = 37.5 Hz), 182.0 (d, JC,P = 2.0 Hz),
218.0 (d, JC,P = 14.8 Hz), 218.3 (d, JC,P = 21.3 Hz).
Oxidative Decomplexation of 6a
To a stirred solution of 6a (0.232 g, 0.441 mmol) in absolute MeOH
(9.6 mL), was added in portions CAN (5 × 0.24 g, 2.2 mmol) over
a period of 30 min. After stirring for 5 min after the last addition,
the red mixture was poured onto brine (15 mL) and extracted with
EtOAc (3 × 20 mL). The combined extracts were washed sequen-
tially with H2O (3 × 20 mL) and brine (20 mL). The organic phase
was then dried (MgSO4) and carefully concentrated. Purification by
column chromatography (SiO2, hexanes–Et2O, 20:1) gave a volatile
colorless oil. The 1H NMR spectrum indicated the oil to be a ~1:1
mixture of 16a and 17a (total yield: 0.0390 g, 58%). Heating this oil
(NMR probe, CDCl3) resulted in complete conversion to the
known12 compound 17a.
31P NMR (CDCl3): d = 62.9.
Anal. Calcd for C31H31FeO4P: C, 67.22; H, 5.64. Found: C, 66.95;
H, 5.80.
Reaction of 3 with Grignard Reagent Prepared from trans-1-
Bromopropene
Methyl cis-2,3-Divinylcyclopropanecarboxylate (16a)
1H NMR (CDCl3): d (as a mixture with 17a) = 1.86 (t, J = 4.9 Hz, 1
H), 2.27–2.35 (m, 2 H), 3.70 (s, 3 H), 5.12 (dd, J = 1.7, 10.3 Hz, 2
H), 5.24 (ddd, J = 0.6, 1.8, 17.0 Hz, 2 H), 5.49–5.62 (m, 2 H).
To a solution of 3 (820 mg, 2.0 mmol) in anhyd CH2Cl2 (20 mL) at
–70 °C was added a solution of the Grignard reagent freshly pre-
pared from trans-1-bromopropene (0.35 mL, 4.0 mmol) and Mg
ribbon (0.20 g, 8.2 mmol) in THF (3.6 mL) with a crystal of I2 added
to initiate the Grignard formation. After stirring –70 °C for 1 h, the
mixture was worked up in a fashion similar to the preparation of 5a.
Column chromatography (SiO2, hexanes–EtOAc, 40:1) afforded an
orange oil (345 mg, 56%). This was determined to be a mixture of
13C NMR (CDCl3): d = 28.2, 31.8, 52.2, 117.4, 133.8, 173.1.
(3-Methoxycarbonyl)cyclohepta-1,4-diene (17a)
IR (neat): 3028, 2951, 2907, 2836, 1743, 1644, 1435, 1266, 1171,
1030, 822, 796 cm–1.
1H NMR (CDCl3): d = 2.13–2.39 (m, 4 H), 3.74 (s, 3 H), 4.17–4.25
(m, 1 H), 5.77–5.93 (m, 4 H).
1
trans-5g and cis-5b (1.5:1) by H NMR spectroscopy; cis-5b was
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York

PAPER
Preparation and Rearrangement of Divinylcyclopropanes
3645
13C NMR (CDCl3): d = 26.2, 44.8, 52.5, 127.1, 132.4, 173.8.
added a methanolic solution of NaOH (0.59 g, 15 mmol NaOH dis-
solved in a minimal volume of MeOH). The mixture was stirred at
–78 °C for 30 min, the cold bath was removed, and the mixture
stirred for an additional 30 min while warming to r.t. During this pe-
riod, the reaction bubbled and became brown. The muddy mixture
was diluted with H2O (50 mL) and extracted with Et2O (4 × 50 mL).
The combined extracts were washed with H2O (3 × 50 mL) fol-
lowed by brine (50 mL). The organic phase was dried (MgSO4) and
concentrated to give a volatile yellow oil. The 1H NMR spectrum in-
dicated the residue to contain a mixture of cis and trans-divinylcy-
clopropane carboxylates (cis-16e:trans-16e = ~1:2.2). The crude
mixture was reduced with LiAlH4 in Et2O (4.2 mL, 1.0 M, 4.2
mmol) at 0 °C in a fashion similar to that above to give a tan oil.
Oxidative Decomplexation (CAN)/Reduction/Cope Rearrange-
ment Sequence; General Procedure
To a stirred solution of 5a (0.500 g, 1.71 mmol) in absolute MeOH
(17 mL) was added in portions CAN (6 × 0.95 g, 99%, 10.3 mmol)
over a period of 14 min. Shortly after the last addition and cessation
of effervescence, the red mixture was poured onto brine (20 mL)
and extracted with EtOAc (4 × 20 mL). The combined extracts were
washed sequentially with, H2O (3 × 20 mL), sat. aq NaHCO3 (20
mL) and brine (20 mL). The organic phase was dried (MgSO4) and
carefully concentrated at 20 °C to give a volatile yellow oil. The oil
was dissolved in Et2O (~1.0 mL) and added dropwise to a stirring
solution of LiAlH4 in Et2O (3.0 mL, 1.0 M, 3.0 mmol) at 0 °C. The
solution was stirred at 0 °C for 3 h, then quenched with sat. aq
NaHCO3 (5 mL). After warming to r.t. and diluting with aq 2 M
NaOH (10 mL), the mixture was extracted with Et2O (4 × 15 mL).
The combined extracts were dried (MgSO4) and concentrated. Puri-
fication by column chromatography (SiO2, hexanes–EtOAc, 4:1)
gave 18a as a colorless oil (0.177 g, 83%).
1
Analysis of the residue by H NMR spectroscopy indicated that it
consisted of a mixture of cis and trans-divinylcyclopropylmetha-
nols (1:2.2). None of the rearrangement product was observed. The
mixture of divinylcyclopropylmethanols was dissolved in mesity-
lene (12 mL) and heated under N2 in a sealed reaction tube at 195–
200 °C for 1 h. After cooling to r.t., purification by column chroma-
tography (SiO2, hexanes–EtOAc gradient, 10:1 → 4:1) provided
18e as a pale yellow oil (0.296 g, 80%).
(Cyclohepta-2,6-dienyl)methanol (18a)
IR (neat): 3346 (br), 3014, 2906, 1646, 1449, 1430, 1033, 817, 784,
736 cm–1.
Methyl 2-(2-Methylprop-1-enyl)-3-vinylcyclopropanecarboxy-
late (cis-16e)
IR (neat): 3085, 2926, 2855, 1730, 1638, 1444, 1284, 1213, 1159,
1H NMR (CDCl3): d = 1.52 (br s, 1 H), 2.12–2.25 (m, 2 H), 2.27–
2.40 (m, 2 H), 3.29–3.40 (m, 1 H), 3.67 (app d, J = 6.0 Hz, 2 H),
5.51–5.59 (m, 2 H), 5.84–5.93 (m, 2 H).
13C NMR (CDCl3): d = 26.9, 42.3, 67.1, 130.2, 132.3.
985, 902 cm–1.
1H NMR (CDCl3): d = 1.69 (dd, J = 4.8, 4.8 Hz, 1 H), 1.71 (s, 3 H),
1.72 (s, 3 H), 2.23–2.37 (m, 2 H), 3.69 (s, 3 H), 4.87 (qdd, J = 1.5,
1.5, 8.5 Hz, 1 H), 5.10 (ddd, J = 0.6, 1.8, 10.3 Hz, 1 H), 5.22 (ddd,
J = 0.8, 1.8, 17.2 Hz, 1 H), 5.47–5.60 (m, 1 H).
GC/MS: m/z = 124.
13C NMR (CDCl3): d = 18.8, 26.0, 27.8, 29.3, 31.8, 52.1, 117.0,
119.7, 134.7, 136.4, 173.6.
Anal. Calcd for C8H12O·0.3H2O: C, 74.15; H, 9.80. Found: C,
74.05; H, 9.66.
cis-(4-Methylcyclohepta-2,6-dienyl)methanol (18f)
Methyl 2-(2-Methylprop-1-enyl)-3-vinylcyclopropanecarboxy-
late (trans-16e)
Oxidative decomplexation of 14 (354 mg, 1.15 mmol) in MeOH
with excess CAN was carried out in a fashion similar to the decom-
plexation of 5a. The resultant oil was reduced with LiAlH4 in Et2O
at 0 °C. After warming to r.t. and diluting with aq 2 M NaOH (6
mL), the mixture was extracted with Et2O (4 × 10 mL). The com-
bined extracts were dried (MgSO4) and concentrated to give a clear,
colorless residue. The crude 1H NMR spectrum indicated the pres-
ence of 18f along with small amounts of unrearranged divinylcyclo-
propylmethanol. Therefore, the residue was dissolved in EtOAc (15
mL) and warmed to 50 °C for 1 h. Concentration followed by col-
umn chromatography (SiO2, hexanes–EtOAc, 4:1) gave 18f as a
colorless oil (0.114 g, 71%).
1H NMR (CDCl3): d = 1.69 (d, J = 1.0 Hz, 3 H), 1.73 (d, J = 1.1 Hz,
3 H), 1.83–1.95 (m, 2 H), 2.23–2.37 (overlapped m, 1 H), 3.68 (s, 3
H), 4.66 (qdd, J = 1.5, 1.5, 8.7 Hz, 1 H), 5.04 (dd, J = 1.8, 10.3 Hz,
1 H), 5.22 (dd, J = 1.8, 17.1 Hz, 1 H), 5.89 (dddd, J = 1.1, 8.2, 10.3,
17.1 Hz, 1 H).
(4,4-Dimethylcyclohepta-2,6-dienyl)methanol (18e)
IR (neat): 3337 (br), 3001, 2955, 2866, 1653, 1469, 1375, 1359,
1079, 1031, 807, 746, 699 cm–1.
1H NMR (CDCl3): d = 1.02 (s, 3 H), 1.05 (s, 3 H), 1.46 (br s, 1 H),
2.08 (dddd, J = 1.1, 1.1, 6.8, 14.1 Hz, 1 H), 2.38 (dd, J = 6.5, 14.2
Hz, 1 H), 3.21–3.31 (m, 1 H), 3.66 (app d, J = ~6.1 Hz, 2 H), 5.28
(ddd, J = 1.3, 3.3, 12.0 Hz, 1 H), 5.46 (ddd, J = 1.3, 2.6, 12.0 Hz, 1
H), 5.74 (dddd, J = 1.3, 1.3, 4.1, 10.6 Hz, 1 H), 5.85 (dtd, J = 2.0,
6.5, 10.6 Hz, 1 H).
IR (neat): 3340 (br), 3011, 2956, 2876, 1645, 1456, 1427, 1375,
1194, 1060, 1027, 789, 735, 700 cm–1.
1H NMR (CDCl3): d = 1.04 (d, J = 7.0 Hz, 3 H), 1.50 (br s, 1 H),
1.97–2.11 (m, 1 H), 2.18–2.30 (m, 1 H), 2.62–2.77 (m, 1 H), 3.31–
3.41 (m, 1 H), 3.66 (app d, J = 6.2 Hz, 2 H), 5.46–5.57 (m, 2 H),
5.63 (dddd, J = 0.6, 2.3, 4.4, 11.1 Hz, 1 H), 5.76 (dddd, J = 2.4, 4.2,
6.5, 11.4 Hz, 1 H).
13C NMR (CDCl3): d = 22.3, 31.9, 35.3, 41.6, 67.1, 129.3, 129.4,
131.0, 138.8.
13C NMR (CDCl3): d = 29.7, 30.9, 35.3, 39.9, 41.5, 66.9, 125.1,
130.2, 132.7, 142.2.
GC/MS: m/z = 152.
Anal. Calcd for C10H16O·0.1H2O: C, 77.97; H, 10.60. Found: C,
77.73; H, 10.55.
GC/MS: m/z = 138.
Anal. Calcd for C9H14O·0.25H2O: C, 75.75; H, 10.24. Found: C,
75.68; H, 10.09.
Acknowledgment
Financial support from the National Science Foundation (CHE-
0415771). High-resolution mass spectral data were obtained at the
Washington University Resource for Mass Spectrometry.
Oxidative Decomplexation (H2O2/HO–)/Reduction/Cope Rear-
rangement Sequence; General Procedure
To a stirred solution of complex 5e (0.780 g, 2.44 mmol) in absolute
MeOH (50 mL) and 30% aq H2O2 (15 mL) at –78 °C under N2, was
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York

3646
N. J. Wallock et al.
PAPER
(6) (a) Morey, J.; Gree, D.; Mosset, P.; Toupet, L.; Gree, R.
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application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK [fax (+44)1223 336033; email:
deposit@ccdc.cam.ac.uk].
Synthesis 2006, No. 21, 3639–3646 © Thieme Stuttgart · New York