BF3·OEt2-catalyzed intermolecular reactions of yinylidenecyclopropanes with bis(p-alkoxyphenyl)methanols: A novel cationic 1,4-aryl-migration process

Article abstract of DOI:10.1002/chem.200903131

BF₃·OEt₂-catalyzed reactions of vinylidenecyclopropanes (VDCPs) 1 with bis(aryl)methanols 2 were thoroughly investigated. When VDCPs 1 reacted with electron-rich bis(aryl)methanols 2, diastereomeric retamers of indene derivatives formed in excellent yields by a novel cationic 1,4aryl migration between two carbon atoms and the subsequent intramolecular Friedel-Crafts reaction pathways in the presence of BF ₃·OEt₂ under mild conditions. As for electron-deficient or less-electron-rich bis(aryl)methanols 2, either trialkene products formed in good yields by direct deprotonation, or another type of indene derivative was produced by direct intramolecular Friedel-Crafts reaction, depending on the substituents on the cyclopropane of VDCPs. In addition, DFT calculations were carried out to explain the experimental results. Plausible mechanisms for all these transformations are proposed on the basis of the experimental and computational results.

Full text of DOI:10.1002/chem.200903131

FULL PAPER
DOI: 10.1002/chem.200903131
BF3·OEt2-Catalyzed Intermolecular Reactions of Vinylidenecyclopropanes
with Bis(p-alkoxyphenyl)methanols: A Novel Cationic 1,4-Aryl-Migration
Process
Lei Wu, Min Shi,* and Yuxue Li*[a]
Abstract: BF3·OEt2-catalyzed reactions
of vinylidenecyclopropanes (VDCPs) 1
the presence of BF3·OEt2 under mild
conditions. As for electron-deficient or
less-electron-rich bisACTHUGNETRNNU(G aryl)methanols 2,
either trialkene products formed in
good yields by direct deprotonation, or
another type of indene derivative was
produced by direct intramolecular Frie-
del–Crafts reaction, depending on the
substituents on the cyclopropane of
VDCPs. In addition, DFT calculations
were carried out to explain the experi-
mental results. Plausible mechanisms
for all these transformations are pro-
posed on the basis of the experimental
and computational results.
with bis
oughly investigated. When VDCPs 1
reacted with electron-rich bis-
ACHTUNGTRENNUNG(aryl)methanols 2, diastereomeric ro-
A
N
tamers of indene derivatives formed in
excellent yields by a novel cationic 1,4-
aryl migration between two carbon
atoms and the subsequent intramolecu-
lar Friedel–Crafts reaction pathways in
Keywords: allenes · aryl migration ·
carbocatios
calculations
·
density functional
diastereomeric
·
rotamers · Friedel–Crafts reaction
Introduction
and others have reported Lewis acid or Brønsted acid cata-
lyzed/mediated [3+2] or [3+3] cycloaddition reactions of
VDCPs with imines, aldehydes, nitriles, a,b-unsaturated ke-
tones, and so forth, to give the corresponding functionalized
tetrahydrofuran and 3,6-dihydropyran derivatives as well as
some other cycloadducts in moderate to good yields under
mild conditions.[3–5] All these interesting results have stimu-
lated us to further explore such cascade ring-opening reac-
tions of VDCPs in the presence of other electrophiles cata-
lyzed by Lewis acids.
During our ongoing investigation on the reactions of
VDCPs with some electrophiles catalyzed by Lewis acids,
we found that bisACTHNUTRGNEUNG(aryl)methanols 2 could act as excellent
precursors of electrophiles, affording active cationic inter-
mediates to initiate the reactions with VDCPs 1. Interesting-
ly, a novel cationic 1,4-aryl migration from carbon atom to
carbon atom in the presence of BF3·OEt2 was observed if
electron-rich bis(p-alkoxyphenyl)methanols were used as
the electrophiles under mild conditions (Scheme 1). Further-
more, we also found that if electron-deficient or less-elec-
Vinylidenecyclopropanes (VDCPs),[1] containing an allene
moiety and a connected cyclopropane ring, belong to the
most remarkable organic families in the chemistry of highly
strained small rings. These highly strained cyclopropanes are
thermally stable, yet reactive substances, which easily under-
go novel intramolecular rearrangements or intermolecular
reactions with many electrophiles, either upon heating and
photoirradiation, or catalyzed by a variety of Lewis or
Brønsted acids. For example, VDCPs can react with carbon–
carbon or carbon–heteroatom multiple bonds to produce
[3+2] or [2+2] cycloaddition products in good yields upon
heating or photoirradiation.[2] Moreover, very recently, we
[a] L. Wu, Prof. Dr. M. Shi, Prof. Dr. Y. Li
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (China)
Fax : (+86)21-64166128
tron-rich bisACTHNUGTRNEUNG(aryl)methanols were used as the electrophiles,
E-mail: mshi@mail.sioc.ac.cn
two other kinds of products were obtained by different reac-
tion pathways, depending on the substituents on the cyclo-
propane of the substrates. More specifically, this interesting
1,4-aryl migration is highly dependent on the electronic
nature of the bisACTHNUTRGNEUNG(aryl)methanols employed, as well as the
substituents on the cyclopropane of VDCPs. To gain more
Supporting information for this article is available from the author or
on the WWW under http://dx.doi.org/10.1002/chem.200903131. It con-
tains spectroscopic data of all the new compounds shown in Tables 1–
5; detailed descriptions of the experimental procedures; X-ray data
for compounds 3aA, 3eB, 6a, and 7e; the optimized structures in
Figure 6; and the total energies and geometrical coordinates of the
structures in Scheme 2 and Figure 7.
Chem. Eur. J. 2010, 16, 5163 – 5172
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Table 1. Optimization of the reaction conditions.[a]
Scheme 1. BF3·OEt2-catalyzed reactions of VDCP 1a with bis(p-
methoxyphenyl)methanol (2a).
ACHTUNGTRENNUNG
insight into the reaction mechanisms and the related transi-
tion states, DFT calculations were carried out to explain the
experimental results. On the basis of the results thus ob-
tained, plausible reaction mechanisms were deduced, and
these are described in this paper.
Catalyst
Solvent
Yield of 3a [%][b]
(ratio 3aA/3aB)
1
2
3
4
5
6
7
8
9
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
CH2Cl2
CH3CN
toluene
DCE
DCE
DCE
DCE
DCE
DCE
DCE
73 (8.1:1.0)
78 (7.9:1.0)
53 (8.2:1.0)
96 (8.3:1.0)
88 (8.0:1.0)
94 (7.7:1.0)
trace[c]
91 (8.1:1.0)
92 (8.2:1.0)
92 (8.0:1.0)
Regarding aryl migration,[6] besides the most extensively
investigated 1,2-aryl migration (neophyl rearrangement),[7]
1,4-aryl-migration reactions have been also observed and
studied. It has been known that this migration can take
place between two carbon atoms, or between a carbon atom
and a heteroatom, as well as between two heteroatoms.
However, most of the 1,4-aryl-migration reactions proceed
by a radical mechanism.[8] The examples concerning a cat-
ionic intermediate are rare.[9] Herein, we wish to report the
full details of these interesting findings.
[Sc
[Zr
[Yb
[In(OTf)3]
[Nd(OTf)3]
TfOH
T
N
N
N
G
10
[a] Reaction conditions: 1a (0.20 mmol), 2a (0.24 mmol, 1.2 equiv), cata-
lyst (10 mol%), solvent (2.0 mL), RT (208C), 5 min (unless specified oth-
erwise). [b] Isolated yield. [c] The reaction mixtures were stirred for 12 h.
Results and Discussion
BF3·OEt2-catalyzed reactions of VDCP 1a with bis(p-meth-
ACHTUNGTRENNUNGoxyphenyl)methanol 2a: First, the reaction was carried out
by using VDCP 1a (0.20 mmol) and bis(p-methoxyphenyl)-
methanol (2a; 0.24 mmol, 1.2 equiv) in dichloromethane
(2.0 mL) at room temperature (208C) in the presence of
BF3·OEt2 (10 mol%). The reaction was completed within
5 minutes to afford product 3a as a pair of diastereomeric
rotamers 3aA and 3aB in 73% yield in a ratio of 8.1:1.0
(Table 1, entry 1). The structure of the major rotamer 3aA
was determined by X-ray diffraction (the CIF data are pre-
sented in the Supporting Information).[10] The ORTEP draw-
ing is shown in Figure 1. It can be seen that one of the p-
methACHTUNGTRENNUNGoxyphenyl rings of 2a has migrated to the ring-opened
dimethyl site to give the product 3a.
Next, we attempted to determine the best reaction condi-
tions for this reaction, and the results of these experiments
are summarized in Table 1. Examination of the solvent ef-
fects when BF3·OEt2 (10 mol%) was used as the catalyst re-
vealed that 1,2-dichloroethane (DCE) was the solvent of
choice, affording 3a in 96% total yield (Table 1, entries 1 to
Figure 1. ORTEP drawing of 3aA.
We also examined the effect of the temperature on this
reaction under the tentatively defined optimal conditions,
and the results are outlined in Table 2. Reaction tempera-
tures between ꢀ108C and 508C did not significantly affect
the reaction outcomes. Therefore, the best reaction condi-
tions were determined to consist of the use of the solvent
DCE (2.0 mL) at room temperature (208C), VDCPs 1
(0.20 mmol) and bisACTHNUTRGNEUNG(aryl)methanols 2 (0.24 mmol, 1.2 equiv)
as the substrates, and BF3·OEt2 (10 mol%) as the catalyst.
Under these optimal conditions, the reaction went to com-
4). When other Lewis acids such as [Sc
ACHUTNGTNERNUG(OTf)3], [ZrACHTUNGTERN(NUGN OTf)4],
[In(OTf)3], and [Nd(OTf)3] (10 mol%) were used as the cat-
A
ACHTUNGTRENNUNG
alysts in DCE, 3a was produced in 88–94% total yield and
in diastereoselectivities of 7.7:1.0 to 8.2:1.0, but almost no
reaction occurred in the presence of [YbACTHNURTGNENG(U OTf)3] (10 mol%)
under identical conditions (Table 1, entries 5–9). Use of the
Brønsted acid trifluoromethanesulfonic acid (TfOH) led to
the formation of 3a in 92% total yield under the standard
conditions (Table 1, entry 10).
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Aryl Migration
FULL PAPER
Table 2. Examination of the effect of the temperature under the standard
conditions.[a]
T
Yield of 3a [%][b]
Ratio 3aA/3aB[c]
1
2
3
ꢀ108C
RT
508C
97
96
99
10.0:1.0
8.3:1.0
9.1:1.0
[a] Reaction conditions: 1a (0.20 mmol), 2a (0.24 mmol, 1.2 equiv),
BF3·OEt2 (10 mol%), DCE (2.0 mL). [b] Total isolated yield of 3aA and
1
3aB. [c] Ratio determined from H NMR spectroscopic data.
Figure 2. ORTEP drawing of 3eB.
pletion within 5 minutes, to afford the corresponding ad-
ducts 3 in excellent yield.
BF3·OEt2-catalyzed reactions of VDCPs 1b–k with bis(p-
methoxyphenyl)methanol 2a: With the standard reaction
conditions in hand, we next investigated the scope and limi-
tations of this reaction by using
not identical (R1 =H and R2 ¼R3 or R1 ¼H), the situation is
more complicated (Table 3, entries 5–10). Regioisomers can
form in addition to their corresponding diastereomeric ro-
a variety of VDCPs 1b–k with
Table 3. Reactions of VDCPs 1b–k with 2a under the optimal reaction conditions.[a]
2a. The results of these experi-
ments are summarized in
Table 3. As can be seen from
Table 3, all of these VDCPs 1
reacted smoothly with 2a to
give the corresponding reaction
products 3 in excellent yields,
regardless of whether they have
electron-rich or electron-defi-
cient substituents on the aro-
matic rings. Furthermore, the
structure of another minor dia-
stereomeric rotamer was unam-
biguously determined by the X-
ray diffraction of 3eB (Fig-
1
Products 3
Total yield of 3 [%][b]
(Ratio A/B)[c]
Major rotamer A[d]
ratio 3/3’/3’’
Minor rotamer B[d]
ratio 3/3’/3’’
R1/R2/R3
1
2
3
4
5
6
7
8
1b
H/F/F
1c
H/Cl/Cl
1d
H/Me/Me
1e
H/OMe/OMe
1 f
H/H/F
1g
H/H/Cl
1h
H/H/OMe
1i
3b (3bA, 3bB)
3c (3cA, 3cB)
3d (3dA, 3dB)
3e (3eA, 3eB)
99 (4.5:1.0)
99 (5.1:1.0)
99 (4.9:1.0)
99 (3.8:1.0)
97 (7.1:1.0)
95 (6.0:1.0)
95 (5.2:1.0)
99 (5.7:1.0)
–
–
–
–
–
–
–
–
ACHTUNGTRENNUNGure 2; CIF data presented in
the Supporting Information).[11]
The two diastereomeric rotam-
ers have the same molecular
structure, but have different
spatial configurations, because
3 f (3 fA, 3 fB)
3 f’ (3 f’A, 3 f’B)
3g (3gA, 3gB)
3g’ (3g’A, 3g’B)
3h (3hA, 3hB)
3h’ (3h’A, 3h’B)
3i (3iA, 3iB)
3i’ (3i’A, 3i’B)
3i’ (3i’A, 3i’B)
3j (3jA, 3jB)
3j’ (3j’A, 3j’B)
3j’ (3j’A, 3j’B)
3k (3kA, 3kB)
3k’ (3k’A, 3k’B)
3k’ (3k’A, 3k’B)
1.0:1.0
1.0:1.0
1.5:1.0
8.0:1.0[e]
1.6:1.0
2.1:1.0
3.3:1.0
6.2:1.0[e]
ꢀ
free rotation around the Ca Cb
F/F/H
bond is blocked by steric hin-
drance between the aryldi
ACHTUNGTNERmNUNG eth-
9
1j
Cl/Cl/H
90 (6.2:1.0)
97 (4.9:1.0)
12.0:4.0:1.0
3.8:1.0[e]
10.0:2.9:1.0
5.1:1.0[e]
ACHTUNGTRENNUNG
omatic rings in the product.
10
1k
Me/Me/H
When the two aromatic rings
are identical (R1 =H, R2 =R3),
a pair of diastereomeric rotam-
ers was obtained in 99% yield
(Table 3, entries 1–4). However,
[a] Reaction conditions: 1 (0.20 mmol), 2a (0.24 mmol, 1.2 equiv), BF3·OEt2 (10 mol%), DCE (2.0 mL), RT,
5 min. [b] Isolated yield. [c] The ratio of the two diastereomeric rotamers. [d] The ratio of the regioisomers in
separated major diastereomeric or minor diastereomeric rotamers. [e] The ratio 3i/3i’ or 3i/3i’’ as well as 3k/
1
if the two aromatic rings are 3k’ or 3k/3k’’ (one of the regioisomers could not be found in the H NMR spectra).
Chem. Eur. J. 2010, 16, 5163 – 5172
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5165

M. Shi, Y. Li, and L. Wu
tamers; these form in different regioselectivities according
to the electronic nature of the aromatic rings—the intramo-
lecular Friedel–Crafts reaction can take place at different ar-
omatic rings (taking place more easily at electron-rich aro-
matic rings), as well as at different sites of the aromatic
rings (Table 3, entries 5–10). The regioselectivities and dia-
stereoselectivities were determined on the basis of NMR
spectroscopic data (presented in Supporting Information).
reaction outcome need to be examined. Consequently, it
was found that when the electron-deficient bis(p-chlorophe-
nyl)methanol (2e) was applied in the reaction under the op-
timal reaction conditions, the reaction became sluggish, af-
fording the corresponding product 5b exclusively in 91%
yield after a week (Table 3, entry 4). We assumed that this is
because an electron-rich aromatic ring favors cationic aryl
migration and an electron-poor aromatic ring disfavors such
a cationic aryl-migration process.[12]
BF3·OEt2-catalyzed reactions of VDCP 1a with bis-
A
BF3·OEt2-catalyzed reactions of VDCPs 1l–q with bis-
A
ACHTUNGTREN(NUGN aryl)methanols 2: VDCPs 1l–q, containing two aromatic
standard reaction conditions (Table 4), and found a distinct
effect of the electronic nature of the aromatic rings of 2 on
groups on the cyclopropane, were also examined in this re-
action under the standard reaction conditions, and we found
that, apart from the similar 1,4-aryl-migration products, a
new set of products was obtained by a different reaction
pathway. The results are outlined in Table 5. The reactions
with electron-rich bis(p-methoxyphenyl)methanol 2a gave
the corresponding product mixtures of diastereomeric ro-
tamers 6 as the major products in good yields by a similar
cationic 1,4-aryl-migration process along with the minor
products 7 derived from a direct intramolecular Friedel–
Crafts reaction (Table 5, entries 1–6). The structure of the
major rotamer of 6a, obtained from the product mixtures of
the diastereomeric rotamers 6a formed by the reaction of 1l
with 2a, as well as the structure of the minor product 7e
formed in the reaction of 1p with 2a were determined by
X-ray diffraction (Figures 3 and 4; CIF data in Supporting
Information).[13,14]
Table 4. Reactions of 1a with bisACHTUNTRGNEU(GN aryl)methanols 2 under the optimal re-
action conditions.[a]
R1
Reaction
time
Yield [%][b]
4 (ratio 4A/4B)
5
1
2
3
4
2b: OEt
2c: OCH2CH=CH2
2d: H
5 min
5 min
72 h
4aA, 4aB: 97 (8.1:1.0)
4bA, 4bB: 98 (7.8:1.0)
4cA, 4cB: 58[c]
–
–
–
5a: 33
5b: 91
2e: Cl
1 week
The use of electron-neutral bisACTHUNTGRNEUNG(phenyl)methanol (2d),
[a] Reaction conditions: 1a (0.20 mmol),
2
(0.24 mmol, 1.2 equiv),
electron-deficient bis(p-chlorophenyl)methanol (2e), or less-
electron-rich bis(p-methylphenyl)methanol (2 f) as the elec-
trophiles to react with VDCP 1l under the standard condi-
tions afforded the corresponding products 7g–i exclusively
in >99% yields (Table 5, entries 7–9). As mentioned above,
the electron-rich aromatic ring in 2 favors cationic 1,4-aryl
BF3·OEt2 (10 mol%), DCE (2.0 mL), RT (unless specified otherwise).
[b] Isolated yields. [c] Only one rotamer was obtained.
the final reaction outcomes. Electron-rich bis(p-alkoxyaryl)-
methanols 2b and 2c, bearing an ethoxy group and an ally-
loxy group on the phenyl ring,
gave the corresponding diaste-
Table 5. Reactions of VDCPs 1l–q with bisACTHNUTRGNEUNG
(aryl)methanols 2 under the optimal reaction conditions.[a]
reomeric rotamers 4a and 4b in
excellent yields by a similar cat-
ionic 1,4-aryl-migration process
(Table 4, entries 1 and 2). Elec-
tron-neutral
bis-
ACHTUNGTRENNUNG(phenyl)methanol 2d as the
electrophile in the reaction with
1a under the standard condi-
tions produced the correspond-
ing aryl-migration product 4c in
58% total yield, after a signifi-
cantly prolonged reaction time,
along with a trialkene product
5a, resulting from simple
proton elimination (Table 4,
entry 3). This showed us that
the influence of the electronic
properties of the aryl rings of
1
R1
R2
R3
2
Ar
Yield of 6 [%][b,c]
Yield of 7 [%][b]
1
2
3
4
5
6
7
8
9
1l
H
H
Me
Cl
H
H
H
H
H
H
H
H
H
Me
Cl
H
H
H
Me
Et
2a
2a
2a
2a
2a
2a
2d
2e
2 f
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
C6H5
6a: 88 (26.0:9.7:4.7:1.0)
6b: 84 (46.2:33.8:5.3:1.0)
6c: 81 (30.0:9.0:4.0:1.0)
6d: 88 (35.8:12.1:3.8:1.0)
6e: 85 (10.0:4.8:1.0:–)
6 f: 79 (9.6:3.5:1.0:–)
–
–
–
7a: 11
7b: 15
7c: 17
7d: 11
7e: 13
7 f: 19
7g: >99
7h: >99
7i: >99
1m
1n
1o
1p
1q
1l
Me
Me
Me
Me
Me
Me
Me
1l
1l
p-ClC6H4
p-MeC6H4
[a] Reaction conditions: VDCP
1 (0.20 mmol), bisACHTUNRTGENG(NUN aryl)methanol 2 (0.24 mmol, 1.2 equiv), BF3·OEt2
(10 mol%), DCE (2.0 mL), RT, several minutes (unless specified otherwise). [b] Total isolated yield. [c] The
values in parentheses are the ratios of the diastereomeric rotamers, determined from 1H NMR spectroscopic
the bis
ACHTUNGTRENNUNG(aryl)methanols 2 on the data.
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Aryl Migration
FULL PAPER
Figure 5. Four diastereomeric rotamers in the formation of 6.
gas-phase fully optimized structures. For each structure, har-
monic vibration frequency calculations were carried out and
thermal corrections were made. All structures were shown
as the transition states (with one imaginary frequency) or
the stationary points (with no imaginary frequency).
Figure 3. ORTEP drawing of the major rotamer of 6a.
The rotation barrier between 3aA and 3aB: Simplified
models 3aA’ and 3aB’ without the OMe groups were used
for calculating the rotation barrier between 3aA and 3aB
(Figure 6). On the basis of a relaxed potential energy sur-
face (PES) scan on the B3LYP/6-31G* level along the dihe-
dral angle D1234 (see Supporting Information SI-Figure 1),
the rotational transition state TS-rotation was located. Then,
3aA’, 3aB’, and TS-rotation were reoptimized at the
B3LYP/6-31+G** level (Figure 6). The rotational barrier is
quite high (up to 36.2 kcalmolꢀ1), suggesting that rotamers
3aA and 3aB cannot isomerize into one another at room
temperature or even upon heating. This phenomenon was
identified by our experimental results, because we found
that heating 3aB in refluxing toluene only resulted in de-
composition of the product mixtures without formation of
3aA. Rotamer 3aA’ is 1.0 kcalmolꢀ1 more stable than 3aB’.
The relative stabilities of 3aA and 3aB are depicted qualita-
tively by the illustrations shown in Figure 6. In 3aA, the 1-
aryl group is adjacent to the smaller G1 group, whereas in
3aB, it is adjacent to the larger G2 group, consequently
leading to larger steric repulsion and higher energy. The
congested structure of 3aB is also disfavored in terms of en-
tropy.
Figure 4. ORTEP drawing of 7e.
migration. However, in this case, the BF3·OEt2-catalyzed in-
tramolecular Friedel–Crafts reaction of the carbocation de-
rived from the ring opening of cyclopropane with the aro-
matic ring at the cyclopropane takes place easily to give the
corresponding products 7. This reaction pathway became
dominant when less-electron-rich bisACTHNUTRGNEUNG(aryl)methanols 2 were
used as the electrophiles (Table 5, entries 7–9). It should be
noted that, because product 6 has two chiral centers, four
diastereomeric rotamers could be formed, as shown in
Table 5, and their ratios were determined from the NMR
spectroscopic data (Figure 5). The stereochemistry of the
major diastereomeric rotamer was determined by X-ray
crystallography.
The reaction pathways: We started the calculation with the
allylic cationic intermediate D (Scheme 2). All of the opti-
mized structures are collected in the Supporting Informa-
tion, and some selected structures are shown in Figure 7. In
intermediate D, Ph1 is parallel to the plane of the allylic
cation, and Ph2 is far from the two Ar groups, thus avoiding
steric repulsion. Theoretical studies suggest that aryl migra-
tions from I to O[19a,b] and alkyl-group migrations from Cl to
O[19c] are by concerted one-step mechanisms without any in-
termediates. However, our calculations indicate that the 1,4-
aryl migration from C1 to C4 is a two-step process. In TS-
DE, one of the aryl groups is migrating from C1 to C4, lead-
Theoretical study: To understand the mechanism of this re-
action, DFT[15] studies were performed by use of the GAUS-
SIAN03 program[16] using the B3LYP[17] method and the 6-
31+G** basis set. In the study of the reaction pathways, the
solvent effect was estimated with IEFPCM[18] (UAHF
atomic radii) method (DCE, e =10.36) on the basis of the
ing to the spirocyclopentylbenzenium intermediate
E
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5167

M. Shi, Y. Li, and L. Wu
(Scheme 2). Then, intermediate
E transforms into the allylic
cation F via transition state TS-
EF over
a small barrier of
1.9 kcalmolꢀ1. The two transi-
tion states TS-DE and TS-EF
and intermediate E all have
five-membered-ring structures.
In intermediate D, the net Mul-
liken charge of the methoxy
group in the leaving aryl group
ꢀ
is +0.016, and the C O dis-
tance is 1.353 ꢁ. In contrast, in
the phenonium intermediate E,
the net Mulliken charge of the
methoxy group increases to
ꢀ
+0.096, and the C O distance
decreases to 1.315 ꢁ. Similar
changes can also be found in
the two transition states TS-DE
and TS-EF. These results indi-
cate that the p-methoxy group
has a great effect on the stabili-
zation of the phenonium inter-
mediate and the transition
states involved, as proposed by
Tahir Khan et al.[9a]
Figure 6. The optimized structures of 3aA’, 3aB’, TS-rotation and graphics to help understand the relative sta-
bilities of 3aA and 3aB. The relative free energies in the gas phase [DGgas (298 K), kcalmolꢀ1] and the dihe-
dral angles [D, 8] are provided. The geometries were fully optimized at the B3LYP/6-31+G** level.
Along path A, the carbocat-
ion attacks Ph1 from below. In-
termediate F undergoes an in-
tramolecular Friedel–Crafts re-
action to form intermediate G-
A via TS-FG-A; G-A is then
transformed into the final
major product 3aA after depro-
tonation. As mentioned above,
the products 3aA and 3aB
cannot transform into one an-
other by rotation around the
ꢀ
C2 C3 bond at room tempera-
ture.
A
relaxed potential
energy surface scan shows that
intermediate also cannot
F
transform into F’’ by rotation
ꢀ
around the C2 C3 bond, which
leads to 3aB. On the basis of
another
relaxed
potential
energy surface scan, the rota-
tional transition state TS-Rot-
F-F’ was located. In this transi-
tion state, the orientation of
Ph1 and Ph2 flips over at the
same time. Thus, along path B,
F can transform into F’ over a
small barrier of 5.0 kcalmolꢀ1.
Then the carbocation attacks
Ph1 from above via Ts-F’G-B
Scheme 2. Reaction pathways and the relative free energies including solvent effect DGsol [kcalmolꢀ1, 298 K].
These were calculated on the B3LYP/6-31+G** level.
5168
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Chem. Eur. J. 2010, 16, 5163 – 5172

Aryl Migration
FULL PAPER
pathways are available for the
next steps in this case. In
path A, the intramolecular Frie-
del–Crafts reaction takes place
directly to afford the final prod-
uct 7 via intermediate G’’. In
path B, however, intermediate
D’ undergoes the same 1,4-aryl
migration via a five-membered
cationic transition state E’ to
furnish intermediate F’ when
the aromatic ring (Ar) has an
electron-donating group, and
the next intramolecular Frie-
del–Crafts reaction takes place
to produce product 6 via inter-
mediate G’. It can be seen that
besides the electronic nature of
the employed bis
ACHTUNGTREN(GNUN aryl)ACHTUNGTRENNUNGmethACHTUNGTRENNUNGan-
AHCTUNGTRENNUNG
clopropane of the employed
VDCPs 1 can also affect the re-
action pathways and the subse-
quent final reaction outcomes.
Figure 7. Selected optimized structures along the reaction pathways. Some bond lengths [ꢁ], bond angles [8],
and relative free energies including the solvent effect [DGsol (298 K), kcalmolꢀ1] are shown. The geometries
were fully optimized at the B3LYP/6-31+G** level.
Conclusion
to form intermediate G-B, which is then transformed into
the final minor product 3aB after deprotonation.
The BF3·OEt2-catalyzed reactions of VDCPs 1 with bis-
(aryl)methanols 2 under mild conditions were examined.
Depending on the electronic nature of the employed bis-
(aryl)methanols and the substituents on the cyclopropane of
the VDCPs, the reaction underwent different pathways to
afford three kinds of products. From electron-rich bis-
The preference for the major product 3aA originates
from the relative stabilities of Ts-FG-A and Ts-F’G-B,
which can also be understood qualitatively by the steric ef-
fects shown in Figure 7.
On the basis of the above experimental and computation-
al results, a plausible mechanism for the formation of 3, 4,
and 5 was found, and is outlined in Scheme 3 using 1a as an
example. The reaction is initiated by the generation of the
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTREG(NNNU aryl)methanols 2a–c, indene derivatives 3, 4, and 6 were
carbocationic intermediate A from bisACTHNUTRGNE(NUG aryl)methanol 2 in
the presence of BF3·OEt2, which attacks the central carbon
of the allene moiety of 1a to produce the cationic inter-
mediate B. Intermediate B then immediately undergoes ring
opening to afford allylic cation C or its resonance structure
D. In intermediate D, when the aromatic ring (Ar) has an
electron-donating group, the 1,4-aryl migration takes place
via the cationic transition state E (a five-membered-ring in-
termediate) to afford another allylic cation F, which under-
goes intramolecular Friedel–Crafts reaction to furnish the
final product 3. When the aromatic ring (Ar) in intermedi-
ate D has an electron-withdrawing group, the aromatic ring
does not undergo aryl migration, but instead undergoes
direct deprotonation to afford product 5.
In contrast, in the case of VDCPs 1l–q, the plausible reac-
tion mechanism outlined in Scheme 4, using 1l as a model,
was derived. The reaction undergoes the same process for
the formation of the carbocationic intermediate A, followed
by electrophilic attack and subsequent ring opening of the
cyclopropyl cation to produce intermediate D’. Two reaction
Scheme 3. A plausible reaction mechanism of VDCPs 1a–k with bis-
AHCTUNGTREG(NNNU aryl)methanols 2.
Chem. Eur. J. 2010, 16, 5163 – 5172
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5169

M. Shi, Y. Li, and L. Wu
General procedure for BF3·OEt2-catalyzed reactions of VDCPs 1 with
bisACHTGNUTRENNUNG
bisACHTGNUTRENNUNG
loaded into a Schlenk tube. The reaction mixture was stirred at RT
(208C), and then BF3·OEt2 (10 mol%) was added. After 5 min, the sol-
vent was removed under reduced pressure and the residue was purified
by flash column chromatography.
Compound 3aA: White solid; m.p. 194–1978C; 1H NMR (300 MHz,
CDCl3): d=0.91 (s, 3H; CH3), 0.98 (s, 3H; CH3), 1.00 (s, 3H; CH3), 1.64
(s, 3H; CH3), 3.71 (s, 3H; CH3), 3.78 (s, 3H; CH3), 4.74 (s, 1H; CH), 6.52
(d, , J=9.0 Hz, 2H; Ar), 6.67 (d, J=9.0 Hz, 2H; Ar), 6.77 (d, J=9.0 Hz,
2H; Ar), 7.00 (d, J=9.0 Hz, 2H; Ar), 7.20–7.25 (m, 1H; Ar), 7.31–7.33
(m, 3H; Ar), 7.40–7.49 ppm (m, 5H; Ar); 13C NMR (75 MHz, CDCl3):
d=21.3, 25.6, 28.6, 33.1, 43.4, 55.0, 55.2, 59.9, 112.9, 113.3, 120.0, 124.5,
125.1, 126.8, 127.3, 128.4, 129.1, 130.6, 131.5, 134.0, 134.3, 136.3, 138.6,
144.6, 145.8, 147.3, 149.9, 156.6, 158.4 ppm; IR (CH2Cl2): n˜ =2961, 2930,
2906, 2835, 1608, 1509, 1463, 1442, 1374, 1299, 1249, 1179, 1036, 830, 785,
740, 702, 656 cmꢀ1; MS (EI): m/z (%): 500 (9) [M]+, 352 (30), 351 (92),
244 (21), 243 (100), 228 (13), 149 (78), 121 (28), 91 (9), 41 (10); elemental
analysis calcd (%) for C36H36O2: C 86.36, H 7.25; found: C, 86.41, H 7.32.
Compound 3aB: White solid; m.p. 190–1958C; 1H NMR (300 MHz,
CDCl3): d=0.60 (s, 3H; CH3), 0.74 (s, 3H; CH3), 1.18 (s, 3H; CH3), 1.87
(s, 3H; CH3), 3.69 (s, 3H; CH3), 3.81 (s, 3H; CH3), 4.82 (s, 1H; CH), 6.44
(d, J=9.0 Hz, 2H; Ar), 6.51 (d, J=9.0 Hz, 2H; Ar), 6.84–6.87 (m, 2H;
Ar), 7.15–7.28 (m, 5H; Ar), 7.39–7.52 ppm (m, 5H; Ar); IR (CH2Cl2):
n˜ =2932, 2906, 2834, 1733, 1609, 1510, 1463, 1248, 1179, 1036, 830, 760,
702 cmꢀ1; MS (EI): m/z (%): 500 (7) [M]+, 351 (100), 321 (10), 273 (12),
243 (85), 149 (73), 121 (25), 85 (20), 71 (22), 57 (30), 43 (20), 41 (17);
HRMS (EI): m/z: calcd for C36H36O2: 500.2715; found: 500.2718.
Scheme 4. A plausible reaction mechanism of VDCPs 1l–q with bis-
(aryl)methanols 2.
Compound 5a: Colorless liquid; 1H NMR (300 MHz, CDCl3): d=1.08 (s,
3H; CH3), 1.41 (s, 3H; CH3), 1.46 (s, 3H; CH3), 4.43 (d, J=2.1 Hz, 1H;
CH2 =), 4.74 (d, J=2.1 Hz, 1H; CH2 =), 5.39 (s, 1H; CH), 7.08–7.26 ppm
(m, 20H; Ar); 13C NMR (75 MHz, CDCl3): d=21.4, 23.2, 23.8, 55.7,
116.9, 125.6, 126.0, 126.4, 127.2, 127.6, 128.2, 128.8, 129.0, 129.3, 130.7,
133.6, 133.9, 139.1, 141.9, 142.5, 143.1, 143.3, 143.7, 145.6 ppm; IR
(CH2Cl2): n˜ =3057, 3025, 2923, 2852, 1797, 1598, 1493, 1443, 1075, 898,
743, 698, 586 cmꢀ1; MS (EI): m/z (%): 440 (100) [M]+, 349 (11), 321 (26),
291 (19), 273 (44), 243 (32), 207 (21), 167 (60), 91 (16); HRMS (EI): m/z:
calcd for C34H32: 440.2504; found: 440.2504.
ACHTUNGTRENNUNG
obtained in good to excellent yields as mixtures of disaster-
ACHTUNGTRENNUNGeomeric rotamers by a novel cationic 1,4-aryl migration be-
tween carbon atoms and a subsequent intramolecular Frie-
del–Crafts reaction. On the other hand, in the case of elec-
tron-deficient, electron-neutral, or less-electron-rich bis-
ACHTUNGTRENNUNG(aryl)methanols 2d–f, other reaction processes took place
Compound 7a: White solid; m.p. 247–2508C; 1H NMR (300 MHz,
CDCl3): d=1.10 (d, J=7.5 Hz, 3H; CH3), 3.56 (q, J=7.5 Hz, 1H; CH),
3.76 (s, 3H; CH3), 3.82 (s, 3H; CH3), 5.53 (s, 1H; CH), 6.35 (d, J=
6.9 Hz, 2H; Ar), 6.39 (d, J=7.5 Hz, 2H; Ar), 6.75–6.87 (m, 7H; Ar),
6.94 (d, J=8.4 Hz, 2H; Ar), 7.02–7.41 ppm (m, 14H; Ar); 13C NMR
(75 MHz, CDCl3): d=17.4, 47.1, 53.2, 55.2, 55.3, 113.4, 114.1, 120.4, 122.5,
124.7, 125.6, 126.1, 126.4, 126.8, 127.0, 127.7, 128.3, 128.5, 129.0, 129.4,
129.7, 131.2, 132.1, 135.0, 135.5, 142.2, 142.8, 143.5, 143.7, 144.0, 147.2,
149.0, 158.0, 158.2 ppm; IR (CH2Cl2): n˜ =3055, 2955, 2927, 1608, 1509,
1462, 1249, 1178, 1034, 747, 699 cmꢀ1; MS (EI): m/z (%): 610 (8) [M]+,
383 (10), 367 (5), 305 (9), 291 (10), 227 (100), 167 (4), 91 (2); ; HRMS
(EI): m/z: calcd for C45H38O2: 610.2872; found: 610.2873.
rather than the aryl migration, and the corresponding trial-
kene compounds 5, derived from direct deprotonation, as
well as another kind of indene derivatives 7, derived from
direct intramolecular Friedel–Crafts reaction, were pro-
duced in moderate to excellent yields. On the basis of the
experimental results in conjunction with further computa-
tional studies, plausible mechanisms for all these reactions
were proposed. Efforts are in progress to elucidate further
mechanistic details of these reactions and to understand
their scope and limitations.
Compound 6a: Contains a trace of diastereomeric isomers; white solid;
1
m.p. 216–2188C; H NMR (300 MHz, CDCl3): d=1.48 (d, J=7.5 Hz, 3H;
CH3), 3.76 (s, 3H; CH3), 3.86 (s, 3H; CH3), 4.44 (q, J=7.5 Hz, 1H; CH),
4.52 (s, 1H; CH), 5.89 (d, J=8.1 Hz, 2H; Ar), 6.04 (d, J=8.1 Hz, 2H;
Ar), 6.25–6.40 (m, 1H; Ar), 6.56 (d, J=8.4 Hz, 2H; Ar), 6.73 (t, J=
7.8 Hz, 2H; Ar), 6.89 (d, J=8.4 Hz, 2H; Ar), 6.90–7.78 ppm (m, 16H;
Ar); 13C NMR (75 MHz, CDCl3): d=17.0, 39.9, 55.1, 55.4, 57.1, 112.6,
114.0, 120.0, 123.9, 125.3, 125.7, 126.2, 126.4, 126.7, 128.0, 128.2, 128.87,
129.1, 129.4, 129.6, 130.7, 130.9, 135.5, 136.2, 136.4, 142.76, 142.83, 142.9,
144.2, 146.8, 149.5, 158.0, 158.3 ppm; IR (CH2Cl2): n˜ =3055, 2955, 2928,
1608, 1509, 1490, 1462, 1302, 1249, 1178, 1036, 830, 743, 702, 572,
535 cmꢀ1; ; MS (EI): m/z (%): 500 (9) [M]+, 352 (30), 351 (92), 244 (21),
243 (100), 228 (13), 149 (78), 121 (28), 91 (9), 41 (10); elemental analysis
calcd (%) for C45H38O2: C 88.49, H 6.27; found: C 88.40, H 6.21.
Experimental Section
General: 1H and 13C NMR spectra were recorded at 300 (or 400) and 75
(or 100) MHz, respectively. MS and HRMS were carried out by the EI
method. Organic solvents that were used were dried by standard methods
when necessary. Satisfactory CHN microanalyses were obtained with an
analyzer. Commercially obtained reagents were used without further pu-
rification. All these reactions were monitored by TLC (silica gel coated
plates). Flash column chromatography was carried out on silica gel at in-
creased pressure.
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 5163 – 5172

Aryl Migration
FULL PAPER
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Acknowledgements
We thank the Shanghai Municipal Committee of Science and Technology
(06XD14005, 08dj1400100-2), the National Basic Research Program of
China (973-2009CB825300), and the National Natural Science Founda-
tion of China (20872162, 20672127, 20821002, 20732008, 20702059) for fi-
nancial support, and also thank Mr. Sun Jie for performing the X-ray dif-
fraction.
[1] For the synthesis of VDCPs, see: a) K. Isagawa, K. Mizuno, H.
Sugita, Y. Otsuji, J. Chem. Soc. Perkin Trans. 1 1991, 2283–2285,
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[10] Crystal data of 3aA: formula: C36H36O2; Mr: 500.65; crystal color,
habit: colorless, prismatic; crystal system: triclinic, lattice type:
primitive; lattice parameters: a=10.9712(10), b=11.1396(10), c=
12.7340(12) ꢁ; a=84.184(2), b=71.499(2), g=74.938(2)8; V=
3
1424.8(2) ꢁ ; space group: P1; Z=2; 1calcd =1.167 gcmꢀ3; F000 =536;
¯
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M. Hashiguchi, K. Shima, Tetrahedron Lett. 2001, 42, 3363–3366;
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1975, 97, 4413–4414.
diffractometer: Rigaku AFC7R; residuals: R, Rw: 0.0528, 0.1286.
CCDC-678274 contains the supplementary crystallographic data for
3aA. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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[11] Crystal data of 3eB: formula: C38H40O4; Mr: 560.70; crystal color,
habit: colorless, prismatic; crystal system: monoclinic, lattice type:
primitive; lattice parameters: a=12.3292(13), b=8.2702(9), c=
31.355(3) ꢁ; a=90, b=98.443(2), g=908; V=3162.5(6) ꢁ3; space
group: P21/n; Z=4; 1calcd =1.178 gcmꢀ3; F000 =1200; diffractometer:
Rigaku AFC7R; residuals: R, Rw: 0.0625, 0.1397. CCDC-682291
contains the supplementary crystallographic data for 3eB. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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11593.
[13] Crystal data of 6a: formula: C45H38O2; Mr: 610.75; crystal color,
habit: colorless, prismatic; crystal system: monoclinic; lattice type:
primitive; lattice parameters: a=8.5135(9), b=21.504(2), c=
9.8635(10) ꢁ; a=90, b=110.542(2), g=908; V=1691.0(3) ꢁ3; space
group: P21; Z=2; 1calcd =1.200 gcmꢀ3
; F000 =648; diffractometer:
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Eur. J. Org. Chem. 2009, 270–274; k) W. Li, M. Shi, J. Org. Chem.
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3875–3881; m) W. Li, M. Shi, Y.-X. Li, Chem. Eur. J. 2009, 15,
8852–8860; for the Lewis acid catalyzed reactions of bis(4-alkoxy-
phenyl)methanol with (diarylmethylene)- and (dialkylmethylene)cy-
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Rigaku AFC7R; residuals: R, Rw: 0.0449, 0.0815. CCDC-686803
contains the supplementary crystallographic data for 6a. These data
can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] Crystal data of 7e: formula: C1.74H1.56O0.07; Mr: 23.66; temperature:
296(2) K; wavelength: 0.71073 A; crystal system, space group, tri-
¯
clinic, P1; unit cell dimensions: a=10.7752(2), b=12.1157(2), c=
15.6991(3) ꢁ; a=81.8540(10), b=86.4030(10), g=65.3660(10)8;
volume: 1844.16(6) ꢁ3; Z=54; 1calcd =1.150 Mgmꢀ3; m: 0.069 mmꢀ1
;
F
000 =680; crystal size: 0.37ꢂ0.25ꢂ0.13 mm; q range for data collec-
tion: 1.86–25.018; limiting indices: ꢀ12ꢁhꢁ9, ꢀ14ꢁkꢁ14, ꢀ18ꢁ
lꢁ18; final R indices [I>2s(I)], R1=0.0434, wR2=0.1032, R indi-
ces (all data): R1=0.0901, wR2=0.1324. CCDC-731461 contains the
supplementary crystallographic data for 7e. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Received: November 15, 2009
Published online: March 22, 2010
5172
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 5163 – 5172