Gold(I) and bronsted acid catalyzed intramolecular rearrangements of vinylidenecyclopropanes

Article abstract of DOI:10.1002/chem.201001433

(Chemical Equation Presented) Spot the difference! Different intramolecular rearrangements of vinylidenecyclopropanes containing a secondary alcohol chain were observed depending on whether the reaction was catalyzed by a gold(I) complex or a Bronsted acid (HOTf; OTf=trifluoromethanesulfonate). The corresponding adducts were obtained in moderate to good yields (see scheme). The scope and limitations of these reactions, as well as plausible mechanisms, have been discussed.

Full text of DOI:10.1002/chem.201001433

COMMUNICATION
DOI: 10.1002/chem.201001433
Gold(I) and Brønsted Acid Catalyzed Intramolecular Rearrangements of
Vinylidenecyclopropanes
Bei-Li Lu, Yin Wei,* and Min Shi*[a]
Vinylidenecyclopropanes (VDCPs) are highly strained but
thermally stable and adequately reactive substances that
serve as extremely versatile building blocks in many organic
reactions. Over the last few decades, due to their unique
structural and electronic properties, the chemistry of
VDCPs has attracted much attention from mechanistic,
spectroscopic, and synthetic viewpoints and a wide variety
of transformations have been disclosed.[1,2] For example, nu-
merous ring-opening/cycloaddition and intramolecular rear-
rangement reactions of VDCPs catalyzed or mediated by
transition metals or Lewis and Brønsted acids have been dis-
closed extensively thus far.[3,4] Meanwhile, gold catalysts, be-
cause of their unique properties of activating alkynes, al-
lenes, and alkenes toward attack from a variety of nucleo-
philes, have now become a well-established choice for many
see the Supporting Information). To our delight, we found
that in the presence of gold(I) or a Brønsted acid, VDCPs 2
could undergo different intramolecular rearrangements to
give, respectively, enone or enyne derivatives in moderate to
good yields under mild conditions. To the best of our knowl-
edge, this is the first example of a gold(I)-catalyzed intramo-
lecular rearrangement of VDCPs containing a secondary al-
cohol chain. Herein, we wish to report the results of these
interesting reactions.
Initially, the reactions were carried out by using VDCP
2a (only one diastereoisomer with anti-configuration as de-
termined by a NOESY spectrum) as the substrate in the
presence of a gold(I) catalyst and the results are summar-
ized in Table 1. We found that (E)-1-cyclohexyl-3-methyl-2-
styrylbut-2-en-1-one (3a) was obtained stereospecifically as
a single stereoisomer in 63% yield in THF at room tempera-
ture in the presence of Ph3PAuCl (10 mol%) and AgOTf
chemical transformations.[5] Recently, we synthesized
a
series of vinylidenecyclopropanes 2[6] containing an addition-
al secondary alcohol chain on the cyclopropanes from vinyli-
denecyclopropanes 1 (Scheme 1, for a detailed description,
(10 mol%;
OTf=trifluoromethanesulfonate)
(Table 1,
Table 1. Optimization of the reaction conditions for the gold(I)-catalyzed
rearrangement of VDCP 2a.
Scheme 1. Synthesis of VDCPs 2 containing a secondary alcohol chain;
IBX=2-iodoxybenzoic acid.
Entry[a] Solvent Au cat.
AgX
PPh3AuCl AgOTf
CH2Cl2 PPh3AuCl AgOTf
T [8C] t [h] Yield [%][b]
1
THF
RT
RT
RT
RT
RT
RT
RT
RT
RT
20
20
20
20
20
20
24
20
20
20
6
63
33
trace
37
89
2
3
4
DCE
MeCN
PPh3AuCl AgOTf
PPh3AuCl AgOTf
[a] B.-L. Lu, Y. Wei, M. Shi
5
toluene PPh3AuCl AgOTf
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
6
7
MeOH
toluene
PPh3AuCl AgOTf
–
78
[c]
AgOTf
–
–
8
toluene PPh3AuCl
22[d]
85
9
toluene PPh3AuCl AgBF4
10
11
toluene PPh3AuCl AgSbF6 RT
toluene PPh3AuCl AgOTf 40
26
89
E-mail: Weiyin@mail.sioc.ac.cn
Mshi@mail.sioc.ac.cn
Supporting information for this article, including experimental proce-
dures, compound characterization data, and X-ray crystal data of 3a
is available on the WWW under http://dx.doi.org/10.1002/
chem.201001433.
[a] All reactions were carried out using 2a (0.1 mmol) in the presence of
catalyst (10 mol%) in various solvents (1.0 mL). [b] Yield of the isolated
product. [c] Product 4a was obtained in 40% yield and 5a was formed in
31% yield. [d] 63% of 2a was recovered. DCE=1,2-dichloroethane.
Chem. Eur. J. 2010, 16, 10975 – 10979
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10975

Table 2. Optimization of the reaction conditions for the Brønsted acid
catalyzed rearrangement of VDCP 2a.
entry 1). The crystal structure of 3a was unambiguously de-
termined by X-ray diffraction (Figure 1) and its CIF data
are presented in the Supporting Information.[7] Screening of
Entry[a]
Solvent
Catalyst
T [8C]
t [h]
Yield [%][b]
4a
5a
1
2
3
4
5
6
7
toluene
THF
HOTf
HOTf
HOTf
HOTf
HOTf
MeSO3H
CF3CO2H
RT
RT
RT
RT
RT
RT
60
3
3
3
3
3
49
53
44
33
33
26
MeOH
CH2Cl2
MeCN
MeCN
MeCN
complex mixture
82
41
23
–
26
52
3
12
[a] All reactions were carried out using 2a (0.1 mmol) in the presence of
catalyst (10 mol%) in various solvents (1.0 mL). [b] Yield of the isolated
product.
room temperature with 10 mol% of HOTf (Table 2,
entry 5).
Figure 1. ORTEP drawing of 3a ellipsoids at 30% probability.
Having optimized the reaction conditions, we next exam-
ined the generality of these interesting intramolecular rear-
the solvents revealed that toluene was the optimal solvent,
affording 3a in 89% yield (Table 1, entries 2–6). Further-
more, we found that using AgOTf alone as the catalyst in
toluene at room temperature afforded enyne derivative 4a
and alcohol derivative 5a (see Table 2) in 40% and 31%
yield, respectively, instead of 3a (Table 1, entry 7). When
using PPh3AuCl alone as the catalyst, compound 3a was ob-
tained in 22% yield along with 63% of recovered substrate,
suggesting that the in situ generated gold(I) species is the ef-
ficient catalyst (Table 1, entry 8). Another silver salt, namely
AgBF4 also produced 3a in an excellent yield as a single ste-
reoisomer (Table 1, entry 9). The combination of AuPPh3Cl/
AgSbF6 did not have efficient catalytic activity in this reac-
tion (Table 1, entry 10). Increasing the reaction temperature
to 408C resulted in the same yield as that obtained at room
temperature with a shorter reaction time (Table 1, entry 11).
Therefore, the best reaction conditions have been identified
as a combination of AuPPh3/AgOTf as the catalyst in tolu-
ene at 408C (Table 1, entry 11).
On the other hand, VDCP 2a could undergo a totally dif-
ferent intramolecular rearrangement in toluene at room
temperature in the presence of trifluoromethanesulfonic
acid (TfOH, 10 mol%). The enyne derivative 4a was pro-
duced in 49% yield along with the alcohol derivative 5a in
33% yield (Table 2, entry 1). Using THF or MeOH as the
solvent, the products 4a and 5a were obtained in good total
yields (Table 2, entries 2 and 3). In dichloromethane, com-
plex product mixtures were formed under identical condi-
tions (Table 2, entry 4). Interestingly, in MeCN, 4a was
formed in 82% yield as the sole product, suggesting that the
effects of the solvent significantly affected the reaction out-
come (Table 2, entry 5). Other Brønsted acids, such as
MeSO3H and CF3CO2H are not as effective as HOTf under
the similar conditions (Table 2, entries 6 and 7). Thus, the
best conditions for the formation of the corresponding
enyne derivative 4a were found to be those in MeCN at
2
2
À
rangement reactions of VDCPs 2 (in which R , R =
1
À
A
ence of gold(I) or a Brønsted acid and the results were sum-
marized in Tables 3 and 4, respectively. VDCPs 2b–2e and
2g–2h containing an electron-withdrawing or electron-do-
Table 3. Gold(I)-catalyzed rearrangement of VDCPs 2 under the optimal
conditions.
Entry[a]
2, R1, (d.r.)
Yield [%][b]
1
2
3
4
5
6
7
8
2b, p-BrC6H4, (7.1:1.0)
2c, p-MeC6H4, –[c]
2d, p-MeOC6H4, –[c]
2e, p-C6H5C6H4, –[c]
2 f, b-C10H7, –[c]
3b, 87
3c, 84
3d, 83
3e, 85
3 f, 75
3g, 76
3h, 77
complex mixture
complex mixture
complex mixture
complex mixture
2g, m-ClC6H4, –[c]
2h, 3,5-Me2C6H3, –[c]
2i, o-MeC6H4, –[c]
2j, Et (1.1:1.0)
9
10
11
2k, CH2C6H11, –[c]
2l, CH2C6H5, –[c]
[a] All reactions were carried out using 2 (0.1 mmol) in the presence of
PPh3AuCl/AgOTf (10 mol%) in toluene (1.0 mL) at 408C. [b] Yield of
the isolated product. [c] Only one diastereoisomer was isolated.
nating group at the para- or meta-position of the phenyl ring
produced the corresponding products 3 in 76 to 87% yields
under the optimized conditions of the gold(I)-catalyzed re-
action (Table 3, entries 1–4, 6, and 7). In addition, for
VDCP 2 f in which R1 =2-naphthyl, the reaction also pro-
ceeded smoothly to give 3 f in 75% yield (Table 3, entry 5).
However, when there was a methyl group at the ortho-posi-
10976
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Chem. Eur. J. 2010, 16, 10975 – 10979

Gold(I) and Brønsted Acid Catalyzed Rearrangements
COMMUNICATION
Table 4. Brønsted acid catalyzed rearrangement of VDCPs 2 under the
Table 5. Gold(I)-catalyzed rearrangement of VDCPs 2 under the opti-
optimal conditions.
mized conditions.
Entry[a]
2, R1, (d.r.)
Yield [%][b]
Entry[a] 2, R2, R2, (d.r.)
Yield [%][b]
1
2
3
4
5
6
7
8
9
2b
2c
2d
2e
2 f
2h
2i
4b, 80
4c, 81
4d, 75
4e, 76
4 f, 67
4h,76
4i, 72
4j, 45
4k, 55
4l, 53
1
2
2m, Bu, Bu, –[c]
3i, 84
3j, 74
[c]
À
À
2n,
2o,
2p,
ACHTUGNRTNE(NUNG CH2)6 , –
3
(several diastereomeric mixtures) 3k, 88
[c]
À
À
4
5
G
3l, 88
3m, 80
2q, Me, Me, –[c]
2j, Et (1.1:1.0)
2k, CH2C6H11, –[c]
2l, CH2C6H5, –[c]
[a] All reactions were carried out using 2 (0.1 mmol) in the presence of
PPh3AuCl/AgOTf (10 mol%) in toluene (1.0 mL) at 408C. [b] Yield of
the isolated product. [c] Only one diastereoisomer was isolated.
10
[a] All reactions were carried out using 2 (0.1 mmol) in the presence of
HOTf (10 mol%) in MeCN (1.0 mL) at RT. [b] Yield of the isolated
product. [c] Only one diastereoisomer was isolated.
Table 6. Brønsted acid catalyzed rearrangement of VDCPs 2 under the
optimal conditions.
tion of the benzene ring, the expected rearrangement prod-
uct could not be obtained, presumably due to the steric hin-
drance of the aromatic ring (Table 3, entry 8). For the
VDCPs 2j–2l (R1 =aliphatic group), complex product mix-
tures were formed under identical conditions without forma-
tion of the desired products, presumably due to the fact that
the corresponding cationic intermediates could not be stabi-
lized by the aliphatic group in these substrates.
In the case of the Brønsted acid catalyzed reaction,
VDCPs 2b–2e and 2h with electron-withdrawing or elec-
tron-donating groups at the para- or meta-position of the
phenyl ring produced the enyne derivatives 4b–4e and 4g in
75–81% yields under the standard conditions (Table 4, en-
tries 1–4 and 6). Even for VDCP 2i in which R1 =o-
MeC6H4, the rearrangement reaction could also proceed
smoothly to give 4h in 72% yield (Table 4, entry 7). In the
case of VDCP 2 f, the product 4 f was afforded in 67% yield
(Table 4, entry 5). Furthermore, we found that this reaction
was also suitable for the substrates 2j–2l with aliphatic sub-
stituents, producing enyne derivatives 4i–4k in moderate
yields, (Table 4, entries 8–10).
To extend the scope of these two reactions, other VDCPs
2m–2q (R1 =phenyl group, R2 =aliphatic group) were also
examined under the standard conditions and the results of
these experiments are summarized in Tables 5 and 6. It was
found that by either expanding the six-membered ring to a
seven-membered ring or contracting it to a five-membered
ring, the gold(I)-catalyzed reactions could still proceed
smoothly to give 3j and 3l in 74 and 88% yield, respectively
(Table 5, entries 2 and 4). The substrate 2o containing a cy-
clohexyl group with three methyl substituents afforded the
product 3k in 88% yield (Table 5, entry 3). As for the sub-
strate 2m (R2 =butyl), the product 3i was obtained in 84%
yield (Table 5, entry 1). The substrate 2q with a methyl sub-
stituent also gave the corresponding enone derivative 3m in
80% yield (Table 5, entry 5).
Entry[a]
1
2
Yield [%][b]
2m
4l, 82[c]
2
3
4
5
2n
2o
2p
2q
4m, 80
4n, 77[d]
4o, 81
4p, 79 (R2’ =H)
[a] All reactions were carried out using 2 (0.1 mmol) in the presence of
HOTf (10 mol%) in MeCN (1.0 mL) at RT. [b] Yield of the isolated
product. [c] The corresponding enyne was obtained with trans-configura-
tion. [d] Diastereomeric mixtures with 1.1:1.0 ratio.
Moreover, when the Brønsted acid catalyzed reaction was
carried out using VDCPs 2n, 2p (R2, R2 =cyclic substitu-
ents) or 2m, 2q (R2, R2 =non-cyclic substituents) as the sub-
strates, it was found that the corresponding rearranged prod-
ucts were produced in 79–82% yields under the standard
conditions (Table 6, entries 1, 2, 4, and 5). It should be
noted that using VDCP 2o (several diastereoisomeric mix-
tures) as the substrate led to the enyne derivative 4n as a
diastereoisomeric mixture in a good combined yield with a
1.1:1.0 ratio (Table 6, entry 3).
To clarify the reaction mechanism of the gold(I)-catalyzed
intramolecular rearrangement, an isotopic labeling experi-
ment was conducted wherein the 18O-enriched secondary al-
cohol [18O]2a (18O content 44%) was synthesized and used
in the gold(I)-catalyzed reaction under the standard condi-
tions.[8] It was found that the corresponding product [18O]3a
was obtained in 89% yield with an 18O content of 45%.
Using 2a as the substrate with the addition of 1.0 equiv of
H218O in the reaction system, it was found that enone deriv-
ative 3a was formed in 85% yield with an 18O content of
Chem. Eur. J. 2010, 16, 10975 – 10979
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10977

B.-L. Lu, Y. Wei and M. Shi
0%, indicating that the oxygen atom of the product 3a is
indeed derived from substrate 2a and therefore the intramo-
lecular cyclization is responsible for the formation of 3a
(Scheme 2).
taining intermediate D, which affords the final product 3
through the elimination of gold(I).
In Scheme 4, in the presence of the Brønsted acid HOTf,
VDCP 2 is first protonated by TfOH at the hydroxyl group
along with the liberation of one molecule of water to give a
stabilized cationic intermediate I, which undergoes cyclopro-
pane ring opening to afford the corresponding intermediate
II and a resonance-stabilized intermediate III. Intermediate
III provides product 4 by deprotonation or through attack
by water to give the product 5.
In conclusion, we have developed two distinct gold(I) and
Brønsted acid catalyzed intramolecular rearrangements of
VDCPs 2 containing a secondary alcohol chain to produce
enone and enyne derivatives in moderate or good yields
under mild conditions. The scope and limitations of the two
reactions have been carefully examined and the reaction
mechanisms have been discussed on the basis of 18O-isotope
labeling experiments. The rearrangement products 3 and 4
are important building blocks in organic chemistry. The po-
tential utilization and extension of the scope of this synthet-
ic methodology are currently under investigation.
Scheme 2. Isotopic labeling experiment using [18O]2a as the substrate
and adding H218O into the reaction under the standard conditions.
Plausible mechanisms for the formation of the enone and
enyne derivatives are outlined in Scheme 3 and Scheme 4,
respectively. In the gold(I)-catalyzed reaction, the allene
functionality of 2 is activated by gold(I) to form intermedi-
ate A. The intramolecular nucleophilic addition by the hy-
droxyl group onto the electrophilic carbon center in inter-
mediate A[9] gives intermediate B,[10] which undergoes a
cation-induced rearrangement to produce intermediate C.
Subsequent ring-opening of the cyclopropane forms Au-con-
Experimental Section
General procedure for the gold-catalyzed rearrangement: Under an
argon atmosphere, vinylidenecyclopropane 2a (0.1 mmol), AuPPh3Cl
(0.01 mmol), AgOTf (0.01 mmol), and toluene (1 mL) were added to a
Schlenk tube. The reaction mixture was stirred at 408C for 6 h and the
reaction was monitored by TLC. Then, the solvent was removed under
reduced pressure and the residue was purified by a flash column chroma-
tography (silica gel; eluent: EtOAc/petroleum ether (PE)=1:20). Com-
pound 3a was isolated as a yellow oil in 89% yield.
General procedure for the Brønsted acid catalyzed rearrangement:
Under an argon atmosphere, vinylidenecyclopropane 2a (0.1 mmol),
HOTf (0.01 mmol), and MeCN (1 mL) were added to a Schlenk tube.
The reaction mixture was stirred at RT until the reaction was complete.
Then, the solvent was removed under reduced pressure and the residue
was purified by flash column chromatography (silica gel; eluent: EtOAc/
PE=1:50). Compound 4a was isolated as a yellow oil in 82% yield.
Acknowledgements
We thank the Shanghai Municipal Committee of Science and Technology
(08dj1400100-2), National Basic Research Program of China ACHTUNGTRENNUNG(973)-
Scheme 3. A plausible mechanism for the formation of 3.
2009CB825300, and the National Natural Science Foundation of China
for financial support (20472096, 20872162, 20672127, 20821002 and
20732008).
Keywords: Brønsted acid · cyclopropanes · gold · reaction
mechanisms · rearrangement
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Gold(I) and Brønsted Acid Catalyzed Rearrangements
COMMUNICATION
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Received: May 24, 2010
Published online: August 5, 2010
Chem. Eur. J. 2010, 16, 10975 – 10979
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10979