Gold(I)-catalyzed tandem oxidative ring-opening/C-C bond cleavage reactions of vinylidenecyclopropanes with secondary amines under an oxygen atmosphere

Article abstract of DOI:10.1002/chem.201100862

A Midas touch! Gold(I)-catalyzed tandem oxidative ring-opening/C-C bond cleavage reactions of vinylidenecyclopropanes with a variety of secondary amines proceeded smoothly in toluene or 1,1,2,2-tetrachloroethane (TCE) to give the corresponding amides in moderate to good yields under oxygen atmosphere (see scheme; DCE=1,2-dichloroethane). The scope and limitations as well as the plausible mechanisms have been discussed.

Full text of DOI:10.1002/chem.201100862

DOI: 10.1002/chem.201100862
À
Gold(I)-Catalyzed Tandem Oxidative Ring-Opening/C C Bond Cleavage
Reactions of Vinylidenecyclopropanes with Secondary Amines Under an
Oxygen Atmosphere
Bei-Li Lu and Min Shi*[a]
À
Gold(I)-catalyzed tandem oxidative ring-opening/C C
bond cleavage reactions of vinylidenecyclopropanes with a
variety of secondary amines proceeded smoothly in toluene
or 1,1,2,2-tetrachloroethane to give the corresponding
amides in moderate to good yields under an oxygen atmos-
phere. The scope and limitations as well as the plausible
mechanism have been discussed.
During the past several years, we and others have been
engaging in the chemical transformations of vinylidenecyclo-
propanes (VDCPs), which have a strained cyclopropyl
group connected with an allene moiety and yet are thermal-
ly stable and active substances in organic chemistry. Vinyli-
denecyclopropanes 1 were prepared from easily available di-
bromocyclopropane and an excess of alkene under mild con-
ditions (Scheme 1, for the detailed procedure please see the
Compared with conventional stoichiometric oxidation re-
actions, environmentally friendly oxidative transformations
catalyzed by a transition metal have received the growing
interest of organic chemists.[1] Recently, gold-catalyzed
tandem reactions have been widely investigated and have
achieved great progress during the last decade,[2] in which
the gold-catalyzed additions of nucleophiles to unsaturated
carbon–carbon bonds[2a,f,g,h,j] have been significantly devel-
oped. However, a few successful examples have been report-
Scheme 1. Synthesis of VDCPs 1.
À
ed for gold-catalyzed homogeneous oxidative C C bond
cleavage reactions;[3] the typical successful examples are as
follows: In 2006, Shi and co-workers reported an interesting
Supporting Information).[4] It has been found that these
highly strained small-ring compounds are extremely useful
in organic reactions as versatile building blocks.[5] Moreover,
oxidation reactions of VDCPs using ozone,[6] peracetic
acid,[7] and cerium(IV) ammoniumnitrate (CAN)[8] as oxi-
dants have been successfully developed to deliver the corre-
sponding oxidized products in good yields under mild condi-
tions. However, to the best of our knowledge, there are no
examples using oxygen as a terminal oxidant and gold as the
catalyst in the transformation of VDCPs. In this paper, we
wish to report a novel gold(I)-catalyzed tandem oxidative
À
gold(I)-catalyzed oxidative C C bond cleavage of alkenes
to afford the corresponding ketones or aldehydes in good
yields using tert-butyl hydrogen peroxide (TBHP) as the oxi-
dant;[3e] following that, Liu et al. reported a novel tandem
cyclization/oxidative CACTHNUTRGENUCGN cleavage reaction of (Z)-enynols
catalyzed by a gold(I) catalyst with molecular oxygen.[3f] In
2007, Toste and co-workers reported the gold(I)-catalyzed
oxidative rearrangement reactions of alkynes using sulfox-
ides as oxidants.[3 g] Moreover, Liu and Hashmi also report-
À
À
ed the intramolecular oxidative C C bond cleavage cata-
ring-opening/C C bond cleavage reaction of VDCPs with
various secondary amines under mild conditions using mo-
lecular oxygen as a terminal oxidant.
lyzed by AuI in 2008, respectively.[3j,k] As can be seen from
À
these successful examples, the C C bond disconnection ap-
proaches have important synthetic applications in organic
synthesis.
Initially, VDCP 1a and morpholine 2a were used as the
substrates to examine the reaction outcomes in the gold(I)-
catalyzed oxidation under an oxygen atmosphere and subse-
quently optimize the reaction conditions. The results of
these experiments are summarized in Table 1. It was found
that amide 3a was formed in 68% yield in 1,2-dichloro-
ethane (DCE) at 508C for 24 h by using 1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene gold(I) chloride (iPrAuCl, 10
mol%) as the catalyst (Table 1, entry 1). The crystal struc-
ture of 3a was unambiguously determined by X-ray diffrac-
tion (Figure 1) and its CIF data are presented in the Sup-
porting Information.[9] The examination of solvent effects re-
vealed that 1,1,2,2-tetrachloroethane (TCE) was the optimal
solvent, affording 3a in 74% yield under the standard con-
[a] B.-L. Lu, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road, Shanghai 200032 (P.R. China)
Fax : (+86)21-64166128
E-mail: Mshi@mail.sioc.ac.cn
Supporting information (including experimental procedures, com-
pound characterization data, and X-ray crystal data of 3a (CCDC
740118) and anti-5 (CCDC 808598)) for this article is available on the
WWW under http://dx.doi.org/10.1002/chem.201100862.
9070
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Chem. Eur. J. 2011, 17, 9070 – 9075

COMMUNICATION
Table 1. Optimization of the reaction conditions for gold(I)-catalyzed
À
catalyst in this tandem oxidative ring-opening/C C bond
À
tandem oxidative ring-opening/C C bond cleavage reactions of VDCP
cleavage reaction (Table 1, entry 11). Therefore, the best re-
action conditions were identified when using PPh3AuCl as
the catalyst in TCE at 508C under an oxygen atmosphere.
The yield of 3a was lower in toluene, probably due to the
higher solubility of molecular oxygen in TCE than that in
toluene[10] and the higher polarity of TCE favoring this
1a with morpholine 2a under an oxygen atmosphere.
À
gold(I)-catalyzed tandem oxidative ring-opening/C C bond
cleavage reaction.
Entry[a] Solvent
Catalyst
[mol%] Additive
ACHTUNGTRENUN[NG mol%] Yield
[%][b]
On the basis of above results, it was expected that aceto-
phenone should be also isolated as a byproduct. Since the
boiling point of TCE is quite high (1478C), the residue was
directly purified by using flash column chromatography
without the removal of the solvent under reduced pressure.
Furthermore, acetophenone has the almost same Rf value
with that of TCE for the employed eluent (ethyl acetate/pe-
troleum ether=1:10). Therefore, it is difficult to identify
acetophenone under the optimal conditions. When using tol-
uene as the solvent and iPrAuCl as the catalyst, we could
isolate acetophenone 4a in 50% yield, which was almost
similar to the yield of product 3a (Scheme 2). In the case of
VDCPs 1b, 1c, and 1d, the corresponding ketones 4b–4d
were also isolated as the byproducts in similar yields as the
major product amides 3 (Scheme 2, for details please see the
Supporting Information).
1
2
3
4
5
6
7
8
DCE
THF
MeCN
TCE
toluene
iPrAuCl
iPrAuCl
iPrAuCl
iPrAuCl
iPrAuCl
10
10
10
10
10
10
–
–
–
–
–
–
–
–
–
–
–
–
–
–
10
10
–
68
47
55
74
71
65
81
77
37
64
18
CH3NO2 iPrAuCl
TCE
TCE
TCE
TCE
TCE
PPh3AuCl 10
PPh3AuCl 10
–
PtCl2
AuCl3
AgOTf
AgOTf
–
–
9
10
11
10
10
–
[a] All reactions were carried out using 1a (0.1 mmol), 2a (0.2 mmol),
catalyst (10 mol%), and additive (10 mol%) in solvent (2.0 mL) under
O2 atmosphere. [b] Yields of isolated products.
À
Scheme 2. Gold(I)-catalyzed tandem oxidative ring-opening/C C bond
cleavage reactions of VDCPs 1a, 1b, 1c, and 1d with morpholine 2a in
toluene under oxygen atmosphere.
Having these optimized reaction conditions in hand, we
next examined the generality of this interesting gold(I)-cata-
Figure 1. ORTEP Drawing of 3a.
À
lyzed tandem oxidative ring-opening/C C bond cleavage re-
ditions (Table 1, entries 2–6). Furthermore, we also found
that this reaction proceeded efficiently in the presence of
PPh3AuCl, giving 3a in 81% yield in TCE (Table 1, entry 7).
The combination of PPh3AuCl/AgOTf was less efficient for
the production of amide 3a (Table 1, entry 8). Using AgOTf
alone as the catalyst afforded product 3a in 37% yield
(Table 1, entry 9). Treatment of the substrates 1a and 2a
with PtCl2 in TCE could also produce the corresponding
amide 3a in 64% yield (Table 1, entry 10). To examine if
the goldACHTUNGTRENNUNG(III) species is a potential catalyst under such an ox-
idation conditions, AuCl3 was used as the catalyst to replace
PPh3AuCl; it was found that 3a was formed only in 18%
yield, suggesting that the goldACTHNUTRGENUGN(III) species is not an efficient
action of VDCPs 1 with morpholine 2a in the presence of
PPh3AuCl and the results are summarized in Table 2. As for
vinylidenecyclopropanes 1b–1g and 1j–1l (R1, R2, R3 =aryl
groups, and R4 =methyl or aryl groups), the reactions with
2a proceeded smoothly to afford the corresponding amides
3b–3g and 3h–3j in 67–82% yields whether they have elec-
tron-withdrawing or electron-donating groups on their ben-
zene rings (Table 2, entries 1–6 and 9–11). When R1 and R2
or R3 and R4 are alkyl groups such as vinylidenecyclopro-
panes 1h and 1i (R1 and R2 =Bu, R3 and R4 =Bu), no reac-
tion occurred under the standard conditions, suggesting that
R1, R2, and R3 should be aromatic groups in this reaction
(Table 2, entries 7 and 8).
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9071

M. Shi and B.-L. Lu
À
À
Table 2. Gold(I)-catalyzed tandem oxidative ring-opening/C C bond
cleavage reactions of VDCPs 1 with 2a under the optimal conditions.
Table 3. Gold(I)-catalyzed tandem oxidative ring-opening/C C bond
cleavage reactions of VDCP 1a with various the secondary amines 2
under the optimal conditions.
Entry[a]
2
RR’NH
Product
Yield
[%][b]
Entry[a]
1
R1/R2/R3/R4
Product
Yield
[%][b]
1c
2
2b
2c
2d
3k
3l
68
68
59
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1 f
1g
1h
1i
C6H5/C6H5/C6H5/C6H5
3b
3c
3d
3e
3 f
3g
–
80
67
82
74
78
71
N.R
N.R.
70
C6H5/C6H5/pMeC6H4/p-MeC6H4
C6H5/C6H5/p-BrC6H4/Me
p-FC6H4/p-FC6H4/C6H5/C6H5
p-ClC6H4/p-ClC6H4/C6H5/C6H5
pMeC6H4/p-MeC6H4/C6H5/C6H5
Bu/Bu/C6H5/C6H5
3
3m
4[c]
5
2e
2 f
2g
2h
2i
3n
3o
3p
3q
3r
77
61
63
55
48
C6H5/C6H5/Bu/Bu
–
9
10
11
1j
1k
1l
C6H5/C6H5/p-ClC6H4/p-ClC6H4
C6H5/C6H5/pMeC6H4/Me
C6H5/C6H5/p-ClC6H4/Me
3h
3i
3j
75
78
6[d]
7[d]
8
[a] All reactions were carried out using 1 (0.2 mmol), 2a (0.4 mmol),
PPh3AuCl (10 mol%) in TCE (4.0 mL) under O2 atmosphere. [b] Yields
of isolated products.
9[e]
2j
3s
40
Encouraged by these results, we further investigated the
reactions of VDCP 1a with various secondary amines 2b–2j
under the standard conditions and the results are summar-
ized in Table 3. We found that using piperidine 2b and its
derivatives 2c and 2d as the nucleophiles produced the cor-
responding amides 3k, 3l, and 3m in 68 and 59% yields, re-
spectively (Table 3, entries 1–3). As for pyrrolidine 2e, the
reaction should be carried out in DCE, which afforded the
corresponding amide 3n in 77% yield (Table 3, entry 4).
Even for imidazole 2 f, the reaction also proceeded smoothly
to give the corresponding amide 3o in 61% yield (Table 3,
entry 5). Using the aliphatic secondary amines 2g–2i as nu-
cleophiles, the corresponding amides 3p–3r were afforded
in moderate yields, presumably
[a] All reactions were carried out using 1a (0.2 mmol), 2 (0.4 mmol),
PPh3AuCl (10 mol%) in TCE (4.0 mL) under O2 atmosphere. [b] Yields
of isolated products. [c] The solvent is DCE. [d] K2CO3 (2.0 equiv) was
added. [e] The product 3s is a mixture of two stereoisomers (1.2:1.0).
Interestingly, the corresponding amide 3t was obtained in
34% yield as stereoisomeric mixtures (1.2:1.0) along with an
indene derivative 5 in 25% yield as anti- and syn isomeric
mixtures (anti/syn=7.3:1.0). When the reaction was carried
out in toluene, the product 5 was formed solely in 32%
yield in a 20.2:1.0 diastereomeric ratio (Scheme 3; also see
the Supporting Information). When the reaction was carried
due to their steric hindrance
and
lower
nucleophilicity
(Table 3, entries 6–8). Further-
more, we also found that this
reaction was also suitable for
the asymmetrical secondary
amine isoquinoline 2j under
the standard conditions, pro-
ducing the corresponding prod-
uct 3s in 40% yield as stereo-
isomeric
mixtures
(1.2:1.0,
Table 3, entry 9; also see the
Supporting Information).
We next performed the
gold(I)-catalyzed tandem reac-
tion of VDCP 1a with N-me-
thoxy-N-methylamine
hydro-
chloride 2k (2.0 equiv) under
the standard conditions in the
presence of K2CO3 (2.0 equiv). with amine 2k in the presence of K2CO3.
À
Scheme 3. Gold(I)-catalyzed tandem oxidative ring-opening/C C bond cleavage reaction of VDCPs 1a and 1b
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Chem. Eur. J. 2011, 17, 9070 – 9075

Gold(I)-Catalyzed Tandem Oxidative Ring-Opening
COMMUNICATION
out in TCE using VDCP 1b as the substrate, the products
3t and 5 could be also obtained in 32 and 21% yields, re-
spectively. The X-ray diffraction of the major isomer of 5
was carried out to unambiguously determine its anti configu-
ration. The CIF data is presented in the Supporting Infor-
mation and the ORTEP drawing of
Figure 2.[11]
5
is shown in
On the basis of the above results, a plausible mechanism
for the formation of amide 3b is outlined in Scheme 4 using
1b as a model.[12] The VDCP 1b is activated by a gold(I)
complex (the cationic species PPh3Au+ can also catalyze
this reaction) to give intermediate A, which is then attacked
by nucleophile 2a to afford intermediate B. Intermediate B
then forms the corresponding dioxo-gold intermediate C,
which is similar to the dioxogold intermediate proposed in
the previous literature in the presence of molecular O2.[13]
Subsequent isomerization and rearrangement of cyclopro-
pane take place to give the corresponding intermediate D,
in which the C=C bond is further activated by the gold(I)
complex. Then, the intramolecular nucleophilic attack and
the ring-opening of cyclopropane takes place to give the cor-
responding allylic gold intermediate E, according to the di-
oxetane intermediate proposed in the previous litera-
ture,[14,15] which undergoes protonation and the ring-opening
of the dioxetane via intermediate F to produce the corre-
sponding intermediate G along with the generation of one
molecule of benzophenone. Finally, the product 3b is
Figure 2. ORTEP Drawing of anti-5.
formed through
a tautomerization of intermediate G
Furthermore, we conducted three control experiments on
the mechanistic investigations of this novel gold(I)-catalyzed
(Scheme 4). The plausible mechanism for the formation of
indene derivative 5 using N-methoxy-N-methylamine hydro-
chloride 2k as the nucleophile might be similar as that men-
tioned above (for the details, please see the Supporting In-
formation).
À
tandem oxidative ring-opening/C C-bond-cleavage reaction.
The first control experiment has confirmed that this reac-
tion, under the optimized conditions, was unaffected by the
addition of the radical inhibitors such as 2,2,6,6-tetramethyl-
piperidine-1-oxyl (TEMPO, 1.0 equiv), which renders the in-
tervention of a radical pathway unlikely (reaction 1).
Second, an isotope labeling experiment was conducted in
which H218O (1.0 equiv) was added into the reaction system
under the standard conditions (reaction 2). It was found that
the corresponding product 3b was obtained in 64% yield
along with 0% of 18O content, which indicates that the
oxygen atom of the product 3b does not come from the
H2O of the system. Third, the reaction of VDCP 1b with 2a
in the catalysis of PPh3AuCl under absolute anaerobic con-
ditions did not take place and the substrates were complete-
ly recovered, suggesting that molecular oxygen is involved
in a much earlier stage of the reaction (reaction 3).
To clarify the reaction mechanism of this novel gold(I)-
À
catalyzed tandem oxidative ring-opening/C C bond cleav-
age reaction, we further conducted two deuterium labeling
experiments to examine the reaction outcomes (Scheme 5).
The first deuterium labeling experiment was performed by
subjecting the corresponding deuterated substrate [D]1b
(84% of the deuterium content at the cyclopropane; please
see the Supporting Information for the preparation) to the
reaction system under the standard conditions. It was found
that the resulting deuterated product [D]3b was obtained in
70% yield along with 57% deuterium content at the methyl
group of [D]3b (Scheme 5, reaction 1). Second, using
VDCP 1b as the substrate and adding D2O (1.0 equiv,
>99% deuterium content) into the reaction system under
the standard conditions, the corresponding deuterated prod-
uct 1.0 equiv [D]3b’ was formed in 54% yield along with
32% deuterium content (Scheme 5, reaction 2). These two
experiments unambiguously confirm that the three hydrogen
atoms of methyl group in amides 3 both come from the cy-
clopropane of substrate 1 and the proton source in the reac-
tion system. These results suggested that an isomerzation of
the cyclopropane in substrate 1 indeed took place and the
allylic gold(I) species existed in the reaction system.
In summary, we have developed a novel gold(I)-catalyzed
À
tandem oxidative ring-opening/C C bond cleavage reaction
of VDCPs 1 with a variety of secondary amines 2 to produce
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9073

M. Shi and B.-L. Lu
prop-1-en-2-ylbenzene
(1.77 g,
15 mmol). The reaction mixture was
vigorously stirred at room temperature
for 24 h. Then the residue was purified
by a flash column chromatography di-
rectly (silica gel, PE=petroleum
ether). Compound 1a was isolated as
a yellow solid in 54% yield.
The gold-catalyzed tandem oxidative
À
ring-opening/C C bond cleavage reac-
tion of VDCP 1a with morpholine 2a:
Under
an
argon
atmosphere,
PPh3AuCl (5 mg, 0.01 mmol) was
added into a Schlenk tube. Then the
argon atmosphere was pumped off and
a balloon filled with O2 was connected
to the reaction flask. A solution of vi-
nylidenecyclopropane
1a
(31 mg,
0.1 mmol) in 1,1,2,2-tetrachloroethane
(TCE, 2.0 mL) and morpholine 2a
(18 mL, 0.2 mmol) were added into the
Schlenk tube. The reaction mixture
was stirred at 508C until the reaction
was complete. Then the residue was
purified by a flash column chromatog-
raphy directly. (silica gel, EtOAc/PE=
1:4). Compound 3a was isolated as a
white solid in 81% yield.
Scheme 4. A plausible mechanism for the formation of 3b.
The gold-catalyzed tandem oxidative
À
ring-opening/C C bond cleavage reac-
tion of VDCP 1a with dimethylamine
hydrochloride 2g: Under an argon at-
mosphere, dimethylamine hydrochlo-
ride 2g (32 mg, 0.4 mmol) and potassi-
um
carbonate
(K2CO3,
55 mg,
0.4 mmol) were added into a Schlenk
tube. Then the atmosphere was
pumped off and a balloon filled with
O2 was connected to the reaction
flask.
1,1,2,2-tetrachloroethane
(2.0 mL) was added into the Schlenk
tube and the reaction mixtures was
stirred at room temperature for
20 min. After that, PPh3AuCl (10 mg,
Scheme 5. Deuterium labeling experiments.
0.02 mmol) and a solution of vinylide-
necyclopropane 1a (62 mg, 0.2 mmol)
in 1,1,2,2-tetrachloroethane (TCE, 2.0 mL) were added. The reaction
mixture was stirred at 508C until the reaction was complete. Then, the
residue was purified by a flash column chromatography directly. (silica
gel, EtOAc/PE=1:4). Compound 3p was isolated as a white solid in
63% yield.
the corresponding amides in moderate to good yields using
molecular oxygen as a terminal oxidant. The amides are im-
portant building blocks in organic synthesis; however, the
fully substituted acrylamides are difficult prepare according
to the previous methods. Our finding in this paper offered
an alternative approach for the efficient construction of
these important acrylamides. The scope and limitations of
this reaction have been carefully examined and the possible
reaction mechanism has been discussed. The potential uti-
À
lization and extension of the scope of this novel C C bond
disconnection approach are currently under investigation.
Acknowledgements
We thank the Shanghai Municipal Committee of Science and Technology
(08dj1400100–2), National Basic Research Program of China ACHTUNGTRENNUNG(973)-
2009CB825300, and the National Natural Science Foundation of China
for financial support (21072206, 20472096, 20872162, 20672127, 20821002
and 20732008).
Experimental Section
General procedures: The preparation of vinylidenecyclopropane 1a: A
mixture of Bu4NHSO4 (3.4 g, 10 mmol) and powdered NaOH (10 g,
250 mmol) was added to a tetrahydrofuran (THF) solution containing
1,1-dibromo-2,2-diphenylcyclopropane (1.75 g, 5 mmol) and an excess of
À
Keywords: amides · amines · gold · oxidative C C bond
cleavage · ring-opening · vinylidenecyclopropanes
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Gold(I)-Catalyzed Tandem Oxidative Ring-Opening
COMMUNICATION
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Received: March 21, 2011
Revised: June 10, 2011
Published online: July 8, 2011
Chem. Eur. J. 2011, 17, 9070 – 9075
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9075