Gold-catalyzed reactions of enynals/enynones with norbornenes: Generation and trapping of cyclic o-quinodimethanes (o-QDMs)

Article abstract of DOI:10.1002/chem.201300232

Fan-like structures: An efficient Au^III-catalyzed method to generate the highly reactive cyclic o-quinodimethane (o-QDM) species from easily available enynals or enynones is presented (see scheme). This method produced a variety of structurally unique fan-like products with the advantages of mild reaction conditions, excellent diastereoselectivities, and high functional-group tolerance. Copyright

Full text of DOI:10.1002/chem.201300232

COMMUNICATION
DOI: 10.1002/chem.201300232
Gold-Catalyzed Reactions of Enynals/Enynones with Norbornenes:
Generation and Trapping of Cyclic o-Quinodimethanes (o-QDMs)
Shifa Zhu,* Zhicai Zhang, Xiaobing Huang, Huanfeng Jiang,* and Zhengjiang Guo[a]
o-Quinodimethane (o-QDM), a transient short-lived and
highly reactive species, has attracted great curiosity from
chemists over the past 40 years, both from theoretical and
synthetic points of view.[1] As cis-dienes, o-QDMs are much
more reactive than the related ꢀclassical dienesꢁ (such as bu-
tadiene and cyclopentadiene) in Diels–Alder reactions (DA)
with olefins. They can be classified as two main types based
on their structures: acyclic o-QDMs and cyclic o-QDMs
(Scheme 1). During the past decades, the acyclic o-QDM
sembly.[5] However, the generation and trapping of the cyclic
o-QDM was reported to be much scarcer than the acyclic
one due to its higher reactivity and greater steric hin-
drance.[6,7] For example, 2,3-dihydronaphthalene (2,3-DHN),
a simple cyclic o-QDM, was typically generated by the irra-
diation of o-divinyl benzene (o-DVB; Scheme 1B, path a).[6]
But the resulting cyclic o-QDM would rearrange to 1,2-
DHN quickly by a [1,5]-H-shift before being trapped by the
dienophiles (Scheme 1B).[6] Furthermore, the photolysis re-
action is highly limited because the reaction conditions are
not applicable for irradiation-sensitive groups or sub-
strates.[6,7] Therefore, the efficient generation and trapping
of the cyclic o-QDM with dienophiles still remained a chal-
lenge in organic synthesis. In past decades, transition metals
have shown exceptional efficiency in catalyzing a variety of
chemical transformations.[8] We were curious about the pos-
sibility of generating the cyclic o-QDM from an easily avail-
able starting material through a transition-metal-catalyzed
pathway, with the ultimate aim of synthesizing the fan-like
structures by trapping such highly reactive species
AHCTUNGTRENNUNG
method to generate the highly reactive hydroxy-o-QDM,
which was trapped by the alkenes to form polysubstituted
tetrahydronaphthols (Scheme 2).[9] It was the first example
of transition-metal-catalyzed generation of acyclic hydroxyl-
o-QDM species. Owing to the greater challenge to generate
Scheme 1. DA reactions involving o-QDMs.
has been used as a versatile building block in the synthesis
of lignans, terpenes, anthracyclines alkaloids, steroids, and
other natural products by inter- or intramolecular DA reac-
tions (Scheme 1A).[2] Similarly, trapping of the latter would
allow rapid access to the rigid fan-like structures
ACHTUNGTRENNUNG(Scheme 1B). These interesting skeletons, which were gener-
ally difficult to acquire by conventional methods, have been
extensively utilized as rigid building blocks for molecular
tweezers and clips,[3] rigid cavitands,[4] and molecular self-as-
[a] Prof. Dr. S. Zhu, Z. Zhang, X. Huang, Prof. Dr. H. Jiang, Z. Guo
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou 510640 (P.R. China)
E-mail: zhusf@scut.edu.cn
jianghf@scut.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300232.
Scheme 2. Transition-metal-catalyzed generation of o-QDMs.
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Table 1. Optimization of reaction conditions.[a]
the highly reactive and sterically-hindered cyclic o-QDM,
also as part of our continuing efforts to develop o-QDM
chemistry, we then decided to set out to search for an effi-
cient, practical, and transition-metal-catalyzed method to
generate and trap the cyclic o-QDM.
Enynal has been used as a versatile synthon in the Lewis
acid catalyzed benzannulation in the past decade.[10] We no-
ticed that in the Lewis acid catalyzed benzannulation of
enynal with alkene, a well-accepted intermediate B, which
came from the pyrylium A, was frequently proposed.[10] It
eventually led to the formation of 1,2-DHN, which was
often believed to proceed through b-H elimination
(Scheme 2, path a). However, we envisioned that path b, in
which a cyclic o-QDM intermediate was generated, would
also be possible. The product 1,2-DHN could also be
formed from the cyclic o-QDM by a [1,5]-H-shift process, or
at least in part. If this was the case, we anticipated the cyclic
o-QDM could be trapped by the dienophiles to form differ-
ent fan-like molecules through DA reactions. Herein, we
would like to report the results of our investigations.
Initial efforts were made to systematically investigate var-
ious catalytic reaction conditions for the reaction of enynal
1a and alkenes 2. By following the conditions reported in
our previous work,[9] different alkenes were then tested. Dis-
appointedly, both the styrene derivatives and aliphatic ole-
fins, which were good substrates in our previous work, did
not afford the desired products 3 (Table 1, entry 1). Even
when norbornene (NB), a naked and more reactive alkene,
was used as the substrate, no desired product 3 was detected
either (entry 2). Other metal salts (Cu, Pd, Pt, Rh, Hg),
which are traditionally used to activate the CꢀC triple
bond, were also proven ineffective. In the past decade, gold
catalysis has emerged as a powerful method for activating
alkynes toward nucleophiles.[11,12] The reduced oxophilicity
of gold relative to other transition metals allowed for high
functional-group tolerance and presented an ideal platform
for selective functionalization of alkynes. Therefore, we then
turned to different inorganic and organic gold sources as the
catalysts. To our delight, when NaAuCl4·2H2O or
KAuCl4·2H2O were applied as the catalyst, both afforded
the desired product 3a in 21% yield (entries 3, 4). Addition-
ally, 2-picolinic acid and N-heterocyclic carbene (NHC)-sup-
Entry Catalyst
Additive
Alkene
styrene
norbornene n.d.
norbornene 21
norbornene 21
norbornene 15
norbornene 34
norbornene n.d.
norbornene 20
norbornene 21
Yield 3
[%]
Yield 4
[%]
1
AgSbF6
–
–
–
–
–
–
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
25
2
AgSbF6
3
4
5
NaAuCl4·2H2O
KAuCl4·2H2O
A
2A
6
N
G
7[c]
8
AgSbF6
–
AgSbF6
ACHTUNGTRENNUNG
9[d]
10
11
12
13
14[g]
15[g,h]
16[h]
N
G
Selectfluor norbornene 47
17
3
14
Selectfluor norbornene 35
Selectfluor norbornene 52
Selectfluor norbornene 58
Selectfluor norbornene 60[i]
Selectfluor norbornene n.d.
33
N
29[i]
n.d.
[a] Unless otherwise noted, the reactions were performed in DCE at
808C for 24 h by using 5 mol% cat. and 10 mol% add. under N2, 1/2=
1:5, [1]=0.05m. The yield was determined by 1H NMR spectroscopy
(n.d.=not determined). [b] Pic=2-picolinate. [c] 15 mol% AgSbF6.
[d] 5 mol% AgSbF6. [e] SIMes=1,3-dimesityl-4,5-dihydroimidazol-2-yli-
dene. [f] IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. [g] 1/2=
1:10. [h] 15 mol% add. [i] The isolated yields were 58 and 28%, respec-
tively.
facts, we then tried to produce the NHC–AuIII in situ
through the reaction of NHC–AuI and Selectfluor. Interest-
ingly, a significant positive effect was observed when the
combination of [AuClACTHNUTRGNE(NUG IMes)]/Selectfluor was applied; the
yield of 3 was improved to 47% (entry 10). Additionally, a
side product 4 was also isolated in 25% yield. The product 4
was formed presumably through the tandem 1,3-dipolar ad-
dition/cyclopropanation of gold carbene species. Among
four different [AuCl
N
ACHTUNGTRENNUNG
diisopropylphenyl)dihydroimidazol-2-ylidene)/Selectfluor
functioned better than the other two (entries 10–13). Al-
though the yield of [AuCl
ported gold
well (entries 5, 6). Among them, [AuCl
N,N’-bis[2,4,6-(trimethyl)phenyl]imidazol-2-ylidene) afford-
ACHTUNGTRENNUNG
E
ACHTUNGTRNE(NUNG SIPr)]/Selectfluor was slightly
C
higher than [AuCl(IMes)]/Selectfluor, however, the latter
ACHTUNGTRENNUNG
ed the product 3a in 34% yield (entry 6). Trying to improve
the reaction efficiency by adding a silver salt to scavenge
one had better reproducibility. Increasing the amount of NB
or Selectfluor could improve the yields of 3 and 4 further
(entries 14, 15). For example, the overall yield of 3 and 4
was up to 91% when 10.0 equivalents of NB was applied
(entry 14). The best results were obtained when the reaction
was conducted in 1,2-dichloroethane (DCE) at 808C for
the chloride atoms of [AuCl
(IMes)] failed (entry 7). The
(IMes)], with or without
analogues AuI complex, [AuCl
AHCTUNGTRENNUNG
silver salts, could catalyze the reaction as well (entries 8, 9).
But the yields were lower than the corresponding AuIII com-
plexes. With the fact that AuIII was a better catalyst to pro-
mote this transformation than the others being tested, we
then focused our attention to find better AuIII sources. Re-
cently, Selectfluor was extensively used as a mild organic ox-
idant in the reactions to oxidize the transition metals, espe-
cially in the conversion of AuI to AuIII.[12] Inspired by these
24 h by using 5 mol% [AuClACTHNUTRGNE(UNG IMes)] as the catalyst and
15 mol% Selectfluor as an additive under N2, and the molar
ratio of 1/2=1:10 (entry 15). Under the optimized reaction
conditions, the yields of 3 and 4 climbed to 60 and 29%, re-
spectively. Additionally, the reaction did not occur without
catalyst (entry 16).
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Gold-Catalyzed Reactions of Enynals/Enynones with Norbornenes
Table 2. Substrate scope of enynals/enynones and norbornene.[a]
COMMUNICATION
(Table 2, entries 1–6), the reactions occurred smoothly with
high conversion (54–100%). The conversions for those sub-
strates substituted with electron-withdrawing groups were
higher than the electron-donor ones (entries 2–5). For exam-
ple, NO2-substituted enynal 1e gave quantitative reaction
conversion (entry 5), whereas the MeO-substituted 1b gave
only 55% (entry 2). Notably, the reaction could proceed
smoothly even for the pyridine-containing substrate 1 f,
albeit with a slightly lower yield (entry 6). The fan-like mol-
ecules 3 dominated in all these cases. Moreover, the reac-
tions have excellent diastereoselectivities. Only one isomer
was formed in all cases, the stereochemistry of which was
determined by the X-ray diffraction analysis of the single
crystals 3a and 4e (Figure 1).
Figure 1. X-ray diffraction analysis of A) 3a and B) 4e.
It is interesting to note both norbornanyl substructures in
compounds 3a adopted the exo-configuration; such a config-
uration would efficiently reduce the steric repulsion between
the perpendicular phenyl group and diethylene groups
(Figure 1).
For the enynals bearing the alkyl or alkenyl alkynes, the
reactions proceed efficiently as well (Table 2, entries 7–9).
For these substrates, the reactions selectively gave product
3. The reaction functioned particularly efficiently for the cy-
clopropyl substituted 1i, which gave the corresponding
product 3 in quantitative yield (entry 9). Importantly, the re-
action selectivity was completely reversed when a terminal
alkyne was applied (entry 10). In this case, only the cyclo-
propanation product 4j was obtained in 55% yield. We also
noted that the reactions could proceed efficiently as well
when varying R1 groups of the enynals 1 (entries 11–13). A
similar reaction trend was observed for these substrates:
electron-deficient substrates had better conversions and se-
lectivity. For example, 5-MeO-enynal 1k furnished the cor-
responding products 3k and 4k in almost a 1:1 ratio
(entry 11), whereas the corresponding electron-deficient
ones (5-F and 5-CF3) gave 3l and 3m dominantly. In addi-
tion to the enynals, enynonoes can serve as efficient sub-
strates as well (entries 14 and 15). For example, both the
methyl ketone 1n and phenyl ketone 1o furnished the cor-
responding fan-like molecules 3n and 3o in 70 and 76%
yields, respectively. The reactions had excellent selectivities
and no cyclopropanation products 4n and 4o were detected.
[a] The reactions were carried out under N2 with a molar ratio of 1/2=
1:10, [1]=0.05m. The yield refers to the isolated yield.
With the optimized reaction conditions (Table 1, entry 15)
in hand, the substrate scope was then examined. As sum-
marized in Table 2, the catalytic process could be successful-
ly applied to a variety of enynals 1 when NB was used as
the substrate. For example, when different aromatic alkynyl-
substituted benzaldehydes were used as the substrates
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S. Zhu, H. Jiang et al.
After having established the reactions of NB with enynals
and enynones as a reliable and efficient synthetic process,
we then proceeded to the other NB derivatives. When 2,5-
norbornadiene (NBD) was subjected to the same reaction
conditions, however, the desired fan-type products 5 were
obtained only as minor products (Table 3). The yields of
Actually, another unexpected 1-naphthyl ketone 6 domi-
nated the product distribution. Such ketones substituted at
the 1-position of naphthalenes were typically difficult to
access through traditional Friedel–Crafts acylation.[13] The
yields of 6 were ranging from 40–95%. In this reaction, a
tandem DA/retro-DA reaction was proposed. The retro-DA
process was accompanied by the release of one molecule of
cyclopentadiene (see the mechanistic discussion for details).
Therefore, the NBD here served as the acetylene (HCꢀ
CH) equivalent, a gas which is not easily accessible and op-
erationally inconvenient in common labs. Due to the excel-
lent substrate scopes and selectivities, this reaction could be
regarded as a reliable alternative method to prepare such 1-
naphthyl ketones.
Table 3. Substrate scope of enynals/enynones and 2,5-norbornadiene.[a]
More importantly, this gold-catalyzed system also had ex-
cellent functional-group tolerance (Table 4). For example, in
Table 4. Substrate scope of the enynals/enynones and norbornenes.[a]
[a] 1/2=1:5, [1]=0.05m. The yield refers to the isolated yield.
addition to the simple NB and NBD, the norbornenes sub-
stituted with a terminal or internal alkenyl group could also
be used as the substrates and furnished the desired products
7a and 7b in 75 and 78% yields, respectively. When benzo-
norbornene was applied as the substrate, the corresponding
product 7c was formed in 95% yield. The structure and
[a] The reactions were carried out under N2 with a molar ratio of 1/2=
1:10, [1]=0.05m. The yield refers to the isolated yield.
compounds 5 were typically lower than 20%, except in the
case of 5i (Table 3, entry 6). Compound 5 has the same skel-
eton as 3 but with different stereochemistry, in which one
norbornenyl group took the endo- and another took the
exo-configuration. This orientation may be partially attribut-
ed to the balance between steric hindrance and electronic
effects.
stereoACHTNUTRGNEcNUG hemistry of 7c were also confirmed by X-ray diffrac-
tion analysis (see the Supporting Information). We also
noted that even the functional groups of ether, ester, and
halogen atoms can also be well-tolerated; in these cases, the
corresponding products (7d–f) could be formed in moderate
yields (38–45%).
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Gold-Catalyzed Reactions of Enynals/Enynones with Norbornenes
COMMUNICATION
lectivities, and high functional-group tolerance, this system
holds considerable potential in the design of structurally in-
teresting fan-like molecules, which are frequently encoun-
tered in molecular self-assembly and materials chemistry.
Experimental Section
General experimental procedure: The corresponding enynals/enynones
(0.2 mmol) and norbornene (188.3 mg, 2.0 mmol) were added to a solu-
tion of the catalyst combination of [AuClACTHNUTRGNEU(GN IMes)] (5.37 mg, 0.01 mmol)
and Selectfluor (10.62 mg, 0.03 mmol) in DCE (4 mL, 0.05m). The reac-
tion mixture was stirred under a nitrogen atmosphere at 808C for 24 h.
After the reaction was finished, the mixture was filtered by short silica,
and then the solvent was evaporated under reduced pressure and the res-
idue was purified by flash chromatography on silica gel to afford the de-
sired products 3 and 4.
Acknowledgements
We are grateful for funding from the NNSFC (20902028 and 21172077),
the Program for New Century Excellent Talents in University (NCET-
10–0403), Guangdong NSF (10351064101000000), the National Basic Re-
search Program of China (973) (2011CB808600), SRF for ROCS, State
Education Ministry, and the Fundamental Research Funds for the Cen-
tral Universities, SCUT (2012ZZ0038).
Scheme 3. Proposed reaction mechanism.
A plausible mechanism is shown in Scheme 3. Initially,
ACTHNUTRGNEUNG
the catalytically active species, [AuIIIClF(NHC)]+ (simplified
as [Au] in the catalytic cycle), was generated in situ from
Keywords: carbenes
reaction · gold catalysis · QDM
· diastereoselectivity · Diels–Alder
the oxidation of [AuCl
AHCTUNGTRENNUNG
dination of the triple bond of enynal 1 to [Au] enhanced the
electrophilicity of the alkyne, and the subsequent nucleo-
philic attack of the carbonyl oxygen atom to the electron-
deficient alkyne would form the intermediate pyrylium A,
which was in equilibrium with another intermediate carbene
C. Two different pathways would then be followed based on
two different intermediates: 1) tandem DA/DA or DA/
retro-DA based on pyrylium A and 2) tandem 1,3-dipolar
addition/cyclopropanation based on carbene C. Pyrylium A
served as an electron-deficient diene that was trapped by
NB or NBD to form intermediate B. Due to the instability
of B, the cleavage of the carbon–oxygen bond, followed by
the release of catalyst [Au], formed the key intermediate, o-
QDM E. In the presence of NB, a second DA reaction then
occurred to give product 3. While in the presence of NBD, a
retro-DA reaction occurred instead to furnish naphthyl
ketone 6, accompanied by the release of pentadiene. When
it came to the carbene C, the 1,3-dipolar addition and cyclo-
propanation reactions proceed sequentially to form product
4.
In conclusion, we have developed an efficient gold-cata-
lyzed method to generate the highly reactive cyclic o-QDM
species from the easily available enyals or enynones. This
method allowed rapid access to a variety of structurally
unique fan-like products. Meanwhile, an important retro-
DA reaction was also observed when NBD was applied as
the dienophile of the DA reaction. In this case, all kinds of
1-naphthyl ketones were formed in good to excellent yields.
Owing to the mild reaction conditions, excellent diastereose-
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Received: January 21, 2013
Published online: March 4, 2013
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