Platinum-catalyzed cyclization/[1,2]-alkyl migration/allyl shift/cyclization cascade of epoxy alkynyl allyl ethers: A step-economical route to spirobenzo[h]chromanones

Article abstract of DOI:10.1002/chem.201200126

In tandem: A PtI₄-catalyzed tandem reaction of epoxy alkynyl allyl ethers involving a cyclization, [1,2]-alkyl migration, O→C allyl shift, aromatic cyclization sequence has been achieved to synthesize spirobenzo[h]chromanones. The reaction simultaneously forms two adjacent stereocenters, one of which is a quaternary carbon atom (see scheme). Copyright

Full text of DOI:10.1002/chem.201200126

COMMUNICATION
DOI: 10.1002/chem.201200126
Platinum-Catalyzed Cyclization/ACHTNUTRGNEUNG[1,2]-Alkyl Migration/Allyl Shift/Cyclization
Cascade of Epoxy Alkynyl Allyl Ethers: A Step-Economical Route to
Spirobenzo[h]chromanones
Yan-Fang Yang, Xing-Zhong Shu, Jian-Yi Luo, Shaukat Ali, and Yong-Min Liang*[a]
The use of transition-metal-catalyzed tandem reaction to
enhance the molecular complexity by combining several re-
actions into one operation provides great opportunities for
the development of novel catalytic protocols.[1] The semi-pi-
nacol rearrangement is an important and reliable reaction,
which has undergone fast development in the past few de-
cades.[2] The application of a semi-pinacol rearrangement in
a tandem reaction allows a rapid construction of relatively
complex products with multiple stereocenters and a quater-
nary carbon center.[2a,3] The significance of this combination
has been highlighted in the synthesis of several natural pro-
ducts.[2c,4] Exploration of new, efficient, and atom-economi-
cal tandem reactions in this area is often desirable.
Scheme 1. Design a tandem process to spirobenzo[h]chromanone.
As an important type of semi-pinacol rearrangement, in-
vestigations have largely focused on the rearrangement of
a-hydroxy epoxide.[5] In this case, the migration is driven by
acid-promoted ring-opening of epoxide. The reaction always
leads to the formation of a hydroxyl ketone product. During
our search for the ring-opening reactions of epoxide, we
found that an oxonium ion intermediate could be formed by
using a p-acidic catalyst to induce epoxide attack on an
alkyne moiety.[6] Previously, we have successfully applied
this oxonium ion intermediate to synthesize various furans
by trapping nucleophiles (Scheme 1, [Eq. (1a)]).[6a,c,f] We
also found that this intermediate could induce the migration
of an adjacent group to undergo a semi-pinacol type rear-
rangement (Scheme 1, [Eq. (1b)]). Various spiropyranones
rings with a spirocarbon can be efficiently formed in one re-
action.
We designed epoxy alkynyl allyl ether 1a for our initial
study. To our delight, when 1a was treated with PtCl2
(10 mol%) and 5 ꢀ MS in dry anisole at 808C under O2 at-
mosphere, an aromatized product spirobenzo[h]chromanone
2a was isolated in 13% yield after 18 h (Table 1, entry 1).
The structure was further established by X-ray diffraction
(see the Supporting Information).[11] A variety of PtII and
PtIV catalysts were then investigated. Among them PtI4 was
proved to be the most efficient catalyst and the desired
product 2a was obtained in 31% yield (Table 1, entry 6). In-
creasing the PtI4 loading from 5 to 10 mol% led to decom-
position of the starting material. The presence of both
oxygen and 5 ꢀ MS was found to be useful, because the re-
action resulted in inferior yields either under argon atmos-
phere or in wet solvents.[12] Gold catalysts, such as AuCl3
were then synthesized efficiently by
a gold-catalyzed
tandem cyclization/[1,2]-alkyl migration reaction of epoxy
ACHTUNGTRENNUNG
alkynes.[6b] To explore a more efficient and atom-economical
tandem process based on the oxonium ion intermediate trig-
gered semi-pinacol rearrangement, we herein report a plati-
num-catalyzed reaction of an epoxy alkynyl allyl ether to
a spirobenzo[h]chromanone (Scheme 1, [Eq. (2)]).[7] The re-
action proceeds through several challenging steps including
epoxy alkyne cyclization,[8] [1,2]-alkyl migration,[9] allyl
shift,[10] aromatic cyclization in a cascade manner. Three
and AuCl, as well as PdACHTUNTRGNEUNG(OAc)2 were inefficient and did not
give any desired product (Table 1, entries 7–9). Further ex-
amination of the solvent effect revealed that the use of 1,2-
dichloroethane (DCE), 1,4-dioxane, or CH3NO2 did not
afford expected product 2a (Table 1, entries 10–12). Solvents
such as toluene, chlorobenzene, and acetonitrile (MeCN) af-
forded inferior yields (Table 1, entries 13–15). When CuBr2,
AgOTf, or BQ was used as an additive, no superior result
was obtained (Table 1, entries 16–18). Interestingly, when
[a] Dr. Y.-F. Yang, X.-Z. Shu, J.-Y. Luo, S. Ali, Prof. Dr. Y.-M. Liang
State Key Laboratory of Applied Organic Chemistry
Lanzhou University
Lanzhou 730000 (China)
Fax: 0086-931-8912582
E-mail: liangym@lzu.edu.cn
Fe
creased to 62% (Table 1, entry 19). Increasing Fe
loading to 5 mol%, the yield of 2a was slightly declined
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200126.
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Table 1. Optimization of the platinum-catalyzed tandem reaction.[a]
Table 2. Platinum-catalyzed tandem reactions of various epoxy alkynyl
allyl ethers.[a]
Entry Catalyst
[(mol%)]
Additive
[(mol%)]
Solvent
T
Yield
G
N
[8C] [%][b]
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
PtCl2 (10)
PtBr2 (10)
PtI2 (10)
K2PtCl4 (10)
PtCl4 (10)
PtI4 (5)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
anisole
anisole
anisole
anisole
anisole
anisole
anisole
anisole
anisole
CH3NO2
DCE
1,4-dioxane
toluene
chlorobenzene 120
MeCN
anisole
anisole
anisole
80
13
17
NR
NR
15
Entry
Substrate
t
Product
Yield
[%][b]
62
41
49
35
33
45
120
120
80
80
120
80
80
80
120
120
120
120
R1
Ph
R2
H
H
H
H
H
H
H
H
H
[h]
15
12
24
12
33
45
13
16
30
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2 f
2g
2h
2i
p-CH3C6H4
p-ClC6H4
p-BrC6H4
m-CH3C6H4
m-ClC6H4
m-BrC6H4
Bn
31
AuCl3 (5)
AuCl (5)
NR
NR
NR
NR
NR
NR
16
25
trace
26
9
PdN
63
10
11
12
13
14
15
16
17
18
19
20
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
PtI4 (5)
37[c]
32
isopropyl
10
11
11
11
58
45
120
120
120
120
120
120
CuBr2(5)
AgOTf(5)
BQ(5)
Fe
Fe
decomp.
38[c]
62[d]
55[e]
Ph
Ph
4-CH3
2k
G
(2.5) anisole
anisole
AHCTUNGTRENNUNG
[a] The reaction was conducted by using 1a (0.1 mmol), catalyst (5–
10 mol%), 5 ꢀ MS (50 mg) in solvent (3 mL) under an O2 atmosphere
for 18 h. NR=no reaction. [b] Yield of isolated product. [c] BQ=1,4-
benzoquinone. [d] For 15 h. [e] For 13 h.
12
13
14
20
16
13
46
4-Cl
2m
40[c]
47
(55%, 13 h; entry 20). Thus the use of 5 mol% PtI4 and
2.5 mol% FeACHTUNGTRENNUNG(acac)3 with 5 ꢀ MS in dry anisole at 1208C
under an oxygen atmosphere were considered as the optimal
reaction conditions.
Encouraged by these results, and aiming to obtain a varie-
ty of spirobenzo[h]chromanones, we next examined the
scope of this catalytic process. We firstly tested substituents
on the oxirane ring. It was found that both aryl and alkyl
groups were tolerated. When R1 was an aryl group, the elec-
tron-withdrawing substituent on the para- or meta-position
of aryl group always gave better results than electron-donat-
ing substituent, which might be ascribed to the oxonium ion
intermediate (Table 2, entries 2–7). Not surprisingly, when
R1 was an aliphatic group, such as benzyl or isopropyl
group, the reaction did not proceed well, and the corre-
sponding products were formed in 37 and 32% yields, re-
spectively (Table 2, entries 8 and 9). Then substituents on
phenyl ring of alkyne moiety were also tested. When R2 was
a meta-methyl group, excellent regioselectivity was observed
and only less steric hindrance product 2j was obtained in
58% yield (Table 2, entry 10). Other substituted aryl groups
were also found to be compatible with this reaction
(Table 2, entries 11–13). Methyl-substituted compound 1n
reacted smoothly to afford spiropyranone 2n in 47% yield.
The ring size was then studied. An eight-membered ring
substrate gave a 61% yield of corresponding product after
11 h (Table 2, entry 15). To our disappointment, it was found
15
11
61
[a] The reaction was carried out using
5 mol%), Fe(acac)3 (0.9 mg, 2.5 mol%), 5 ꢀ MS (50 mg) in anisole
(3 mL) under O2 atmosphere. [b] Yield of isolated product. [c] Reaction
run at 1308C.
1 (0.1 mmol), PtI4 (3.5 mg,
AHCTUNGTRENNUNG
that five- and seven-membered-ring substrates decomposed
under this condition.
As shown in Scheme 2, when methyl-substituted com-
pound 1p was subjected to the reaction conditions, we ob-
tained a single isomer of spirobenzo[h]chromanone 2p in
33% yield.[13] It appears that this platinum-catalyzed tandem
rearrangement is stereospecific. Our proposed product for-
mation is consistent with the rearrangement proceeding via
the oxonium ion intermediate B having an axial methyl
group.
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Economical Route to Spirobenzo[h]chromanones
COMMUNICATION
Based on the above results, a plausible mechanism is pro-
posed as shown in Scheme 5. Coordination between the p-
acidic Pt catalyst and epoxy alkynyl allyl ether 1 would lead
to complex A. An intramolecular nucleophilic attack of ep-
oxide on the triple bond would give oxonium ion intermedi-
ate B, which triggers a [1,2]-migration of an adjacent alkyl
group to give another oxonium ion intermediate C. An allyl
ꢀ
cation could be trapped by C Pt bond to form product 4
and intermediate III directly. However, the mechanistic
study in Scheme 5 does not support this pathway. On the
other hand, when the allyl cation is trapped by an intramo-
lecular enol ether group, intermediate D could be formed.
When R2 is an aryl group, intramolecular ligand-exchange
of intermediate D-1 would give compound E. The electron-
poor aryl group should not be favorable in this step and
thus gives an inferior result (Table 2, entry 13). A Heck-type
cyclization of compound E will afford intermediate F. b-Hy-
dride elimination[15] of intermediate F gives compound G,
which isomerizes to the desired product 2. The [Pt]-H spe-
Scheme 2. Stereospecificity of the platinum-catalyzed tandem process.
When the R2 group was an aliphatic group, the allyl-shift
products 4 were obtained in the presence of 5 mol% PtI4.
Addition of FeACHTUNGTRENNUNG(acac)3, 5 ꢀ MS and oxygen did not improve
the result. With the growth of chain length, the yields in-
creased from 35 to 68% (Scheme 3, products 4a and 4b).
cies could be oxidized by O2/FeACHTNUGRTENUNG(acac)3 system to regenerate
the PtIV catalyst.[16] On the other hand, the protodemetalla-
tion of intermediate F will give compound H, which could
also be oxidized to the desired product 2. However, reaction
of 1a under 5 mol% PtI4, 2.5 mol% FeACHTNUGTRNEUNG(acac)3, Ar, 5 ꢀ MS
at 1208C or 5 mol% PtI4, Ar, 5 ꢀ MS at 1208C did not give
any compound I, and only low yield of 2a was obtained.
Therefore, we favor the b-hydride elimination pathway in
this step.
Scheme 3. Platinum-catalyzed tandem approach to spiropyranones.[13]
When R2 is an alkyl or alkenyl group, the two groups are
not nucleophilic enough to undergo an intramolecular
ligand-exchange. Compound D-2 undergoes an intermolecu-
lar ligand exchange to give product 4 and regenerate the
platinum catalyst. No oxidant is needed here as shown by
results in Scheme 3.
When a double bond was introduced instead of an aryl
group, still only the allyl-shift product 4c was obtained in
61% yield.
Because both alkyl and alkenyl substituents at R2 position
gave allyl shift products 4, it is possible that this compound
might be an intermediate in the cascade reaction 1!2. To
test this hypothesis, a possible intermediate III was prepared
in two steps (see Scheme 4).[14] When compound III was sub-
jected to the standard reaction conditions, the 1,3-hydride
migration product IV was obtained in 85% yield after 12 h
and no 2a was observed. This result indicates that the
tandem reaction 1!2 does not pass through an intermediate
III.
In summary, we have developed a new cascade reaction
for the construction of spirobenzo[h]chromanone and spiro-
pyranone derivatives that are not easily available through
traditional methods. The approach utilizes a platinum-cata-
lyzed tandem cyclization/ACHTNUTRGNE[NUG 1,2]-alkyl migration/allyl shift, fol-
lowed by an aromatic cyclization reaction. The reaction pro-
vides good substrate generality, low catalyst loading, and op-
erational simplicity. In this reaction, the construction of ad-
jacent multiple stereocenters with a new quaternary carbon
atom was achieved. Further exploration of the reaction
scope and mechanism is under-
way.
Experimental Section
Typical procedure for the platinum-
catalyzed synthesis of 2a–2p: Epoxy
alkynyl allyl ether 1a–1p (0.10 mmol),
PtI4 (3.5 mg, 5 mol%), FeACHTUNGTRENNUNG(acac)3
(0.9 mg, 2.5 mol%), and powdered
5 ꢀ molecular sieves (50 mg) were
added to a test tube. The test tube was
purged under vacuum and then re-
Scheme 4. Experimental confirmation of the reaction mechanism.
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Typical procedure for the platinum-catalyzed synthesis of 4a–4c: Epoxy
alkynyl allyl ether 3a–3c (0.10 mmol) and PtI4 (3.5 mg, 5 mol%) were
added to a test tube. The test tube was purged under vacuum and then
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Acknowledgements
Financial support from National Science Foundation NSF 21072080), Na-
tional Basic Research Program of China (973 Program) 2010CB833203
and the “111” program of MOE are greatly acknowledged.
Keywords: cascade reaction · cyclization · platinum · semi-
pinacol rearrangement · spirobenzo[h]chromanone
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Economical Route to Spirobenzo[h]chromanones
COMMUNICATION
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Received: January 12, 2012
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Y. M. Liang et al.
Cascade Reactions
Y.-F. Yang, X.-Z. Shu, J.-Y. Luo, S. Ali,
Y.-M. Liang* .................. &&&& — &&&&
Platinum-Catalyzed Cyclization/ACTHNUTRGENUG[N 1,2]-
In tandem: A PtI4-catalyzed tandem
reaction of epoxy alkynyl allyl ethers
involving cyclization, [1,2]-alkyl migra-
tion, O!C allyl shift, aromatic cycliza-
tion sequence has been achieved to
synthesize spirobenzo[h]chromanones.
The reaction simultaneously forms two
adjacent stereocenters, one of which is
a quaternary carbon atom (see
scheme).
Alkyl Migration/Allyl Shift/Cyclization
Cascade of Epoxy Alkynyl Allyl
Ethers: A Step-Economical Route to
Spirobenzo[h]chromanones
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www.chemeurj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!