Gold-catalyzed [4+3]-Annulations of Benzopyriliums with Vinyldiazo Carbonyls to Form Bicyclic Heptatriene Rings with Skeletal Rearrangement

Article abstract of DOI:10.1002/adsc.202000292

We report gold catalyzed [4+3]-annulations between benzopyriliums and 3-alkyl-2-diazo-3-vinyl carbonyls, yielding 7H-benzo[7]annulene products efficiently. Notably, the carbon skeletons of resulting 7H-benzo[7]annulenes are structurally rearranged, accompanied by migrations of their alkyl and ketone motifs. Apart from applicable substrates over a wide scope, these annulatios are applicable to pyriliums and 3-alkyl-2-diazo-3-vinyl esters to increase their reaction significance. We postulate a mechanism involving an initial [4+2]-cycloaddition between benzopyriliums and 3-alkyl-2-diazo-3-vinyl carbonyl species, followed by formation of gold carbenes to induce a ring expansion and group migrations. (Figure presented.).

Full text of DOI:10.1002/adsc.202000292

Title: Gold-catalyzed [4+3]-Annulations of Benzopyriliums with
Vinyldiazo Carbonyls to Form Bicyclic Heptatriene Rings with
Skeletal Rearrangement
Authors: Rai-Shung Liu and Antony Sekar Kulandai Raj
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000292
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10.1002/adsc.202000292
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Gold-catalyzed [4+3]-Annulations of Benzopyriliums with
Vinyldiazo Carbonyls to Form Bicyclic Heptatriene Rings with
Skeletal Rearrangement
Antony Sekar Kulandai Raj and Rai-Shung Liu*
a
Frontier Research Center for Fundamental and Basic Science of Matters, Department of Chemistry, National Tsing-
Hua University, Hsinchu, Taiwan, ROC
e-mail:rsliu@mx.nthu.edu.tw
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. We report gold catalyzed [4+3]-annulations
between benzopyriliums and 3-alkyl-2-diazo-3-vinyl
carbonyls, yielding 7H-benzo[7]annulene products
efficiently. Notably, the carbon skeletons of resulting 7H-
benzo[7]annulenes
are
structurally
rearranged,
accompanied by migrations of their alkyl and ketone
motifs. Apart from applicable substrates over a wide scope,
these annulatios are applicable to pyriliums and 3-alkyl-2-
diazo-3-vinyl esters to increase their reaction significance.
We postulate a mechanism involving an initial [4+2]-
cycloaddition between benzopyriliums and 3-alkyl-2-diazo-
3-vinyl carbonyl species, followed by formation of gold
carbenes to induce a ring expansion and group migrations.
Keywords: Annulations; 2-Alkynylbenzaldehyde;
Benzopyriliums; Rearrangement; Vinyldiazo carbonyls
Stable vinyldiazo species[1-2] are versatile to undergo
various cycloadditions with -bond motifs, forming
carbo- or heterocyclic rings of various sizes.
Vinyldiazo carbonyl species are considered as
electron-rich alkenes to serve as effective
nucleophiles. Such species provide 2- or 3-carbon
building units to undergo cycloadditions with
electrophilic -bond motifs, thus furnishing products
with five- or six-membered rings.[3] Alternatively,
these diazo species can be transformed into
electrophilic vinylmetal carbenes that work as 1- or
3-carbon building units.[4] Benzopyrilium species In-
1 are versatile electrophilic 1,4-carbon dipoles[5] that
undergo [4+2]-cycloadditions with alkynes,[6]
alkenes,[7] furans,[8] aldehydes,[9] benzofurans[10] and
isoxazoles,[11] leading exclusively to the formation of
six-membered rings. Eq 1 shows a typical path for
metal-catalyzed reactions of benzopyriliums In-1
with alkenes.[7] We reported[12] gold-catalyzed
reactions of vinyldiazo esters with benzopyriliums to
yield [4+2]-cycloadducts II, following a typical path
(eq 2). Notably, the use of vinyldiazo ketones in these
reactions surprisingly afforded bicyclic azacyclic
Scheme 1. Reaction modes for benzopyriliums.
products[12] III; vinyldiazo ketones here become 5-
atom building units (3C+2N) in these bicyclic
annulations. We postulate[12] that the conformation of
vinyldiazo ketones retains a U-shape to enable an
initial [5+4]-cycloaddition, followed by an
intramolecular cyclization (eq 2). In that work we
mentioned[12] one specific example of [4+3]-
annulations between benzopyriliums and 2-diazo-3-
methyl-1-phenylbut-3-en-1-one (R2 = Me) to yield
one annulation product (IV) with skeletal
rearrangement (eq 3). We proposed the change of
chemoselectivity
arose
from
an
increased
concentration of sickle-shaped conformation for this
diazo species as the U-shape suffers steric interaction
between alkyl and carbonyl groups. To verify this
hypothesis and to test reaction generality, this work
reports
distinct
[4+3]-annulations
between
1
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10.1002/adsc.202000292
Advanced Synthesis & Catalysis
benzopyriliums and 3-alkyl-2-diazo-3-vinyl ketones,
which are accompanied by alkyl and ketone
migrations. Apart from 3-alkyl-2-diazo-3-vinyl
ketones and benzopyriliums, these new annulations
are extensible to pyriliums and 3-alkyl-2-diazo-3-
vinyl esters, reflecting applicable substrates over a
wide scope.
Table 1 shows the optimizations of reaction
conditions between 2-alkynylbenzaldehyde 1a and 2-
diazo-3-methyl-1-phenylbut-3-en-1-one 2a using
various gold catalysts. Formation of compound 3a
has been mentioned in our previous work using
(PhO)3PAuCl/AgOTf (10 mol %, entry 5).[12] Our
initial test with IPrAuCl/AgSbF6 (10 mol %) in DCM
at 25 °C (18 h) led to formation of 7H-
benzo[7]annulene 3a in 61% (entry 1).
IPrAuCl/AgOTf (10 mol %) increased the yield of
compound 3a to 86% yield (entry 2) whereas
IPrAuCl/AgNTf2 (10 mol %) afforded products in a
complicated mixture (entry 3). Other phosphine gold
species such as LAuCl/AgOTf [L = P(t-Bu)2(o-
biphenyl), P(OPh)3 and PPh3] afforded compound 3a
in 63-77% yields (entries 4-6). For IPrAuCl/AgOTf
(10 mol %), the yield of compound 3a in DCE was
83% (entry 7) whereas MeCN and THF were
ineffective for the reactions (entries 8-9). AgOTf
alone was entirely catalytically inactive (entry 10).
The structure of compound 3a was inferred from X-
ray diffraction of its related 3d[14] (Table 2, entry 3).
We assessed the substrate scope of these reactions
with various 1-acyl-2-alkynylbenzenes 1; the results
are summarized in Table 2. For those aldehyde
substrates 1b-1f bearing 4-substituted phenylalkynes
(R = 4-XC6H4; X = Me, Cl, Br, tert-butyl, and OMe),
their gold-catalyzed reactions gave the desired
products 3b-3f in 83–85% yields (entries 1-5). The
molecular structure of 3d was confirmed with X-ray
diffraction.[14] For 2-thienyl substituted analogue 1g,
its corresponding product 3g formed in 83% yield
The significance of this reaction is to provide an
easy
access
to
6,7,8,9-tetrahydro-5H-
benzo[7]annulene frameworks (V) that are commonly
found as the structural cores of bioactive molecules
V-1 to V-7 (Figure 1). Compounds V-1 and V-2 have
been demonstrated to be potent inhibitors of tubulin
polymerization and of growth of human cancer cell
lines, thus showing anti-mitotic activity.[13a]
Compounds V-3 and V-4 are natural products
isolated from hairy root cultures of Salvia
broussonetii.[13b-c] Compound V-5 is biogenetically
produced on oxidation of pyrogallol; its analogues
compound V-6 is natural pigments.[13d] Compounds
V-7 are potent and specific αv integrin
antagonists.[13d-e]
Figure 1. Representative bioactive molecules.
Table 1. Optimization of the reaction conditions.
Table 2. Scope with various 1-acyl-2-alkynylbenzenes.
Entry Catalyst
Solvent Time Yield
(h)
(%)[b]
[c]
1
2
3
4
IPrAuCl/AgSbF6
IPrAuCl/AgOTf
DCM
DCM
DCM
DCM
18
2
21
4
3
3
61
86
-
63
71
77
83
-
IPrAuCl/AgNTf2
LAuCl/AgOTf [d]
5[12]
6
(PhO)3PAuCl/AgOTf DCM
PPh3AuCl/AgOTf
IPrAuCl/AgOTf
IPrAuCl/AgOTf
IPrAuCl/AgOTf
AgOTf
DCM
DCE
MeCN
THF
7
8
3
18
13
23
9
-
-
10[e]
DCM
[a]
[b]
1a = 0.15M.
Product yields are obtained after
[c]
purification from
a
silica column,
IPr
=
1,3-
L = P(t-
82% of the starting alkynal 1a was
recovered in entry 10, Tf = trifluoromethanesulfonyl.
[d]
[a]
[b]
bis(diisopropylphenyl)imidazol-2-ylidene,
1 = 0.15M.
Product yields are obtained after
[e]
Bu)2(o-biphenyl),
purification from a silica column, Tf = trifluoromethane-
sulfonyl.
2
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10.1002/adsc.202000292
Advanced Synthesis & Catalysis
(entry 6). We prepared also 1-acetyl-2-alkynyl
benzenes 1h (R1 = Me), which proved to be an
applicable substrate, further affording compound 3h
in 81% yields (entry 7). The reactions were also
amenable to alkyl-substituted alkynes 1i-1k (R
=cyclopropyl, n-butyl, and isopropyl), yielding
desired 3i-3k in 81-84% (entries 8-10). We
performed the reactions also the 5-phenyl derivatives
1l-1m (R2 = Me and Cl), affording compounds 3l-3m
in 79% and 76% yields respectively (entries 11-12).
For compounds have substituents at phenyl ring 1n–
1o (R2 = Me and Cl), their resulting product 3n-3o
were obtained in 78% and 80% yields respectively
(entries 13-14). For 4-methoxyphenyl analogue 1p,
its resulting product 3p was obtained in 77% (entry
15). This [4+3]-annulation is extensible also to
methyl and n-propyl), affording compounds 4k and 4l
in 73%-76% yields (entries 11-12).
The scope of these reactions was further expanded
with its applicability to various nonaromatic enynals
5; the results are summarized in Table 4. For those
cyclohexane bridged enynals 5a-5d bearing 4-
substituted phenylalkynes (R = 4-XC6H4; X = H, Me,
Cl and Br), their gold-catalyzed reactions provided
desired products 6a-6d in 73-78% yields (entries 1-4).
For dimethyl cyclohexane analogue 5e, its
corresponding product 6e was formed in 77% yield
(entry 5). We tested the reactions also on a
cycloheptane-bridged enynal 5f, further affording
compound 6f in 71% yield.
Table 4. Scope with various nonaromatic enynals.
dioxolo-substituted
benzene
1q,
generating
compound 3q in 76% yield (entry 16).
We further examined these annulations with various
3-alkyl-2-diazo-3-vinyl carbonyls 2; the results are
summarized in Table 3. We prepared 2-diazo-3-vinyl
species 2b-2f bearing varied 4-phenylketone
substituents (R3 = 4-XC6H4; X = Me, Cl, Br, tert-
butyl and OMe); their resulting products 4a-4e were
obtained in satisfactorily yields 80-84% (entries 1-5).
The molecular structure of 4d was confirmed with X-
ray diffraction.[14] For 2-thienylketone diazo
derivative 2g, its resulting product 4f was obtained in
78% yield (entry 6). The reactions were further
compatible with alkylketone derivatives 2h to 2i (R3
= n-butyl and cyclohexyl), delivering the desired
products 4g-4h in 79-80% yields (entries 7-8). We
tested the reactions also on 3-alkyl-2-diazo-3-vinyl
ketone as in species 2j-2k (R4 = n-Pr and n-Bu),
affording compounds 4i and 4j in 78% and 81%
yields respectively (entries 9-10). This cycloaddition
reaction is extensible to 3-alkyl-2-diazo-3-vinyl esters
2l and 2m bearing various 3-alkyl groups (R4 =
[a]
[b]
5 = 0.15M.
Product yields are obtained after
silica column, Tf =
purification
trifluoromethanesulfonyl.
from
a
As shown in scheme 2, we conducted the reactions
of 2-diazo-3-methyl-1-phenylbut-3-en-1-one 2a with
species 18O-1j bearing 66% 18O-content at its
aldehyde; the corresponding product 18O-3j contained
36% 18O, indicative a partial loss of 18O (eq 4). We
tested the typical reaction also in the presence of
Table 3. Scope with 3-alkyl-2-diazo-3-vinyl carbonyls.
H2 O; the resulting product O18-3j comprised 17%
18
18O content (eq 5). In these two reactions, we
[a]
[b]
1a = 0.15M.
Product yields are obtained after
purification from a silica column, Tf = trifluoromethane-
sulfonyl.
Scheme 2. Isotope labeling experiments.
3
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10.1002/adsc.202000292
Advanced Synthesis & Catalysis
assigned the 18O position to the alkylketone because
its phenyl ketone does not participate in the reaction.
In a separate experiment, we added D2O, which
afforded a product in which CH2 moiety contained no
or one deuterium according to the mass analysis (eq
6).
We also used density functional theory (see
computational details in the SI) to investigate the
effects of 3-alkyl substituents (R = Me versus H) on
the U- versus sickle-shaped conformations (Table 5).
Three different methods (B3LYP, CAM-B3LYP and
M11) all indicate that U-shape is more stable than the
sickle-shape by ~2.0 kcal/mol for R = H; but this
energy differences become smaller (0.4-0.6 kcal/mol)
for R = Me. In other words, sickle conformation is
unlikely to form for unsubstituted 2-diazo-2-vinyl
ketone (R = H) and the concentration of sickle-
shaped conformation will become significant for R =
Me.
Scheme 3. A Plausible reaction mechanism.
In summary, we report gold-catalyzed [4+3]-
annulations between benzopyriliums and 3-alkyl-2-
diazo-3-vinyl
ketones,
yielding
7H-
Table 5. The relative Gibbs free energy for U- and Sickle-
shape. The unit for energy is kcal/mol.
benzo[7]annulenes efficiently. In this process, the
carbon skeletons of the resulting products become
rearranged, together with migrations of alkyl and
ketone groups. Apart from applicable substrates over
a wide scope, these new annulations are applicable to
pyrilium intermediates and 3-alkyl-2-diazo-3-vinyl
esters, furnishing cycloheptatrienes fused with six- or
seven-carbocyclic rings. We performed 18O and H
2
isotopic labeling experiments to elucidate the reaction
mechanism, which involves an initial [4+2]-
cycloadditions of benzopyriliums with 3-alkyl-2-
diazo-3-vinyl ketones, followed by formation of gold
carbenes to induce a ring expansion, further leading
to skeletal rearrangement and substituent migrations.
R = CH3
U-shape
Sickle-shape
B3LYP
CAM-B3LYP
M11
-0.42
-0.59
-0.53
0.00
0.00
0.00
R = H
B3LYP
CAM-B3LYP
M11
-2.02
-2.06
-2.06
0.00
0.00
0.00
Experimental Section
Typical procedure for synthesis of (6-methyl-7H-
benzo[7]annulene-5,8-diyl)bis(phenylmethanone) (3a):
Scheme 3 depicts a mechanism to rationalize
compound 3a resulting from gold-catalyzed
annulations between benzopyriliums and 2-diazo-3-
methyl-1-phenylbut-3-en-1-one 2a. In this process,
gold-containing benzopyriliums A undergo [4+2]-
cycloadditions with 2-diazo-3-methyl-1-phenylbut-3-
en-1-one 2a to yield species B that loses a proton to
yield 1,2-dihydronaphthalene species C. In the
presence of gold catalyst, species C becomes gold
carbenes D to induce an alkene migration to form
tertiary carbocations E to generate species F in which
the C-H proton is highly acidic to induce a 1,2-alkene
migration. A subsequent 1,7-carbonyl migration[15] of
species G forms species H that undergoes a 1,7-
hydrogen migration[16] to yield the observed product
3a. This main path rationalizes well our HC=18O and
D2O labeling experiments in eqs 4 and 6. In eq 5, we
A suspension of IPrAuCl (30 mg, 0.048 mmol) and AgOTf
(12 mg, 0.048 mmol) in dry DCM (1 mL) was fitted with
N2 balloon and the mixture was stirred at 25 °C for 5 min.
To this mixture was added dry DCM (2 mL) solution of 2-
(phenylethynyl)benzaldehyde 1a (100 mg, 0.485 mmol)
and 2-diazo-3-methyl-1-phenylbut-3-en-1-one 2a (108 mg,
0.582 mmol) at 25 °C. The resulting mixture was stirred
for 2 h at 25 °C. The solution was filtered over a short
celite bed and evaporated under reduced pressure. The
residue was purified on a silica gel column using ethyl
acetate/hexane (10:90) as eluent to give compound 3a as
brown oil (151 mg, 0.413 mmol, 86%).
Acknowledgements
We thank the Ministry of Education (MOE 106N506CE1) and the
Ministry of Science and Technology (MOST 107-3017-F-007-
002), Taiwan, for financial support of this work.
18
note also that the H2 O also contributes to the
oxygen source of the alkyl ketone of compound 18O-
3j. Accordingly, we postulate that H2O also attacks
oxonium species B to form ketal intermediates I, then
species J; this path also yields intermediate D to
furnish a catalytic circle.
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Advanced Synthesis & Catalysis
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10.1002/adsc.202000292
Advanced Synthesis & Catalysis
UPDATE
Gold-catalyzed [4+3]-Annulations of
Benzopyriliums with Vinyldiazo Carbonyls to
Form Bicyclic Heptatriene Rings with Skeletal
Rearrangement.
Adv. Synth. Catal. Year, Volume, Page – Page
Antony Sekar Kulandai Raj and Rai-Shung Liu*
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