A facile access to substituted benzo[a]fluorenes from o- alkynylbenzaldehydes via in situ formed acetals

Article abstract of DOI:10.1039/c4cc03934c

In situ formed acetal changes the course of Br?nsted acid-catalyzed reaction of ortho-alkynylbenzaldehydes with arylalkynes altogether. By utilizing this, an efficient domino approach for the regioselective synthesis of substituted benzo[a]fluorenes has been developed under mild reaction conditions. In situ formed acetal facilitates the intermolecular heteroalkyne metathesis and subsequent trans to cis isomerization of a double bond to effect the intramolecular annulation.

Full text of DOI:10.1039/c4cc03934c

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A facile access to substituted benzo[a]fluorenes from o-
alkynylbenzaldehydes via in situ formed acetals
Cite this: DOI: 10.1039/x0xx00000x
Seetharaman Manojveer and Rengarajan Balamurugan*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
either BA along with LA catalyst or BA alone (Scheme 1, eq i).5 In
In situ formed acetal changes the course of Brønsted acid-
catalyzed reaction of ortho-alkynylbenzaldehydes with
arylalkynes altogether. By utilizing this, an efficient domino
approach for the regioselective synthesis of substituted
benzo[a]fluorenes has been developed under mild reaction
conditions. In situ formed acetal facilitates the
intermolecular heteroalkyne metathesis and subsequent
trans to cis isomerization of double bond to effect the
intramolecular annulation.
our endeavor to develop reactions using in situ generated acetals,6
we have found that reaction of oꢀalkynylbenzaldehydes and alkynes
in the presence of trimethyl orthoformate and catalytic Brønsted acid
results in benzo[a]fluorene derivatives efficiently (Scheme 1, eq ii).
At the outset, oꢀalkynylbenzaldehyde 1a was reacted with 1.5 equiv
of phenylacetylene 2a in the presence of 2.0 equiv of trimethyl
orthoformate and 50 mol% of TfOH in dichloromethane solvent at
room temperature. Interestingly, the reaction completed in 30 min
and resulted in benzo[a]fluorene 6a in 72% yield (Table 1, entry 1).
Impressed by this result and knowing the importance of fluorene
derivatives, our attention then turned to improve the yield of the
product 6a by changing various parameters. The results are
summarized in the Table 1. Unfortunately, there were no reactions
with trifluoroacetic acid and pꢀtoluenesulfonic acid even when the
reactions were allowed to stir for 24 h at room temperature in
dichloromethane solvent (Table 1, entries 2 and 3). With super acid
Polycyclic aromatic hydrocarbons (PAHs) such as naphthalenes,
fluorenes, antharacenes, phanthrenes etc. play an important role in
the fields of medicinal as well as materials chemistry due to their
unique properties.1 For their synthesis in a regioselective manner,
cyclization reactions of type [4+2], [2+2], [2+2+2] and domino
reactions have been developed.2 Limited methods are only available
in the literature for the construction of fluorene3 framework in
3i, 3j
general and benzo[a]fluorenes in particular.3g,
Among PAHs,
.
HSbF6 6H2O as catalyst the product 6a was isolated in 70% yield
fluorene derivatives have been widely utilized in the field of material
science in developing optical materials.4 Hence, development of
simple method for the construction of fluorenes from readily
available starting materials is highly desirable.
(Table 1, entry 4). Since the reaction required strong Brønsted acids
like TfOH, we then focused on finding the better solvent for TfOHꢀ
catalyzed reaction. Only moderate yields were obtained when the
reaction was carried out separately in dichloroethane, nitromethane
and toluene (Table 1, entries 5ꢀ7). In dioxane, as solvent, the
expected product did not form and some unidentified products were
obtained. There was no reaction in methanol even after 24 h (Table
1, entry 9). Interestingly, clean reaction was observed when the
reaction was performed in acetonitrile for 4 h the product was
isolated in 86% yield (Table 1, entry 10). For further optimization,
acetonitrile was used as solvent. The amount of TfOH catalyst was
reduced to 20 mol% and found that there was hardly any appreciable
reduction in the product yield (85%) (Table 1, entry 11). Yield of 6a
slightly dropped to 80% when the reaction was conducted with 10
mol% TfOH (Table1, entry 12). There was no change in the yield of
the product when 2a was taken in 1.2 equiv (Table 1, entry 13).
Scheme 1. Reaction of oꢀalkynylbenzaldehydes with alkynes under
Lewis/Brønsted acid conditions
Finally,
a control reaction was conducted without trimethyl
Yamamoto and coꢀworkers studied extensively the cyclization
orthoformate using TfOH in dichloromethane solvent. Under this
condition, the reaction resulted in 36% of 2ꢀphenylnaphthalene
(Scheme 3, eq. 1). This observation clearly indicates that the
benzo[a]fluorene formation does not take place through the
intermediate proposed in the mechanism by Yamamoto group for
naphthalene synthesis and the importance of trimethyl orthoformate
involving
oꢀalkynylbenzaldehydes
and
alkynes
under
Lewis/Brønsted (LA/BA) acidꢀcatalyzed conditions for the synthesis
of naphthalenes via [4+2] benzannulation pathway. In this reaction
naphthyl ketone is the major product with LA catalyst alone whereas
debenzoylated naphthalene was the major product in the presence of
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for the formation benzo[a]fluorene. The in situ formed acetal would comparatively shorter reaction times than that in acetonitrile.10
generate more electrophilic oxonium ion species in the presence of Substrates with substituents like F, Cl, Br, OMe, acetal, cyano and
BA catalyst which undergoes fast [2+2] cycloaddition reaction than CF3 tolerated and corresponding products were obtained in good to
their aldehyde form with alkyne to afford enone.7 Saá group has excellent yields. The benzo[a]fluorene with halo substitution (6d, 6e,
utilized intramolecular HBF4ꢀpromoted alkyne/acetal coupling in the 6i and 6n) could be obtained using this method and the products can,
synthesis of hydroazulenones.8 Luo and coꢀworkers have utilized its in principle, be functionalized further by transition metalꢀcatalyzed
intermolecular version catalyzed by In(OTf)3/PhCOOH to synthesize coupling reactions. This method is applicable to aliphatic
indanones.9
Table 1. Optimization studya
alkynylaldehydes as well and products 6j and 6k were obtained in
good yields. Only one of the possible regioisomeric products (6v-6x)
were obtained in good yields with internal unsymmetrical
diarylalkynes. To our delight, pharmaceutically important group
such as CF3 could be incorporated in benzo[a]fluorene using this
methodology (6w). Unfortunately, this transformation is not
applicable when R1 is aliphatic group. Only corresponding acetal
was obtained when oꢀalkynylbenzaldehyde with R1 = C6H13 was
treated with phenylacetylene under the reaction condition. In
addition, there was no reaction when aliphatic terminal alkynes were
used as substrates as [2+2] cycloaddition is not possible.
Yield (%)b
Entry
Solvent
CH Cl
Catalyst
Cat.
(mol %)
Time
(hours)
6a
7
1
2
TfOH
PTSA
TFA
50
50
50
50
50
50
50
50
50
50
20
10
20
5
0.5
24.0
24.0
12.0
0.5
72
2
2
2
2
2
CH Cl
NR
NR
2
3
CH Cl
2
4
CH Cl
HSbF6.6H2O
TfOH
70
57
54
48
ꢀꢀꢀ
2
5
C2H4Cl2
CH NO
6
TfOH
0.5
3
2
7
Toluene
Dioxane
TfOH
0.5
8
TfOH
5.0
ꢀꢀꢀ
9
CH OH
TfOH
24.0
4.0
NR
3
10
11
12
13
14
CH CN
TfOH
86
85
80
3
CH CN
TfOH
4.0
3
CH CN
TfOH
4.0
3
c
CH CN
TfOH
4.0
85
ꢀꢀꢀ
3
d
AuCl
12.0
94
99
NR
CH CN
3
3
d
15
16
17
18
Cu(OTf)
5
5
5
5
12.0
12.0
12.0
24.0
ꢀꢀꢀ
CH CN
2
3
d
Fe(OTf)
CH CN
2
3
d
Yb(OTf)
NR
NR
CH CN
3
3
d
AgSbF
CH CN
6
3
a
All reactions were carried out using 1a (1.0 equiv) 2a (1.5 equiv) and
b
trimethyl orthoformate (2.0 equiv) at rt unless otherwise stated. isolated
yield. c 1.2 equiv of 2a was used. d The reaction was performed under reflux
condition. NR: no reaction
Interestingly, no naphthalene or fluorene derivatives were obtained
when the reaction was performed under different Lewis acids such as
AuCl3, Cu(OTf)2, Fe(OTf)2, Yb(OTf)3 and AgSbF6 (Table 1, entries
14ꢀ18). Under these conditions only acetal formation was observed
at room temperature, which did not give the desired product at reflux
conditions. In the reactions employing Cu(OTf)2 and AuCl3
catalysts, the acetal 7 was isolated in 99% and 94% respectively. On
the other hand, the got acetal converted back to aldehyde with
Fe(OTf)2 and Yb(OTf)3 under reflux condition. No reaction was
observed with AgSbF6 catalyst even under both reflux conditions.
Fig 1. Substrate scope
A plausible mechanism is illustrated in Scheme 2. The acetal I is
formed from aldehyde 1 in the presence of trimethyl orthoformate
and TfOH. Intermolecular alkyneꢀacetal coupling ([2+2]
cycloaddition) would take place between arylalkyne and
oxocarbenium ion intermediate III generated from I to give oxetene
intermediate IV. Outcome of the reaction of oꢀalkynylbenzaldehyde
The substrate scope was then studied and the results are shown in
Figure 1. This cascade reaction worked with both terminal as well as
internal aromatic alkynes. In general, the reaction completed in 4 h
with terminal alkynes and took more time with internal alkynes. It
has to be mentioned that dichloromethane is better solvent for
reactions involving diarylalkynes and high yields were obtained in
2 | J. Name., 2012, 00, 1-3
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with R1
=
C6H13 suggests that the triple bond of oꢀ and its conversion to oxocarbenium intermediate V to effect the trans
alkynylbenzaldehyde having aryl as R1 group might participate at the to cis isomerization. In order to confirm the formation of carbocation
early stage of the reaction.12 With R′ = Ar, the formed acetal can IX during the reaction, 5.0 equiv of ethanol was added intentionally
attack the triple bond under the influence of acid to give intermediate as external nucleophile to trap the carbocation intermediate IX.
II which can result in the formation of oxocarbenium ion III Indeed, ethanol quenched product 9a formed along with 6a in the
required for the [2+2] cycloaddition. The oxetene formation is ratio of 1:0.44 as an inseparable mixture (eq 6). Reaction of
regioselective and the oxygen of oxocarbenium will be attached to phenylacetylene alone under the reaction condition in the absence of
the alkyne carbon which is attached to electron rich aryl ring.9 This oꢀalkynylbenzaldehyde did not give any product even after 24 h.
is the reason for the formation of benzo[a]fluorene derivatives 6q-6t These control experiments demonstrate that in situ formed acetal not
from internal alkynes having both aryl and alkyl substituents as the only helps for [2+2] cycloaddition reaction but also assists the trans
final cyclization to result in benzo[a]fluorene derivative is possible to cis isomerization13 to undergo annulation in the present
only with this regioselectivity. Again it is the reason for the transformation. The isomerization is favored as the cis product will
regioselective formation of products 6v-6x where diarylalkynes cyclize to result in more stable benzo[a]fluorene system.
having aryl rings with different electronic properties were used.
Scheme 3. Control experiments
Then, [2+2] cycloreversion of IV will afford oxocarbenium ion
intermediate V which will be in equilibrium with its cis isomer VI.13
Subsequently, annulation reaction is initiated by intramolecular
attack of alkyne on activated carbonyl group.3g, 3i The formed vinyl
carbocation VII will be trapped by the aryl group by aromatic
electrophilic substitution reaction to give VIII. Finally, the
benzo[a]fluorene 6 is formed by isomerization of VIII through
carbocation intermediate IX in the presence of TfOH. The structure
of 6f was confirmed by COSY and NOESY experiments. The other
possible regioisomer which can form in the second annulation step
(5ꢀmembered ring formation) was not detected. The structure of 6u
was confirmed by singleꢀcrystal xꢀray analysis.14
Scheme 2. Plausible mechanism
To summarize, a simple and efficient method has been developed for
the regioselective synthesis of substituted benzo[a]fluorenes from
readily accessible starting materials under mild reaction conditions.
This transformation takes place via [2+2] cycloaddition/alkyne
acetal coupling of in situ generated acetal followed by double bond
isomerization which facilitates the intramolecular cyclization to
construct benzo[a]fluorene framework. Thus the in situ formed
acetal can advantageously be used to develop new reactions leading
to interesting molecular frameworks.
Control experiments were carried out to check the proposed
mechanistic pathway and intermediates (Scheme 3). Acetal 7 was
prepared separately and reacted with phenylacetylene (1.2 equiv)
and 20 mol% TfOH in acetonitrile solvent. It resulted in only 48% of
Acknowledgements
benzo[a]fluorene derivative 6a and 21% of 1a was recovered (eq 2).
In another experiment, acetal 7 was subjected to standard reaction
conditions, which yielded 87% of 6a (eq 3). Hence, in the presence
of trimethyl orthoformate, the concentration of acetal will be kept
high as aldehyde formed by deprotection under acidic condition will
be converted back to acetal. To evaluate the formation of
intermediate VI, the chalcone 8 was prepared and treated with TfOH
and HC(OMe)3 in dichloromethane. Gratifyingly, the product 6a was
isolated in 91% yield after 24 h (eq 4) which is quite higher than that
took for direct conversion of 1a into 6a. The same reaction resulted
in 69% of 6a when the reaction was carried out in acetonitrile for 24
h and 20% of 7 was recovered. No reaction was observed in the
experiment in which 8 was treated with TfOH catalyst alone in
dichloromethane (eq 5). Shorter reaction time required for the direct
transformation of 1a into 6a (Fig. 1) than 8 into 6a might be due to
ease of intermediate V to isomerize to cis form than the trans enone
8. Further the trans enone 8 requires time for the formation of acetal
The authors thank Department of Science and Technology (DST),
India for the financial support. SM is grateful to CSIR, India for a
fellowship.
Notes and references
a School of Chemistry, University of Hyderabad, Gachibowli,
Hyderabad, India. Eꢀmail: rbsc@uohyd.ernet.in;
Fax: 9140 2301 2460; Tel: 91 40 2313 4817
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/c000000x/
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DOI: 10.1039/C4CC03934C
TOC
Brønsted acid catalysed reaction of oꢀalkynylbenzaldehydes with
arylalkynes in the presence of trimethyl orthoformate takes a
different reaction pathway and results in benzo[a]fluorenes.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5