Indium-catalyzed cycloisomerizations of cyclopropene-3,3-dicarbonyl compounds: Efficient access to benzo-fused heteroaromatics and heterobiaryls

Article abstract of DOI:10.1002/anie.201107717

Rapid access: The title reaction generates highly functionalized benzo-annulated heterocycles and heterobiaryls. This method is amenable to a diverse array of heteroaromatics, thus allowing rapid access to benzothiophenes, benzofurans, indoles, indolizine

Full text of DOI:10.1002/anie.201107717

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DOI: 10.1002/anie.201107717
Diazo Compounds
Indium-Catalyzed Cycloisomerizations of Cyclopropene-3,3-Dicar-
bonyl Compounds: Efficient Access to Benzo-Fused Heteroaromatics
and Heterobiaryls**
Lien H. Phun, Joel Aponte-Guzman, and Stefan France*
Benzo-fused heteroaromatics and heterobiaryls[1] are
common structural motifs termed as “privileged”[2] by medic-
inal chemists because of their presence in a diverse range of
pharmaceutically relevant small molecules and bioactive
natural products. Both have been used as probes to explore
biochemical pathways and to understand biological function.
Because of their interesting photophysical properties, each
has found application in materials science as organic light-
emitting diodes (OLEDs) and organic photovoltaics
(OPVCs).[3] Furthermore, they both serve as ligands for
many metal complexes. Given this diverse utility, the devel-
opment of efficient methods for the synthesis of benzo-fused
heteroaromatics and heterobiaryls has been an important
goal for synthetic chemists. Whereas alkynes have been
widely used as reactive units to generate benzo-fused
substrates,[4] cyclopropenes have received limited attention
in this area despite their unique reactivity that results from
their substantial ring strain.
As part of our ongoing efforts toward the reactions of
small strained rings in the presence of Lewis acids,[11] we were
keenly interested in utilizing cyclopropene-3,3-dicarbonyl
compounds as templates for intramolecular cyclizations.
Although this class of cyclopropenes has not been employed
as a substrate for cycloisomerizations, it has been shown to
undergo ring-opening reactions in the presence of halides[12]
or organometallic reagents.[13] These reagents serve as nucleo-
philes to promote an SN2-like ring-opening of the cyclo-
propene. However, to the best of our knowledge, ring-
opening of cyclopropene-3,3-dicarbonyl compounds pro-
moted by Lewis acids have not been disclosed. Herein, we
report the first example of a Lewis acid catalyzed cyclo-
isomerization of cyclopropene-3,3-dicarbonyl compounds to
give a wide array of benzo-fused heteroaromatics and
heterobiaryls (Scheme 1).
Cyclopropenes readily participate in an assortment of
additions, substitutions, cycloadditions, and metal-promoted
transformations.[5] In particular, the metal-catalyzed rear-
rangement, or cycloisomerization, of 2-acyl- and 2-iminocy-
clopropenes to afford heterocyclic compounds has recently
garnered a lot of attention from both a synthetic and
mechanistic viewpoint. This method has become a powerful
way to prepare furans,[6] indolizines,[7,8] imidazopyridines,[7]
and pyrrolo[2,1-b]oxazoles.[8] In contrast, only few examples
of the analogous metal-catalyzed cycloisomerizations to form
benzenoid rings have been disclosed. In a seminal report, Shi
and co-workers demonstrated that arylvinylcyclopropenes
rearrange in the presence of Lewis acids to give functional-
ized naphthalenes.[9] While this example (and subsequent
publications)[10] has provided invaluable insight into the
potential of using cyclopropenes as substrates for benzannu-
lation reactions, only carbocyclic products were generated.
Scheme 1. In(OTf)3-catalyzed cycloisomerizations of 3,3-dicarbonyl
cyclopropene substrates. Tf=trifluoromethanesulfonyl.
Cyclopropene-3,3-dicarbonyl substrates (3) were readily
synthesized by using the general protocol disclosed by
Gonzales-Bobes et al.[14] in which the corresponding a-
diazo-b-keto esters 2 are added to solutions of alkynes in
the presence of Duboisꢀ catalyst,[15] [Rh2esp2] (dirhodium
a,a,a’,a’-tetramethyl-1,3-benzene dipropanoate; Scheme 2).
[*] L. H. Phun, J. Aponte-Guzman, Prof. S. France
School of Chemistry and Biochemistry
Georgia Institute of Technology
901 Atlantic Drive, Atlanta, GA 30332 (USA)
E-mail: stefan.france@chemistry.gatech.edu
Homepage: http://www.chemistry.gatech.edu/faculty/France
Scheme 2. Rhodium(II)-catalyzed cyclopropenation.
[**] S.F. gratefully acknowledges financial support from the National
Science Foundation (CAREER Award CHE-1056687). L.H.P. thanks
the Georgia Tech Center for Drug Design, Development and
Delivery (CD4) for a GAANN fellowship. J.A.-G. thanks Georgia Tech
for a Presidential Fellowship.
However, while the expected cyclopropenes 3 were observed
as the major products in all cases, varying amounts of the
cycloisomerization products 4 (for structure see Table 1) were
also obtained.[16] Attempts to reverse the product ratio by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107717.
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increasing the catalyst loading (2.5 to 5.0 mol%) only
provided a ratio of approximately 1.4:1 in favor of the
cycloisomerization product.[17] Encouraged by the ability of
Gevorgyan and co-workers to selectively modulate product
ratios in his reported cycloisomerizations of 3-iminocyclo-
propenes,[7] we examined other rhodium(II) dimers as cata-
lysts. However, in the end, these efforts failed to give
appreciable amounts of the desired cycloisomerization prod-
ucts.
a strongly electron-deactivating substituent is employed on
the aryl ring, such as the 4-CF3 group in 3d, no cyclo-
isomerization product is obtained (Table 1, entry 4). Similarly,
having an n-alkyl substituent on the cyclopropene, as in 3e,
provided no benzannulated product (Table 1, entry 5).
Attempts to promote cyclization by increasing the temper-
ature (1,2-dichloroethane or toluene at reflux) or catalyst
loading (up to 30 mol%) failed to yield any appreciable
conversion.
Consequently, we focused our efforts on probing the
cycloisomerizations of the cyclopropenes in the presence of
Lewis acids. Given our success in the homo-Nazarov cycliza-
tions of heteroaryl cyclopropyl ketones, In(OTf)3 was the first
Lewis acid screened. To optimize the reaction conditions, we
chose to utilize the cyclopropene-3,3-dicarbonyl compound
3a, derived from 2-thiophene, as the model substrate.[18] To
our delight, In(OTf)3 effectively catalyzed the cycloisomeri-
zation of the cyclopropene 3a to give the anticipated
heterobiaryl benzo[b]thiophene derivative 4a in 86% yield
(Table 1, entry 1).[19]
This lack of reactivity can be explained in terms of
localized charge stabilization. If we envision the extreme case
of the actual formation of a vinylic cation and consider
reaction enthalpies (energies of stabilization), a phenyl sub-
stituent provides approximately 25 kcalmolÀ1 of increased
stability, over a simple n-alkyl substituent, on the positively
charged vinylic position.[20] While we do not believe a formal
vinyl cation is being generated, we argue that there must be
a significant partial positive charge at C1 for cycloisomeriza-
tion to occur. Hence, aryl substituents (ones that can stabilize
the charge) will promote the reactions, whereas simple n-alkyl
substituents will not. To probe this hypothesis, we synthesized
the cyclopropene 3 f, derived from propargyltrimethylsilane.
Localization of positive charge at C1 should be favorable, as it
will be stabilized by the b-silyl effect. We were pleased to find
that although the cycloisomerization did not proceed at room
temperature, upon heating 3 f in 1,2-dichloroethane, the
desired product 4 f was obtained in 68% yield (Table 1,
entry 6).
Table 1: In(OTf)3-catalyzed cyclopropene cycloisomerizations.[a]
3
4
t
[h]
Yield
[%][b]
1
2
3
3a
3b
3c
4a
4b
4c
9
10
10
86
86
86
Lastly, when the cyclopropene 3g, derived from 2-ethynyl
thiophene was employed, the anticipated heterobiaryl thio-
phene derivative 4g was afforded in 83% yield (Table 1,
entry 7).
After establishing the reactivity of the cyclopropenes
derived from 2-thiophene, the cycloisomerization reactions
with cyclopropenes containing different heteroaromatic sub-
stituents were subsequently investigated (Table 2). Given that
the 2-thiophene substrate 3a served as our model substrate,
other thiophene derivatives were examined first. The 3-
thienyl cyclopropene 3h readily cyclized to give the isomeric
benzo[b]thiophene 4h in 82% yield (Table 2, entry 1). When
the 2-bromo-3-thienyl cyclopropene 3i was employed, the
anticipated benzo[c]thiophene (isobenzothiophene) product
4i was obtained in 55% yield (Table 2, entry 2). This
represents an important result given that benzo[c]thiophene
analogues have been extensively utilized in the field of
organic materials as components in OLEDs, as well as
photovoltaics.[21]
[c]
4
5
3d
3e
–
–
24
24
–
[c]
–
6
7
3 f
4 f
7.5
68[d]
3g
4g
7
83
Given the established importance of the benzofuran
moiety in medicinal chemistry, we were interested in examin-
ing the reactivity of various furanyl derivatives in the
cycloisomerization reaction. Both 2-furyl and 3-furyl cyclo-
propenes 3j and 3k provided the functionalized benzo[b]fur-
ans 4j and 4k in 78 and 82% yield, respectively (Table 2,
entries 3 and 4). Similarly, the 2-benzofuranyl cyclopropene
3l readily generated dibenzofuran 4l in 69% yield (Table 2,
entry 5). It is important to note that the dibenzofuran moiety
is present in a number of natural products.
[a] Reactions run with cyclopropene (1 equiv) and In(OTf)3 (5 mol%) in
CH2Cl2 at 258C. [b] Yield of product isolated after column chromatog-
raphy. [c] No reaction after 24 h. [d] Performed in 1,2-dichloroethane at
reflux. TMS=trimethylsilyl.
Next, the effects of the cyclopropene substituent were
studied by changing the alkyne used during the cycloprope-
nation. Both the phenyl and 4-bromophenyl cyclopropenes
3b and 3c afforded their respective cycloisomerization
products in 86% yield (Table 1, entries 2 and 3). When
Along the same lines, N-methylpyrrole was employed as
the heteroaryl substituent. Upon subjecting cyclopropene 3m
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Table 2: Effects of changing the heteroaryl moiety.[a]
amides (Table 3). We anticipated that these groups would
have some effects on the reaction based on changes in the
Lewis basicity of the carbonyl groups. During our attempts to
synthesize cyclopropene-3,3-diketones (containing a ketone
in place of an ester), we obtained an unforeseen result.
Treatment of the a-diazo 1,3-diketones 5a and 5b (derived
from 2-thiophene and 3-furan, respectively) with [Rh2esp2]
and 4-ethynylanisole, only provided the benzo[b]thiophene
6a and benzo[b]furan 6b in 62 and 31% yield, respectively
(Table 3, entries 1 and 2).[23]
Similarly, when the N-pyrrolyl a-diazo b-amidoester 7a
was used,[24] only the cycloisomerization product 8a, a func-
tionalized indolizine, was generated in 51% yield (Table 3,
entry 3). Surprisingly, when the N-indolyl a-diazo b-ami-
doester 7b was employed, only the cyclopropene 9 was
produced in 67% yield (Table 3, entry 4). Ultimately, 9 could
be cycloisomerized in the presence of In(OTf)3 to afford the
pyrido[1,2-a]indole product 8b in 69% yield [Eq. (1)]. Given
3
4
t
Yield
[h] [%][b]
1
2
3
4
5
6
3h
3i
4h
4i
9
8
9
82
55
3j
4j
78[c]
3k
3l
4k 10 82
4l
12 69
3m
4m 12 78
[a] Reactions run with cyclopropene (1 equiv) and In(OTf)3 (5 mol%) in
CH2Cl2 at 258C. [b] Yield of product isolated after column chromatog-
raphy. [c] Performed in 1,2-dichloroethane at reflux.
that the electronics of the N-pyrrolyl and N-indolyl a-diazo b-
amidoesters should be similar, this dramatic difference in
reactivity can be rationalized based on steric interference or
a p-stacking interaction that may arise between the ligands on
the rhodium catalyst and the benzenoid portion of the indole.
Mechanistically, we envisioned that the heteroaryl group
can serve as an intramolecular nucleophile according to
a Friedel–Crafts-type mechanism (Scheme 3). The Lewis acid
catalyst will polarize the C1–C3 bond upon coordination with
the carbonyl groups in the cyclopropene to form a six-
membered chelate. Next, the pendant heteroaryl moiety will
initiate an intramolecular SN1-like nucleophilic cyclopropene
ring-opening. This sequence ultimately generates the final
to the reaction conditions, the highly functionalized N-
methylindole 4m was obtained in 78% yield (Table 2,
entry 6). This result was particularly satisfying given that
indole is considered a privileged scaffold by medicinal
chemists and our method allows the facile formation of
indolyl derivatives with a high degree of substitution.[22]
Finally, we wanted to probe the effects of varying the
carbonyl groups on the cyclopropenes to include ketones and
Table 3: Effects of varying the carbonyl acceptor groups.[a]
Substrate
Product
t
Yield
[h] [%][b]
1
2
5a
5b
6a
6b
4
5
2
62[c]
31
3
4
7a
7b
8a
9
51
2.5 67
[a] Reactions run with diazo compound (1.3 equiv), 4-ethynylanisole
(1 equiv), and [Rh2esp2] (0.1 mol%) in CH2Cl2 at 258C. [b] Yield of
product isolated after column chromatography. [c] 1 mol% [Rh2esp2] was
employed.
Scheme 3. Mechanistic rationalization for the indium-catalyzed cyclo-
isomerization.
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An interesting result was observed when cyclopropene 3n
(derived from TMS acetylene) was subjected to the reaction
conditions (Scheme 3). The only product observed was the 4-
unsubstituted benzo[b]thiophene derivative 10 (58% yield),
which does not contain the TMS group. We postulate that
heteroaryl attack on the cyclopropene occurs with an inverse
regioselectivity to afford the corresponding TMS-substituted
aromatic product, which under the reaction conditions, will
undergo protiodesilylation[25] to afford the benzo[b]thiophene
product 10.
In summary, we have developed a novel Lewis acid
catalyzed cycloisomerization of heteroaryl cyclopropene-3,3-
dicarbonyl compounds. The method is efficient (up to 86%
yield), highly modular (amenable to various heteroaromatics,
N-acylindoles, and N-acylpyrroles), and allows the facile
formation of functionalized benzo-fused heteroaromatics and
heterobiaryls, including aryl- or heteroaryl-substituted ben-
zothiophenes, benzofurans, dibenzofurans, indoles, indoli-
zines, and pyrido[1,2-a]indoles. Efforts to employ cyclopro-
penes derived from internal alkynes are currently underway.
Received: November 2, 2011
Revised: December 3, 2011
Published online: February 16, 2012
Keywords: arenes · diazo compounds · heterocycles ·
.
small ring systems · strained molecules
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[16] In certain cases, [Rh2esp2] rapidly degraded the diazo compound.
To circumvent this issue, [Rh2(OAc)4] was employed instead (see
the Supporting Information).
[17] Attempts to add in more RhII catalyst failed to generate more
cycloisomerization product. Similarly, adding a Lewis acid to the
mixture of cyclopropene and cycloisomerization product failed
to promote further cycloisomerization.
[18] This approach was motivated in part by our recent publication
on the In(OTf)3-catalyzed ring-opening/intramolecular Friedel –
Crafts alkylation (known as the heteroaromatic homo-Nazarov
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compared to In(OTf)3, AgOTf provided similar reactivity with
slightly longer reaction times (8 h vs. 4.5 h). Both Sc(OTf)3 and
Cu(OTf)2 furnished the desired benzo[b]thiophene in even
longer reaction times (ca. 10 h). Al(OTf)3 and Zn(OTf)2 were
inefficient at catalyzing the reaction, as the reactions did not go
to completion even after 24 h. Finally, no cycloisomerization was
observed, when catalysts derived from RhI and AuI were
employed.
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[23] As compared to the reactions with the keto esters, no cyclo-
propene products were observed in the case of the diketones or
amido esters.
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