Gold-catalyzed cycloisomerization of pyridine-bridged 1,8-diynes: An expedient access to luminescent cycl[3.2.2]azines

Article abstract of DOI:10.1021/acs.orglett.9b02677

Gold-catalyzed diyne cycloisomerizations involving carbene/alkyne metathesis have been the focal point of attention for the past few years as it offers great potential to build complex polycyclic architectures. However, the design of novel cycloisomerizations has been mostly limited to 1,5/1,6-diynes and has remained very challenging to apply for higher 1,n-diynes. Herein, we disclose an unprecedented cycloisomerization of pyridine-bridged 1,8-diynes involving carbene/alkyne metathesis to access luminescent cycl[3.2.2]azines.

Full text of DOI:10.1021/acs.orglett.9b02677

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Gold-Catalyzed Cycloisomerization of Pyridine-Bridged 1,8-Diynes:
An Expedient Access to Luminescent Cycl[3.2.2]azines
Chetan C. Chintawar,† Manoj V. Mane,‡,§ Akash G. Tathe,† Suprakash Biswas,† and Nitin T. Patil*,†
†Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhauri, Bhopal 462 066, India
‡Physical Chemistry Division, CSIR − National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
Academy of Scientific and Innovative Research (AcSIR), New Dehli 110 025, India
§KAUST Catalysis Centre, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
S
* Supporting Information
ABSTRACT: Gold-catalyzed diyne cycloisomerizations in-
volving carbene/alkyne metathesis have been the focal point
of attention for the past few years as it offers great potential to
build complex polycyclic architectures. However, the design of
novel cycloisomerizations has been mostly limited to 1,5/1,6-
diynes and has remained very challenging to apply for higher
1,n-diynes. Herein, we disclose an unprecedented cyclo-
isomerization of pyridine-bridged 1,8-diynes involving carbene/alkyne metathesis to access luminescent cycl[3.2.2]azines.
he design and synthesis of organic fluorophores,
continuously evolving with time, are essential for
seamless progress in the areas of chemistry, biology, and
functional-material research.1 In particular, cycl[3.2.2]azines
(Figure 1a), a class of bridged annulenes, are known for their
Hashmi, and Liu reported an elegant protocol for cyclo-
isomerization of 1,5/1,6-diynes involving vinyl gold−carbenes8
whose formation is triggered by 1,2-acyloxy migration of
propargylic esters (Scheme 1a)9 or 1,2-sulfur migration (for
1,6-diyne thionates).10 An attractive alternative for genesis of
gold−carbenes in such carbene/alkyne metathesis is provided
by selective oxidation of one of the alkynes by N-oxides or
azides. In this context, the research groups of Hashmi, Zhang,
Ye, and Ji have meticulously explored the utilities of these
oxidatively generated highly electrophilic α-oxo/imino gold
carbenes in triggering a 1,5/1,6-diyne cycloisomerization
cascade giving rise to diversely functionalized polycyclic
structures (Scheme 1b,c).11 A careful literature survey reveals
that the design of cycloisomerization reactions has mostly been
limited to 1,5/1,6-diynes and remains very challenging to apply
for higher 1,n-diynes, probably because of the high energy
transition states associated with the formation of products.
Moreover, such a 1,n-diyne cycloisomerization technique has
never been developed for accessing any bridged annulenes. To
address this bottleneck in the field and further expand the
repertoire of this chemistry, we sought a new design of
pyridine-bridged 1,8-diynes, i.e., 2-(arylethynyl)-6-(2-
(arylethynyl)phenyl)pyridines 1 (Scheme 1d), which can be
conveniently prepared by a series of Pd-catalyzed cross-
coupling reactions.12 It was envisaged that under the gold
catalysis such empirically designed pyridine-bridged 1,8-diynes
would undergo a selective 5-exo-dig imino-auration followed by
back-bonding to generate an α-indolizidino gold-carbene (Int
A).13 The gold−carbene, thus generated, would undergo
carbene/alkyne metathesis via nucleophilic attack of pendant
T
Figure 1. Structures of cycl[3.2.2]azine and its derivatives.
high fluorescence efficiency and often find application in
OLEDs (Figure 1b).2 Of note, one of its derivatives is known
as a nonsteroidal agonist of estrogen receptor (Figure 1c).
Moreover, a partially hydrogenated framework is often found
in several myrmicarin-based natural products which were
isolated from the poison gland of the African ant species
Myrmicaria opaciventris.3 However, most of the methods4 to
access these cycl[3.2.2]azines rely on metal-catalyzed annula-
tion reaction of indolizines with alkynes.5 Unfortunately, these
reactions suffer from regioselectivity issues when unsym-
metrical alkynes are employed. To this end, efficient synthetic
approach to access multiply substituted cycl[3.2.2]azines from
easily available starting materials would be highly desirable.
Gold-catalyzed cycloisomerization of alkynes is considered
as one of the most efficient and atom-economical strategies for
building complex molecules.6 In particular, gold-catalyzed
cycloisomerization of 1,n-diynes involving carbene/alkyne
metathesis7 has emerged as a powerful tool for generating
diverse polycyclic architectures.6b,c,g For instance, Chan,
Received: July 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02677
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 1. Cycloisomerization of 1,n-Diynes: Previous and
Table 1. Optimization of the Reaction
Present Work
entry
cat. M
AuCl
AuCl3
solvent
toluene
toluene
toluene
toluene
toluene
benzene
chlorobenzene
acetonitrile
DCE
benzene/DMSO
benzene/DMF
benzene/DMSO
benzene/DMSO
benzene/DMSO
yield (%)
1
2
3
4
5
6
7
8
22
29
31
34
40
48
42
25
22
AuCl3·pyridine
HAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·.2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
NaAuCl4·2H2O
9
c
10
11
12
13
14
55
d
46
e
51
f
65
g
57
a
Reaction conditions: 0.1 mmol of 1, 5 mol % of Cat M, solvent (2
b
mL), 100 °C, 12 h. Yields of isolated products. Solvent system used.
c
d
Benzene/DMSO (95:5). Benzene/DMF (95:5). Additives used:
10 mol % of NaNTf2, 10 mol % of NaOTf and 10 mol % of NaBF4.
e
f
g
DMF would result in better yields (entries 10 and 11).
Accordingly, when 1a was treated with NaAuCl4·2H2O in
benzene/DMSO (95:5) solvent system, 3a was obtained in
55% yield (entry 10). It should be noted only DMSO and
DMF are not viable solvents for the reaction. Out of the
additives screened (entry 12−14), the influence of NaOTf on
the reaction outcome was found to be the best, giving 3a in
65% yield (entry 13).
alkyne to generate a β-gold vinyl cation species (Int B) which
can be trapped by external nucleophile to furnish cycl[3.2.2]-
azines 2.9c,11a,c−e Alternatively, Int A is susceptible to undergo
1,6-carbene transfer via a cyclopropenation/ring opening
cascade to generate a new gold-carbene species (Int
B′),9a,b,d−f,10,11b,f which can then be captured by an external
nucleophile to offer bridged annulenes 2. Thus, an expedient
and straightforward synthetic route to luminescent cycl[3.2.2]-
azines is conceived for the first time from cycloisomerization of
newly designed pyridine-bridged 1,8-diynes. Herein, we
describe the detailed implementation of this diyne cyclo-
isomerization along with judicious mechanistic studies
supported by DFT calculations.
At the outset, we employed pyridine-bridged 1,8-diyne 1a,
for reaction discovery and optimization, some results of which
are shown in Table 1. After having no success with ligated
gold(I) complexes in achieving the desired diyne cyclo-
isomerization, we switched to simple gold salts. Accordingly,
much to our delight, when 5 mol % of AuCl was employed,the
desired benzo-fused cycl[3.2.2]azine product 3a was indeed
formed, albeit in 22% yield (entry 1). While other gold salts
such as AuCl3, AuCl3·Py and HAuCl4·2H2O were also effective
(entries 2−4), NaAuCl4·2H2O yielded an encouraging 40%
yield of 3a (entry 5). Next, the effects of various other solvents
were evaluated (entries 6−9), and an increase in the yield of
the product was noted with benzene (48%, entry 6). As a little
turbidity was observed in this case, we were rather curious
whether increasing the homogeneity of reaction mixture by
using solvent systems such as benzene/DMSO and benzene/
To define the scope of diyne cyclization, we next proceeded
to assess the reaction on a series of pyridine-bridged diynes,
and the results are summarized in Scheme 2. To start with,
when R1 was substituted with 1-Np and cyclohexenyl groups
the corresponding products 3b and 3c were obtained in 71%
and 46% yields. Surprisingly, no product was obtained with
cyclopropyl group as a variant (1d). When substrates with
variation on the aryl ring at the alkyne terminus, R1 (1e-l),
were put to the test, electron-donating groups and functional
groups such as alkyl- or halo- fared very well irrespective of the
substitution pattern, whereas a small decrease in yield (52%)
was observed with an electron-withdrawing −CN group (1i).
Next, we directed our attention to examine the tolerance of
substituents at the C-6 phenyl ring of the pyridyl moiety (1m−
t). Pleasingly, introduction of diverse substitutions on the
previously mentioned aryl moiety were well tolerated to afford
the respective cycl[3.2.2]azines 3m−t in good to excellent
yields (54−81%). In contrast, the substrates with naphthalene
and cyclohexenyl substitutions at R2 position did not suit the
reaction well and provided respective products 3u−v in lower
yields (26% and 32%). On the other hand, varying the
substitution pattern (ortho-, meta-, para-) on the phenyl ring as
R2 did not compromise the yields of diyne-cyclized products.
In these experiments, respective cycl[3.2.2]azines 3w−ac were
obtained in decent yields (45−65%). Switching the pyridyl
ring with an isoquinyl provided respective product 3ad in
substantial yield (75%).
B
DOI: 10.1021/acs.orglett.9b02677
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
between α-cyclyzino gold-carbene Int B′ and β-gold vinyl
cation Int B (Scheme 1) the probability of the former
operating as a reaction intermediate is significantly less. This is
again supported by the fact that no cyclopropanation product 4
was obtained when 1a was subjected to styrene under the
standard reaction conditions (eq 2). Next, when excess H218O
was added under the standard reaction conditions, incorpo-
ration of more than 60% 18O (3a′)12 suggests that oxygen in
product 3 might be originating from adventitious water present
in reaction mixture (eq 3). Therefore, we presumed that
alcohol 2a should exist in the reaction mixture to undergo
aerial oxidation leading to the final product 3a. Accordingly,
when 2a was synthesized and allowed to stir overnight at 100
°C in benzene/DMSO (95:5), it was indeed oxidized to
anticipated product 3a almost quantitatively (eq 4), confirming
our presumption.
Scheme 2. Scope of the Reaction
With these results we were curious to know if the
intermediate Int B or Int B′ can be trapped with nucleophiles
other than H2O. Gratifyingly, under slightly modified
conditions, electron-rich aromatics like indole and azulene
were found to be suitable nucleophiles providing direct access
to novel triarymethanes−an important class of organic
compounds often found in natural products and medicinally
relevant molecules15 (eq 5).
a
Reaction conditions: 0.1 mmol of 1, 5 mol % of NaAuCl4·2H2O, 10
b
mol % of NaOTf, benzene/DMSO (95:5), 100 °C, 12 h. Isolated
yields. Reaction was stirred for 24 h.
c
To gain a better understanding of the mechanistic premise,
the following control experiments were performed (Scheme 3).
Consulting literature precedents, a plausible mechanism
consistent with the above control experiments is proposed in
Scheme 4. As discussed previously, the carbene/alkyne
metathesis of α-indolizidino gold−carbene B, generated from
back-bonding in intermediate A,16 can lead to β-gold vinyl
cation intermediate C through the nucleophilic attack of
pendant alkyne. The intermediate C can be better represented
as more stabilized enylium-cationic intermediate D.11c
Nucleophilic attack of H2O on the intermediate C/D followed
by an isomerization/protodeauration sequence would furnish a
cycl[3.2.2]azine 2. Alternatively, the β-gold vinyl cation
intermediate C would undergo cyclopropenation/ring opening
cascade (cf. C′) to generate a new gold−carbene species D′
which would react with H2O to produce cycl[3.2.2]azines 2.
The cycl[3.2.2]azines 2, thus generated, would undergo aerial
oxidation to produce cycl[3.2.2]azines 3.
Scheme 3. Mechanistic Investigation
The most challenging task here would be to understand
clearly whether the reaction proceeds through either a β-gold
vinyl cation or a gold−carbene intermediate.9−11 It should be
noted that the reactivity exhibited by both species is
complementary,17 and hence, it is relatively difficult to propose
the existence of either of them by running control experiments.
Hence, to shed some light on these mechanistic aspects, we
considered quantum-chemical calculations based on density
functional theory.12 Our calculations suggest that direct attack
of water molecule on the β-gold vinyl cation intermediate C
To examine the posited participation of DMSO to act as a
source of oxygen in the product, we charged analogous
diphenyl sulfoxide (Ph2SO) in the reaction mixture in place of
DMSO,14 but no diphenyl sulfide (Ph2S) was observed. This
suggests that oxygen transfer from a sulfoxide might not be the
case here (eq 1). Further, this control experiment implies that
C
DOI: 10.1021/acs.orglett.9b02677
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
actively operating intermediate in lieu of the alternative gold−
carbene species generated via 1,6-carbene transfer pathway. In
light of the fact that there exists a paucity of general methods
that provide direct and efficient access to the luminescent
cycl[3.2.2]azines, the strategy documented herein proves to be
a powerful tool to access these class of compounds extending
their application in the domain of material and biological
sciences.
Scheme 4. Plausible Reaction Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02677.
1
Experimental procedures, analytical data, copies of H,
13C NMR spectra of all newly synthesized products, X-
ray data of 3l and 3v, photophysical data, and
computational details (PDF)
Accession Codes
CCDC 1882296 and 1905761 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
has a step barrier of 1.5 kcal/mol, whereas the formation of α-
cyclizino gold−carbene intermediate D′ via cyclopropenation/
ring opening cascade requires a step barrier of 22.6 kcal/mol
which is almost 19.8 kcal/mol higher than the direct water
attack which renders it energetically disfavored (see Figure
S13).
Next, we evaluated the photophysical properties of selected
cycl[3.2.2]azine derivatives which exhibited intense fluores-
cence. Emission spectra of 10 μM solution of these
fluorophores in DCM were recorded by exciting at 410 nm,
and it is found that all the probes showing emission in the
visible region at 450−520 nm (Figure 2). The quantum yields
(ϕf) of these fluorophores were calculated to be 0.06−0.36.12
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: npatil@iiserb.ac.in.
ORCID
Nitin T. Patil: 0000-0002-8372-2759
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous financial support by the Council of Scientific and
Industrial Research (CSIR), New Delhi [File No. 02(0336)/
18/EMR-II] is gratefully acknowledged. We thank Dr. Apurba
Lal Koner, IISER Bhopal, for helping us in the determination
of photophysical properties of fluorophores. C.C.C. and S.B.
thank UGC for the award of junior and senior research
fellowships, respectively. A.G.T. thanks IISER Bhopal for a
junior research fellowship.
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DOI: 10.1021/acs.orglett.9b02677
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