Photoinduced oxidative cyclopropanation of ene-ynamides: Synthesis of 3-aza[: N.1.0]bicycles via vinyl radicals

Article abstract of DOI:10.1039/d1cc02016a

The first photoinduced synthesis of polyfunctionalized 3-aza[n.1.0]bicycles from readily available ene-ynamides and 2,6-lutidine N-oxide using an organic acridinium photocatalyst is reported. Applying a photocatalytic strategy to the reactive distonic cation vinyl radical intermediate from ynamide, a series of bio-valuable 3-azabicycles, including diverse 3-azabicyclio[4.1.0]heptanes and 3-azabicyclo[5.1.0]octanes that are challenging to accomplish using traditional methods, have been successfully synthesized in good to high yields under mild and metal-free conditions. Mechanistic studies are consistent with the photocatalyzed single-electron oxidation of ene-ynamide and the intermediacy of a putative cationic vinyl radical in this transformation. Importantly, this strategy provides new access to the development of photocatalytic vinyl radical cascades for the synthesis of structurally sophisticated substrates. This journal is

Full text of DOI:10.1039/d1cc02016a

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Pages 5225–5348
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Yongming Deng et al.
Photoinduced oxidative cyclopropanation of ene-ynamides:
synthesis of 3-aza[n.1.0]bicycles via vinyl radicals

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Photoinduced oxidative cyclopropanation of
ene-ynamides: synthesis of 3-aza[n.1.0]bicycles
via vinyl radicals†
Cite this: Chem. Commun., 2021,
57, 5254
Received 16th April 2021,
Accepted 5th May 2021
Yongming Deng, *a Jason Zhang,b Bradley Bankhead,b Jonathan P. Markhamb
and Matthias Zellerc
DOI: 10.1039/d1cc02016a
rsc.li/chemcomm
The first photoinduced synthesis of polyfunctionalized 3-aza[n.1.0] for the construction of carbon–carbon and carbon–heteroatom
bicycles from readily available ene-ynamides and 2,6-lutidine N-oxide bonds that otherwise would be difficult to accomplish using
using an organic acridinium photocatalyst is reported. Applying a traditional ynamide chemistry.4 Nevertheless, such reactions,
photocatalytic strategy to the reactive distonic cation vinyl radical especially those catalyzed by photocatalysis, have been rela-
intermediate from ynamide, a series of bio-valuable 3-azabicycles, tively less explored.5 Recently, our group demonstrated con-
including diverse 3-azabicyclio[4.1.0]heptanes and 3-azabicyclo[5.1.0] venient and catalytic access to the reactive distonic cation vinyl
octanes that are challenging to accomplish using traditional methods, radical intermediate through a mild photocatalyzed single-
have been successfully synthesized in good to high yields under mild electron oxidation of ynamides and arylacetylenes in the
and metal-free conditions. Mechanistic studies are consistent with the presence of pyridine N-oxide (Scheme 1a).6,7 Accordingly, a
photocatalyzed single-electron oxidation of ene-ynamide and the inter- visible-light photocatalyzed ortho-alkylation reaction of pyri-
mediacy of a putative cationic vinyl radical in this transformation. dine N-oxide with alkynes was developed. In this transforma-
Importantly, this strategy provides new access to the development of tion, the pyridine N-oxide functioned as an oxygen transfer
photocatalytic vinyl radical cascades for the synthesis of structurally agent and a trapping substrate of the oxypyridinium tethered
sophisticated substrates.
vinyl radical intermediate, which underwent an intramolecular
Minisci-type pathway leading to the ortho-alkylation product.
We thus questioned whether the cationic vinyl radical inter-
Ynamides are exceptionally versatile building blocks for the
synthesis of diverse organic molecules, especially nitrogen mediate could react with additional radical acceptor to initiate
containing structures and N-heterocycles.1 The conjugation tandem radical transformations instead of engaging in the intra-
and polarization effects afforded by the amide group in yna- molecular Minisci reaction (Scheme 1a). In this regard, we envi-
mides allows for the development of highly efficient and sioned that an alkene substituent in the ene-ynamide could serve
selective new reactions and synthetic sequences.1–3 For example, as a viable radical-trapping unit for the photocatalytically
a variety of transition metal- or Brønsted acid-catalyzed selective
addition reactions of ynamides with diverse nucleophiles have
been studied extensively.1,2 Ynamides with the polarized p-system
have recently emerged as valuable a-oxo metal carbene precursors
through transition metal-catalyzed oxygenative transformations
with N–O bond oxidants.3
Alternatively, radical-mediated ynamide transformations
present complementary and appealing synthetic approaches
a Department of Chemistry and Chemical Biology, Indiana University–Purdue
University Indianapolis, 402 N Blackford St, Indianapolis, Indiana 46202, USA.
E-mail: yongdeng@iupui.edu
b Chemistry Department, Western Kentucky University, Bowling Green, KY, 42101,
USA
c Department of Chemistry, Purdue University, IN, 47907, USA
† Electronic supplementary information (ESI) available. CCDC 2064066–2064068.
Scheme 1 (a) Our reported photocatalytic generation of vinyl radicals
from ynamides. (b) Proposed concept. (c) This work.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1cc02016a
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generated vinyl radical intermediate (Scheme 1b). Additionally, we possessing electron-rich and halogen groups on the aromatic
hypothesized that the use of an ortho-substituted pyridine N-oxide, substituents of the N-allylynesulfonamide occurred smoothly
such as 2,6-lutidine N-oxide, could suppress the competing Minisci generating the corresponding bicyclic products in chemo-
reaction. Following radical cascades, formal cyclopropanation, and selectivity. The structure of product 2f was identified spectro-
N–O bond scission would release 2,6-lutidine and lead to a scopically and confirmed by X-ray diffraction analysis.
3-aza[n.1.0]bicyclic skeleton. These highly functionalized motifs Moderate yield (2h, 58%) was obtained from the reaction of
are commonly found in natural products, bioactive compounds, ynamide with an electron-withdrawing cyano group at the para-
and important synthetic intermediates.8 The construction of position of the aryl substituent. In addition to the N-tosylyna-
3-azabicyclio[3.1.0]hexanes has been studied in a variety of mides, mesyl-, nosyl-, and tert-butyloxycarbonyl-protected
reactions.9 However, catalytic synthesis of 3-azabicyclio[4.1.0] substrates were ideal reagents as well, yielding products with
heptanes and 3-azabicyclo[5.1.0]octanes is relatively rare and its high yields (2i–2k, 84–90%). The scope with respect to the
development is in great demand.10 Noteworthily, to the best of our ynamides bearing an allyl unit with different degrees of sub-
knowledge, a catalytic photoredox synthetic methodology to stitution was then explored. The reaction of the b-methallyl
3-azabicyclio[n.1.0]hexanes has never been reported. In this con- tethered substrate delivered the desired bicyclic product 2l
text, we describe here a photoinduced oxygenative cyclopropana- in good yield; product 2m was isolated in 61% yield from
tion of ene-ynamides for the synthesis of
a series of the reaction performed in acetone with gem-dimethylallyl-
3-aza[n.1.0]bicycles via catalytically generated cationic vinyl radi- substituted ynamide. Both trans-crotyl and trans-cinnamyl
cals with 2,6-lutidine N-oxide as an oxygen donor. This methodol- tethered ynamides participated in the photoinduced cyclo-
ogy represents not only the first photocatalytic approach of propanation furnishing products in good yields, 74 and 86%
3-aza[n.1.0]bicycle synthesis, but also a complementary metal- respectively.
free method with mild reaction conditions and general scope to
the traditional transition metal-catalyzed processes.
We then sought to generalize this photoinduced cyclopro-
panation to the synthesis of various 3-azabicyclio[4.1.0]hep-
tanes and 3-azabicyclo[5.1.0]octanes. Our initial examination
of N-3-butenylynamide 4a was performed under the optimized
conditions for N-allylynamide (Table 1). To our delight, the
desired 3-azabicyclio[4.1.0]heptane 6a was furnished in 70%
yield. Continuing the optimization with N-3-butenylynamide 4a
(Table S2, ESI†), we observed an increase in the yield of 6a
(89%) by using 3,6-di-tert-butyl-substituted acridinium12 Mes-
(tBu)2Acr-PhBF4 in place of Mes-Acr-PhBF4 as the photocatalyst.
To test our hypothesis, the model substrate, N-allylynesulfo- As summarized in Table 2a, a series of 3-azabicyclio[4.1.0]hep-
namide 1a, was subjected to 5 mol% of 9-mesityl-10-phenyl tanes bearing various arene substituents were successfully
acridinium perchlorate (Mes-Acr-MeClO4) and 2,6-lutidine produced (6a–6g, 64–90%) catalyzed by Mes-(tBu)2Acr-PhBF4.
N-oxide (1.0 equiv.) in acetonitrile with blue LED light irradiation The N-3-butenylmeylynamide was a suitable substrate as well,
(lmax = 450 nm) at room temperature (eqn (1)). As proposed, the 3-
azabicyclo[3.1.0]hexane-2-one 2a was generated smoothly in 71%
yield under aerobic conditions. A ketoimide compound 3 was
Table 1 Scope of N-allylynamidesa
isolated in 10% yield, which is speculated to be generated from
over-oxidation of the vinyl radical intermediate. Visible-light
irradiation and the acridinium photocatalyst were indispensable
for the reaction (Table S1, ESI†). Systematic examination of
the reaction variants, including photocatalysts, N-oxides, and
solvents, was performed (Table S1, ESI†). The optimized reaction
conditions were identified with the use of 9-phenyl substituted
acridinium11 Mes-Acr-PhBF4 and 2,6-lutidine N-oxide in aceto-
nitrile, providing the best results to date under aerobic conditions
(18 hours, 88% yield, and eqn (1)). Notably, under the
standard conditions, but in the absence of oxygen, product 2a
Entry
Substrate 1
2b (%)
1
Ar = Ph, E = Ts, R1 = R2 = R3 = H
2a, 88
2b, 90
2c, 91
2d, 92
2e, 86
2f, 75
2g, 83
2h, 58
2i, 90
2j, 84
2k, 81
2l, 80
2m, 61
2n, 74
2o, 86
2
Ar = p-MeC6H4, E = Ts, R1 = R2 = R3 = H
Ar = p-MeOC6H4, E = Ts, R1 = R2 = R3 = H
Ar = p-ClC6H4, E = Ts, R1 = R2 = R3 = H
Ar = p-BrC6H4, E = Ts, R1 = R2 = R3 = H
Ar = o-ClC6H4, E = Ts, R1 = R2 = R3 = H
Ar = m-ClC6H4, E = Ts, R1 = R2 = R3 = H
Ar = p-CNC6H4, E = Ts, R1 = R2 = R3 = H
Ar = Ph, E = Ms, R1 = R2 = R3 = H
3c
4
5
6
7
8
was obtained, however, in lower yield (52%) under
a
9c
10
11
12
13d
14e
15f
Ar = Ph, E = p-Ns, R1 = R2 = R3 = H
Ar = Ph, E = Boc, R1 = R2 = R3 = H
prolonged reaction time (48 h), suggesting that dioxygen
may play an important role in the regeneration of the photo-
redox catalyst.
Applying the optimized conditions, a variety of N-allylyna-
mides with various substitution patterns and functional groups
were subjected to the photocatalyzed intramolecular cyclopro-
panation reaction. The reactions performed with substrates
Ar = Ph, E = Ts, R1 = CH3, R2 = R3 = H
Ar = Ph, E = Ts, R1 = H, R2 = R3 = CH3
Ar = Ph, E = Ts, R1 = H, R2 = CH3, R3 = H
Ar = Ph, E = Ts, R1 = H, R2 = Ph, R3 = H
a
b
c
d
For experimental details, see ESI. Isolated yield. 10 h. Solvent =
e
f
acetone. d.r. = 2.7 : 1. d.r. = 17 : 1.
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Table
2
Synthesis of 3-azabicyclio[4.1.0]heptanes 6 and 3-azabicy-
rearrangement cascades driven by aromatization lead to
compound 8.
clo[5.1.0]octanes 7a
In further probing the synthetic utility of this photoinduced
transformation, we were pleased to find that the photocatalytic
reaction of N-propargyl ynamide 9a produced an oxidative
cycloisomerization product 10a in 61% yield, when 2.5 equiv.
of 2,6-lutidine N-oxide was used (eqn (S1), ESI†). For R = CH3
(9b), besides the generation of 2-oxopyrrolidine 10b, a diene
product 10c was isolated in 41% yield. It is proposed that the
catalytically generated cationic vinyl radical through single-
electron oxidation of ynamide and 2,6-lutidine N-oxide reacted
with the tethered alkyne to afford a new vinyl radical inter-
mediate D, which can be oxidized by 2,6-lutidine N-oxide to
yield product 10. When R was a methyl group (9b), 1,2-hydro-
gen migration may produce the diene compound 10c.
To lend further insight into the mechanism of the transfor-
mation, mechanistic studies including fluorescence quenching
studies, electrochemical studies, and radical inhibition experi-
ments, were performed. Stern–Volmer fluorescence quenching
studies (Fig. S3 to S6, ESI†) of the N-allylynamide 1a and 2,
6-lutidine N-oxide system demonstrated that the light-excited
photocatalyst Mes-Acr+* was more significantly quenched
by ynamide 1a (Ksv = 80.90) than by 2,6-lutidine N-oxide
(Ksv = 52.29). And the mixed solution of 1a and 2,6-lutidine
N-oxide exhibited similar fluorescence quenching efficiency
(Ksv = 83.85) to the solo N-allylynamide 1a solution, which
indicated that the irradiated photocatalyst might be quenched
by ynamide 1a rather than by 2,6-lutidine N-oxide. Notably,
besides the expected suppression of the photocatalyzed reaction
a
For experimental details, see ESI.
producing 6h in 87% yield. Moreover, we were pleased to find
that 3-azabicyclo[5.1.0]octanes 7 could also be readily accessed
from the corresponding N-4-pentenylynesulfonamide 5 in good
to high yields under the photocatalytic conditions (Table 2b,
7a–7h, 69 to 87%). The structures of 6 and 7 were unambigu-
ously identified spectroscopically and confirmed by X-ray crys-
tallography of 6f and 7d respectively. Noteworthily, this
photoinduced reaction provides a mild metal-free and conve-
niently achieved synthesis of valuable azabicyclic compounds,
which were only accessible from piperidine/azepane precursors
or precious metal catalyzed reactions.
With the successful construction of cyclopropane fused
five/six/seven-membered bicycles, we set out to further test
the photocatalyzed reaction of N-allylynamides with an
alkyl substituent at the alkyne terminus. Intriguingly, when
n-hexyl or cyclopropyl substituted ynamides were subjected
to the photocatalytic reaction, unexcepted meta-alkylation
products of 2,6-lutidine N-oxide with ynamide were
obtained albeit in moderate isolated yields (8a and 8b,
eqn (2)). The alkyl substituted bicyclic compounds were
not detected. We rationalize that the formation of these
oxyalkylation products was on account of the switched site-
selective generation of vinyl radical intermediate A, in
which the allyl tethered sulfonamide provided greater
resonance stabilization than the alkyl substitution. Such
switched selectivity is consistent with our previously
reported ortho-alkylation of pyridine N-oxide with alkyl
substituted ynamides.6 It is proposed that the photocata-
lytically generated pyridinium vinyl radical A could then
undergo the Minisci-type transformation affording the
bicyclic aminyl radical cation B. Recently, the Maulide
group reported a meta-selective oxyarylation of alkynes
with pyridine N-oxide.13 The mechanistic and computa-
tional studies of the meta-oxyarylation revealed that the
reaction proceeded through pseudopericyclic [3,5] rearran-
gements and a key cyclopropane intermediate. Accordingly,
in our meta-alkylation we proposed that the cyclopropane
intermediate C was generated from aminyl radical cation B
via simultaneous or successive-stepped [3,5] rearrange-
ment, N–O bond scission, and a SET process. Subsequent
Scheme 2 Plausible mechanism.
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with addition of radical scavenger 2,2,6,6-tetramethylpiperidin-1- References
oxyl (TEMPO), the formation of 11 from ynamide cation radical I
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and TEMPO was detected by ESI-MS analysis (Scheme 2 and
Fig. S1, ESI†). Moreover, N-allylynamide 1a (irreversible half-peak
ox
potential, E = 1.58 V vs SCE) possesses much lower oxidation
1/2
ox
1/2
potential than 2,6-lutidine N-oxide does (E = 1.82 V vs SCE).
Based on the aforementioned results, it is more feasible that the
generation of ynamide cation radical I from photocatalyzed
single-electron oxidation of N-allylynamide 1a may initiate this
photoinduced transformation. Given the high excited-state
reduction potential of catalyst Mes-Acr-PhBF4 (Erꢀed ¼ 2:20 V),12
the pathway of photoinduced single-electron oxidation of 2,
6-lutidine N-oxide cannot completely be excluded at this stage.7
Herein, in accordance with the experimental evidence and our
previous work,6 a plausible mechanism is proposed in Scheme 2.
We hypothesized that the N-allylynamide 1 could be formally
oxidized by the excited acridinium photocatalyst affording a yna-
mide cation radical I. Subsequent nucleophilic a-addition of the
resultant cation radical I by 2,6-lutidine N-oxide would lead to the
formation of the key oxypyridinium tethered vinyl radical intermedi-
ate II. Alternatively, a photoinduced single-electron oxidation of 2,
6-lutidine N-oxide is also possible. Nevertheless, subsequent radical
addition of ene-ynamide 1 and 2,6-lutidine N-oxide radicals would
generate the oxypyridinium tethered vinyl radical intermediate II as
well. Guided by the reported experimental and computational
studies of radical ring closure,14 we propose that the vinyl radical
may undergo a 5-exo-trig radical cyclization with tethered alkene
producing intermediate III. This open-shell radical is subsequently
trapped intramolecularly by the enol frame to close the cyclopropane
ring forming a ketyl radical IV. The resultant intermediate IV could
then undergo a b-N–O bond scission yielding the oxygenative
cyclopropanation product 2 and 2,6-lutidine radical cation. Based
on the literature survey and our experimental results, it is proposed
that the acridine radical Mes-Acr could be oxidized by O2 regenerat-
5 For recent representative examples, see: (a) Z.-S. Wang, Y.-B. Chen,
H.-W. Zhang, Z. Sun, C. Zhu and L.-W. Ye, J. Am. Chem. Soc., 2020,
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and J. Wu, Org. Lett., 2019, 21, 5321–5325.
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and T. Librowski, Curr. Med. Chem., 2010, 17, 2338–2347;
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.
superoxide should readily reduce the 2,6-lutidine radical cation,
although the 2,6-lutidine radical cation intermediate might be
capable of catalyst turnover. Another putative pathway with direct
reduction of ketyl radical IV by reactive superoxide and subsequent
N–O bond scission may account for the product formation as well.
In summary, we have developed a photoinduced oxidative
cyclopropanation of ene-ynamides via photocatalytically gener-
ated vinyl radical involved radical cascades. The protocol provides
´
´
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´
(c) A. Lopez-Rodrıguez, G. Domınguez and J. Perez-Castells, J. Org.
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a
convenient metal-free access to a variety of valuable
3-aza[n.1.0]bicycles. Notably, this photocatalytic strategy of catio-
nic vinyl radical generation and the successful radical trapping by
the tethered alkene demonstrates its synthetic potential of devel-
oping versatile cascade/tandem radical transformations. Further
investigation of this transformation and the expansion of this
strategy with other radical acceptors to allow the synthesis of
molecular complexity are currently underway.
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Conflicts of interest
There are no conflicts to declare.
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