Visible-light-promoted iminyl-radical formation from Acyl oximes: A unified approach to pyridines, quinolines, and phenanthridines

Article abstract of DOI:10.1002/anie.201411342

A unified strategy involving visible-light-induced iminyl-radical formation has been established for the construction of pyridines, quinolines, and phenanthridines from acyl oximes. With fac-[Ir(ppy)₃] as a photoredox catalyst, the acyl oximes were converted by 1 e^- reduction into iminyl radical intermediates, which then underwent intramolecular homolytic aromatic substitution (HAS) to give the N-containing arenes. These reactions proceeded with a broad range of substrates at room temperature in high yield. This strategy of visible-light-induced iminyl-radical formation was successfully applied to a five-step concise synthesis of benzo[c]phenanthridine alkaloids.

Full text of DOI:10.1002/anie.201411342

Angewandte
Chemie
DOI: 10.1002/anie.201411342
Photochemistry
Visible-Light-Promoted Iminyl-Radical Formation from Acyl Oximes:
A Unified Approach to Pyridines, Quinolines, and Phenanthridines**
Heng Jiang, Xiaode An, Kun Tong, Tianyi Zheng, Yan Zhang,* and Shouyun Yu*
Abstract: A unified strategy involving visible-light-
induced iminyl-radical formation has been estab-
lished for the construction of pyridines, quinolines,
and phenanthridines from acyl oximes. With fac-
[Ir(ppy)3] as a photoredox catalyst, the acyl oximes
were converted by 1eÀ reduction into iminyl radical
intermediates, which then underwent intramolecu-
lar homolytic aromatic substitution (HAS) to give
the N-containing arenes. These reactions proceeded
with a broad range of substrates at room temper-
À
Scheme 1. N O cleavage of acyl oximes to form N-containing heterocycles.
MW=microwave radiation.
ature in high yield. This strategy of visible-light-
induced iminyl-radical formation was successfully
applied to a five-step concise synthesis of benzo[c]-
phenanthridine alkaloids.
transfer mechanism from acyl oximes in several copper-
I
À
A
cyl oximes are versatile building blocks for the synthesis of
catalyzed reactions,[2g,4] the oxidative addition of Cu to N O
bonds through a CuI/CuIII cycle is an inevitable competitive
pathway, especially at high temperature.[5] Therefore, efficient
methods for the exclusive 1eÀ reduction of acyl oximes under
mild (ideally at room temperature) and catalytic conditions
are still in high demand owing to the potential application of
iminyl-radical intermediates in the construction of N-con-
taining heterocycles.
structurally diverse and biologically active N heterocycles
[1]
À
À
through N O bond cleavage and C N bond formation.
During the past few years, transition-metal catalysis has
been demonstrated as a powerful tool for breaking the N O
bond of acyl oximes through oxidative addition of the metal
to the N O bond and the subsequent formation of various
À
À
multisubstituted heterocycles, such as pyridines, indoles,
isoquinolines, and imidazoles (Scheme 1a).[2] Another path-
The renaissance of visible-light photoredox catalysis has
been witnessed since 2008.[6] By taking advantage of the 1eÀ
redox potential of a photoexcited catalyst, this environ-
mentally friendly and sustainable synthetic technology can
promote versatile organic transformations, including the
construction of arenes and heteroarenes.[7,8] Recently, we
À
way for cleavage of the N O bond is homolysis under
microwave (> 1608C) or UV irradiation. This method has
been employed to produce several heterocycles by radical
cyclization of iminyl-radical intermediates (Scheme 1b).[3]
À
Furthermore, the N O bond of acyl oximes can also be
À
cleaved to give iminyl radicals, as well as acyloxy anions,
through single-electron reduction with the assistance of an
oxidant owing to the weak bond energy and electronic
reported a visible-light-promoted direct C H bond amination
of heteroarenes with hydroxylamine derivatives (Sche-
me 2a).[9a] The key step in this transformation is the formation
of amidyl radicals from hydroxylamine derivatives through
properties of the N O bond (Scheme 1c).[1] Although iminyl
À
radicals were considered to be formed by a CuI/CuII 1eÀ
1e reductive cleavage of the N(sp ) O bond.[10] Meanwhile,
À
3
À
the research groups of Sanford,[9b] Lee,[9c] Luo,[9d] and Kçnig[9e]
À
also reported visible-light-induced C H bond amination
[*] H. Jiang, X. An, K. Tong, T. Zheng, Prof. Dr. Y. Zhang, Prof. Dr. S. Yu
State Key Laboratory of Analytical Chemistry for Life Science and
Collaborative Innovation Center of Chemistry for Life Sciences
School of Chemistry and Chemical Engineering, Nanjing University
Nanjing 210093 (China)
reactions of arenes and heteroarenes with different amidyl-
3
À
radical precursors. This newly exploited strategy of N(sp ) O
bond cleavage inspired us to consider the feasibility of visible-
light-induced N(sp ) O bond cleavage and iminyl-radical
2
À
E-mail: njuzy@nju.edu.cn
formation. Accordingly, we speculated that the iminyl radical
could be generated by the 1eÀ reduction of acyl oximes by
photoredox catalysis and could be further converted into
a class of ubiquitous azaarenes, including pyridines,[11] quino-
lines,[12] and phenanthridines.[13]
According to the rationale outlined in Scheme 2b, the
catalytic cycle begins with the visible-light-promoted photo-
excitation of a photocatalyst PC to its excited state (PC*).
Reductive cleavage of an acyl oxime I, as assisted by PC*,
produces an iminyl radical II, an acyl anion V, and the
yushouyun@nju.edu.cn
Homepage: http://hysz.nju.edu.cn/yanzhang/
http://hysz.nju.edu.cn/yusy/
[**] Financial support from the National Basic Research Program of
China (2011CB935800), the 863 Program (2013AA092903), the
National Natural Science Foundation of China (21121091,
21272113, 21372115), and the Natural Science Foundation of
Jiangsu Province (BK2012012, BK20131266) is acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411342.
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Scheme 3. Screening of acyl oxime substrates. Reaction conditions: A
mixture of 1 (0.1 mmol) and fac-[Ir(ppy)3] (0.001 mmol, 1.0 mol%) in
dry DMF (2.0 mL) was irradiated by 5 W white light-emitting diodes
(LEDs) for 10 h. Yields were determined by 1H NMR spectroscopic
analysis. [a] The reaction time was prolonged to 24 h. [b] Result
observed (no reaction) without a photocatalyst or irradiation with
visible light. n. r.=no reaction. DMF=N,N-dimethylformamide.
state photocatalyst fac-[Ir(ppy)3]*. No conversion of 1h was
observed without the photocatalyst or visible-light irradia-
tion, which further demonstrates the rationality of the
proposed
(SET) mechanism.
Under the optimized conditions, we first tested the
transformation of a variety of biphenyl acyl oximes 1 to
assess the scope of the reaction for the formation of
phenanthridines 2 (Scheme 4). Generally, the desired phen-
visible-light-induced
single-electron-transfer
Scheme 2. Rationale for the synthesis of six-membered azaarenes.
À
a) Previously reported visible-light-promoted C H amination via
amidyl radicals (Bs=benzenesulfonyl). b) Visible-light-promoted syn-
thesis of azarenes via iminyl radicals (present study).
oxidized photocatalyst (PC+). Radical II undergoes intra- anthridine derivative was isolated in high to excellent yield
molecular homolytic aromatic substitution (HAS)[14] to form (79–95%) irrespective of the position and electric nature of
a radical intermediate III, which is then oxidized by PC+ to substituents on the biphenyl moiety (products 2a–h). The
forge a cationic intermediate IV and regenerate the photo- substituent at C6 could be varied to give 6-alkyl and 6-aryl
catalyst. Ultimately, deprotonation of IV by the acyl anion V phenanthridines 2i–n in excellent yield (87–99%). Phenan-
yields a six-membered heteroarene. Alternatively, III can also thridines 2n–2aa without a substituent at the 6-position could
lose a proton to form a radical-anion intermediate VI,[13e,14] also be produced by means of this method with high
which is then oxidized by PC+ to give a heteroarene and functional-group tolerance and in high yield (82–99%). We
regenerate the photocatalyst.
carried out the reaction on a gram scale (3.0 mmol) to
We chose fac-[Ir(ppy)3] (ppy = 2-phenylpyridine) as the demonstrate the practicability of this methodology, and
photocatalyst owing to its superior reduction capacity in the isolated 2a in similarly high yield (97%).
IV/
excited state (E1/2
*
III = À1.73 V vs. SCE).[15] A series of O-
We next sought to examine the applicability of this
acyl oximes 1a–h with different acyl groups were prepared, strategy to other important functionalized azaarenes. Various
and their ability to form phenanthrine 2a via an iminyl radical acyl oximes 3a–s were prepared from benzaldehyde deriva-
under visible-light catalysis was evaluated (Scheme 3). When tives and 1,3-dicarbonyl compounds to evaluate the possibil-
O-acyl oximes 1a–e were irradiated with visible light in the ity of constructing quinoline derivatives 4. As shown in
presence of fac-[Ir(ppy)3] in DMF, the starting materials were Scheme 5, electron-withdrawing or electron-donating groups,
fully recovered. To our delight, we observed the formation of as well as halides or a fused ring on the phenyl moiety, did not
phenanthridine 2a in 91% yield (as determined by NMR affect this reaction significantly (products 4a–m, 62–88%
spectroscopy) when the electron-deficient benzoate 1 f was yield). The substituent at the C3 position could be an ester
employed. Further exploration showed that the perfluoro- (product 4n), amide (product 4o), or keto group (products
benzoate 1g and p-trifluoromethylbenzoate 1h were superior 4q–s). Products 4p–s were obtained in satisfactory yield when
to 1 f and could be converted into 2a quantitatively (see the the substituent at C2 was a propyl or phenyl group. C4-
Supporting Information for details of further optimization of substituted quinolines 4t and 4u were also synthesized in
the reaction conditions). The cyclic voltammogram recorded moderate yield.
for 1h contained an irreversible reduction wave at À0.79 V vs.
Polysubstituted pyridines, which are ubiquitous in phar-
SCE, thus indicating that 1h is readily reduced by the excited- maceuticals and natural products, could also be accessed by
2
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Scheme 5. Scope of the transformation of acyl oximes into quinolines.
Reaction conditions: A mixture of 3 (0.2 mmol) and fac-[Ir(ppy)3]
(0.002 mmol, 1.0 mol%) in dry DMF (4.0 mL) was irradiated by 5 W
white LEDs for 10 h. Yields are for the isolated product. [a] The asterisk
Scheme 4. Scope of the transformation of acyl oximes into phenanthri-
dines. Reaction conditions: A mixture of 1 (0.2 mmol) and fac-[Ir(ppy)3]
(0.002 mmol, 1.0 mol%) in dry DMF (4.0 mL) was irradiated by 5 W
white LEDs for 10 h. Yields are for the isolated product. [a] The
reaction was carried out on a 3.0 mmol scale. [b] The major diastereo-
À
indicates the position of the C N bond to this ring in the minor
regioisomer. [b] The yield was obtained from the 2Z configuration of 3.
À
isomer is shown, and the asterisk indicates the position of the C N
bond to this ring in the minor regioisomer. Bn=benzyl.
this strategy from acyl oximes 5 (Scheme 6). The substituent
at the 3-position of 5 could be an ester (products 6a–c) or an
amide group (product 6d), and the products were isolated in
good yield (78–89%). Products 6e and 6 f with R2 as an n-
propyl or a benzyl group were obtained in 91 and 87% yield,
respectively. R3 could also be an aryl group instead of an alkyl
group. Thus, tetrasubstituted pyridines 6g–i were obtained in
83–90% yield. Trisubstituted pyridines 6j–n with different
functionalities could also be synthesized in moderate yield
(45–55%).[16]
Having established this strategy for generating phenan-
thridines, quinolines, and pyridines, we next sought to
demonstrate the utility of this method in rapid total syntheses
of the benzo[c]phenanthridine alkaloids noravicine (11) and
nornitidine (12), which show a wide range of pharmacological
properties, in particular, antitumor activity (Scheme 7).[17]
Previously reported routes to 11 and 12 required 11 steps
and provided the products in about 20% overall yield.[18] Our
Scheme 6. Scope of the transformation of acyl oximes into pyridines.
Reaction conditions: A mixture of 5 (0.2 mmol) and fac-[Ir(ppy)3]
(0.002 mmol, 1.0 mol%) in dry DMF (4.0 mL) was irradiated by 5 W
white LEDs for 10 h. Yields are for the isolated product.
À
strategy began with the iridium-catalyzed C H borylation of
7,[19] followed by Suzuki coupling, to give biphenyl aldehydes
9a and 9b. The treatment of 9a with hydroxylamine
hydrochloride and then acylation with p-trifluoromethylben-
zoyl chloride provided acyl oxime 10a in near-quantitative
yield. Ultimately, the alkaloid noravicine (11) was completed
in 93% yield by the cyclization of 10a under our established
conditions. Similarly, nornitidine (12) was synthesized from
9b in 90% overall yield for three steps.
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In summary, we have described the visible-light-promoted
formation of iminyl radicals from acyl oximes. This trans-
formation was utilized to construct a range of heteroarenes
with N-containing six-membered rings, including pyridines,
quinolines, and phenanthridines. These sophisticated N-con-
taining heterocycles were constructed in satisfactory to
excellent yields from a broad range of substrates simply by
irradiation with visible light in the presence of 1.0 mol% of
a photocatalyst at room temperature. Moreover, five-step
rapid and highly efficient total syntheses of the biologically
active alkaloids noravicine and nornitidine were accom-
plished by using this strategy as the key step. We expect this
powerful protocol to be of broad utility in the synthesis of
biologically important N-containing heterocycles, including
natural products.
Received: November 23, 2014
Published online: && &&, &&&&
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Keywords: acyl oximes · N heteroarenes · iminyl radicals ·
photochemistry · visible light
.
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Photochemistry
H. Jiang, X. An, K. Tong, T. Zheng,
Y. Zhang,* S. Yu*
&&&& — &&&&
Visible-Light-Promoted Iminyl-Radical
Formation from Acyl Oximes: A Unified
Approach to Pyridines, Quinolines, and
Phenanthridines
See the light: A unified strategy involving
visible-light-induced iminyl-radical for-
mation followed by intramolecular
homolytic aromatic substitution (HAS)
has been established for the efficient
construction of heteroarenes with a N-
containing six-membered ring (see
scheme; ppy=2-phenylpyridine, SET=
single-electron transfer). This strategy
was applied to a five-step concise syn-
thesis of benzo[c]phenanthridine alka-
loids.
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!