Organoselenium-Catalyzed Regioselective C?H Pyridination of 1,3-Dienes and Alkenes

Article abstract of DOI:10.1002/anie.201610657

An efficient approach for organoselenium-catalyzed regioselective C?H pyridination of 1,3-dienes to form pyridinium salts has been developed. This method was also successfully applied to direct C?H pyridination of alkenes. Fluoropyridinium reagents, or initially loaded pyridine derivatives, acted as pyridine sources in the pyridination reactions. The obtained pyridinium salts could be further converted under different conditions. This work is the first example of catalytic C-2 direct C?H functionalization of 1,3-dienes and the first case of organoselenium-catalyzed C?H pyridination.

Full text of DOI:10.1002/anie.201610657

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International Edition: DOI: 10.1002/anie.201610657
German Edition: DOI: 10.1002/ange.201610657
Pyridinium Functionalization
À
Organoselenium-Catalyzed Regioselective C H Pyridination of
1,3-Dienes and Alkenes
Lihao Liao, Ruizhi Guo, and Xiaodan Zhao*
Abstract: An efficient approach for organoselenium-catalyzed
À
regioselective C H pyridination of 1,3-dienes to form pyridi-
nium salts has been developed. This method was also success-
À
fully applied to direct C H pyridination of alkenes. Fluoro-
pyridinium reagents, or initially loaded pyridine derivatives,
acted as pyridine sources in the pyridination reactions. The
obtained pyridinium salts could be further converted under
different conditions. This work is the first example of catalytic
À
C-2 direct C H functionalization of 1,3-dienes and the first
À
case of organoselenium-catalyzed C H pyridination.
À
D
irect C H functionalization is an ideal strategy to quickly
synthesize valuable compounds. Numerous methods, includ-
ing metal catalysis and radical pathways, have been developed
for this purpose.[1] However, regioselectivity and efficiency
are still two challenging issues in this field when it comes to
introduction of different functional groups into parent
molecules. It is well-known that 1,3-dienes are versatile
precursors in organic synthesis. The direct functionalization of
À
À
their vinylic C H bonds has suffered from similar problems.
Scheme 1. Regioselective C H functionalization of 1,3-dienes.
À
In general, direct C H functionalization occurred at the
C-1 position of 1,3-dienes and hardly at other positions;
limited types of functional groups, such as aryl,[2] boryl,[3]
fluoroacyl,[4] silyl,[5] and so on,[6] could be incorporated
through Heck-type reactions (Scheme 1a, left). Owing to
the highly reactive nature of conjugated double bonds, it was
derivatives with N-fluorobenzenesulfonimide (NFSI) and
internal alkenes.[9d] Shortly after, our group discovered that
allylic alcohols could be aminated regio- and stereoselectively
by organoselenium catalysis with NFSI when we pursued the
[10b]
À
À
difficult to directly functionalize vinylic C H bonds at the C-2
method with respect to C H functionalization.
As a con-
position of 1,3-dienes. Until now, no efficient methods have
been demonstrated with respect to this transformation,
although the functionalized compounds can act as good
synthons for accessing targets that are difficult to make by
known methods.[7] Thus, developing an efficient approach for
sequence of the significance of C-2 functionalized 1,3-dienes,
we questioned whether 1,3-dienes could be functionalized
directly at the C-2 position by selenium catalysis (Scheme 1a,
right). According to the selenenylation–deselenenylation
process,[9,10] we rationalized that a species R’SeX formed
from the corresponding diselenides could react selectively
with the double bond of 1,3-dienes to form seleniranium ion I
(Scheme 1b). An equilibrium might exist between I and II. It
was expected that seleniranium ion I could be attacked by
nucleophiles selectively at the C-2 position to produce
intermediate III, followed by oxidation to generate C-2
functionalized products.
Pyridinium salts are an important class of pyridine
derivatives[11] with wide applications in materials,[12] and are
good precursors for the construction of piperidine frame-
works.[13] Therefore, functionalization of pyridine groups with
1,3-dienes is highly valuable. With this in mind, we envisioned
that pyridine could serve as a nucleophile to attack a sele-
niranium ion, resulting in the formation of pyridinium salt as
the product. Pyridine could be initially loaded or derived from
fluoropyridinium salts.[14] Although there is no report related
to the attack of free pyridine towards seleniranium ion in the
literature,[15] the stability of pyridinium salts might drive the
À
direct C-2 C H functionalization of 1,3-dienes is highly
desirable for the synthesis of 1,3-diene derivatives.
Electrophilic selenium catalysis has been paid much
attention in organic synthesis in recent years because of
mild conditions and good selectivities.[8–10] The technique has
emerged as a powerful tool for functionalization of alkenes,
especially in selective vinylic C H functionalization.
[9b–e,10a,b]
À
For example, in 2013 Breder et al. reported selenium-
catalyzed allylic and vinylic amination to generate imide
[*] L. Liao, R. Guo, Prof. Dr. X. Zhao
Institute of Organic Chemistry & MOE Key Laboratory of Bio-
inorganic and Synthetic Chemistry, School of Chemistry
Sun Yat-Sen University
Guangzhou 510275 (China)
E-mail: zhaoxd3@mail.sysu.edu.cn
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610657.
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occurrence of oxidation–elimination. Herein, we report our
discovery that 1,3-dienes could be directly pyridinated at the
C-2 position by electrophilic selenium catalysis with fluoro-
tetrahydrofuran (THF) led to the formation of the desired
product in 76% isolated yield (Table 1, entry 11). Higher
reaction concentration resulted in lower yield (Table 1,
entry 12). Using 10 mol% dibenzyl diselenide, 2a was gen-
erated in a reduced yield of 71% and no product was formed
in the absence of catalyst C5 (Table 1, entries 13 and 14).
With the optimized conditions in hand, the substrate
scope of 1,3-dienes was evaluated (Scheme 2). 1-Aryl-1,3-
pyridinium salts (Scheme 1c). The method could also be
[16]
À
applied to C H pyridination of alkenes.
We began our study with trans-1-phenyl-1,3-butadiene
(1a) as the model substrate. Fluoropyridinium salt P1 was
tentatively utilized as a pyridine source and oxidant. Satisfac-
torily, when 1a was treated with 1.2 equiv of P1 in the
presence of 20 mol% diphenyl diselenide in acetonitrile
(MeCN) at room temperature, the desired product was
À
butadienes underwent selective C H pyridination to give the
corresponding products in good yields (2b–e, 62–74%).
À
formed in 38% yield (Table 1, entry 1). The C H function-
alization took place selectively at the expected position, and
not at the carbon connecting to the phenyl group. Since
electronic effects have a profound impact on the catalytic
activity of a catalyst, diselenides with different groups were
screened (Table 1, entries 2–5). It was found that dibenzyl
diselenide was the most effective, giving the product in 69%
yield. Pyridination was affected slightly by using fluoro-
pyridinium salt with the anion BF4À (Table 1, entry 6). When
sterically hindered P3 or P4 was used, 1,3-dienes did not
undergo pyridination to give 2ab or 2ac (Table 1, entry 7).
Different solvents were less effective than MeCN (Table 1,
entries 8–10). However, the mixed solvents of MeCN and
Table 1: Evaluation of experimental conditions.[a]
À
Scheme 2. Substrate scope for regioselective C H pyridination of
1,3-dienes. Reaction conditions: 1 (0.10 mmol), P1 (1.2 equiv),
BnSeSeBn (20 mol%), MeCN–THF (v/v=1:1, 2 mL), RT, 18 h.
Entry Cat. PyF reagent
Solvent
Product Yield [%][b]
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C5
C5
C5
C5
C5
C5
C5
C5
–
P1
P1
P1
P1
P1
MeCN
MeCN
MeCN
MeCN
MeCN
2a
2a
2a
2a
2a
38
43
6
51
69
64
0
50
55
61
81(76)
68
71
0
Reactions of alkyl- substituted 1,3-dienes also proceeded
efficiently to form the desired pyridinium salts in excellent
yields (2 f, 89%; 2g, 89%).[17] It is noteworthy that a halogen
substituent tolerated the reaction conditions (2h, 60%). To
test the effect of the steric hindrance of substrates,
3,4-disubstituted dienes were synthesized for pyridination.
They transformed very efficiently to afford products in good
yields (2i, 81%; 2j, 71%). When 3,4-disubstituted dienes
derived from natural products (À)-myrtenal and dihydro
cuminyl alcohol were utilized, the corresponding products
were produced in moderate to good yields (2k, 76%; 2l,
53%). This result implies that the method has potential
applicability in modification of natural products. Moreover,
P2
MeCN
MeCN
2aa
2ab or 2ac
2a
P3 or P4
P1
Acetone
MeOCH2CH2OMe
THF
MeCN + THF (1:1)
MeCN + THF (1:1)
MeCN + THF (1:1)
MeCN + THF (1:1)
9
P1
P1
P1
P1
P1
P1
2a
2a
2a
2a
2a
2a
10
11
12[c]
13[d]
14
[a] Reaction conditions: 1a (0.05 mmol), PyF reagent (1.2 equiv), cat.
(20 mol%), solvent (1 mL), RT, 18 h. [b] Refers to NMR yield with
3,5-dimethylanisole as the internal standard; yield of isolated products is
in parentheses in 0.1 mmol scale. [c] Solvent (total 0.5 mL). [d] Cat.
(10 mol%).
À
C-2 selective C H pyridination of 4,4-disubstituted trienes
proceeded efficiently to generate product 2m in good yield
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(66%). Importantly, the isolated double bond on the sub-
strate was not affected under the conditions. When the steric
hindrance further increased at the C-4 position, pyridination
became sluggish. For example, 2n was formed only in
a moderate yield of 40%. Subsequently, pyridination of
1,4-disubstituted dienes was studied. 1-Methyl-4-phenyl-1,3-
butadiene (E/Z = 1.7:1) gave product 2o in 57% yield with
E/Z = 1:1.6. Furthermore, cycloheptatriene (3) underwent
pyridination to form the corresponding product 4 in 57%
yield with excellent regioselectivity [Eq. (1)].
This result clearly indicates that a conjugated aryl or vinyl
À
group heavily impacts on selective C H pyridination of
double bonds.
To generalize the method, we turned our attention to C-2
À
selective C H pyridination of 1,3-dienes using different
pyridines as the nitrogen sources (Scheme 4). Reaction
À
Scheme 4. Regioselective C H pyridination of 1a using different
pyridines. Reaction conditions: 1a (0.10 mmol), pyridine derivatives
(1.0 equiv), BnSeSeBn (5 mol%), [TMPyF]+[BF4]À (1.2 equiv), Select-
fluor (0.5 equiv), MeCN (2 mL), RT, 24 h.
À
This method could be applied to selective C H pyridina-
tion of alkenes (Scheme 3). Styrene derivatives with various
functional groups on aromatic rings worked very efficiently to
give the corresponding products in 62–96% yields (6a–j).
conditions were optimized with 1a as the substrate (see
Supporting Information). The desired products could be
efficiently afforded with N-fluoro-2,4,6-trimethylpyridinium
tetrafluoroborate ([TMPyF]+[BF4]À) and Selectfluor as the
co-oxidant.[18] For example, 2aa was formed in 86% yield.
When pyridinyl 1,3-dienes 7 were utilized as the substrates,
products 8 with a newly formed six-membered ring were
obtained in good yields [Eq. (2)]. The bicyclic pyridinium
salts are potential precursors for the synthesis of natural
products.[19]
À
Scheme 3. Regioselective C H pyridination of styrenes. Reaction con-
ditions: 5 (0.10 mmol), P1 (1.2 equiv), BnSeSeBn (20 mol%), MeCN–
THF (v/v=1:1, 2 mL), RT, 18 h.
Pyridinium salts can be easily converted into multifunc-
tional compounds.[13,20] For example, when 2a was treated
with ethyl acrylate, cycloadduct 9 was formed by the
Diels–Alder reaction in 75% yield with excellent stereo-
selectivity (Scheme 5).[7a] It could undergo depyridination to
give substituted 1,3-diene 10 in high yield with DBU. When
product 2a was treated with piperidine, the valuable amina-
tion product 11 (a potential precursor of diamine) was
obtained by the addition of piperidine into the conjugated
double bond. Additionally, vinyl pyridinium salts could be
further transformed. For example, reaction of 6a with
TMSCF3 gave the CF3-substituted dihydropyridine derivative
12 in 85% yield. When bicyclic pyridinium salt 8b was treated
with Et3N, an aromatization product 13 was formed in 97%
yield, which could be converted to quinolizidine derivative by
the known method.[21]
Electron-rich styrenes gave the desired products in higher
yields than that obtained with electron-deficient styrenes.
À
Similarly, (E)-internal alkene 5k underwent C H
pyridination to afford product 6k in excellent regio- and
stereoselectivity (6k, 94%, single isomer). When 1,2-dihy-
dronaphthalene 5l was utilized under similar conditions,
product 6l was formed in 87% yield. Electron-rich 2-vinyl-
naphthalene 5m and a heterocyclic aryl alkene such as
2-vinylthiophene 5n were also suitable substrates for this
transformation (6m, 79%; 6n, 92%). Moreover, when alkyl
terminal alkene was employed as the substrate under similar
conditions, a mixture of terminal pyridinated products were
generated in good total yield (see Supporting Information).
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possible that partial BnSeOTf reacts with pyridine to give 15.
Subsequently, reactive BnSeX interacts with 1a to form
intermediate I and II. Pyridine regioselectively attacks I to
generate III. After its oxidation and the following elimina-
tion, product 2a is formed and catalytic species BnSeX is
regenerated.
In summary, we have developed an efficient approach for
À
selective C H pyridination of 1,3-dienes to form pyridinium
salts by organoselenium catalysis. Trienes and alkenes also
À
underwent direct C H pyridination to efficiently give the
corresponding products. Mechanistic studies revealed that
a selenenylation–deselenenylation process is feasible. This
À
work enables direct C-2 C H functionalization of 1,3-dienes
and pyridination with fluoropyridinium salts. The develop-
ment of new reactions by organoselenium catalysis is ongoing
in our laboratory.
Scheme 5. Further transformations of pyridinium salts.
NMR experimental studies were conducted to further
elucidate the pyridination mechanism. Upon treatment of
dibenzyl diselenide with P1, followed by 1,3-diene 1a,
a selenenylated intermediate 14 was afforded in 65% NMR
yield (Scheme 6a). N-benzylpyridinium salt (15) was
concomitantly formed in a small amount of 5%. Salt 14
could be oxidized to afford product 2a and byproduct 15
(Scheme 6b). These results revealed that a selenenylation–
deselenenylation pathway is feasible for the aforementioned
Acknowledgements
We thank Sun Yat-Sen University, the “One Thousand Youth
Talents” Program of China and the Natural Science Founda-
tion of Guangdong Province (Grant No. 2014A030312018) for
financial support.
Conflict of interest
À
C H pyridination, and that the observed salt 15 is derived
from the decomposition of dibenzyl diselenide, which might
lead to somewhat high catalyst loading. Based on the above
studies, a possible mechanism for pyridination is depicted in
Scheme 6c. First, diselenide is oxidized by P1 to form BnSeX,
BnSeY (X = OTf, if Y= F or vice versa), and pyridine. It is
The authors declare no conflict of interest.
À
Keywords: 1,3-dienes · C H functionalization · pyridination ·
selenium catalysis · synthetic methods
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Manuscript received: November 1, 2016
Revised: December 9, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Pyridinium Functionalization
L. Liao, R. Guo, X. Zhao*
&&& — &&&
Organoselenium-Catalyzed
À
Regioselective C H Pyridination of 1,3-
Dienes and Alkenes
Pyridinium salts are formed in an orga-
alkenes. Fluoropyridinium reagents or
initially loaded pyridine derivatives acted
as pyridine sources in the pyridination
reactions.
À
noselenium-catalyzed regioselective C H
pyridination of 1,3-dienes. The efficient C-
2 selective method was successfully
À
applied to direct C H pyridination of
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!