Palladium-Catalyzed 6-Endo Selective Alkyl-Heck Reactions: Access to 5-Phenyl-1,2,3,6-tetrahydropyridine Derivatives

Article abstract of DOI:10.1021/acs.orglett.6b01787

A new type of palladium-catalyzed 6-endo-selective alkyl-Heck reaction of unactivated alkyl iodides has been described. This strategy provides efficient access to a variety of 5-phenyl-1,2,3,6-tetrahydropyridine derivatives, which are important structural

Full text of DOI:10.1021/acs.orglett.6b01787

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed 6‑Endo Selective Alkyl-Heck Reactions: Access
to 5‑Phenyl-1,2,3,6-tetrahydropyridine Derivatives
Xu Dong,† Ying Han,† Fachao Yan,† Qing Liu,*,† Ping Wang,† Kexun Chen,† Yueyun Li,†
Zengdian Zhao,† Yunhui Dong,† and Hui Liu*,†,‡
†School of Chemical Engineering, Shandong University of Technology, 266 West Xincun Road, Zibo 255049, P. R. China
‡College of Materials Science and Engineering, Hunan University, Changsha, Hunan 410082, China
S
* Supporting Information
ABSTRACT: A new type of palladium-catalyzed 6-endo-selective
alkyl-Heck reaction of unactivated alkyl iodides has been
described. This strategy provides efficient access to a variety of
5-phenyl-1,2,3,6-tetrahydropyridine derivatives, which are impor-
tant structural motifs for bioactive molecules. This process
displays a broad substrate scope with excellent 6-endo selectivity.
Mechanistic investigations reveal that this alkyl-Heck reaction
performs via a hybrid palladium-radical process.
Scheme 1. Palladium-Catalyzed Intramolecular Alkyl-Heck-
Type Reactions
he palladium-catalyzed Mizoroki−Heck reaction is a
Tfundamental synthetic tool throughout organic chemistry
and related disciplines, achieving the direct cross-coupling of
alkenes with halides or sulfonates to valuable molecules.1 The
significant contribution of the Heck reaction to synthetic
chemistry led to R. Heck, A. Suzuki (Suzuki Coupling) and E.
Negishi (Negishi Coupling) being awarded the 2010 Nobel
Prize in chemistry.2 Compared with the Heck reaction of Csp2
halides, the analogues reaction with Csp3 halides, in particular,
unactivated substrates bearing β-hydride, has been less
investigated.3 The harsh conditions for the oxidative addition
of Csp3 halides to low-valent transition metals and premature β-
hydride elimination of putative alkylmetal are significant
challenges for chemists.4
Despite the difficulties in alkyl-Heck-type transformation,
some elegant strategies have emerged recently. In 2007, Fu and
co-workers developed the first palladium-catalyzed intra-
molecular Heck reactions of unactivated, β-hydrogen-contain-
ing alkyl halides (Br, Cl) via 5-exo-trig cyclization (Scheme
1a).5 NHC (N-heterocyclic carbene) ligands were considered
to promote migratory insertion over the competitive β-hydride
elimination, in which an SN2 oxidative addition mechanism was
proposed but not a radical process. In 2011, the Alexanian
group described 5-exo-trig and 6-exo-trig types of intra-
molecular alkyl-Heck reactions, and a hybrid palladium-radical
mechanism was demonstrated to explain the suppression of β-
hydride elimination (Scheme 1a).6 Both protocols underwent
conventional exo selective cyclization (Scheme 1a). However,
an endo-trig Heck reaction is quite rare;7 to date, there is only
one example of an endo-selective alkyl-Heck reaction reported
by the Gevorgyan group in 2014.8 The 7,8,9-endo-selective
alkyl-Heck reactions were described, which were suggested to
occur via a hybrid palladium-radical process (Scheme 1b). Two
crucial factors promote the transformations: (a) the use of the
silyl tether favor endo-selective cyclization without substrate
bias; (b) no competitive β-hydride elimination issue exists.
However, to the best of our knowledge, there are no reports on
the 6-endo-trig-selective alkyl-Heck reaction, which inspires us
to investigate this untouched chemistry. Herein, we report a
highly 6-endo-selective alkyl-Heck reaction with unactivated
alkyl iodides bearing a β-hydride, delivering 5-phenyl-1,2,3,6-
tetrahydropyridine structural motifs efficiently (Scheme 1c).
The 5-exo-trig cyclization is not detected.
Received: June 20, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01787
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Organic Letters
Letter
Our laboratory has been interested in developing palladium-
promoted radical reactions with unactivated alkyl iodides
bearing a β-hydride.9 We are aware that 5-exo cyclization is a
much more tendentious route than the 6-endo style, while 6-
endo forms a relatively stable secondary or tertiary radical
intermediate.10 To reconsider the radical mechanism, we
hypothesize that if the stability of radical intermediate
generated from the radical addition to the double bond is
increased, the reaction might be impelled to proceed as a 6-endo
cyclization. As designed in Scheme 1c, substrate I is supposed
to generate a fairly stable tertiary-benzyl radical intermediate II
via 6-endo-trig cyclization (VS 5-exo), followed by β-hydride
elimination to furnish 5-phenyl-1,2,3,6-tetrahydropyridine
derivates III. Notably, this transformation might provide a
distinct strategy to convert facile synthesized substrates into
various 5-phenyl-1,2,3,6-tetrahydropyridine derivatives, a struc-
tural motif that has been widely found in natural products and
pharmaceutical compounds, such as the P75NTR receptor
inhibitor, Fms-like tyrosine kinase 3 inhibitor, C-KIT inhibitor,
Chemokine receptor CXCR3 inhibitor, and (+)-Ipalbidine (for
details see Supporting Information).11
5). By screening the reaction temperature, we found that
decreasing the reaction temperature to 110 °C could give 2a in
80% yield; however, the yield decreased suddenly to a 26%
yield together with high recovery of the starting material at 80
°C, indicating the temperature was crucial to this trans-
formation (entries 6 and 7). Employment of different catalysts/
ligands, such as Pd(OAc)2/dppf, Pd(OAc)2/PPh3, Pd(PPh3)4,
and PdCl2(PPh3)2, did not give positive results (entries 8−11),
except the combination of Pd(PPh3)4 and dppf afforded a 73%
yield (entry 12). Furthermore, the screening of solvent showed
that toluene was also the better choice (entries 13−16). On the
basis of these results, the optimal conditions were determined
to include PdCl2(dppf) combined with Cy2NMe in toluene at
110 °C (entry 6).
With the optimized conditions in hand, we next aimed to
explore the substrate scope of this transformation. Excellent
functional group compatibility was obtained among the aryl
rings (Scheme 2). The substrates with strong electron-donating
ab
,
Scheme 2. Scope of 6-Endo Heck-Type Cyclyzation
To test the hypothesis, our efforts commenced with the
investigation of unactivated iodide 1a (Table 1). Substrate 1a
Table 1. Alkyl-Heck Reaction of an Unactivated Alkyl
a
Iodides: Influence of Reaction Parameters
b
entry
catalyst
base
solvent
t (°C) yield (%)
1
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
Pd(OAc)2/dppf
K2CO3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
PhCF3
xylene
130
130
130
130
130
110
80
NR
NR
48
2
Cs2CO3
Et3N
3
4
DIPEA
33
5
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
Cy2NMe
79
6
80
c
7
26
d
e
8
110
110
110
110
110
110
110
110
110
ND
ND
20
9
Pd(OAc)2/PPh3
Pd(PPh3)4
10
11
12
13
14
15
16
PdCl2(PPh3)2
Pd(PPh3)4/dppf
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
PdCl2(dppf)
24
d
73
39
53
dioxane
PhCl
45
ND
a
Reaction conditions: 1a (0.2 mmol), [Pd] (0.02 mmol), base (0.4
a
b
c
Reaction Conditions: 1 (0.2 mmol), PdCl2(dppf) (0.02 mmol),
mmol), solvent (2.0 mL). Isolated yields. The starting material was
recovered in 71% yield. 30 mol % dppf was used. 25 mol % PPh3 was
used.
b
d
e
Cy2NMe (0.4 mmol), toluene (2.0 mL). Isolated yields.
groups, such as alkoxy at the ortho and meta positions, resulted
in formation of the corresponding products smoothly.
Interestingly, the ortho-OEt group led to a decrease in yield;
however, the disubstituted substrate could furnish the desired
tetrahydropyridine derivates 2d in excellent yield. Furthermore,
the substrate possessing the strong electron-withdrawing
substituent CN efficiently produced 6-endo product 2e.
Remarkably, the reaction showed great tolerance to the halides;
in particularly, bromo could survive very well under these
conditions,12 which was potentially transformed to more
valuable molecules via cross-coupling (2h and 2i).
was subjected to the conditions comprised of 10 mol %
PdCl2(dppf) and 2.0 equiv of K2CO3 at 130 °C in toluene
under N2. However, no reaction was detected (entry 1). The
same result was obtained from Cs2CO3 (entry 2). Gratifyingly,
when Et3N was used as a base, the desired 6-membered
tetrahydropyridine derivate 2a was obtained in 48% yield (entry
3). Following these promising results, we continued with
further exploration of organic bases. To our delight, when N-
cyclohexyl-N-methylcyclohexanamine instead of Et3N was used
as the base, the yield was increased dramatically to 79% (entry
B
DOI: 10.1021/acs.orglett.6b01787
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
To further explore the substrate scope of this strategy, the
substitutions at alkyl tether were investigated. Introduction of
different groups, such as Ph, Me, iPr, iBu, and Bn at the C2-
position of the alkyl tether, resulted in good to excellent yields
(Scheme 2, 2j−2n). In addition, the secondary alkyl iodide also
performed successfully, affording the endo cyclized product 2o
in 58% yield (Scheme 2, 2o).
Scheme 4. Deuterium Scrambling Investigation
To gain a better understanding of this transformation, we are
eager to explore the reaction mechanism. As demonstrated in
Scheme 1c, alkyl iodide is supposed to generate a radical
intermediate easily in the presence of Pd(0), discussed by
Cook,10e−g Alexanian,13 Gevorgyan,8 Zhou,14 Ryu,15 Tong,16
Jiang,17 and so on.18 These prior works led us to propose a
hybrid palladium-radical mechanism as shown in path a
(Scheme 3). Taking the deuterated substrate 1a-D as an
In order to obtain direct evidence for the formation of
carbon-centered radicals in the current reaction, we performed
this cyclization in the presence of TEMPO (2,2,6,6-tetramethyl-
piperidine 1-oxyl) (Scheme 5). After screening several
ab c
, ,
Scheme 5. TEMPO Testing Experiment
Scheme 3. Proposed Reaction Mechanism for 6-Endo Alkyl-
Heck Reaction
a
Reaction conditions: 1a (0.2 mmol), toluene (2.0 mL), 130 °C. N2,
b
c
16 h. Isolated yields. No 1a was recovered.
conditions (for details see SI), we were pleased to isolate 3a
in 31% yield by using 100 mol % Pd(PPh3)4, accompanying
12% cyclization product 2a. This evidence strongly proved the
generation of a carbon-centered radical intermediate. So the
radical mechanism, path a in Scheme 3, was preliminarily
established.
In summary, we have developed a highly 6-endo-selective
alkyl-Heck reaction catalyzed by palladium. This strategy is
applicable to the synthesis of valuable 5-phenyl-1,2,3,6-
tetrahydropyridine scaffolds and tolerates a variety of alkyl
iodides. A hybrid palladium-radical process is strongly
supported by the results of a TEMPO testing experiment and
deuterium scrambling investigation. The stable tertiary-benzyl
radical intermediate is crucial for the highly 6-endo selectivity.
Further applications are in progress in our laboratory.
example, alkyl radical 1a-D-I will be generated under the
standard conditions, which subsequently affords a fairly stable
tertiary-benzyl radical 1a-D-II via 6-endo cyclization. Next,
recombination of 1a-D-II with ·PdI produces racemic
alkylpalladium intermediate 1a-D-III, which undergoes β-
hydride or β-deuterium elimination delivering products 2a-D-I
with both H and D. Alternatively, hydrogen abstraction from
1a-D-II by ·PdI could also be responsible for 2a-D-I. In
addition, the classical Heck-type mechanism path b is also an
alternative choice for this reaction (Scheme 3). Followed by
oxidative addition of 1a-D, stereospecifically intramolecular
insertion of olefin to the C−Pd bond via transition state 1a-D-
IV gives intermediate 1a-D-V with a well-defined stereo-
chemistry. Finally, syn selective β-hydride elimination should
perform to yield 2a-D-II, where deuterium is reserved fully.
Basing on the hypothesis of deuterium scrambling, the
deuterated substrate 1a-D (90% D) with (E)-configuration
terminal olefin was synthesized, which was then subjected to
the optimized conditions (Scheme 4). To our delight, the
deuterated product 2a-D was isolated in 77% yield with
deuterium erosion to 56%. Apparently, the observation of
deuterium erosion supported the formation of radical
intermediate 1a-D-II which might undergo configuration
scrambling naturally. This process was responsible for the
deuterium erosion of the product, demonstrating the 6-endo-
alkyl-Heck reaction was more likely to undergo a hybrid
palladium-radical process.
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.6b01787.
Experimental procedures and compound characterization
data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: qingliu0315@163.com.
*E-mail: huiliu1030@sdut.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by NSFC (21302057 and
21405095), the Young Talents Joint Fund of Shandong
Province (ZR2015JL005), and Special Funding for Postdoc-
C
DOI: 10.1021/acs.orglett.6b01787
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Organic Letters
Letter
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We are grateful to the support from Zibo Positive Additive Co.,
Ltd.
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Org. Lett. XXXX, XXX, XXX−XXX