Merging " Anti-Baldwin" 3- Exo-Dig Cyclization with 1,2-Alkynyl Migration for Radical Alkylalkynylation of Unactivated Olefins

Article abstract of DOI:10.1021/acs.orglett.8b01382

A new combination of "anti-Baldwin" 3-exo-dig cyclization with 1,2-alkynyl migration of 1,4-enynes with simple cycloalkanes was established, enabling C-C breaking and reconstruction to access a wide range of α-alkynyl ketones with generally good yields by

Full text of DOI:10.1021/acs.orglett.8b01382

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Merging “Anti-Baldwin” 3-Exo-Dig Cyclization with 1,2-Alkynyl
Migration for Radical Alkylalkynylation of Unactivated Olefins
Qi Zhao, Xiao-Shuang Ji, Yi-Yun Gao, Wen-Juan Hao,* Ke-Ying Zhang, Shu-Jiang Tu,* and Bo Jiang*
School of Chemistry & Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu
Normal University, Xuzhou, 221116, P. R. China
*
S Supporting Information
ABSTRACT: A new combination of “anti-Baldwin” 3-exo-dig cycliza-
tion with 1,2-alkynyl migration of 1,4-enynes with simple cycloalkanes
was established, enabling C−C breaking and reconstruction to access a
wide range of α-alkynyl ketones with generally good yields by FeCl /di-
2
tert-butyl peroxide (DTBP) as a catalytic oxidation system. Radical-
3
induced C(sp )−H functionalization of cycloalkanes was realized,
3
3
3
leading to the direct formation of C(sp )−C(sp ) and C(sp)−C(sp ) bonds. The mechanism for forming α-alkynyl ketones was
proposed.
ue to low polarity and high bond-dissociation energy
Scheme 1. Radical-Induced Alkynyl Migration
3
D
(BDE), C(sp )−H functionalization has become a
challenging and attractive mechanism and has aroused the
interest of many chemists. Over the years, substantial significant
1
advances have been achieved in this field, in which the vast
3
majority of efforts had been made on C(sp )−H functionaliza-
2
3
4
tion adjacent to heteroatoms, phenyl, alkenyl, and electron-
5
3
withdrawing groups. In contrast, research on inert C(sp )−H
functionalization of simple alkanes has been relatively less
popular, but is full of challenge. Recently, oxidative coupling
6
reactions (OCRs) have proven to be a powerful and promising
tool for forging carbon−carbon and carbon−heteroatom bonds
in the organic community, because of its step and atom
economy without prefunctionalizations, therefore providing
3
high-efficient pathways for inert C(sp )−H functionalization of
7
simple alkanes. For example, Li’s group reported an interesting
radical is more stable than the secondary carbon radical. Thus,
we considered preparing the 1,4-enynes with the replacement
of an isopropenyl group for the vinyl moiety. In view of the
abundance and easily availability of cycloalkanes and the
ruthenium catalyzed oxidative coupling reaction between
3
cycloalkanes and chelating arenes for direct inactive C(sp )−
8
H functionalization. Later, our group discovered an
3
unprecedented dual α,α-C(sp )−H difunctionalization of
3
9
continuation of our efforts on the direct C(sp )−H
cycloalkanes under oxidative conditions. Despite these limited
functionalization, we attempted to employ the preformed 1,4-
enynes with an isopropenyl unit to react with simple
cycloalkanes to evaluate the possibility of 1,2-alkynyl migration.
significant advances, the development of a new and facile
3
protocol for inert C(sp )−H bond functionalization of
cycloalkanes toward functionalized molecules is still highly
desirable.
Interestingly, with FeCl /di-tert-butyl peroxide (DTBP) as the
2
catalytic oxidation system, the reaction worked well to afford
the corresponding α-alkynyl ketones with generally good yields
via radical 3-exo-dig cyclization/1,2-alkynyl migration. Notably,
in Baldwin’s rules, 3-exo-dig cyclization is generally considered
to be unfavorable. During our reaction process, this reaction
involving “anti-Baldwin” 3-exo-dig cyclization enabled C−C
breaking and reconstruction via radical-induced alkylalkynyla-
tion of unactivated alkenes under the mild catalytic oxidative
conditions. Here, we describe a new and attractive1,2-alkynyl
Over the past decade, we have witnessed a growing boom in
1
0
the radical-induced difunctionalization of alkenes, especially
11
via the functionality migration strategy. Specifically, Zhu and
other groups reported the difunctionalization of alkenes by
1
2
13
14
15
16
17
aryl, cyano, formyl, heteroaryl, alkynyl, and alkenyl
migration, respectively. Notably, Zhu’s group pioneered radical
alkynylation of unactivated alkenes through distal 1,4- and 1,5-
alkynyl migration, but 1,2- and 1,3-alkynyl migration did not
16a
work (Scheme 1a).
They suggested that the migration
prefers five- or six-membered cyclic transition states thermo-
dynamically rather than three- or four membered cyclic
Received: May 1, 2018
1
6a
transition states. It is well-known that the tertiary carbon
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01382
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
,
ab
migration of 1,4-enynes via radical relay for achieving the
C(sp )−H bond alkylation of cycloalkanes. To the best of our
knowledge, there is no precedent report on the 1,2-alkynyl
Scheme 2. Substrate Scope of 1,2-Alknyl Migration
3
migration of 1,4-enynes through radical-triggered “anti-
18
Baldwin” 3-exo-dig cyclization.
Our initial attempts focused on identifying the optimized
conditions for the reaction of the preformed 1,4-enyne 1a and
cyclohexane 2a as both a reaction component and reaction
media. With FeCl as the catalyst and tert-butyl peroxybenzoate
2
(
TBPB, 4.0 equiv) as the oxidant, the reaction proceeded
readily at 120 °C, leading to the clean formation of the
migration product 3a in 52% yield (Table 1, entry 1). This
a
Table 1. Optimization of the Reaction Conditions
b
entry
cat. (10 mol %)
oxidant (equiv)
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
9
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
TBPB (4.0)
TBHP (4.0)
DTBP (4.0)
DTBP (4.0)
DTBP (4.0)
DTBP (3.0)
DTBP (2.0)
DTBP (4.0)
DTBP (4.0)
DTBP (4.0)
DTBP (4.0)
DTBP (4.0)
120
120
120
110
100
120
120
120
120
120
120
120
52
trace
65
47
37
58
35
60

FeCl3
49
10
11
12
CuI
53
Cu O
2
47
Cu(OTf)2
trace
a
Reaction conditions: 1,4-enyne 1a (0.2 mmol, 1.0 equiv), cyclo-
hexane 2a (2.0 mL), catalyst (10 mol %), oxidant (x equiv), under the
b
air conditions, for 12 h. Isolated yield based on 1a.
satisfactory result prompted us to further search for the optimal
reaction conditions. Changing the oxidant from TBPB to tert-
butyl hydroperoxide (TBHP) completely suppressed the
reaction process (entry 2). The use of di-tert-butyl peroxide
(
3
DTBP) as the oxidant gave the best outcome as the product
a was offered in 65% yield (entry 3). We discovered that the
reaction temperature was an important factor in affecting the
reaction efficiency. The decrease of the reaction temperature is
not beneficial to the transformation, and inferior results were
observed (entries 4 and 5). Moreover, the yield of product 3a
dropped remarkably when lowering the amount of DTBP
a
Reaction conditions: 1 (0.2 mmol, 1.0 equiv), cyclohexane 2a (2.0
mL), FeCl (10 mol %), DTBP (4.0 equiv), under the air conditions,
1
2
b
2 h. Isolated yield in parentheses based on 1.
(
entries 6 and 7). Without FeCl , the transformation could give
yields. Diverse functionalities, such as methyl (1b and 1c), ethyl
(1d), tert-butyl (1e), methoxy (PMP = p-methoxyphenyl, 1f),
chloro (1g, 1h and 1i), and bromo (1j), were proven to be
favorable for this alkynyl migration. Notably, 1,4-enynes
carrying thienyl (1k) and n-butyl (1l) on the alkynyl moiety
could be readily transformed into α-alkynyl ketones 3k and 3l,
respectively, albeit with 40% and 30% yields. The electronic
2
a 60% yield of product, but necessitated the prolonging of the
reaction time to 32 h (entry 8). The following screening of
metal salts revealed that FeCl , CuI, Cu O, and Cu(OTf) were
3
2
2
found to be inferior in terms of catalytic performance in the
transformation as compared with FeCl (entries 9−12).
2
With the optimized reaction conditions in hand, we set out
to investigate the generality of the migration reaction of 1,4-
enynes by utilizing cycloalkanes as the solvent and C-radical
precursors (Scheme 2). 1,4-Enynes 1 with diverse function-
alities were first examined in combination with cyclohexane
2
nature of substituents (R ) attached by a quaternary carbon of
1,4-enynes was then evaluated. The alkynyl migration reaction
still worked well with various functional groups, including
methyl, tert-butyl, methoxy, fluoro, chloro, and bromo, locating
2
(2a) under the optimal conditions. Both electron-rich and
at the different positions of the phenyl ring (R ), enabling
electron-poor groups at different positions of the arylalkynyl
moiety (R ) can all tolerate this radical relay system well,
delivering the corresponding products 3b−3j in 30%−74%
radical-induced alkylalkynylation of unactivated alkenes to
access the corresponding α-alkynyl ketone products 3m−3w
with yields from 46% to 66%. Moreover, a sterically
1
B
DOI: 10.1021/acs.orglett.8b01382
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
encumbered 2-naphthyl (2-Np) analogue 1x was an appropriate
reaction partner, which underwent the similar radical relay
alkynyl migration process toward the corresponding product 3x
in 41% yield. Alternatively, thienyl and ethyl functionalities
linked by quaternary carbon (1y and 1z) were found to show
high reactivity, resulting in thienyl and ethyl substituted α-
alkynyl ketones 3y and 3z in 54% and 46% yields, respectively.
Next, the scope with respect to other cycloalkane
components as C-radical precursors, such as cyclopentane
the methyl group linked to the terminal olefin unit was the key
in the success of this transformation (Scheme 4b).
On the basis of the above results and previous reports,
9
,16a
a
reasonable mechanism is depicted in Scheme 5. Initially, Fe(II)
Scheme 5. Plausible Reaction Pathways
(
2b) and cycloheptane (2c), was investigated. As anticipated,
both of these cycloalkanes were proven to be suitable
substrates, thus enabling the C-radical relay approach to furnish
the corresponding α-alkynyl ketones 3aa−3dd in moderate
yields (Scheme 3). The structures of these α-alkynyl ketones
Scheme 3. Application of 1,2-Alknyl Migration ,
a b
catalyzes the decomposition of DTBP to afford a tert-butoxy
radical intermediate and tert-butoxy anion, which is further
oxidized by Fe(III) to form a tert-butoxy radical intermediate
9
and release Fe(II). Then, cyclohexane is converted into
cyclohexane radical A by H-abstraction of the tert-butoxy
radical. The addition of cyclohexane radical A to the terminal
alkene unit of 1,4-enyne 1a gives radical intermediate B,
followed by 3-exo-dig cyclization (“anti-Baldwin” rules) and
alkynyl migration (the homolysis of the C−C bond) to render
hydroxyalkyl radical D. Finally, a single electron oxidation of
the intermediate D leads to the carbon cation E, which loses a
proton to give the desired product 3a.
a
Reaction conditions: 1 (0.2 mmol, 1.0 equiv), cycloalkanes 2b−c (2.0
mL), FeCl (10 mol %), DTBP (4.0 equiv), under the air conditions,
1
2
b
2 h. Isolated yield in parentheses based on 1.
In summary, starting from easily available 1,4-enynes and
simple cycloalkanes, we have established a new combination of
were fully characterized by their NMR spectroscopy and
HRMS data. Furthermore, sulfonyl hydrazone 4a derived from
α-alkynyl ketone 3a and p-tosylhydrazine was confirmed by X-
ray diffraction analysis (CCDC 1844500).
“
anti-Baldwin” 3-exo-dig cyclization with 1,2-alkynyl migration
for the practical synthesis of a wide range of α-alkynyl ketones
with generally good yields under the catalytic oxidative
conditions. The C-centered radical-induced alkylalkynylation
To understand the mechanism of this reaction, 2,2,6,6-
3
of unactivated olefins enabled the direct formation of C(sp )−
19
tetramethyl-1-piperidinyloxy (TEMPO) as the radical scav-
enger was subjected to the reaction system under the standard
conditions; the desired α-alkynyl ketone 3a was not observed
and the starting materials were recovered, indicating that this
transformation may include a radical process (Scheme 4a).
When the methyl group on the terminal olefin unit was
replaced with hydrogen, the reaction of 1,4-enyne 1a′ gave a
complex mixture under the standard conditions, showing that
3
3
C(sp ) and C(sp)−C(sp ) bonds with simultaneous realization
3
of C(sp )−H functionalization in one step. This chemistry is
very rarely seen in chemical science. Further application of this
attractive radical 1,2-alkynyl migration strategy is underway in
our laboratory.
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.8b01382.
Scheme 4. Control Experiments
Experimental procedures and spectroscopic data for all
new compounds 3a−3dd (PDF)
Accession Codes
CCDC 1844500 contains 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
1
EZ, UK; fax: +44 1223 336033.
C
DOI: 10.1021/acs.orglett.8b01382
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
1
m) Palmer, W. N.; Zarate, C.; Chirik, P. J. J. Am. Chem. Soc. 2017,
AUTHOR INFORMATION
Corresponding Authors
■
39, 2589.
4) (a) Rakshit, S.; Patureau, F. W.; Glorius, F. J. Am. Chem. Soc.
(
*
*
*
E-mail: wjhao@jsnu.edu.cn.
E-mail: laotu@jsnu.edu.cn.
E-mail: jiangchem@jsnu.edu.cn.
2010, 132, 9585. (b) Ramirez, T. A.; Zhao, B.; Shi, Y. Chem. Soc. Rev.
2012, 41, 931. (c) Shi, E.; Shao, Y.; Chen, S.; Hu, H.; Liu, Z.; Zhang,
J.; Wan, X. Org. Lett. 2012, 14, 3384. (d) Sekine, M.; Ilies, L.;
Nakamura, E. Org. Lett. 2013, 15, 714. (e) Cuthbertson, J. D.;
MacMillan, D. W. C. Nature 2015, 519, 74. (f) Lee, S. Y.; Hartwig, J. F.
J. Am. Chem. Soc. 2016, 138, 15278. (g) Zang, Z.-L.; Zhao, S.;
Karnakanti, S.; Liu, C.-L.; Shao, P.-L.; He, Y. Org. Lett. 2016, 18, 5014.
ORCID
Bo Jiang: 0000-0003-3878-515X
Notes
(
h) Bao, W.; Kossen, H.; Schneider, U. J. Am. Chem. Soc. 2017, 139,
4362. (i) Michigami, K.; Mita, T.; Sato, Y. J. Am. Chem. Soc. 2017, 139,
094. (j) Mita, T.; Hanagata, S.; Michigami, K.; Sato, Y. Org. Lett.
The authors declare no competing financial interest.
6
ACKNOWLEDGMENTS
2017, 19, 5876. (k) Zhou, R.; Liu, H.-W.; Tao, H.-R.; Yu, X.-J.; Wu, J.
Chem. Sci. 2017, 8, 4654.
■
We are grateful for financial support from the NSFC (Nos.
1472071 and 21602087), PAPD of Jiangsu Higher Education
Institutions, the Outstanding Youth Fund of JSNU
YQ2015003), NSF of Jiangsu Province (BK20160212), and
the Qing Lan Project of Jiangsu Education Committee.
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