Regioselective Copper-Catalyzed Oxidative Coupling of α-Alkylated Styrenes with Tertiary Alkyl Radicals

Article abstract of DOI:10.1021/acs.orglett.8b01600

A radical-mediated oxidative cross-coupling of readily accessible α-alkylated styrenes with 1,3-dicarbonyl compounds utilizing a combination of Cu(OAc)₂ and air as a catalytic system is described. Rather than requiring α-halocarbonyl compounds, this efficient approach enables direct installation of tertiary functionalized alkyl motifs to olefins with simple carbonyl derivatives. The novel protocol is characterized with high allylic selectivities via a competing β-H elimination. Both radical-clock and -trapping experiments provided clear-cut evidence for the intermediacy of an α-keto carbon-centered radical.

Full text of DOI:10.1021/acs.orglett.8b01600

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Regioselective Copper-Catalyzed Oxidative Coupling of α‑Alkylated
Styrenes with Tertiary Alkyl Radicals
Cong Wang,† Rui-Hua Liu,† Ming-Qing Tian,† Xu-Hong Hu,*,† and Teck-Peng Loh*,†,‡
†Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center
for Advanced Materials, Nanjing Tech University, Nanjing 211816, China
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: A radical-mediated oxidative cross-coupling of
readily accessible α-alkylated styrenes with 1,3-dicarbonyl
compounds utilizing a combination of Cu(OAc)2 and air as a
catalytic system is described. Rather than requiring α-
halocarbonyl compounds, this efficient approach enables
direct installation of tertiary functionalized alkyl motifs to
olefins with simple carbonyl derivatives. The novel protocol is
characterized with high allylic selectivities via a competing β-H elimination. Both radical-clock and -trapping experiments
provided clear-cut evidence for the intermediacy of an α-keto carbon-centered radical.
irect functionalization of olefins with simple coupling
Dpartners offers a particularly attractive tool for the rapid
construction of highly substituted olefins and analogues.1 By
virtue of atom and step economy, substantial progress has been
accomplished on the subject of radical-mediated carbon−
carbon bond-forming reactions,2 which often involve the
addition of in situ generated carbon-centered radical
intermediates from activated precursors to an olefin.
Classically, prefunctionalized alkyl halides are widely utilized
for coupling with olefins in alkylic Heck-type reactions,3 as
have been discovered by Oshima,4 Fu,5 Alexanian,6 Lei,7
Nishikata,8 Zhou,9 Li,10 and others.11 However, a highly
appealing alternative approach that remains a considerable
challenge would be a direct oxidation of hydrocarbon12 via
single-electron-transfer (SET) to generate sp3-hybridized
Figure 1. (a) Four possible challenges impeding our proposed
mechanistic pathway (path d): (i) oxidative cleavage of olefins via
path e; (ii) protonolysis of intermediate I via path a; (iii) radical
coupling via path b; (iv) competitive β-H elimination of intermediate
carbon-centered radicals.
Inspired by the extensive versatility of 1,3-dicarbonyl
compounds in synthetic chemistry, as well as Snider’s seminal
work13 on intramolecular oxidative addition of an α-keto
II via path c. (b) Our work for oxidative coupling of α-alkylated
carbon-centered radical14 into olefins, we designed an efficient
olefins with tertiary alkanes.
transformation for oxidative cross-coupling reactions of α-
alkylated styrenes with 1,3-dicarbonyl derivatives. In principle,
(Figure 1, path d).20 We hypothesized that allylic selectivity
might be achieved by sterically controlled β-H elimination on
the premise that congested carbon−carbon formation occurs.
Recently, Nishikata and co-workers reported the installation of
tertiary alkyl motifs with α-halocarbonyl compounds to olefins
through a copper/triamine catalyzed Heck-type reaction.8
Herein, we wish to report a complementary method to existing
alkylic Heck-type reactions, which represents the first example
the strategy for the sp3-hybridized carbon-centered radical
addition to olefins, mainly relying on the corresponding
halides, has been well documented in recent decades, generally
resulting in alkylation products by protonolysis15 (Figure 1,
path a) or interception by other radical species to yield
difunctionalizations10,16,17 (Figure 1, path b). Another note-
worthy pathway is that the intermediate I undergoes oxidation
to the intermediate II18 following competitive β-H elimination
to preferentially generate the thermodynamically controlled
vinyl alkylation7,9b,19 (Figure 1, path c), whereas reports on the
synthesis of the allyl products have not yet been established
Received: May 21, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01600
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
of copper-catalyzed dehydrogenative couplings of α-alkylated
styrenes with simple tertiary carbonyl compounds under
aerobic oxidative conditions.21 This unprecedented strategy
allowed access to densely functionalized olefins with the
unsaturation of exomethylene retained, although several
fundamental challenges mentioned above might impede such
a process, including problematic oxidative cleavage of olefins22
(Figure 1, path e) which has been previously illustrated in the
literature.
Building on our mechanistic postulate of this transformation,
we set out to realize this proposal with the coupling of both
commercially available α-methyl styrene (1a) and diethyl
methylmalonate (2a) (Table 1). By varying a range of
byproduct from oxidative cleavage of 1a, 4-methoxyacetophe-
none, was also formed during our investigation. It was believed
that the presence of a base could facilitate β-H elimination to
produce an olefin in oxidative dehydrogenative coupling
reactions. When different bases were examined (entries 8−
14), we were pleased to observe the formation of 3a in 70%
isolated yield (entry 14), with dicyclohexylamine (Cy2NH) as
the base in the presence of Cu(OAc)2 under air in DMSO,
which were identified as the optimal conditions. Lowering the
reaction temperature or reducing the loading of Cu(OAc)2
decreased the yield (entries 15 and 16). Importantly, the
concentration of the air also impacted the yield of the desired
product (entries 17 and 18). Conducting the reaction in 10
and 50 mL reactors led to an incomplete conversion and an
increased amount of byproducts, respectively.
Table 1. Optimization of Cu-Catalyzed Olefination of
a
The initial success prompted us to evaluate the substrate
scope of α-methyl styrenes in the Cu-catalyzed dehydrogen-
ative coupling reactions with 2a. As shown in Scheme 1, the
Tertiary Dicarbonyl Compounds
a
Scheme 1. Scope with Regard to α-Methyl Styrenes
b
c
yield (%)
ratio
entry
catalyst
CuX2
oxidant
base
3a + 3a′
3a/3a′
1
2
3
4
5
6
7
8
O2
O2
air
−
−
−
−
−
−
−
0
−
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
20
52
27
trace
45
21
28
56
67
55
66
64
70
55/45
88/12
86/14
−
87/13
88/12
91/9
88/12
88/12
82/18
87/13
87/13
88/12
86/14
88/12
90/10
82/18
d
BQ
d
BPO
d
DTBP
d
TBHP
air
air
air
air
air
air
air
air
air
air
air
KOAc
DMAP
Et3N
iPr2NH
Bn2NH
Et2NH
Cy2NH
Cy2NH
Cy2NH
Cy2NH
Cy2NH
9
10
11
12
13
14
15
16
17
18
e
48
67
51
f
g
h
45
a
Reactions run with 1a (0.2 mmol), 2a (0.3 mmol), Cu(II) catalyst
(0.06 mmol), base (0.4 equiv) in DMSO (0.1 M) at 130 °C for 24 h
b
c
in 25 mL Schlenk tube unless otherwise noted. Isolated yield. Ratios
1
of 3a/3a′ were determined by H NMR analysis of crude reaction
d
e
mixtures. 2 equiv of oxidants were used under nitrogen. At 120 °C.
a
b
f
g
Isolated yields. See Table 1 for the standard reaction conditions. 1
20 mol % of catalyst was employed. 10 mL Schlenk tube was used as
c
d
h
mmol scale. 40 mol % of Cu(OAc)2 was employed. At 150 °C.
the reactor. 50 mL Schlenk tube was used as the reactor. BQ = 1,4-
benzoquinone, BPO = benzoyl peroxide, DTBP = di-tert-butyl
peroxide, TBHP = tert-butyl hydroperoxide.
t
electron-rich (R = OBn, Me, Bu) and -neutral (R = H)
styrenes gave the desired allyl products (3b−e) as the major
isomers with higher yields than the eletron-poor styrenes (R =
Br). Thus, the electronic nature of substituents on the phenyl
group had a profound impact on the reactivity, as elucidated by
the additional results of multimethoxy substituted styrenes (3g
and 3h). When the fact that the α-keto carbon radical
possesses electrophilic characteristics is considered,23 the
observed phenomena are not surprising, since the presence
of an electron-donating group increases the nucleophilicity of
styrenes to undergo the free-radical addition process. Notably,
α-methyl styrenes bearing interesting functional groups, such
as hydroxyl (3i), morpholino (3j), and acetal (3k), were
copper(II) salts in a DMSO medium under a dioxygen
atmosphere at 130 °C (see Table S1, entries 1−6 in the
Supporting Information), we quickly determined Cu(OAc)2 to
be the only catalyst to trigger the radical process, despite its
poor catalytic activity. An oxidant is the critical component in
controlling the radical pathway, as evidenced by a survey of
common oxidants (entries 3−7). To our delight, air, serving as
an environmentally benign oxidant that is abundant and
inexpensive, is the perfect choice for this process, effectively
producing the allyl product 3a as the major regioisomer. It is
noteworthy to mention that a significant amount of undesired
B
DOI: 10.1021/acs.orglett.8b01600
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
compatible for this process. Specifically, to better showcase the
functional group tolerance, we sought to elaborate a series of
α-methyl styrenes tethered with synthetically useful handles for
postfunctionalizations. Epoxide, cyclopropyl, nitrile, ester,
cyclopentyl, alkynyl, and alkenyl functionalized alkoxyl-
substituted styrenes participated in the reaction without
difficulty to provide the corresponding coupled products in
59−85% isolated yields with good allylic selectivities (3l−r).
Subsequently, the present catalytic protocol was successfully
extended to a variety of functionalized tertiary alkanes with the
styrene 1g (Scheme 2). Dimethyl methylmalonate reacted
Scheme 4. Radical-Clock and -Trapping Experiments
a
Scheme 2. Scope with Regard to Tertiary Alkanes
and sequential cyclization, thus implicating the addition of a
carbon-centered radical intermediate to the olefin. In addition,
we found that addition of 2,2,6,6-tetramethylpiperidine N-
oxide (TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT),
as commonly used radical inhibitors, to the reaction system
completely suppressed the dehydrogenative coupling process.
Instead, the TEMPO- and BHT-trapped complex were
detected by HRMS analysis, respectively (Scheme 4b).
These findings suggested that the carbon-centered radical
intermediates are likely involved in the process.
a
b
Isolated yields. See Table 1 for the standard reaction conditions. 40
c
mol % of Cu(OAc)2 was employed. 50 mol % of Cu(OAc)2 was
employed.
In summary, we have established a copper-catalyzed
dehydrogenative coupling which assembled α-alkylated styr-
enes with simple tertiary carbonyl compounds under aerobic
oxidative conditions. This new approach for tertiary alkylation
of olefins bears advantages over the traditional Heck-type
reaction by obviating the need for alkyl halides and expensive
ligands. Control experiments implied that the process might
involve the α-keto carbon radical intermediate addition to an
olefin, followed by a competitive β-H elimination from the
distal carbon from steric tertiary alkyl group, which accounts
for the observed allylic selectivity. Further studies on expansion
of the substrate scope and synthetic application of this protocol
are currently underway in our laboratory.
smoothly with 1g to afford the desired product 3s in 87:13
selectivity. Subjecting diethyl α-substituted malonate deriva-
tives (ethyl, isopropyl, phenyl, and cyanoethyl) to the reaction
system delivered the corresponding allyl products 3t−x with
moderate to good yields and remarkably higher selectivities as
compared to 2a, except for the benzyl group (3w). The results
concerning the regioselectivity are consistent with our principal
hypothesis that sterically demanding alkyl substitution is
beneficial to the terminal β-H elimination process. Further-
more, α-sulfonyl and cyano propionates also appeared to be
viable coupling partners to furnish 3y−z with high allylic
selectivity.
Remarkably, the exocyclic olefin 4 was also proven to be a
suitable substrate for this reaction protocol, affording the
corresponding allyl product 5 exclusively, albeit in low yield
(Scheme 3).
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.8b01600.
Scheme 3. Additional Example of α-Alkyl Styrenes
Experimental procedures and characterization data
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
The intermediacy of an α-keto carbon radical was generally
involved in the oxidative functionalization of simple 1,3-
dicarbonyl compounds. To further clarify the possible
mechanism for this transformation, control experiments with
radical-clock and -trapping reagents were investigated. As
shown in Scheme 4a, using 2a to react with 1-phenylvinyl-
cyclopropane,24 the rearranged product 7 was observed
through ring opening of cyclopropylmethyl radical species
*E-mail: ias_xhhu@njtech.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
ORCID
Teck-Peng Loh: 0000-0002-2936-337X
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.8b01600
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
Financial support from the NNSFC (21702106), the Natural
Science Foundation of Jiangsu Province (BK20170967), and
the Start-up Grant from Nanjing Tech University (39839101
and 39837101) are gratefully acknowledged. We also thank the
SICAM Fellowship by Jiangsu National Synergetic Innovation
Center for Advanced Materials.
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