Oxidative cyclization of 2-allenyl-1,1′-biphenyls with α-carbonyl alkyl bromides: Facile access to functionalized phenanthrenes

Article abstract of DOI:10.1039/c7cc00483d

A new copper-facilitated oxidative cyclization of 2-allenyl-1,1′-biphenyls with α-carbonyl alkyl bromides for producing functionalized phenanthrenes is presented, which represents the first allene 2,3-dicarbofunctionalization triggered by oxidative radical-medicated C3-addition of the terminal allene moiety and C-Br/C-H functionalization.

Full text of DOI:10.1039/c7cc00483d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Oxidative Cyclization of 2-Allenyl-1,1'-biphenyls with α-Carbonyl
Alkyl Bromides: Facile Access to Functionalized Phenanthrenes
Received 00th January 20xx,
Accepted 00th January 20xx
a+
a+
*b
*a,c
Cheng-Yong Wang, Gao-Hui Pan, Fan Chen, and Jin-Heng Li
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new copper-facilitated oxidative cyclization of 2-allenyl-1,1'-
biphenyls with α-carbonyl alkyl bromides for producing
functionalized phenanthrenes is presented, which represents the
first allene 2,3-dicarbofunctionalization triggered by oxidative
radical-medicated C3-addition of the terminal allene moiety and
C-Br/C-H functionalization.
Scheme 1. Naturally Occurring Phenanthrenes.
Phenanthrenes are a class of key structural motifs in many natural
1
2
products, biologically active compounds and organic functional
3
are scarce, such transformations are particular fascinating which
allow the incorporation of new functional groups across the allenes
materials.
For
1
example,
many
1d
naturally
occurring
1d
phenanthrenes, such as Confusarin, Ephemeranthol B, Fimbriol
1
0
1
e
1f
1g
(Scheme 2a). Notably, these approaches are achieved via the
initial C2-addition of the in-situ generated radicals across the
internal carbon of the allenes followed by reaction with various
nucleophiles. We envisioned that such oxidative radical-mediated
versions might be employed to build functionalized phenanthrenes
by selective annulation of allene-contained biaryls. Herein, we
report a novel copper-facilitated oxidative cyclization of 2-allenyl-
A, Boldine, Tylophorine and Antofine , which display a broad
spectrum of biological activities (e.g. antiviral and anticancer)
(Scheme 1). As a result, considerable efforts have been devoted to
development of efficient methods for building such functionalized
4
-8
phenanthrene frameworks.
Classically, attractive methods
4
include the Pschorr reaction, Mallory cyclizations of stilbenes,
5
6
7
cyclization of alkynylated biaryls, [4+2] annulation of biphenyls,
1
1
12
8
1,1'-biphenyls with α-carbonyl alkyl bromides using Ag
oxidant that initiated by C3-addition of the terminal allene moiety
Scheme 2b); The reaction proceeds via oxidative radical-mediated
CO
2 3
and Ullman coupling−McMurry cyclization; however, many of
which suffer from harsh reaction conditions, limited substrate
scope and unavailable introduction of new functional groups.
Therefore, new efficient strategies for producing functionalized
phenanthrenes, especially involving simultaneous incorporation of
new functional groups during the phenanthrene backbone
formation process, are interesting.
(
C-Br/C-H functionalization, and enables the two C-C bond formation
to assemble functionalized phenanthrenes using an allene 2,3-
dicarbofunctionalization strategy.
Recently, oxidative radical-mediated reactions have been
9
becoming a powerful tool to prepare complex molecules. Although
examples of oxidative radical-mediated transformations of allenes
a.
State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
Changsha 410082, China, and Key Laboratory of Jiangxi Province for Persistent
Pollutants Control and Resources Recycle, Nanchang Hangkong University,
Nanchang 330063, China
b.
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou,
3
25035, China
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
30000, China
c.
7
E-mail: jhli@hnu.edu.cn and fanchen@wzu.edu.cn
These authors contributed equally to this work.
Electronic Supplementary Informaꢀon (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
+
†
Scheme 2. Oxidative radical transformations of Allenes
.
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We initiated the studies by investigating the cyclization reaction (2k) furnished 3ak in 59% yield. Tertiary α-bromoalkyl ester 2l also
of 2-(buta-2,3-dien-2-yl)-1,1'-biphenyl (1a) with methyl 2- showed high reactivity, albeit giving a diminishing yield of 3al. We
bromopropanoate (2a) (Table 1). Reaction of substrate 1a with 2a were pleased to find that substrates 1b-c, bearing a substituent
and Ag
phenanthrene 3aa in 38% yield (entry 1). Other silver salts, consistent with the optimal conditions, and afforded 3ba-ca in
including AgO, AgOAc and AgNO , were examined: they showed moderate yields. The substitution effect on the phenyl ring of the
high reactivity, but AgO and AgOAc were less effective than Ag CO 1’-phenyl moiety were subsequently examined: a wide range of
or AgNO (entries 2-4). Among the effect of solvents examined, substituents, such as Me, MeO, Cl, F and CO Me, were well
2 3
CO was successfully performed, giving the desired (ethyl or cyclohexyl) at the 1 position of the allene moiety, were
1
3
3
2
3
3
2
dioxane was turned out to be preferred by comparison to DMF and tolerated, and their electronic and steric hindrance nature had a
PhCl (entries 1, 5 and 6). However, the reaction could not take place slight effect on the reaction. While substrates with an electron-
when using a base, such as K
2
CO
3
and Cs
2
CO
3
, instead of Ag
CO do not efficiently furnished 3da-ga in 60%-65% yields, substrate 1h
base. Gratifyingly, metal salts, such as CuCl, having a strong withdrawing group was also successful to construct
, CuCl , Cu(acac) , FeBr and Fe(OAc) , were found to 3ha with a diminishing yield (53%). For substrate 1i with a methyl
facilitate the reaction, and CuCl had the highest catalytic activity group at the meta position, the reaction afforded a mixture of the
entries 9-14). Notably, some side-products, including 9,10- regioselective products 3ia/3ia’ with 2:1 ratio. Ortho-substituted
dimethylphenanthrene (4aa), were observed by GC-MS analysis substrate 1l also showed high reactivity, providing 3ja in 48% yield.
2
CO
3
donating group (1d-e) or a weak electron-withdrawing group (1f-g)
1
3
(entries 7 and 8). The results suggest that the roles of Ag
2
3
act only as
a
Cu(MeCN)
4
PF
6
2
2
2
2
(
o
(entry 9). A screen of the reaction temperatures revealed 120 C as
the best choice (entry 9 versus entries 15-16).
Table 2. Cyclization of 2-Allenyl-1,1'-biphenyls (1) with α-Carbonyl
a
Alkyl Bromides (2a)
a
Table 1. Screening of optimal reaction conditions
o
Entry
[M]
Additive solvent T ( C) Isolated Yield (%)
1
2
3
4
5
6
7


Ag CO dioxane 120
38
35
34
38
8
29
0
0
57
43
44
40
55
40
45
55
2
3
AgO dioxane 120
AgOAc dioxane 120
AgNO dioxane 120
Ag CO


3

DMF
PhCl
120
120
2
3
3

Ag
2
CO

K2CO3 dioxane 120
Cs CO dioxane 120
8

2
3
b
9
CuCl
Ag CO dioxane 120
2
3
1
1
1
1
1
1
0
1
2
3
4
5
6
Cu(MeCN)
CuCl2
Cu(acac)
FeBr2
Fe(OAc)
CuCl
CuCl
4
PF
6
Ag
2
CO
3
dioxane 120
Ag CO dioxane 120
2
3
2
Ag
2
CO
3
dioxane 120
Ag CO dioxane 120
2
3
2
Ag
2
CO
3
dioxane 120
Ag CO dioxane 110
2
3
1
Ag
2
CO
3
dioxane 130
a
Reaction conditions: 1a (0.2 mmol), 2a (3 equiv), [M] (5 mol%),
b
additive (2 equiv), solvent (1 mL), argon and 16 h. 9,10-
Dimethylphenanthrene (4aa), was obtained in 17% yield.
With the optimal reaction conditions in hand, we turned our
attention to investigate the scope of the cyclization reaction with
respect to 2-allenyl-1,1'-biphenyls with α-carbonyl alkyl bromides
(
Table 2). Initially, a variety of α-carbonyl alkyl bromides 2b-l,
a
Reaction conditions: 1 (0.2 mmol), 2 (3 equiv), CuCl (5 mol%),
namely primary, secondary and tertiary esters, amide and ketones,
were compatible with the optimal conditions, affording the
corresponding phenanthrenes 3ab-al in moderate yields. Other
secondary α-bromoalkyl esters 2b-d, especially dimethyl 2-
bromomalonate (2d), were viable for the construction of 3ab-ad by
o
Ag
2
CO
3
(2 equiv), dioxane (1 mL), 120 C, argon and 16 h.
As shown in Scheme 3, the reaction of 1a with 2a was
completely suppressed when using a stoichiometric amount of
radical scavengers, including TEMPO and BHT, in which products 5
or 6 were obtained from the direct reaction of 2a with TEMPO or
BHT. The results suggest that the reaction involves a radical process.
Consequently, the mechanisms for the oxidative cyclization
2 3
reacting with substrate 1a and Ag CO . Using amide 2e also
delivered 3ae in useful yield. Secondary α-bromoalkyl ketones 2f-g
also exhibited high reactivity, providing 3af-ag in moderate yields.
The primary α-bromoalkyl ester 2h and ketones 2i-j were suitable
substrates for accessing 3ah–aj. Gratifyingly, 2-bromoacetonitrile
9
-12
3
reaction was proposed (Scheme 3). Initially, the α-C(sp )-Br bond
2
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under heating to afford the alkyl radical A, the Cu species and the
Ag species via single-electron transfer (SET).
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moiety in substrate 1a produces the vinyl radical B due to both
conjugation effect and the steric effect, which sequentially
undergoes cyclization with the 1’-phenyl ring to give the radical
intermediate C. Oxidation of intermediate C by both the Cu species
and the Ag species generates the cation intermediate D. Finally,
hydrogen abstraction of the cation intermediate D takes place to
form the desired product 3aa.
Notably, substrate 1a can also undergo a 6π cycloaddition-
isomerization process to form the by-product 4aa under basic
II
9,12
Selective addition
2
016, 52, 381.
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Scheme 3
.
Control Experiments and Possible reaction mechanisms
.
,
1
In summary, we have developed a new copper-facilitated
oxidative cyclization of 2-allenyl-1,1'-biphenyls with α-carbonyl alkyl
bromides through C-Br/C-H functionalization and allene 2,3-
dicarbofunctionalization cascades, which represents the first
example of allene oxidative radical transformation triggered by C3-
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0
addition of the terminal allene moiety. The reaction proceeds via
an oxidative radical strategy, and offers a straightforward access to
phenanthrenes with broad substrate scope and excellent selectivity.
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3 CCDC 1537709
7
,
4
1
(
3ea
)
contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
4
| J. Name., 2012, 00, 1-3
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