Manganese-Catalyzed Intermolecular Oxidative Annulation of Alkynes with γ-Vinyl Aldehydes: An Entry to Bridged Carbocyclic Systems

Article abstract of DOI:10.1021/acs.orglett.7b03086

Manganese-catalyzed intermolecular oxidative annulation of alkynes with γ-vinyl aldehydes involving acylation and alkylation is described, thus providing a scenario for the divergent synthesis of bridged carbocyclic systems. By means of this manganese-catalyzed alkyne dicarbofunctionalization strategy, three chemical bonds, including two C-C bonds and one C-H bond, are formed via an aldehyde C(sp²)-H oxidative functionalization/[4 + 2] annulation/protonation cascade.

Full text of DOI:10.1021/acs.orglett.7b03086

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Manganese-Catalyzed Intermolecular Oxidative Annulation of
Alkynes with γ‑Vinyl Aldehydes: An Entry to Bridged Carbocyclic
Systems
Yang Li,†,‡,∥ Jin-Xia Li,†,‡,∥ Xuan-Hui Ouyang,†,‡ Qiu-An Wang,*,† and Jin-Heng Li*,†,‡,§
†State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China
‡Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University,
Nanchang 330063, China
§State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: Manganese-catalyzed intermolecular oxidative
annulation of alkynes with γ-vinyl aldehydes involving
acylation and alkylation is described, thus providing a scenario
for the divergent synthesis of bridged carbocyclic systems. By
means of this manganese-catalyzed alkyne dicarbofunctional-
ization strategy, three chemical bonds, including two C−C
bonds and one C−H bond, are formed via an aldehyde
C(sp2)−H oxidative functionalization/[4 + 2] annulation/
protonation cascade.
ridged carbocyclic systems, including bridged bicyclic and
tricyclic carbocycles, are unique structural motifs that
selectively building such frameworks are restricted to the
intramolecular fashion with unavailable limited substrates via
multiple synthetic steps, and an alternative one-step entry that
allows the divergent construction of different bridged cyclic
systems from simple starting materials is very challenging.
The cycloaddition reaction with aldehydes represents a
powerful method for the construction of diverse cyclic systems,
with the vast majority of examples concerning destruction of
the carbonyl group via CO bond cleavage as a carbon
monoatom unit or a C/O two-atom unit for construction of the
cycle.5 To the best of our knowledge, an approach to radical-
mediated cycloaddition with the aldehyde C(sp2)−H bond, to
avoid the destruction of the carbonyl group, has yet to be
reported. In recent years, radical-mediated addition of
aldehydes across the unsaturated C−C bonds has become an
important tool for directly incorporating an acyl group into
simple unsaturated hydrocarbons,6−8 in which acyl radical A is
generated by cleavage of the aldehyde C(sp2)−H bond using
photocatalysis7 or oxidation8 (Scheme 2a,b). We envisioned
that if an aldehyde inherently contains a functional group to
trap radical intermediate B, the cycloaddition process would
not destroy the carbonyl group, allowing the assembly of
carbonyl-containing cycles (Scheme 2c). Herein we report a
new MnSO4-catalyzed intermolecular oxidative annulation of
alkynes with γ-vinyl aldehydes via acylation and alkylation for
the divergent synthesis of 5H-2,4-methanoinden-5-ones,
bicyclo[3.3.1]non-3-en-2-ones, and cyclohex-2-en-1-ones
(Scheme 2d), which rely on the γ-vinyl aldehyde reaction
B
widely exist in numerous natural products, pharmaceuticals, and
other bioactive molecules (Scheme 1).1,2 For example,
Scheme 1. Examples of Important Bridged Carbocycles
trichagmalin A and xyloccensins,2a−f which were isolated from
the leaves of Cedrela odorata or the fruits of the Chinese
mangrove plant, are constituted by an octahydro-1H-2,4-
methanoindene skeleton. Other natural products, including
spirovibsanin A,2g,h hyperforin,2i−k α-cedrene,2l (−)-huperzine
A,2m−o and lycodoline,2p,q all feature a bicyclo[2.3.2]nonane
ring system. Furthermore, bridged carbocyclic systems have a
multitude of applications in the organic and medicinal
community, including use as chiral ligands3 and versatile
synthetic intermediates.1,4 Efficient construction of a bridged
dicarbocyclic or tricarbocyclic framework is therefore in
demand and has attracted ongoing interest from scientific
researchers.1,2,4 However, the majority of strategies for
Received: October 2, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03086
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
peroxide (DTBP) and tert-butyl perbenzoate (TBPB) exhibited
lower reactivity (entries 8 and 9). It was noted that the reaction
could not occur without an oxidant (entry 10). Screening of
both the amount of TBHP and the reaction temperature
revealed that 3 equiv of TBHP at 100 °C was the best option
(entries 11−14). Two other alternative solvents, MeCN and
DMF, both were less reactive than PhCF3 (entries 15 and 16).
With the optimal reaction conditions identified, we turned
our attention to an investigation of the scope of this tandem
annulation protocol with respect to alkynes 1 (Scheme 3) and
Scheme 2. Radical Transformations of Aldehydes
a
Scheme 3. Variation of Alkyne 1
partners. This radical-mediated reaction is achieved by
simultaneously incorporating an acyl group and a vinyl group
across the CC bond through the formation of two C−C
bonds and one C−H bond and represents a new, alternative
entry to bridged carbocyclic systems from branched aldehydes
by avoiding the tendency for the competing decarbonylation.9
Our initial studies started with the reaction between
phenylaceylene (1a) and norborn-5-ene-2-carbaldehyde (2a)
for optimization of the reaction conditions (Table 1).
a
Table 1. Optimization of the Reaction Conditions
entry
[M] (mol %)
[O] (equiv)
solvent
t (°C) yield (%)
b
1
2
MnSO4 (10)
MnSO4 (5)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
CHP (3)
DTBP (3)
TBPB (3)
−
TBHP (2)
TBHP (4)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
PhCF3
MeCN
DMF
100
100
100
100
100
100
100
100
100
100
100
100
110
90
73 (68 )
55
3
MnSO4 (20)
MnCl2 (10)
Mn(OAc)2 (10)
MnO2 (10)
75
a
Reaction conditions: 1 (0.2 mmol), 2 (2 equiv), MnSO4 (10 mol %),
4
54
TBHP (3 equiv), PhCF3 (2 mL), argon, 100 °C, 12 h. The dr value of
each product was >20:1.
5
50
6
58
7
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
MnSO4 (10)
63
8
<5
a
9
<5
Scheme 4. Variation of γ-Vinyl Aldehyde 2
10
11
12
13
14
15
16
trace
37
71
73
55
100
100
47
33
a
Reaction conditions: 1a (0.2 mmol), 2a (2 equiv), [M], [O], PhCF3
1
(2 mL), argon, 12 h. The dr values were >20:1, as determined by H
NMR analyses of the crude products. 1a (1 mmol) and PhCF3 (3
b
mL) for 24 h.
Treatment of alkyne 1a with aldehyde 2a, MnSO4, and
TBHP in PhCF3 at 100 °C succeeded in accessing the desired
product 3aa in 73% yield (entry 1). Notably, a reaction scaled
up to 1 mmol of 1a also afforded 3aa in good yield (entry 1).
The amount of MnSO4 affected the reaction: a lower loading of
MnSO4 (5 mol %) decreased the yield of 3aa (entry 2),
whereas a higher loading of MnSO4 (20 mol %) gave an
identical yield as 10 mol % MnSO4 (entry 3). Other Mn
catalysts, namely, MnCl2, Mn(OAc)2, and MnO2, displayed
high activity (entries 4−6), but they all were less efficient than
MnSO4. Using cumene hydroperoxide (CHP) also delivered a
satisfactory yield (entry 7). However, both di-tert-butyl
a
Reaction conditions: 1 (0.2 mmol), 2 (2 equiv), MnSO4 (10 mol %),
b
TBHP (3 equiv), PhCF3 (2 mL), argon, 100 °C, 12 h. The dr value of
the product was >20:1.
γ-vinyl aldehydes (Scheme 4). As shown in Scheme 3, the
optimal conditions were compatible with a wide range of
alkynes, including terminal aryl alkynes (1b−l), alkyl alkynes
(1m and 1n), ethynyltrimethylsilane (1o), propiolamide (1p),
propiolate (1q) and internal alkynes (1r−t). With terminal
arylalkynes 1b−l, a series of substituents, namely, Me, MeO, Cl,
B
DOI: 10.1021/acs.orglett.7b03086
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Br, and CN, on the aryl ring were well-tolerated (3ba−la). Me-
substituted aryl alkyne 1b, for example, was converted into 3ba
in 65% yield. MeO-substituted aryl alkyne 1c delivered 3ca in
70% yield. Importantly, halogen groups, including Cl and Br,
are consistent with the optimal conditions, thus offering facile
handles for further synthetic elaborations of the halogenated
positions (3da, 3ea, 3ga, and 3ha). Aryl alkynes 1d, 1g, and 1h
with a Cl group at the para, meta, and ortho positions,
respectively, were suitable substrates, giving 3da, 3ga, and 3ha
in 58−75% yield. Alkyne 1f having an electron-withdrawing CN
group was also successful at accessing 3fa. With bis(MeO)-
substituted alkyne 1i and 1-ethynylnaphthalene (1j), this
tandem annulation protocol afforded 3ia and 3ja in 55% and
70% yield, respectively. The heteroaryl alkynes 3-ethynylpyr-
idine (1k) and ethynylferrocene (1l) both showed high
reactivity (3ka and 3la). Alkyl alkynes 1m and 1n and
electron-poor terminal alkyne 1p could undergo the annulation
reaction (3ma, 3na, and 3pa), but all were less reactive than
terminal aryl alkynes. To our delight, 1o proved amenable to
the optimal conditions, giving silyl-substituted product 3oa in
50% yield. Gratifyingly, the optimal conditions were applicable
to internal alkynes, including 1,2-diphenylethyne (1q), diethyl
but-2-ynedioate (1r), and N,N-dimethyl-3-phenylpropiolamide
(1s), affording 3qa−sa in 46%−85% yield.
The optimal conditions were also suitable for other γ-vinyl
aldehydes 2b−d (Scheme 4). Cyclohex-3-ene-1-carbaldehyde
(2b), the other secondary aldehyde, exhibited high reactivity
and executed the acylalkylation reaction with alkynes 1a, 1c,
and 1r to deliver bicyclo[3.3.1]non-3-en-2-ones 3ab, 3cb, and
3rb in 52%−65% yield, respectively. For pent-4-enal (2c), a
primary aldehyde, the acylalkylation reactions with alkynes 1a
and 1c proceeded smoothly, giving the corresponding cyclohex-
2-en-1-ones 3ac and 3cc in good yields. However, 2,2-
dimethylpent-4-enal (2d), a tertiary aldehyde, underwent
decarbonylative annulation with alkynes 1a and 1c, resulting
in the formation of the five-membered cyclic products 3ad and
3cd in high yields.
Scheme 5. Control Experiments and Possible Mechanism
with the CC bond, and protonation. The reaction is
applicable to a wide range of alkynes, including aryl alkynes,
alkyl alkynes, ethynyltrimethylsilane, and electron-poor alkynes,
and represents a new alkyne annulation alternative to access
diverse bridged carbocycles, such as 5H-2,4-methanoinden-5-
ones, bicyclo[3.3.1]non-3-en-2-ones, and common five- or six-
membered cycles. Studies on expanding this annulation strategy
in total synthesis and developing new synthetic methodologies
are currently underway.
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.7b03086.
Notably, the reaction of alkyne 1a with aldehyde 2a was
completely inhibited when radical inhibitors were used,
implying that the reaction includes a radical process (Scheme
5, eq 1).
Descriptions of experimental procedures for compounds
and analytical characterization (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
A possible mechanism for the intermolecular oxidative
acylalkylation protocol is outlined in Scheme 5.4−8 Initially,
the splitting of the O−O bond in TBHP occurs when the active
MnII species is heated to generate the tert-butoxyl radical and
the MnIII(OH) species.6,7 The MnIII(OH) species can be
readily converted into the MeIII(OOtBu) species in the
presence of TBHP. Selective hydrogen abstraction of the
aldehyde group in 2a with the tert-butoxyl radical produces
silicon-centered radical intermediate A and tBuOH,6,7 and
subsequent addition across the CC bond in alkyne 1a affords
vinyl radical intermediate B. Cyclization of intermediate B with
the CC bond gives rise to radical alkyl intermediate C.
Finally, oxidation of C through hydrogen atom transfer to
TBHP or single-electron oxidation by the MeIII(OOtBu)
species, followed by protonation, provides access the desired
product 3aa.
In summary, we have described the first example of a
manganese-catalyzed intermolecular oxidative [4 + 2] annula-
tion of alkynes with γ-vinyl aldehydes for constructing bridged
carbocyclic systems. The reaction is achieved through a
sequence of carbonyl C−H oxidative functionalization, acyl
radical generation, addition across the CC bond, annulation
*E-mail: wangqa@hnu.edu.cn.
*E-mail: jhli@hnu.edu.cn.
ORCID
Jin-Heng Li: 0000-0001-7215-7152
Author Contributions
∥Y.L. and J.-X.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of China (21625203
and 21472039) and the Jiangxi Province Science and
Technology Project (20171ACB20015 and 20165BCB18007)
for financial support.
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DOI: 10.1021/acs.orglett.7b03086
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