Alkynylation of Tertiary Cycloalkanols via Radical C-C Bond Cleavage: A Route to Distal Alkynylated Ketones

Article abstract of DOI:10.1021/acs.orglett.5b02353

An efficient Na₂S₂O₈-promoted radical coupling of tertiary cycloalkanols with alkynyl hypervalent iodide reagents via C-C bond cleavage was developed. This tandem ring-opening/alkynylation procedure showed some advantages,

Full text of DOI:10.1021/acs.orglett.5b02353

Letter
pubs.acs.org/OrgLett
Alkynylation of Tertiary Cycloalkanols via Radical C−C Bond
Cleavage: A Route to Distal Alkynylated Ketones
Shun Wang, Li−Na Guo, Hua Wang, and Xin-Hua Duan*
Department of Chemistry, School of Science and MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed
Matter, Xi’an Jiaotong University, Xi’an 710049, China
*
S Supporting Information
ABSTRACT: An efficient Na S O -promoted radical cou-
2
2
8
pling of tertiary cycloalkanols with alkynyl hypervalent iodide
reagents via C−C bond cleavage was developed. This tandem
ring-opening/alkynylation procedure showed some advan-
tages, including mild conditions and wide substrate scope,
thus providing a simple synthetic method for β-, γ- and δ-
alkynylated ketones.
11a
he catalytic ring-opening coupling of strained tertiary
α-keto acids with AHI.
Although significant progress on
T
cycloalkanols has emerged as an efficient strategy for the
synthesis of functionalized acyclic organic molecules. In this
alkynylation reactions using AHI as the alkynylation reagent has
been made, the catalytic radical alkynylation of strained tertiary
cycloalkanols with AHI is virtually unknown. As a part of our
ongoing interest in the chemistry of AHI, we surmise that the γ-
keto radical generated from the oxidative C−C bond cleavage of
strained cyclobutanol might be trapped by the electrophilic
alkynyl hypervalent iodide reagent to afford the γ-alkynylated
ketone (Scheme 1). Herein, we report a tandem radical ring-
1
context, the palladium-catalyzed cross-coupling of tertiary
cycloalkanols with various electrophiles has attracted much
attention and has proven to be a powerful tool to obtain carbonyl
2
compounds. Recently, significant effort has also been made in
the catalytic enantioselective C−C bond activation of cyclo-
alkanols.
2
d,3
In addition, the tandem radical ring-opening
coupling of cycloalkanols under different one-electron oxidants
has been developed and has provided another alternative method
for accessing functionalized ketones. For example, Narasaka’s
Scheme 1. Alkynylation of Tertiary Cyclobutanols
4
group reported Mn(III)-promoted and Ag(I)-catalyzed tandem
radical coupling of cyclopropanols with alkenes, respectively. In
those transformations, the β-carbonyl radical intermediates
generated from the oxidative ring-opening of cyclopropanols
4
b−d
opening/alkynylation of strained tertiary cycloalkanols with
hypervalent iodide reagents under mild transition-metal-free
conditions which provides an efficient route to distal alkynylated
ketones in good yields.
are involved.
Subsequently, several research groups applied
this tandem radical ring-opening/coupling strategy for the
5a,b
5c−e
synthesis of heterocycles and organofluorine compounds.
More recently, Dai and Lopp have independently described the
Initially, we selected 1-phenylcyclobutanol (1a) and 1-[(tert-
butyldimethylsilyl)ethynyl]-1,2-benziodoxol-3(1H)-one (TBS-
EBX, 2a) as model substrates to optimize the ring-opening/
Cu-catalyzed ring-opening electrophilic trifluoromethylation and
6
amination of cyclopropanols. However, only a few studies on
the ring-opening coupling of less strained cyclopentanols have
4
,5
5
e
2b,d,i
alkynylation reaction. Treatment of 1a with 2a in the presence
been reported. Although catalytic arylation
tion
and alkyla-
of tertiary cycloalkanols have been well documented,
reports on the alkynylation of tertiary cycloalkanols are still
4
b−d,6d
of 10 mol % of AgNO and 2.0 equiv of K S O in CH CN/H O
3
2
2
8
3
2
(
1:1) at 50 °C afforded the desired product 3a in 49% yield
(Table 1, entry 1). We were delighted to find that the reaction of
a with 2a also resulted in a similar yield even in the absence of
2
h,6f
rare.
1
The use of alkynyl hypervalent iodide (AHI) as an efficient
catalyst (entry 1). The yield of 3a was increased to 65% when
alkynylating reagent has recently received increasing attention in
7
acetone/H O (1:1) was used as the solvent (entry 2). Other
organic synthesis due to its excellent reactivity and versatility. A
2
8
a−e
8f−h
8i
8j
homogeneous phase systems such as AcOH/H O and DMF/
series of elegant Au-,
catalyzed
Pd-,
Pt-, Cu-, and Rh-
2
8
k−n
H O were also effective for this reaction (entries 3 and 4). In
2
alkynylation reactions involving AHI have been
contrast, H O and CH Cl /H O just gave a trace amount of the
2
2
2
2
developed for the synthesis of functionalized alkynes. In 2012, Li
and co-workers reported an efficient Ag(I)-catalyzed radical
decarboxylative alkynylation of aliphatic carboxylic acids with
desired product 3a (entries 5 and 6). Gratifyingly, a much better
yield of 3a was obtained by increasing the amount of 2a to 1.8
9
AHI. In addition, some transition-metal-free alkynylation
10
reactions have also been reported. In this respect, we reported
a mild K S O -promoted radical decarboxylative alkynylation of
Received: August 14, 2015
2
2
8
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02353
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Optimization of the Ring-Opening/Alkynylation of
Cyclobutanol
Cyclobutanols bearing a methoxy group on the ortho or meta
position of the phenyl ring were also smoothly converted into the
corresponding products 3g and 3h in 88% and 76% yields,
respectively. Moreover, the heteroaromatic-substituted cyclo-
butanol 1i was also compatible with the reaction conditions and
produced the desired γ-alkynylated ketone 3i in 65% yield. More
importantly, alkyl-substituted cyclobutanol 1j also worked well
with 2a to afford the desired product 3j in good yield. Other
alkynylating reagents such as TIPS-EBX and aryl-EBX were also
efficient to give the corresponding alkynylated ketones 3k and
a
b
entry
oxidant (equiv)
K S O (2.0)
solvent
yield (%)
c
1
2
3
4
5
6
7
8
9
CH CN/H O (1:1)
47 (49)
2
2
8
3
2
K S O (2.0)
acetone/H O (1:1)
65
2
2
8
2
K S O (2.0)
AcOH/H O (1:1)
45
2
2
8
2
3m in 67% and 42% yields, respectively. It should be noted that
K S O (2.0)
DMF/H O (1:1)
32
2
2
8
2
other alkynylating reagents, such as phenylacetylene, phenyl
phenylethynyl sulfone, and phenylethynyl phenyliodonium
tosylate failed to afford the desired product 3m under the
present conditions. Unfortunately, the alkyl-EBX furnished a
complex mixture (3l). Finally, this ring-opening/alkynylation
reaction could also be applied to 1,2-disubstituted cyclobutanols,
leading to the desired product 3n in 43% yield with high
regioselectivity (eq 1).
K S O (2.0)
H O
trace
trace
2
2
8
2
K S O (2.0)
CH Cl /H O (1:1)
2
2
8
2
2
2
d
K S O (2.0)
acetone/H O (1:1)
76
2
2
8
2
e
K S O (2.0)
acetone/H O (1:1)
82
2
2
8
2
f
K S O (2.0)
acetone/H O (1:1)
75
2
2
8
2
e
g
1
1
1
1
1
0
1
2
3
4
Na S O (2.0)
acetone/H O (1:1)
87 (83)
2
2
8
2
e
(NH ) S O (2.0)
acetone/H O (1:1)
59
4
2
2
8
2
e
oxone (2.0)
acetone/H O (1:1)
trace
e,h
2
Na S O (2.0)
acetone/H O (1:1)
64
2
2
8
2
i
acetone/H O (1:1)
nr
2
a
Reaction conditions: 1a (0.20 mmol, 1.0 equiv), 2a (0.24 mmol, 1.2
equiv), oxidant (0.40 mmol, 2.0 equiv), solvent (2 mL), 50 °C, 24 h
b
c
under N . Yield of isolated product. 10 mol % of AgNO was used as
2
3
d
e
f
a catalyst. 2a (0.30 mmol, 1.5 equiv). 2a (0.36 mmol, 1.8 equiv). 2a
g
(
0.40 mmol, 2.0 equiv). Yield on a 1 mmol scale is given in
parentheses. Room temperature. nr = no reaction.
Encouraged by the above results, we then investigated the
reaction of tertiary cyclopropanols with TBS-EBX 2a. To our
delight, the ring-opening/alkynylation reaction proceeded
smoothly to afford the expected β-alkynylated ketones 5 in
moderate to good yields under the slightly modified conditions
h i
equiv (entries 7 and 8). However, a further increase of the
amount of 2a resulted in a decreased yield (entry 9). Further
investigation revealed that Na S O was the best oxidant for this
transformation (entries 10−12). It should be noted that the
reaction still gave 64% yield of 3a at room temperature (entry
2
2
8
(
Scheme 3). When 1-phenylcyclopropanol 4a was subjected to
Scheme 3. Scope of Cyclopropanols
13). Finally, no reaction was observed in the absence of oxidant
(
entry 14).
With the optimized conditions in hand, the scope and
limitations of this tandem ring-opening/alkynylation reaction
were then investigated (Scheme 2). A variety of cyclobutanols
Scheme 2. Scope of Cyclobutanols and Alkynylating Agents
the reaction, the product 5a was isolated in 78% yield. 1-Aryl-
substituted cyclopropanols having either an electron-rich or
-
poor groups at the para position of the aromatic ring also reacted
well with 2a to give moderate yields of the corresponding
products 5b−e. The 1-benzylcyclopropanol 4f was also
successfully engaged in this reaction, delivering the product 5f
reacted with TBS-EBX efficiently to afford the corresponding γ-
alkynylated ketones in moderate to good yields (3b−j). Aryl-
substituted cyclobutanols containing an electron-withdrawing or
t
in 72% yield. Substituents such as MeO, Bu, and Br on the
benzene ring of cyclopropanols were also well-tolerated in this
transformation (5g−i). Remarkably, we found that the less
strained tertiary cyclopentanols also underwent the ring-
opening/alkynylation process in the presence of silver catalyst
to afford the desired δ-alkynylated ketones 7a−c in moderate
yields (eq 2). Unfortunately, the reaction of 1-phenylcyclohex-
-
donating substituent at the para positon of the aromatic ring
were good substrates to give the desired products 3b−e in 56−
0% yields. However, the strong electron-withdrawing effect of
the CF group led to the desired γ-alkynylated ketone 3f in only
8
3
3
1% yield along with a large amount of 1f recovered.
B
DOI: 10.1021/acs.orglett.5b02353
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
anol with 2a afforded a complex mixture under the same
conditions.
radical intermediate III. Finally, β-elimination of radical III gives
the desired product 3a as well as the 2-iodobenzoic acid, which is
generated by a reduction−protonation of the benziodoxolonyl
radical IV.
In summary, we have developed a new tandem Na S O -
2
2
8
mediated radical ring-opening/alkynylation of strained tertiary
cyclobutanols with alkynyl hypervalent iodide reagents in which
a sequence of C−C bond cleavage, radical rearrangement, and a
C−C bond formation process were involved. Remarkably, this
protocol could be further extended to other strained cyclo-
alkanols, thus providing a straightforward approach to a series of
β- and γ-alkynylated ketones. Moreover, less strained cyclo-
pentanols have also been applied successfully in this reaction.
This transformation provided a rapid and efficient strategy for
the carbon chain growth, and the products could further undergo
diverse transformations.
At last, some synthetic utilities of 3a and 5a are shown in
Scheme 4 (for details, see the Supporting Information).
Scheme 4. Derivatization of the Product 3a and 5a
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02353.
Experimental procedures and spectroscopic data of new
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Treatment of 3a with a stoichiometric amount of TBAF led to
the terminal γ-alkynylated ketone 8a in 90% yield. Satisfactorily,
the triazole 10a could be synthesized easily in 76% yield via the
sequential desilylation and cycloaddition of 3a with benzyl azide.
Moreover, 3a could also undergo tandem desilylation/
Sonogashira coupling with 4-bromo- and 4-fluoroiodobenzene
to afford the products 12 in moderate yields. This one-pot
procedure offers a good opportunity to synthesize diverse
alkynylated ketones. Finally, in the presence of 1.2 equiv of
*
E-mail: duanxh@mail.xjtu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Natural Science Basic Research Plan
in Shaanxi Province of China (No. 2014JQ2071) and the
Fundamental Research Funds of the Central Universities (No.
2015qngz17) is greatly appreciated.
t
KO Bu, the β-alkynylated ketone 5a could be easily converted to
2,5-disubstituted furan 13a in good yield.
To gain some insight into the mechanism of this reaction,
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Org. Lett. XXXX, XXX, XXX−XXX