Ag-Catalyzed difluorohydration of β-alkynyl ketones for diastereoselective synthesis of 1,5-dicarbonyl compounds

Article abstract of DOI:10.1039/c7cc02088k

A new catalytic difluorohydration of β-alkynyl ketones using NFSI as the fluorinating reagent has been established, diastereoselectively furnishing a range of structurally diverse difluoride 1,5-dicarbonyl products through C(sp³)-H fluorination. Notably, the sterically encumbered t-butyl functionality located at the α-position of the carbonyl group of substrates 1 showed excellent diastereoselectivity (up to >99:1 dr). The reaction enabled multiple bond-forming events including two C(sp³)-F formation through Ag-catalysis to provide a highly efficient and practical method toward difluoride 1,5-dicarbonyls, some of which were successfully converted into difluorinated isoquinolines.

Full text of DOI:10.1039/c7cc02088k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Ag-catalyzed difluorohydration of -alkynyl ketones for
β
diastereoselective synthesis of 1,5-diconboyl compounds
Yi-Long Zhu,a,b Ai-Fang Wang,a Jian-Yu Du,a Bo-Rong Leng,b Shu-Jiang Tu,*,a De-Cai Wang,b Ping
Wei,*,b Wen-Juan Hao,a and Bo Jiang*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new catalytic difluorohydration of
β
-alkynyl ketones using NFSI difluoride molecules.11 For instance, the group of Jacobsen11e,f
catalytic 1,2-
as the fluorinating reagent has been established, and Gilmour11g independently reported
a
diastereoselectively furnishing a range of structurally diverse difluorination of alkenes using suitable F-source and
difluoride 1,5-dicarbonyl products through C(sp3)-H fluorination. substituted iodobenzenes as a catalyst. Xu and co-workers
Notably, the sterically encumbered t-butyl functionality located at described the synthesis of gem-difluoromethlylenes by gold-
α-position of carbonyl group of substrates 1 behaved the excellent catalyzed dihydrofluorination of alkynes and DMPU/HF
diastereoselectivity (up to >99:1 dr). The reaction enabled complex (Scheme 1b).11h Despite these significant advances,
multiple bond-forming events including two C(sp3)–F formation however, catalytic triple functionalization of
β-alkynyl ketones
through Ag-catalysis to provide a high-efficient and practical involved difluorination for diastereoselective synthesis of
method toward difluoride 1,5-dicarbonyls, some of which were difluoride 1,5-dicarbonyl compounds, to the best of our
successfully converted into difluorinated isoquinolines.
knowledge, has not been documented yet.
R1
F
F-source
R1
R3
The incorporation of fluorine into organic molecules can
modulate their abilities including lipophilicity, bioavailability
and metabolic stability.1 As a result, the fluorine-containing
compounds have been widely applied in material science,
chemical biology and pharmaceutical chemistry.2 Furthermore,
some fluorine-containing compounds behaved as key
intermediates for the preparation of bioactive substances3 and
served as organocatalysts in catalytic chemistry.4 With these
attributes in mind, substantial effort aimed toward identifying
general methods for the synthesis of these fluoric molecules
has been made, mainly depending on the use of fluorination
reagents, such as Selectfluor,5 Et3N·HF,6 N-fluoropyridinium
R3
(a)
(b)
R2
Iodobenzenes
R2
F
Jacobsen and Gilmour's work
F
F
R2
DMPU/HF
[Au], KHSO4
R1
R1
R2
Xu's work
Me
Ar
Ar
Ar
O
Ar
O
P
P
O
R1
[Ag]
R1
O
P(O)Ar2
R1
(c)
(d)
(e)
O
R2
R2
R2
Our previous work
This work
R3
R3
F
O
R3
NFSI
[Ag]
R1
O
O
R1
R1
salts,7
diethylaminosulfur
trifluoride,8
N-
?
R2
R2
fluorobenzenesulfonimide (NFSI).9 With these fluorination
reagents, the mono-fluorination involved the C(sp)−H,
C(sp2)−H or C(sp3)−H functionalization has been extensively
studied, providing an efficient strategy for the collection of
fluorine-containing compounds.10 Recently, chemists turned
their attention to the dual fluorination for constructing
R2
4
H2O
F
NFSI
1
R3
N
F
R3
F
O
R2
O
NH4OAc
R1
R2
(±)
F
R1
2
3
Scheme 1. Profile application of difluorination
Catalytic cyclization of β-alkynyl ketones has proven to be a
exceptionally efficient methodology to construct synthetically
significant cyclic compounds in a convergent manner.12,13 Very
recently, we reported silver-mediated oxo-cyclization and
C(sp3)–H biphosphinylation of
β-alkynyl ketones to afford
functional isochromenes. For this reaction, the in-situ-
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generation of methyleneisochromene intermediates is a key cyclobutyl, cyclohexyl (Cy), aryl, and 2-propenyl, would be
step to capture diarylphosphine oxide radicals to form the accommodated, confirming the efficiency of the reaction, as
target products (Scheme 1c).14 On the basis of the above the corresponding 2n-2w were generated with 46%-91% yields
successful transformation, we reasoned that under suitable and up to >99:1 dr. Interestingly, the sterically bulky t-butyl
catalytic conditions,
into electron-rich
electrophilic substitution with NFSI to give fluoride 67% yield (>99:1 dr). In view of this attractive outcome, we
isochromenes (Scheme 1d). Surprisingly, we found that the considered to exploit other t-butyl derivatives to evaluate the
Ag-catalyzed reaction of the preformed -alkynyl ketones diastereoselectivity. To our delight, the purposefully
underwent unexpected difluorohydration process in the preformed -alkynyl ketones 1x-1dd enabled Ag-catalyzed
β
-alkynyl ketones are easily converted counterpart 1p was a good reaction component, delivering the
methyleneisochromenes, enabling diastereoenriched product 2p as a single diastereoisomer in
4
β
1
β
presence of H2O, leading to the diastereoselective formation difluorohydration to exhibit the excellent diastereoselectivity
of difluoride 1,5-dicarbonyls. Herein, we would like to report (up to 99:1 dr), except for 1y. In general, the current protocol
this interesting transformation of catalytic difluorohydration represents
involved C(sp3)-H bond fluorination (Scheme 1c). The resulting diastereoselective construction of richly decorated 1,5-
1,5-dicarbonyls were subjected to the reactions of ammonium dicarbonyls. The structures of these 1,5-dicarbonyls were
acetate, allowing a facile dehydrated [5 + 1] aza-cyclization to characterized by their NMR spectroscopy and HRMS.
a
new and practical pathway for the
2
access difluorinated isoquinolines with excellent yields.
Our initial investigation commenced with the reaction of
Furthermore, stereoscopic structures of 2w and 2z were
β
-
confirmed by carrying out single crystal X-ray diffraction (see
alkynyl ketones 1a with NFSI in a 1:2 mole ratio in the Supporting Information).
presence of water (2.0 equiv) using AgOAc (20 mol%) as a
catalyst.15 The reaction was performed in 1,4-dioxane solvent
at room temperature under Ar conditions, delivering the
unexpected difluoride product 2a in 49% yield (Table 1, entry
S1, See Supporting Information). After careful optimizations,
we found that the reaction of 1a with NFSI in a 1:3 mole ratio
in 1,4-dioxane at room temperature together with 1.2
equivalents of water and 10 mol% of AgNO3 as a catalyst was
proven to be most effective, affording product 2a in 82% yield
(entry S18).
After determining the optimal reaction conditions, we then
investigated the generality of this catalytic difluorohydration
by examining
β
variety of -alkynyl ketones
-alkynyl ketone components (Scheme 2). A
readily reacted with NFSI to
β
1
furnish the corresponding inseparably diastereoisomeric 1,5-
dicarbonyls 2a−2dd with generally good yields. With the ethyl
group on the carbonyl anchor, various substituents on the
arylalkynyl moiety (R2), including Cl (1a and 1b), Br (1c), F (1d),
Me (1f and 1g), Et (1h), and t-Bu (1i), can tolerate the catalytic
conditions well, providing the 1,5-dicarbonyls 2a−2i with 41%-
82% yields and poor diastereoselectivity (1.3:1 to 8:1 dr). The
presence of functional groups at the para-position of phenyl
ring seemed to improve the reaction efficiency, as the
corresponding products 2a and 2f could be obtained in higher
yields than those resided at meta-position (Scheme 2, 2a vs 2b
,
2f vs 2g). Alternatively, 3-thienyl substituted β-alkynyl ketone
1j still showed high reactivity in current difluorohydration,
giving access to the corresponding product 2j with 68% yield
and 7:1 dr. Unluckily, β-alkynyl ketone 1k bearing n-butyl (n-
Bu) group on the alkynyl moiety was not an adaptable reaction
partner, as the reaction did not proceed at all under the
standard conditions (Scheme 2, 2k). The substrates
1 carrying
Scheme 2 Substrate scope for forming products
2.
(i) Yields of
fluoro functionality at 4- or 5-position of the internal arene
ring can also lead to the formation of trifluoride product 2l or
2m, respectively. Next, we decided to change the substituents
isolated products based on
analysis of 19F NMR.
β-alkynyl ketone 1. (ii) dr value based on the
After the successful formation of difluoride 1,5-dicarbonyl
products, we attempted to employ them as starting materials
to react with ammonium acetate to investigate the dehydrated
located at α-position of carbonyl group to expand its synthetic
utility. As we had expected, a large variety of different
substituents, including n-butyl, i-proply (i-Pr), t-butyl,
2 | J. Name., 2012, 00, 1-3
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[5 + 1] aza-cyclization toward the expected difluorinated alkynyl unit to form isobenzopyrylium intermediate is a key
isoquinolines since the fluorinated isoquinolines have been step for the success of this transformation. The reaction of 1,2-
found to show a broad spectrum of biological activities.16 Upon diphenylethyne (
) failed to give the target product (Scheme
treatment with representative 1,5-dicarbonyls with 4b), showing without carbonyl group at -position of alkynyl
3
7
9
2
β
ammonium acetate proceeded readily in MeOH at room moiety fluorohydration of 1,2-diphenylethyne did not work
temperature under air conditions, providing access to the under the same conditions. In addition, the conversion of 2-
corresponding difluorinated isoquinolines 3a-3f in 89%-95% phenylacetophenone (8) into product 9 did not proceed
yields (Scheme 3).
(Scheme 4b). In the presence of phenylhydrazine, substrate 1a
was converted into mono-fluoride 1,5-dicarbonyl compound
10 (Scheme 4c), but the transformation of compound 10 into
product 2a did not occur under the standard conditions
(Scheme 4d). These results revealed that fluorination of alkynyl
unit occurs prior to the hydration and the second C(sp3)-H
fluorination. The deuterium-labeling experiments based on the
use of 1a (Scheme 4e) indicated that a hydrogen atom of the
new forming C-H bond was generated from H2O.
Scheme 3 Synthesis of difluorinated isoquinolines 2. Yields of
isolated products based on compounds 2.
Scheme 5. Plausible mechanism for forming 2
Combining the above investigations and the literature
precedents of silver-catalyzed redox system17 and the oxygen
transfer process,12 we proposed a reasonable mechanism for
this difluorohydration (Scheme 5). First, Ag-catalyzed 6-endo-
dig oxo-cycilzation of β-alkynyl ketones generates
isobenzopyrylium intermediate
NFSI to afford vinyl–AgIII fluoride
Supporting Information). Next, intermediate
into fluoride isobenzopyran (detected by LC-MS) through
A
, followed by oxidation with
(detected by LC-MS, see
is converted
B
B
D
reductive elimination and H-transfer, which undergoes
oxidation-coordination with AgI and NFSI to afford AgIII
complex intermediate
and reductive elimination give difluoride isobenzopyrylium
Finally, the ring-open of isobenzopyrylium attacked by water
diastereoselectively affords difluoride 1,5-dicarbonyl products
E, and the following ligand exchange
G
.
G
2.
Scheme 4 Control experiments
In conclusion, we have established a new silver-catalyzed
difluorohydration of -alkynyl ketones using NFSI as the
To understand the reaction mechanism, several control
β
experiments were conducted. Frist, O-protected or
fluorinating reagent, by which structurally diverse difluoride
1,5-dicarbonyl products can be diastereoselectively
synthesized through C(sp3)-H fluorination under mild catalytic
oxidation conditions. Specifically, the sterically encumbered t-
unprotected 1,2-diarylethynes
standard conditions, but no expected products
observed with the starting materials remaining (Scheme 4a),
5
were subjected with the
6
were
5
suggesting silver-catalyzed addition of carbonyl group into
butyl functionality located at
α
-position of carbonyl group of
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substrates behaved the excellent diastereoselectivity (up to
Journal Name
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forming events including one C(sp3)-H, two C(sp3)–F and two
C–O bond formation through Ag-catalysis, providing a high-
efficient and practical protocol toward difluoride 1,5-
dicarbonyls, and some of them were successfully converted
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cyclization. Further assessment of the biological activity of
these difluoride compounds is underway in our laboratory.
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We are grateful for financial support from the NSFC (No.
21232004, 21472071 and 21602087), PAPD of Jiangsu Higher
Education Institution, the Outstanding Youth Fund of JSNU
(YQ2015003), NSF of Jiangsu Province (BK20151163 and
BK20160212), the Qing Lan Project and NSF of Jiangsu
Education Committee (15KJB150006).
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