Regio- and Stereoselective Radical Perfluoroalkyltriflation of Alkynes Using Phenyl(perfluoroalkyl)iodonium Triflates

Article abstract of DOI:10.1021/acs.orglett.7b01215

A method for regio- and stereoselective anti-addition of the perfluoroalkyl and the triflate group of phenyl(perfluoroalkyl)iodonium triflates to alkynes is presented. The radical reaction uses cheap CuCl as a smart initiator and can be conducted in gram scale. The perfluoroalkyltriflated products are readily further functionalized, rendering this transformation valuable.

Full text of DOI:10.1021/acs.orglett.7b01215

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Letter
pubs.acs.org/OrgLett
Regio- and Stereoselective Radical Perfluoroalkyltriflation of Alkynes
Using Phenyl(perfluoroalkyl)iodonium Triflates
Xi Wang and Armido Studer*
Institute of Organic Chemistry, University of Munster, Corrensstrasse 40, 48149 Munster, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A method for regio- and stereoselective anti-addition of the perfluoroalkyl and the triflate group of
phenyl(perfluoroalkyl)iodonium triflates to alkynes is presented. The radical reaction uses cheap CuCl as a smart initiator
and can be conducted in gram scale. The perfluoroalkyltriflated products are readily further functionalized, rendering this
transformation valuable.
luorinated building blocks have been widely used for
preparation of pharmaceuticals, agrochemicals, and func-
Scheme 1. Synthesis of Vinyl Perfluoroalkanes
F
tional materials.1 This is due to the unique properties exerted by
the F-substituents such as increased lipophilicity, higher
metabolic stability, and better bioavailability.2 Therefore, the
development of methods to introduce perfluoroalkyl groups (Rf)
into organic components is important.3
Recently, remarkable developments to attach Rf groups to
(hetero)arenes,4 benzylic positions,5a allylic positions,5b the α-
position of carbonyl groups,5c,d and heteroatoms6 have been
achieved. Enantioselective perfluoroalkylation was established,7
and the synthesis of vinyl perfluoroalkanes is also in high demand.
Common strategies to access them involve transition metal
catalyzedormediatedcross-couplingofhalidesorcarboxylicacids
with organometallic reagents (Scheme 1A).8 Nevado disclosed a
Pd-catalyzed three-component arylperfluoroalkylation of termi-
nal alkynes (Scheme 1B),9a and the Chaładaj group reported
arylperfluoroalkylation of internal alkynes using Pd-catalysis.9b
Other straightforward approaches to vinyl perfluoroalkanes
include direct radical C−H perfluoroalkylation of alkenes,10
hydroperfluoroalkylation of alkynes,11 and alkyne perfluoro-
alkyliodination (Scheme 1C).12 Notably, trans-selective
trifluoromethyltriflation of internal alkynes via photoredox
catalysis has been developed by Akita and Koike (Scheme
1D).13f Althoughmanyvaluablemethodsforthesynthesisofvinyl
perfluoroalkanes have been developed, some problems still
remain to be solved. Most of the reported perfluoroalkylations
suffer at least from one of the following limitations: harsh reaction
conditions, narrow substrate scope, low E/Z selectivity, and
moderate yield. Therefore, novel approaches to address these
drawbacks are desirable.
AgOTf13c as triflate sources. Recently, electrophile induced
triflation of alkynes with electrophilic reagents bearing −OTf as
the counteranion has been developed (aryl and vinyl triflation by
Gaunt13d and cyanotriflation by our group13e).
Hypervalent iodine(III) reagents14 of the benziodoxole type or
the iodonium salt type are generally used as electrophiles where
only one I(III)-ligand gets transferred to the substrate. Sequential
transfer of two ligands from an iodine(III) compound to a
carbon−carbon unsaturated bond would certainly improve atom
Alkenyl triflates have been intensively applied as vinyl-
organometallic precursors in cross-coupling reactions. The
traditional route for their preparation proceeds via trapping of a
deprotonated ketone with a triflating reagent (usually Tf2O or
PhNTf2). Such triflates can also be accessed using HOTf13a,b or
Received: April 21, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
economy, but such transformations are rare.13d,e,15a−e For
example, the Waser group described an elegant strategy for
oxyalkynylation of diazo compounds with alkynylbenziodox-
oles.15f We herein report highly regio- and stereoselective
perfluoroalkyltriflation of various alkynes with phenyl(perfluoro-
alkyl)iodonium triflates16 using radical chemistry (Scheme 1D).
The method shows remarkable substrate scope and proceeds
under mild conditions. The potential of the sequence is
documented by various follow-up transformations and by the
successful preparation of a fluorinated drug.
was obtained at 50 °C using 10 mol % of CuCl to afford 3a in 94%
isolated yield (Table 1, entry 15).
Having established the optimized conditions, we first explored
the scope with respect to the alkyne component and investigated
internal alkynes (Scheme 2). Various 1-aryl-1-pentynes bearing
a b c
, ,
Scheme 2. Alkyne and I(III)-Reagent Scope
Initial screening was performed with ethyl 4-(pent-1-yn-1-
yl)benzoate 1a and 1.0 equiv of iodine(III) reagent 2a
(PhI(C3F7)OTf) in various solvents at room temperature for 4
h. Pleasingly, we found that in dichloroethane (DCE) the
perfluoroalkylated triflate 3a was obtained in 49% yield with
complete regio- and E-selectivity (Table 1, entry 6). The relative
Table 1. Reaction Optimization
2a
(equiv)
time
(h)
a
b
entry
additive
solvent
yield(%)
1
none
none
none
none
none
none
none
none
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.7
1.7
1.7
1.7
1.7
1.7
1.7
1.7
MeOH
Et2O
4
0
2
4
0
3
MeCN
DCM
CHCl3
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
4
11
40
25
49
53
69
86
78
75
53
86
91
4
4
5
4
6
4
7
15
15
15
15
15
15
15
15
15
8
9
pyridine (1.7 equiv)
K2CO3 (1.7 equiv)
BF3·Et2O (1.7 equiv)
TfOH (1.7 equiv)
TBAI (10 mol %)
CuCl (10 mol %)
CuCl (10 mol %)
10
11
12
13
14
d
c
15
96 (94)
a
Reaction conditions: 1a (0.10 mmol, 1.0 equiv), reagent 2a, additive,
b
solvent (1 mL), room temperature. Yield determined by 19F NMR
analysis using PhCF3 as an internal standard; isomer ratio determined
by 19F NMR and GC-MS analysis on the crude product, E/Z > 20:1.
a
Reaction conditions: 1 (0.10 mmol, 1.0 equiv), 2 (0.17 mmol, 1.7
equiv), CuCl (0.01 mmol, 10 mol %), DCM (1 mL), 45 °C, 15 h.
c
d
b
c
Isolated yield. E/Z isomer ratio determined by 19F NMR analysis
Isolated yield. Conducted at 50 °C.
and GC-MS analysis on the crude product. Unless noted otherwise, E/
d
Z > 20:1. 1 (0.10 mmol, 1.0 equiv), 2 (0.15 mmol, 1.5 equiv), CuCl
(0.01 mmol, 10 mol %), and DCM (1 mL), room temperature, 24 h.
After 15 h, renewed CuCl (0.01 mmol, 10 mol %), 2 (0.17 mmol, 1.7
e
configuration of 3e, 3r, and 3v (see below) was assigned by NOE
experiments, and all other compounds were assigned in analogy.
Reaction did not work in MeOH or Et2O, and lower yields were
achievedinMeCN, dichloromethane (DCM), andCHCl3 (Table
1, entries 1−5). Prolonging the reaction time to 15 h slightly
improved the yield (Table 1, entry 7), and increasing the amount
of 2a from 1.0 to 1.7 equiv let to a better result (Table 1, entry 8).
The yield was further improved upon adding 1.7 equiv of a base
such as pyridine or K2CO3 providing the target product 3a in 86%
and 78% yield, respectively (Table 1, entries 9−10). A similar
effect was achieved with BF3·Et2O as an additive (Table 1, entry
11), whereas perfluoroalkyltriflation in the presence of TfOH was
lower yielding (Table 1, entry 12). Both tetrabutylammonium
iodide (TBAI) and CuCl as additives (10 mol %), which are often
used for SET reduction of iodine(III) reagents,17 enhanced
reaction efficiency (Table 1, entries 13−14). The highest yield
equiv), and DCM (1 mL) addition and continued stirring at 45 °C for
f
another 15 h. After 24 h, renewed CuCl (0.01 mmol, 10 mol %), 2
(0.15 mmol, 1.5 equiv), and DCM (1 mL) addition and continued
g
stirring at room temperature for another 24 h. Yield determined by
h
19F NMR analysis using PhCF3 as an internal standard. E:Z = 17:1.
substituents at the para-, ortho-, and meta-position of the aryl
group were successfully converted in good yields with excellent
regio- and stereoselectivity (3a−n). Heteroaryl substituents such
as pyridyl and thienyl are tolerated, and 3o and 3p were obtained
in moderate to high yields. Symmetrical diaryl alkynes are suitable
substrates as documented by the preparation of 3q and 3r.
Complete regioselectivity and E-selectivity were also observed
with methyl (3s) and cyclopropyl (3t) aryl alkynes.
B
DOI: 10.1021/acs.orglett.7b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Although functionalization of electron-deficient alkynes with
electrophilic reagents is challenging our method proved to be
applicable to such alkynes as documented by the successful
transformation of an ynone (3u), a propiolic ester (3v), and a
propiolic amide (3w). We next attempted to expand the newly
established method to the conversion of terminal alkynes and
could show that arylacetylenes bearing either electron-with-
drawing or -donating groups reacted efficiently to afford the
corresponding perfluoroalkyltriflation products with complete E-
selectivity in moderate to good yields (3aa−3ag). Unfortunately,
reactions with dialkyl alkynes failed and starting materials
remained unreacted. Notably, our reaction can be conducted on
gram scale without compromising the yield (3a, 3b, 3z). We
found that the perfluoroalkyl group in the I(III) reagent can be
readilyvariedasshownfortheperfluorobutyl, perfluorohexyl, and
perfluorooctyl congeners in the reaction with the alkyne 1a. The
corresponding products 3x−z were obtained in good yields and
excellent selectivities.
To demonstrate the synthetic value of the method, we
investigatedfollow-upchemistryandcouldshowthatcompounds
of type 3 engage in various stereospecific Pd-catalyzed cross-
coupling reactions, including alkenyl-aryl and alkenyl−alkenyl
Suzuki couplings (4, 5), the Sonogashira reaction (6), and
methoxycarbonylation (7) (Scheme 3). Moreover, pentafluori-
nated Tamoxifen 8 was prepared in acceptable overall yield and
high E/Z selectivity via a pentafluoroethyltriflation/Suzuki
coupling sequence.18
The suggested mechanism is depicted in Scheme 4. Initiation
likely occurs by SET reduction of 2 with CuCl to give the Rf-
Scheme 4. Suggested Mechanism
radical along with Cu(II)ClOTf.16,17 Rf-radical addition to the
alkyne leads to the vinyl radical A which further reacts via SET
oxidationbytheI(III)-reagent 2toform thevinyliccationB along
with PhI and the Rf-radical sustaining the chain. The observation
that without a Cu-salt reaction proceeds in acceptable yields
clearly shows that oxidation of A with reagent 2 is rather efficient,
and hence the sequence belongs to an electron-catalyzed
process19 where the Cu-salt acts as a smart initiator.19b Thus,
regeneration of the initiating Cu(I)-complex is occurring via
occasional reaction of A with the Cu(II)-salt. Trapping of B with
thetriflateanioneventuallyaffords3.ThehighE-selectivitycanbe
explained by the larger steric hindrance of the Rf group as
compared to the R2 substituent and by the electrostatic repulsion
of the Rf-substituent and the incoming triflate during
trapping.13f,20
Scheme 3. Further Functionalization of Perfluoroalkyltriflated
Products
In summary, we have demonstrated a general and practical
protocol for direct vicinal alkyne perfluoroalkyltriflation.
Prominent features of this process include mild reaction
conditions, compatibility with both internal and terminal alkynes,
excellent E-selectivity and complete regioselectivity, generally
high yields, and easy scale up. Various follow-up reactions
document that the tetrasubstituted vinyl triflates obtained as
products are valuable building blocks in synthesis.
ASSOCIATED CONTENT
■
S
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b01215.
Experimental procedures and characterization data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: studer@uni-muenster.de.
ORCID
Armido Studer: 0000-0002-1706-513X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Alexander von Humboldt
Foundation (postdoctoral fellowship to X.W.) and the European
Research Council (ERC Advanced Grant Agreement No.
692640).
C
DOI: 10.1021/acs.orglett.7b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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