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Chemistry Letters Vol.37, No.12 (2008)
Preparation, Structure, and Diels–Alder Reaction of
Phenyl(trifluoromethanesulfonate)(3,3,3-trifluoropropynyl)-ꢀ3-iodane
Masaki Shimizu,ꢀ Youhei Takeda, and Tamejiro Hiyama
Department of Material Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510
(Received October 9, 2008; CL-080972; E-mail: m.shimizu@hs2.ecs.kyoto-u.ac.jp)
Phenyl(trifluoromethanesulfonate)(3,3,3-trifluoropropynyl)-
ꢀ3-iodane was prepared from tributyl(3,3,3-trifluoropropynyl)-
stannane and PhI(CN)OTf. The molecular structure was dis-
closed by X-ray diffraction analysis to form T-shape geometry
with the hypervalent iodine atom as the central point. The iodo-
nium salt underwent smoothly the Diels–Alder reaction with 1,3-
dienes, furans, and pyrrole to give trifluoromethyl-substituted
cyclohexadienes in good to high yields, respectively, in contrast
to the corresponding nonfluorinated counterpart.
1) F3CC≡CSnBu3
CH2Cl2
–42 to 0 °C, 2 h
PhICN • –OTf
F3C
IPh • –OTf
ð1Þ
2) hexane, –42 °C
15 min
1
60%
Trifluoromethylated compounds have attracted great inter-
est in the fields of pharmaceuticals and materials science because
the presence of fluorine atoms in organic molecules often indu-
ces superior biological, chemical, and/or physical properties
than those of the corresponding non-fluorinated counterparts.1
Therefore, development of versatile synthetic methods/reagents
for CF3-containing compounds is of great importance for explo-
ration of fluorine-containing biologically active substances and
functional organic materials. For example, 3,3,3-trifluoropropy-
nylmetals have emerged as useful nucleophilic reagents for fac-
ile incorporation of the CF3-containing C3 moiety into organic
molecules.2 In contrast, electrophilic reagents consisting of a
3,3,3-trifluoropropynyl group such as 3,3,3-trifluoro-1-halopro-
pynes remain unexplored probably owing to volatility and insta-
bility.3 From the view that alkynyl(aryl)iodonium salts are gen-
erally stable, highly reactive, and widely utilized in various or-
ganic reactions,4 we were interested in the preparation and reac-
tions of 3,3,3-trifluoropropynyliodonium salts as novel synthetic
reagents for CF3-containing compounds. Reported herein are
preparation, structure, and the Diels–Alder reaction of phenyl-
(trifluoromethanesulfonate)(3,3,3-trifluoropropynyl)-ꢀ3-iodane
(1). The iodonium salt is stable and easy to handle, and the cy-
cloaddition with dienes smoothly proceeds even at room temper-
ature to give the corresponding CF3-containing alkenyliodonium
salts in good to high yields.
Preparation of 1 was accomplished by slightly modifying
the protocol developed by Stang and co-workers.5 Thus, tribu-
tyl(3,3,3-trifluoropropynyl)stannane6 was added to a solution
of PhI(CN)OTf7 in CH2Cl2 at ꢁ42 ꢂC and the resulting solution
was allowed to warm to 0 ꢂC over a period of 2 h.8 The addition
of hexane to the reaction mixture at ꢁ42 ꢂC gave precipitates
which were purified by recrystallization from the hexane/
CH2Cl2 solution at ꢁ30 ꢂC, giving rise to colorless microcrystals
of 1 (mp 110–111 ꢂC) in 60% yield (eq 1). The structure was
characterized by 1H, 13C, and 19F NMR, IR, and elemental
analysis, and was unambiguously confirmed by X-ray diffraction
analysis of its single crystal (vide infra).9 The salt 1 was soluble
in CH3CN and CH2Cl2, and could be stored without decomposi-
tion under argon atmosphere at ꢁ30 ꢂC for at least three months.
Figure 1. Molecular structure of 1 (hydrogens are omitted for
ꢂ
˚
clarity). Selected bond lengths (A) and angles ( ): C(1)–C(2)
1.451(1), C(2)–C(3) 1.176(5), C(3)–I(1) 2.022(3), I(1)–C(4)
2.109(3), I(1)–O(2) 2.596(3); C(1)–C(2)–C(3) 179.2(4), C(2)–
C(3)–I(1) 178.3(3), C(3)–I(1)–O(2) 176.99(12), C(3)–I(1)–
C(4) 92.67(14), C(4)–I(1)–O(2) 84.33(11).
The molecular structure of 1 disclosed by the X-ray analysis
is shown in Figure 1. Salt 1 adapted T-shape geometry with bond
angles being 92.67(14)ꢂ for C(3)–I(1)–C(4) and 84.33(11)ꢂ for
O(2)–I(1)–C(4). The atomic distance between I(1) and O(2)
˚
was 2.596(3) A longer than the sum of covalent bond radii of io-
˚
dine and oxygen (2.07 A), but shorter than sum of the van der
Waals radii (3.50 A).10 These structural characteristics are clear-
˚
ly consistent with the hypervalent nature of the iodine atom.
To demonstrate the synthetic utility of 1, we carried out a
Diels–Alder reaction (Table 1). Treatment of 1 with 2,3-dimeth-
yl-1,3-butadiene (2a) in CH3CN at room temperature gave the
corresponding adduct 3a in 87% yield (Entry 1). Since phenyl(1-
propynyl)(trifluoromethanesulfonate)-ꢀ3-iodane did not react
with 2a at all under the same conditions, the presence of fluo-
rine atoms apparently enhanced the reactivity of 1. Cyclic 1,3-
dienes such as 2b and 2c also underwent the cycloaddition to
give alkenyliodonium salts 3b and 3c in good yields (Entries 2
and 3). CF3-substituted 9-oxabicyclo[2.2.1]heptanes 3d and 3e
were prepared from 1 with furans 2d and 2e, respectively (En-
tries 4 and 5). It is noteworthy that 1 underwent the Diels–Alder
reaction even with N-methylpyrrole (2f) because cycloadditions
of 2f are quite limited (Entry 6).11
Versatile reactivity of the alkenyliodonium functionality
provides us with various transformations of the Diels–Alder ad-
ducts.12 For example, treatment of 3a with CuI/KI gave iodocy-
clohexadiene 4 that was converted into CF3-containing polysub-
stituted iodobenzene 5 by dehydrogenation with p-chloranil,
while CF3-substituted enyne 6 was synthesized by the Pd/Cu-
Copyright Ó 2008 The Chemical Society of Japan