Communications
phene-2-carboxylate (CuTC) was used for 2-(hex-5-en-1-
yl)oxirane (Table 2, entry 6). In most cases, the E/Z selectivity
was excellent, with an average ratio of 94:6 for the substrates
examined. We found branched terminal olefins and 1,2-
disubstituted olefins to be unsuitable substrates because of
the formation of complex regioisomeric product mixtures.
Furthermore, cyclic substrates furnished only trace amounts
of product.[21]
Scheme 4. Examination of a diallylmalonate cyclization radical clock.
À
5-exo-trig cyclization that proceeds after the C CF3 bond-
In order to demonstrate the robustness of this trans-
formation, we conducted the trifluoromethylation of 4-
phenyl-1-butene on a 10 mmol scale [Eq. (1)]. All reagents
were weighed on the benchtop, open to the air, and the setup
was conducted using standard Schlenk techniques. The results
from this experiment indicate that the method described
herein can be set up on the benchtop without an accompany-
ing sacrifice of the reaction efficiency.
forming event. It is unclear if the trifluoromethylation results
in the generation a free-radical intermediate (5a) or an
alkylcopper species (5b). After cyclization, termination
occurs by a second trifluoromethylation or elimination to
generate products 4a or 4b, respectively. Of note, we found
that conducting the trifluoromethylation reaction in the
presence of selected radical scavengers provided variable
results that did not aid our understanding of the reaction
mechanism.[22] Further analysis will be necessary to elucidate
the nature of this transformation more accurately.
In conclusion, we have developed an allylic trifluorome-
thylation of unactivated terminal olefins. This method allows
À
for the preparation of allyl CF3 products that were previously
difficult to access in a straightforward and efficient manner.
The mild conditions for this transformation enable the
trifluoromethylation of a range of substrates that bear
numerous functional groups. A preliminary analysis suggests
that the reaction mechanism is complex and multiple path-
Similar to the proposed mechanism of the Kharasch–
Sosnovsky CuI/II-catalyzed oxidation of olefins to generate
allyl esters,[18] we wanted to probe whether this transforma-
tion proceeded via an allylic radical intermediate. We were
intrigued, however, by the high selectivity for the linear
trifluoromethylated products obtained by using the method
described herein. This result is in contrast to most reports of
Kharasch–Sosnovsky-type oxidative alkene functionaliza-
tions, and therefore suggests a possible divergence from this
mechanistic pathway. In order to determine whether this
transformation did indeed proceed via a free allylic radical,
we conducted the trifluoromethylation of cyclopropane
radical clock 3a [Eq. (2)]. Subjecting this substrate to our
standard conditions provided the trifluoromethylated cyclo-
propane 2o in moderate yield; this result suggests that a
mechanism involving the formation of an allylic radical is
unlikely. However, we note that other trifluoromethylated
side products were present but unidentifiable (ꢀ 3% yield
each), thus precluding us from conclusively stating that no
ring-opened product was formed.
The results with cyclopropane 3a prompted us to consider
an alternative mechanistic possibility, wherein the trifluoro-
methylation event occurs through an atom transfer radical
addition type pathway by homolytic cleavage of the alkene.[19]
Data to support or refute this mechanism was sought by
examining diethyl diallylmalonate as a cyclization radical
clock (Scheme 4). The major products obtained under these
conditions were cyclopentane derivatives 4a and 4b. The
presence of these species is explained by the occurrence of a
À
ways leading to the desired allyl CF3 products may be
operating.[23] Future efforts will focus on examining the
mechanistic details more extensively on the way to expanding
the generality and increasing the efficiency of this trans-
formation.
Experimental Section
(E)-(5,5,5-trifluoropent-2-en-1-yl)benzene (2a) on a 10.0 mmol scale:
A 100 mL Schlenk flask was flame-dried under high vacuum and
backfilled with argon. On the benchtop, open to air, [(MeCN)4Cu]PF6
(0.559 g, 1.50 mmol, 0.15 equiv) and 1 (3.16 g, 10.0 mmol, 1.0 equiv)
were weighed and added to the Schlenk flask. The flask was then
sealed with a rubber septum, evacuated, and backfilled with argon
(this process was repeated a total of three times) and cooled to 08C in
an ice–water bath. The flask was charged successively with anhydrous
methanol (50 mL) and 4-phenyl-1-butene (1.65 g, 1.88 mL,
12.50 mmol, 1.25 equiv) by syringe (a bright green-blue color was
observed upon solvent addition). The reaction mixture was stirred for
30 min at 08C, after which the ice-water bath was removed and
stirring was continued for an additional 23 h. The reaction mixture
was partitioned between CH2Cl2 (75 mL) and sat. aq. NaHCO3
(75 mL). The aqueous layer was separated and extracted with
CH2Cl2 (2 ꢀ 50 mL). The combined organic extracts were washed
with saturated aqueous NaHCO3 (75 mL), dried over Na2SO4, and
concentrated in vacuo. The resultant oil was purified by column
chromatography (pentane) on silica gel to afford 2a (1.503 g, 75%) as
a clear colorless oil (E/Z = 97:3) contaminated with 2.5 mol% of a
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9120 –9123