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10.1002/chem.202101090
Chemistry - A European Journal
FULL PAPER
wherein its use as the Lewis acid component of frustrated Lewis
pairs (FLPs) is the most representative.[8] FLPs consisting of
B(C6F5)3 and Lewis bases exhibit synergic reactivities between an
electrophilic boron-centered vacant orbital and a nucleophilic
filled orbital of the Lewis base,[9] which facilitates the heterolytic
cleavage of H2 and reactions with a variety of small molecules.[10]
B(C6F5)3 also promotes several distinguishable reactions of
alkynes.[11–15] More specifically, the 1,2-additions of B(C6F5)3 and
Lewis bases such as phosphines, pyrroles, and amines to alkynes
were reported.[11] We therefore surmised that if B(C6F5)3 and
allylsilanes can add to alkynes, the subsequent coupling of the
dichloroethane (DCE) at 60 °C (Scheme 2), wherein the
unpredicted three-component coupling product 3a was obtained.
1
The structure of 3a was determined by H/13C/19F and 2D NMR
spectroscopy, and by mass spectrometry. In addition, the
treatment of 3a with TBAF promoted the intramolecular
nucleophilic aromatic substitution reaction of the Z-isomer to
afford tetrafluoronaphthalene
8
together with desilylated
compound 9. A single crystal of 8 was obtained and its structure
was unambiguously confirmed by X-ray diffractometry (Figure S5).
The structure of 3a reveals that two C–C bonds were formed
during the reaction, namely one bond between the benzylic C1
resultant alkenylborates will facilitate the 1,2-allylboration process. position of 1a and the β-position to the silicon atom of 2, which is
However, to the best of our knowledge, allylsilanes are less
nucleophilic than the Lewis bases that have been reported for
such 1,2-additions.[16] B(C6F5)3 was reported to readily facilitate
the 1,1-carboborations of alkynes in the absence of a Lewis
base,[14] thereby indicating that the nucleophilicity of the Lewis
base is important for promoting the 1,2-addition prior to the 1,1-
carboboration reaction.[13c,14e] In this article, we report that
B(C6F5)3 and allylsilanes 2 do add arylacetylenes 1 to yield novel
1,2-carbopentafluorophenylation products 3 rather than the
simple allylsilylated or hydroallylated compounds which are
produced from alkenylborate intermediates (Scheme 1B).
Interestingly, C–C bonds are formed at the β-position to silicon of
the allylsilane, and this is accompanied by C–H bond scission
without elimination of the silyl group to afford conjugated 1,3-
dienes instead of skipped dienes. We also demonstrate that the
resultant pentafluorophenyl-substituted 1,3-dienes can be
converted into the corresponding tetrafluoronaphthalenes by a
photochemical 6π-electrocyclization process. In addition,
reactions using 2-substituted furans 4 or silyl enolate 5 as Lewis
accompanied by cleavage of the Cβ–H bond, and a second bond
between the terminal C2 position of 1a and the C6F5 group from
B(C6F5)3.
Because
2 was consumed by the dehydration of
B(C6F5)3•nH2O, an excess amount of 2 was used for the initial
experiment.[15] Hence, we examined the reaction using anhydrous
B(C6F5)3 and 2 equiv of 2 in a glove box, which efficiently
proceeded at room temperature to afford 3a in 56% yield after 2
h (Scheme 2).
Several control experiments were conducted to gain insight
into the mechanism of the novel 1,2-carbopentafluorophenylation
reaction (Schemes 3 and S1, Figures S1-S4). Initially, to evaluate
the possibility of a reaction through 1,1-carboboration,[14] 2 was
added to a solution of 1,1-carboboration product 10 in CD2Cl2,
which was prepared in situ from 1a and B(C6F5)3 in the absence
of a Lewis base; unreacted 10 remained as a major compound
after 4 h (Scheme 3a, Figure S1). This result suggests that 1,1-
carboboration is, at least, not a major reaction pathway for the
1,2-carbopentafluorophenylation. Subsequently, the reaction was
performed using deuterium-labeled phenylacetylene (1a-d), with
deuterium-labeled coupling product 3a-d being formed (Scheme
3b). The level of deuterium incorporation did not decrease in the
product (Figure S2), which suggests that the methine proton of 1a
is not abstracted during the reaction. After stirring the reaction
mixture of B(C6F5)3•nH2O, 1a, and 2 in DCE for 0.5 h at rt, basic
aqueous H2O2 was then added, and alcohol 11 was
bases
were
also
examined,
with
novel
1,2-
carbopentafluorophenylation products 6 and 7 being obtained. To
reasonably understand the mechanism of the observed novel 1,2-
carbopentafluorophenylations,
we
herein
introduce
a
metallomimetic “pull-push” reactivity concept involving B(C6F5)3
toward alkynes (see Scheme 1B). This “pull-push” reactivity is
known to be responsible for the reactivities of carbophilic
transition metal catalysts, such as gold and platinum catalysts, in
reactions with alkynes.[17] The transition-metal catalyst not only
electrophilically activates the alkyne (“pull”), but the electron
density is also back-donated to the vicinal carbon atom (“push”).
This “pull-push” nature imparts a consecutive electrophillic and
nucleophilic characteristics to the vicinal carbon atom of the
alkyne substrate and enables characteristic reactions to take
place such as the cyclopropanation reaction of enynes and the
acetylenic
Schmidt
reaction.[17b,c]
Similarly,
B(C6F5)3
electrophilically activates an alkyne (“pull”) to facilitate the
addition of a Lewis base. The electron density is consecutively
donated to the vicinal carbon atom by the migration of a C6F5
group from the boron to the adjacent carbon atom (“push”) to form
a cycopropane ring. This reactivity enables the observed novel
1,2-carbopentafluorophenylations. Several previously reported
reactions also implicate such a metallomimetic pull-push reactivity.
[13a,b]
Results and Discussion
Scheme 2. Initial results. a) Reaction of B(C6F5)3•nH2O and allylsilane 2 with
phenylacetylene (1a) and the treatment of the obtained product 3a with TBAF.
b) Reaction using anhydrous B(C6F5)3 in glove box.
We initially examined the reaction of B(C6F5)3•nH2O (1 equiv) and
allylsilane 2 (12 equiv) with phenylacetylene (1a) (1 equiv) in
2
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