V. V. Levin et al. / Tetrahedron Letters 52 (2011) 281–284
283
We were also interested in reactions of cationic electrophiles,
which would be expected to interact rapidly with an anionic borate
nucleophile.16 For this purpose, the iminium salt 11 was treated
with borate 1a (Eq. 2).
compounds.20 In this regard, owing to the significant electron-
withdrawing effect of fluorines, the trifluoromethyl group may
bear some similarities to an alkynyl fragment.
In summary, we have demonstrated that borate salts can be
used as efficient reagents for nucleophilic trifluoromethylation.
Reactions of non-enolizable carbonyl compounds and N-tosyli-
mines with CF3-borates proceeded under mild conditions furnish-
ing products in good yields. The search for processes involving
transfer of the CF3-group from boron to other electrophiles will
constitute the subject of our future work.
Me
Me
Me
Me
1.2 eq.
(1a)
F3C B(OMe)
3 K
N
N
ð2Þ
Ph
Ph
CF3
DMF, 50 °C, 1 h
TfO
11
75%
12
Though the anticipated product 12 was formed, virtually no
acceleration compared to the reaction with benzaldehyde was
noted. Indeed, when reactions of benzaldehyde and 11 with 1a
were performed under identical conditions (40 °C, 1 h), approxi-
mately 50% conversions were observed in both cases. This is sur-
prising, given that the iminium ion is expected to be more
electrophilic than the neutral aldehyde. To explain this phenom-
enon we combined salts 11 and 1a in DMF-d7 in an NMR tube.
Significant line-broadening of most of the signals was observed,
as is clearly seen in the 1H and 11B NMR spectra (Fig. 1). This sug-
gests an equilibrium interaction of iminium and borate ions to
generate either semi-aminal-type species or a hydrogen-bonded
complex,17 thereby decreasing the reactivity of the iminium elec-
trophile (Eq. 3).
Acknowledgments
This work was supported by the Russian Foundation for Basic
Research (project 08-03-00428), and the Federal program ‘Scien-
tific and educational personnel of innovative Russia’ (project
02.740.11.0258).
Supplementary data
Supplementary data associated with this article can be found, in
Me
Me
Me
Ph
Me
Me
Me
N
References and notes
N
N
Me
or
Ph
H
Ph
O
Me
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Figure 1. 1H (left) and 11B (right) NMR spectra of 11 and 1a (top) and their mixture
(bottom) in DMF-d7 at room temperature.