Gabbaı showed similar behavior with triarylfluorosilane
bearing a sulfonium moiety as a stabilizing group.7d The
second approach is the cleavage of a SiꢀO bond by fluoride
activation which liberates a chromophore.8
by the most electronegative ligand at the apical position
(Figure 1).11
Scheme 1. Reactivity of Sensors 1ꢀ3 with Fluoride (1 equiv)
Figure 1. Fluoride sensors based on silicon derivatives.
Spirobisilafluorene 2 was prepared from 2,20-dibromo-
biphenyl that was easily transformed into the dilithiated-
biphenyl intermediate by halogenꢀmetal exchange and re-
acted with SiCl4.12 Dinitro derivative 3 was obtained by
nitration of 2with Cu(NO3)2 in acetic anhydride and purified
by flash chromatography.13 With these four substrates in
hand, we observed their behaviors in the presence of fluoride
sources under different experimental conditions.
As anticipated, reaction of silane 1 with inorganic and
organic sources of fluoride proved to be highly efficient
either in pure organic solvents or in the presence of water
(Scheme 1). Tetrabutylammonium fluoride (TBAF) al-
lowed the almost quantitative formation of silicate 5a
in diethyl ether and dichloromethane (DCM) in <1 h.
Potassium fluoride has also been used in acetone to afford
5b in only 20 min and quantitative yield. This transforma-
tion was also highly efficient in a 1/1 acetone/water
medium (96% yield). With 0.1% of water in acetone,
sodium fluoride showed comparable results with a 96%
yield in 15 min. In anhydrous DCM, silane 2 reacted with
sodium fluoride to generate silicate 6 in 96% yield. In the
presence of water (wet DCM or acetone/water 1/1), fast
protodesilylation occurred in <15 min and biphenyl 7,
presumably via the formation of 6, was obtained in 97%
and 92% yield respectively.14,15 When TBAF was used in a
protic solvent (ethanol), the quantitative formation of 7
was observed. This protodesilylation pathway was also
followed when silane 3 was reacted with TBAF in wet
Herein, we report the use of new silicon-based fluoride
sensors that, upon capture of Fꢀ, undergo structural
alteration which provides useful spectroscopic signatures
(Figure 1). For this study, we selected silanes 1ꢀ3 and also
examined the behavior of hypervalent species 4 for ligand
exchange. Bis(R,R-bis(trifluoromethyl)benzenemethanolato-
(ꢀ2)C2,O) silane 1 has been reported for the first time
by Martin in 1979 and is easily accessed from the reaction
of the dilithiated 1,1,1,3,3,3-hexafluoro-2-phenyl-2-propanol
with SiCl4.9 This stable 8-Si-4 spirosilane derivative has been
reported to be a versatile precursor of 10-Si-5 hypervalent
species subsequently to the addition of a nucleophile. This is
illustrated by the easy formation of 4 resulting from the
reaction of 1 with sodium hydroxide.10 They usually adopt a
trigonal bipyramidal (TBP) geometry with the formation of a
three-center four-electron hypervalent bond that is stabilized
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