56
H.-J. Frohn, V.V. Bardin / Journal of Organometallic Chemistry 631 (2001) 54–58
C F CFꢀCFBF . Similar phenomena were observed for
can dissolve BF . We have observed BF after satura-
3 3
4
9
2
19
cis-BF CFꢀCFBF [6] and CF ꢀC(CF )BF [5]. Also
tion of difluoroborane solutions at −40 °C ( F), but
2
2
2
3
2
19
the introduction of fluorine atoms in the ortho-posi-
tions of aryldifluoroboranes C H F BF2 caused a
BF was not detected ( F) after degasing. Admixtures
3
of BF , which are still present, may be the additional
6
5−n
n
3
similar effect [1]. It is likely, that this effect arises from
an intramolecular fluorine–boron interaction.
source of HF during hydrolysis and responsible for the
−
conversion of compounds 1 and 2 into [RCFꢀCFBF ]
3
−
Little information about the reactivity of fluoro-con-
taining alkenyldifluoroboranes was known. It was re-
ported that the interaction of borane 1 with water at
room temperature resulted neither in carbon–boron
nor in fluorine–boron bond cleavage but the products
were not characterised. Alternatively, the quantitative
protodeborylation of difluoroborane 1 took place at
and the basis of [BF4] in the concurrent absence of
RCFꢀCFH.
However, in anhydrous HF (aHF) the difluorobo-
ranes 1 and 2 underwent protodeborylation and the
corresponding fluoro-containing alkenes RCFꢀCFH
were formed. After shaking the CF ꢀCFBF solution in
2
2
CH Cl with aHF at −40 °C, the acidic phase con-
2
2
1
20 °C (15 h) yielding trifluoroethylene [4]. The hydro-
tained both CF ꢀCFH and difluoroborane 1. It is note-
2
19
lysis of compound CF ꢀC(CF )BF resulted in the
worthy to mention that the F-NMR spectrum of the
2
3
2
polyfluorinated olefin CF ꢀCHCF3 (conditions were
latter did not differ from that of CF ꢀCFBF in
2
2
2
not reported) [5].
CH Cl . This fact verifies that no significant complex
2 2
We have investigated the reactivity of some di-
fluoroboranes towards water and ether and we have
found that the treatment of difluoroboranes 1 and 3
with ether in CH Cl gave as expected the etherates 8
formation between trifluorovinyldifluoroborane and
aHF takes place. A similar picture was observed during
the reaction of difluoroborane 2 with aHF.
2
2
RCF=CFBF +aHFRCF=CFH+BF3
2
and 9 in quantitative yield.
R=F; cis-, trans-Cl.
2 2
CH Cl
RCF=CFBF +OEt ꢁꢁꢁꢁꢁꢁꢁꢁꢂRCF=CFBF ·OEt
Alternatively, the addition of aHF to a solution of
trans-C F CFꢀCFBF in CH Cl at −20 °C followed
2
2
2
2
1
or 3
−40°C to r.t.
4
9
2
2
2
R=F (8); cis-C F (9).
2
5
by warming up to room temperature did not result in
19
the protodeborylation. The F-NMR spectrum of the
The coordination of diethyl ether (neutral n-electron
donor) at the boron atom leads to a dramatic redistri-
bution of the charge in the molecule which is reflected
acidic phase showed resonances at −79.53 (3 F-6),
−
115.83 (2 F-3), −122.83, −124.50 (2 F-4 and 2
F-5), −159.83 (F-1) and −169.17 (F-2) ppm [J(1,2)
32 Hz], whereas the resonance of the fluorine atoms
1
9
in the F-NMR spectra. The spectrum of etherate 8
consists of four resonances at −94.32 (F-2trans),
1
bonded to boron was not observed. The organic phase
contained only traces of HF and the above-mentioned
signals. This spectrum coincides with the spectrum ob-
−
118.82 (F-2cis), −199.71 (F-1) and −149.04
(
BF ·OEt ) ppm [J, Hz: (1,2cis) 114, (1,2trans) 17,
2
2
(
2cis,2trans) 73, (2cis, BF ·OEt ) 14 and (F,B) 46]. The
2 2
1
9
tained from K[trans-C F CFꢀCFBF ] in aHF and dif-
F-NMR spectrum of compound 9 displays signals at
84.85 (3 F-4), −119.77 (2 F-3), −147.79
BF ·OEt ), −137.78 and −146.49 (F-1 and F-2trans)
4
9
3
fers from that of the salt in MeCN [resonances at
−
−
−
80.41, −115.95, −124.12, −125.60, −153.06 and
(
2
2
−
177.16 ppm, respectively, and at −144.03 (BF )
ppm. A comparison of the spectra of the adducts 8 and
with those of the starting difluoroboranes 1 and 3
Table 2) and those of the corresponding salts
3
ppm; J(1,2) 130 Hz] [3]. Presumably, in aHF the forma-
tion of a complex between difluoroborane 5 (Lewis
acid) and HF (F-terminus acts as Lewis base) occurred.
9
(
K[RCFꢀCFBF ] [3], demonstrates the dramatic changes
3
19
In fact, the observed F-NMR spectrum can present
an equilibrium between the neutral difluoroborane 5,
HF and the protonated perfluoro-trans-hexen-1-
yltrifluoroborate.
when the BF2 group [1] with the strongest s- and
p-electron accepting effect is converted to the neutral
BF ·OEt group with a tetra coordinated boron atom
2
2
−
or to the negatively charged BF3 group with the
strongest s-electron donating and negligible mesomeric
effect [1].
trans-C F CFꢀCFBF +3HF
4
9
2
[
H F][trans-C F CFꢀCFBF (F···HF)]
2
4
9
2
When CH Cl solutions of RCFꢀCFBF were reacted
2
2
2
with water at 0 °C (R=F) or at ] −78 °C (R=
Taking into account the complex formation of 5 in
cis,trans-Cl),
the
formation
of
the
anions
aHF and the absence of a related complex of
-
−
[
RCFꢀCFBF ] [3] and [BF ] was detected in the
CF ꢀCFBF2 in aHF, the protodeborylation of di-
3
4
2
1
9
19
aqueous phase ( F-NMR). No F-NMR signals which
fluoroboranes 1 and 2 can be described in terms of the
direct attack of the proton on the carbon atom C-1
bonded to boron.
could be attributed to the alkenes RCFꢀCFH or the
boronic acids RCFꢀCFB(OH) were observed neither
2
in the aqueous nor in the organic phase. It is known
It should be noted that fluoroalkenes RCFꢀCFH
(R=F, cis-, trans-Cl) are significantly soluble in aHF
from literature [4] that difluoroboranes RCFꢀCFBF2