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1524
V. V. Bardin, N. Y. Adonin
The catalytic properties of polyfuorinated aryldihaloboranes
were not studied.
Scheme 1
There are two practically available routes to ArFBX2
(ArF is polyfuoroaryl moiety). The frst is a reaction of
C6F5HgAlk (Alk = CH3, C2H5) with BCl3 without sol-
vent [6, 7] or with BBr3 in CH2Cl2 [8]. The preparation
of C6F5BBr2 by long refuxing of C6F5HgBr and BBr3 in
toluene was claimed without description [9]. The second
route is presented by the formation of C6F5BCl2 from of
C6F5SnMe3 or (C6F5)2SnMe2 and boron trichloride (yields
96 and 74%, respectively) [4, 6], and C6F5BBr2 from BBr3
and (C6F5)2SnBu2 (yield 22%) [5]. The main disadvantage
of the “tin” method is the difcult isolation aryldihalobo-
ranes due to the close boiling points and the solubility of
the reaction by-product, alkyltin halide. This is complicated
by the high sensibility of both C6F5BX2 and AlknSnX4−n to
moisture. The “mercury” method is devoid of these disad-
vantages. Mercurials XHgAlk are solid which are poorly
soluble in non-polar organic solvents. They can be easily
separated from the solutions of formed polyfuoroaryldih-
aloborane and reused in the synthesis of C6F5HgAlk without
the environment pollution.
the formation of 1 and phenyldichloroborane. The same
result was obtained at −60 °C (Scheme 3).
The reaction of pentafuorophenyl(ethyl)mercury (5) with
boron trichloride (twofold excess) in CH2Cl2 at −60 °C for
6 h and subsequent warming to room temperature gives
aryldichloroborane 3 and EtHgCl in quantitative yields.
An addition of BCl3 in CH2Cl2 to 5 at 2–4 °C and stirring
at 22 °C also results in 3 and EtHgCl. Attempt to obtain
bis(pentafuorophenyl)chloroborane (6) using excess of 5
(22 °C, 72 h) led to the incomplete conversion of C6F5HgEt
to C6F5BCl2 and (C6F5)2BCl. The complete conversion
of 5 to 6 was achieved after 1 week, although target com-
pound was contaminated with the hydrolysis products such
as C6F5H, [(C6F5)2B]2O, and (C6F5)2BOH (11B, 19F NMR)
(Scheme 4).
Being interested in pentafuorophenyldihaloboranes as
perspective homogeneous catalysts, we studied reactions
of easily available pentafuorophenylmercurials C6F5HgR
(R=C6F5, C6H5, C2H5, Br, and Cl) with boron trichloride
and boron tribromide to develop a convenient way to pro-
duce pentafuorophenyldichloroborane and pentafuorophe-
nyldibromoborane in solution. To get an objective picture,
these reactions were performed in weakly polar solvents
(CH2Cl2, CH2ClCH2Cl) where arylmercurials (both sub-
strates and products) are soluble. However, organomercury
halides and mercury dihalides are low soluble in non-polar
solvents, and at the end of reaction they can be removed
from of the desired solution of aryldihaloboranes by dilution
with hexane or benzene and the subsequent centrifugation.
Reactions with boron tribromide
Pentafuorophenylmercury bromide (7) reacts with BBr3 (1
equivalent) in DCE at 22 °C very slowly and after 24 h its
conversion does not exceed 10–15%. Refux of the reaction
solution within 7 h leads to the precipitation of HgBr2, but
the complete conversion of 7–8 requires a longer period.
The use of C6F5HgCl instead of C6F5HgBr and heating in
sealed tube at the higher temperature results in a mixture of
C6F5BClnBr2−n (n=0–2) (11B, 19F NMR) that was confrmed
by hydrolysis of these boranes to pentafuorophenylboronic
acid (Scheme 5).
Results and discussion
Taking into account the low reactivity of 7 towards BBr3
the stepwise substitution of C6F5 groups in (C6F5)2Hg with
bromine was expected. Actually, the treatment of 2 with
BBr3 leads to the slow disappearance of the substrate and
formation of C6F5BBr2 and C6F5HgBr. The complete con-
version of 2 was achieved within 24 h. Using a more con-
centrated solution of 2 and excess of BBr3 has a small efect.
The desired borane 8 was obtained by heating of 2 with
tribromoborane in DCE within 3 h (Scheme 6).
Reactions with boron trichloride
Pentafuorophenylmercury chloride (1) does not react with
BCl3 being heated in sealed tube at 60–70 °C (Scheme 1).
No reaction between bis(pentafuorophenyl)mercury (2) and
BCl3 in CH2Cl2 was observed at 22 °C over a period of
24 h. At higher temperature (60–70 °C) pentafuorophenyl-
dichloroborane (3) and pentafuorophenylmercury chloride
are formed (Scheme 2).
When (C6F5)2Hg is combined with one equivalent of
BBr3, the formation of bis(pentafuorophenyl)bromoborane
(9) from intermediate 8 and 7 becomes possible. Unfor-
tunately, this reaction proceeds slowly even at 120 °C in
ampoule that points out the lower reactivity of C6F5BBr2
with respect to reactivity of BBr3 (Scheme 7).
There are two possibilities of C–Hg bond cleavage for
pentafuorophenyl(phenyl)mercury (4). Mixing 4 with excess
BCl3 in CH2Cl2 at 2–4 °C and subsequent warming the reac-
tion mixture to room temperature showed unambiguously
1 3