The first example of the ortho-directing effect of the weakly coordinating substituent [-BF3]- in the catalytic hydrodefluorination of the pentafluorophenyltrifluoroborate anion

Article abstract of DOI:10.1016/j.mencom.2009.09.009

The pentafluorophenyltrifluoroborate anion undergoes hydrodefluorination under the action of zinc in the presence of nickel complexes turning into the 2,3,4,5-tetrafluoroborate anion.

Full text of DOI:10.1016/j.mencom.2009.09.009

Mendeleev
Communications
Mendeleev Commun., 2009, 19, 260–262
The first example of the ortho-directing effect of the
–
weakly coordinating substituent [–BF ] in the catalytic
3
hydrodefluorination of the pentafluorophenyltrifluoroborate anion
a
a
b
a
Nicolay Yu. Adonin,* Sergey A. Prikhod’ko, Vadim V. Bardin and Valentin N. Parmon
a
b
G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk,
Russian Federation. Fax: +7 383 330 8056; e-mail: adonin@catalysis.ru
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of
Sciences, 630090 Novosibirsk, Russian Federation
DOI: 10.1016/j.mencom.2009.09.009
The pentafluorophenyltrifluoroborate anion undergoes hydrodefluorination under the action of zinc in the presence of nickel
complexes turning into the 2,3,4,5-tetrafluoroborate anion.
The highly regioselective activation of aromatic carbon–fluorine
bonds by transition metal complexes is of theoretical and
Table 1 Hydrodefluorination of [C F BF ]– 1 in NMP.
6 5 3
1
Composition of the reaction
mixture (mol%)
practical interest. Polyfluorinated azobenzenes, C F N=NC X
6
5
6
5
LiCl
equiv.)
(
X = H, F), undergo ortho-metaldefluorination (fluorine substi-
Run Catalyst (mol%)
(
2
3
tution by metal atom) with Mn (CO) and CpRu(PPh ) Me.
1
2
3
2
10
3 2
‘
Ligand assistance’ of Schiff bases derived from C F CHO and
a
b
6
5
1
2
3
4
5
6
7
8
9
0
1
NiCl ·2bpy (10)
—
—
—
—
—
—
—
—
—
—
—
—
5
96
95
92
99
83
76
75
78
74
80
62
100
52
40
20
95
—
4
5
8
—
—
—
—
—
—
—
—
—
—
—
—
8
2
some 1,2-diamines occurs in reactions of oxidative addition of
W, Mo, Ni and Pt complexes. Similar effects were observed
for other fluorinated aromatic compounds forming complexes
with palladium, platinum and iridium. Intramolecular dehydro-
fluorinative coupling takes place in the case of Rh and Ir.9
Pentafluoropyridine and 2,3,5,6-tetrafluoropyridine react with
Ni(COD) and PEt in hexane to give predominantly [(2-poly-
fluoropyridyl)Ni(PEt ) F] complexes. Selective ortho-hydro-
defluorination occurs in C F COOH after treatment with
Yb(C F )
in a catalytic way.
Earlier, we reported the highly regioselective catalytic activa-
tion of aromatic C–F bonds in derivatives of pentafluorobenzoic
acid C F COX (X = OH, NH , OEt) and pentafluoroaniline
C F NHY (Y = COMe), proceeding in the presence of nickel
complexes under reductive conditions. In this work, we observed
that pentafluorophenyltrifluoroborate anion [C F BF ]
introduced in the reaction as a potassium salt also reacts with
zinc in the presence of NiCl ·nbpy under the described condi-
tions.1
NiCl ·2bpy (10)
2
4
5
NiCl ·2bpy (10)
2
c
NiCl ·bpy (10)
2
Traces
17
24
25
22
26
20
32
—
44
32
57
—
22
c
NiCl ·3bpy (10)
6
7
8
2
NiCl ·3bpy (20)
2
NiCl ·3bpy (5)d
2
e
f
NiCl2·3bpy (10)
NiCl ·3bpy (10)
2
2
3
g
1
1
NiCl ·3bpy (50)
10
2
3
2
g
NiCl ·3bpy (100)
2
6
5
12
13
—
1
1
12
and YbCp (dme). In the last case, it proceeds
NiCl ·3bpy (10)
NiCl ·3bpy (10)
NiCl ·3bpy (10)
6
5 2
2
2
14
15
16
17
10
15
15
5
24
23
—
8
2
2
h
—
i
NiCl ·3bpy (50)
2
1
3
6
5
2
a
b
c
1
4
DMF was used as solvent. DMA was used as solvent. Duration of 6 h.
Zn dust (10 equiv.) was added in 5 portions. Zn dust (20 equiv.) was added
in one portion. Zn dust (20 equiv.) was added in 5 portions; Zn dust
6
5
d
e
f
g
–
h
1
(10 equiv.) was added in one portion. C F H (5 mol%) was detected.
6
5
3
6 5
i
Additionally, 1,2,3-C F H (33 mol%) and 1,2-C F H (37 mol%) were
6
3
3
6
2
4
detected.
2
3,14
The expected reaction route is the formation of an
–
transition metal atom. At the same time, the [–BF ] group of
3
–
isomeric mixture of [C F HBF ] anions. Surprisingly, the
actual reaction product is a salt of 2,3,4,5-tetrafluorophenyl-
trifluoroborate anion, [2,3,4,5-C HF BF ] 2. The isomers
–
–
6
4
3
[C F BF ] 1, by analogy with structurally close anion [BF ] ,
6
5
3
4
15
belongs to weakly coordinating anions.
The catalyst was prepared in situ from nickel chloride and a
necessary amount of 2,2'-bipyridine. The reaction was carried
–
6
4
3
–
–
[2,3,5,6-C HF BF ] and [2,3,4,6-C HF BF ] , as well as the less
6 4 3 6 4 3
fluorinated phenyltrifluoroborates were not detected (Scheme 1).
The observed fact is contradictory to the earlier published
examples of regioselective activation of C–F bonds, since the
directing effect of the functional groups is determined by
the presence of the strong donor centre able to coordinate the
†
19
out at 80 °C. The reaction mixtures were analyzed by F NMR
†
General procedure of the hydrodeboration. A flask supplied with a
magnetic stirrer bar and heating bath was charged with NiCl ·6H O
10 mol%: 6 mg, 0.025 mmol), the respective amounts of 2,2'-bipyridine
2
2
(
(
NiCl ·bpy: 3.9 mg, 0.025 mmol; NiCl ·2bpy: 7.8 mg, 0.050 mmol;
2
2
BF3
BF3
NiCl ·3bpy: 11.7 mg, 0.075 mmol), 1 ml of the solvent (DMF, DMA or
2
F
F
F
F
F
F
F
F
NMP) and 0.2 ml of water. The reaction mixture was stirred at 80 °C for
NiCl2·nbpy
1
h, then 163.5 mg (2.5 mmol) of zinc dust were added in one portion.
Zn, solvent–H2O
80 °C
After stirring for 10 min, 69 mg (0.25 mmol) of K[C F BF ] were added.
F
F
6
5
3
The resulting reaction mixture was stirred at 80 °C for 8 h and cooled
down to ambient temperature. A quantitative internal standard (C H CF )
F
F
F
6
5
3
1
2
3
19
was added, and the mixture was analyzed by F NMR spectroscopy
(282.4 MHz).
Scheme 1
©
2009 Mendeleev Communications. All rights reserved.
– 260 –

Mendeleev Commun., 2009, 19, 260–262
spectroscopy (Table 1). Conversion of 4–8% of anion 1 was
achieved in the presence of NiCl ·2bpy for 8 h and anion 2
2
F
F
Ni(0)
was the only product of the reaction (Table 1, runs 1–3). The
–
Zn, NMP–H2O
80 °C
anion [2,3,4,5-C HF BF ] 2 possesses five signals in the
6
4
3
F
F
F
1
9
F NMR spectrum attributed to four aromatic fluorine and
–BF ] group whose chemical shifts and multiplicities agree
F
F
–
‡
[
3
3
5
with those for an authentic sample and earlier described salt
1
6
Scheme 3
K[2,3,4,5-C HF BF ].
6
4
3
The course of the hydrodefluorination depends on the nature
of the solvent used as the reaction medium. Among a series of
aprotic dipolar solvents, such as DMF, DMA and NMP, the
most appropriate is the latter (Table 1, run 3). The nature of
the catalytic complex is another factor, which determines the
conversion of starting compound 1 in analogy to pentafluoro-
acetanilide.14 For instance, no transformations of 1 were detected
the hydrodeboration product of pentafluorophenylboronic acid
(Table 1, run 16).18 The increase in the LiCl and catalyst
concentration leads to consumption of the initial compound.
¶
However, significant amounts of 1,2,3-trifluorobenzene 5 were
–
observed along with [2,3,4,5-C6HF4BF3] 2 and 1,2,3,4-tetra-
fluorobenzene 3 (Table 1, runs 14, 15 and 17). There are two
main pathways for the formation of fluorobenzene 5. Compound
5 can be a product of tetrafluorobenzene 3 hydrodeboration
(Scheme 3).19 The other way includes a stepwise transforma-
in the presence of NiCl ·1bpy (10 mol%) and Zn (in excess)
2
(
Table 1, run 4). At the same time, the increase in a number of
the ligands up to 3 relatively to nickel leads to a remarkable rise
of the catalytic activity of the nickel complex (Table 1, run 5).
Nevertheless, the maximum conversion of 24% was achieved
only when 20 mol% of the catalyst was applied (Table 1, run 6).
All efforts to raise the reactivity of 1 by means of the increase
of the amount of zinc or change of the order of its addition
–
tion of anion 2 into [3,4,5-C6H2F3BF3] 6 followed by its trans-
formation into 3,4,5-trifluorophenylboronic acid 7 and hydro-
deboration of the latter (Scheme 2).
Thus, we observed the first example of ortho-directing effect
–
of a weakly coordinating substituent [–BF3] in the transition
–
allowed to rise the conversion of [C F BF ] only up to 25%
metal catalysed hydrodefluorination of the polyfluoroaryltrifluoro-
borate anion. We can assume that the most plausible reason of
the ortho-directing effect is the interaction of the Ni atom with
the negatively charged fluorine atom bonded to boron.
6
5
3
(
Table 1, runs 7–10). Surprisingly, the complete conversion of
anion 1 was not obtained even in the presence of 100 mol%
of nickel complex (Table 1, run 11). In the absence of a
–
catalyst, [C F BF ] does not react with Zn in aqueous NMP
6
5
3
(
Table 1, run 12).
During investigation of impact of different additives on the
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1
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BF
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F
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Ni(0)
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X = F
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2 X = F
6 X = H
B(OH)
2
X
F
X
F
8
9
LiCl
NMP–H2O
0 °C
1
995, 233, 103.
NMP–H2O
80 °C
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Scheme 2
‡
Structure of anion [C F BF ] was proved by 19F NMR spectroscopy
–
6
5
3
3
5
(
1
NMP) d: –135.59 (m, 2-F, J 20 Hz, J 15 Hz), –139.36 (q, BF ,
JBF 45 Hz), –143.31 (m, 5-F, J 19.7 Hz), –160.05 (m, 3-F, J 20 Hz,
3J 20 Hz), –162.69 (m, 4-F, J 19 Hz, J 9 Hz) [lit., (CD CN) d:
135.25 (m, 2-F, J 22 Hz, J 9 Hz), –139.16 (m, BF ), –143.01 (m,
FF FF 3
-F, JFF 19 Hz), –159.72 (m, 3-F, JFF 19 Hz, JFF 22 Hz), –162.04
FF FF 3
2
002, 21, 5726.
3
3
FF
FF
1
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4
16
FF
FF
FF
3
3
4
–
5
3
3
3
3
(
m, 4-F, J 19 Hz)].
FF
§
19
¶
19
1
,2,3,4-Tetrafluorobenzene: F NMR (NMP) d: –139.93 (m, 2F, 1,4-F,
1,2,3-Trifluorobenzene: F NMR (NMP) d: –135.85 (m, 2F, 1,3-F,
3
JFF 18 Hz, J 7 Hz), –157.07 (m, 2F, 2,3-F, 3JFF 18 Hz, J 19 Hz,
3
3
3JFF 21 Hz, JHF 8 Hz), –162.45 (m, 1F, 2-F, JFF 21 Hz, JHF 6 Hz)
3
3
4
HF
20
FF
4J 5 Hz) [lit., d: –139.8 (m, 2F; 1,4-F), –156.8 (m, 2F, 2,3-F)].
[lit., d: –136.2 (m, 2F, 1,3-F), –163.0 (m, 1F, 2-F)].
20
HF
–
261 –

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Received: 20th March 2009; Com. 09/3302
–
262 –