C-H and C-F Bond Activations at a Rhodium(I) Boryl Complex: Reaction Steps for the Catalytic Borylation of Fluorinated Aromatics

Article abstract of DOI:10.1021/om500952x

Treatment of the rhodium(I) boryl complex [Rh(Bpin)(PEt₃)₃] (1, pin = pinacolato = O₂C₂Me₄) with pentafluorobenzene, 1,3,5-trifluorobenzene, 1,3-difluorobenzene, or 3,5-difluoropyridine led to C-H activation reactions to give the aryl complexes [Rh(C₆F₅)(PEt₃)₃] (4), [Rh(2,4,6-C₆F₃H₂)(PEt₃)₃] (5), [Rh(2,6-C₆F₂H₃)(PEt₃)₃] (6), and [Rh{4-(3,5-C₅NF₂H₂)}(PEt₃)₃] (8). For 5, 6, and 8 consecutive reactions with in situ generated HBpin occurred to yield [Rh(H)(PEt₃)₃] (7) and boronic esters. The boryl complex 1 gave with hexafluorobenzene or perfluorotoluene the C-F activation products [Rh(C₆F₅)(PEt₃)₃] (4) and [Rh(4-C₆F₄CF₃)(PEt₃)₃] (9), respectively. The complexes 5, 6, and 9 react with B₂pin₂ to yield 1 and boronic ester derivatives. On the basis of these stoichiometric reactions catalytic C-H and C-F borylation reactions using 1 or 7 were developed to generate 2-Bpin-1,3,5-C₆F₃H₂, 2-Bpin-1,3-C₆F₂H₃, and 4-Bpin-C₆F₄CF₃ from 1,3,5-trifluorobenzene, 1,3-difluorobenzene, or perfluorotoluene and B₂pin₂. On treatment of pentafluoropyridine with B₂pin₂ in the presence of 1 or 7 as catalyst 2-Bpin-C₅NF₄ was synthesized by C-F borylation at the 2-position. Using 2,3,5,6-tetrafluoropyridine, B₂pin₂, and catalytic amounts of 7 led to a C-H borylation reaction at the 4-position. 4-Bpin-C₅NF₄ can also be prepared by the reaction of 2,3,5,6-tetrafluoropyridine with stoichiometric amounts of HBpin or by the reaction of pentafluoropyridine with an excess of HBpin in the presence of 7, whereas the reaction of pentafluoropyridine with stoichiometric amounts of HBpin and 5 mol % 7 resulted in the formation of 2,3,5,6-tetrafluoropyridine via hydrodefluorination reaction at the 4-position. This regioselectivity contrasts the borylation of pentafluoropyridine at the 2-position with 1 as catalyst. Overall, the obtained fluorinated aryl boronic ester derivatives might serve as versatile building blocks. (Chemical Equation Presented).

Full text of DOI:10.1021/om500952x

Article
pubs.acs.org/Organometallics
C−H and C−F Bond Activations at a Rhodium(I) Boryl Complex:
Reaction Steps for the Catalytic Borylation of Fluorinated Aromatics
Sabrina I. Kallane, Michael Teltewskoi, Thomas Braun,* and Beatrice Braun
̈
Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: Treatment of the rhodium(I) boryl complex
[Rh(Bpin)(PEt3)3] (1, pin = pinacolato = O2C2Me4) with
pentafluorobenzene, 1,3,5-trifluorobenzene, 1,3-difluoroben-
zene, or 3,5-difluoropyridine led to C−H activation reactions
to give the aryl complexes [Rh(C6F5)(PEt3)3] (4), [Rh(2,4,6-
C6F3H2)(PEt3)3] (5), [Rh(2,6-C6F2H3)(PEt3)3] (6), and
[Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8). For 5, 6, and 8
consecutive reactions with in situ generated HBpin occurred
to yield [Rh(H)(PEt3)3] (7) and boronic esters. The boryl
complex 1 gave with hexafluorobenzene or perfluorotoluene
the C−F activation products [Rh(C6F5)(PEt3)3] (4) and
[Rh(4-C6F4CF3)(PEt3)3] (9), respectively. The complexes 5,
6, and 9 react with B2pin2 to yield 1 and boronic ester
derivatives. On the basis of these stoichiometric reactions catalytic C−H and C−F borylation reactions using 1 or 7 were
developed to generate 2-Bpin-1,3,5-C6F3H2, 2-Bpin-1,3-C6F2H3, and 4-Bpin-C6F4CF3 from 1,3,5-trifluorobenzene, 1,3-
difluorobenzene, or perfluorotoluene and B2pin2. On treatment of pentafluoropyridine with B2pin2 in the presence of 1 or 7
as catalyst 2-Bpin-C5NF4 was synthesized by C−F borylation at the 2-position. Using 2,3,5,6-tetrafluoropyridine, B2pin2, and
catalytic amounts of 7 led to a C−H borylation reaction at the 4-position. 4-Bpin-C5NF4 can also be prepared by the reaction of
2,3,5,6-tetrafluoropyridine with stoichiometric amounts of HBpin or by the reaction of pentafluoropyridine with an excess of
HBpin in the presence of 7, whereas the reaction of pentafluoropyridine with stoichiometric amounts of HBpin and 5 mol % 7
resulted in the formation of 2,3,5,6-tetrafluoropyridine via hydrodefluorination reaction at the 4-position. This regioselectivity
contrasts the borylation of pentafluoropyridine at the 2-position with 1 as catalyst. Overall, the obtained fluorinated aryl boronic
ester derivatives might serve as versatile building blocks.
pentafluoropyridine at [Rh(SiPh3)(PMe3)3] gave the C−F
activation products [Rh(2-C5NF4)(PMe3)3] and [Rh(4-
C5NF4)(PMe3)3]. Subsequent treatment with B2cat2 (cat =
catecholato = O2C6H4) resulted in the formation of pyridyl
boronic ester derivatives. A catalytic C−F borylation using the
highly reactive rhodium boryl complex [Rh(Bpin)(PEt3)3] (1)8
as catalyst was also reported (Scheme 1). The latter activates
selectively the C−F bond at the 2-position of pentafluoro-
pyridine to yield [Rh(2-C5NF4)(PEt3)3] (2). DFT calculations
showed that the C−F bond cleavage step proceeds via a four-
centered boryl-assisted transition state in which C−F bond
cleavage occurs over the Rh−B bond.8a Catalytic reactions of
pentafluoropyridine with B2pin2 (pin = pinacolato = O2C2Me4)
and 1 as catalyst give access to the 2,3,4,5-tetrafluoropyridyl
boronic ester (Scheme 1). In contrast, in [Rh(H)(PEt3)3] (7)
the C−F bond cleavage at pentafluoropyridine proceeded at the
4-position to give [Rh(4-C5NF4)(PEt3)3] (3).9 Treatment of
pentafluoropyridine with the silyl complex [Rh{Si(OEt)3}-
(PEt3)3]10 or [Rh(SiPh3)(PMe3)3]7 at room temperature
INTRODUCTION
■
Fluorinated organic compounds and building blocks play a key
role in many applications because of their exceptional
properties. They are employed, for example, in agrochemicals,
pharmaceuticals and radiotracers, polymers, optoelectrics, and
refrigerants.1 The transition-metal-mediated activation of a C−
F2 or a C−H bond is a promising strategy to convert
commercially available highly fluorinated aromatics into useful
starting materials.2n Thermodynamically, the breaking of a C−F
bond is often hampered by the exceptionally high bond
dissociation energy,3 but C−F activation reactions are often
feasible given that a thermodynamically more stable bond is
generated, such as H−F, Si−F, or B−F. However, examples of
aromatic C−F bond functionalizations, i.e., fluorine atom
replacement by a new group to access higher value fluorinated
compounds, are limited.4 In most of these functionalization
reactions a hydrodefluorination reaction takes place.2m,n,5
The replacement of a C−F bond by a boryl group as a viable
entity is particularly beneficial for further conversions. For
transition-metal-mediated transformations, intermediate boryl
complexes6 might play a fundamental role. Borylation of a C−F
bond was reported by Marder and Perutz et al.7 The reaction of
Received: September 18, 2014
© XXXX American Chemical Society
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Scheme 1. C−F Activation of Pentafluoropyridine and C−H Activation of 2,3,4,5-Tetrafluoropyridine at 1 as Well as Catalytic
C−F Borylation of Pentafluoropyridine8a
Scheme 2. C−H Activation of Partially Fluorinated Aromatics at 1
yielded a product mixture, whereas [Rh{Si(OMe)3}(PEt3)3]
The catalytic borylation via C−H activation11 also represents
an efficient, convenient, and economical way for the synthesis
furnished only 2.10
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Scheme 3. Reactivity of 7 toward Fluorinated Aromatics
Figure 1. ORTEP diagrams of complexes 5, 6, 8, and 9. The ellipsoids are drawn at the 50% probability level. The hydrogen atoms are omitted for
clarity. (a) ORTEP diagram of complex 5. Selected distances [Å] and angles [deg]: Rh1−C19 = 2.1000(18), Rh1−P2 = 2.2818(5), Rh1−P1 =
2.2971(4), Rh1−P3 = 2.3115(4), P2−Rh1−C19 = 164.96(5), P1−Rh1−P3 = 162.704(17), dihedral angle P1−P2−P3−C19−Rh1 plane versus aryl
plane = 86.33(7). (b) ORTEP diagram of complex 6. Selected distances [Å] and angles [deg]: Rh1−C19 = 2.0904(11), Rh1−P2 = 2.2861(3), Rh1−
P1 = 2.2961(3), Rh1−P3 = 2.3096(3), P2−Rh1−C19 = 164.10(3), P1−Rh1−P3 = 161.745(11), dihedral angle P1−P2−P3−C19−Rh1 plane versus
aryl plane = 84.39(5). (c) ORTEP diagram of complex 8. The asymmetric unit cell contains two crystallographically independent molecules, which
show only minor differences in the bond lengths and angles, but differ in the conformation of the PEt3 ligand. Only one of them is shown. Selected
distances [Å] and angles [deg]: Rh1−C7 = 2.062(3), Rh1−P2 = 2.2995(8), Rh1−P1 = 2.3161(8), Rh1−P3 = 2.2993(8), P2−Rh1−C7 = 163.32(9),
P1−Rh1−P3 = 161.76(3), dihedral angle P1−P2−P3−C7−Rh1 plane versus aryl plane = 88.78(14). (d) ORTEP diagram of complex 9. Selected
distances [Å] and angles [deg]: Rh1−19 = 2.0773(13), Rh1−P1 = 2.2874(3), Rh1−P2 = 2.3045(4), Rh1−P3 = 2.3331(4), P1−Rh1−C19 =
174.67(4), P2−Rh1−P3 = 168.399(13), dihedral angle P1−P2−P3−C19−Rh1 plane versus aryl plane = 89.09(6).
of organoboronic acid derivatives,12 and the conversion of C−
H into C−B bonds is thermodynamically favorable.13 However,
the presence of C−H and C−F bonds in the substrate
generates a competition between C−H and C−F activa-
tion.2w,5a,b,7,14 Johnson et al. reported that with {Ni(PEt3)2} a
kinetically controlled C−H activation of aromatic compounds
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can occur reversibly, whereas a subsequent C−F oxidative
addition leads to the thermodynamic products.15 With partially
fluorinated pyridines only the C−F oxidative addition products
have been found.14a,15c For rhodium, there is a preference for
C−H activation.16 For example, the cyclopentadienyl and
tris(3,5-dimethylpyrazolyl)borato rhodium complexes give with
fluorinated benzenes the C−H activation products.17 However,
for conversions that involve an initial nucleophilic attack of a
rhodium center at highly fluorinated substrates C−F bond
cleavage reactions were found.18 The reaction of the hydrido
complex [Rh(H)(PEt3)3] (7) with 2,3,5,6-tetrafluoropyridine
resulted in the formation of the C−H activation product 3.
However, with 2,3,4,5-tetrafluoropyridine a product mixture
was obtained.9 The reaction of the boryl complex [Rh(Bpin)-
(PEt3)3] (1) with 2,3,5,6-tetrafluoropyridine proceeds with a
preference for C−H over C−F bond activation to yield [Rh(4-
C5NF4)(PEt3)3] (3, Scheme 1).8a Note that the capability of
the boryl complex 1 to undergo a C−H bond activation was
also demonstrated by a stoichiometric reaction with benze-
ne.8a,d
Although the generation of 7 occurred before the boryl
complex 1 was completely converted, complexes 5 and 6 were
isolated successfully by crystallization at low temperature from
the reaction mixtures in n-hexane. Both compounds were
characterized by NMR spectroscopy and elemental analysis.
The molecular structures of 5 and 6 were confirmed by single-
crystal X-ray analysis (Figure 1a and b, Table S1) as well. The
31P{1H} spectra show a resonance for the phosphorus atom in
the trans position to the fluorinated ligand at δ 19.6 ppm with a
rhodium−phosphorus coupling of 132 Hz for 5 and at δ 18.9
ppm with a coupling of 129 Hz for 6. The fluorine−phosphorus
4
couplings of JF,P = 13 Hz for both compounds are in the
expected range.9 The phosphine ligands in a mutually trans
arrangement give a doublet of doublets at δ 15.0 ppm for 5 and
at δ 14.6 ppm for 6 with similar coupling constants of 145 Hz
to the rhodium atom and of 39 Hz to the other phosphorus
atom. The magnitude of the rhodium−phosphorus and
phosphorus−phosphorus coupling constants is comparable to
those for other square planar fluorinated (hetero)aryl rhodium-
(I) phosphine complexes.8a,9,10,21 The 19F NMR spectrum of 5
displays two multiplets at δ −77.2 and −121.1 ppm, which
integrate to the ratio of 2:1. For the fluorine atoms of complex
6, a resonance at δ −79.3 ppm was detected in the 19F NMR
spectrum, which exhibits two phosphorus−fluorine and a
rhodium−fluorine coupling. The chemical shifts are in
agreement with other complexes bearing a 2,4,6-trifluoro- or
2,6-difluorophenyl ligand.17a
In accordance with the results described above, treatment of
1 with stoichiometric amounts of 3,5-difluoropyridine led to
C−H activation at the 4-position, and [Rh{4-(3,5-C5NF2H2)}-
(PEt3)3] (8) and [Rh(H)(PEt3)3] (7) were formed in a ratio of
1:0.16 as main products (Scheme 2). In addition, minor
amounts of 3% and 4% mer-[Rh(H)3(PEt3)3] and cis-fac-
[Rh(H)2(Bpin)(PEt3)3] were observed by 31P{1H} NMR
spectroscopy (see below). Furthermore, HBpin and 4-Bpin-
3,5-C5NF2H2 were detected by GC-MS analysis. The reaction
of 1 with 3,5-difluoropyridine is significantly faster than those
with the fluorinated benzene derivatives despite the use of
stoichiometric amounts of pyridine, which results in complete
conversion within 3 h.
In this contribution we report on C−H and C−F bond
activation reactions of fluorinated aromatics at 1 or 7.
Challenging issues concern the chemoselectivity as well as the
regioselectivity of the reactions. The development of
unprecedented catalytic borylation processes using B2pin2 or
HBpin as a source of the boryl group is also described.
RESULTS AND DISCUSSION
■
1. C−H Activation of Partially Fluorinated Aromatics.
Treatment of the rhodium(I) boryl complex [Rh(Bpin)-
(PEt3)3] (1) with an excess of pentafluorobenzene led after
15 min at room temperature to the quantitative generation of
the C−H activation product [Rh(C6F5)(PEt3)3] (4) and HBpin
(Scheme 2). The latter can be removed under vacuum to
obtain 4 in pure form. The pentafluorophenyl complex 4 was
identified by comparison of its NMR data with the literature.10
In accordance with the above-mentioned reaction of 1 with
2,3,5,6-tetrafluoropyridine, which yields the C−H activation
product [Rh(4-C5NF4)(PEt3)3] (3)8a and HBpin (Scheme 1),
the C−H bond activation is favored over the C−F bond
activation at the boryl complex. Note that Milstein et al. found
that [Rh(SiMe2Ph)(PMe3)3] reacts in pentafluorobenzene at
higher temperature to give 1,2,4,5-tetrafluorobenzene and
[Rh(C6F5)(PMe3)3].5a,b
Complex 8 was isolated after crystallization in n-hexane at
−30 °C and was characterized by NMR spectroscopy,
elemental analysis, and single-crystal X-ray analysis (Figure
1c). The 31P{1H} spectrum of 8 displays two signals at δ 17.1
and 11.8 ppm in a ratio of 1:2. The resonance for the
phosphorus atom in the position trans to the fluorinated ligand
at δ 17.1 ppm exhibits coupling to rhodium, two phosphorus
atoms, and two ortho fluorine atoms. The signal at δ 11.8 ppm,
which can be assigned to the phosphorus atoms that are in a
mutually trans position, shows a doublet of doublets splitting
pattern. The chemical shifts and the coupling constants are in
agreement with those found for the aryl complexes 3−6 and
confirm the oxidation state of Rh(I). A resonance at δ −94.1
ppm in the 19F NMR spectrum as well as the signals in the
Reaction of [Rh(Bpin)(PEt3)3] (1) with an excess of 1,3,5-
trifluorobenzene or 1,3-difluorobenzene led to the C−H
activation products [Rh(2,4,6-C6F3H2)(PEt3)3] (5) and [Rh-
(2,6-C6F2H3)(PEt3)3] (6), respectively, as well as HBpin
(Scheme 2). Subsequently, the aryl complexes reacted further
with HBpin to give the aryl boronate ester 2-Bpin-1,3,5-C6F3H2
or 2-Bpin-1,3-C6F2H3 and [Rh(H)(PEt3)3] (7),19 which can
undergo additional reactions (see below and Scheme 3). The
formation of HBpin and the aryl boronate esters20 was
confirmed by GC-MS analysis as well as by 11B and 19F
NMR spectroscopy. The conversion with 1,3,5-trifluoroben-
zene seems to be slightly faster: after 7 h 14% of 1 remained in
the reaction solution after treatment with 1,3,5-trifluoroben-
zene, whereas 27% of 1 was left after a reaction with 1,3-
difluorobenzene (after 1 d: ratio of 7:5 = 2.5:1; ratio of 7:6 =
2.2:1). The addition of PEt3 to a solution of 1 and the arene
inhibited the reaction, which indicates that the dissociation of
phosphine from the metal center plays a crucial role in the rate-
determining step.
1
aromatic region in the H and 13C NMR spectra indicate the
presence of the heteroaryl ligand.
All the reactions described above show that at 1 C−H bond
activation is preferred over C−F bond activation. Note that the
products of the C−H activation reactions of the fluorinated
substrates are aryl complexes, whereas, as reported in the
literature, the reaction of 1 with benzene or SCF3 arenes
resulted in the formation of the hydrido complex [Rh(H)-
(PEt3)3] (7) as well as of borylated aromatics. One reason
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might be the stabilizing effect of the fluorinated ligand.22 In
addition, the results demonstrate that the C−H activation
occurs regioselectively at the carbon atom adjacent to two
fluorine atoms in each case. In combined experimental and
computational studies it was demonstrated that for C−H
activation reactions of fluorinated substrates at rhodium the
products with the fluorine atoms at the ortho position to the
metal center are favored.2n,17a,d,e,23
The hypothesis that the aryl complexes [Rh(2,4,6-C6F3H2)-
(PEt3)3] (5), [Rh(2,6-C6F2H3)(PEt3)3] (6), and [Rh{4-(3,5-
C5NF2H2)}(PEt3)3] (8) react with HBpin in a consecutive
reaction to give [Rh(H)(PEt3)3] (7) and borylated fluoroar-
omatics is supported by independent reactions (Scheme 2).
Treatment of 5, 6, or also 8 with one equivalent of HBpin in
cyclohexane for 1 d generated 7 as the main product in varying
amounts. Mechanistically, the reactions of the aryl complex 5,
6, or 8 with HBpin might involve the generation of a
rhodium(III) intermediate, which eliminates the aryl boronic
ester due to the favored formation of a C−B bond. Reductive
elimination of a tolyl boronate ester after oxidative addition of
B2cat2 or HBcat was observed at cis-[Os(Bcat)(o-tolyl)-
(CO)2(PPh3)2].24 However, a concerted reaction pathway25
for the generation of 7 and the borylated product would also be
conceivable.
1,3-difluorobenzene, and 4% cis-fac-[Rh(H)2(Bpin)(PEt3)3] as
well as 3% mer-[Rh(H)3(PEt3)3] after reaction with 3,5-
difluoropyridine.
2. C−F Activation of Perfluorinated Aromatics. The
reported C−F activation capability of [Rh(Bpin)(PEt3)3] (1)
with pentafluoropyridine (Scheme 1) and perfluoropropene8a
encouraged us to probe its reactivity toward other perfluori-
nated aromatics. Treatment of the rhodium boryl complex 1
with an excess of hexafluorobenzene gave the complex
[Rh(C6F5)(PEt3)3] (4) and the fluoroborane FBpin via C−F
bond cleavage (Scheme 4). As described above, 4 is also
Scheme 4. C−F Activation of Perfluorinated Aromatics at 1
With regard to the generation of 7 and the formation of
borylated aromatics that are shown in Scheme 2, the reactivity
of the hydrido complex 7 toward partially fluorinated aromatics
was studied. Note that DFT calculations for the rhodium-
catalyzed benzylic borylation of toluenes on using [Rh(Cl)-
(PiPr3)2(N2)] as catalyst suggest that the C−H activation
reaction step also occurs at a hydrido species, {Rh(H)-
(PiPr3)2}.26
accessible from pentafluorobenzene. Note that Milstein et al.
reported the C−F activation of hexafluorobenzene at [Rh(H)-
(PMe3)3].5b Furthermore, rhodium silyl complexes undergo
C−F activation with hexafluorobenzene.5a,b,10
Treatment of 1 with stoichiometric amounts of perfluoro-
toluene yielded [Rh(4-C6F4CF3)(PEt3)3] (9) as well as FBpin
by C−F activation at the position para to the CF3 group
(Scheme 4). The same regioselectivity in the activation reaction
of perfluorotoluene was obtained at [Rh{Si(OR)3}(PEt3)3] (R
= Me, Et).10 Note that the activation of the C−F bond at the
para position of perfluorotoluene was also achieved at Ni or
other metals.4d,14c,27 Complex 9 was identified by comparison
of its NMR data with the literature data.10 In addition its
molecular structure in the solid state was determined by X-ray
diffraction analysis (Figure 1d).
Several mechanistic possibilities for C−F activation reactions
are discussed in the literature.2b,j,n For instance, the formation
of a rhodium(III) fluoro intermediate by oxidative addition of
perfluorinated substrates at 1 and subsequent reductive
elimination of FBpin is conceivable. However, concerted
reaction pathways that involve a C−F bond cleavage step
that is assisted by a boryl8a or phosphine ligand28 are also
possible.2n The regioselectivity of the activation of perfluoro-
toluene at the 4-position would in addition be in accordance
with other reaction mechanisms such as a nucleophilic reaction
pathway or an electron transfer step.2d,f,g,p
Remarkably, on treatment of 7 with 1,3,5-trifluorobenzene,
1,3-difluorobenzene, or 3,5-difluoropyridine, the formation of
the aryl complex 5, 6, or 8 was observed, which demonstrates
that 7 is also capable of undergoing aromatic C−H bond
activation reactions (Scheme 3). With 3,5-difluoropyridine the
C−H activation reaction at 7 appears to be significantly faster
than with the fluorinated benzene derivatives and resulted in a
mixture of 7, [Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8), and small
amounts of mer-[Rh(H)3(PEt3)3] and fac-[Rh(H)3(PEt3)3] in a
ratio of 1:1:0.06:0.01 after 2 h at room temperature. Treatment
of 7 with an excess of 1,3,5-trifluorobenzene gave a mixture of
7, [Rh(2,4,6-C6F3H2)(PEt3)3] (5), and small amounts of mer-
[Rh(H)3(PEt3)3] and fac-[Rh(H)3(PEt3)3] in a ratio of
3.4:1:0.3:0.05 after 1 d at room temperature, and with an
excess of 1,3-difluorobenzene the reaction mixture contained 7
and [Rh(2,6-C6F2H3)(PEt3)3] (6) in a ratio of 10:1 after 4 d at
50 °C. The results suggest that the reaction of 7 with 1,3-
difluorobenzene is much slower than with 1,3,5-trifluoroben-
zene, and such considerable difference in the reaction rates was
not found using 1.
The reactivity observed for 7 also explains the subsequent
reactions in the conversions of 5, 6, and 8 into 7 and borylated
aromatics, which are described above (Scheme 2). The
presence of the additional product cis-fac-[Rh(H)2(Bpin)-
(PEt3)3] as well as of mer-[Rh(H)3(PEt3)3] and fac-[Rh-
(H)3(PEt3)3] can be explained by reactions of 7 with HBpin or
H2 as reported before.5c,21 After completion of the reactions
and bringing the solution to dryness, the solids contained 4%
mer-[Rh(H)3(PEt3)3] as well as 1% fac-[Rh(H)3(PEt3)3] after
reaction with 1,3,5-trifluorobenzene, 6% cis-fac-[Rh(H)2(Bpin)-
(PEt3)3] as well as 1% mer-[Rh(H)3(PEt3)3] after reaction with
3. Development of Catalytic C−F and C−H Bond
Borylation Reactions. To develop a catalytic borylation
process, the aryl complexes [Rh(4-C5NF4)(PEt3)3] (3),
[Rh(2,4,6-C6F3H2)(PEt3)3] (5), [Rh(2,6-C6F2H3)(PEt3)3]
(6), [Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8), and [Rh(4-C6F4CF3)-
(PEt3)3] (9) were treated stoichiometrically with B2pin2
(Scheme 5). The phenyl complexes, 5, 6, and 9 were converted
to the boryl complex 1 and the borylated aromatics 2-Bpin-
1,3,5-C6F3H2, 2-Bpin-1,3-C6F2H3, and 4-Bpin-C6F4CF3, but the
reactions appear to be very slow at room temperature. For 5
and 6 a complete conversion was observed after 10 or 5 d,
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Scheme 5. Treatment of the Aryl Complexes 3, 5, 6, 8, and 9 with B2pin2
a
Scheme 6. Catalytic Borylation of Fluorinated Benzenes via C−H and C−F Bond Activation with B2pin2 on Using 1 or 7
a
TON based on borylation steps/mol catalyst.
respectively. For 9 a conversion of 50% was observed after 5 d.
Treatment of the pyridyl complexes 3 and 8 with B2pin2 led to
no reaction, not even at 50 °C. The stability and sometimes
weak reactivity of the pyridyl complexes in comparison to
phenyl counterparts are mainly caused by the strength of the
metal−carbon bond to the perfluorinated ligand.9,29 As
F
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Scheme 7. Possible Pathways for the Catalytic Formation of Aryl Boronic Esters via C−H Borylation at 1 or 7
mentioned above, Marder and Perutz et al.7 achieved a
borylation of the heteroaryl complexes at [Rh(Ar)(PMe3)3]
(Ar = 2-C5NF4, 4-C5NF4, 2-(4-Me-C5NF3), 6-(2,3,5-C5NF3H))
with B2cat2 yielding borylated pyridines as well as the trisboryl
complex fac-[Rh(Bpin)3(PMe3)3], which is presumably a
thermodynamic sink.
With regard to the C−H activation reactions at [Rh(H)-
(PEt3)3] (7) with di- and trifluorinated benzenes that are
described above, complex 7 was also tested as catalytic
precursor. Indeed, treatment of 1,3,5-trifluorobenzene or 1,3-
difluorobenzene with B2pin2 in the presence of 5 mol % 7 at 50
°C led to the formation of the aryl boronate esters 2-Bpin-1,3,5-
C6F3H2 and 2-Bpin-1,3-C6F2H3 in good, but slightly lower,
yields when compared to the reactions with 1 as catalyst
(Scheme 6).
On the basis of the stoichiometric reactions, we investigated
the catalytic borylation of partially fluorinated benzene
derivatives. In the presence of 3.5 mol % [Rh(Bpin)(PEt3)3]
(1), B2pin2 and 1,3,5-trifluorobenzene or 1,3-difluorobenzene
can be converted into the aryl boronate esters by C−H
activation (Scheme 6). The reactions were carried out at 50 °C
in cyclohexane with a 6-fold excess of the arene to give the
monoborylated benzene derivatives as main products in a yield
of formally 120% (2-Bpin-1,3,5-C6F3H2) or 98% (2-Bpin-1,3-
C6F2H3) after 24 h. The yields, which are based on the amount
of B2pin2, larger than 100% indicate that the generated HBpin
also acts as a borylation agent in the catalytic processes.
However, the formation of HBpin was detected by GC-MS
Mechanistically, we suggest that an active species in the
catalytic process on using 7, which is probably the boryl
complex 1, can be generated from fac-[Rh(H)(Bpin)2(PEt3)3]
(10). The latter compound might be furnished from 7 and
B2pin2. Alternatively, the catalysis can be started by the
generation of the aryl complex 5 or 6 from 7 by C−H
activation (Scheme 7, (d)). The aryl complex reacts with
diborane under release of the aryl boronic ester and the
formation of 1 (b), which undergoes C−H activation with
another substrate molecule to yield HBpin and the aryl
complex 5 or 6 (a). However, this species is capable of reacting
with B2pin2 (b) or with the generated HBpin (on a slower time
scale) to give 7 (c). For both reactions of 5 or 6 with B2pin2 or
HBpin the borylated arene would be generated.
On the basis of the stoichiometric C−F activation reactions,
the catalytic C−F borylation of perfluorotoluene was
developed. Treatment with B2pin2 in benzene at 55 °C for 2
d in the presence of 3.5 mol % [Rh(Bpin)(PEt3)3] (1) led to
the formation of 4-Bpin-C6F4CF3 in a yield of 57% (Scheme 6).
Only the monoborylated product was observed, and a
consecutive second borylation did not take place. The
borylation of pentafluoropyridine, which was reported before,8a
and the conversion of perfluorotoluene represent unique
examples for catalytic borylation of perfluorinated benzene
derivatives via C−F bond cleavage. The compound 4-Bpin-
C6F4CF3 was characterized by NMR spectroscopy and GC-MS
analysis. The 19F NMR spectrum exhibits three multiplets at δ
−57.3, −129.2, and −142.1 ppm, which integrate in a ratio of
3:2:2. For the boryl group, the 11B NMR spectrum shows a
resonance at δ 29 ppm, which is in a typical range for aryl
boronic esters.8a,20,35 Whereas the C−F borylation reaction of
perfluorotoluene with B2pin2 is selective, the conversion of
1
analysis and H NMR spectroscopy. We were not able to
observe the formation of H2. On using 1,3,5-trifluorobenzene,
the diborylated compound 2,4-(Bpin)2-1,3,5-C6F3H was found
in small amounts (8%), whereas the use of 1,3-difluorobenzene
also led to a variety of additional products. GC-MS data
indicate the presence of mono- and diborylated difluoroben-
zenes (approximately overall 37%, based on the amount of
B2pin2, according to a 19F NMR spectrum), which were not
further characterized. C−H bond borylation reactions to access
fluorinated aryl boronic are documented in the literature.12b,30
31
Note that 2-Bpin-1,3,5-C6F3H2 can also be synthesized by
iridium- or rhodium-catalyzed C−H borylations of 1,3,5-
trifluorobenzene with HBpin at 150 °C.30a,32 With 2 mol %
[(Cp*)Rh(η4-C6Me6)] and a 2-fold excess of arene a mixture of
C6F3H2(Bpin) (46%), C6F3H(Bpin)2 (7%), and C6F2H3(Bpin)
(6%) was obtained.30a The compound 2-Bpin-1,3-C6F2H3 was
generated as a minor product in the iridium-catalyzed
borylation of 1,3-difluorobenzene.20b,33 The major product is
the isomer 5-Bpin-1,3-C6F2H3, which suggests that the
regioselectivity for iridium-mediated C−H borylation reactions
is predominantly determined by steric factors.11,34
G
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hexafluorobenzene and B2pin2 in the presence of 1 gives only a
product mixture, which was not analyzed further.
with B2pin2 to furnish the boryl complex 1. Subsequent
reactions lead to a borylation of pentafluoropyridine at the 2-
position. Alternatively, there are indications that the initial
formation of fac-[Rh(H)(Bpin)2(PEt3)3] (10) from 7 and
B2pin2 is possible. The diborylhydrido complex 10 activates the
C−F bond at the 2-position of pentafluoropyridine to give
[Rh(2-C5NF4)(PEt3)3] (2), shown in an independent reaction.
This observation suggests that 10 can be converted into 1.8d
Although [Rh(4-C5NF4)(PEt3)3] (3) does not react with
B2pin2 in a stoichiometric reaction, we considered the catalytic
C−H borylation of 2,3,5,6-tetrafluoropyridine with B2pin2
catalyzed by 1 or 7 in order to synthesize 4-Bpin-C5NF4. On
using 1 the reaction yielded a product mixture of mono- and
diborylated pyridine derivatives, which could not be identified
further. A GC-MS analysis of the reaction solution indicated
that C−H and C−F borylation as well as hydrodefluorination
steps occur. Surprisingly, with 7 as catalyst 4-Bpin-C5NF4 was
obtained as the main product under release of HBpin (Scheme
8). After 1 week at 50 °C 2,3,5,6-tetrafluoropyridine was
converted into 4-Bpin-C5NF4 in a yield of 44%. The compound
4-Bpin-C5NF4 was identified by NMR spectroscopy and GC-
MS analysis. In the 19F NMR spectrum two signals at δ −92.8
and −132.6 ppm in a ratio of 1:1 were detected. The chemical
shifts are in agreement with those found for the corresponding
boronic acid and 4-Bcat-C5NF4.7,36 A signal at δ 29 ppm in the
11B NMR spectrum as well as the resonance at δ 1.09 ppm in
The catalytic C−F borylation of pentafluoropyridine at the
boryl complex 1 results selectively in the formation of 2-boryl-
tetrafluoropyridine as mentioned above (Scheme 1). Since 1
undergoes C−F activation at the 2-position and 7 activates
stoichiometrically the C−F bond at the 4-position,8a,9 we were
surprised to find that 7 can also be used as catalytic precursor to
achieve mainly a borylation at the 2-position (Scheme 8). With
Scheme 8. Catalytic Borylation of Fluorinated Pyridines via
C−F or C−H Bond Activation with B2pin2 on Using 7
1
the H NMR spectrum indicates the presence of the boryl
group.
We also employed HBpin as reagent in C−F activation
reactions at pentafluoropyridine. With 5 mol % 7, HBpin, and
an excess of pentafluoropyridine, the hydrodefluorina-
tion2m,o,5,14d,37 product 2,3,5,6-tetrafluoropyridine was obtained
as the main product in a yield of 91% (Scheme 9).
Furthermore, small amounts of 4-Bpin-C5NF4 were generated.
At room temperature the reaction was completed within 1 day.
5 mol % of the hydrido complex 7, pentafluoropyridine and
B2pin2 were converted into 2-Bpin-C5NF4 in a yield of 86%
after 1 d at 50 °C. Only minor amounts (3%) of 4-Bpin-C5NF4
were observed. We assume that in the first step of the catalytic
process 7 reacts with pentafluoropyridine to give 3 via C−F
activation reaction at the 4-position, followed by reaction of 3
Scheme 9. Catalytic Borylation and Hydrodefluorination of Fluorinated Pyridines via C−H and C−F Bond Activation with
HBpin on Using 7
H
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On the basis of C−F bond cleavage steps at the 4-position a
turnover number of 19 was determined. We assume that
dihydrogen, which can be generated from HBpin and HF,
serves mainly as hydrogen source for the hydrodefluorination.
Note that the activation of pentafluoropyridine at 7 at the 4-
position as well as a catalytic hydrodefluorination of
pentafluoropyridine with dihydrogen to give 2,3,5,6-tetrafluor-
opyridine was reported before.5c,9
However, 4-Bpin-C5NF4 was prepared in high yield from the
reaction of a 2:1 mixture of HBpin and pentafluoropyridine in
the presence of 5 mol % 7 in benzene (Scheme 9). The
reaction time at room temperature had to be extended to 3
weeks, until pentafluoropyridine was completely converted. We
suggest that intermediately formed 2,3,5,6-tetrafluoropyridine
reacts with a second equivalent of HBpin to the boronic ester
derivative under release of H2. Remaining 2,3,5,6-tetrafluoro-
pyridine was detected in a yield of 5%. An independent reaction
supports this hypothesis. Treatment of 2,3,5,6-tetrafluoropyr-
idine with HBpin in the presence of 5 mol % 7 led to the
formation of 4-Bpin-C5NF4. A conversion of 40% was observed
after 3 d at 50 °C.
tetrafluoropyridine was generated by catalytic hydrodefluorina-
tion with a substoichiometric amount of HBpin.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out in an
argon atmosphere. [D6]Benzene, [D8]toluene, [D12]cyclohexane,
cyclohexane, hexane, and hexamethyldisilane were dried by stirring
over Na/K and then distilled. PFA NMR tubes were used for highly
sensitive compounds to inhibit reactions at glass surfaces. Complex
[Rh(Bpin)(PEt3)3] (1) was prepared according to the literature.8a,b
The NMR spectra were recorded at 300 K (if not stated otherwise) on
a Bruker Avance 400, a Bruker DPX 300, or a Bruker Avance III 300
1
NMR spectrometer. The H NMR chemical shifts were referenced to
residual C6D5H at δ 7.16 ppm, [D7]toluene at δ 2.09 ppm, or
[D11]cyclohexane at δ 1.43 ppm. The 11B{1H} NMR spectra were
referenced to external BF3·OEt2 at δ 0.0 ppm, the 19F NMR spectra to
external CFCl3 at δ 0.0 ppm, and the 31P{1H} NMR spectra to external
2
H3PO4 at δ 0.0 ppm. In order to get a H lock signal, C6D6 was
introduced in the space between the glass NMR tubes and the PFA
inliners, which contained the reaction mixture with hexamethyldisilane
or cyclohexane as a solvent. GC-MS spectra were measured at an
Agilent 6890N gas-phase chromatograph (Agilent 19091S-433
Hewlett-Packard), which was equipped with an Agilent 5973 Network
mass selective detector at 70 eV. The microanalyses were obtained
with a Euro EA HEKAtech elemental analyzer.
In the catalytic borylation reaction of 1,3,5-trifluorobenzene and
1,3-difluorobenzene the yields or the sum of the yields of borylated
products is larger than 100% (based on the amount of B2pin2 as a
source of one boryl group). This is caused by the fact that the
generated HBpin can act as a borylation agent. TONs are defined as
number of borylation steps or C−F bond cleavage steps/mol of Rh
complex.
Overall, the treatment of pentafluoropyridine with B2pin2
results in the C−F borylation at the 2-position. For 2,3,5,6-
tetrafluoropyridine with 7 as catalyst a C−H borylation reaction
takes place. In contrast, with HBpin as reagent the bond at the
4-position of 2,3,5,6-tetrafluoropyridine or pentafluoropyridine
is activated. Therefore, the regioselectivity of the C−F
borylation of pentafluoropyridine can be controlled by the
choice of the borylation agent.
Treatment of [Rh(Bpin)(PEt3)3] (1) with Pentafluorobenzene:
Synthesis of [Rh(C6F5)(PEt3)3] (4). A solution of [Rh(Bpin)(PEt3)3]
(1) (9.9 mg, 17 μmol) in Me6Si2 (0.5 mL) in a PFA tube was treated
with pentafluorobenzene (50 μL, 0.45 mmol). After 15 min the NMR
spectroscopic data of the reaction solution revealed the complete
conversion of 1 and the formation of [Rh(C6F5)(PEt3)3] (4) and
HBpin. The latter was detected by GC-MS analysis and can be
removed under vacuum. Complex 4 was identified by comparison of
the NMR data with those in the literature.10 Additional analytical data:
CONCLUSION
■
Reactions of the boryl complex [Rh(Bpin)(PEt3)3] (1) with
partially fluorinated benzene and pyridine derivatives resulted
in C−H activation products, which reveals a preference for C−
H over C−F activation. Treatment of 1 with the perfluorinated
substrate hexafluorobenzene or perfluorotoluene led to the C−
F activation products. The regiochemistry of the C−H
activation reactions seems to be dominated by the strength of
metal−carbon bond interaction which is linked with a
maximum number of fluorine atoms ortho to the metal center.
Initial formation of fluoroaryl complexes in stoichiometric
reactions instead of a borylation of the fluoroaryl moiety might
in part be attributed to the stabilizing effect of fluorinated
organyl ligands. In contrast to the pyridyl complexes, the
fluorophenyl complexes are prone to a stoichiometric
borylation with B2pin2. The hydrido complex [Rh(H)(PEt3)3]
(7) is also capable of C−H activation reactions of partially
fluorinated benzenes and 2,3,5,6-tetrafluoropyridines. Based on
the stoichiometric transformations, catalytic C−H and C−F
borylation reactions using 1 or 7 as catalyst were developed.
These reactions show a good selectivity and proceed under
mild conditions when compared to other rhodium-catalyzed
borylation processes. Although rhodium-catalyzed arene
borylation reactions are known,13,30a,b they usually require
fairly harsh reaction conditions and are regarded to be
less selective, in contrast to the iridium-catalyzed
processes.30a−d,32,38 The synthesized boronic esters can be
valuable starting compounds for further transformations. The
regioselectivity of the rhodium-catalyzed C−F borylation
reaction of pentafluoropyridine can be controlled by the choice
of the borylating reagent; that is, using 5 mol % 7, B2pin2 leads
to a borylation at the 2-position, whereas HBpin yields the 4-
borylpyridine derivative. However, in the latter case 2,3,5,6-
1
31P{1H} NMR (121.5 MHz, C6D6) δ 18.6 (dttt, J(Rh,P) = 133 Hz,
4
5
2J(P,P) = 40 Hz, J(F,P) = 15 Hz, J(F,P) = 8 Hz, 1P), 14.1 (dd,
1J(Rh,P) = 140 Hz, J(P,P) = 40 Hz, 2P).
2
Treatment of [Rh(Bpin)(PEt3)3] (1) with 1,3,5-Trifluoroben-
zene: Synthesis of [Rh(2,4,6-C6F3H2)(PEt3)2] (5). A solution of
[Rh(Bpin)(PEt3)3] (1) (34.6 mg, 59 μmol) in cyclohexane (0.2 mL)
in a PFA tube was treated with 1,3,5-trifluorobenzene (122 μL, 1.18
mmol). After 1 d the NMR spectroscopic data of the reaction solution
revealed the complete conversion of 1 and the formation of
[Rh(H)(PEt3)3] (7) and [Rh(2,4,6-C6F3H2)(PEt3)3] (5) in a ratio
of 2.5:1 (according to the 31P{1H} NMR spectrum). HBpin and 2-
Bpin-1,3,5-C6F3H2 were detected by GC-MS analysis. 2-Bpin-1,3,5-
C6F3H2 and 5 were formed in a ratio of 2:1 (according to the 19F
NMR spectrum). The volatiles were removed and the residue was
dissolved in toluene-d8. The complexes mer-[Rh(H)3(PEt3)3] and fac-
[Rh(H)3(PEt3)3] were found in traces (4% and 1% based on the
amounts of 1 as starting compound) by NMR spectroscopy at 203 K.
2-Bpin-1,3,5-C6F3H2 was identified by comparison of the NMR data
with those in the literature.30a,31,32 Orange crystals of [Rh(2,4,6-
C6F3H2)(PEt3)3] (5) were obtained at −30 °C by crystallization from
a solution of the reaction products in n-hexane. Analytical data for
1
[Rh(2,4,6-C6F3H2)(PEt3)3] (5): H NMR (300.1 MHz, C6D6) δ 6.60
(m, br s in the 1H{19F} NMR spectrum, 2H, CHar), 1.46 (m, q in the
1H{31P} NMR spectrum, 3J(H,H) = 8 Hz, 6H, CH2), 1.36 (m, q in the
3
1H{31P} NMR spectrum, J(H,H) = 8 Hz, 12H, CH2), 1.06 (m, t in
the 1H{31P} NMR spectrum, 3J(H,H) = 8 Hz, 9H, CH3), 1.03 (m, t in
1
3
the H{31P} NMR spectrum, J(H,H) = 7 Hz, 18H, CH3); 13C{1H}
NMR (75.5 MHz, C6D6) δ 167.8 (ddd, 1J(F,C) = 234 Hz, J = 31 Hz,
I
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3J(F,C) = 14 Hz, CF), 161.3 (dt, J(F,C) = 237 Hz, J(F,C) = 14 Hz,
(m, q in the 1H{31P} NMR spectrum, 3J(H,H) = 8 Hz, 6H, CH2), 1.29
1
3
1
3
(m, q in the H{31P} NMR spectrum, J(H,H) = 8 Hz, 12H, CH2),
2
2
CF), 98.0 (dd, J(F,C) = 38 Hz, J(F,C) = 22 Hz, CHar), 20.1 (dt,
1J(P,C) = 18 Hz, J = 3 Hz, CH2), 18.3 (t, J = 10 Hz, CH2), 8.8 (s,
CH3), 8.6 (s, CH3), the signal for the Rh−C atom was not observed;
19F{1H}NMR (282.4 MHz, C6D6) δ −77.2 (ddm, J = 16 Hz, J = 10
Hz, 2F), −121.1 (m, 1F); 31P{1H} NMR (121.5 MHz, C6D6) δ 19.6
(dtt, J(Rh,P) = 132 Hz, J(P,P) = 39 Hz, J(F,P) = 13 Hz, 1P), 15.0
(dd, J(Rh,P) = 145 Hz, J(P,P) = 39 Hz, 2P). Anal. Calcd (%) for
C24H47F3P3Rh: C, 48.99; H, 8.05. Found: C, 49.08; H, 8.25.
Treatment of [Rh(Bpin)(PEt3)3] (1) with 1,3-Difluorobenzene:
Synthesis of [Rh(2,6-C6F2H3)(PEt3)3] (6). A solution of [Rh(Bpin)-
(PEt3)3] (1) (43.3 mg, 74 μmol) in cyclohexane (0.2 mL) in a PFA
tube was treated with 1,3-difluorobenzene (145 μL, 1.48 mmol). After
1 d the NMR spectroscopic data of the reaction solution revealed the
complete conversion of 1 and the formation of [Rh(H)(PEt3)3] (7)
and [Rh(2,6-C6F2H3)(PEt3)3] (6) in a ratio of 2.2:1 (according to the
31P{1H} NMR spectrum). HBpin and 2-Bpin-1,3-C6F2H3 were
1.03 (m, t in the 1H{31P} NMR spectrum, 3J(H,H) = 7 Hz, 9H, CH3),
1
3
0.95 (m, t in the H{31P} NMR spectrum, J(H,H) = 8 Hz, 18H,
CH3); 13C{1H} NMR (75.5 MHz, Me6Si2) δ 167.3 (ddm, J(F,C) =
1
2
235 Hz, J = 20 Hz, CF), 130.3 (d, J(F,C) = 36 Hz, CHar), 19.9 (dt,
2J(P,C) = 18 Hz, J = 3 Hz, CH2), 18.3 (tt, J = 11 Hz, J = 4 Hz, CH2),
8.8 (s, CH3), 8.5 (s, CH3) (the signal for the Rh−C atom was not
observed); 19F{1H} NMR (282.4 MHz, Me6Si2) δ −94.1 (dd, 3J(Rh,F)
≈ 4J(P,F) ≈ 12 Hz); 31P{1H} NMR (121.5 MHz, Me6Si2) δ 17.1 (dtt,
1
2
4
1
2
1J(Rh,P) = 126 Hz, J(P,P) = 41 Hz, J(F,P) = 13 Hz, 1P), 11.8 (dd,
1J(Rh,P) = 142 Hz, 2J(P,P) = 41 Hz, 2P). Anal. Calcd (%) for
C23H47F2NP3Rh: C, 48.34; H, 8.29; N, 2.45. Found: C, 48.31; H, 8.28;
2
4
1
N, 2.10. Analytical data for 4-Bpin-3,5-C5NF2H2: H NMR (300.1
MHz, cyclohexane) δ 8.39 (s, 2H, CHar), 1.39 (s, 12H, CH3); 11B
NMR (96.3 MHz, cyclohexane) δ 30 (s); 13C{1H} NMR (75.5 MHz,
1
2
cyclohexane) δ 162.3 (d, J(F,C) = 262 Hz, CF), 133.9 (d, J(F,C) =
25 Hz, CHar), ∼113.5 (br s, CB), 84.8 (s, C(CH3)2), 24.8 (s,
C(CH3)2); 19F{1H} NMR (282.4 MHz, cyclohexane) δ −116.8 (s).
Treatment of [Rh(2,4,6-C6F3H2)(PEt3)3] (5) with HBpin. A
solution of [Rh(2,4,6-C6F3H2)(PEt3)3] (5) (4.4 mg, 7 μmol) in
cyclohexane (0.1 mL) was treated with HBpin (1.1 μL, 7 μmol) in a
NMR tube. After 1 d the volatiles were removed under vacuum, and
the residue was dissolved in toluene-d8. The NMR spectroscopic data
of the reaction solution at 203 K revealed the presence of a mixture
containing 5 and [Rh(H)(PEt3)3] (7) as well as small amounts of mer-
[Rh(H)3(PEt3)3] and fac-[Rh(H)3(PEt3)3] in a ratio of 1:1.5:0.2:0.03
(according to the 31P{1H} NMR spectrum). Furthermore, the
formation of 2-Bpin-1,3,5-C6F3H2 and 1,3,5-trifluorobenzene was
detected by 19F NMR spectroscopy and GC-MS analysis. 2-Bpin-
1,3,5-C6F3H2 was identified by comparison of the NMR data with
those in the literature.30a,31,32
Treatment of [Rh(2,6-C6F2H3)(PEt3)3] (6) with HBpin. A
solution of [Rh(2,6-C6F2H3)(PEt3)3] (6) (2.2 mg, 4 μmol) in
cyclohexane (0.1 mL) was treated with HBpin (0.6 μL, 4 μmol) in
a NMR tube. After 1 d the volatiles were removed under vacuum and
the residue was dissolved in toluene-d8. The NMR spectroscopic data
of the reaction solution at 203 K revealed the presence of a mixture
containing 6 and [Rh(H)(PEt3)3] (7) as well as small amounts of mer-
[Rh(H)3(PEt3)3] in a ratio of 1:1.1:0.15 (according to the 31P{1H}
NMR spectrum). Furthermore, the formation of 2-Bpin-1,3-C6F2H3
and 1,3-difluorobenzene was detected by 19F NMR spectroscopy and
GC-MS analysis. 2-Bpin-1,3-C6F2H3 was identified by comparison of
the NMR data with those in the literature.20b,33
detected by GC-MS analysis. 2-Bpin-1,3-C6F2H3 and 6 were formed
in a ratio of 2:1 (according to the 19F NMR spectrum). Furthermore,
after removing the volatiles and dissolving the residue in toluene-d8
mer-[Rh(H)3(PEt3)3] and cis-fac-[Rh(H)2(Bpin)(PEt3)3] were found
in traces (2% and 6% based on the amounts of 1 as starting
compound) by NMR spectroscopy at 203 K. 2-Bpin-1,3-C6F2H3 was
identified by comparison of the NMR data with those in the
literature.20b,33 Orange crystals of [Rh(2,6-C6F2H3)(PEt3)3] (6) were
obtained at −30 °C by crystallization from a solution of the reaction
products in n-hexane. Analytical data for [Rh(2,6-C6F2H3)(PEt3)3]
1
(6): H NMR (300.1 MHz, C6D6) δ 6.87 (m, 1H, CHar), 6.77 (dm,
3J(H,H) = 6 Hz, 2H, CHar), 1.60 (m, q in the 1H{31P} NMR
spectrum, 3J(H,H) = 7 Hz, 6H, CH2), 1.51 (m, q in the 1H{31P} NMR
3
1
spectrum, J(H,H) = 7 Hz, 12H, CH2), 1.20 (m, t in the H{31P}
NMR spectrum, 3J(H,H) = 7 Hz, 9H, CH3), 1.17 (m, t in the 1H{31P}
NMR spectrum, J(H,H) = 7 Hz, 18H, CH3); 13C{1H} NMR (75.5
3
MHz, C6D6) δ 169.1 (dd, 1J(F,C) = 223 Hz, J = 25 Hz, CF), 124.1 (t,
3J(F,C) = 8 Hz, CHar), 109.3 (d, J(F,C) = 34 Hz, CHar), 20.2 (d,
2
1J(P,C) = 17 Hz, CH2), 18.4 (t, J = 11 Hz, CH2), 8.9 (s, CH3), 8.6 (s,
CH3), the signal for the Rh−C atom was not observed; 19F{1H} NMR
3
4
(282.4 MHz, Me6Si2) δ −79.3 (ddt, J(Rh,F) ≈ J(P,F) ≈ 13 Hz,
4J(P,F) = 5 Hz); 31P{1H} NMR (121.5 MHz, C6D6) δ 18.9 (dtt,
1J(Rh,P) = 129 Hz, 2J(P,P) = 39 Hz, 4J(F,P) = 13 Hz, 1P), 14.6 (bdd,
1J(Rh,P) = 145 Hz, 2J(P,P) = 39 Hz, 2P). Anal. Calcd (%) for
C24H48F2P3Rh: C, 50.53; H, 8.48. Found: C, 50.96; H, 8.61.
Comparison of Conversion in the C−H Activation Reaction
of 1,3,5-Trifluorobenzene and 1,3-Difluorobenzene. A solution
of [Rh(Bpin)(PEt3)3] (1) (29.8 mg, 51 μmol) in cyclohexane (0.2
mL) in a PFA tube was treated with 1,3,5-trifluorobenzene (105 μL,
1.02 mmol) or 1,3-difluorobenzene (100 μL, 1.02 mmol). After 7 h the
NMR spectroscopic data of the reaction solution revealed that 86% of
1 was converted into 5 and 7 from 1,3,5-trifluorobenzene vs 73%
conversion into 6 and 7 from 1,3-difluorobenzene according to the
31P{1H} NMR spectra.
Treatment of [Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8) with HBpin. A
solution of [Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8) (3.0 mg, 5 μmol) in
cyclohexane (0.1 mL) in a NMR tube was treated with HBpin (0.8 μL,
5 μmol). After 1 d the volatiles were removed under vacuum, and the
residue was dissolved in toluene-d8.The NMR spectroscopic data of
the reaction solution at 203 K revealed the presence of a mixture
containing 8 and [Rh(H)(PEt3)3] (7) in a ratio of 1:3 (according to
the 31P{1H} NMR spectrum).
Treatment of [Rh(Bpin)(PEt3)3] (1) with 3,5-Difluoropyridine:
Synthesis of [Rh{4-(3,5-C5NF2H2)}(PEt3)3] (8). A solution of
[Rh(Bpin)(PEt3)3] (1) (36.4 mg, 62 μmol) in cyclohexane (0.2
mL) in a PFA tube was treated with 3,5-difluoropyridine (6 μL, 62
μmol). After 3 h the NMR spectroscopic data of the reaction solution
revealed the complete conversion of 1 and the formation of [Rh{4-
(3,5-C5NF2H2)}(PEt3)3] (8) and [Rh(H)(PEt3)3] (7) in a ratio of
1:0.16 (according to the 31P{1H} NMR spectrum). HBpin and 4-Bpin-
3,5-C5NF2H2 were detected by GC-MS analysis. 4-Bpin-3,5-C5NF2H2
and 7 were formed in a ratio of 0.1:1 (according to the 19F NMR
spectrum). Furthermore, after removing the volatiles and dissolving
the residue in toluene-d8 mer-[Rh(H)3(PEt3)3] and cis-fac-[Rh-
(H)2(Bpin)(PEt3)3] were determined in traces (3% and 4% based
on the amounts of 1 as starting compound) by NMR spectroscopy at
203 K. Orange crystals of [Rh{4-(3,5-C5NF2H2)}(PEt3)2] (8) were
obtained at −30 °C by crystallization from a solution of the reaction
products in n-hexane. Analytical data for [Rh{4-(3,5-C5NF2H2)}-
(PEt3)3] (8): 1H NMR (300.1 MHz, C6D6) δ 8.20 (s, 2H, CHar), 1.43
Treatment of [Rh(H)(PEt3)3] (7) with 1,3,5-Trifluorobenzene.
A solution of [Rh(H)(PEt3)3] (7) (22.4 mg, 49 μmol) in cyclohexane
(0.15 mL) in a PFA tube was treated with 1,3,5-trifluorobenzene (101
μL, 0.98 mmol). After 1 d the volatiles were removed under vacuum
and the residue was dissolved in toluene-d8. The NMR spectroscopic
data of the reaction solution at 203 K revealed the presence of a
mixture containing 7, [Rh(2,4,6-C6F3H2)(PEt3)3] (5), and small
amounts of mer-[Rh(H)3(PEt3)3] and fac-[Rh(H)3(PEt3)3] in a ratio
of 3.4:1:0.3:0.05 (according to the 31P{1H} NMR spectrum).
Treatment of [Rh(H)(PEt3)3] (7) with 1,3-Difluorobenzene. A
solution of [Rh(H)(PEt3)3] (7) (18.0 mg, 39 μmol) in cyclohexane
(0.2 mL) in a PFA tube was treated with 1,3-difluorobenzene (77 μL,
0.79 mmol). After 4 d at 50 °C the volatiles were removed under
vacuum and the residue was dissolved in toluene-d8. The NMR
spectroscopic data of the reaction solution at 203 K revealed the
presence of a mixture containing 7 and [Rh(2,6-C6F2H3)(PEt3)3] (6)
in a ratio of 10:1 (according to the 31P{1H} NMR spectrum).
J
DOI: 10.1021/om500952x
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Article
Treatment of [Rh(H)(PEt3)3] (7) with 3,5-Difluoropyridine. A
solution of [Rh(H)(PEt3)3] (7) (22.6 mg, 49 μmol) in cyclohexane
(0.15 mL) in a PFA tube was treated with 3,5-difluoropyridine (4.5 μL,
49 μmol). After 2 h the volatiles were removed under vacuum and the
residue was dissolved in toluene-d8. The NMR spectroscopic data of
the reaction solution at 203 K revealed the presence of a mixture
containing 7, [Rh(4-{3,5-C5NF2H2})(PEt3)3] (8), and small amounts
of mer-[Rh(H)3(PEt3)3] and fac-[Rh(H)3(PEt3)3] in a ratio of
1:1:0.06:0.01 (according to the 31P{1H} NMR spectrum).
Treatment of [Rh(Bpin)(PEt3)3] (1) with Hexafluorobenzene:
Synthesis of [Rh(C6F5)(PEt3)3] (4). A solution of [Rh(Bpin)(PEt3)3]
(1) (9.9 mg, 17 μmol) in Me6Si2 (0.5 mL) in a PFA tube was treated
with hexafluorobenzene (100 μL, 0.45 mmol). After 2 h the NMR
spectroscopic data of the reaction solution revealed the complete
conversion of 1. The volatiles were removed under vacuum, and the
residue was extracted with n-hexane. Removing the solvent from the
extract led to a solid, which was [Rh(C6F5)(PEt3)3] (4) according to
its NMR spectra.
(based on the amount of B2pin2), which was determined by integration
of the signals in the 19F NMR spectrum (TON = 34, based on
borylation steps). Small amounts (8% based on the amount of B2pin2)
of 2,4-(Bpin)2-1,3,5-C6F3H were determined by 19F NMR spectros-
1
copy and GC-MS analysis. Furthermore, HBpin was detected by H
NMR spectroscopy and GC-MS analysis. Analytical data for 2,4-
(Bpin)2-1,3,5-C6F3H: 19F NMR (282.4 MHz, cyclohexane) δ −84.4 (t,
4J(F,F) = 7 Hz, 1F), −93.6 (dd, 3J(H,F) = 9 Hz, 4J(F,F) = 7 Hz, 2F).
Formation of 2-Bpin-1,3-C6F2H3 from 1,3-Difluorobenzene
and B2pin2 with 3.5 mol % [Rh(Bpin)(PEt3)3] (1). A solution of
[Rh(Bpin)(PEt3)3] (1) (6.4 mg, 11 μmol) in cyclohexane (0.15 mL)
in a PFA tube was treated with B2pin2 (79.8 mg, 314 μmol) and 1,3-
difluorobenzene (185 μL, 1.88 mmol) and heated to 50 °C. After 24 h
the NMR spectroscopic data of the reaction solution revealed the
formation of 2-Bpin-1,3-C6F2H3 in a yield of 98% (based on the
amount of B2pin2), which was determined by integration of the signals
in the 19F NMR spectrum (TON = 28, based on borylation steps). In
addition, mono- and diborylated difluorobenzene derivatives in a yield
of 37% (based on the amount of B2pin2) were determined by
integration of the signals in the 19F NMR spectrum (based on the
assumption that no C−F bond cleavage steps occur, FBpin or HF
could not be observed) and identified by GC-MS analysis.
Furthermore, HBpin was detected by 1H NMR spectroscopy and
GC-MS analysis.
Treatment of [Rh(Bpin)(PEt3)3] (1) with Perfluorotoluene:
Synthesis of [Rh(4-C6F4CF3)(PEt3)3] (9). A solution of [Rh(Bpin)-
(PEt3)3] (1) (18.1 mg, 31 μmol) in Me6Si2 (0.5 mL) in a PFA tube
was treated with perfluorotoluene (10 μL, 71 μmol). After 4 h the
NMR spectroscopic data of the reaction solution revealed the
complete conversion of 1. The volatiles were removed under vacuum,
and the residue was extracted with n-hexane. After removal of the
solvent from the extract [Rh(4-C6F4CF3)(PEt3)3] (9) was obtained as
a yellow solid (yield: 19 mg, 90%). Yellow crystals of 9 were obtained
by crystallization from a solution in n-hexane. Complex 9 was
identified by comparison of the NMR data with those in the
literature.10 Additional analytical data: 31P{1H} NMR (121.5 MHz,
C6D6) δ 17.7 (dttt, 1J(Rh,P) = 130 Hz, 2J(P,P) = 40 Hz, 4J(F,P) = 14,
Formation of 2-Bpin-1,3,5-C6F3H2 from 1,3,5-Trifluoroben-
zene and B2pin2 with 5 mol % [Rh(H)(PEt3)3] (7). A solution of
[Rh(H)(PEt3)3] (7) (6.9 mg, 15 μmol) in cyclohexane (0.1 mL) in a
PFA tube was treated with B2pin2 (76.5 mg, 301 μmol) and 1,3,5-
trifluorobenzene (187 μL, 1.80 mmol) and heated to 50 °C. After 24 h
the NMR spectroscopic data of the reaction solution revealed the
formation of 2-Bpin-1,3,5-C6F3H2 in a yield of 112% (based on the
amount of B2pin2), which was determined by integration of the signals
in the 19F NMR spectrum (TON = 22, based on borylation steps).
Small amounts (7% based on the amount of B2pin2) of 2,4-(Bpin)2-
1,3,5-C6F3H were determined by 19F NMR spectroscopy and GC-MS
1
2
5J(F,P) = 18 Hz, 1P), 13.4 (dd, J(Rh,P) = 140 Hz, J(P,P) = 40 Hz,
2P); coupling constants were determined by simulation with gNMR.39
Anal. Calcd (%) for C25H45F7P3Rh: C, 44.52; H 6.73. Found: C, 44.17;
H, 6.85.
1
Treatment of [Rh(2,4,6-C6F3H2)(PEt3)3] (5) with B2pin2. A
solution of [Rh(2,4,6-C6F3H2)(PEt3)3] (5) (7.5 mg, 13 μmol) in
cyclohexane (0.2 mL) in a NMR tube was treated with B2pin2 (9.9 mg,
39 μmol). After 10 d the NMR spectroscopic data of the reaction
solution revealed the complete conversion of 5 and the formation of
[Rh(Bpin)(PEt3)3] (1). The formation of 2-Bpin-1,3,5-C6F3H2 was
confirmed by 19F NMR spectroscopy and GC-MS analysis. 2-Bpin-
1,3,5-C6F3H2 was identified by comparison of the NMR data with
those in the literature.30a,31,32
Treatment of [Rh(2,6-C6F2H3)(PEt3)3] (6) with B2pin2. A
solution of Rh(2,6-C6F2H3)(PEt3)3] (6) (12.1 mg, 21 μmol) in
Me6Si2 (0.15 mL) in a PFA tube was treated with B2pin2 (10.7 mg, 42
μmol). After 5 d the NMR spectroscopic data of the reaction solution
revealed the complete conversion of 6 and the formation of
[Rh(Bpin)(PEt3)3] (1). The formation of 2-Bpin-1,3-C6F2H3 was
confirmed by 19F NMR spectroscopy and GC-MS analysis. 2-Bpin-1,3-
C6F2H3 was identified by comparison of the NMR data with those in
the literature.20b,33
analysis. Furthermore, HBpin was detected by H NMR spectroscopy
and GC-MS analysis.
Formation of 2-Bpin-1,3-C6F2H3 from 1,3-Difluorobenzene
and B2pin2 with 5 mol % [Rh(H)(PEt3)3] (7). A solution of
[Rh(H)(PEt3)3] (7) (4.4 mg, 12 μmol) in cyclohexane (0.1 mL) in a
PFA tube was treated with B2pin2 (48.8 mg, 192 μmol) and 1,3-
difluorobenzene (113 μL, 1.15 mmol) and heated to 50 °C. After 24 h
the NMR spectroscopic data of the reaction solution revealed the
formation of 2-Bpin-1,3-C6F2H3 in a yield of 83% (based on the
amount of B2pin2), which was determined by integration of the signals
in the 19F NMR spectrum (TON = 17, based on borylation steps). In
addition, mono- and diborylated difluorobenzene derivatives in a yield
of 34% (based on the amount of B2pin2) were determined by
integration of the signals in the 19F NMR spectrum (based on the
assumption that no C−F bond cleavage steps occur, FBpin or HF
could not be observed) and identified by GC-MS analysis.
Furthermore, HBpin was detected by 1H NMR spectroscopy and
GC-MS analysis.
Treatment of [Rh(4-C6F4CF3)(PEt3)3] (9) with B2pin2. A solution
of [Rh(4-C6F4CF3)(PEt3)3] (9) (19.5 mg, 29 μmol) in cyclohexane
(0.15 mL) in a PFA tube was treated with B2pin2 (14.7 mg, 58 μmol).
After 5 d the NMR spectroscopic data of the reaction solution revealed
that 50% of 9 was converted into [Rh(Bpin)(PEt3)3] (1). The
formation of 4-Bpin-C6F4CF3 was confirmed by 19F NMR spectros-
Formation of 4-Bpin-C6F4CF3 from Perfluorotoluene and
B2pin2 with 3.5 mol % [Rh(Bpin)(PEt3)3] (1). A solution of
[Rh(Bpin)(PEt3)3] (1) (5.3 mg, 9 μmol) in C6D6 (0.5 mL) in a PFA
tube was treated with B2pin2 (65.8 mg, 259 μmol) and
perfluorotoluene (36 μL, 433 μmol). After 2 d at 55 °C the NMR
spectroscopic data of the reaction solution revealed the formation of 4-
Bpin-C6F4CF3 in a yield of 57% (based on the amount of B2pin2),
which was determined by integration of the signals in the 19F NMR
spectrum (TON = 16, based on borylation steps).
Formation of 2-Bpin-C5NF4 from Pentafluoropyridine and
B2pin2 with 5 mol % [Rh(H)(PEt3)3] (7). A solution of [Rh(H)-
(PEt3)3] (7) (5.3 mg, 12 μmol) in cyclohexane (0.2 mL) in a NMR
tube was treated with B2pin2 (58.9 mg, 232 μmol) and
pentafluoropyridine (25 μL, 232 μmol). After 1 d at 50 °C the
NMR spectroscopic data of the reaction solution revealed the
formation of 2-Bpin-C5NF4 in a yield of 86% and 4-Bpin-C5NF4 in
a yield of 3% (based on the amount of B2pin2, determined by
1
copy and GC-MS analysis. Analytical data for 4-Bpin-C6F4CF3: H
NMR (300.1 MHz, C6D6) δ 1.18 (s, CH3); 11B NMR (96.3 MHz,
C6D6) δ 29 (s); 19F NMR (282.4 MHz, C6D6) δ −57.3 (t, 4J(F,F) = 21
Hz, 3F, CF3), −129.2 (m, 2F, 3/5-CF), −142.1 (m, 2F, 2/6-CF).
Formation of 2-Bpin-1,3,5-C6F3H2 from 1,3,5-Trifluoroben-
zene and B2pin2 with 3.5 mol % [Rh(Bpin)(PEt3)3] (1). A solution
of [Rh(Bpin)(PEt3)3] (1) (5.8 mg, 10 μmol) in cyclohexane (0.15
mL) in a PFA tube was treated with B2pin2 (72.6 mg, 286 μmol) and
1,3,5-trifluorobenzene (177 μL, 1.72 mmol) and heated to 50 °C.
After 24 h the NMR spectroscopic data of the reaction solution
revealed the formation of 2-Bpin-1,3,5-C6F3H2 in a yield of 120%
K
DOI: 10.1021/om500952x
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Article
integration of the signals in the 19F NMR spectrum, TON = 18, based
on borylation steps). Furthermore, FBpin was detected by 19F NMR
spectroscopy.
NMR spectrum, TON = 8, based on C−H cleavage steps) as the only
product, which was detectable in a significant amount.
Treatment of fac-[Rh(Bpin)2(H)(PEt3)3] (10) with Pentafluoro-
pyridine: Formation of [Rh(2-C5NF4)(PEt3)3] (2). A solution of fac-
[Rh(Bpin)2(H)(PEt3)3] (10) (4.2 mg, 6 μmol) in C6D6 (0.2 mL) in a
NMR tube was treated with pentafluoropyridine (0.8 μL, 7 μmol).
After 5 min the NMR spectroscopic data of the reaction solution
revealed the quantitative formation of [Rh(2-C5NF4)(PEt3)3] (2) and
FBpin.
Catalytic Reaction of 2,3,5,6-Tetrafluoropyridine with B2pin2
on Using 3.5 mol % [Rh(Bpin)(PEt3)3] (1). A solution of
[Rh(Bpin)(PEt3)3] (1) (6.4 mg, 11 μmol) in cyclohexane (0.2 mL)
in a PFA tube was treated with B2pin2 (79.8 mg, 314 μmol) and
2,3,5,6-tetrafluoropyridine (32 μL, 314 μmol). After 2 d at 50 °C the
19F NMR spectroscopic data of the reaction solution revealed the
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information contains the crystallographic data
of compounds 5, 6, 8, and 9. This material is available free of
charge via the Internet at http://pubs.acs.org. Crystallographic
data are also available from the Cambridge Crystallographic
Database as file numbers CCDC 1023372−1023375.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thomas.braun@chemie.hu-berlin.
Notes
formation of a product mixture. A GC-MS analysis of the reaction
solution confirms the presence of mono- and diborylated pyridine
derivatives and indicates that C−H and C−F borylation as well as
hydrodefluorination steps occurred.
The authors declare no competing financial interest.
Formation of 4-Bpin-C5NF4 from 2,3,5,6-Tetrafluoropyridine
and B2pin2 with 5 mol % [Rh(H)(PEt3)3] (7). A solution of
[Rh(H)(PEt3)3] (7) (5.5 mg, 12 μmol) in cyclohexane (0.2 mL) in a
PFA tube was treated with B2pin2 (61.0 mg, 240 μmol) and 2,3,5,6-
tetrafluoropyridine (22 μL, 240 μmol). After 7 d at 50 °C the 19F
NMR spectroscopic data of the reaction solution revealed the
formation of 4-Bpin-C5NF4 in a yield of 44% (based on the amount
of B2pin2, determined by integration of the signals in the 19F NMR
spectrum, TON = 9, based on C−H borylation steps). Small amounts
(6% based on the amount of B2pin2, determined by integration of the
signals in the 19F NMR spectrum) of additional products such as 3
were detected, but were not identified further. Furthermore, HBpin
was detected by 1H NMR spectroscopy and GC-MS analysis.
ACKNOWLEDGMENTS
■
We would like to acknowledge the Cluster of Excellence
“Unifying Concepts in Catalysis” funded by the Deutsche
Forschungsgemeinschaft (DFG) for financial support. M.T. was
a fellow of the graduate school “Fluorine as a Key Element”
financed by the DFG.
REFERENCES
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Formation of 4-Bpin-C5NF4 from 2,3,5,6-Tetrafluoropyridine
and HBpin with 5 mol % [Rh(H)(PEt3)3] (7). A solution of
[Rh(H)(PEt3)3] (7) (5.7 mg, 12 μmol) in cyclohexane (0.2 mL) in a
NMR tube was treated with HBpin (36 μL, 0.25 mmol). After 1 min
2,3,5,6-tetrafluoropyridine (23 μL, 0.25 mmol) was added. After 3 d at
50 °C the NMR spectroscopic data of the reaction solution revealed
the formation of 4-Bpin-C5NF4 in a yield of 40% (based on the
amount of HBpin, determined by integration of the signals in the 19F
L
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