A highly reactive rhodium(I)-boryl complex as a useful tool for C-H bond activation and catalytic C-F bond borylation

Article abstract of DOI:10.1002/anie.201001070

Chemical Equation Presentation C-F bond borylation: A 16-electron rhodium (1)-boryl complex was synthesized by borylation of a rhodium (1)-fluorine complex. The former reacts with benzene or 2,3,5,6-tetrafluoropyridine by C-H activation. A catalytic C-F borylation reaction of pentafluoropyridine was also developed, which uses [Rh(Bpin) (PEt₃)₃] as a catalyst and Me₃SiSiMe₃ as a solvent. pin = pinacol.

Full text of DOI:10.1002/anie.201001070

Angewandte
Chemie
DOI: 10.1002/anie.201001070
Boryl Complexes
A Highly Reactive Rhodium(I)–Boryl Complex as a Useful Tool for
À
À
C H Bond Activation and Catalytic C F Bond Borylation**
Michael Teltewskoi, Julien A. Panetier, Stuart A. Macgregor,* and Thomas Braun*
Transition-metal–boryl complexes are key intermediates in
catalytic hydroboration and borylation reactions and find
applications in boron-based materials.[1] Rhodium complexes
can be applied in the rhodium-mediated hydroboration or
dehydrogenative borylation reactions of olefins or even in the
borylation of alkanes.[2,3] Rhodium and iridium boronates also
play a crucial role in the borylation of arenes to give, for
instance, Bpin derivatives.[4,5] Thus, it has been reported that
rhodium complexes with boryl and phosphine ligands are
intermediates in the catalytic borylation reactions of benzene,
toluene, para-xylene, and mesitylene with HBpin (HBpin =
detected. The thermodynamic driving force in this case is
apparently the generation of B F bonds.[8] Rhodium(I)–boryl
À
complexes are almost without any precedent in the litera-
ture,[1,9] although [Rh(Bcat)(PMe3)4] has been synthesized by
Marder and co-workers from the treatment of [Rh(Me)-
(PMe3)4] with B2cat2 (Bcat = B{1,2-O2C6H4}).[10]
Herein, we report the identification of a unique 16-
electron rhodium(I) species, [Rh(Bpin)(PEt3)3] (3). This
À
complex is highly reactive and can effect the C H activation
À
of benzene, as well as the stoichiometric C F activation and
À
catalytic C F borylation of fluorinated substrates.
4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
pinacolborane).[4]
We reasoned that a phenoxo complex might be an ideal
starting compound for the formation of a rhodium(I)–boryl
complex, partly because of the high stability of a boron–
oxygen bond.[11,12] Treatment of the chloro complex [Rh(Cl)-
(PEt3)3] (1) with NaOPh gave the rhodium phenoxide [Rh-
(OPh)(PEt3)3] (2; Scheme 1). The 1H and 31P NMR spectra of
À
Although it is conceivable that the C H activation step
occurs at the 14-electron species {Rh(Bpin)(PiPr3)2}, density
functional theory (DFT) calculations indicate that benzylic
activation takes place at {Rh(H)(PiPr3)2}.[4b] In this case, RhI–
RhIII cycles have been suggested, whereas iridium-based
catalysts may operate through IrIII–IrV cycles.[4,5]
À
Examples of C F bond functionalization, i.e. where
fluorine is replaced by a new group to access higher-value
fluorinated compounds, are very limited,[6,7] and most exam-
ples involve hydrodefluorination.[6] C F bond activation is
À
generally thermodynamically favorable, because of the
À
À
À
strength of the H F and Si F bonds, or even of the M F
bonds that are formed.[6] We have developed a catalytic
process for the conversion of hexafluoropropene and HBpin
into Bpin derivatives of trifluoropropane using [Rh(H)-
(PEt3)3] as a catalyst.[8] Mechanistic considerations suggest
the involvement of a rhodium(I)–boryl species in some of the
À
C F activation steps, but such a compound could not be
Scheme 1. Synthesis of a rhodium(I)–boryl complex.
[*] Dipl.-Chem. M. Teltewskoi, Prof. Dr. T. Braun
Institut fꢀr Chemie, Humboldt-Universitꢁt zu Berlin
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
complex 2 showed the expected splitting patterns; the
phosphorus–rhodium coupling constants in the 31P NMR
spectrum of 169 and 142 Hz are characteristic of a rhodium(I)
compound.[13] Reaction of 2 in benzene with excess B2pin2
(3 equiv) gave [Rh(Bpin)(PEt3)3] (3) after 16 hours, along
with PhOBpin (Scheme 1). Moreover, we found that 3 can be
generated in a comparable reaction, starting from the fluoro
compound [Rh(F)(PEt3)3] (4).[14] The fluoroborane FBpin
that was also formed can be removed in vacuo. Note that
boryl complex 3 does not react with another equivalent of
B2pin2 to give a RhIII complex that would be comparable to
fac-[Rh(Bcat)3(PMe3)3].[10]
J. A. Panetier, Prof. S. A. Macgregor
School of EPS – Chemistry, Heriot-Watt University
Edinburgh EH14 4AS (UK)
E-mail: s.a.macgregor@hw.ac.uk
[**] We would like to acknowledge the Humboldt-Universitꢁt as well as
Heriot-Watt University and the EPSRC for financial support. We also
thank Dr. Marcel Ahijado Salomon, Ann-Katrin Jungton (Berlin), Kai
Altenhꢂner (University of Bielefeld), and Prof. D. Lentz (FU Berlin)
for help with NMR spectroscopic analysis, preliminary experiments,
and helpful discussions. The Cluster of Excellence “Unifying
Concepts in Catalysis” funded by the DFG is gratefully acknowl-
edged. We thank the Solvay Fluor GmbH for a gift of hexafluor-
opropene. M.T. is a fellow of the graduate school “Fluorine as a Key
Element” financed by the DFG.
Complex 3 was only characterized in solution; the
31P NMR spectrum at room temperature revealed only one
broad signal at d = 15.1 ppm. This indicates a dynamic
behavior, which involves the exchange of the phosphine
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001070.
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ligands, similar to the fluxionality which has been observed
develop a catalytic process for the generation of PhBpin from
benzene and B2pin2. Complex 6 can also be synthesized
independently, starting from [Rh(Bpin)(PEt3)3] (3;
Scheme 2). A solution of 3 was treated with HBpin to give
6 plus considerable amounts of cis-fac-[Rh(H)2(Bpin)(PEt3)3]
(7). The formation of 7 can be explained by the reductive
elimination of B2pin2 from 6 to furnish the hydrido complex
[Rh(H)(PEt3)3] (5). It has been previously reported that
treatment of 5 with HBpin affords 7.[8]
for [Rh(H)(PEt3)3] (5).[15] Variable-temperature 31P NMR
analysis shows that the free phosphine has no influence on
the dynamic behavior, which suggests that the process is
intramolecular. The 31P NMR spectrum at 203 K exhibits a
signal at d = 20.5 ppm for the phosphine atoms that are in a
mutually trans position. The resonance features characteristic
couplings for a rhodium(I) compound, with doublet couplings
to rhodium and to phosphorus of 166 Hz and 30 Hz,
respectively.[12,13] A second resonance at d = 9.1 ppm, with a
coupling to rhodium of 110 Hz, can be assigned to the
phosphorus ligand that is in the position trans to the Bpin
group. We could not resolve any couplings to the 11B nucleus.
The 11B NMR spectrum of 3 shows one broad signal at d =
A signal at d = À10.18 ppm in the 1H NMR spectrum of 6
confirms the presence of the hydrido ligand. The 31P NMR
spectrum shows rhodium–phosphorus coupling constants of
98 Hz and 75 Hz for the doublet of triplets at d = 8.1 ppm and
the doublet of doublets at d = 2.3 ppm, respectively, reveal the
presence of a RhIII compound.[12] The relatively small
coupling constant of 75 Hz suggests that the two equivalent
phosphine ligands are located trans to the boryl ligands. This
arrangement is compatible with a fac configuration for 6; for
1
46.5 ppm (Dn = = 338 Hz); this chemical shift is typical for a
2
rhodium derivative of a 1,3,2-dioxaborolane.[16]
NMR spectroscopic investigations revealed that 3 is not
very stable in benzene and converts slowly into the hydride
[Rh(H)(PEt3)3] (5) at room temperature, with concomitant
formation of PhBpin (Scheme 2). In the presence of B2pin2,
complex 5 reacts further to give the RhIII complex cis-fac-
[Rh(H)(Bpin)2(PEt3)3] (6).
1
complex 7, J(Rh,P) = 78 Hz for the phosphorus atom in the
position trans to the boryl ligand has been observed.[8] The
11B NMR spectrum of 6 shows one broad signal at d =
1
47.1 ppm (Dn= = 633 Hz), which confirms the presence of
2
the boryl ligands.[8]
À
To avoid activation of the aromatic C H bonds on 3, the
phenoxo complex [Rh(OPh)(PEt3)3] (2) and the fluoro
compound [Rh(F)(PEt3)3] (4) were treated stoichiometrically
with B2pin2 in Me3SiSiMe3 as a solvent. Under these reaction
conditions, boryl complex 3 was observed as the sole product
(Scheme 1). The stability of 3 in Me3SiSiMe3 enabled us to
perform further studies on the reactivity of 3 towards the
activation of carbon–fluorine bonds.[6]
Treatment of 3 with perfluoropropene in Me3SiSiMe3 gave
=
the propenyl compound [Rh{(Z)CF CF(CF3)}(PEt3)3] (8a)
=
and the isomeric complex [Rh{C(CF3) CF2}(PEt3)3] (8b) in a
ratio of 2:7 (Scheme 3). Therefore, compound 3 might indeed
be an intermediate in the conversion of hexafluoropropene
and HBpin into Bpin derivatives of trifluoropropane, as has
been previously proposed.[8] The formation of complex 8a has
been previously reported from the treatment of [Rh(H)-
(PEt3)3] (5) with perfluoropropene.[14c] The NMR spectra of
8b show the expected splitting pattern with a geminal
fluorine–fluorine coupling constant of 61 Hz in the
19F NMR spectrum.[18]
Scheme 2. Formation and reactivity of rhodium–boryl complexes.
À
These observations indicate that C H activation of
Fluorinated pyridines are also interesting substrates for
À
benzene may occur at [Rh(Bpin)(PEt3)3] (3), possibly via a
RhIII intermediate, [Rh(H)(Ph)(Bpin)(PEt3)3], although this
could not be detected.[4] Reaction of 3 in C6D6 furnished
(D5C6)Bpin and [Rh(H)(PEt3)3] (5). 1H NMR EXSY spectra
(400 MHz; EXSY= exchange spectroscopy) of 5 confirmed
exchange between the hydrogen atom that is bound to the
metal and the CH2 and CH3 hydrogen atoms of the cis
phosphine ligands. It appears that intramolecular cyclometa-
lation processes are occurring, which result in the transfer of
deuterium atoms from being bound to the metal onto the
phosphine alkyl groups.[17]
investigating C F activation reactions with boryl complex
3.[19–21] For instance, the reaction of C5NF5 with [Rh(H)-
(PEt3)3] (5) gives [Rh(4-C5NF4)(PEt3)3] (9a) and HF,[20] whilst
reaction with [Rh(SiPh3)(PMe3)3] revealed the isomeric
compounds [Rh(2-C5NF4)(PMe3)3] and [Rh(4-C5NF4)-
(PMe3)3] in a 3:1 ratio, along with FSiPh3.[19,22] When 2,3,5,6-
tetrafluoropyridine was used as the substrate, a delicate
À
À
balance between C H and C F activation was observ-
ed.[6,21e,23] Thus, reaction with [Rh(H)(PEt3)3] (5) furnished
[7]
À
9a by C H activation, whereas reaction with [Rh(SiPh3)-
(PMe3)3] gave a mixture of products that were derived from
[19]
À
À
We have no indication for the generation of HBpin. It
seems that reductive elimination from 6 to afford rhodium(I)–
boryl complex 3 does not occur under the reaction conditions.
This observation might explain why it was not possible to
activation at the 2-(C F) and 4-(C H) positions.
Reaction of 3 with 2,3,5,6-tetrafluoropyridine gave the
À
C H activation product [Rh(4-C5NF4)(PEt3)3] (9a) and
À
HBpin, with no observed C F bond cleavage (Scheme 3).
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the Lewis acidity of the accepting phosphine ligand. By
analogy, we reasoned that an equivalent boryl-assisted
process may be possible for complex 3, wherein the formally
sp2-hybridized boron atom may be a particularly effective
Lewis acid.
The mechanism and selectivity of the reaction of penta-
fluoropyridine with 3 were investigated using DFT calcula-
tions on model complex [Rh(Bpin)(PMe3)3] (3’).[27] Both
concerted oxidative addition and boryl-assisted processes
À
were considered, and the lowest energy C F activation
transition states for reaction at the 2- and 4-positions are
shown schematically in Figure 1.
Figure 1. Computed transition states and energies [kJmolÀ1] for C F
activation of pentafluoropyridine at [Rh(Bpin)(PMe3)3] (3’).
À
À
Scheme 3. C F Activation at the rhodium–boryl complex 3.
However, treatment of in situ generated 3 with pentafluoro-
The most accessible oxidative addition process corre-
sponds to activation at the 4-position (DE° = + 24.2 kJmolÀ1
cf. + 46.9 kJmolÀ1 at the 2-position). These processes give
stable mer-[Rh(Bpin)(F)(4-C5NF4)(PMe3)3] and mer-[Rh-
(Bpin)(F)(2-C5NF4)(PMe3)3] complexes, in which the fluoride
ligand is trans to the Bpin group. However, subsequent
isomerization and reductive elimination of FBpin to give
[Rh(4- or 2-C5NF4)(PMe3)3] (9’a or 9’b) is facile (see the
Supporting Information). Therefore, oxidative addition of the
À
pyridine selectively gave the C F activation product [Rh(2-
C5NF4)(PEt3)3] (9b), with FBpin by-product at room temper-
À
ature. Thus, C F activation at the boryl species 3 occurs
exclusively at the 2-position of the fluorinated substrate. The
31P NMR spectrum of 9b showed a doublet of doublets at d =
16.0 ppm and a resonance at d = 19.7 ppm, with a complex
splitting pattern arising from coupling with phosphorus,
rhodium, and fluorine atoms. The four signals at d = À87.7,
À125.0, À155.4, and À175.9 ppm in the 19F NMR spectrum
indicate the presence of the pyridyl ligand, with the metal at
the 2-position.[19,21]
À
C F bond is the overall rate-determining step and, most
importantly, indicates a kinetic preference for reaction at the
4-position, which is at odds with experimental observations.
À
À
C F bond activation of pentafluoropyridine most fre-
In contrast, boryl-assisted C F activation clearly favors
quently involves reaction at the 4-position, para to the
nitrogen, and such selectivity is often associated with radical
pathways or those involving initial nucleophilic attack.
Activation at the 2-position is less common, although it has
been suggested that it may occur by a concerted oxidative
addition process.[6,21,24] Therefore, we wanted to investigate
this unusual selectivity in the reaction of 3 with pentafluoro-
pyridine.
the 2-position (DE° = + 17.2 kJmolÀ1 cf. + 74.9 kJmolÀ1 at
the 4-position). Both processes lead directly to 9’a/b and
À
FBpin. Moreover, boryl-assisted C F activation at the 2-
position is now more accessible than oxidative addition at the
4-position, and so this pathway accounts for the experimen-
tally observed selectivity. One factor that might contribute to
the greater accessibility of the boryl-assisted process at the 2-
position is the short Rh···N contact in the transition state
(2.22 ꢀ); donation of electron density from the nitrogen atom
lone pair serves to stabilize the rhodium center (Figure 2).[25c]
We also experimentally investigated the reactivity of 3 in
catalytic borylation reactions of pentafluoropyridine. In
Me3SiSiMe3 solvent, we anticipated a selective borylation
reaction at the 2-position of pentafluoropyridine, because the
formation of [Rh(H)(PEt3)3] (5) would not occur. As
À
Recently, a novel mechanism for aromatic C F bond
activation has been characterized at electron-rich iridium and
platinum metal centers.[25] This process involves the addition
À
À
of a C F bond across a {M PR3} moiety via a four-centered
transition state, which can account for the selective activation
at the 2-position of pentafluoropyridine and {Ni(PEt3)2}
fragments.[25,26] Key to such phosphine-assisted processes is
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Communications
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Figure 2. Key computed distances [ꢃ] in the boryl-assisted transition
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À
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Scheme 4. Catalytic formation of a pyridyl boronate ester.
À
C F bond has also been reported by Marder, Perutz, and co-
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À
[Rh(2-C5NF4)(PMe3)3] or [Rh(4-C5NF4)(PMe3)3], with B2cat2
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À
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In conclusion, a 16-electron rhodium(I)–boryl complex
À
(3) has been synthesized which undergoes C H activation at
room temperature with benzene to give PhBpin. Reaction
with fluorinated substrates, such as pentafluoropyridine and
À
perfluoropropene, results in C F activation products. DFT
À
calculations suggest that C F activation proceeds along a
boryl-assisted pathway that involves direct transfer of fluorine
onto the boron center via a four-membered transition state.
This mechanism also shows that the selective activation at the
2-position arises because of a stabilizing Rh···N interaction in
the transition state. Investigation of the catalytic borylation
reaction led to the formation of tetrafluoropyridyl boronate
esters, which can provide new fluorinated building blocks.[28]
Note that it is extremely difficult to access tetrafluoropyr-
idines that are further functionalized at the 2-position.[6,21]
Received: February 22, 2010
Published online: April 23, 2010
À
Keywords: boryl ligands · borylation · C F activation ·
C H activation · rhodium
.
À
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