Photoinduced Cobalt(III)?Trifluoromethyl Bond Activation Enables Arene C?H Trifluoromethylation

Article abstract of DOI:10.1002/anie.201711693

Visible-light capture activates a thermodynamically inert Co^III?CF₃ bond for direct C?H trifluoromethylation of arenes and heteroarenes. New trifluoromethylcobalt(III) complexes supported by a redox-active [OCO] pincer ligand were prepared. Coordinating solvents, such as MeCN, afford green, quasi-octahedral [(^SOCO)Co^III(CF₃)(MeCN)₂] (2), but in non-coordinating solvents the complex is red, square pyramidal [(^SOCO)Co^III(CF₃)(MeCN)] (3). Both are thermally stable, and 2 is stable in light. But exposure of 3 to low-energy light results in facile homolysis of the Co^III?CF₃ bond, releasing ^.CF₃ radical, which is efficiently trapped by TEMPO^. or (hetero)arenes. The homolytic aromatic substitution reactions do not require a sacrificial or substrate-derived oxidant because the Co^II by-product of Co^III?CF₃ homolysis produces H₂. The photophysical properties of 2 and 3 provide a rationale for the disparate light stability.

Full text of DOI:10.1002/anie.201711693

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Accepted Article
Title: Photoinduced Co-CF3 Bond Activation Enables Arene C-H
Trifluoromethylation
Authors: Caleb F. Harris, Christopher S. Kuehner, John Bacsa, and
Jake D. Soper
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711693
Angew. Chem. 10.1002/ange.201711693
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http://dx.doi.org/10.1002/ange.201711693

Angewandte Chemie International Edition
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COMMUNICATION
Photoinduced Co–CF Bond Activation Enables Arene C–H
3
Trifluoromethylation
[
a]
[a]
[a,b]
[a]
Caleb F. Harris, Christopher S. Kuehner, John Bacsa,
and Jake D. Soper*
Abstract: Visible light capture activates a thermodynamically inert
this foregoes metal-mediated selectivity. Accordingly, new
methods to activate M–CF intermediates are being pursued to
access the full power of organometallic C–H activation in
selective fluoroalkylations. For instance, C–CF reductive
elimination from high-valent electrophilic Group 10-11 metals
III
Co –CF
3
bond for direct C–H trifluoromethylation of arenes and
3
heteroarenes. New trifluoromethylcobalt(III) complexes supported by
a redox-active [OCO] pincer ligand were prepared. Coordinating
solvents, such as MeCN, afford green, quasi-octahedral
3
S
III
19,21,31-38
[
( OCO)Co (CF
complex is red, square pyramidal [( OCO)Co (CF
Both are thermally stable, and 2 is stable in light. But exposure of 3
3
)(MeCN)
2
] (2), but in non-coordinating solvents the
continues to garner significant interest.
Baker and co-
complexes
S
III
3
)(MeCN)] (3).
workers are using fluoride abstraction from Co–CF
3
3
9,40
to circumvent the stability of Co perfluororalkyls.
Cobalt–alkyls are among the most studied organometallic
bonds. Biological activity in 12-dependent mutases is
predicated on facile, reversible Co–C homolysis.
III
3
to low-energy light results in facile homolysis of the Co –CF bond,
●
●
releasing CF
3
radical, which is efficiently trapped by TEMPO or
B
4
1
(
hetero)arenes. The homolytic aromatic substitution reactions do not
It was
II
III
II
require a sacrificial or substrate-derived oxidant because the Co by-
proposed that reduction of methylcobalamin from Co to Co
weakens the Co–CH bond and increases the rate of Co–C
homolysis by populating a Co–CH σ* MO, which lowers the Co–
III
product of Co –CF
3
homolysis produces H
2
. The photophysical
3
properties of 2 and 3 provide a rationale for the disparate light
3
4
2
stability. Design principles for photoactivation of other strong M–CF
complexes for catalysis are discussed.
3
C bond order to 0.5. Based on the well-known proclivity of
LCo(III) complexes with redox-active phenoxyl or dioxolene
●
43
ligands to form L Co(II) redox isomers, we speculated that
intravalence charge transfer from a properly oriented, oxidizable
Introduction of
a
CF
3
group into organic molecules can
ligand might similarly activate inert cobalt–perfluoroalkyl bonds,
substantially affect biological properties including lipophilicity,
bioavailability, and capacity to penetrate the blood-brain barrier,
III
thereby opening new avenues to utilize Co –CF
3
intermediates
in trifluoromethylation catalysis. These design criteria are met by
our recently reported redox-active [OCO] pincer ligand Co ET
1
-7
making it a potent tool in agricultural and medicinal chemistry.
Numerous, varied methods for the preparation of CF -containing
Efforts to avoid
3
44
series, providing entry to novel photolytic trifluoromethylations,
8
-22
molecules have been developed.
as described below.
prefunctionalized or electronically activated starting materials
have focused attention on direct functionalization of unactivated
S
II
Treating four-coordinate [( OCO)Co (MeCN)] (1) with AgCF
3
in MeCN gave a dark green suspension (2) which became
burgundy (3) upon removal of solvent in vacuo (Scheme 1). The
dark red color of 3 is preserved in non-coordinating solvents
8
,19,23
C–H bonds,
spurring
a
“Renaissance” in radical
1
8
trifluoromethylations. These include redox and photoredox
●
methods to generate electrophilic CF
3
radicals via single
The photoredox catalysis was pioneered
using precious metal chromophores, but Cu and metal-free
2 2 2
such as CH Cl or Et O, but dissolution in coordinating solvents
1
8,22-26
electron transfer.
such as THF or MeCN regenerates the emerald green color of 2.
Green crystals of 2 were isolated from a mixture of MeCN and
benzene; red crystals of 3 were obtained from a concentrated
toluene solution at –25 ˚C. The gross geometric features of 2
and 3 are the same aside from the number of coordinated MeCN
2
7-29
variants have been described.
Efforts to develop organometallic base metal catalysts for
trifluoromethylation are challenged by the stability of M–CF
3
1
2
bonds to low valent later 3d metals. In this respect, an
molecules (Figures 1a, b). Both have the newly installed CF
3
●
advantage of the outer-sphere redox methods for CF
generation is that they bypass thermodynamically stable, inert
M–CF complexes. This is exemplified by a recently reported
Cu-catalyzed trifluoromethylation protocol, wherein formation of
3
S
group orthogonal to the ( OCO)Co plane. The Co–CF
3
bond
length of 1.8692(16) Å in 3 is the shortest Co–CF bond in the
3
3
45
CCDC database (cf. trifluoromethylcobalamin 1.878(12) Å),
indicating significant covalency. The aryl C–C bonds in the
3
0
a Cu–CF
3
bond renders the complex catalytically inactive. But
S
[
OCO] ligands (Figure S1) are consistent with fully reduced
4
6
phenoxides. Both complexes are diamagnetic. The sum of the
data suggests 2 and 3 are best formulated as low-spin Co(III)
S
2–
[
a]
b]
Dr. C. F. Harris, C. Kuehner, Dr. J. Bacsa, Prof. J. D. Soper
School of Chemistry and Biochemistry
Georgia Institute of Technology
Atlanta, GA 30332-0400 (USA)
E-mail: jake.soper@gatech.edu
Dr. J. Bacsa
X-ray Crystallography Center, Department of Chemistry
Emory University
Atlanta, GA 30322 (USA)
centers supported by closed-shell [ OCO]
bis(phenolate)
ligands, as illustrated in Scheme 1.
Consistent with the pronounced visible color changes
described above, loss of the MeCN ligand from 2 results in a
significantly altered UV–vis spectrum for 3 (Figure 1c). Five-
coordinate 3 is characterized by a broad CT absorption band
[
2
centered at 785 nm. Cooling red Et O solutions of 3 to –60 ˚C
gives a color change to green, which is completely reversed
upon warming to ambient temperature. Variable temperature
Supporting information for this article is given via a link at the end of
the document.
2
UV–vis spectroscopy of 3 in Et O showed that the green
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Angewandte Chemie International Edition
10.1002/anie.201711693
COMMUNICATION
Scheme 1.
1
9
spectrum observed at low temperatures bears strong similarities
determined by F NMR spectroscopy (Figure S8). The balance
of the CF -containing products were unreacted 3 (9%) and
to six-coordinate 2 (Figure S2), suggesting weak, reversible
3
Et
2
O binding. Heating 2 in MeCN produced an opposite effect.
α,α,α-trifluorotoluene (4%). Control reactions confirmed that (i) 3
●
Near the boiling point of the solvent, a color change from green
to orange occurred, consistent with formation of 3 (Figure S3).
These temperature-dependent phenomena are also captured in
and TEMPO are unreactive in the dark over 18 h, and (ii) no
●
reaction is observed between 2 + TEMPO after 3 h photolysis
(Figure S9). These indicate that photoexcitation of 3 is required
1
●
the H NMR spectra of 2 and 3 (Figure S5-S7). Finally, diluting a
for CF
3
transfer, but 2 is inert under the photolysis conditions
concentrated green MeCN solution of 2 with toluene, benzene,
employed herein.
CH
2
Cl
2
or Et
2
O affords red 3. In total, these data suggest that
The observation of α,α,α-trifluorotoluene in the photoinduced
●
the solution structures of 2 and 3 parallel those observed in the
solid state. Interconversion of 2 and 3 is fully reversible, and the
solution coordination geometry is highly sensitive to both solvent
composition and temperature. The labile MeCN ligand in 2 is a
reaction of
3
with TEMPO
prompted us to pursue
trifluoromethylations of other (hetero)arenes. Photolysis of 3
using the same 16W CFL for 16 h in neat benzene, mesitylene,
or pyrrole gave quantitative conversions to the corresponding
1
2
19
consequence of the strong trans influence of the CF
3
group.
mono trifluoromethylated (hetero)arenes, as determined by
F
Most importantly, the color changes upon ligand loss reflect
perturbations of the frontier molecular orbitals, which lead to
divergent photochemical reactivity.
NMR spectroscopy. Further screening experiments revealed that
the expected trifluoromethylated products are obtained in high
yields with arene loadings as low as 20 equiv vs. 3 in N-methyl-
2-pyrrolidone (NMP) solvent (Figure 2). The substrate scope and
selectivity parallels that observed in other arene functionalization
2
The UV–vis spectrum of 3 in Et O is unchanged over 18 h in
the dark at room temperature, but exposure of 3 to ambient light
gave clean conversion to 1, as shown in Figure 1d. In contrast,
subjecting 2 to the same conditions gave no measurable change
●
24,26
reactions that occur via radical CF
3
delivery.
In each
1
9
reaction, the
F
NMR resonance for 3
was completely
in the UV–vis spectrum (Figure S4). Conversion of 3 → 1 is
3
consumed after 16 h, and a signal for trifluoromethane (CHF )
●
●
balanced by loss of CF
3
, so photolysis experiments were
was observed that accounted for the balance of the CF
was not incorporated into the (hetero)arene (Figure S10). Use of
the halogenated solvents CH Cl2, CHCl or CCl gave
exclusively CF Cl after 6 h of photolysis. Aliphatic substrates,
such as pentane or cyclohexane, gave only high yields of CHF
When the reaction was performed in C 12 the characteristic
3
that
●
3
performed to assess the viability of 3 as a radical CF reagent.
●
Using 10 equiv 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO )
as the radical acceptor in benzene gave the expected
2
3
,
4
3
trifluoromethylated TEMPO–CF
3
(85%) after 3 h under a 16W
3
.
2
700K (soft white) compact fluorescent lamp (CFL), as
6
D
S
III
S
III
Figure 1. ORTEP plots of (a) [( OCO)Co CF
non-coordinated solvent molecules omitted for clarity. UV–vis absorption spectra for (c) 2 in MeCN and 3 in Et
Et O upon exposure to ambient light. Spectra are shown at t = 0 (red line), and at 2 h intervals to t = 18 h (orange line). A spectrum of isolated
3
(MeCN) ] (2) and (b) [( OCO)Co CF (MeCN)] (3). Ellipsoids are drawn at 50% probability. Hydrogen atoms and
2
3
–5
2
O at 23 °C, and (d) a sample of 6.3 × 10 M 3 in
2
S
II
[
( OCO)Co (MeCN)] (1) in Et
2
O (black line) is shown for comparison. (e) Qualitative MO diagrams showing the expected reordering of the frontier orbitals upon
loss of MeCN from 2 to generate 3.
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Angewandte Chemie International Edition
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COMMUNICATION
derive from its five-coordinate geometry as well as the redox-
S
44
activity of [ OCO] ligand.
CT absorption bands in the long-wavelength visible or near
IR are characteristic of redox-active ligand cobalt(II)/(III)
complexes that show solution bistability associated with ligand–
4
3
metal electron transfer. In this context, the intense absorption
band at 785 nm in 3 is the likely origin of its photochemical
behavior. Assuming no significant M–L π bonding, ligand-field
theory predicts the Co–CF
3
σ* MOs in both 2 and 3—dz2 when
bond—are empty in their low
spin d ground states (Figure 1e). Accordingly, in either 2 or 3,
half occupancy of dz2 weakens the Co–CF
bond by reducing the
bond order to 0.5. Loss of the MeCN ligand trans to CF
lowers the energy of dz2 relative to the redox-active ligand p
Thus photoinduced transfer of 1e from the [ OCO] ligand to dz2
is predicted to occur at a significantly lower energy in five-
coordinate 3. By this reasoning, the stability of 2 in visible light
reflects the absence of a strong CT absorptions at wavelengths
the z-axis is colinear with Co–CF
3
6
3
Figure 2. Photoinduced trifluoromethylations of arenes and heteroarenes by 3.
Reactions were performed with 20 equiv. (hetero)arene vs. 3. Yields were
determined by integration of F NMR resonances (Figures S11-S16), as
described in the Supporting Information.
3
in 2
MO.
19
π
–
S
doublet at –78.3 ppm for CHF
3
was replaced by a triplet at –79.2
ppm for CDF
of H/D.
3
(Figure S17), confirming the solvent as the source
>
470 nm. Computational studies on the photophysical
The observed trifluoromethylated arene products are
●
properties of 2 and 3 are in progress.
consistent with the reactivity expected of a free CF
3
radical,
The arene trifluoromethylations reported herein are not
inherently advantageous as compared to the many other known
which is a strong electrophile prone to reaction with electron rich
1
8
alkenes.
Re-aromatization
of
the
initially
formed
1
8,22-29
●
radical methods.
and fundamentally
thermodynamically strong, inert base metal M–CF
Rather, this approach represents a new
cyclohexadienyl radical formally requires loss of H . We
●
distinct
strategy
to
activate
speculated that H trapping by the Co(II) center in 1 (the metal
III
3
bonds for
byproduct of Co –CF
3
homolysis in 3) generates an unobserved
and
regenerate 1, as shown in Scheme 2. Related Co(III) hydrides
S
III
applications in small molecule transformations. Photoinduced
arene trifluoromethylation by 3 is to our knowledge the first
example of Co-mediated C–H trifluoromethylation, and a rare
(
OCO)Co (H) intermediate, which could produce H
2
4
7-49
are well-known intermediates in catalytic H
2
production,
but
●
28
1
base metal chromophore for photoredox CF
3
transfer. Key
molecular H
2
is not observable in the H NMR spectra of the
S
features of this system are the geometric flexibility of ( OCO)Co
arene trifluoromethylation reactions because the solubility is low
and the signal is obscured by a paramagnetic, ligand resonance.
S
44
core and the redox non-innocence of the [ OCO] ligand, which
II
S
III
3
together facilitate generation of the active Co –CF upon low
To assess the competency of ( OCO)Co (H) for H
2
production,
the proposed Co –H was generated in situ by treating a green
III
energy photoexcitation. The specific chemistry reported herein
bears strong resemblance to alkyl cobalamins, but the design
S
III
THF solution of [( OCO)Co (THF)
effervescence and clean conversion to orange 1. Transferring
the headspace to a toluene solution of Vaska’s IrCl(CO)(PPh
2
]OTf with NaH, which gives
S
principles are transferrable beyond the ( OCO)Co framework.
For instance, low-energy LMCT has already been described for
numerous metal–redox-active ligand combinations across the
3 2
)
1
complex yields the H NMR resonances characteristic of the
4
3
5
0
transition elements,
reaction chemistry.
but is underexplored in photoredox
dihydrido IrCl(H)
with a peak at 4.28 ppm for molecular H
The products—1 and H —are the same as those expected from
the homolytic aromatic substitution in Scheme 2, suggesting
2
(CO)(PPh
3
)
2
oxidative addition product, along
2
(Figures S18-S19).
S
Other properties of ( OCO)Co suggest opportunities to
2
advance
beyond
established
methods
for
radical
S
III
trifluoromethylation. First, the capacity of 1 to catalyze H
2
(
OCO)Co (H) is a viable intermediate in both reactions.
production permits direct arene C–H trifluoromethylation without
a sacrificial or substrate-derived oxidant for the cyclohexadienyl
Scheme 2.
5
1
radical intermediates. Second, the substitutional lability of the
solvent-derived ligands to Co(III) in 3 opens avenues to pursue
●
selectivity in CF
3
transfer that is inaccessible with the outer
sphere free radical methods. For instance, we have already
S
shown the ( OCO)Co core is capable of binding an array of N-
4
4
and O-donors, including nitriles, furans, and pyridines.
Radical C–H (hetero)aryl trifluoromethylation by
3
is
homolysis. But these bonds are
not inherently weak. In fact, the exceptionally short Co–CF
III
Accordingly, we envision using these functionalities as ligand
predicated on facile Co –CF
3
directing groups for intramolecular photolytic arene C–H
3
S
trifluoromethylations, where the ( OCO)Co(CF
3
) functions as
bond lengths in 2 and 3 suggest thermodynamically strong
both a chromophore and a scaffold to impart regiospecificity in
bonds, and the complexes are thermally robust in the absence
●
C–H functionalization that occurs via CF
3
transfer. These, and
of light. Moreover, the stability of 2 in visible light shows that
III
catalytic variants, are a focus of ongoing work in our lab.
photosensitivity is not a feature intrinsic to the Co –CF
3
linkage.
●
Rather, the ability of
3
3 to deliver CF upon low-energy
photoexcitation is a consequence of electronic properties that
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Angewandte Chemie International Edition
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We gratefully acknowledge financial support of this work from
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Angewandte Chemie International Edition
10.1002/anie.201711693
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Caleb F. Harris, Christopher S. Kuehner,
John Bacsa, Jake D. Soper*
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3
Photoinduced Co–CF Bond
Activation Enables Arene C–H
Trifluoromethylation
III
Visible light activates a strong Co –CF
3
bond for direct trifluoromethylation of
arenes and heteroarenes. The pincer (OCO)Co complex functions as both the
●
chromophore for CF
aromatic substitution. Co –CF
intramolecular electron transfer from the redox-active [OCO] ligand.
3
radical generation as well as the oxidant for C–H homolytic
III
3
bond activation apparently derives from facile
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