Selective C-F bond activation of tetrafluorobenzenes by nickel(0) with a nitrogen donor analogous to N-heterocyclic carbenes

Article abstract of DOI:10.1002/anie.200806048

(Chemical Equation Presented) N, not NHC: A neutral, basic, strong σ-donor nitrogen ancillary ligand with properties analogous to those of N-heterocyclic carbenes (NHCs) was developed to aid in the oxidative additions of challenging substrates to late tra

Full text of DOI:10.1002/anie.200806048

Angewandte
Chemie
DOI: 10.1002/anie.200806048
À
C F Bond Activation
À
Selective C F Bond Activation of Tetrafluorobenzenes by Nickel(0)
with a Nitrogen Donor Analogous to N-Heterocyclic Carbenes**
Meghan E. Doster and Samuel A. Johnson*
N-heterocyclic carbenes (NHCs) have gained recent prom-
inence as alternatives to the ubiquitous phosphine donors as
ancillary ligands for both stoichiometric and catalytic trans-
formations.[1–5] The strong s-donor abilities[6] of this class of
ligands have a profound effect on reactivity; for example, the
use of these donors often permits the oxidative addition of
substrates that are otherwise unreactive.[7] Although these
donors are commonly described as predominantly carbene-
like in character, ylid resonance structures with carbanion
character may also be drawn (Scheme 1).
Scheme 2. Synthesis of donor 1 and its structure.[14]
deprotonation of the amino nitrogen atom with NaH provides
1 as a white, sublimable, toluene-soluble powder in 85%
yield. Species 1 has two viable resonance structures that could
B. Mes=2,4,6-Me3C6H2
Scheme 1. Carbene Structure A and zwitterionic resonance structures
describe its ground state. The imine form has minimized
charge separation, whereas the zwitterionic form benefits
We sought to modify nitrogen donors using a similar
strategy. Amido donors (R2NÀ) are well known to stabilize
high-oxidation-state early-transition-metal complexes by
virtue of the fact that they are hard donors capable of both
s and p donation.[8] These donors should be ideal for
promoting oxidative addition reactions with the late transi-
tion metals. However, the excessively hard donor properties
of amido ligands and the strongly p-antibonding interactions
between the occupied metal d orbitals and the nitrogen-based
lone pair often renders these donors too reactive for use as
ancillary ligands with these low-valent soft metals.[9–13] The
anionic charge of amido donors also impedes the utility of
these donors in catalysis; the low oxidation state of the
majority of active species in late-transition-metal catalysis
mandates the use of neutral ancillary ligands to maintain
sufficient reactive sites. A nitrogen-donor ligand with amido-
donor-like properties but a net neutral charge and diminished
p-donor abilities could have an impact similar to NHCs.
A synthetic route to such a nitrogen donor is shown in
Scheme 2. The alkylation of 4-(isopropylamino)pyridine at
the pyridine nitrogen atom with MeI and subsequent
from aromatic stabilization.
The solid-state structure of 1 was determined by X-ray
crystallography, and an ORTEP depiction is shown in
À
Scheme 2. The C(4) N(1) bond length of 1.3044(15) ꢀ
confirms that 1 displays considerable imine character; typical
À
bond lengths for single and double C N bonds are 1.47 and
1.28 ꢀ, respectively. Likewise, the C(4) C(5) bond of
1.4532(16) ꢀ is longer than a typical aromatic C C bond,
À
À
À
and the C(5) C(6) bond of 1.3474(17) ꢀ is much shorter. The
À
C(6) N(2) bond of 1.3709(15) ꢀ is longer than the expected
1
À
1.34 ꢀ for pyridine N C bonds. The H NMR spectrum of 1 in
C6D6 displays four proton environments for the nitrogen-
containing ring, owing to the considerable double-bond
À
character of the C(4) N(1) bond. Irrespective of this consid-
erable imine character, the reactivity of 1 resembles that of an
amido salt, as exemplified by its behavior as a strong base in
aqueous solution.
Adducts generated from [{(CO)2Rh(m-Cl)}2] have been
used to determine the donor properties of NHCs by
measurement of the CO stretching frequencies.[15] To provide
a comparison, donor 1 was treated with half an equivalent
of [{(CO)2Rh(m-Cl)}2] to generate cis-[(CO)2RhCl-
(iPrNC5H4NMe)] (2, Scheme 3). The solid-state structure of
2 was determined by X-ray crystallography and confirms
binding of the nitrogen donor to the rhodium center with
minimal perturbation of the C N and C C bond lengths
compared to those of the free ligand 1. The IR spectrum of 2
displays two CO stretching frequencies at 2077 and 1998 cmÀ1
(av 2038 cmÀ1). By this measure, 1 is a significantly stronger
donor than the pyridine analogue, which displays an average
nCO = 2052 cmÀ1.[16] Remarkably, the average nCO value for 2 is
[*] M. E. Doster, Prof. Dr. S. A. Johnson
Department of Chemistry and Biochemistry, University of Windsor
Windsor, ON, N9B 3P4 (Canada)
À
À
Fax: (+1)519-973-7098
E-mail: sjohnson@uwindsor.ca
[**] We acknowledge NSERC for a research discovery grant and for a
postgraduate scholarship for M.E.D.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806048.
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Scheme 3. Synthesis of the rhodium carbonyl adduct 2 and its
structure.[14]
similar to that of the analogous complex of the N-heterocyclic
carbene A, which displays nCO values of 2081 and 1996 cmÀ1
(av 2039 cmÀ1).[17] This comparison indicates that the donor
properties of 1 could resemble those of the NHCs.
To test the ability of these ligands to aid in the oxidative
addition of challenging substrates, we investigated the acti-
[18–21]
À
vation of the strong and relatively inert C F bonds
in
fluorinated aromatics by nickel(0).[22–30] It is known that
sources of the {(PEt3)2Ni} moiety react with the tetrafluor-
obenzenes, but the reaction takes weeks at room temperature
and can provide unwanted byproducts by rapid and reversible
C H bond activation.[31] Very recently it has been shown that
À
the use of N-heterocyclic carbenes rather than phosphines as
the ancillary ligand in these reactions allows for more rapid
À
selective activation of C F bonds in a variety of polyfluor-
obenzene species; however, to date no nickel complex
capable of the selective activation of the tetrafluorobenzenes
has been reported.[32]
À
Scheme 4. C F bond activation with C6F6, C6F5H, and all three isomers
of C6F4H2. Site of selective activations shown in bold; chemical shifts
from 19F{1H} NMR spectra are given with respect to CFCl3 at
d=0.0 ppm.
As monitored by 1H NMR spectroscopy, toluene solutions
of [Ni(cod)2] (cod = 1,5-cyclooctadiene) and 1 showed no
significant replacement of the 1,5-cyclooctadiene donors, as
might be expected for a hard donor such as 1. However,
solutions of two equivalents 1 and one equivalent [Ni(cod)2]
react over the course of 0.5–5 h at room temperature with
C6F6, C6F5H, 1,2,4,5-, 1,2,3,4-, and 1,2,3,5-tetrafluorobenzene
À
to form C F bond-activated products 3–7, respectively
(Scheme 4). The products precipitated from the reaction
mixtures as red crystalline solids and were isolated in 70–80%
yields.
All three components (Ni0 source, ligand, and fluorinated
substrate) are necessary to observe a reaction; no reaction
was observed in solutions of either [Ni(cod)2] with the
Figure 1. Structure of 3 as determined by X-ray crystallography.[14]
Selected bond lengths [ꢀ]: Ni(1)–N(1) 1.9256(15), Ni(1)–F(1)
1.8589(15), Ni(1)–C(10) 1.903(3).
polyfluoroaromatics or
1 with the polyfluoroaromatics.
Also, no reaction was observed in these systems when
alternate nitrogen donors such as bipyridine or the imine
tBuCHNPh[33] were used in place of 1.
The solid-state structure of 3 was determined by X-ray
crystallography, and an ORTEP depiction is shown in
Figure 1. Product 3 is square-planar, with the ancillary ligands
trans disposed. The nitrogen-containing and pentafluoro-
phenyl rings all lie out of the coordination plane, and the
isopropyl substituents of the ancillary ligands are situated on
1
Both the H and 19F{1H} NMR spectra for 3–7 in CD2Cl2
À
are consistent with regioselective C F bond activation, with
no detectable impurities or byproducts. The 19F{1H} shifts for
3–7 are summarized in Scheme 3. Notably, the chemical shift
for the fluoride resonance is dramatically affected by the
substitution pattern of the fluoroaryl group, with the presence
ortho and meta fluorine substituents having a larger effect
than the para fluorine substituents on the fluoride shift. The
similarity of the 1H NMR spectra for the ancillary ligand
À
opposites faces of the square plane. The Ni(1) N(1) bond
length is 1.9256(15) ꢀ, which lies within the range of nickel
amide bonds (1.93–1.82 ꢀ)[34] and is shorter than nickel imine
bonds, which are typically longer than 2.0 ꢀ.
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The site of activation in 4–7 was readily confirmed by
reaction with a solution of 10% HCl in D2O and subsequent
extraction into C6D6 and filtration through silica gel to
remove any protonated 1 and nickel-containing byproducts.
The site of deuteration in the resultant fluorobenzene
derivatives was readily determined by NMR spectroscopy
by using the deuterium isotope effect on the 19F chemical
shifts. The regioselectivity observed is identical to the
preferred products of typical nucleophilic aromatic substitu-
tion reactions with these substrates, but with significantly
improved regioselectivity.[35]
[17] W. A. Herrmann, K. Oefele, D. Von Preysing, E. Herdtweck, J.
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This donor set facilitates regioselective C F bond activa-
À
tion, which includes the first example of selective C F bond
activation of the tetrafluorobenzenes by a nickel complex.
These results illustrate the strong N-heterocyclic-carbene-like
s-donor properties of 1 and the ability of this hard ligand to
facilitate difficult oxidative additions. In light of the remark-
able scope and impact of N-heterocyclic carbenes, the
investigation of these simple nitrogen donors as alternative
ligands in similar applications is warranted.
Received: December 11, 2008
Published online: February 6, 2009
À
Keywords: amides · C F activation · fluorides · nickel ·
.
zwitterions
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