Addition of Carbon-Fluorine Bonds to a Mg(I)-Mg(I) Bond: An Equivalent of Grignard Formation in Solution

Article abstract of DOI:10.1021/jacs.6b08104

Addition of the carbon-fluorine bond of a series of perfluorinated and polyfluorinated arenes across the Mg-Mg bond of a simple coordination complex proceeds rapidly in solution. The reaction results in the formation of a new carbon-magnesium bond and a new fluorine-magnesium bond and is analogous to Grignard formation in homogeneous solution.

Full text of DOI:10.1021/jacs.6b08104

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Communication
Addition of Carbon–Fluorine Bonds to a Mg(I)–Mg(I)
bond: An Equivalent of Grignard Formation in Solution
Clare Bakewell, Andrew J. P. White, and Mark R. Crimmin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08104 • Publication Date (Web): 16 Sep 2016
Downloaded from http://pubs.acs.org on September 16, 2016
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Addition of Carbon–Fluorine Bonds to a Mg(I)–Mg(I) bond:
An Equivalent of Grignard Formation in Solution
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Clare Bakewell, Andrew J. P. White and Mark R. Crimmin*
Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, UK.
Supporting Information Placeholder
Silylene, germylene, stannylene and related alumi-
ABSTRACT: Addition of the carbon–fluorine bond
of a series of perfluorinated and polyfluorinated
arenes across the Mg–Mg bond of a simple coordina-
tion complex proceeds rapidly in solution. The reac-
tion results in the formation of a new carbon–
magnesium bond and a new fluorine–magnesium
bond and is analogous to Grignard formation in ho-
mogeneous solution.
num(I) reagents react with fluoroarenes by addition
of the carbon–fluorine bond to the main group center
(Figure 1, eq. 2-3).9
Figure 1: Carbon–fluorine bond borylation, silylation,
germylation, stannylation and alumination.
Grignard reagents are ubiquitous in synthesis.1
Widely employed and often taught in practical exper-
iments to undergraduate students, the preparation of
Grignard reagents by the simple addition of an alkyl
or aryl halide to magnesium metal has stood the test
of a century of scientific advances.1 The vast majority
of preparations employ substrates containing R–X (X
= I, Br, Cl) bonds. The use of finely divided (Rieke)
magnesium powder,2 or metal vapor synthesis,3 how-
ever, has allowed extension of the reaction scope to a
handful of challenging substrates including those
containing carbon–fluorine bonds.3 In more recent
years there has been growing interest in the use of
main group reagents for the functionalization of
strong carbon–fluorine bonds in organic molecules.4
Reactions that transform an unreactive carbon–
fluorine bond to a reactive and polarized carbon–
element bond (element = boron, aluminum, silicon)
are gaining increasing attention.5-8
For example, diborane reagents react with either
monofluoroarenes or partially fluorinated arenes in
the presence of nickel precatalysts, in combination
with either phosphine or N-heterocyclic carbene lig-
ands and additives.6a-c Similarly, the reaction of
bis(pinacolato)diborane with fluoroarenes bearing a
2-pyridyl directing group can be catalyzed by
[Rh(1,5-COD)2][BF4] in the presence of 2 equiv.
KOAc.6d These reactions can be categorized in terms
of a formal 1,2-addition of the carbon–fluorine bond
across the B–B bond of the diborane reagent (Figure
1, eq. 1). In related studies, a number of low-valent
main group species have been shown to be capable of
the oxidative addition of carbon–fluorine bonds.
The seminal discovery by Jones, Stasch and Green
of 1, a coordination complex containing a Mg–Mg
bond (Figure 1, BDI = κ2-{2,6-iPr2C6H3NCMe}2CH),10
offers the possibility of studying the addition of car-
bon–fluorine bonds across a Mg–Mg bond in homo-
geneous solution. Despite important work demon-
strating that 1 can act as a net 2-electron reductant,11
reactions with alkyl or aryl halides are to the best of
our knowledge unreported. Herein, we demonstrate
that 1 undergoes facile reactions with fluoroarenes to
generate the corresponding magnesium fluoride and
aryl reagent (Figure 1, eq. 4). The reaction does not
require a directing group in the substrate, nor toxic
or expensive catalysts.
Upon addition of 10 equiv. of C6F6 to a 0.02 M so-
lution of 1 in C6D6 an instant color change from yel-
low to colorless was observed. Monitoring the reac-
tion by 19F NMR spectroscopy revealed clean for-
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mation of new Mg–C6F5 and Mg–F moieties as evi-
–2.176(2) Å. Close contact of a fluorine atom with the
1
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8
magnesium center of 2b is evidenced by the asym-
metry in the Mg---F distances to the two ortho-
fluorines of 3.2 and 3.4 Å, accompanied by a small
canting of the fluoroarene of ~ 5o toward the side
with the closest Mg---F contact.
denced by new resonances at δ = -119.6, -156.0, -
1
156.9 ppm and δ = -187.8 ppm respectively. H NMR
data at the same time point demonstrated a set of
broad resonances consistent with a single ligand en-
vironment. The complexity of these observations was
resolved by adding a drop of THF at which point the
equilibrium mixture collapses to 2:1 mixture of
[BDIMg(THF)(C6F5)] and [BDIMg(µ-F)(THF)]2. The
same mixture was obtained upon reaction of the THF
solvate of 1 with C6F6.
Table 1: Scope of the addition of fluoroarenes to 1.
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A synthetic procedure was developed using these
observations. The scope of this reaction is presented
in Table 1 and includes a series of perfluorinated and
partially fluorinated arenes. Yields measured by NMR
spectroscopy are consistently high (80-95%). The
products could be separated by fractional crystalliza-
o
tion of the magnesium fluoride at -35 C and further
purification of the magnesium aryl complexes
achieved by recrystallization from n-hexane (38-
54%). In the case of perfluoroarenes, the reaction
occurs at a single carbon–fluorine bond, and the rea-
gent can be forced to the ortho-position by inclusion
of a 2-pyridyl directing group. Preparations using
partially fluorinated arenes are less selective and
while mixtures of regioisomeric products are ob-
tained, in all cases the carbon–fluorine bond that
breaks is that with at least one ortho-fluorine sub-
stituent.12 This regiochemistry complements that
reported by Marder, Radius and coworkers, and C–F
borylation using Ni-catalysis proceeds at positions
adjacent to an ortho-hydrogen substituent.6d For 2j
and 2k the major products are accompanied by a
side-reaction
and
small
amounts
of
C–H bond cleavage of the acidic proton was observed
(~5% NMR scale; 15-20 % preparative scale - see
supporting information).
Complexes 2b-d where subject to single-crystal X-
ray studies, these are rare examples of crystallo-
graphically characterized magnesium fluoroaryl spe-
cies. The Mg–C bond lengths ranges from 2.1381(15)
aNMR scale reactions use 10 equiv. of fluorocarbon, bpreparative scale
reactions use 1 equiv. of fluorocarbon. a,bFor regioisomeric products
F1 is the major and F2 the minor product selectivities are consistent
across NMR and preparative scale reactions.cIsolated without THF
coordinated.
Figure 2: Structures of (a) 2b, (b) 2c and (c) 2d. Selected bond lengths (Å) and angles (o) 2b: Mg–C 2.176(2), 2c:
2.1719(18), 2d: Mg–N(31) 2.1293(12), Mg–C 2.1381(14), N(31)–Mg(1)–C 79.62(5).
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Figure 3: Gas-phase calculations using the ωB97X or ωB97XD functional (values in bold) and 6,31G+d,p (C,H,N,O,F) /
Lanl2DZ (Mg) basis set. (a) Thermodynamics of C–F bond cleavage and Mg solvation, (b) Relative stability of dimeric spe-
cies. Gibbs free energies at 298 K, all values in kcal mol-1.
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bond cleavage may not proceed through radical in-
termediates and the concerted pathway remains
most likely.15 Although a cross-over experiment be-
tween 1 and 3 results in slow formation of the
The subtle β-fluorine interaction is reflected in
DFT calculations (NBO: 2a-b, ortho-F –0.34 and –
0.35, av. F, –0.315) and VT NMR studies in which hin-
dered rotation about the Mg–C bond in 2a is ob-
served below 283 K in toluene-d8 (ΔH‡ = 7.6 kcal
o
asymmetric species 4 at 25 C (Figure 4a & 5), this
mol-1, ΔS‡ = -15.0 cal K-1 mol-1, ΔG‡
= 12.1 kcal
result could be explained by a ligand exchange reac-
tion in which the Mg–Mg core remains intact.
298K
mol-1). This weak interaction may represent an in-
termediate along a reaction coordinate to β-fluoride
elimination. In line with this finding, thermal stability
tests on reaction mixtures show that decomposition
of the products occurs slowly at 25 oC or more rapid-
ly at elevated temperatures. For example, heating the
Figure 4: Preliminary experimental support for
a
concerted pathway for C–F bond cleavage.
o
reaction mixture of 1 with C6F6 for 50 C in C6D6 re-
sulted in slow decomposition of the magnesium aryl
species over a period of 4 weeks.13 Attempts to ex-
pand the scope of carbon–fluorine bond functionali-
zation to substrates with lower fluorine content (e.g
1,2,3-trifluorobenzene) required elevated tempera-
tures and resulted in complex, inseparable, reaction
mixtures. No reaction was observed between 1 and
either fluorobenzene or α,α,α-trifluorotoluene.
The thermodynamics of carbon–fluorine bond
functionalization and THF solvation were evaluated
by DFT. Both the carbon–fluorine bond functionaliza-
tion and the subsequent THF solvation are highly ex-
ergonic (Figure 3a). These calculations provide some
insight into the complex Schlenk equilibria at play
following C–F bond cleavage; the steric demands of
the β-diketiminate ligand are such that the magnesi-
um aryl cannot form symmetric dimeric species con-
taining 3-centre 2-electron Mg–C–Mg bonds. Instead
a number of asymmetric dimers are potentially ac-
cessible (Figure 3b).
Figure 5: Crystal structure of 4. Selected bond lengths (Å)
Mg1–Mg2 2.8700(9), Mg(1)-N(1) 2.0644(12).
Jones, Stasch and coworkers have demonstrated
that 1 reacts with benzophenone, 1,3-cyclohexadiene
or tBuNC by single-electron transfer.11 Recent calcula-
tions on the addition of CO2 to 1 are consistent with a
concerted 2-electron pathway.14 Based on prelimi-
nary observations we suggest that carbon–fluorine
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To probe production of [BDIMg]•, the reaction of 1
with 9,10-dihydroanthracene or 1,4-cyclohexadiene
was conducted and no evidence for hydride abstrac-
tion observed under the conditions presented in Ta-
ble 1 (Figure 4b). Further to this, the reaction of 1
with C6F6 is not inhibited by addition of either radical
trap (Figure 4c). In combination these experiments
suggest that neither [BDIMg]• nor organic radicals
are reaction intermediates in carbon–fluorine bond
cleavage. In summary, we report the first example of
a homogeneous equivalent of Grignard formation.
Addition of carbon–fluorine bonds of fluorinated
arenes across the Mg–Mg bond of a simple coordina-
tion complex proceeds rapidly in solution. We are
continuing to study the mechanism and scope of this
reaction. We aim develop further chemical transfor-
mations of this new generation of Grignard reagents
and are currently investigating reactions with a se-
ries of electrophiles.
C.; Martin, R. J. Am. Chem. Soc., 2015, 137, 12470. (c) Niwa,
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ASSOCIATED CONTENT
The Supporting Information is available free of
charge on the ACS Publications website. Experi-
mental procedures, details of the DFT studies, single
crystal Xray data and multinuclear NMR spectra
(PDF). X-ray crystallographic data for 2a-d and 4 (cif)
AUTHOR INFORMATION
Corresponding Author *m.crimmin@imperial.ac.uk
ACKNOWLEDGMENT
We are grateful to the Royal Society for provision of a
University Research Fellowship (MRC) and to the
Leverhulme
Trust
(RPG-2015-248)
and
ERC
12. Currently we cannot discriminate a kinetic effect, that the
ortho fluorine directs the Mg–Mg reagent to an adjacent
C–F, bond from a thermodynamic one, that the weakest C–F
bonds are those flanked by fluorine atoms: see for compari-
son: (a) Macgregor, S. A.; Mckay, D; Panetier, J. A.; Whittle-
sey, M. K. Dalton Trans. 2013, 42, 7386. For activation of C–
F bonds flanked by hydrogen atoms see: (b) Selmeczy, A. D.;
Jones, W. D.; Partridge, M. G.; Perutz, R. N. Organometallics
1994, 13, 522. (c) Evans, M. E.; Burke, C. L.; Yaibuathes, S.;
Clot, E.; Eisenstein, O.; Jones, W. D. J. Am. Chem. 2009, 131,
13464. (d) Clot, E.; Eisenstein, O.; Jasim, N.; Macgregor, S. A.;
McGrady, J. E.; Perutz, R. N. Acc. Chem. Res. 2011, 44, 333.
(e) Clot, E.; Mégret, C.; Eisenstein, O,; Perutz, R. N. J. Am.
Chem. Soc, 2009, 131, 7817.
13. The fate of the organic fragment was not clear. No evidence
for a benzyne intermediate or [4+2]-cycloaddition products
was collected.
14. (a) Lalrempuia, R.; Stasch, A.; Jones, C. Chem. Sci. 2013, 4,
4383. (b) Kefalidis, C. E.; Stasch, A.; Jones, C.; Maron, L.
Chem. Commun. 2014, 50, 12318.
(FluoroFix:677367) for generous funding.
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32x28mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 11 of 12
Journal of the American Chemical Society
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190x272mm (600 x 600 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 12
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77x32mm (300 x 300 DPI)
ACS Paragon Plus Environment