Reactions of Fluoroalkenes with an Aluminium(I) Complex

Article abstract of DOI:10.1002/anie.201802321

A series of industrially relevant fluoroalkenes react with a monomeric Al^I complex. These reactions break strong sp² and sp³ C?F bonds, and result in the formation of a diverse array of organoaluminium compounds. Mechanistic studies show that two mechanisms are likely in operation: 1) direct oxidative addition of the C?F bond to Al^I occurs with retention of alkene stereochemistry, and 2) stepwise formation and decomposition of a metallocyclopropane intermediate occurs with inversion of alkene stereochemistry. As part of this mechanistic analysis, we have isolated the first aluminium metallocyclopropane complex from oxidative addition of an alkene to Al^I. Remarkably this reaction is reversible and reductive elimination of the alkene occurs at higher temperature reforming Al^I. Furthermore, in selected cases the organoaluminium products are susceptible toward β-fluoride elimination to yield a double C?F activation pathway.

Full text of DOI:10.1002/anie.201802321

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Accepted Article
Title: Reactions of Fluoroalkenes with an Aluminium(I) Complex
Authors: Mark Richard Crimmin, Clare Bakewell, and Andrew White
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802321
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10.1002/anie.201802321
Angewandte Chemie International Edition
Reactions of Fluoroalkenes with an Aluminium(I) Complex
Clare Bakewell, Andrew J. P. White and Mark R. Crimmin*
catalytic copper salts in the presence of a base.18,19 In related
studies, Braun and co-workers have documented the
Abstract: A series of industrially relevant fluoroalkenes react with a
monomeric Al(I) complex. These reactions break either strong sp2 or
sp3 C–F bonds and result in the formation of a diverse array of
hydroboration, hydrosilylation and hydrogermylation of a series of
organoaluminium compounds. Mechanistic studies show that two
mechanisms are likely in operation: Oxidative addition of the C–F
bond to Al(I) occurs with retention of alkene stereochemistry, while
stepwise formation and decomposition of a metallocyclopropane
fluoroalkenes, including HFOs, by Lewis acidic heterogeneous
catalysts or homogeneous rhodium catalysts. These
hydrofunctionalisation reactions often lead to complex mixtures of
products due to competing addition and elimination pathways.20-
intermediate occurs with inversion of alkene stereochemistry. As part
22
of this mechanistic analysis, we have isolated the first aluminium
In this paper, we report a method to convert HFOs in to
metallocyclopropane complex from oxidative addition of an alkene to
organoaluminium reagents. While we and others have previously
Al(I). Remarkably this reaction is reversible and reductive elimination
of the alkene occurs at higher temperature reforming Al(I).
Furthermore, we show that, in selected cases, the organoaluminium
shown that the monomeric Al(I) reagent 1 reacts with C–F bonds
of fluoroalkanes and fluoroarenes,23,24 here we study industrially
relevant fluoroalkenes. The methodology is an example of a
products are susceptible toward β-fluoride elimination yielding a
growing number of approaches in which a low-valent main group
double C–F activation pathway.
metal mimics the redox behaviour of a transition metal.24-30 As part
of this analysis we have isolated an unprecedented aluminium
In 2015 the European Union made a commitment to contain,
metallocyclopropane complex and show that the reaction of an
alkene with 1 is reversible.
reduce, and replace gaseous hydrofluorocarbons (HFCs).1
Commonly used as refrigerants and aerosols, HFCs are potent
contributors to global warming whose use has expanded 10-15 %
per annum in recent years.2 Fluoroalkenes, specifically
hydrofluoroolefins (HFOs), have been billed as next generation
refrigerants and are being marketed as direct replacements for
HFCs.3,4 While HFOs possess significantly lower global warming
potentials than HFCs,5 questions have been raised about their
long term impact in the environment. HFOs are known to degrade
to form trifluoroacetic acid (TFA).6 The ecological effects of TFA
accumulation are unsubstantiated, but given its long lifetime and
high Brønstead acidity, it may become detrimental to the
environment if the concentration remains unchecked.7 In parallel
Scheme 1. Reaction of 1 with the stereoisomers of HFO-1234ze
to these practical applications of fluorocarbons, synthetic
sp2 C–F Bond Activation of Fluoroalkenes: The addition of an
excess of (E)-1,3,3,3-tetrafluoropropene (HFO-1234ze) to a 18-
75 mM solution of 1, in C6D6 at 298 K, resulted in the immediate
formation of a 4:1 mixture of 2-E and 2-Z (Scheme 1). The
products could be separated by fractional crystallisation in n-
hexane and were characterized by multinuclear NMR
chemists have developed increasingly sophisticated methods to
install fluorine containing groups into pharmaceuticals or
agrochemicals. For example, reagents that effect trifluoro-,
difluoro- and fluoromethylation of aromatic and heteroaromatic
rings are now well established.8-10
Based on the emerging trends, there is a clear advantage to
developing new chemical transformations that use fluoroalkenes
spectroscopy and X-ray diffraction (Figure 1). The product ratio
as fluorinated chemical building blocks. To date however, the
majority of reactions of HFOs involve either C–C or C–H bond
and repeating the reaction in the presence of a radical inhibitor
formation, limiting the opportunity for further synthetic
was found to be independent of the initial reaction concentration
(9,10-dihydroanthracene) did not affect the selectivity. Monitoring
elaboration.11-17 Only recently have methods emerged to convert
unreactive C–F bonds of HFOs into C–B, C–Si or C–Ge bonds.
For example, Hosoya and co-workers presented an elegant
defluoroborylation of (E)-1,3,3,3-tetrafluoropropene and 2,3,3,3-
tetrafluoropropene with B2pin2 (bis(pinacolato)diborane) using
reactions by 19F and 1H NMR spectroscopy revealed that ratios of
2-E:2-Z do not change as a function of time. Moreover, attempts
to equilibrate a pure sample of 2-E to a mixture of 2-E and 2-Z by
heat (353 K in C6D6) or light (sunlight) were ineffective. These
data all suggest that isomerization of the C=C bond of the
fluoroalkene must occur on the pathway to C–F bond activation.
In line with this hypothesis, the reaction of 1 with (Z)-1,3,3,3-
tetrafluoropropene led to the formation of a mixture of 2-E and 2-
Z, in a 0.9:1 ratio (Scheme 1). Hence the reaction occurs with
partial erosion of the original stereochemistry of the alkene.
[a]
Dr. Clare Bakewell, Dr. Andrew J. P. White, Dr. Mark R. Crimmin
Department of Chemistry, Imperial College London, South
Kensington, London, SW7 2AZ, UK.
E-mail: m.crimmin@imperial.ac.uk
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201802321
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COMMUNICATION
Scheme 3. Reaction of 1 with trifluoropropene
sp3 C–F Bond Activation of Trifluoropropene: Expanding the
investigation to fluoroalkenes that do not contain sp2 C–F bonds
leads to equally facile sp3 C–F bond activation. The reaction of 1
with 3,3,3-trifluoropropene yields 5 (Scheme 3). The 19F NMR
spectrum of 5 displays distinctive doublet resonances of the
2
germinal difluoroalkene moiety at δ = –94.8 (d, JF–F = 61.7 Hz)
2
Scheme 2. Reaction of 1 with HFO-123yf and hexafluoropropene
and –98.0 (d, JF–F = 61.7 Hz) along with the broad singlet
1
resonance of the Al–F moiety at δ = –168.3 ppm. The H NMR
The scope of C–F bond activation was expanded. Addition of
2,3,3,3-tetrafluoropropene (HFO-1234yf) to 1 led to exclusive
activation of the sp2 C–F bond to form 3 (Scheme 2, Figure 1).
The reaction of 1 with hexafluoropropene proved more complex
with 1H and 19F NMR spectroscopy revealing the formation of two
new compounds 4a and 4b in a 1:1.7 ratio (Scheme 2).31 4a is the
result of breaking the internal sp2 C–F bond, while 4b is formed
from reaction of the terminal sp2 C–F bond, trans to the CF3 group.
Again, the products could be separated by fractional
crystallisation from n-hexane (Figure 1). In both cases the C=C
bond of hexafluoropropene is retained in the organoaluminium
compound, with comparable bond lengths of 1.310(6) and
1.325(4) Å in 4a and 4b respectively. While the reactivity of
hexafluoropropene with 1 is consistent with many examples of
addition of nucleophiles to this substrate, 14,32-36 the precise origin
of this selectivity remains somewhat unclear.
spectrum of 5 has a distinctive multiplet at δ = 0.54 ppm, indicative
of a methylene group in close proximity to a Lewis acidic metal.
C–F bond activation has occurred with transposition of the C=C
bond. The structure of 5 was elucidated by single crystal X-ray
diffraction (Figure 1), the presence of the sp2 CF2 group is
evidenced by a short carbon–carbon bond length (C32-C33,
1.284(4) Å) and geometries around the terminal carbon of the
fluorinated chain. The formation of a related organometallic
species from trifluoropropene has previously been reported upon
reaction with [Cp*Ir(PMe3)H]Li and [(PEt3)3Rh(GePh3)],
respectively.34,37
A similar fluorine elimination followed by
transposition of the double bond is observed in Lewis acid
mediated 3,3-difluoroallylation reactions between 2-bromo-3,3,3-
trifluoropropene and arenes.38
Figure 1. Solid state structures of 2-E, 2-Z, 3, 4a, 4b and 5.
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10.1002/anie.201802321
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COMMUNICATION
Following metallocyclopropane formation, both α-fluoride (ΔG‡ =
15.2 kcal mol-1 and β-fluoride elimination (ΔG‡ = 10.7 kcal mol-1)
steps are calculated to occur by low-energy barriers.
Mechanistic Studies: In combination, the reactions of 1 with
fluoroalkenes possess a series of interesting mechanistic features
that require comment. These include: (i) the partial erosion of
alkene E/Z-stereochemistry during C–F bond activation, (ii) the
facile activation of both sp3 and sp2 C–F bonds of fluoroolefins
with 1, and (iii) the potential for C=C bond migration in C–F
activation. DFT calculations using the ωB97x functional and a
hybrid SDD/6,31G** basis set were undertaken to investigate the
possible C–F activation pathways. Our preliminary study focused
on both (E)-1,3,3,3-tetrafluoropropene and trifluoropropene as in
combination they allow the mechanistic points above to be
addressed.
Although the calculations support the formation of
metallocyclopropane intermediates in C–F bond activation with 1
there is limited precedent for such species. While 1 is known to
react with alkynes,26,41 reactions with alkenes have not been
reported. Attempts to identify a metallocyclopropane intermediate
during C–F bond activation were unsuccessful. The reaction
between 1 and trifluoropropene is fast even at 193 K leading only
to the formation of 5. To add weight to the mechanistic hypothesis,
the reaction of 1 with a non-fluorinated alkene was investigated.
Norbornene reacts with 1 to form the metallocyclopropane 6 over
5 h at 298 K in C6D6 solution (Figure 2c). The relief of ring-strain
undoubtedly provides a thermodynamic driving force for this
reaction and 6 could be isolated and fully characterized. The
Two viable mechanisms for C–F bond activation were
identified (Figure 2a). The first involves a direct oxidative addition
of the sp2 C–F bond to 1 (∆G‡ = 23.5 kcal mol-1). This mechanism
occurs with retention of the alkene stereochemistry.39,40 The
alternative pathway involves reaction of 1 with the alkene to form
a metallocyclopropane intermediate (∆G‡ = 27.1 kcal mol-1) that
undergoes subsequent α-fluoride elimination. α-Fluoride
elimination was calculated to occur through a concerted transition
state (Figure 2a, Newman projection). The formation and
decomposition of the metallocyclopropane destroys and
reconstructs the C=C bond with inversion of the stereochemistry.
Consistent with experiment the difference in the calculated
activation energies between the retention and inversion pathways
is small (∆∆G‡ = 3.6 kcal mol-1) with the former pathway having
the lower barrier. While based on this method alone it would be
expected that the retention pathway should dominate, upon
exploring a series of functionals (Table S3) it was found that the
∆∆G‡ varies between 0.6 – 3.8 kcal mol-1. The lower end of this
range is consistent with competitive retention and inversion
pathways which, in combination, explain the partial stereoerosion
observed in experiments of HFO-1234-ze with 1.
single crystal X-ray structure shows that
6 contains a
metallocyclopropane unit with a long C–C bond of 1.623(6) Å. The
Al–N bond distances of 1.8924(13) Å in 6 are 0.07 Å shorter than
those found in 1, while the N–Al–N angle of 96.30(8) is ~6o is more
obtuse.25 The solid-state data are consistent with
metallocyclopropane formation being defined as an oxidative
addition reaction. The bond lengths and angles around aluminium
reflect a change from the +1 to +3 oxidation state and the higher
charge density at the metal, an assignment that is supported by
DFT calculations which show that the charge on Al in 6 is far more
electropositive than in 1 (supporting information, Table S5).
The addition of norbornene to 1 is reversible and at high
temperatures (>333 K) the alkene is released and
1 is
regenerated, as demonstrated in the variable temperature 1H
NMR spectrum (Figure S1). While reversible alkene coordination
to transition metals is a fundamental step often invoked in
catalysis, examples of reversible reaction with main group
reagents are exceptionally rare.42,43 Due to the reductive
elimination of norbornene from 6 being accessible, at high
temperatures 6 can act as a ‘masked’ Al(I) reagent and will react
with 3,3,3-trifluoropropene to yield 5.24
The reaction of 1 with 3,3,3-trifluoropropene is also calculated
to proceed by a metallocyclopropane intermediate (Figure 2b). In
this case β-fluoride elimination results in the formation of 5 and
invoking this intermediate provides an explanation for the
transposition of the C=C double bond observed in experiment.
Figure 2. (a) Proposed mechanism for the reaction of HFO-1234ze with 1, (b) proposed mechanism for the reaction of trifluoropropene with 1, (c)
preparation and reactivity of a metallocyclopropane complex 6, (d) calculated reaction pathway for the reaction of 1 with norbornene and solid-state
structure of 6.
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10.1002/anie.201802321
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COMMUNICATION
Thermal Stability and Double C–F Bond Activation: Transition
metal and s-block complexes bearing partially fluorinated groups
are well known to decompose by β-fluoride elimination.17,44-47 But
while β-hydride elimination occurs in tri-iso-butylaluminium at high
temperatures,48 β-fluoride elimination is less well studied in
organoaluminium chemistry. The thermal stability of 2-5 in
benzene solution was probed to gauge the ease of fluoride
elimination. While 2-E, 2-Z, 4b and 5 were found to be stable at
high temperatures for prolonged periods of time (353 K; > 1
week), both 3 and 4a fragmented when heated to 353 K. The
organic fragments were identified as 1,1-difluoroallene (from 3)
and tetrafluoroallene (from 4a); both form alongside the expected
aluminium difluoride 1-F2 which has also been isolated and fully
characterized (Scheme 4).49-51
3D-shapes. We are currently investigating these compounds as
nucleophilic sources of {C3F5}–, {C3F3H2}– and {C3F2H3}– in
synthesis.
Acknowledgements
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 (FluoroFix: 677367) for generous funding.
Bryan J. Ward is thanked for assistance with DFT calculations.
Keywords: Carbon–Fluorine Bond Activation • HFOs •
Aluminium • Fluoride Elimination • Fluoroolefins
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