Selective Hydrodefluorination of Hexafluoropropene to Industrially Relevant Hydrofluoroolefins

Article abstract of DOI:10.1002/adsc.201900234

The selective hydrodefluorination of hexafluoropropene to HFO-1234ze and HFO-1234yf can be achieved by reaction with simple group 13 hydrides of the form EH₃ ? L (E=B, Al; L=SMe₂, NMe₃). The chemoselectivity varies depending on the nature of the group 13 element. A combination of experiments and DFT calculations show that competitive nucleophilic vinylic substitution and addition-elimination mechanisms involving hydroborated intermediates lead to complementary selectivities. (Figure presented.).

Full text of DOI:10.1002/adsc.201900234

Title: Selective Hydrodefluorination of Hexafluoropropene to
Industrially Relevant Hydrofluoroolefins
Authors: Nicholas Phillips, Andrew White, and Mark Crimmin
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900234
Link to VoR: http://dx.doi.org/10.1002/adsc.201900234

10.1002/adsc.201900234
Advanced Synthesis & Catalysis
Very Important Publication
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Selective Hydrodefluorination of Hexafluoropropene to
Industrially Relevant Hydrofluoroolefins
Nicholas A. Phillips, Andrew. J. P. White, and Mark. R. Crimmin*
Department of Chemistry, Molecular Sciences Research Hub, Imperial College London, 80 Wood Lane, London,
W12 0BZ, UK. E-mail: m.crimmin@imperial.ac.uk
: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract.
The
selective
hydrodefluorination
of A combination of experiments and DFT calculations show
hexafluoropropene to HFO-1234ze and HFO-1234yf can be that competitive nucleophilic vinylic substitution and
achieved by reaction with simple group 13 hydrides of the addition-elimination mechanisms involving hydroborated
form EH3L (E = B, Al; L = SMe2, NMe3). The intermediates lead to complementary selectivities.
chemoselectivity varies depending on the nature of the group
13 element.
Keywords: Hydrodefluorination; HFO-1234ze; HFO-
1234yf; Alane; Hydroboration
selective hydrodefluorination of hexafluoropropene
(HFP) is an attractive route to a number of
commercially relevant HFOs including HFO-1234ze
and HFO-1234yf (Figure 1). HFP itself is bulk
commodity used in the manufacture of
poly(fluoro)olefins.
Introduction
Synthetic refrigerants have improved our quality of
life and contributed the year-on-year growth of the
fluorocarbon industry.[1] The most widespread
synthetic refrigerants are volatile molecules of low
molecular mass that contain at least one halogen atom.
Refrigerants are applied in sealed compressor units in
household refrigerators, climate control systems in
cars or industrial air-conditioning units. Despite their
immediate benefit to humanity, early generations of
synthetic refrigerants and aerosols (CFCs and HFCs)
have been a disaster for the environment. This led to
legislation such as the Montreal Protocol restricting
their use.[2,3] The fluorocarbon industry has responded
by beginning the manufacture, marketing and supply
of hydrofluoroolefins (HFOs). HFOs have global
warming potentials similar to CO2 and do not deplete
ozone. These volatile molecules are our most
advanced refrigerants and hold promise as a long-term
solution to a long-standing environmental problem.[4]
Figure 1. Selective hydrodefluorination of HFP as a route
to HFOs.
The majority of known syntheses of industrially
relevant HFOs rely on multistep processes involving
partial chlorination of propane, halogen exchange to
introduce the fluorine atom and a hydrodehalogenation
step to obtain the olefin.[5-7] An approach involving the
hydrogenation and hydrodefluorination of HFP using
a chromium-based heterogeneous catalyst has also
been reported.[8] Given that the majority of the
patented industrial processes involve the redundant
construction and destruction of a C–Cl bond along
with toxic, corrosive and expensive materials, the
The selective hydrodefluorination of HFP to form
tetrafluoroolefins has limited precedent. A number of
researchers have shown that boranes react with
fluoroolefins via either catalysed or non-catalysed
pathways that can involve the generation of mixtures
from
a
combination of hydroboration and
hydrodefluorination steps.[9-13] Related alanes are also
able to hydrodefluorinate these substrates. In 2018, it
was reported that AlH3NHC (NHC = N-heterocyclic
carbene) can selectively hydrodefluorinate HFP to
form HFO-1234yf in 79% yield.[14,15]
1
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
Herein, we detail the hydrodefluorination of HFP to
commercially relevant HFO-1234ze and HFO-1234yf.
The selectivity can be guided by careful choice of
Lewis base and group 13 hydride (EH3L; L = THF,
SMe2, NMe3; E = B, Al). We show that the tailored
and expensive N-heterocyclic carbene ligand systems
are actually unnecessary for selective reactions. While
these reactions rely on the use of stoichiometric
quantities of the main group reagents, the by-products
(EF3L) are industrially relevant materials in their own
right, typically used as Lewis Acids.[16,17] Through
investigation of the mechanism we provide insight into
the origin of selectivity and as such this work may act
as a foundation for future (catalytic) routes from HFP
to industrially relevant HFOs.
concentration yielding
a
larger amount of
pentafluoropropenes. The boron containing side-
product contains diagnostic resonances in the 11B and
19F NMR spectra at δB = 3.2 (s) ppm, δF = –136.67 (br)
ppm consistent with those reported for BF3SMe2.
In contrast, the reaction of HFP with AlH3NMe3 leads
to the efficient and highly selective formation of
2,3,3,3-tetrafluoropropene (HFO-1234yf) under mild
conditions in high yield and selectivity (1 atm. 40 °C,
72 h). Initial mixing of AlH3NMe3 with HFP in
benzene-d6 results in rapid formation of a colourless
precipitate (assumed to be AlF3) and high conversion
to E-2 and Z-2, in ratio 2:1 as evidenced by 1H NMR
spectroscopy after 5 mins at 25 °C. Further HDF
reactivity is observed even at ambient temperature, but
warming the reaction to 40 °C gives optimum
selectivity and rate, yielding HFO-1234yf as the major
F-containing product (> 98%) after 72 h at 40 °C.
Small quantities of 3,3,3-trifluoropropene (< 2%) are
observed from over-reduction (Figure 2b).
Results and Discussion
Hydrodefluorination
of
Hexafluoropropene:
Surprisingly there are no reports investigating the
reaction of hexafluoropropene (HFP) with the simple
borane adducts BH3L (1, L = THF, SMe2, NMe3).
Heating a sealed J Young NMR tube containing a 0.8
mM solution of BH3SMe2 in benzene-d6 charged to 1
atm. of HFP at 100 °C forms isomers of 1,2,3,3,3-
pentafluoropropene, E-2 and Z-2 after 24 h. Ultimately
E-/Z-mixtures of 1,3,3,3-tetrafluoropropene (HFO-
1234ze) are generated as the major product after
heating for 5 d (Figure 2a).
Mechanisms of HDF: Several observations are
consistent with the alane and borane reagents reacting
by different pathways. The first of which is the ligand
dependence of reactivity. Despite the strongly bound
amine ligand, AlH3NMe3 reacts readily and with the
same selectivity as the AlH3NHC system.[14] In
contrast, the borane system BH3L shows a strong
ligand dependence with L = NMe3 being ineffectual
for HDF and L = THF highly inefficient. The latter
borane was used as a THF solution and it is likely that
exogeneous THF is acting as an inhibitor in the
reaction. Only hydrocarbon solutions of BH3L L =
SMe3 proved useful for HDF. The experiments are
consistent with the borane reagents requiring a ligand
dissociation step to react and the alane reagents not.
As part of detailed studies into the mechanism
hydroboration with BH3L, Brown and co-workers
have concluded that the reaction can be considered a
combination of a reversible ligand dissociation step to
form BH3 followed by extremely facile addition to the
alkene.[18,19] While solvent effects have historically
been somewhat contentious in this field, the
mechanism is now widely accepted. Brown and co-
workers demonstrated that strong donor ligands, such
as amines, decrease the rate of reaction.[18]
The second observation which would be consistent
with a switch in mechanism is the switch in selectivity
which is observed in changing the reagent from
AlH3L to BH3L. The latter main group hydride gives
a mixture of HFO-1234ze in low selectivity as the
major product, while the former gives almost
exclusively HFO-1234yf.
Figure 2. Hydrodefluorination of HFP with (a) BH3•L and
(b) AlH3•NMe3.
The final product distribution represents an 86% yield
of HFO-1234ze formed in a 2:1 ratio of E:Z isomers.
The remaining mass balance has been characterised as
E/Z-2 (13%) and trifluoropropene (<2%). The ratio of
products proved dependent on the reaction scale, with
preparative scale reactions albeit at the same absolute
Alane
Pathway
(concerted
SNV):
The
hydrodefluorination of HFP with AlH3NMe3 was
observed to proceed without the formation of defined
reaction intermediates. No experimental evidence was
obtained to suggest that hydroalumination of the
2
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
alkene occurs in the reaction mechanism. DFT
calculations were used to probe the plausible pathways
alongside a series of experiments investigating the
formation and reactivity of reaction intermediates.
Calculations were implemented in Gaussian09. A
series of functionals B97xD, M06L, M062x,
B3PW91-GD3BJ) were investigated to confirm that
the trends were reproducible across a number of
methods. While all the computational approaches led
to the same qualitative outcomes for every single
mechanism and reaction step, data for the B3PW91
functional are presented herein.
energy (G‡
= 35 – 37 kcal mol-1) than those for
298K
the terminal position (G‡
= 25 – 28 kcal mol-1)
298K
and are unlikely to be accessible at 40 °C (Figure 3d).
Similarly, the generation of 1,1,1-trifluoropropene by
a 3rd HDF step of HFO-1234yf at the internal site
occurs by a high energy transition state (G‡298K = 36
kcal mol-1 see supporting information TS-A8). The
relative barriers can be explained by considering the
fluorinated substrates as Michael acceptors. HFP is
known to undergo nucleophilic attack at the terminal
position with a number of reactions occurring trans to
the electron-withdrawing CF3 group.[20] In the cSNV
mechanism the CF3 moiety acts as the electron-
withdrawing group to activate the electrophile and the
hydride takes the role of the nucleophile. Nucleophilic
attack occurs at the terminal position due to a
combination of inductive (–F and –CF3) and
mesomeric effects (–F). These effects result in the
polarisation of HFP in a manner consistent with a
Michael acceptor, with the terminal carbon atom being
considerably more electrophilic than the internal one.
Comparing TS-A1 and TS-A3 (Figure 3e) it becomes
clear that the most positively charged sp2 carbon centre
is that of the terminal position. Transfer of the hydride
atom from Al to C occurs with transfer of charge to the
alkene moiety. The aforementioned polarisation of
HFP results in charge being best accommodated by the
internal carbon atom and again favours attack at the
terminal position. In TS-1 nucleophilic attack occurs
with H---C bond formation at the terminal carbon but
charge transfer to the internal carbon as evidenced by
a perturbation of the geometry of this carbon atom
from sp2 toward sp3 hybridisation (TS-A1,  =
111.1°).
In line with the lack of a significant effect of the ligand
L on reactivity, HDF was calculated to proceed by a
concerted SNV pathway involving the 4-coordinate
aluminium reagent (Figure 3a-c). Formation of E-2 is
preferred over Z-2 with activation barriers of G‡
298K
= 24.8 and 27.7 kcal mol-1 respectively. E-2 and Z-2
can undergo a subsequent concerted HDF step with
barriers of 28.3 and 28.2 kcal mol-1 respectively, both
yielding HFO-1234yf. The calculations are consistent
with the experimental data and predict the observation
of pentafluoropropenes E-2 and Z-2 as intermediates
due to the more challenging second HDF step, the
preferential formation of intermediate E-2, and
ultimately the high selectivity for the formation of
HFO-1234yf. Ligand dissociation from AlH3NMe3 is
calculated to be exergonic by 22 kcal mol-1 and is not
required for the cSNV HDF pathway to operate. In fact,
the lowest barriers for a direct HDF by the concerted
pathway are already within a reasonable range of the
ligand dissociation Go before considering any bond
making or breaking steps involving AlH3 and the
fluoroolefin.
Transition states involving hydride transfer to the
internal position of HFP, E-2, or Z-2 are higher in
Figure 3. DFT calculations for AlH3NMe3 with (a) HFP, (b) E-PFP and (c) Z-PFP. (d) Comparison of TS geometries. (e)
Comparison of NPA charges in the anionic fluoroalkene fragments of TS-A1 and TS-A3. alane = AlH3NMe3 by-product
AlFH2NMe3 not shown.
3
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
Figure 4. Reactions of BH3L with (a) HFP, (b) E/Z-2 and (c) HFO-1234yf. Along with selected data that support the
assignment of the products and regioselectivity of hydroboration. J values from simulated spectra. Peaks marked with an *
are the impurity 4PPh3 in samples of 5PPh3.
Hence, the HDF of HFP by a concerted SNV
mechanism involving a metal hydride reagent occurs
with a substrate bias for the formation of HFO-1234yf
in high selectivity. This realisation may set the
foundation for routes from HFP to HFO-1234yf by
catalytic hydrodefluorination using hydrogen as a
reductant and it is plausible that coordinatively
saturated transition metal hydride complexes may
behave in a similar manner to AlH3NMe3.
SMe2 these reactions proceeded in high-yield allowing
the in-situ generation of organoborane products as a
single regioisomer. In contrast, for NMe3 the reaction
only proceeded in low conversion, consistent with the
need for ligand dissociation prior to hydroboration and
the formation of BH3 as an intermediate. 3SMe2 was
characterised by the appearance of a diagnostic
1
resonance in the H NMR spectrum corresponding to
the –CF2H group at δH = 5.85 (ddd, 2JHF = 56.1, 54.0
3
Hz, JHF = 8.5 Hz) ppm. The data are consistent with
Borane Pathway (Addition-Elimination): The HDF of
HFP by BH3SMe2 did not lead to HFO-1234yf but
rather a mixture of E/Z-isomers of HFO-1234ze. This
reaction required more forcing conditions (100 oC for
5 d versus 40 °C for 3 d) to reach high conversion and
proceeds with a lower selectivity than observed for the
alane. Calculations show that the direct reaction of
BH3SMe2 with HFP by a concerted SNV mechanism
requires the formation prohibitively high energy
formation of a single regioisomer and Markovnikov
addition of the borane to HFP. While this intermediate
could not be isolated due to its volatility, reaction of
3SMe2 with PPh3 at 40 °C in toluene for 18 h cleanly
generated the phosphine adduct, 3PPh3, which can be
isolated as a crystalline, colourless solid (Figure 4a).
The crystal structure clearly shows the expected
Markovnikov regioselectivity with no evidence for the
formation of the anti-Markovnikov isomer in this
instance (Figure 4a). The P–B distance of 1.950(5) Å
in 3PPh3 is unremarkable and slightly elongated over
BH3PPh3 (av. 1.917 Å).[21]
transition states (G‡
= 50 – 60 kcal mol-1),
298K
effectively ruling out this pathway (see supporting
information). In order to gain further insight into role
of the borane in the HDF of HFP, and the potential for
alkene hydroboration in the mechanism, a series of
stoichiometric experiments were conducted.
The 1:1 reaction of BH3L with HFP (L = NMe3, THF,
SMe2) led to the clean formation of hydroborated
intermediates 3L (Figure 4). For both L = THF and
Reaction of BH3THF with a mixture of E-/Z-2
(generated from the partial reduction of HFP by
AlH3NMe3 followed by vacuum transfer to remove
the involatile inorganic side-products) also resulted in
hydroboration generating 4THF which could be
4
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
trapped as 4PPh3. The major product is again that of
Markovnikov addition. 4PPh3 is characterised by a
distinct multiplet at δH ~ 4.50 ppm which could be
modelled as the two diastereotopic 1H environments of
the –C(F)CFH2 group considered as an ABMX spin
system insulated from the –CF3 and B-containing
moieties (Figure 4b). Equally diagnostic was the triplet
groups primarily lead to stabilisation of the negative
charge at the adjacent carbon centre in the transition
state for hydroboration, and hence Markovnikov
selectivity. For HFO-1234yf it would seem that the
influence of the electron-withdrawing substituents is
counter-balanced by the mesomeric effect of the sp2 C–
F moiety favouring the anti-Markovnikov addition and
connection of the boron to a carbon atom that bears no
fluorine atoms.
2
resonance at δF = –220.55 (t, JF-H = 50.4 Hz) ppm in
19
the F NMR which collapsed to a singlet on proton
decoupling. Despite 4PPh3 being the major product,
it is not generated cleanly, in part this is due to the
difficulty in the selective reduction of HFP to E/Z-2,
which in our hands contains small amounts of HFP and
HFO-1234yf. The analysis is further complicated by
the realisation that E-2 and Z-2 would give rise to two
possible diastereomers of the anti-Markovnikov
isomer of 4PPh3. Based on total integration of
quantitative 19F{1H} NMR experiments it can be
determined that Markovnikov 4PPh3 forms in at least
81 % yield giving a minimum selectivity of 4:1 for the
major product. The true selectivity for the
Markovnikov isomer is likely much higher.
Accurate computation modelling of hydroboration
pathways with BH3 by DFT calculations is
surprisingly complex. Due to the extremely low
energy local barrier for the hydroboration step,
Singleton and others have commented that the
approximations of transition state theory may not
hold.[28-30] Precise determination of the regioselectivity
of the addition of boranes to even simple alkenes
requires the consideration of reaction dynamics and
modelling of reaction trajectories in place of more
standard computational approaches.[31-33] As such, we
are reluctant to use DFT calculations to model the
complete reaction mechanism of the HDF of HFP with
BH3. The complexity described above is further
confounded by the realisation that a mixture of
Commercial samples of HFO-1234yf react with
BH3THF to form an approximate 1:1 mixture of
Markovinkov and anti-Markovinkov products 5PPh3, reagents that may be involved in the HDF step, viz BH3,
following trapping with PPh3. In this instance, the
analysis is simplified due to the lower fluorine content
of the substrate, along with its availability in high
purity. Markovnikov 5PPh3 is characterised by a
diagnostic doublet for the –CH3 group δH = 1.58 (d,
3JF–H = 23.8 Hz) ppm, while the anti-Markovnikov
isomer shows a distinct resonance at δH = 4.49 ppm
which is assigned to the terminal proton of the –
CH2CHFCF3 moiety with appropriate coupling (Figure
4c). The assignments have been confirmed by 19F,
19F{1H}, 1H–1H and 1H–19F NMR spectroscopy. In the
case of the anti-Markovnikov product of 5PPh3 the
data are further supported by a known germanium
analogue Et3GeCH2CHFCF3 formed from the
hydrogermylation of HFO-1234yf under catalytic
conditions.[22]
BH2F and BHF2 which may perform the hydroboration
step with different selectivities. [12] The concentrations
of the latter boron fluorides are expected to increase at
higher conversion. Furthermore, bimolecular
pathways involving the reaction of hydroborated
intermediates with a further equiv. of borane cannot be
excluded at this point.
What is clear from the experiments is that both
Markovnikov and anti-Markovnikov products are
accessible under the reaction conditions, with the
selectivity for the major Markovnikov isomer
decreasing with decreasing fluorine content of the
olefin. Heating solutions of 3SMe2 in C6D6 led to
slow HDF. Similarly, both 4THF and 5THF are
competent reaction intermediates giving rise to HDF
products. In the latter case it is notable that both
Markovnikov and anti-Markovnikov isomers are
consumed in the reaction to form predominantly 3,3,3-
trifluoropropene.
In combination, these reactions show that, while the
hydroboration of HFP proceeds in high-selectivity, the
regioselectivity is sensitive to the fluorine content of
the olefin. Hence, although E-2 and Z-2 give
predominantly Markovnikov products, HFO-1234yf
reacts unselectively with both modes of addition of
BH3 to the olefin observed. Substituent effects on the
regioselectivity of alkene hydroboration are well
understood.[23-25] Expanding on the work of Stone,[13,26]
Brown and co-workers have demonstrated previously
that trifluoropropene reacts with HBX2 (X = Cl, Br, H)
to give primarily Markovnikov substitution products
with the selectivity being sensitive to the number of
halogen ligands on the borane and ranging from 7:1 to
>9:1.[12] Similarly, vinyl chloride has been proposed to
react with BH3 by Markovnikov addition.[27] As with
the cSNV pathway, selectivity is dictated by a
combination of inductive (–F and –CF3) and
mesomeric effects (–F). The halogen containing
Two potential mechanisms explain the observed
intermediates and the selectivity for HFO-1234ze in
the HDF of HFP. Both involve addition-elimination
and BH3 as a reaction intermediate. A reaction
sequence involving, Markovnikov addition / -
fluoride elimination followed by anti-Markovnikov
addition / -fluoride elimination would yield mixtures
of E/Z-2 and E/Z-HFO-1234ze with limited preference
for the E- or Z-isomer as observed experimentally
(Figure 5 – Pathway 1).
5
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
Figure 5. Plausible addition-elimination reaction mechanisms originating from hydroborated intermediate 3.
Alternatively, Markovnikov hydroboration of HFP of a 1:1 mixture of regioisomers for HFO-1234yf
with BH3 to generate 3 followed by intramolecular shows that the selectively in these reactions is finely
hydride / fluoride exchange and subsequent -fluoride tuned. It is plausible that the substituted boranes BH2F
elimination would yield mixtures of E/Z-HFO-1234ze and BHF2 react with higher selectively for anti-
directly (Figure 5 – Pathway 2). While unusual, this Markovnikov addition favouring Pathway 1.
type of intramolecular rearrangement has been Alternatively, the hydroboration step may be reversible
proposed before.[27]
and selectivity determined by the -fluoride
elimination step.
In order to probe the viability of these steps the local
energy barriers for the -fluoride elimination and
hydride / fluoride exchange reactions were calculated
by DFT (B3PW91 functional). Although the resulting
data are of limited value in assessing the complete
reaction sequence, they do provide some insight into
whether these steps are accessible under the reaction
conditions. -Fluoride eliminations steps from either
Markovnikov or anti-Markovnikov hydroborated
intermediates 3 and 4 proceed with local Gibbs
activation energies of G‡298K = 15 – 25 kcal mol-1. The
hydride / fluoride exchange mechanism originates
from intermediate 2 which is formed exclusively as a
single regioisomer based on the experimental data. The
local Gibbs activation energy for the exchange
Conclusion
In summary, we report a new synthetic route to HFO-
1234yf and HFO-1234ze that exploits the selective
hydrodefluorination of hexafluoropropene. This
method uses a bulk chemical commodity and does not
rely on sequential chlorination and dichlorination
sequences. Using main group hydrides of the form
EH3L (E = B, L = SMe2; E = Al, L = NMe3) leads to
complementary
chemoselectivity
in
the
hydrodefluorination sequence. With the borane reagent
yielding a ~ 2:1 mixture of E- and Z-isomers of HFO-
1234ze and the alane almost exclusively HFO-1234yf.
The reaction by-products are boron and aluminium
fluorides, compounds of commercial interest in their
own right.
transition state is G‡
= 32.3 kcal mol-1. This
298K
transition state involves a concerted migration of the
hydride from boron-to-carbon and the fluoride from
carbon-to-boron and develops cationic character on the
carbon centre adjacent to boron.
Exploration of the plausible reaction mechanisms by
experiments and DFT calculations suggests that the
complementary selectivities may have a mechanistic
origin. For the alane, a concerted SNV mechanism
occurs with high substrate bias for H/F-exchange at the
terminal position of HFP. For the borane, ligand
dissociation followed by an addition elimination
sequences proceeds with selectivity for H/F-exchange
at the terminal and internal positions of HFP.
Based on the calculations neither Pathway 1 nor
Pathway 2 can be discounted at this stage. While it is
possible that both may be operating under the reaction
conditions (days at 100 °C) as Pathway 2 does not yield
E/Z-2 as intermediates it is unlikely to be the only
mechanism in action. Experimentally E/Z-2 mixtures
were found to form Markovnikov hydroboration
products with high selectivity, however the formation
6
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
Acknowledgements
Our findings lay out an experimental and mechanistic
template for the synthesis of modern refrigerants by a
selective hydrodefluorination approach.
We are grateful to the European Research Council
(FluoroFix:677367) and the Royal Society (UF090149).
Experimental Section
Declaration of Interest
General Procedure for HDF with a Borane: Me2S·BH3 (0.4
mmol) dissolved in C6D6 (0.5 ml) in a J-Young NMR tube
and was degassed by the freeze-pump-thaw method.
Hexafluoropropene (1 atm, 2 ml, ca. 0.08 mmol) was
allowed to fill the tube and the reaction mixture was heated
to 100 °C for 96 h. The hydrodefluorination products were
characterised in situ and by vacuum transfer of the volatile
species to a clean NMR tube. Integrations were carried out
against an internal standard of 1-fluorohexane and showed
59% E-HFO-1234ze, 27% Z-HFO-1234ze along with 10%
Z-2 and 3% E-2. NMR data are provided in the supporting
information.
Aspects of this work are the subject of UK patent
application number 1810647.6 filed 28th June 2018 in
collaboration with Imperial Innovations. A version of
this manuscript was filed on the preprint server
ChemRxiv.[34]
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[2] International Actions – The Montreal Protocol on
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actions-montreal-protocol-substances-deplete-ozone-layer.
(accessed 5th Feb. 2019)
General Procedure for HDF with an Alane: Me3N·AlH3
(0.2 mmol) dissolved in C6D6 (0.5 ml) in a J-Young NMR
tube and was degassed by the freeze-pump-thaw method.
Hexafluoropropene (1 atm, 2 ml, ca. 0.08 mmol) was
allowed to fill in the flask and the reaction mixture was
heated to 40 °C for 72 h. The hydrodefluorination product
(HFO-1234yf) was characterised in situ and by vacuum
transfer of the volatile species to a clean NMR tube.
Integrations were carried out against an internal standard of
durene and showed >98% yield.
[3] Eu legislation to control F-gases,
HFO-1234yf (2,3,3,3-tetrafluoropropene)1H NMR (C6D6,
https://www.ec.europa.eu/clima/policies/f-
gas/legislation_en. (accessed 5th Feb. 2019)
[4] Honeywell Blowing Agents,
3
2
400 MHz, 298 K): δH 4.45 (1H, dd, JHF = 13.2 Hz, JHH
=
4.9 Hz, CH2), 4.32 (1H, ddq, 3JHF = 14.8 Hz, 2JHH = 4.8 Hz,
4JHF = 1.6 Hz, CH2). 19F NMR (C6D6, 376 MHz, 298 K): δF
-73.34 (m, CF3), -124.33 (q, 3JFF = 10.3 Hz, CF).
https://www.fluorineproducts-
honeywell.com/blowingagents/regulation. (accessed 5th
Feb. 2019)
Synthesis of 3a: Me2S·BH3 (48 µl, 0.5 mmol) dissolved in
toluene (5 ml) was degassed by the freeze-pump-thaw [5] H. K. Nair, R. R. Singh, A. J. Poss, D. Nalewajek, US
method. Hexafluoropropene (1 atm, 15 ml, 0.6 mmol) was
allowed to fill in the flask and the reaction mixture was
heated to 100 °C for 1 h. PPh3 (131 mg, 0.5 mmol) was
added and the reaction was heated to 40 °C for 16 h.
Removal of the volatiles under reduced pressure and
trituration with pentane yielded the title complex as a
colourless solid (133 mg, 63% yield). Dissolution in Et2O
and storage at -30 °C gave colourless crystals suitable for X-
ray diffraction analysis. CCDC-1866058 contains the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Pat. 8889924B2, 2012; Honeywell International, Inc.
[6] H. S. Tung, H. K. Nair, S. S. M. Mukhopadhyay, M.
Van der Puy, WO Pat.2005108332A1, 2004, Honeywell
International, Inc.
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Pat. 9255046B2, 2003, Honeywell International, Inc.
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Crystallographic
Data
Centre
via
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Teltewskoi, S. Hinze, Angew. Chem. Int. Ed., 2009, 48,
www.ccdc.cam.ac.uk/data_request/cif.
1H NMR (C6D6, 400 MHz, 298 K): δH 7.55 (6H, m, CH-Ph), 1818–1822.
2
3
6.93 (9H, m, CH-Ph), 5.86 (1H, t d, JHF = 55.0 Hz, JHF
=
[10] T. Braun, F. Wehmeier, K. Altenhöner, Angew. Chem.
Int. Ed., 2007, 46, 5321–5324.
8.7 Hz, CF2H), 2.71 (2H, br, BH2).
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24, 6769–6777.
13C NMR (C6D6, 100 MHz, 298 K): δC 134.2 (d, 1JCP = 19.8
Hz, C-Ph), 133.9 (d, 3JCP = 9.1 Hz, CH-Ph), 131.6 (CH-Ph),
128.9 (d, 2JCP = 10.6 Hz, CH-Ph), 126.1 (m, CF3), 114.6 (t d,
1JCF = 246.2 Hz, 2JCF = 29.1 Hz, CF2H), 96.4 (br, CF).
1
11B NMR (C6D6, 128 MHz, 298 K): δB -28.3 (br d, JBP
=
78.6 Hz, BH2).
19F NMR (C6D6, 376 MHz, 298 K): δF -74.43 (d, 3JFF = 7.3
2
2
Hz, CF3), -127.16 (dd, JFF = 295.3 Hz, JFH = 55.3 Hz,
CF2H), -128.17 (dd, 2JFF = 295.3 Hz, 2JFH = 56.5 Hz, CF2H),
-191.87 (m, CF).
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1
31P NMR (C6D6, 162 MHz, 298 K): δP 14.68 (br d, JBP
=
76.6 Hz).
Elemental Analysis: found %C 59.30, %H 4.38; calc. %C
59.19, %H 4.26.
7
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
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10.1002/adsc.201900234
Advanced Synthesis & Catalysis
FULL PAPER
Selective Hydrodefluorination of
Hexafluoropropene to
Industrially Relevant Hydrofluoroolefins
Adv. Synth. Catal. Year, Volume, Page – Page
N. A. Phillips, A. J. P. White, M. R. Crimmin
9
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