Synthesis of hydrofluorocycloolefins through dehydrofluorination of hydrofluorocycloalkanes in amide solvents

Article abstract of DOI:10.1016/j.jfluchem.2019.06.008

Hydrofluorocycloolefins have outstanding economic and environmental advantages, as well as huge application potentials owing to their zero ozone depletion potentials (ODP) values and low global warming potentials (GWP) values. The synthesis of hydrofluorocycloolefins through dehydrofluorination of hydrohalocycloalkanes in N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAC) was investigated. It was found that the presence of [sbnd]CHF[sbnd] is critical for efficient elimination of HF. The reaction using reagents containing [sbnd]CHF[sbnd] group proceeded very well, whereas elimination was much more difficult to occur for reactants without [sbnd]CHF[sbnd] group. Based on these results, a rational reaction mechanism was proposed.

Full text of DOI:10.1016/j.jfluchem.2019.06.008

JournalofFluorineChemistry226(2019)109342
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis of hydrofluorocycloolefins through dehydrofluorination of
hydrofluorocycloalkanes in amide solvents
⁎
Wenni Zhanga, Chengping Zhanga, Qin Guoa, Fengniu Lua, Hengdao Quanb,
b National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-8565, Ibaraki, Japan
a Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing, 100081, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydrofluorocycloolefins have outstanding economic and environmental advantages, as well as huge application
potentials owing to their zero ozone depletion potentials (ODP) values and low global warming potentials (GWP)
values. The synthesis of hydrofluorocycloolefins through dehydrofluorination of hydrohalocycloalkanes in N,N-
dimethylformamide (DMF) or N,N-dimethylacetamide (DMAC) was investigated. It was found that the presence
of eCHFe is critical for efficient elimination of HF. The reaction using reagents containing eCHFe group
proceeded very well, whereas elimination was much more difficult to occur for reactants without eCHFe group.
Based on these results, a rational reaction mechanism was proposed.
Hydrofluorocycloolefins
Dehydrofluorination
DMF
DMAC
Elimination
1. Introduction
fixed bed reactor [20] in the presence of catalyst whose active component
was palladium; (3) Catalytic isomerisation of hydrofluorocycloolefins
The promulgation of the Montreal and Kyoto protocols brought for-
ward the global issue of ozone depletion caused by chlorofluorocanbons
(CFCs) and hydrochlorofluorocarbons (HCFCs) [1]. To address the issue,
novel substances for zero ozone depletion potentials (ODP) values and
low global warming potentials (GWP) values are extensively explored.
Hydrofluorocarbons (HFCs) are one of the developed ozone depletion
substitutes (ODS), among which hydrofluoroolefins have attracted tre-
mendous attention by virtue of their zero ODP values and short atmo-
spheric lifetimes [2–4]. Herein, particular interest was paid to hydro-
fluorocycloolefins owing to their outstanding economic and
environmental advantages, as well as huge application potentials. As
basic raw materials, hydrofluorocycloolefins are also widely used for the
production of fluorine-containing fine chemicals and polymer materials
such as rocket propellant composites [5], refrigerants [6,7], blowing
agents [8,9], etchants [10], cleaning agents [11], fire extinguishing
agents [12], sprays [13], and heat pumping fluids [14,15].
[21]; (4) HX (X = F, Cl, Br) elimination from hydrohalofluorocycloalk-
anes in strong base [9,22–25]. In reality, the harsh reaction condition in
method (1) prevents its popularity. Meanwhile, methods (2) and (3) al-
ways suffer low selectivity for hydrofluorocycloolefins, generating a mass
of by-products that are difficult to separate. For example, the gas-phase
HeX exchange between 1,2-dichlorotetrafluorocyclobutene and H2 with
chromium-nickel catalyst at 250 °C produces a mixture of 1,2-dihydrote-
trafluorocyclobutene (HFO-c1334zz, selectivity of 44%, boiling point of
54 °C), 1,1,2,2-tetrafluorocyclobutane (HFC-c354cc, selectivity of 24%,
boiling point of 50 °C), and 1-chloro-3,3,4,4-tetrafluorocyclobutene
(HCFO-c1324xz, selectivity of 25%, boiling point of 59 °C) that are dif-
ficult to separate due to their azeotropic feature [19]. As a result, method
(4), namely, dehydrofluorination of hydrofluorocycloalkanes, remains a
primary approach for the synthesis of hydrofluorocycloolefins.
In this study, we reported a facile method for the synthesis of hydro-
fluorocycloolefins through dehydrofluorination of hydrofluorocycloalkanes
in N,N-dimethylformamide (DMF) or N,N-dimethyl acetamide (DMAC)
under mild conditions. So far, it has rarely been reported that amide solvent
is able to serve as a dehydrofluorinated reagent. DMF and DMAC are
employed because their unique acid-base properties [26] enabled suc-
cessful elimination of HF as reported by our team [27]. To verify the ap-
Generally, hydrofluorocycloolefins can be prepared through several
different ways as follows, which were summarized in Scheme 1. (1) Li-
quid-phase HeX (X = F, Cl) exchange, a reaction occurs between fluor-
ocycloolefins and sodium borohydride, which provides hydrogen
[16–18], under an ultra-low temperature condition in mixing solvents of
lithium tetrahydrogen aluminum (LAH) and ether; (2) Catalytic gas-phase
HeCl exchange, which occurs between hydrofluorocycloolefins and hy-
drogen gas (H2) in quartz reactor [19] catalysed by chromium-nickel or in
plicability of this novel synthetic strategy,
a number of hydro-
fluorocycloalkanes, including 1,1,2,2,3,3,4-heptafluorocyclopentane (HFC-
c447ef), cis-1,1,2,2,3,3,4,4-octafluorocyclopentane (HFC-c438ee (cis)),
⁎ Corresponding author.
E-mail address: hengdao-quan@aist.go.jp (H. Quan).
https://doi.org/10.1016/j.jfluchem.2019.06.008
Received 22 April 2019; Received in revised form 25 June 2019; Accepted 30 June 2019
Availableonline02July2019
0022-1139/©2019ElsevierB.V.Allrightsreserved.

W. Zhang, et al.
JournalofFluorineChemistry226(2019)109342
DMAC than DMF [26], because DMF could be decomposed into a strong
alkaline dimethylamine after heating. As a result, the elimination reaction
occurred more easily in DMF than in DMAC. However, both HFC-c447ef
and HFC-c438ee (cis) exhibited similar conversion rates in DMF and
DMAC, which could be attributed to easier removal of the β-hydrogen of
fluorine in five-membered hydrofluorocycloalkane than that in four-
membered one [17,25,36]. Therefore, the basicity of solvent was not a
decided factor for dehydrohalogenation in five-membered hydro-
fluorocycloalkane.
The failure of dehydrohalogenation for HFC-c456ff and HCFC-
c353cfb illustrated that dehydrohalogenation would not occur for re-
actants containing only eCH2e, eCFCle or eCF2e but no eCHFe
group. This was probably because the extremely weak acidity of hy-
drogen induced by fluorine from eCH2eCH2eCFXe group prevented
itself from leaving and the strong strength of CeCl bond in eCFCle
group and CeF bond in eCF2e group prevented the removal of halogen
from eCFCle and eCF2e groups [28,29,33–35].
Scheme 1. Existing routes to synthesize hydrofluorocycloolefins.
1,1,2,2,3-pentafluorocyclobutane (HFC-c345ef), cis-1,1,2,2,3,4-hexa-
2.2. Mechanism of dehydrofluorination
fluorocyclobutane (HFC-c336ee (cis)), 1,1,2,2,3,3-hexafluorocyclopentane
(HFC-c456ff), and 1-chloro-1,3,3-trifluorocyclobutane (HCFC-c353cfb),
were carefully investigated. The results illustrated that dehydrofluorination
occurred very well in reactants containing eCHFe group. Elimination of
HF was much more difficult to take place in reactants without eCHFe
group. We therefore concluded that the presence of eCHFe group was
indispensable for efficient dehydrofluorination of hydrofluorocycloalkanes,
and a reaction mechanism was proposed.
In view of the above experimental results, the mechanism of de-
hydrofluorination
of
hydrofluorocycloalkanes
containing
eCFR1eCHR2e (R1 = F, H; R2 = F, H) group in DMF or DMAC was
proposed in Scheme 3.
It should be noted that DMAC (CH3CON(CH3)2) is thermally stable
but DMF (HCON(CH3)2) easily decomposes into dimethylamine (HN
(CH3)2) and carbon monoxide (CO) under heating [37,38]. Therefore,
under the current reaction condition (160 °C in DMF and 170 °C in
DMAC) (Scheme 2), the actual species coming into play are dimethy-
lamine and DMAC (see Scheme 3, R3 = H and CH3CO in R3N(CH3)2).
Initially, the lone pair electrons on the nitrogen of R3N(CH3)2 [26] at-
tacked and removed the acidic β-hydrogen of fluorine (the hydrogen
atom located on the carbon adjacent to the carbon substitued with
fluorine) in hydrofluorocycloalkane [34,35,39] by forming a proto-
nated intermediate, namely, R3(HN+)(CH3)2 [39]. The elimination of
hydrogen led to the formation of a carbanion whose electron pair was
stabilized by the strong induction effect of fluorine [31,32,39,41–44].
This fluorine was then removed as fluoride anion (F−), generating
hydrofluorocycloolefin as the final product. Notably, the F- was stabi-
lized by forming electrically neutral [R3(HN+)(CH3)2]F− with the
protonated intermediate (R3(HN+)(CH3)2) [40,45]. In other words, the
protonated intermediate made fluorine a much better leaving group. In
short, the hydrogen fluoride was eliminated from the hydro-
fluorocycloalkane via E1cB elimination.
2. Results and discussion
2.1. Dehydrofluorination of hydrofluorocycloalkanes
The details of dehydrofluorination for all the hydro-
fluorocycloolefins, including reaction route, conversion rate, and se-
lectivity for each corresponding product, were summarized in Scheme
2. The conversion rate of HFC-c447ef (83.3% in DMF and 83.2% in
DMAC) (Eq 1) is higher than that of HFC-c438ee (cis) (42.1% in DMF
and 45.7% in DMAC) (Eq 2). Similarly, the conversion rate of HFC-
c345ef (36.8% in DMF and 11.9% in DMAC) (Eq 3) is higher than that
of HFC-c336ee (cis) (8.5% in DMF and 1.3% in DMAC) (Eq 4). No
dehydrofluorination occurred for HFC-c456ff and HCFC-c353cfb in
both DMF and in DMAC as illustrated (Eq 5) and (Eq 6), respectively.
According to the comparison of the reaction outcomes between (Eq
1) and (Eq 2), as well as those between (Eq 3) and (Eq 4), the dehy-
drofluorination from eCHFeCH2e located at e(CF2)neCHFeCH2e
(n = 2, 3) occurred more easily than the that from eCHFeCHFe lo-
cated at e(CF2)neCHFeCHFe (n = 2, 3), which can be rationalized by
The eCF2eCHFeCH2eCF2e (in HFC-c447ef and HFC-c345ef) and
eCHFeCHFeCF2e (in HFC-c438ee (cis) and HFC-c336ee (cis)) are
both highly fluorinated groups. In these groups, the relative acidity of
β-hydrogen ranked in an order of eCH2e > eCHFe [33–35]. There-
fore, the loss of hydrogen from eCH2e was more preferred than that
from eCHFe group. On the other hand, due to the strong strength of
CeF bond, the leaving ability of fluorine followed an order of
eCHFe > eCF2e [33–35], which resulted in easier removal of fluorine
from eCHFe than from eCF2e group.
stronger elimination reactivity of eCHFeCH2e located at
e
(CF2)neCHFeCH2e (n = 2, 3) than that of eCHFeCHFe located at e
(CF2)neCHFeCHFe (n = 2, 3) [28,29]. In (Eq 2), a small amount of
HFO-c1427yyce was formed due to the isomerization of HFO-c1427yye
in the presence of fluorine anion [30].
Comparison of the selectivity for each product in (Eq 2) and (Eq 4)
implies that the loss of a fluorine from eCHFe group was easier than the
loss of a fluorine from eCF2e group in e(CF2)neCHFeCHFe (n = 2, 3)
[28,31,32]. Besides, the selectivity of each products in (Eq 1) and (Eq 3)
suggested easier removal of a hydrogen from eCH2e group than that
from eCHFe group in e(CF2)neCHFeCH2e (n = 2, 3) [33–35]. Fur-
thermore, the higher conversion rates of (Eq 1) (or (Eq 2)) than that of (Eq
3) (or (Eq 4)) confirmed a lower reactivity of four-membered hydro-
fluorocycloalkane than that of five-membered hydrofluorocycloalkane.
This is probably because the carbanion intermediates are less stable in
four-membered ring than in five-membered ring [17,36].
According to the above rules, the ease of dehydrohalogenation of
the eCF2eCHFeCH2eCF2e group followed an order of
eCF2eCH=CHeCF2e > eCF2eCHFeCH = CFe > > eCF=CFeCH2
eCF2e, which agreed well with the higher selectivity for HFO-c1436zz
(97.0% in DMF and 94.8% in DMAC) than HFO-c1436yze (2.5% in DMF
and 4.7% in DMAC) in the dehydrohalogenation of HFC-c447ef (Eq 1),
as well as the higher selectivity for HFO-c1334zz (97.4% in DMF and
97.8% in DMAC) and HFO-c1334yze (2.6% in DMF and 2.8% in DMAC)
in the dehydrohalogenation of HFC-c345ef (Eq 3). Similarly, the ease
of dehydrohalogenation of eCHFeCHFeCF2e group is in an order of
eCF = CHeCF2e > eCHFeCF = CFe, which was also in accordance
with the higher selectivity for HFO-c1427yz (69.3% in DMF and 66.8%
in DMAC) than HFO-c1427yye (25.6% in DMF and 27.2% in DMAC) in
As to dehydrofluorination of HFC-c345ef in (Eq 3) and that of HFC-
c336ee (cis) in (Eq 4), there both were higher conversion rate in DMF
than in DMAC. Such a trend might be caused by a lower alkalinity of
2

W. Zhang, et al.
JournalofFluorineChemistry226(2019)109342
Scheme 2. Novel methods for preparation of hydrofluorocycloolefins.
the dehydrohalogenation of HFC-c438ee (cis) (Eq 2) as well as the
higher selectivity for HFO-c1325yz (62.4% in DMF and 54.7% in
DMAC) than HFO-c1325yyc (37.6% in DMF and 45.3% in DMAC) in the
dehydrohalogenation of HFC-c336ee (cis) (Eq 4).
trans- elimination of the fluorine and hydrogen atoms that of both co-
planar and antiperiplanar positions.
In the case of eCH2eCH2eCF2e (in HFC-c456ff) and
eCF2eCH2eCFCle (in HCFC-c353cfb) groups, among which the hy-
drogen had such extremely weak acidity owing to the strong induction
effect of fluorine that it was quite difficult to leave. In addition, the
strong CeF and CeCl bonds prevented the leaving of fluorine and
chlorine [28,48,49]. Therefore, no elimination of hydrogen halide oc-
curred, as displayed in (Eq 5) and (Eq 6).
For cyclic halides, conformational effect should not be neglected
because it dominates the ratio of elimination products with different
conformations [32], for instance, trans- and cis- conformations. Al-
though trans-elimination was generally easier, generation of compar-
able cis- and trans- products from the elimination of cyclic halides is
also frequently reported [16,41,46,47]. We therefore examined the
conformation effect for the elimination of our molecular system. It was
found that main products from the elimination of HFC-c438ee (cis)
and HFC-c336ee (cis), namely, HFO-c1427yz (Eq 2) and HFO-c1325yz
(Eq 4), were both derived from trans-elimination of fluorine and hy-
drogen that are of antiperiplanar conformation. However, it seems that
all the products from (Eq 1) to (Eq 4) originated from both cis- and
3. Conclusions
The dehydrohalogenation of hydrohalocycloalkanes in DMF or
DMAC was systematically investigated by using a series of four-mem-
bered and five-membered hydrofluorocycloalkanes with different ex-
tents of fluorination. It was found that the dehydrofluorination
3

W. Zhang, et al.
JournalofFluorineChemistry226(2019)109342
(400 MHZ) at 25 °C in CDCl3. The patterns were compared with those of
authentic samples.
The gas chromatograph was a Shimadzu GC-2014S with DB-WAX
column with 30 m length and 0.25 mm inner diameter (film: 0.25 μm)
from Agilent Technologies Inc. The operating conditions of the GC were
as follows: 40 °C for 10 min; 10 °C/min to 240 °C; hold for 10 min. Both
the injection port and the thermal conductively were maintained at
240 °C, and the carrier gas was a mixture of H2, He and air introduced at
a rate of 3 mL/min.
4.3. Experiment procedure
The dehydrofluorination reactions of all reactants including HFC-
c447ef, HFC-c345ef, HFC-c438ee (cis), HFC-c336ee (cis), HFC-c456ff
and HCFC-c353cfb were identically designed and conducted according
to following conditions. The molar ratio of DMF or DMAC and each
reactant was 2/1. The mixture of DMF or DMAC and each reactant were
placed into a 2000 mL stainless steel reactor equipped with an electric
heater and an agitating device. The reactor was heated to a temperature
of 160 °C in DMF solvent and being stirred for 6 h under magnetic
stirring, while the reactor was heated to a temperature of 170 °C in
DMAC solvent and being stirred for 6 h under magnetic stirring. The
products from the above system were detected by GC and NMR, re-
spectively.
Scheme 3. The mechanism of dehydrofluorination of hydrofluorocycloalkanes.
proceeded very well in reactants containing eCHFe group. Elimination
was much more difficult to occur in reactants without eCHFe group.
We thus notified the importance of the presence of eCHFe group for
smooth dehydrofluorination of hydrohalocycloalkanes. Base on the
experimental results, a rational reaction mechanism was proposed. Our
study not only demonstrates a facile approach for synthesizing hydro-
fluocycloolefins with high selectivity under relatively mild condition,
but also provides in-depth understanding of the reaction mechanism
which will guide efficient production of various hydrofluocycloolefins
in future.
4.4. Analytic results
4.4.1. HFC-c345ef
4. Experimental section
4.1. Chemicals
The reagents 1,1,2,2,3,3,4-heptafluorocyclopentane (HFC-c447ef,
99.0+%), cis-1,1,2,2,3,3,4,5-octafluorocyclopentane (HFC-c438ee
(cis), 99.0+%) and 1,1,2,2,3,3-hexafluorocyclopentane (HFC-c456ff,
99.0+%) were purchased from Shaanxi Shenguang Chemical Industry
Chemical Co., Ltd. The reagents 1,1,2,2,3-pentafluorocyclobutane
(HFC-c345ef, 99.0+%), cis-1,1,2,2,3,4-hexafluorocyclobutane (HFC-
c336ee (cis), 99.0+%), 1,1,3-pentafluoro-3-chloro-cyclobutane
(HCFC-c353cfb, 99.0+%), N,N-dimethylformamide (DMF) and N,N-
dimethylacetamide (DMAC) were purchased from Wako Pure Chemical
Industries, Ltd. Chloroform-d (CDCl3, 99.8 atom%D) at was obtained
from Aldrich Chem. Co. (Japan). CFCl3 (CFC-11, 99.0+%) was pur-
chased from Synquest Labs, Lnc.
MS data (m/z): 146 (M+), 127 (M+–F), 113 (M+–CH2F), 107
(M+–HF2), 100 (M+–C2H3F), 95 (M+–CHF2), 93 (M+–CH3F2), 82
(M+–C2H2F2), 77 (M+–CF3), 75 (M+–CH2F3), 69 (M+–HF4), 63
(M+–C2H2F3), 57 (M+–CHF4), 45 (M+–C2HF4), 37 (M+–CH2F5).
19F NMR (377 MHz, CDCl3): δ −202.77 (s, F5, 1 F), −135.21 (d,
J = 220.2 Hz, F6, 1 F), −118.81 (d, J = 219.8 Hz, F8, 1 F), −116.01
(d, J = 217.5 Hz, F7, 1 F), −113.95 (d, J = 204.0 Hz, F12, 1 F).
1H NMR (400 MHz, CDCl3): δ 2.69 (m, H9 and H10, 2 H), 5.16 (d, J
= 52.4 Hz, H11, 1 H).
13C NMR (101 MHz, CDCl3): δ 37.32 (qd, J = 22.67 Hz, C3, 1C),
84.05 (dm, J = 224.12 Hz, C2, 1C), 115.60 (tm, J = 277.14 Hz,C4, 1C),
115.61 (tq, J = 290.88 Hz,C1, 1C).
4.2. Instrument
4.4.2. HFO-c1334zz
The 2000 mL stainless steel reactor was used for all experiments
with magnetic stirring using a Magmix stirrer tough mixer (MRK Co.,
LTD.).
The 1H, 13C and 19F NMR spectra of HFC-c447ef, HFC-c438ee (cis)
and HFC-c456ff were cited from the literature [50]. The 1H, 13C and 19
F
NMR spectra of HFO-c1436zz, HFO-c1436yze, HFO-c1427yz, HFO-
c1427yye, HFO-c1427yyce and HFO-c1427yyce were cited from the
literature [30]. The 19F NMR spectra of HFC-c345ef, HFO-c1334zz,
HFC-c336ee (cis), HFO-c1325yz, HFO-c1325yyc and HCFC-c353cfb
were recorded on a Bruker AVANCE 400 (400 MHZ) at 25 °C with CFCl3
as internal references in CDCl3 solvent. The 13C and 1H NMR spectra of
HFC-c345ef, HFO-c1334zz, HFC-c336ee (cis), HFO-c1325yz, HFO-
c1325yyc and HCFC-c353cfb were recorded on a Bruker AVANCE 400
MS data (m/z): 126 (M+), 107 (M+–F), 100 (M+–C2H2), 88
(M+–F2), 87 (M+–HF2), 75 (M+−CHF2), 69 (M+–F3), 63 (M+–C2HF2),
50 (M+–C3H2F2), 44 (M+–C2HF3), 38 (M+–CF4), 37 (M+–CHF4).
19F NMR (377 MHz, CDCl3): δ −111.02 (d, J = 3.77 Hz, F6, F7, F8
and F9, 4 F).
4

W. Zhang, et al.
JournalofFluorineChemistry226(2019)109342
1H NMR (400 MHz, CDCl3): δ 6.82 (m, H5 and H10, 2 H).
4.4.6. HCFC-c353cfb
13C NMR (101 MHz, CDCl3): δ 119.28 (tm, J = 286.64 Hz, C1 and
C4, 2C), 142.56 (quint, J = 19.19 Hz, C2 and C3, 2C).
4.4.3. HFC-c336ee (cis)
MS data (m/z): 144 (M+), 125 (M+–F), 109 (M+–CH4F), 105
(M+–HF2), 95 (M+–CH2Cl), 89 (M+–HFCl), 80 (M+–C2H2F2), 75
(M+–CH3FCl), 64 (M+–C2H2FCl), 59 (M+–CF2Cl), 57 (M+–CH2F2Cl),
51 (M+–HF3Cl), 44 (M+–C2H3F2Cl), 38 (M+–CH2F3Cl).
19F NMR (377 MHz, CDCl3): δ −99.64 (dm, J = 200.19 Hz, F12,
1 F), –114.28 (m, F7, 1 F), −114.79 (m, F8, 1 F).
MS data (m/z): 164 (M+), 145 (M+–F), 113 (M+–CHF2), 100
(M+–C2H2F2), 95 (M+–CF3), 82 (M+–C2HF3), 75 (M+–CHF4), 64
(M+–C2F4), 57 (M+–CF5), 44 (M+–C2HF5), 37 (M+–CHF6).
19F NMR (377 MHz, CDCl3): δ –224.03 (d, J = 50.90 Hz, F10 and
F12, 2 F), −134.11 (d, J = 233.36 Hz, F6 and F7, 2 F), −119.56 (d,
J = 233.36 Hz, F5 and F8, 2 F).
1H NMR (400 MHz, CDCl3):δ 2.546 (m, H5 and H6, 2 H), 2.602 (m,
H9 and H10, 2 H).
13C NMR (101 MHz, CDCl3):δ 28.02 (td, J = 22.12 Hz, C1 and C3,
2C), 106.97 (ddd, J = 294.01 Hz, C2, 1C), 117.00 (ddd, J = 288.25 Hz,
C4, 1C).
1H NMR (400 MHz, CDCl3): δ 5.12 (dm, J = 52 Hz, H9 and H11,
2 H).
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13C NMR (101 MHz, CDCl3): δ 83.99 (dm, J = 232.81 Hz, C2 and
C3, 2C), 113.92 (tm, J = 297.24 Hz, C1 and C4, 2C).
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6