Pyrolysis reactions of nonafluorobiphenyl-4-yl prop-2-enyl ether: A remarkable rearrangement reaction of an intramolecular Diels-Alder product

Article abstract of DOI:10.1039/b001356k

The pyrolysis reaction of nonafluorobiphenyl-4-yl prop-2-enyl ether were studied. The reaction was proposed as a remarkable rearrangement reaction of an intramolecular Diels-Alder product. The flash vapor pyrolysis of the compound performed at 350 °C gave a complex mixture containing the products obtained from Diels-Alder reactions. The molecular structure of the compounds obtained was characterized by x ray crystallography.

Full text of DOI:10.1039/b001356k

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Pyrolysis reactions of nonafluorobiphenyl-4-yl prop-2-enyl ether:
a remarkable rearrangement reaction of an intramolecular
Diels–Alder product
1
Andrei S. Batsanov, Gerald M. Brooke,* Darren Holling and Alan M. Kenwright
Chemistry Department, Science Laboratories, South Road, Durham, UK DH1 3LE
Received (in Cambridge, UK) 18th February 2000, Accepted 24th March 2000
Published on the Web 9th May 2000
The title compound 16 on flash vapour pyrolysis (FVP) at 350 ЊC gives a complex mixture which includes 20, the
product of one of the two possible intramolecular Diels–Alder reactions of the cyclohexa-2,4-dienone intermediate
17 formed via a Claisen rearrangement reaction. FVP of 16 at 420 ЊC gives the bicyclic compound 30, formed not
from the other possible Diels–Alder adduct 27 but from an isomer 31 having exactly the same carbon skeleton, but
produced as a transient intermediate via a rare retro-cyclisation reaction of 20 to a tethered ketene 32 and
recyclisation via the alternative mode.
In an earlier paper,1 a novel route to 5,6,7,8-tetrafluoro-2H-
chromene 4 was conceived whereby under flash vapour-phase
conditions at low pressure through a silica tube packed with
silica wool, pentafluorophenyl prop-2-enyl ether 1 would
undergo the Claisen rearrangement to the intermediate 2 which
would be followed first by the elimination of hydrogen fluor-
ide and then electrocyclisation of the o-quinomethane-type
material 3 (Scheme 1).
(invoked in the formation of 8) and its skeletally related isomer
13 containing a plane of symmetry, isolated as its 1,1-diol
hemihydrate 14.6 The formation of 13 was rationalised by
invoking an overall [1,3]sigmatropic shift occurring in a step-
wise manner from 6; the species 12 containing two conjugated
radicals was proposed as an intermediate (Scheme 3).
The purpose of the present work was to investigate the
pyrolysis reactions of the 4-pentafluorophenyl derivative of 1,
namely compound 16, and in particular, the possibility of
inducing not only the Claisen and Cope rearrangements via 17
to 18, but also two further Cope rearrangement reactions to 19
(Scheme 2), the first of which would involve an aromatic ring
in the [3,3]sigmatropic reaction. This latter type of reaction
has been studied in another system, but no rearrangement
occurred.7 In the present work, no isomerisations of 16 to 18 or
19 were detected: some chemistry analogous to our earlier work
was found, but another truly remarkable rearrangement reac-
tion was discovered which has been reported in a preliminary
communication.8
Results and discussion
The starting material 16 was prepared from the known
nonafluoro-4-hydroxybiphenyl 159 and prop-2-enyl bromide
(Scheme 2) and was subjected to FVP at 350 ЊC and 0.01
mmHg as before to give a complex mixture of products, clearly
evidenced from the 19F NMR spectrum. The major component
(67%) of the products was unreacted 16 (Scheme 3) which was
efficiently removed in the supernatant liquid from solid
materials which had crystallised from light petroleum at
Ϫ18 ЊC. Chromatography of the solid enabled the Diels–Alder
adduct 20 to be isolated, the structure of which was determined
by X-ray crystallography.8 The 19F NMR spectra of the frac-
tions from the chromatography immediately following 20
showed the presence of another closely related compound
having three of the resonances at low frequencies shifted to even
lower frequencies by ca. 1.3–4.5 ppm. In a separate experiment,
chromatography of 20 on silica resulted in its being converted
to this new species 21, contaminated with 15% of 20, and it was
assigned the gem-diol structure (the result of facile hydration
due to adventitious water on the stationary phase during separ-
ation) on the basis of the strong O–H absorptions in the IR
spectrum. The next material isolated by chromatography was a
mixture of closely related compounds 23 and 24 which on
Scheme 1
At 365 ЊC and 0.05 mmHg the cyclohexa-2,5-dienone 5 was
formed via 2 followed by a Cope rearrangement,2 (Scheme 2).
Under more vigorous conditions at 440–480 ЊC, compound 1
gave the 1-fluorovinyl ketone 8 the formation of which was
proposed to occur from 2 via the decomposition of the intra-
molecular Diels–Alder adduct 6 to 7 followed by loss of HF3
(Scheme 3). The intermediacy of the alternative intramolecular
Diels–Alder adduct 9 was proposed to explain the formation of
another isomer, the bicyclic compound 11, via the ring-opened
species 101 containing two conjugated radicals (Scheme 4). The
original objective of synthesising the heterocyclic compound 4
in Scheme 1 was realised eventually by heating 1 in refluxing
DMF with potassium fluoride as a base to effect the required
dehydrofluorination reaction of the intermediate 2.4
In order to increase significantly the time of reaction, the
ether 1 was heated in vacuo at 137–141 ЊC over 13 days and
gave two products: the intramolecular Diels–Alder adduct 65
DOI: 10.1039/b001356k
J. Chem. Soc., Perkin Trans. 1, 2000, 1731–1734
This journal is © The Royal Society of Chemistry 2000
1731

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Scheme 2
Scheme 3
crystallisation from water gave the stable gem-diol monohydrate
24 and which in turn could be dehydrated with P2O5 to the
ketone 23 contaminated with 4% 24. The structures of 23 and
24 were determined by 19F NMR spectroscopy which showed
the presence of a plane of symmetry in the molecules because
the intensities of the absorptions at Ϫ186.4 and Ϫ194.6 ppm
respectively were twice those at Ϫ201.4 and Ϫ202.2 ppm (the
bridgehead fluorines), and by analogy of these shifts with those
in 14 whose structure had been determined by X-ray crystal-
lography (the details of which were not reported),6 and which
has since been repeated (Fig. 1).10
When the pyrolysis of 16 was carried out at 420 ЊC and 0.01
mmHg as before, the 19F NMR spectrum of the crude product
showed the presence of three major components, one of which
was identified by the presence of a unique absorption at ca.
Ϫ140 ppm as the phenol 15 (presumably formed by homolytic
cleavage of the O–CH2 bond in the ether followed by a hydro-
gen atom abstraction reaction), as well as very small propor-
tions of 20, 21, 23 and 24. Chromatography of the material gave
a liquid readily identified as the 1-fluorovinyl ketone 26 by 19F
and 1H NMR spectroscopy and by analogy with data found for
compound 8 studied earlier3 (Scheme 3). The pyrolysis reac-
1
–
Fig. 1 X-Ray crystal structure of 14ؒ2 H2O.
tion was repeated under identical conditions and the crude
product was crystallised from light petroleum at Ϫ18 ЊC to
give a solid which had all the superficial 19F and H NMR
1
spectral properties (principally chemical shifts and a gem-
CHF group J5αH–5βF 49.3 Hz) expected of compound 29,
1732
J. Chem. Soc., Perkin Trans. 1, 2000, 1731–1734

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Scheme 4
Scheme 5
analogous to 11 (Scheme 4). However, the 19F COSY data
indicated some unusual values for JF–F for structure 29. The
structure of the bicyclic compound 11 had been assigned on
the basis of further chemical evidence,1 but an X-ray crystal-
lographic analysis was carried out on the suspected bicyclic
compound which, surprisingly, showed it to be the isomer 30
(Scheme 4); this structure immediately rationalised the NMR
data and also showed that the Diels–Alder adduct 27 cannot
have been formed from 17 during the reaction and therefore a
new mechanism has to be proposed. At 350 ЊC, new products
identified by 19F NMR were present in the crude product to
the extent of 39%: 15, 20, 21, 23, 24, 26 and 30 being present
in the ratio 19:28.5:0.5:35:4.5: 4.5:7; at 420 ЊC, 70% of the
following identifiable compounds were present in the crude
product: 15, 16, 20, 21, 23, 24, 26 and 30 being present in the
ratio 41:8:1:0.5:1:0.5:22:26. All five fluorines in the C6F5
groups in compounds 20, 21 and 30 were observed, which is
indicative of restricted rotation.
In the earlier work, the formation of 11 was readily explained
as arising from the alternative Diels–Alder adduct 9 via homo-
lytic cleavage of a C–C bond to give the diradical 10, the flexi-
bility of which allowed the close approach of the radical in the
six-membered ring to abstract hydrogen from the CH2 group in
the five-membered ring. It was also pointed out that simple
molecular models indicate that the formation of adducts of the
type 6 (and therefore 20) should be much easier than those of
type 9 (and therefore 27).1 Nevertheless, an intermediate 31
having the same basic carbon skeleton as 27 but having the
alkenic F and R groups interchanged, is required as the precur-
sor to 30 and we propose its formation from 20 via a retro-
Diels–Alder reaction to give the cyclohexa-2,4-dienylmethyl
fluoroketene 32, a rare reaction type,11 followed by the altern-
ative Diels–Alder cyclisation as shown in Scheme 5; compound
30 is racemic, but has the enantiomeric structure shown when
formed in this way from 20.
the structures 6 and 20 does take place (the former type occur-
ring even under relatively mild conditions5) and the formation
of the alternative one 27 does not arise from the 2,4-dienone 17
for steric reasons, the alternative circuitous route to 31 starting
from 20 is favourable nevertheless, though it is not obvious why
this should be so; even the formation of 11 is likely to proceed
via this new molecular rearrangement reaction. Further substi-
tuted materials analogous to 16 are currently being investigated
to assess the generality of the rearrangement.
Experimental
NMR spectra were recorded on the following instruments at
the frequencies listed: Varian Mercury 200 (1H, 199.991 MHz;
19F, 188.179 MHz) and Varian VXR 400 (1H, 399.952 MHz;
19F, 376.346 MHz). Chemical shifts are reported using the
high-frequency positive convention from TMS and CFCl3,
hence 19F resonance values are negative; J values are in Hz; 19
F
COSY experiments were carried out on compounds 16, 20, 26
and 30 to establish connectivities. Elemental analyses were
performed on an Exeter Analytical Inc CE440 elemental
analyser.
Nonafluoro-4-hydroxybiphenyl 15
Compound 15 was prepared in 68% yield from decafluoro-
biphenyl by the method given in the literature9 (41%);
δF(CDCl3) Ϫ137.6 (m, 2Ј-F, 6Ј-F), Ϫ139.4 (m, 3-F, 5-F),
Ϫ150.8 (tt, 4Ј-F), Ϫ160.7 (tm, 3Ј-F, 5Ј-F), Ϫ162.0 (dm, 2-F,
6-F).
Nonafluorobiphenyl-4-yl prop-2-enyl ether 16
A mixture of nonafluoro-4-hydroxybiphenyl 15 (14.91 g), allyl
bromide (15 ml) and potassium carbonate (20.0 g) in acetone
(150 ml) was heated under reflux for 16 h, filtered, the solvent
removed in vacuo at 30 ЊC and the residue distilled to give
nonafluorobiphenyl-4-yl prop-2-enyl ether 16 (16.44 g, 98%); bp
82 ЊC at 0.02 mmHg (Found: C, 48.22; H, 1.30. C15H5F9O
requires C, 48.41; H, 1.35%); δF(CDCl3) Ϫ137.5 (m, 2Ј-F, 6Ј-F),
Ϫ139.6 (m, 3-F, 5-F), Ϫ150.8 (tt, 4Ј-F), Ϫ155.4 (dm, 2-F, 6-F),
Ϫ160.8 (tm, 3Ј-F, 5Ј-F); δH(CDCl3) 4.84 (d, CHaHa), 5.36 (d,
Intermolecular (4ϩ2) π reactions of ketenes to form six-
membered carbocyclic rings (as opposed to oxygen-containing
rings by involvment of the C᎐O bond) are uncommon.12
᎐
However, there is one recorded example of an intramolecular
process of this type in the literature.13
The conclusion to be drawn from these experiments has to be
that while the formation of internal Diels–Alder adducts having
J. Chem. Soc., Perkin Trans. 1, 2000, 1731–1734
1733

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H ), 5.46 (d, H ), 6.07 (m, H ) in –CH H CH ᎐CH H where H
b
left for 15 h in a freezer at Ϫ18 ЊC, and the solid (0.271 g)
᎐
d
c
b
a
a
b
c
d
and Hd are Z-configuration.
which separated was recrystallised from light petroleum (bp
80–100 ЊC) to give 7-pentafluorophenyl-2,5β,6,7aβ-tetrafluoro-
3aβ,4,5,7a-tetrahydro-1H-inden-1-one 30, mp 135–136 ЊC
(Found: C, 48.35; H, 1.25. C15H5F9O requires C, 48.41; H,
1.35%); δF(CDCl3) Ϫ99.2 (d, 6-F), Ϫ132.9 (d, 2-F), Ϫ137.3 (tt,
2Ј-F), Ϫ137.8 (d, 6Ј-F), Ϫ151.0 (tt, 4Ј-F), Ϫ154.5 (t, 7aβ-F),
Ϫ160.9 (td, 3Ј-F), Ϫ161.3 (td, 5Ј-F), Ϫ194.4 (dm, 5β-F);
δH(CDCl3) 2.48 (m, two 4-H), 3.59 (dm, 4α-H), 5.16 (dm, 5α-H,
Pyrolysis of nonafluorobiphenyl-4-yl prop-2-enyl ether 16
(a) At 350 ؇C. The ether 16 (2.45 g) was distilled in vacuo, in
five consecutive experiments using ca. 0.5 g per experiment,
from a silica boat through a silica tube (510 × 20 mm) packed in
the middle 170 mm with silica wool. The packing only in the
tube was heated to 350 ЊC with the boat initially at one end of
the closed tube at room temperature, the other end being con-
nected to a high vacuum system via a trap cooled in liquid air.
When the pressure had reached 0.01 mmHg, the pyrolysis tube
was moved to allow the boat and contents to enter the hot zone
of the heating oven and a hot air gun was used to vaporise any
condensate beyond the exit of the oven into the trap. Each
experiment took approximately 15 minutes and each pyrolysate
was washed from the trap with ether. The solvent was distilled
from the combined solutions and a 19F NMR was run of a
sample in CDCl3 to allow, ultimately, an analysis of the com-
plex composition (see later). The crude reaction product was
dissolved in light petroleum (bp 40–60 ЊC) and left for 15 h in a
freezer at Ϫ18 ЊC. A solid (0.81 g) was separated by decanting it
from the supernatant solution which contained a high propor-
tion of unreacted starting material 16 and was not examined
further. The solid was chromatographed on silica using diethyl
ether as eluant to give four compounds: (i) 3-pentafluorophenyl-
2,4,5,7-tetrafluorotricyclo[3.3.1.02,7]non-3-en-6-one 20, mp 140–
141 ЊC (from light petroleum bp 80–100 ЊC) (Found: C, 48.22;
H, 1.35. C15H5F9O requires C, 48.41; H, 1.35%); δF(CDCl3)
Ϫ116.1 (s, 4-F), Ϫ136.4 (br s, 2Ј-F), Ϫ138.1 (br d, 6Ј-F),
Ϫ150.6 (t, 4Ј-F), Ϫ160.2 (td, 3Ј-F), Ϫ160.8 (td, 5Ј-F), Ϫ175.0
(m, 7-F), Ϫ193.0, (m, 5-F), Ϫ198.1 (d, 2-F); δH(CDCl3) 2.16 (d,
1-H), 2.34 (m, one H), 2.42 (m, one H), 3.10 (m, one H), 3.24
J5αH–5βF 49.3), 6.94 (t, 3-H); νmax 3083 (᎐C–H), 1747 (C᎐O), 1701
᎐
᎐
(CF᎐CC F ), 1651 cmϪ1 (CF᎐CH).
᎐
᎐
6
5
In a separate experiment, the crude product from the
pyrolysis of the ether 16 (1.965 g) was chromatographed on
silica using light petroleum (bp 40–60 ЊC)–diethyl ether (95:5%
v/v) to give 1-fluorovinyl 3-pentafluorophenyl-2,4-difluorophenyl
ketone 26 (0.309 g), a liquid which rapidly polymerises in air
(Found: C, 51.23; H, 1.18. C15H4F8O requires C, 51.16; H,
1.14%); δF(CDCl3) Ϫ102.0 (m, 4-F), Ϫ107.3, (m, 2-F), Ϫ114.9
(m, Fa), Ϫ137.5 (m, 2Ј-F, 6Ј-F), Ϫ151.1 (tt, 4Ј-F), Ϫ161.0 (tm,
3Ј-F, 5Ј-F); δH(CDCl3) 5.40 (s, Ha), 5.54 (dd, Hb, JFa–Hb 32,
JHa–Hb 2 in C ᎐CH H where F and H are E), 7.10 (t), 7.68
᎐
Fa
a
b
a
b
(q) are unassigned H5, H6; νmax 1691 cmϪ1 (C᎐O).
᎐
Product ratios from the pyrolysis reactions
(a) At 350 ؇C. In the 19F NMR spectrum of the crude
product, the integrations equivalent to one fluorine of a unique
absorption in every identifiable component in the mixture were
related to the integrations of all the 4Ј-F’s (all triplets) in C6F5
groups between Ϫ150.7 and Ϫ152.8 ppm (which provided a
measure of all the molecular species present) and gave the total
proportion of these components in the mixture as 39%, com-
pounds 15, 20, 21, 23, 24, 26 and 30 being present in the ratio
19:28.5:0.5:35:4.5:4.5:7.
(m, one H); νmax 1775 cmϪ1 (C᎐O). (ii) the next fractions con-
᎐
tained mixtures of 20 and a closely related compound (from the
19F NMR spectra) which in separate chromatography on silica
provided an enriched mixture containing 20 (15 parts) and
3-pentafluorophenyl-2,4,5,7-tetrafluorotricyclo[3.3.1.02,7]non-3-
ene-6,6-diol 21 (85 parts), δF(CDCl3) Ϫ114.6 (s, 4-F), Ϫ137.2
(br s, 2Ј-F), Ϫ138.5 (br d, 6Ј-F), Ϫ152.5 (t, 4Ј-F), Ϫ161.4 (td,
3Ј-F), Ϫ161.7 (td, 5Ј-F), Ϫ178.0 (m, 7-F), Ϫ197.4, (m, 5-F),
Ϫ199.3 (d, 2-F); νmax 3566 and 3427 cmϪ1 (O–H). (iii) the
third and fourth components were a mixture of a ketone and
its related 1,1-diol: recrystallisation from water gave 3-penta-
fluorophenyl-1,2,4,7-tetrafluorotricyclo[3.3.1.02,7]non-3-ene-8,8-
diol monohydrate 24, mp 98–99 ЊC (Found: C, 44.46; H, 1.96.
C15H7F9O2ؒ1H2O requires C, 44.13; H, 2.22%); δF(CDCl3)
Ϫ98.6 (s, 4-F), Ϫ138.1 (m, 2Ј-F, 6Ј-F), Ϫ152.6 (t, 4Ј-F), Ϫ161.7
(td, 3Ј-F, 5Ј-F), Ϫ194.6 (d, 1-F, 7-F), Ϫ202.2 (m, 2-F);
δH(CDCl3) 1.59 (s, H2O), 1.79 (m, two H), 2.72 (m, two H), 2.95
(m, two OH), 3.92 (d, 5-H); νmax 3548 cmϪ1 (gem-diol); sublim-
ation from a mixture of 24 and four times its weight of P2O5 in
vacuo at 55 ЊC/0.01 mmHg gave an enriched mixture containing
24 (4 parts) and 3-pentafluorophenyl-1,2,4,7-tetrafluorotricyclo-
[3.3.1.02,7]non-3-en-8-one 23 (96 parts); δF(CDCl3) Ϫ98.1 (s, 4-
F), Ϫ137.9 (m, 2Ј-F, 6Ј-F), Ϫ151.4 (t, 4Ј-F), Ϫ161.0 (td, 3Ј-F,
5Ј-F), Ϫ186.4 (d, 1-F, 7-F), Ϫ201.4 (m, 2-F); δH(CDCl3) 2.18
(b) At 420 ؇C. 70% of the following identifiable compounds
were present in the crude product: 15, 16, 20, 21, 23, 24, 26 and
30 being present in the ratio 41:8:1:0.5:1:0.5:22:26.
References
1 G. M. Brooke, J. Chem. Soc., Perkin Trans. 1, 1974, 233.
2 G. M. Brooke, Tetrahedron Lett., 1971, 2377.
3 G. M. Brooke and D. H. Hall, J. Chem. Soc., Perkin Trans. 1, 1976,
1463.
4 G. M. Brooke, J. Fluorine Chem., 1983, 22, 483.
5 G. M. Brooke and D. H. Hall, J. Fluorine Chem., 1982, 20, 163.
6 G. M. Brooke, D. H. Hall and H. M. M. Shearer, J. Chem. Soc.,
Perkin Trans. 1, 1978, 780.
7 A. Nickon and B. R. Aaronoff, J. Org. Chem., 1964, 29, 3014.
8 A. S. Batsanov, G. M. Brooke, D. Holling and A. M. Kenwright,
Chem. Commun., 1999, 1549.
9 M. W. Buxton, R. H. Mobbs and D. E. M. Wotton, J. Fluorine
Chem., 1972/73, 2, 231.
10 Crystal data: C9H7F5O2ؒ¹H2O, M = 251.15, T = 120 K, mono-
¯
²
clinic, space group C2/c (no. 15) a = 25.133(7), b = 6.757(2),
c = 11.032(4) Å, β = 105.44(2)Њ, U = 1806(1) Å3, Z = 8 (H2O on axis
2), Dcalc = 1.85 g cmϪ3, SMART CCD area detector, Mo-Kα
radiation, SHELXL97 software, 6231 reflections (2θ р 55Њ), 2064
unique, R = 0.034 [1703 data with F 2 у 2σ(F 2)], wR(F 2) = 0.088.
Full data (excluding structure factors) have been deposited at the
Cambridge Crystallographic Data Centre. CCDC reference
number 207/419. See http://www.rsc.org/suppdata/p1/b0/b001356k/
crystallographic files in .cif format.
(td, two H), 2.68 (d, two H), 3.13 (d, 5- H); νmax 1831 (C᎐O),
᎐
1697 cmϪ1 (CF᎐CC F ).
᎐
6
5
(b) At 420 ؇C. The ether 16 (2.349 g) was pyrolysed in five
batches of ca. 0.5 g at 420 ЊC as described in (a) and again the
19F NMR was run in CDCl3 to allow, ultimately, an analysis of
the complex composition (see later). The crude reaction
product was dissolved in light petroleum (bp 40–60 ЊC) and
11 P. J. Pollart and H. W. Moore, J. Org. Chem., 1989, 54, 5444;
T. T. Tidwell, Ketenes, Wiley, New York, 1995.
12 J. A. Hyatt and P. W. Raynolds, Org. React., 1994, 45, 159.
13 T. Mujashi, H. Kawamoto, T. Nakajo and T. Mukai, Tetrahedron
Lett., 1979, 155.
1734
J. Chem. Soc., Perkin Trans. 1, 2000, 1731–1734