Reactions of polyfluorinated cyclohexadienones with diazoalkanes. Part 1. Formation of cyclopropanes from polyfluorinated cyclohexa-2,4-dienones with diazomethane and phenyldiazomethane

Article abstract of DOI:10.1039/b001618g

The reaction of 6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-dienone (1) with diazomethane gives a mixture of two diastereoisomers of 6-chloro-3a,4,5,6,7a-pentafluoro-3,3a,6,7a-tetrahydrospiro[indazole-7,2′-o xirane] 2a and 2b. In contrast phenyldiazomethane reacts with polyfluorinated cyclohexa-2,4-dienones 1, 9, and 10 in acetonitrile to form the fluorinated 7-phenylbicyclo[4.1.0]hept-4-en-2-ones 4a,b, 11a,b, and 12a,b as the main products. This reaction shows a remarkable dependence on the polarity of the solvent. While a complex mixture of products was formed in pentane, the reaction in acetonitrile proceeds with high stereoselectivity giving only the two endo-isomers.

Full text of DOI:10.1039/b001618g

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Reactions of polyfluorinated cyclohexadienones with diazoalkanes.
Part 1. Formation of cyclopropanes from polyfluorinated
cyclohexa-2,4-dienones with diazomethane and phenyldiazomethane
1
Vladimir N. Kovtonyuk,a Ljubov S. Kobrina,*a Yurij V. Gatilov,a Irina Yu. Bagryanskaya,a
Roland Fröhlichb and Günter Haufe*b
a N.N.Vorozhtsov Institute of Organic Chemistry, 630090 Novosibirsk, Russian Federation
b Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstraße 40,
D-48149 Münster, Germany
Received (in Cambridge, UK) 28th February 2000, Accepted 10th April 2000
Published on the Web 23rd May 2000
The reaction of 6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-dienone (1) with diazomethane gives a mixture of two
diastereoisomers of 6-chloro-3a,4,5,6,7a-pentafluoro-3,3a,6,7a-tetrahydrospiro[indazole-7,2Ј-oxirane] 2a and 2b. In
contrast phenyldiazomethane reacts with polyfluorinated cyclohexa-2,4-dienones 1, 9, and 10 in acetonitrile to form
the fluorinated 7-phenylbicyclo[4.1.0]hept-4-en-2-ones 4a,b, 11a,b, and 12a,b as the main products. This reaction
shows a remarkable dependence on the polarity of the solvent. While a complex mixture of products was formed
in pentane, the reaction in acetonitrile proceeds with high stereoselectivity giving only the two endo-isomers.
Introduction
It is known, that alkyl substituted cyclohexa-2,4-dienones react
with diazomethane and alkyl- or phenyldiazomethanes only at
the carbon–carbon double bonds.1 Acetoxyalkylcyclohexa-
dienone cycloadducts, formed in these reactions, can be easily
transformed into 7-hydroxyindazoles1a or to methyl substituted
Scheme 1
cyclohexa-2,4-dienones.1c,2 The influence of the substituent’s
position on the regioselectivity of cycloaddition has been inten-
Results and discussion
sively studied. Usually the cycloaddition occurs on the C2᎐C3
᎐
The reaction of 6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-
bond.1a,c,d Alkyl groups (except the tert-butyl group) in
dienone (1)9 with diazomethane in ether at 0 ЊC results in the
formation of a 58:42 mixture of two isomers of 6-chloro-
3a,4,5,6,7a-pentafluoro-3,3a,6,7a-tetrahydrospiro[indazole-
7,2Ј-oxirane] (2a,b) in 52% overall yield (Scheme 2). The struc-
position 2 do not hinder the cycloaddition on the C2᎐C3 double
᎐
bond.1a,c In contrast an alkyl group in position 3 steers the
reaction to the C4᎐C5 double bond.1a If the geminal substitu-
᎐
ents in position 6 of cyclohexa-2,4-dienones are relatively small
(e.g. methyl groups or one hydroxy and one methyl group) bis-
adducts can be formed with both double bonds.1b,d,3 A few
more inter- and intramolecular cycloadditions of cyclohexa-
2,4-dienones with specific diazo compounds giving polycyclic
pyrazoles and cyclopropanes in subsequent steps have been
described.4 To the best of our knowledge cycloaddition reac-
tions of fluorinated cyclohexa-2,4-dienones and diazomethane
or its derivatives are not known.
Scheme 2
Fluorine-containing, especially polyfluorinated, cyclohexa-
dienones, are of great interest due to their high and diverse
reactivity. The nucleophilic substitution of fluorine in poly-
fluorinated cyclohexadienones and subsequent reduction of the
products to form phenols are well known.5 We also investigated
nucleophilic reactions of the carbonyl group of polyfluorinated
cyclohexa-2,5-dienones.6 Recently it has been found that
polyfluorinated cyclohexa-2,4-dienones undergo cycloadditions
with substituted acetylenes, alkenes and vinyl fluorides to give
the [2 ϩ 4] cycloadducts with high regio- and stereoselectivity.7
Earlier we have shown, that diazomethane reacts with both
the carbonyl groups and the double bonds of tetrafluoro-
benzoquinone giving a spirocyclohexadienone as the main
product and a small amount of a pentacycle (Scheme 1).8 Here
we present our results of the reactions of several fluorinated
cyclohexa-2,4-dienones with diazomethane or phenyldiazo-
methane, respectively.
ture of compounds 2a and 2b was attributed on the basis of the
1
analysis of H and 19F NMR spectra of a mixture of isomers
(Table 1). One of the isomers, 2a, shows three signals in the 19
F
NMR spectrum, at Ϫ173.4 (F3a), Ϫ100.6 (F6) and Ϫ155.1 (F7a)
ppm with corresponding hyperfine splitting constants of
JF3aF6 = JF6F3a = 7 and JF6F7a = JF7aF6 = 12.5 Hz. For the other
isomer, 2b, these constants are <2 Hz. These data allowed us to
assume that the isomers 2a and 2b differ in spatial orientation
of the geminal fluorine and chlorine atoms.
The reaction of 6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-
dienone (1)9 with phenyldiazomethane in acetonitrile (addition
of the solution of diazoalkane to the solution of 1) also gave a
mixture of the isomeric 6-chloro-3a,4,5,6,7a-pentafluoro-3,3Ј-
diphenyl-3,3a,6,7a-tetrahydrospiro[indazole-7,2Ј-oxirane] (3a
and 3b, 67:33) but in low yield (5%). The main products of this
reaction are the isomeric 3-chloro-1,3,4,5,6-pentafluoro-7-
DOI: 10.1039/b001618g
J. Chem. Soc., Perkin Trans. 1, 2000, 1929–1933
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1929

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Fig.
1
X-Ray crystal structure of the diastereomeric 3-chloro-
1,3,4,5,6-pentafluoro-7-phenylbicyclo[4.1.0]hept-4-en-2-ones (4a and
4b).
Scheme 3
phenylbicyclo[4.1.0]hept-4-en-2-ones (4a and 4b, 62:38)
(Scheme 3). These compounds can be viewed as masked
σ-homobenzo-1,2-quinones10 and could give access to fluorin-
1
ated tropolones. The H and 19F NMR spectra of the isomeric
compounds 3a and 3b (Table 1) are similar to those for
compounds 2a and 2b, however the signals of fluorine
atoms 3a next to the phenyl group in the pyrazoline ring of
compounds 3a and 3b exhibit a down field shift of about 10
ppm.
The bicycloheptenones 4a and 4b were isolated in pure form
and their structure was determined by X-ray structural analy-
sis (Fig. 1). The X-ray structures show that the cycloaddition
of 6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-dienone (1)
with phenyldiazomethane proceeds with high selectivity; both
isomers 4a and 4b have the endo-configuration. It is possible to
consider two routes for the formation of these compounds. One
possible way might be the reaction of phenylcarbene with a
double bond of cyclohexadienone 1. However, it is known, for
example, that the reaction of cyclohexene with phenylcarbene
proceeds without stereoselectivity.11 That is why a second
route, the 1,3-dipolar cycloaddition of phenyldiazomethane to
6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-dienone (1), is
more probable in our opinion. The selectivity of this reaction
may be a result of the transition state 5 in which the aromatic
ring of phenyldiazomethane is coordinated to the cyclohexa-
dienone ring of 6-chloro-2,3,4,5,6-pentafluorocyclohexa-2,4-
dienone (1) as a charge transfer complex (Scheme 4). The
cycloadduct 6 formed, evidently from 5, quite possibly reacts
with a second molecule of phenyldiazomethane to give prod-
ucts 3a and 3b or eliminates nitrogen to form the cyclopropane
derivatives 4a and 4b. As for the formation of 4a and 4b it is
known that pyrazoline-type products are quite stable and usu-
ally require heating for transformation to cyclopropanes.12
However, in the diazoalkane reactions with alkenes containing
electron-withdrawing substituents, the cyclopropane derivatives
have been obtained without heating and the formation of
pyrazolines as intermediates was only supposed.13 In certain
cases, particularly in polar solvents, the pyrazolines are easily
transformed to cyclopropanes with retention of configuration14
which is in agreement with the formation of 4a and 4b in our
case.
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phenoxy)tetrafluoro-6-phenylcyclohexa-2,4-dienone (10) with
phenyldiazomethane in acetonitrile gave mixtures of isomeric
cyclopropane derivatives 11a and 11b (67:33) or 12a and 12b
(83:17), respectively, as main products and pentafluorophenyl
benzyl ether (13). Furthermore, (Z)- and (E)-stilbenes are
produced as competitive products. In these reactions no bicyclic
tetrahydroindazoles have been detected. We suggest the
following mechanism of formation of compounds 11a,11b or
12a,12b, respectively, and 13 (Scheme 6).
Scheme 4
If 3a and 3b are formed according to this mechanism, an
excess of phenyldiazomethane or mixing of the reactants in
reverse order (addition of 1 to a solution of the diazo com-
pound) should change the ratio of 3:4. This was not the case.
Therefore we explain the formation of 3a and 3b as a result of
the following sequence. At first phenyldiazomethane attacks the
carbonyl group of dienone 1 giving the zwitterion 8 which
cyclizes to the oxirane 7. The reaction of 7 with a second
molecule of the diazo compound leads to 3a and 3b (Scheme 5).
Scheme 6
The zwitterion 14 formed from 9 or 10 at the first stage
perhaps via a transition state like 5 can be transformed by two
routes: (i) the elimination of nitrogen followed by cyclization
forming the cyclopropane derivatives 11a and 11b or 12a
and 12b, respectively, or (ii) elimination of pentafluorophenol
and formation of dienone 15 which is apparently unstable and
decomposed during isolation. Pentafluorophenol reacts further
with phenyldiazomethane to give pentafluorophenyl benzyl
ether (13) as a by-product. The probability of the last reaction
was checked by an independent experiment. The following
literature data also support the proposed mechanism. It is well
known that diazoketones can be synthesized by reaction of acyl
halides with diazomethane.16 It is also known that polyfluor-
inated cyclohexadienones, which are the vinylogues of acyl
fluorides, have a very nucleophilic mobile fluorine atom at pos-
ition 35 and its orientation in the reaction of diazomethane
with polyfluorinated alkenes is the same as in nucleophilic reac-
tions.17 Furthermore, it is known that the pentafluorophenoxy
group at position 3 of polyfluorinated cyclohexadienones can
be easily substituted by different nucleophilic reagents.18
Scheme 5
This mechanism agrees with the observed influence of solvent
polarity on the course of reaction of 6-chloro-2,3,4,5,6-penta-
fluorocyclohexa-2,4-dienone (1) with phenyldiazomethane. The
reaction in pentane results in the expected change of the ratio
of 3 and 4. According to the 19F NMR spectra of the reaction
mixtures, the ratio of 3a,b and 4a,b is approximately 1:3 in
acetonitrile and 1:1 in pentane, while the ratio of a:b did not
change. Moreover, the reaction in pentane gives a complex
mixture of more than ten compounds. The main pathway of
reaction is the catalytic decomposition of phenyldiazomethane
to form almost equal amounts of (Z)- and (E)-stilbene. This
is similar to the well known decomposition of phenyldiazo-
methane induced by oxidizing agents such as tetrahalobenzo-
quinones.15 Note that a more than three-fold excess of the diazo
compound was necessary to complete the conversion of 1 in
pentane, while the reaction in acetonitrile needed only one
equivalent.
Conclusion
Thus, we have found that polyfluorinated cyclohexa-2,4-
dienones, in contrast to non-fluorinated cyclohexadienones,
react with diazoalkanes not only at C᎐C double bonds but
᎐
also at the carbonyl group. Treatment of 6-chloro-2,3,4,5,6-
pentafluorocyclohexa-2,4-dienone (1) with diazomethane gave
mainly a mixture of two diastereomeric tetrahydrospiro-
[indazole-7,2Ј-oxirane]s 2a and 2b. The reactions of phenyldi-
azomethane with polyfluorinated cyclohexa-2,4-dienones
depended on the polarity of the solvent and gave two diastereo-
meric fluorinated 7-phenylbicyclo[4.1.0]hept-4-en-2-ones 4a,b,
11a,b, or 12a,b with endo-orientation of the phenyl group as the
main products in acetonitrile, while in pentane the diazo com-
pounds interact mainly with the carbonyl group which led to
complex product mixtures and partial decomposition of the
diazo compound.
Moreover, reactions of 6-chloro-3-(pentafluorophenoxy)-
tetrafluorocyclohexa-2,4-dienone (9) and 3-(pentafluoro-
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Experimental
General
Thin layer chromatography was performed with TLC plates 60
F254 by Merck. Column chromatography was performed using
silica gel (Merck, particle size 22–63 µm). Gas chromatographic
analysis was done using a Hewlett Packard 5890 II apparatus,
temperature programme 40 to 280 ЊC, heating rate 10 ЊC minϪ1
.
Mass spectra were measured on a Finnigan MAT-8200 instru-
ment operating at 70 eV. NMR spectra were recorded on a
1
Bruker WP-200SY (200.00 MHz for H and 188.28 MHz for
1
19F) or Bruker WN-300 (300.13 MHz for H and 282.40 MHz
for 19F) with TMS or CFCl3 as internal standards, respectively,
in CDCl3 or CCl4 solution. 6-Chloropentafluorocyclohexa-2,4-
dienone (1) was prepared according to ref. 9. All other starting
materials and reagents were obtained from Fluka or Acros.
Solvents were dried by storage over molecular sieves 0.4 nm.
6-Chloro-3a,4,5,6,7a-pentafluoro-3,3a,6,7a-tetrahydrospiro-
[indazole-7,2Ј-oxirane] (2a,b)
A solution of CH2N2 obtained from N-nitroso-N-methylurea
(0.5 g, 5 mmol) in diethyl ether (10 cm3) was added at 0 ЊC to a
solution of cyclohexadienone 1 (0.53 g, 2.4 mmol) in diethyl
ether (10 cm3). After 2 hours the solvent was evaporated and the
residue was purified by column chromatography on silica gel
using CCl4 as eluent to give a mixture of compounds 2a,b (0.35
1
g, 52%, 58:42). H and 19F NMR spectra of compounds 2a,b
are given in Table 1. In the mass spectra of both isomers 2a and
2b the molecular ion was found at m/z = 274.
General procedure for reactions of fluorinated cyclohexadienones
with phenyldiazomethane
In acetonitrile. To a solution of phenyldiazomethane in pent-
ane (250 cm3) prepared from benzaldehyde tosylhydrazone (cf.
Table 3) according to the protocol given in ref. 19, acetonitrile
(10–20 cm3) was added, the pentane was removed under
reduced pressure and the remaining solution was added at an
appropriate temperature to the solution of the cyclohexa-
dienone in acetonitrile (10 cm3) and stirred for 2–24 h. After
evaporation of the solvent, the residue was purified by column
chromatography on silica gel using cyclohexane–dichloro-
methane (gradient, 20:1 to 1:2) as eluent to afford the
corresponding cycloadducts. The 1H and 19F NMR data for all
products are listed in Tables 1 and 2. More details on the reac-
tion conditions and analytical data for all products are given in
Table 3.
In pentane. A solution of cyclohexadienone 1 (0.6 g, 2.7
mmol) in pentane (5 cm3) was added at 0 ЊC to a pentane
solution (20 cm3) of phenyldiazomethane, obtained from benz-
aldehyde tosylhydrazone19 (3.2 g, 1.2 mmol), and stirred for 20
h. Evaporation of pentane gave a mixture of more than 10
compounds (1.8 g) in an approximately equimolar ratio (GC,
1H and 19F NMR). In the reaction mixture the following
compounds have been identified: (E)- and (Z)-stilbene, benzyl
pentafluorophenyl ether (13), compounds 3a,b and 4a,b
(3:4 ≈ 1:1). This mixture was partially separated by column
chromatography on silica gel using cyclohexane as eluent to
give a mixture (0.32 g) of (Z)- (28%), (E)-stilbenes (30%), 13
(4%) and unidentified products, a mixture (0.12 g) containing
≈60% of compounds 4a and ≈10% of 13, a mixture (0.08 g)
containing ≈9% of compound 4a, ≈46% of compound 4b and
traces of compounds 3a,b.
Benzyl pentafluorophenyl ether
Pentafluorophenol (0.92 g, 5 mmol) was added with stirring to a
solution of phenyldiazomethane obtained from benzaldehyde
tosylhydrazone (2.0 g, 7 mmol) in acetonitrile (10 cm3). The
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Table 3 Analytical data for products of the reaction of polyfluorinated cyclohexadienones with phenyldiazomethane
Analytical data found
(calculated)
Cyclohexadienone/
(solvent) g (mmol)
TsNHN᎐CHPh/
g (mmol)
Products,
Time/h yield g (%)
Mp/ЊC
(solvent)
᎐
Entry
1
T/ЊC
C (%)
H (%)
M
1
3.2
(12)
Ϫ20→rt
2.5
3a,b
0.15 (5)
4a
0.91 (38)
4b
0.42 (18)
13 ϩ stilbenes
0.27a
11a
0.57
11b
0.25
13 ϩ stilbenes
0.47b
12a
Viscous oil
426.05490
(426.05582)
308.00022
(308.00273)
308.00205
(308.00273)
(Acetonitrile)
1.7
(8)
79–81
(Pentane)
92–94
(Pentane)
2
3
9
2.0
(7)
0→rt
4.5
(Acetonitrile)
1.35
(4)
81–84
48.26
(48.25) (1.27)
48.39 1.40
(48.25) (1.27)
1.35
471.99302
(471.99127)
471.99262
(471.99127)
(Pentane)
115–117
(Pentane)
10
3.7
(13.5)
rt
24
(Acetonitrile)
0.8
(1.9)
90–93
(CH3OH ϩ H2O)
Viscous oil
58.46
2.40
514.06261
(514.06152)
514.06209
(514.06152)
0.27
12b
0.05
(58.37) (2.14)
a The mixture contains 48% benzyl pentafluorophenyl ether (13), 14% (E)-stilbene and 38% (Z)-stilbene (1H NMR). b The mixture contains 14%
benzyl pentafluorophenyl ether (13), 33% (E)-stilbene and 53% (Z)-stilbene (1H NMR).
reaction mixture was stirred for 1 h and evaporated. The residue
was purified by column chromatography on silica gel using
pentane as eluent to give a mixture (0.71 g) containing 69% of
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3-Chloro-1,3,4,5,6-pentafluoro-7-phenylbicyclo[4.1.0]hept-4-
en-2-one (4a). Formula C13H6ClF5O, Mr = 308.63; monoclinic
P21/c; a = 8.889(2), b = 10.377(2), c = 13.610(3) Å; α = 90,
β = 95.57(3), γ = 90Њ, V = 1249.5(5) Å3, Z = 4. Data were meas-
ured in a 2θ range from 5.00 to 70.37Њ. A total of 2312 reflec-
tions were collected. Among them, 2056 were considered to be
observed (I > 2.0σ(I)). Final agreement indices are R(F) =
0.0609, wR(F2) = 0.1413, GoF = 1.015, based on anisotropic
refinement of all non-hydrogen atoms. H atom positional
parameters were calculated and refined as riding on their parent
atom.
3-Chloro-1,3,4,5,6-pentafluoro-7-phenylbicyclo[4.1.0]hept-4-
en-2-one (4b). Formula C13H6ClF5O, Mr = 308.63; monoclinic
P21/c; a = 11.205(1), b = 13.755(1), c = 7.725(1) Å; α = 90.0,
β = 91.28(1), γ = 90.0Њ, V = 1190.3(2) Å3, Z = 4. Data were
measured in the 2θ range from 2.96 to 30.48Њ. A total of 9988
reflections were collected. Among them, 3517 were considered
to be independent and 2683 were observed (I > 2.0σ(I)). Final
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1.037, based on anisotropic refinement of all non-hydrogen
atoms. H atom positional parameters were calculated and
refined as riding on their parent atom.
CCDC reference number 207/426. See http://www.rsc.org/
suppdata/p1/b0/b001618g/ for crystallographic files in .cif
format.
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Acknowledgements
The authors would like to thank the Volkswagen-Stiftung for
financial support of this work.
J. Chem. Soc., Perkin Trans. 1, 2000, 1929–1933
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