The multifaceted reactivity of o -fluoranil

Article abstract of DOI:10.1021/jo202193c

In addition to Diels-Alder and hetero-Diels-Alder reactions, tetrafluoro-o-benzoquinone (o-fluoranil) undergoes nucleophilic additions, addition-eliminations, dioxole formation, and charge-transfer complexation, reacting at every site on the molecular skeleton. It also effects dehydrogenations and other oxidations. The quinone can function as a (CF) ₄ synthon.

Full text of DOI:10.1021/jo202193c

Article
pubs.acs.org/joc
The Multifaceted Reactivity of o-Fluoranil
Vivek Kumar, Sudharsanam Ramanathan, Dayong Sang, Xuanyi Chen, and David M. Lemal*
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
S
* Supporting Information
ABSTRACT: In addition to Diels−Alder and hetero-Diels−Alder reactions,
tetrafluoro-o-benzoquinone (o-fluoranil) undergoes nucleophilic additions,
addition−eliminations, dioxole formation, and charge-transfer complexation,
reacting at every site on the molecular skeleton. It also effects dehydrogen-
ations and other oxidations. The quinone can function as a (CF)4 synthon.
Scheme 2
INTRODUCTION
■
Previous publications have described [4 + 2] cycloadditions of
o-fluoranil (1): Diels−Alder and hetero-Diels−Alder reac-
tions.1−3 To date, all acetylenic substrates have yielded
exclusively Diels−Alder adducts, even though the alternative
pathway is strongly favored thermodynamically. However,
alkenes and dienes give either Diels−Alder (2) or hetero-
Diels−Alder adducts (3), or both (Scheme 1).4 Cycloaddition
Scheme 1
hexadienone.5 Because the tetrafluorobenzene yields are
typically better, that method is superior to ours.
Alkene adducts of the cyclohexadienone do not undergo
photoextrusion of the bridge, as judged from the single example
we tested, the norbornene adduct. However, alkene adducts of
o-fluoranil do, yielding tetrafluoro-1,3-cyclohexadienes that can
be aromatized by heating with DDQ. The photochemical step
often proceeds in low yield, probably due to impurities
introduced in the cycloaddition step. That step goes very
well, though, in the case of cyclooctene, which yields
benzocyclooctene 9 via adducts 7 (exo and endo) and
cyclohexadiene 8 (Scheme 3).
o-Fluoranil undergoes another kind of cycloaddition, dioxole
formation.6 Treated with ethyl diazoacetate in refluxing
benzene, for example, the quinone is transformed into dioxole
10 (Scheme 4).
Instead of cycloadding, some especially nucleophilic CC
double bonds add to a carbonyl group of o-fluoranil; for
example, indole and N-alkylindoles give adducts 11 (Scheme
5). That the site of addition is a carbonyl and not a fluorinated
carbon is clear from the 13C NMR spectrum: the lone high-field
(sp3) skeletal carbon signal is not split by a one-bond C−F
coupling, and four fluorinated carbons appear far downfield.
Two reports in the 1980s literature7,8 that describe the
interaction of indoles with o-chloranil claim a different mode of
reaction, addition ortho to a carbonyl group to afford 12
(Scheme 6). This structure should lose HCl easily. By analogy
to its fluorinated counterpart, o-chloranil would be expected to
occurs with both electron-donating and electron-withdrawing
substituents, under more vigorous conditions with the latter.
Via its Diels−Alder adducts, o-fluoranil can serve as a (CF)4
synthon, incorporating a (−CFCF−CFCF−) diene frag-
ment into a variety of molecular frameworks, as described
below. The present account then explores reaction types other
than [4 + 2] cycloaddition in which the quinone engages.
RESULTS AND DISCUSSION
■
o-Fluoranil Diels−Alder adducts undergo extrusion of the
dicarbonyl bridge under UV irradiation. Irradiation is generally
carried out through Pyrex (λ > 280 nm), but if complications
arise from a chromophore elsewhere in the molecule, a cutoff
filter can be used that eliminates nearly all of the UV. In the
case of alkyne adducts, the products are tetrafluorobenzenes. In
addition to several known examples, new compounds 4−6 have
been prepared by this method (Scheme 2). Kobrina obtained
tetrafluorobenzenes by photoextrusion of the [−CClFCO−]
bridge from alkyne adducts of 6-chloroperfluoro-2,4-cyclo-
Received: October 24, 2011
Published: January 5, 2012
© 2012 American Chemical Society
966
dx.doi.org/10.1021/jo202193c | J. Org. Chem. 2012, 77, 966−970

The Journal of Organic Chemistry
Article
Scheme 3
Scheme 7
o-chloranil as 15. However, we found the high-field skeletal
carbon signal for the N-methylpyrrole adducts of o-fluoranil and
o-chloranil at δ 76.1 and δ 80.7, respectively, similar to each
other and almost identical with those found for the N-
hexylindole adducts. We conclude that o-chloranil adducts
actually have the structure 16.
Scheme 4
Scheme 5
Saturated nucleophiles can also add to a carbonyl group of o-
fluoranil. When excess methanol was added to a solution of the
quinone in CDCl3, four new signals corresponding to adduct
17 appeared quickly in the 19F NMR spectrum (Scheme 8).
Scheme 8
Scheme 6
undergo addition at a carbonyl group, a surmise that is
supported by the close similarity of the adducts’ 13C spectra.
We prepared the o-fluoranil and o-chloranil adducts of N-
hexylindole and found the high-field skeletal carbon peaks at δ
76.3 and δ 80.6, respectively. Thus, o-chloranil−indole adducts
have the structure 13, a conclusion reached long ago by
Horner. He reported the adducts of indole and N-alkylindoles
with o-chloranil in 19509 and, in 1955, without benefit of NMR,
correctly assigned their structures primarily on the basis of their
chemical reactivity.10
The fluorine-decoupled 13C spectrum obtained at 0 °C
showed four CF carbons in the δ 132−150 region, establishing
that methanol had added at a carbonyl carbon, which appeared
at δ 90.6. At RT, this initial adduct soon diminished, giving way
to three new signals that represented compound 18, formed by
addition−elimination. These peaks faded in turn, to be replaced
by two signifying trimethoxy compound 19. No further change
occurred, even when the reaction was carried out in methanol
as the solvent.
Formation of 19 from 18 presumably requires temporary loss
of the hemiacetal methanol to activate the molecule for attack
beta to the new carbonyl group. While catalysis by HF probably
plays a role in this chemistry, similar results are obtained when
the reaction mixture is stirred with a large excess of calcium
carbonate.
The location of the remaining fluorine atoms in 19 was
revealed when it was heated with dimethyl acetylenedicarbox-
ylate in refluxing toluene, as the resulting Diels−Alder adduct
Pyrroles also add readily to o-fluoranil at a carbonyl carbon to
give 14, as evidenced in the 13C spectrum again by the absence
of a large CF coupling constant for the high-field (sp2) skeletal
carbon peak and by four fluorinated carbon signals at much
lower field (Scheme 7). Assignments were confirmed with a
fluorine-decoupled 13C spectrum. The two 1980s reports
alluded to above assigned the structures of pyrrole adducts of
967
dx.doi.org/10.1021/jo202193c | J. Org. Chem. 2012, 77, 966−970

The Journal of Organic Chemistry
Article
20 gave rise to a single resonance at δ −202.5 in the 19F
spectrum. Under the reaction conditions, the hemiketal
methanol is expelled, making the two fluorines equivalent.
The very high field location of the signal requires that they be
located at the bridgehead positions in 20; otherwise, they
would appear at lower field by tens of parts per million.11 The
structure of 19 follows from that of 20, and 18 is assigned on
the basis that methanol would undergo conjugate addition to
17. That is, it would attack at the carbon β, not γ, to the
carbonyl group. The structure of 20 is further confirmed by
photodecarbonylation to phthalate ester 21.
oxygen. The tube was irradiated under nitrogen with a Hanovia 450 W
medium pressure mercury lamp until decarbonylation was complete.
The solvent was evaporated, and the product was chromatographed on
20 g of silica gel (200−400 mesh) with hexanes as the eluent. The
yield of tetrafluorobenzene 4 was 48%; mp 35−36 °C. 1H NMR
(CDCl3): δ 0.43 (s, 18H). 19F NMR (CDCl3): δ −120.3 (m, 2F),
−155.1 (m, 2F). 13C NMR (CDCl3): δ 152.5 (1JCF = 239 Hz), 140.3
(1JCF = 261 Hz), 128.4, 2.6. HRMS Calcd for C12H18F4Si2: 294.0883.
Found: 294.0866.
1-Ethyl-2,3,4,5-tetrafluoro-6-methylbenzene (5). The proce-
dure used for 4 was followed with 0.27 g (4 mmol) of 2-pentyne and
5.5 h at reflux. 2-Ethyl-1,4,5,6-tetrafluoro-3-methylbicyclo[2.2.2]octa-
1
As would be expected of a high-potential quinone,12,13 o-
fluoranil is a good dehydrogenating agent. It aromatizes 1,3-
cyclohexadiene, 1,4-cyclohexadiene, and 1,2-dihydronaphtha-
lene rapidly at RT in preference to undergoing cycloaddition.
The quinone oxidizes electron-rich compounds, such as o-
phenylenediamine and 2,5-dimethylpyrrole, to ill-defined
products, forming tetrafluorocatechol in all of these reactions.
o-Fluoranil forms charge-transfer complexes with a wide
variety of donor molecules as indicated, for example, by intense
colors that commonly appear at the start of its Diels−Alder
reactions. The 1:1 complex with hexamethylbenzene (22) was
isolated as shiny brown needles; mp = 121−122 °C.
2,5-diene-7,8-dione: H NMR (CDCl3): 2.44 (m, 2H), 2.00 (m, 3H),
1.05 (m, 3H). 19F NMR (CDCl3): δ −152.5 (m, 1F), −152.9 (m, 1F),
−208.1(m, 1F), −209.4 (m, 1F). Tetrafluorobenzene 5 was obtained
1
in 27% yield. H NMR (CDCl3): δ 2.67 (m, 2H), 2.21 (m, 3H), 1.14
(m, 3H). 19F NMR (CDCl3): δ −143.1 (m, 1F), −146.1 (m, 1F),
−161.4 (m, 2F). 13C NMR (CDCl3): δ 146.5 (1JCF = 242 Hz), 138.5
(1JCF = 250 Hz), 126.0, 119.2, 18.7, 13.7, 9.8. HRMS Calcd for
C9H8F4: 192.0562. Found: 192.0551.
2′,3′,4′,5′-Tetrafluoro-2,3,4,5-tetrahydro-1,1′-biphenyl (6).
The same procedure as used for 4 was employed with 0.42 g (4
mmol) of 1-ethynylcyclohexene, benzene as the solvent, and 0.5 h at
reflux. 5-(Cyclohex-1-enyl)-1,2,3,4-tetrafluorobicyclo[2.2.2]octa-2,5-
diene-7,8-dione: 19F NMR (CDCl3): δ −151.6 (m, 1F), −151.9 (m,
1F), −204.1 (m, 1F), −204.8 (m, 1F). Tetrafluorobenzene 6 was
1
obtained in 21% yield. H NMR (CDCl3): δ 6.83 (s, 1H), 5.97 (s,
1H), 2.31−0.88 (m, 8H). 19F NMR (CDCl3): δ −140.9 (m, 1F),
−142.3 (m, 1F), −156.6 (m, 1F), −159.3 (m, 1F). 13C NMR
(CDCl3): δ 146.6 (1JCF = 246 Hz), 145.0 (1JCF = 247 Hz), 140.8 (1JCF
= 250 Hz), 139.0 (1JCF = 248 Hz), 131.3, 130.4, 127.7, 109.7, 28.4,
1
25.7, 22.7, 21.7. 13C NMR (CDCl3, 19F decoupled, H coupled): δ
146.6 (d, J = 6.4 Hz), 145.0 (d, J = 12.3 Hz), 140.0 (s), 138.9 (d, J =
9.7 Hz), 131.3 (d, J = 6.0 Hz), 130.4 (d, J = 156 Hz), 127.7 (m), 109.7
(d, J = 167 Hz), 28.4 (t, J = 129 Hz), 25.7 (t, J = 127 Hz), 22.7 (t, J =
136 Hz), 21.7 (t, J = 136 Hz). HRMS Calcd for C12H10F4: 230.0719.
Found: 230.0720.
In summary, the quinone is a versatile electrophile that
undergoes reaction at every position on its skeleton, and it is
also a building block that can provide a fluorinated diene
fragment for the construction of more complex molecules.
1,2,3,4-Tetrafluoro-5,6,7,8,9,10-hexahydrobenzo[8]-
annulene (9).16 A solution of o-fluoranil (225 mg, 1.25 mmol) and
cyclooctene (229 mg, 2.08 mmol) in methylene chloride (10 mL) was
refluxed for 12 h, forming exo- and endo-1,4,11,12-tetrafluorodecahy-
dro-1,4-ethanobenzo[8]annulene-2,3-dione (7). 19F NMR (CD2Cl2):
δ −146.9 (m, 1F), −148.1 (m, 1F), −195.2 (m, 1F), −196.0 (m, 1F).
Nitrogen was bubbled through the solution to remove oxygen; then
the adducts were irradiated through Pyrex for 3 h under nitrogen with
EXPERIMENTAL SECTION
■
For some of the following experiments, the o-fluoranil was introduced
as a readily synthesized 60:40 (o:p) mixture with its much less reactive
para isomer, which was removed in the workup.2,14 In other
experiments, the ortho isomer alone was used. It was prepared by
oxidation of tetrafluorocatechol as described below. 19F NMR spectra
were referenced to internal chlorotrifluoromethane, as 1H and 13C
NMR spectra were referenced to TMS.
the 450
W Hanovia lamp to obtain 1,2,3,4-tetrafluoro-
4a,5,6,7,8,9,10,10a-octahydrobenzo[8]annulene (8). 19F NMR
(CD2Cl2): δ −145.2 (m, 2F), −167.3 (m, 2F). The NMR yield,
determined by integration against hexafluorobenzene, was 87%. DDQ
(2,3-dichloro-5,6-dicyano-1,4-benzoquinone, 228 mg, 10.1 mmol) was
added to the solution, which was then refluxed for 3.5 h. The solvent
was evaporated, and chromatography of the residue on 10 g of silica
gel with hexane as the eluent gave 131 mg of tetrafluorobenzene 9
o-Fluoranil (1).1 A solution of tetrafluorocatechol (4.0 g, 22
mmol) in nitromethane (52 mL) was cooled to −30 °C in a
bromobenzene slush bath. Concentrated nitric acid (16 mL) in
nitromethane (5.2 mL) was added dropwise with stirring during 20
min. Cold water (52 mL) was added, stirring was continued for 2 min,
then the mixture was extracted with benzene (22 mL). The aqueous
layer was again extracted with benzene (2 × 15 mL), and the
combined extract was dried over Na2SO4. Concentration of the
solution below 30 °C gave 3.8 g of an orange semisolid product, 90%
pure o-fluoranil (86% yield) that contains some nitromethane and
benzene.15 Pure quinone (mp 66.5−67.5 °C) can be obtained by
sublimation, but the 90% material was used in the experiments
described here. The specified amounts of quinone are corrected for the
presence of impurity.
1
(42% overall yield). H NMR (CDCl3): δ 2.78 (s, 4H), 1.67 (s, 4H),
1.4 (s, 4H). 19F NMR (CDCl3): δ −145.5 (m, 2F), −160.8 (m, 2F).
13C NMR (CDCl3): δ 144.9 (1JCF = 245 Hz), 138.6 (1JCF = 252 Hz),
124.6, 30.0, 25.8, 23.4. HRMS Calcd C12H12F4: 232.0875. Found:
232.0877.
Ethyl 4,5,6,7-Tetrafluorobenzo[d]-1,3-dioxole-2-carboxylate
(10). o-Fluoranil (0.264 g, 1.5 mmol) was refluxed with ethyl
diazoacetate (0.335 g, 2.9 mmol) in benzene (10 mL) for 8 h. The
solvent was evaporated under vacuum, and the crude product was
taken up in diethyl ether (15 mL). To remove unreacted ethyl
diazoacetate, the solution was washed with 1 M HCl (15 mL), then
with saturated sodium bicarbonate solution (15 mL). Evaporation of
the solvent left 263 mg (67% yield) of crude dioxole. This was
chromatographed on 20 g of silica gel using 1% EtOAc/hexanes as the
eluent. A colorless oily product was obtained: 0.154 g (40% yield). 1H
NMR (CDCl3): δ 6.34 (s, 1H), 4.28 (q, J = 7.2 Hz, 2H), 1.28 (t, J =
7.2 Hz, 3H). 19F NMR (CDCl3): δ −162.4 (m, 2F), −164.6 (m, 2F).
1,2,3,4-Tetrafluoro-5,6-bis(trimethylsilyl)benzene (4). In a 25
mL round-bottom flask were placed a 60:40 quinone mixture (1.00 g,
3.3 mmol ortho), 0.68 g (4 mmol) of bis(trimethylsilyl)acetylene, and
3 mL of toluene. The solution was refluxed until the o-fluoranil was
consumed (2.5 h); then the solvent was evaporated. Tetrafluoro-5,6-
bis(trimethylsilylbicyclo[2.2.2]octa-2,5-diene-7,8-dione: 1H NMR
(CDCl3): δ 0.36 (s, 18H). 19F NMR (CDCl3): δ −153.2 (m, 2F),
−193.3 (m, 2F). Unchanged p-fluoranil was observed at −142.0 ppm.
This mixture dissolved in methylene chloride (25 mL) was placed in a
cylindrical Pyrex tube and purged with a stream of nitrogen to remove
968
dx.doi.org/10.1021/jo202193c | J. Org. Chem. 2012, 77, 966−970

The Journal of Organic Chemistry
Article
13C NMR (CDCl3): δ 163.4, 137.2 (1JCF = 249 Hz), 132.8 (1JCF = 251
Hz), 131.6, 105.8, 63.1, 13.7. HRMS Calcd for C10H6F4O4: 266.0202.
Found: 266.0191.
trimethoxy compound 19 at −144.4 (s, 1F) and −160.2 (q, J = 5.2 Hz,
1F, long-range coupled to a methyl group). To establish the structures
of 18 and 19, the reaction sequence was run on a larger scale in
methanol as the solvent.
2,3,4,5-Tetrafluoro-6-(1-hexyl-1H-indol-3-yl)-6-hydroxycy-
clohexa-2,4-dienone (11, R = n-hexyl). A mixture of o-fluoranil
(360 mg, 2.0 mmol) and N-hexylindole (446 mg, 2.2 mmol) in
benzene (10 mL) was stirred for 10 min. The solution was
concentrated to 3 mL and chromatographed on silica gel (10 g,
mesh size 200−400) with benzene. The resulting solid was dissolved
in hot hexanes (30 mL), and the crystal crop obtained at RT was
washed with hexanes (20 mL) and dried at room temperature to give
the dienone as a yellowish solid: mp 86−87 °C (218 mg, 29% yield).
1H NMR (CDCl3): δ 7.83 (d, J = 7.5 Hz, 1H), 7.26 (m, 3H), 7.04 (s,
1H), 4.07 (t, J = 7.3 Hz, 2H), 3.53 (s, 1H), 1.83 (br s, 2H), 1.554 (s,
2H), 1.31 (br s, 4H), 0.88 (br s, 3H). 19F NMR (CDCl3): δ −132.6 (s,
1F), −132.8 (s, 1F), −161.3 (m, 1F), −165.8 (m, 1F). 13C NMR
(CDCl3): δ 187.2, 150.8 (1JCF = 268 Hz), 145.9 (1JCF = 288 Hz), 137.4
(1JCF = 269 Hz), 136.7, 131.6 (1JCF = 259 Hz), 127.2, 124.9, 122.8,
120.8, 120.7, 110.2, 109.5, 76.3, 46.8, 31.2, 29.9, 26.5, 22.4, 13.9. Anal.
Calcd for C20H19F4NO2: C, 62.99; H, 5.02; N, 3.67. Found: C, 62.74;
H, 5.06; N, 3.63.
o-Fluoranil (0.450 g, 2.5 mmol) was dissolved in 5 mL of methanol.
The solution was stirred for 13 h at room temperature to form
trimethoxy compound 19, NMR yield of 84%. The reaction mixture
was cooled in ice and stirred with NaHCO3 (2.25 g, to neutralize HF)
until the pH reached ∼6. Methylene chloride (20 mL) was added, the
mixture was filtered, and the solvent was evaporated to give compound
19. 19F NMR (toluene): δ −142.7 (s, 1F), −161.6 (s, 1F). A small
amount of unreacted dimethoxy compound 18 was present: 19F NMR
(toluene): δ −135.3 (s, 1F), −140.4 (s, 1F), −157.7 (s, 1F). The
product was dissolved in toluene (5 mL) containing dimethyl
acetylenedicarboxylate (0.532 g, 3.75 mmol), and the solution was
refluxed for 33 h, yielding Diels−Alder adduct 20. 19F NMR (toluene):
δ −202.5 (s, 2F). The solvent was removed and replaced with
methylene chloride. The solution was irradiated for 1 h through Pyrex
with the 450 W Hanovia lamp. Evaporation of the solvent was
followed by chromatography on silica gel with 5% EtOAc/hexanes as
the eluent. The yield of phthalate ester 21 was 0.077 g (11% overall).
1
19F NMR (CDCl3): δ −133.4 (s, 2F). H NMR (CDCl3): δ 4.03 (s,
2,3,4,5-Tetrachloro-6-(1-hexyl-1H-indol-3-yl)-6-hydroxycy-
clohexa-2,4-dienone (13, R = n-hexyl). A mixture of o-chloranil
(1000 mg, 4.1 mmol) and N-hexylindole (821 mg, 4.1 mmol) in
benzene (15 mL) was stirred for 10 min. The solution was
concentrated to 5 mL and chromatographed on silica gel (10 g,
mesh size 200−400) with 5% EtOAc/hexanes. The resulting solid was
dissolved in hexanes (30 mL), and the crystal crop obtained at −25 to
−20 °C was washed with cold hexanes (10 mL) and dried at room
temperature to give an orange solid: mp 102−103 °C (155 mg, 8.6%
6H), 3.92 (s, 6H). 13C NMR (CDCl3): δ 163.4, 150.0 (1JCF = 254 Hz),
144.0, 115.5, 61.9, 53.0. HRMS Calcd for C12H12F2O6: 290.0602.
Found: 290.0590.
Dehydrogenation of 1,3-Cyclohexadiene. o-Fluoranil (32 mg)
was dissolved in CDCl3 (0.5 mL) in an NMR tube, and 1,3-
cyclohexadiene (12.8 mg, 0.9 equiv) was added. Within 10 min, the
diene peaks in the 1H spectrum had disappeared, replaced by a
benzene peak at δ 7.37. The 19F spectrum showed signals for
tetrafluorocatechol at δ −165.3 (m, 2F) and −169.7 (m, 2F), and a
small amount of residual quinone.
o-Fluoranil−Hexamethylbenzene Complex (22). To a solution
of o-fluoranil (0.204 g, 1.13 mmol) in chloroform (5 mL) was added
hexamethylbenzene (0.183 g, 1.13 mmol). As the reaction mixture was
stirred for 1 h, a brown solid precipitated. It was collected by filtration,
dried on the filter, and recrystallized from hot hexanes (60 mL).
Brown needles deposited at RT. The crystals were collected and
washed with hexanes: mp 121−122 °C (0.137 g, 35% yield). 19F NMR
(CDCl3): o-fluoranil signals were unperturbed by the benzene. Anal.
Calcd for C18H18F4O2: C, 63.15; H, 5.30; F, 22.20. Found: C, 63.27;
H, 5.25; F, 22.22.
1
yield). H NMR (CDCl3): δ 7.70 (d, J = 7.5 Hz, 1H), 7.22 (m, 4H),
4.08 (m, 2H), 1.80 (s, 1H), 1.29 (m, 8H), 0.87 (s, 3H). 13C NMR
(CDCl3): δ 188.5, 143.8, 140.5, 136.6, 128.4, 127.3, 124.9, 124.6,
122.5, 120.7, 120.5, 110.3, 108.6, 80.6, 46.8, 31.2, 29.8, 26.5, 22.5, 14.0.
Anal. Calcd for C20H19Cl4NO2: C, 53.72; H, 4.28; N, 3.13. Found: C,
53.90; H, 4.30; N, 3.22.
2,3,4,5-Tetrafluoro-6-hydroxy-6-(1-methyl-1H-pyrrol-2-yl)-
cyclohexa-2,4-dienone (14, R = Me). A mixture of o-fluoranil (27
mg, 0.15 mmol) and N-methylpyrrole (13.5 mg, 0.16 mmol) in CDCl3
(0.5 mL) was shaken for 2 min in an NMR tube. The orange quinone
solution had turned dark brown upon addition of the pyrrole due to
charge-transfer complexation, and addition followed rapidly. In our
1
hands, the adduct was too labile to purify. H NMR (CDCl3): δ 6.68
(m, 1H), 6.03 (m, 2H), 3.85 (d, J = 1.2 Hz, 3H), 3.5 (br s, 1H). 19F
NMR (CDCl3): δ −132.5 (m, 1F), −133.9 (m, 1F), −160.6 (m, 1F).
−164.6 (m, 1F). 13C NMR (CDCl3): δ 186.7, 150.4 (1JCF = 293 Hz),
144.9 (1JCF = 282 Hz), 136.8 (1JCF = 264 Hz), 131.9 (1JCF = 260 Hz),
128.3, 123.8, 110.5, 107.2, 76.1, 36.3. 13C NMR (CDCl3, 19F
ASSOCIATED CONTENT
■
S
* Supporting Information
1H, 19F, and 13C NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
decoupled, H coupled): 186.7, 150.4, 144.9, 136.8, 131.9, 128.3 (d,
J = 185 Hz), 123.8, 110.5 (d, J = 170 Hz), 107.2 (d, J = 174 Hz), 76.1,
36.3, (q, J = 140 Hz).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: david.m.lemal@dartmouth.edu.
2,3,4,5-Tetrachloro-6-hydroxy-6-(1-methyl-1H-pyrrol-2-yl)-
cyclohexa-2,4-dienone (16, R = Me).7,8 o-Chloranil (30 mg, 0.12
mmol), N-methylpyrrole (9.9 mg, 0.12 mmol), and CDCl3 (0.5 mL)
were combined in an NMR tube. The orange quinone solution had
turned dark brown upon addition of the pyrrole due to charge-transfer
complexation. The tube was allowed to stand for 3 h to allow
completion of the addition reaction. 13C NMR (CDCl3): δ 188.1,
143.7, 139.8, 128.3, 127.8, 125.6, 122.5, 110.8, 107.3, 80.7, 36.1.
Dimethyl-3,6-difluoro-4,5-dimethoxyphthalate (21). Metha-
nol (4 drops) was added to a solution of 30 mg of o-fluoranil in 0.5 mL
of CDCl3 in an NMR tube cooled to 0 °C. The 19F spectrum revealed
the development of signals for adduct 17 at −132.1 (m, 1F), −141.1
ACKNOWLEDGMENTS
The authors thank the National Science Foundation for
support of this work (Grant No. CHE-0653935).
■
REFERENCES
■
(1) Shteingarts, V. D.; Budnik, A. G. Izv. Sib. Otd. Akad. Nauk SSSR,
Ser. Khim. Nauk 1967, 124.
(2) Lemal, D. M.; Ramanathan, S.; Shellito, J. J. Org. Chem. 2008, 73,
3392.
(3) Lemal, D. M.; Sang, D.; Ramanathan, S. J. Org. Chem. 2009, 74,
7804.
(4) Examples of dienophiles that yield significant amounts of both
kinds of adduct include cyclopentadiene, norbornadiene, cis- and trans-
stilbene, and trans-β-methylstyrene.
(m, 1F), −162.2 (m, 1F), and −163.8 (m, 1F). 13C NMR (CDCl3, 19
F
1
decoupled, H coupled): 183.9, 150.1, 143.7, 136.6, 132.5, 90.6, 50.0
(q, J = 141 Hz). After the tube had warmed to RT, the 19F signals
faded and new peaks for compound 18 came to dominate the
spectrum at −134.9 (1F), −140.9 (1F), and −156.5 (1F), all singlets
under the measurement conditions. With the addition of more
methanol, these peaks disappeared, replaced by two representing
(5) Bogachev, A. A.; Kobrina, L. S. Russ. J. Org. Chem. 1997, 33, 681.
969
dx.doi.org/10.1021/jo202193c | J. Org. Chem. 2012, 77, 966−970

The Journal of Organic Chemistry
Article
(6) The analogous derivative of o-chloranil was prepared by Latif, N.;
Zeid, I.; El-Masry, R. Egypt J. Chem. 1980, 20, 617.
(7) Saito, K.; Horie, Y.; Takahashi, K. Chem. Pharm. Bull. 1988, 36,
4986.
(8) Saito, K.; Horie, Y. Heterocycles 1986, 24, 579.
(9) Horner, L.; Merz, H. Justus Liebigs Ann. Chem. 1950, 570, 7.
(10) Horner, L.; Spietschka, W. Justus Liebigs Ann. Chem. 1955, 591,
1.
(11) The vinyl fluorines in Diels−Alder adducts of o-fluoranil
typically appear at δ −145 to −150 ppm in the 19F NMR spectrum.
(12) o-Fluoranil’s reduction potential in acetonitrile is 0.17 V vs SCE.
Stallings, M. D.; Morrison., M. M.; Sawyer, D. T. Inorg. Chem. 1981,
20, 2655.
(13) The quinone’s reduction potential has recently been estimated
by theoretical methods: 0.219 V vs NHE in acetonitrile and 0.290 V in
dimethyl sulfoxide. Zhu, X.-Q.; Wang, C.-H. J. Org. Chem. 2010, 75,
5037.
(14) Shteingarts, V. D.; Budnik, A. G.; Yakobson, G. G.; Vorozhtsov,
N. N. Jr. Zh. Obshch. Khim. 1967, 37, 1537.
(15) The original Russian method yielded purer quinone but
employed much larger amounts of water and benzene with some loss
of product by volatilization during evaporation of the benzene (ref 1).
(16) This compound had been prepared previously in very low yield
by photolysis of 1,2-diiodobenzene in benzene, followed by catalytic
hydrogenation. Brewer, J. P. N.; Eckhard, I. F.; Heaney, H.; Johnson,
M. G.; Marples, B. A.; Ward, T. J. J. Chem. Soc. C 1970, 2569.
NOTE ADDED AFTER ASAP PUBLICATION
Scheme 3 was published with errors on January 5, 2012; the
corrected version was reposted on January 9, 2012.
■
970
dx.doi.org/10.1021/jo202193c | J. Org. Chem. 2012, 77, 966−970