A re-examination of the methylenation reaction

Article abstract of DOI:10.1016/S0040-4020(03)00619-7

A re-examination of the methylenation reaction of 1-hydroxy-2-mercapto-, 1,2-dihydroxy- and 1,2-dimercapto-substituted benzenes by bromochloromethane with cesium carbonate shows that these substrates give mixtures of five- and ten-membered benzocondensed heterocyclic compounds and in some cases even dibenzodioxines.

Full text of DOI:10.1016/S0040-4020(03)00619-7

Tetrahedron 59 (2003) 4383–4387
A re-examination of the methylenation reaction
Maria Grazia Cabiddu, Enzo Cadoni, Stefania De Montis, Claudia Fattuoni,
*
Stefana Melis and Michele Usai
Dipartimento di Scienze Chimiche, Cittadella Universitaria di Monserrato, SS 554 Bivio per Sestu, I-09042 Monserrato (CA), Italy
Received 3 January 2003; revised 18 March 2003; accepted 10 April 2003
Abstract—A re-examination of the methylenation reaction of 1-hydroxy-2-mercapto-, 1,2-dihydroxy- and 1,2-dimercapto-substituted
benzenes by bromochloromethane with cesium carbonate shows that these substrates give mixtures of five- and ten-membered
benzocondensed heterocyclic compounds and in some cases even dibenzodioxines. q 2003 Elsevier Science Ltd. All rights reserved.
In previous papers we reported a systematic study on
bimetallation of aromatics promoted by the thioethereal
group. Recently, this research has been extended to 1,3-
benzodioxoles and 1,3-benzoxathioles1,2 because these
systems are contained in natural compounds, in bioactive
molecules and in materials showing non-linear optical
properties.3–8
its elemental analysis was equal to 2a, but its mass spectrum
showed a molecular ion Mþ of 276, corresponding to the
dimer 3a. Furthermore, the 1H NMR spectrum of 2a showed
the methylene singlet at d 5.50,17 while the solid product
showed two singlets of equal intensity at d 4.12 and 5.89
attributable to the two methylenes in 3a.
For a better understanding of this methylenation reaction,
we decided to re-examine the reactions with catecholic
derivatives. Using 1,2-benzodiol and its derivatives the
reaction gave a mixture of benzodioxole and tetraoxecine
derivatives. In detail, methyl derivatives 1c and 1d gave a
significant amount of tetraoxecines ranging between 26 and
30%: in fact 1c gave a 1.1:1 mixture of two isomeric
tetraoxecines 3c and 30c; while 1d gave an almost equimolar
mixture of 3d and 30d. Employing the methoxy derivative
1g, we obtained a mixture of the benzodioxole 2g (68%) and
the isomeric tetraoxecine 3g and 30g (17%) beside two
isomeric dibenzodioxines 4 (or 40) and 5 (or 50), in almost
equal amounts (Fig. 1).
It is known that these compounds can be prepared
condensing 1,2-benzodioles or 2-hydroxybenzenethioles
with dichloromethane or dibromomethane in the presence
of alkaline hydroxides or fluorides or potassium carbonate
or sodium hydride in various solvents (DMSO, DMF,
HMPA) with or without catalytic amounts of bronze, nickel
oxide or cupric oxide.9–15 Good results can be obtained
using phase transfer catalysts.16,17 More recently, the
methylenation of a variety of catechols has been reported,
employing cesium carbonate and bromochloromethane in
N,N-dimethylformamide or acetonitrile.18
In this work we intend to verify if this method can be
applied to the methylenation of 2-hydroxybenzenethiol
(1a).
Then we decided to apply this reaction protocol to 1,2-
dimercaptobenzene (1h): 1,3-benzodithiole (2h) (98%) was
obtained almost exclusively beside traces amount of
unknown products.
All reactions were performed in anhydrous N,N-dimethyl-
formamide reacting 1a, cesium carbonate and bromochloro-
methane. Reaction yields were determined by HPLC
analysis of the reaction mixture obtained after extraction
with solvents. The products were isolated as pure by flash-
chromatography (Scheme 1).
The same reactions performed in acetonitrile and at different
concentrations gave analogous results. At the end we can
deduce that when the benzene ring bears two equally
nucleophilic groups (benzodioles and dimercaptobenzenes)
the main product is the one with a five-membered ring;
when the groups have a different nucleophilic power the
main product is the ten-membered ring.
Surprisingly, the expected 1,3-benzoxathiole (2a), liquid at
room temperature,17 was obtained in a 32% yield, while the
main product (65% yield) was a solid with mp 148–1508C:
The structure determination of all products was performed
by comparison with commercial authentic samples or by
spectroscopic techniques. For example all 3 compounds
Keywords: methylenation; benzodioxoles; dioxadithiecins; tetroxecins.
*
Corresponding author; e-mail: stmelis@unica.it
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00619-7

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M. G. Cabiddu et al. / Tetrahedron 59 (2003) 4383–4387
Scheme 1.
show a mass spectrum with a similar fragmentation pattern,
with a low abundance molecular ion and a base peak
represented by a ion I or II, both of which have greater
aromatic character than the molecular ion (Fig. 2).
5.57 and 5.73 in the 1H NMR spectrum, while another
singlet at 5.85 was attributed to the equivalent methylenes
of 30g. On the contrary, we were unable to separate the two
dibenzodioxines 4 and 5 because their HPLC retention times
were too close (5.05 and 5.3 min, respectively): then we
performed several successive flash-chromatographies to
enrich the mixture in one isomer. We reached a mixture
composition of 5:1 and its elemental analysis and mass
spectrum were identical to the ones of the 1:1 mixture. The
mass spectrum showed the molecular ion as base peak and a
very poor fragmentation, typical of aromatic compounds.
The enrichment made easier the assignment of some signals
Two isomers 3c and 30c were identified by 1H NMR analysis
of their mixture. The HPLC analysis revealed two peaks with
very close retention times (4.38 and 4.59 min, respectively)
so that their separation appeared to be very difficult. The 1H
NMR spectrum of their mixture showed two singlets at d 2.26
and 2.28 in the ratio of 1:1.1, attributable to the methyl groups
of the two isomers; two singlets at d 5.57 and 5.62 were due to
the non-equivalents methylenes of the more abundant isomer
3c and another singlet at d 5.56 was attributed to the two
methylenes of the isomer 30c.
1
in the H and 13C NMR spectra, but it was not possible to
attribute them to one of the four possible structures.
The formation of the benzodioxines was completely
unexpected: in fact these compounds are usually obtained
through nucleophilic substitution by phenoxide anion on aryl
halides or by self-condensation of 2-halogenophenols.19,20
We may tentatively justify the formation of these benzo-
condensed compounds by a radical process involving a
semiquinone radical, which can easily be formed in alkaline
solution by oxidation. This anion radical then attacks the
already formed benzodioxole leading to the condensed 4 (or
40) and 5 (or 50).
For 3d and 30d the HPLC analysis of their mixture revealed
only one peak. The two isomers were identified by the H
1
NMR spectrum of their mixture: two singlets at d 5.61 and
5.65 were attributed to the two non-equivalents methylenes
of the more abundant isomer 3d and a singlet at d 5.63 was
attributed to the two methylenes of 30d. The relative
intensity of these signals revealed a ratio between the two
isomers 3d/30d of 1.1:1. The methyl groups of the two
isomers gave only one signal at d 2.33.
Following the reaction of 1g chromatography gave 2g and
an inseparable mixture of 3g and 30g. Also isolated was a
mixture of isomeric dibenzodioxines. The compound 3g
was identified by two singlets, due to the methylenes, at d
The molecular ion of 30f gave a particularly low abundance
peak because the two CHO in para to the OCH2O led to a
predominance of the symmetric fragmentation showed in
the following structure (Fig. 3).
Figure 1.

M. G. Cabiddu et al. / Tetrahedron 59 (2003) 4383–4387
4385
The second fraction was identified as 3a; yield 65%;
crystallised from ethanol as white crystals, mp 156–1588C.
[Found: C, 60.77; H, 4.33; S, 23.13. C14H12O2S2 requires C,
60.84; H, 4.38; S, 23.20%]; nmax (Nujol) 1580, 1495, 1260,
1200, 1135 cm21; dH (300 MHz, CDCl3) 4.12 (2H, s,
SCH2S), 5.89 (2H, s, OCH2O), 7.16 (2H, t, J¼7.5 Hz, ArH),
7.24 (2H, d, J¼9.0 Hz, ArH), 7.42 (2H, t, J¼7.5 Hz, ArH),
7.60 (2H, d, J¼7.8 Hz, ArH); dc (75.4 MHz, CDCl3) 46.7,
95.8, 118.5, 124.9, 125.4, 130.8, 136.9, 159.4; m/z (EI) 276
(4, Mþ), 151 (30), 139 (15), 138 (77), 137 (100), 121 (6),
109 (11), 96 (15), 82 (4), 78 (7), 77 (10).
Figure 2.
1.3.2. Methylenation of 1b. The HPLC analysis showed the
presence of two products. The extract was flash-chromato-
graphed using light petroleum as eluent. The first fraction
was identified as 2b by comparison of its NMR and mass
spectra with those of a commercial sample; yield 88%;
pale yellow oil, bp 65–668C/10 mm Hg (lit.21 bp
608C/9 mm Hg).
Figure 3.
1. Experimental
1.1. General
The second fraction was identified as 3b by comparison with
an authentic sample; yield 11%; crystallised from acetic
acid as white crystals, mp 2628C (lit.21 mp 261–2628C).
[Found: C, 66.78; H, 4.91. C14H12O4 requires C, 66.85; H,
Analyses by HPLC were carried out with a Waters 600
HPLC equipped with a UV Gilson 116 detector and a
Spherisorb NH2 (in the case of aldehydes a Spherisorb
silica) 5m column (25 cm£4.6 mm) using 98:2 hexane/2-
propanol as eluent. IR spectra were recorded on a Perkin–
4.95%]; nmax (Nujol) 1590, 1500, 1250, 1195, 1105 cm21
;
dH (300 MHz, CDCl3) 5.61 (4H, s, OCH2O), 7.11 (8H, s,
ArH); dc (75.4 MHz, CDCl3) 98.5, 121.4, 125.7, 150.0. m/z
(EI) 244 (6, Mþ), 135 (3), 122 (100), 121 (68), 80 (11), 77
(6).
1
Elmer 1310 grating spectrophotometer. H and 13C NMR
spectra were recorded on a Varian VXR-300 spectrometer.
Mass spectra were obtained at 70 eV with a Hewlett–
Packard 5989A mass spectrometer, using the direct inlet
system. Microanalyses were carried out on a Carlo Erba
1106 element analyser.
1.3.3. Methylenation of 1c. The HPLC analysis showed
three peaks: two peaks had very close retention times (4.38
and 4.59 min, respectively) so that their separation appeared
to be very difficult.
1.2. Materials
The extract was flash-chromatographed using light pet-
roleum as eluent. The first fraction was identified as 2c by
comparison of its NMR and mass spectra with those of an
authentic sample; yield 70%; pale yellow oil, bp 608C/
3 mm Hg (lit.22 bp 458C/1.2 mm Hg).
Reagent-grade commercially available reagents and sol-
vents were used. 2-Hydroxybenzenethiol (1a), 1,2-benzo-
diol derivatives (1b–f) and 1,2-dimercaptobenzene (1g)
were purchased (Aldrich).
1.3. General method of methylenation of compounds
1a–h
1
The H NMR analysis of the second fraction showed the
presence of two isomers 3c and 30c in a 1.1:1 ratio; total
yield 26%; crystallised from acetic acid as white crystals,
mp 190–1928C. [Found C, 70.48; H, 5.87. C16H16O4
requires C, 70.57; H, 5.92%]; nmax of the mixture (Nujol)
To a suspension of the starting compound 1a–h (50 mmol),
cesium carbonate (24.4 g, 75 mmol) and anhydrous N,N-
dimethylformamide (120 mL), was added dropwise bromo-
chloromethane (9.6 g, 75 mmol) and the resulting mixture
was vigorously stirred under a nitrogen atmosphere and then
heated at 107–1108C for 2 h. The mixture was poured into
water, the organic layer separated and the aqueous layer
extracted with ether and then with dichloromethane,
because ether is able to extract only the monomeric product
while the dimeric ones are extracted by dichloromethane.
The organic phases were dried (Na2SO4). The solvent was
evaporated in vacuo and the residue was analysed by HPLC.
1570, 1515, 1250, 1200, 1115 cm21
.
Compound 3c. dH (300 MHz, CDCl3) 2.28 (6H, s, CH3),
5.57 (2H, s, OCH2O), 5.62 (2H, s, OCH2O), 7.08 (6H, m,
ArH); dc (75.4 MHz, CDCl3) 20.5, 98.1, 98.3, 120.2, 122.4,
126.0, 135.2, 147.0, 150.3.
Compound 30c. dH (300 MHz, CDCl3) 2.26 (6H, s, CH3),
5.56 (4H, s, OCH2O), 7.08 (6H, m, ArH); dc (75.4 MHz,
CDCl3) 20.5, 98.2, 121.5, 122.6, 125.3, 136.4, 146.6, 150.7;
m/z (EI) of the mixture: 272 (7, Mþ), 149 (4), 137 (9.5), 136
(100), 135 (53), 105 (3), 91 (6), 79 (5), 78 (8), 77 (10).
1.3.1. Methylenation of 1a. The HPLC analysis showed the
presence of two products. The extract was flash-chromato-
graphed using light petroleum as eluent. The first fraction
was identified as 2a by comparison of its NMR and mass
spectra with those of an authentic sample; yield 32%;
pale yellow oil, bp 101–1028C/10 mm Hg (lit.17 bp 93–
958C/5 mm Hg).
1.3.4. Methylenation of 1d. The HPLC analysis showed
two peaks. The extract was flash-chromatographed using
light petroleum as eluent. The first fraction was identified as
2d by comparison of its NMR and mass spectra with those

4386
M. G. Cabiddu et al. / Tetrahedron 59 (2003) 4383–4387
of an authentic sample; yield 68%; pale yellow oil, bp 115–
1178C/3 mm Hg (lit.21 bp 196–1978C/753.5 mm Hg).
1.3.7. Methylenation of 1g. The HPLC analysis showed
four peaks. The mixture was flash-chromatographed using
9:1 ether/light petroleum as eluent. The first fraction was
identified as 2g by comparison of its NMR and mass spectra
with those of an authentic sample; yield 68%; crystallised
from ethanol as white crystals, mp 41–428C (lit.15 mp 39–
418C).
The 1H NMR analysis of the second fraction (corresponding
to the second HPLC peak) showed the presence of two
isomers 3d and 30d in a 1.1:1 ratio; total yield 29%;
crystallised from acetic acid as white crystals, mp 219–
2208C. [Found C, 70.50; H, 5.98 C16H16O4 requires C,
70.57; H, 5.92%]; nmax of the mixture (Nujol) 1580, 1505,
The 1H NMR analysis of the second fraction (corresponding
to the second HPLC peak) shows the presence of two
isomers 3g and 30g in a 1.1:1 ratio; yield 17%; crystallised
from methanol as white crystals, mp 205–2068C. [Found C,
63.07; H, 5.35. C16H16O6 requires C, 63.15; H, 5.30%]; nmax
1255, 1205, 1120 cm21
.
Compound 3d. dH (300 MHz, CDCl3) 2.33 (6H, s, CH3),
5.61 (2H, s, OCH2O), 5.65 (2H, s, OCH2O), 6.97 (2H, d,
J¼8.1 Hz, ArH), 6.99 (2H, s, ArH), 7.06 (2H, d, J¼8.1 Hz,
ArH); dc (75.4 MHz, CDCl3) 20.9, 98.4, 98.6, 120.9, 121.8,
126.2, 135.6, 147.6, 149.5.
of the mixture (Nujol) 1600, 1500, 1250, 1150 cm21
.
Compound 3g. dH (300 MHz, CDCl3) 3.77 (6H, s, OCH3),
5.57 (2H, s, OCH2O), 5.73 (2H, s, OCH2O), 6.79 (4H, m,
ArH), 7.11 (2H, m, ArH); dc (75.4 MHz, CDCl3) 55.8, 94.5,
98.2, 108.4, 113.3, 124.8, 128.3, 151.1, 152.8.
Compound 30d. dH (300 MHz, CDCl3) 2.33 (6H, s, CH3),
5.63 (4H, s, OCH2O), 6.97 (2H, d, J¼8.1 Hz, ArH), 6.99
(2H, s, ArH), 7.06 (2H, d, J¼8.1 Hz, ArH); dc (75.4 MHz,
CDCl3) 20.9, 98.5, 121.1, 121.6, 126.1, 135.5, 147.4, 149.6;
m/z (EI) of the mixture 272 (8, Mþ), 149 (2), 137 (9), 136
(100), 135 (25), 106 (3), 91 (2), 78 (5), 77 (3).
Compound 30g. dH (300 MHz, CDCl3) 3.80 (6H, s, OCH3),
5.85 (4H, s, OCH2O), 6.42 (4H, d, J¼8.0 Hz, ArH), 6.68
(2H, t, J¼7.8 Hz, ArH); dc (75.4 MHz, CDCl3) 56.4, 101.0,
102.3, 107.4, 121.9, 135.1, 143.9, 148.6; m/z (EI) of the
mixture 304 (18, Mþ), 165 (6), 153 (10), 152 (100), 151
(34), 137 (8), 107 (13), 95 (10), 79 (6).
1.3.5. Methylenation of 1e. The HPLC analysis showed the
presence of two products. The extract was flash-chromato-
graphed using 10:1 hexane/ethyl acetate as eluent. The first
fraction was identified as 2e by comparison of its NMR and
mass spectra with those of an authentic sample; yield 88%;
crystallised from ethanol as white crystals, mp 32–348C
(lit.23 mp 338C).
The third fraction was identified as a 1:1 mixture of two
products 4 (or 40) and 5 (or 50). Yield 10%. This mixture was
isolated by chromatography as a white solid with mp 219–
2208C. [Found C, 62.41; H, 4.25. C15H12O6 requires: C,
62.50; H, 4.20%]; m/z (EI) 288 (100, Mþ), 273 (9), 245 (12),
243 (9), 230 (8), 217 (17), 215 (10), 187 (5), 165 (6), 144
(8), 116 (5), 77 (12).
The second fraction was identified as 3e. Yield 10%,
crystallised from acetic acid as a white solid, mp 200–
2028C. [Found C, 63.91; H, 3.97. C16H12O6 requires C,
63.99; H, 4.03%]; nmax (Nujol) 2710, 1695, 1600, 1255,
1105 cm21; dH (300 MHz, CDCl3) 5.69 (2H, s, OCH2O),
5.79 (2H, s, OCH2O), 7.31 (2H, t, J¼7.5 Hz, ArH), 7.42
(2H, d, J¼7.5 Hz, ArH), 7.65 (2H, d, J¼7.5 Hz, ArH), 10.18
(2H, s, CHO); dc (75.4 MHz, CDCl3) 98.4, 99.1, 121.2,
122.7, 124.2, 132.4, 136.8, 148.9, 151.2, 190.1; m/z (EI) 300
(14, Mþ), 270 (5.5), 164 (15), 163 (15), 151 (17), 150 (91),
149 (100), 134 (23), 122 (14), 121 (24), 107 (15), 105 (13),
92 (21), 91 (10), 77 (18).
After repeated column chromatographies on silica using
light petroleum as eluent the mixture was enriched in one of
the two components reaching a ratio of 5:1. The mp of this
mixture was 232–2348C. The elemental analysis and mass
spectrum of the 5:1 mixture were identical to those of the
1:1 mixture. One product: dH (300 MHz, CDCl3) 3.86 (3H,
s, OCH3), 4.08 (3H, s, OCH3), 5.85 (2H, s, CH2), 6.15 (1H,
s, ArH), 6.43 (1H, dd, J¼8.0, 1.8 Hz, ArH), 6.54 (1H, dd,
J¼8.0, 1.8 Hz, ArH), 6.82 (1H, t, J¼8.0 Hz, ArH); dc
(75.4 MHz, CDCl3) 56.11, 60.50, 91.82, 101.04, 107.22,
109.87, 122.82, 128.99, 131.47, 132.78, 133.12, 137.08,
142.47, 143.41, 148.23. Other product: dH (300 MHz,
CDCl3) 3.87 (3H, s, OCH3), 4.05 (3H, s, OCH3), 5.86
(2H, s, CH2), 6.31 (1H, s, ArH), 6.44 (1H, dd, J¼8.2,
1.7 Hz, ArH), 6.54 (1H, dd, J¼8.2, 1.7 Hz, ArH), 6.83 (1H,
t, J¼8.2 Hz, ArH); dc (75.4 MHz, CDCl3) 56.12, 60.51,
92.45, 101.06, 107.22, 109.25, 122.82, 128.98, 131.45,
132.79, 133.11, 137.08, 142.45, 143.41, 147.87.
1.3.6. Methylenation of 1f. The HPLC analysis showed the
presence of two products. The extract was flash-chromato-
graphed using 10:1 hexane/ethyl acetate as eluent. The first
fraction was identified as 2f by comparison of its NMR and
mass spectra with those of an authentic sample; yield 88%;
crystallised from ethanol as white crystals, mp 36–378C
(lit.24 mp 378C).
The second fraction was identified as 30f. Yield 10%;
crystallised from acetic acid as white crystals, mp 260–
2628C. [Found C, 63.88; H, 3.98. C126H12O6 requires C,
63.99; H, 4.03%]; nmax (Nujol) 2715, 1695, 1600, 1260,
1100 cm21; dH (300 MHz, CDCl3) 5.70 (4H, s, OCH2O),
7.37 (2H, d, J¼6.9 Hz, ArH), 7.65 (4H, m, ArH), 9.90 (2H,
s, CHO); dc (75.4 MHz, CDCl3) 98.6, 122.0, 128.8, 130.9,
134.4, 154.7, 167.7, 190.2; m/z (EI) 300 (3, Mþ), 151 (10),
150 (100), 149 (79), 135 (2), 121 (14), 103 (3), 91 (5), 79
(10), 77 (6).
1.3.8. Methylenation of 1h. The HPLC analysis showed the
presence of 2h. The residue was flash-chromatographed
using light petroleum as eluent. The first fraction was
identified as 2h by comparison of its NMR and mass
spectra with those of an authentic sample; yield 98%; pale
yellow oil, bp 104–1068C/2 mm Hg (lit.25 bp 95–
968C/1 mm Hg).
We were not able to identify the trace amount products.

M. G. Cabiddu et al. / Tetrahedron 59 (2003) 4383–4387
4387
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