A Simple Preparation of Diarylmethanes by Oxidative Friedel-Crafts Reaction of Methyl-Substituted Benzenes with o-Chloranil

Article abstract of DOI:10.1081/SCC-120026326

The reaction of o-chloranil with some methyl-substituted benzenes at elevated temperature was found to be a simple and selective route toward diarylmethanes without need for prior preparation of benzyl halides.

Full text of DOI:10.1081/SCC-120026326

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A Simple Preparation of Diarylmethanes by Oxidative
Friedel-Crafts Reaction of Methyl-Substituted Benzenes
with o-Chloranil
Tomoshige Kobayashi a & Sheikh Maksudur Rahman a
a Department of Chemistry , Faculty of Science , Shinshu University , Matsumoto, Japan
Published online: 19 Aug 2006.
To cite this article: Tomoshige Kobayashi & Sheikh Maksudur Rahman (2003) A Simple Preparation of Diarylmethanes
by Oxidative Friedel-Crafts Reaction of Methyl-Substituted Benzenes with o-Chloranil, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:22, 3997-4003, DOI: 10.1081/
SCC-120026326
To link to this article: http://dx.doi.org/10.1081/SCC-120026326
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 22, pp. 3997–4003, 2003
A Simple Preparation of Diarylmethanes
by Oxidative Friedel-Crafts Reaction of
Methyl-Substituted Benzenes with o-Chloranil
*
Tomoshige Kobayashi and Sheikh Maksudur Rahman
Department of Chemistry, Faculty of Science,
Shinshu University, Matsumoto, Japan
ABSTRACT
The reaction of o-chloranil with some methyl-substituted benzenes at
elevated temperature was found to be a simple and selective route
toward diarylmethanes without need for prior preparation of benzyl
halides.
Diarylmethanes have been widely utilized as raw materials for
fragrance,[1] polyurethane resins,[2–4] and medicines.[5–9] A promising
synthetic pathway toward diphenylmethane derivatives is the
Friedel-Crafts type alkylation of aromatic compounds with benzyl
halides.[10] Previously, we eventually found that 1,3-benzodixoles were
*Correspondence: Tomoshige Kobayashi, Department of Chemistry, Faculty of
Science, Shinshu University, 3-1-1 Asahi, Matsumoto, 390-8621 Japan.
3997
DOI: 10.1081/SCC-120026326
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3998
Kobayashi and Rahman
formed by the reaction of o-chloranil and methyl-substituted benzenes in
refluxing tetrachloroethylene albeit in low yields.[11] When we
investigated a similar reaction with the methyl-substituted benzenes as
the solvents, diarylmethanes were obtained in acceptable yields. Thus, we
wish to describe here the synthesis of diarylmethanes without need for
prior preparation of benzyl halides.
A solution of o-chloranil in p-xylene as the solvent was refluxed for
24 h to give 2,40,5-trimethyldiphenylmethane (2) in 71% yield. The
formation of tetrachlorocatechol was also detected by GLC analysis.
Similarly, the reactions of mesitylene and durene with o-chloranil at
140ꢀC provided 2,30,4,50,6-pentamethyldiphenylmethane (3) and
2,20,3,40,5,50,6-heptamethyldiphenylmethane (4) in 94 and 89% yields,
respectively. Treatment of p-methylanisole with o-chloranil afforded
2,40-dimethoxy-5-methyldiphenylmethane (5) in 74% yield, accompanied
by the formation of 2,20,400-trimethoxy-4,40-dimethyltriphenylmethane (6)
in 24% yield. In contrast, the reaction of toluene with o-chloranil under
reflux resulted in the formation of a 1:1 mixture of 4-methyldiphenyl-
methane (7) and 2-methyldiphenylmethane (8) only in 9% yield, along
with the formation of 4,5,6,7-tetrachloro-2-phenyl-1,3-benzodioxole
(9) in 30% yield. The attempted reactions with o-xylene, N,N-dimethyl-
p-toluidine, p-bromotoluene, and 4-methylbenzonitrile resulted in the
formation of a complex mixture.
The formation of diarylmethanes would be explained by the
intermediacy of benzyl cations and the subsequent Friedel-Crafts type
alkylation. Previously, triarylmethanes were described to give triaryl-
methyl cations on treatment with quinones such as DDQ.[12]
p-Chloranil was also reported to abstract a hydride from p-xylene
under the dehydrogenation conditions of cholesterin.[13] However, the
use of DDQ or p-chloranil instead of o-chloranil for the present trans-
Table 1. Products and yields of the oxidative Friedel-Crafts reaction of
methyl-substituted benzenes.
Substrate
Products yielda (%)
p-Xylene
Mesitylene
Durene
2 (71%)
3 (94%)
4 (89%)
p-Methylanisole
Toluene
5 (74%), 6 (24%)
7 þ 8 (1:1, 9%), 9 (30%)
aYields are based on o-chloranil (1). The yields of 6 and 9 are calculated
based on the assumption that two molar equivalents of 1 are required.

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Preparation of Diarylmethanes
3999

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4000
Kobayashi and Rahman
formation resulted in the formation of an inseparable mixture, and the
use of o-chloranil is requisite. Oxidative Friedel-Crafts reactions of
methyl-substituted benzenes were widely investigated with Fe(III),[14–17]
Cu(II),[18] Pd(II),[19] Mn(III) and Mn(IV),[20,21] and NO2þ speices.[22]
Unfortunately, these reactions seem not always useful for the synthesis
of diarylmethanes due to the formation of by-products such as biaryls
and the requirement of acidic conditions. Thus, the present method
would provide one of simple and selective routes to some diarylmethanes
under neutral conditions, without need for prior preparation of the
corresponding benzyl halides.
EXPERIMENTAL
General
All mps were determined with a Yanagimoto hot-stage apparatus.
Infrared spectra were obtained with a JEOL Diamond-20 spectrometer.
NMR spectra were recorded with a JEOL JNM-LA400 (1H: 400 MHz,
13C: 100 MHz) spectrometer using TMS as internal standard.
Assignments of the 1H and 13C signals are based on DEPT, H-H
COSY, and C-H COSY measurements. Mass spectra were measured
with a Shimadzu GCMS-QP1000EX spectrometer operating in the elec-
tron impact mode (70 eV). Elemental analyses were performed with a
Perkin–Elmer Model 240 apparatus. The GLC analyses were carried
out on a Shimadzu GC-12A (FID) instrument equipped with a 10%-
SE30 column at 180ꢀC column temperature using nitrogen as the carrier
gas. Solvents were dried and purified by standards methods. Yields are
based on the isolated products with sufficient purity.
General procedure. A mixture of o-chloranil (1) (246 mg, 1 mmol)
and a methyl-substituted benzene (5 cm3) was heated at 140ꢀC (bath
temperature) for 24 h. The excess aromatic compound was recovered
by vacuum distillation, and the residue was separated by column
chromatography (silica gel, hexane-ethyl acetate 10/1) to give a diphenyl-
methane derivative and other products if available. The remaining parts
of the fractions collected after chromatography was subjected to GLC
analyses, which detected the presence of tetrachlorocatechol.
2,40,5-Trimethyldiphenylmethane[15,23,24] (2). 71%; a colorless oil; bp
130–135ꢀC/2 Torr (bath temp, cold-finger trap distillation). IR (neat)
3045, 3018, 3001, 2972, 2943, 2920, 2861, 1514, 1502, 1444, 1379, 1117,
1
1038, 1022 cmꢁ1. H NMR (CDCl3) ꢀ ¼ 2.17 (3H, s, CH3), 2.26 (3H, s,
CH3), 2.28 (3H, s, CH3), 3.88 (2H, s, CH2) 6.90–7.05 (7H, m). 13C NMR

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Preparation of Diarylmethanes
4001
(CDCl3) ꢀ ¼ 19.2 (CH3), 21.0 (CH3), 39.0 (CH2), 127.0, 128.6, 129.0,
130.1, 130.7, 133.4, 135.2, 135.3, 137.4, 138.9, 1C (CH3) missing. MS
m/z (rel. intensity) 210 (Mþ, 70), 195 (M-CH3, 100), 118 (63), 91
(PhCH2, 13). Found: C, 91.63; H, 8.69%. Calcd. for C16H18: C, 91.37;
H, 8.63%.
2,30,4,50,6-Pentamethyldiphenylmethane (3). 94%; colorless plates
(from chloroform–methanol 1/2). M.p. 68–69ꢀC (Lit.[25] m.p. 67ꢀC). IR
(KBr) 3012, 2964, 2935, 2916, 2858, 2729, 1603, 1579, 1508, 1485, 1454,
1
1442, 1421, 1373, 1036, 1014 cmꢁ1. H NMR (CDCl3) ꢀ ¼ 2.20 (6H, s,
CH3), 2.22 (6H, s, CH3), 2.29 (3H, s, CH3), 3.94 (2H, s, CH2), 6.62
(2H, s), 6.78 (1 H, s), 6.88 (2H, s). 13C NMR (CDCl3) ꢀ ¼ 20.2 (CH3),
20.9 (CH3), 21.3 (CH3), 34.6 (CH2), 125.7, 127.4, 128.8, 133.9, 135.5,
137.0, 137.8, 140.0. MS m/z (rel. intensity) 238 (Mþ, 100), 223 (M-
CH3, 93), 208 (M-2Me, 31), 132 (M-Me2C6H4, 74). Found: C, 90.65;
H, 9.30%. Calcd. for C18H22: C, 90.70; H, 9.30%.
2,20,3,40,5,50,6-Heptamethyldiphenylmethane (4). 89%; colorless plates
(from chloroform–methanol 1/1). M.p. 153–154ꢀC (Lit.[16] m.p.
154–155ꢀC, Lit[22] m.p. 144.6–145.2ꢀC). IR (KBr) 2997, 2960, 2939,
2918, 2862, 2729, 2360, 2341, 1500, 1473, 1460, 1381, 1373, 1020,
1
1001 cmꢁ1. H NMR (CDCl3) ꢀ ¼ 2.04 (3H, s, CH3), 2.06 (6H, s, CH3),
2.18 (3H, s, CH3), 2.26 (6H, s, CH3), 2.36 (3H, s, CH3), 3.87 (2H, s, CH2),
6.28 (1H, s), 6.93 (1H, s), 6.96 (1H, s). 13C NMR (CDCl3) ꢀ ¼ 15.7 (CH3),
19.1 (CH3), 19.2 (CH3), 19.3 (CH3), 20.6 (CH3), 32.7 (CH2), 127.9, 129.8,
131.2, 133.2, 133.5, 133.8, 135.3, 136.7, 2C missing. MS m/z (rel. inten-
sity) 266 (Mþ, 53), 251 (M-CH3, 32), 146 (M-Me3C6H3, 100). Found: C,
90.30; H, 9.93%. Calcd. for C20H26: C, 90.16; H, 9.84%.
2 40-Dimethoy-5-methyldiphenylmethane[20,21] (5). 74%; colorless nee-
dles (from methanol). M.p. 72–73ꢀC (Lit.[26] m.p. 74ꢀC). IR (KBr) 3032,
2997, 2958, 2854, 1610, 1583, 1512, 1466, 1441, 1429, 1300, 1292, 1277,
1
1248, 1227, 1176, 1120, 1032 cmꢁ1. H NMR (CDCl3) ꢀ ¼ 2.22 (3H, s,
CH3), 3.76 (3H, s, OCH3), 3.77 (3H, s, OCH3), 3.87 (2H, s, CH2),
6.74 (1H, d, J ¼ 8 Hz, 3-H), 6.80 (2H, d, J ¼ 9 Hz, 30-H and 50-H),
6.86 (1H, d, J ¼ 2 Hz, 6-H), 6.96 (1 H, dd, J ¼ 8 Hz and 2 Hz, 3-H),
7.12 (2H, d, J ¼ 9 Hz, 20-H and 60-H). 13C NMR (CDCl3)
ꢀ ¼ 20.5 (CH3), 34.9 (CH2), 55.2 (OCH3), 55.5 (OCH3), 110.4,
113.6, 127.5, 129.6, 129.8, 130.9, 133.2, 155.2, 157.7, 1C missing. MS
m/z (rel. intensity) 242 (Mþ, 100), 227 (M-CH3, 49), 211 (M-OCH3,
59), 121 (OCH3C6H4CH2, 50). Found: C, 79.40; H, 7.33%. Calcd. for
C16H18O2: C, 79.31; H, 7.49%.
2,20,400-Trimethoxy-4,40-dimethyltriphenylmethane (6). 24%; colorless
needles (from methanol). M.p. 154–155.0ꢀC. IR (KBr) 2999, 2956, 2941,
2908, 2837, 1608, 1558, 1541, 1510, 1496, 1458, 1436, 1321, 1290, 1238,

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4002
Kobayashi and Rahman
1
1178, 1119, 1107, 1032 cmꢁ1. H NMR (CDCl3) ꢀ ¼ 2.18 (6H, s, CH3),
3.66 (6H, s, OCH3), 3.77 (3H, s, OCH3), 6.09 (1H, s, CH), 6.60 (2H, d,
J ¼ 2 Hz), 6.75 (2H, d, J ¼ 8 Hz), 6.78 (2H, d, J ¼ 8 Hz), 6.95–6.98 (4H,
m). 13C NMR (CDCl3) ꢀ ¼ 20.8 (CH3), 42.2 (CH), 55.1 (OCH3), 56.0
(OCH3), 111.0, 113.3, 127.4, 129.2, 130.3, 130.7, 132.9, 136.1, 155.3,
157.5. MS m/z (rel. intensity) 362 (Mþ, 88), 347 (M-CH3, 45), 331 (M-
OCH3, 100), 241 (M-CH3C6H3OCH3, 14), 121 (CH3C6H3OCH3, 62).
Found: C, 79.48; H, 7.02%. Calcd. For C24H26O3: C, 79.53; H, 7.23%.
A 1:1 mixture of 4-methyldiphenylmethane (7) and 2-methyldiphenyl-
methane[18] (8). 9%; b.p. 108–115ꢀC/2 Torr (bath temp., cold-finger trap
distillation). IR (neat) 3060, 3026, 2918, 1601, 1514, 1495, 1462, 1452,
1
1107, 1074, 1030 cmꢁ1. H NMR (CDCl3) ꢀ ¼ 2.23 (1.5H, s, CH3), 2.30
(1.5H, s, CH3), 3.93 (1H, s, CH2), 3.98 (1H, s, CH2), 7.08–7.28 (9H, m).
13C NMR (CDCl3) ꢀ ¼ 19.7 (CH3), 21.0 (CH3), 39.4 (CH2), 41.5 (CH2),
125.9, 126.0, 126.5, 128.3, 128.4, 128.7, 128.8, 128.9, 129.1, 130.0, 130.3,
135.5, 136.6, 138.1, 138.9, 140.4, 141.4, 1C missing. MS m/z (rel. inten-
sity) 182 (Mþ, 78), 167 (M-CH3, 100), 165 (53), 104 (23). Found: C, 92.14;
H, 7.72%. Calcd. for C14H14: C, 92.26; H, 7.74%.
4,5,6,7-Tetrachloro-2-phenyl-1,3-benzodioxole (9). 30%; m.p. and
mixed m.p. 113.5–115ꢀC (Lit.[11] m.p. 115ꢀC).
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Received in Japan May 7, 2003

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