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tetramethylbenzene afforded the corresponding product
in only 52% GC yield (Table 2, entry 3). In addition,
because the introduction of a Br group deactivated the
benzene ring, only 46% of product was obtained (Table
2, entry 4). In the two-substituted benzene ring, the reac-
tivity was similarly dependent on the orientation of sub-
stituents. For example, m-xylene reacted with trioxane
to give the dehydrated products in 67% GC yield (Table
2, entry 5), but in the case of p-xylene, only 11% of dehy-
1
drated product (determined by H NMR of the crude
reaction mixture) was formed. In the cases of 4-methyl-
anisole and phenetole used, the high yields of the corre-
sponding products were obtained, because the
introduction of an OCH3 or OEt group activated the
benzene ring (Table 2, entries 6 and 7). The reaction
of pentamethylbenzene with trioxane afforded 69% of
dehydrated product (Table 2, entry 8).
However, 4-methoxybenzaldehyde, which bears an
OCH3 group (activating group) and a CHO group
(deactivating group) reacted with trioxane to produce
trace amount of desired product only. Furthermore, in
the cases of chlorobenzene and nitrobenzene used, only
small amount (<5%) of the dehydrated products were
detected by GC. These results indicated that the present
InCl3Æ4H2O-catalyzed dehydration reaction of arenes
with trioxane showed good catalytic activity for elec-
tron-rich arenes only.
6. A typical experiment for the synthesis of 1b and 1b0 (Table
2, entry 1): A mixture of 3,5-dimethylanisole (816.0 mg,
6.0 mmol), 1,3,5-trioxane (18.0 mg, 0.2 mmol, 0.6 mmol of
CH2O) and InCl3Æ4H2O (8.7 mg, 0.03 mmol) were stirred at
120 ꢁC for 6 h. To this resulting reaction mixture, hexane
(5.0 mL) as a diluent and octadecane (52.0 mg) as an
internal standard for GC analysis were added. GC and
GC–MS analyses of the mixture disclosed that bis-(4,6-
dimethyl-2-methoxy-phenyl)methane 1b and (4,6-dimethyl-
2-methoxy-phenyl)-(2,6-dimethyl-4-methoxy-phenyl)meth-
ane 1b0 were formed in a ratio of 83:17. After removal of
the insoluble materials, the filtrate was concentrated.
Compound 1b (136.0 mg, 0.48 mmol, in 80% yield) and
1b0 (22.0 mg, 0.08 mmol, in 13% yield) were isolated by
preparative TLC (silica gel) using a mixture solvent of
cyclohexane and diethyl ether (100:3) as an eluent. The total
GC yield of 1b and 1b0 was 99% confirmed by analysis of
the reaction mixture.
In summary, this letter provided an efficient and envi-
ronmentally benign route to diarylmethanes via the
dehydration of arenes with trioxane. The easy opera-
tion, ready availability and low cost of catalyst as well
as good to excellent yield of diarylmethanes are the
remarkable advantages of this procedure. Further appli-
cation of InCl3Æ4H2O as catalyst in organic synthesis is
now in progress.
7. Compounds 1b and 1b0 are new compounds, 1a and 1c–j
are known compounds, their structures have been con-
firmed by 1H, 13C NMR and GC–MS. The selected
spectroscopic data are reported below.
Acknowledgements
Bis-(2,4,6-trimethyl-phenyl)methane 1a: 1H NMR (300
MHz, CDCl3) d 7.85 (s · 4, 1H · 4), 4.00 (s, 2H), 2.24
(s · 2, 3H · 2), 2.10 (s · 4, 3H · 4); 13C NMR (75 MHz,
CDCl3) d 136.8, 135.0, 134.8, 129.4, 31.1, 20.9, 20.8;
GC–MS m/z (% rel. inten.) 252 (M+, 17), 237 (10), 207 (14),
193 (6), 178 (5), 165 (7), 132 (100), 117 (25), 91 (17).
This project (20573061) was supported by National
Natural Science Foundation of China and Foundation
of Tsinghua University for Ph.D. Candidate’s research.
1
Bis-(4,6-dimethyl-2-methoxy-phenyl)methane 1b: H NMR
References and notes
(300 MHz, CDCl3) d 6.53 (s · 2, 1H · 2), 6.50 (s · 2,
1H · 2), 3.98 (s, 2H), 3.70 (s · 2, 3H · 2), 2.26 (s · 2,
3H · 2), 2.11 (s · 2, 3H · 2); 13C NMR (75 MHz, CDCl3) d
157.9, 138.4, 135.5, 125.5, 123.7, 109.4, 55.6, 22.9, 21.3,
19.7; GC–MS m/z (% rel. inten.) 284 (M+, 43), 237 (11), 223
(6), 193 (6), 165 (10), 148 (100), 135 (40), 119 (31), 91 (23).
Anal. Calcd for C19H24O2: C, 80.24; H, 8.51. Found: C,
80.16; H, 8.51. (4,6-Dimethyl-2-methoxy-phenyl)-(2,6-
dimethyl-4-methoxy-phenyl)methane. Compound 1b0: 1H
NMR (300 MHz, CDCl3) d 6.54–6.53 (m, 4H), 3.97 (s, 2H),
3.76 (s, 3H), 3.68 (s, 3H), 2.29 (s, 3H), 2.18 (s, 6H), 2.04 (s,
3H); 13C NMR (75 MHz, CDCl3) d 157.9, 156.9, 138.2,
137.9, 136.1, 130.9, 125.2, 124.0, 113.5, 109.8, 55.6, 55.1,
26.5, 21.3, 20.9, 20.0; GC–MS m/z (% rel. inten.) 284 (M+,
34), 253 (3), 223 (3), 193 (3), 165 (7), 148 (100), 135 (28), 119
(12), 91 (18).
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Bis-(2,4,5-trimethyl-phenyl)methane 1c: 1H NMR (300
MHz, CDCl3) d 6.96 (s · 2, 1H · 2), 6.67 (s · 2, 1H · 2),
3.79 (s, 2H), 2.23 (s · 2, 3H · 2), 2.21 (s · 2, 3H · 2), 2.16