Organic Syntheses, CV 5, 1128
Submitted by K. Dimroth, A. Berndt, and C. Reichardt
1.
Checked by Saul Cherkofsky and Richard E. Benson.
1. Procedure
In a
1-l. three-necked flask equipped with a
mechanical stirrer, a
reflux condenser, and a
dropping funnel are placed
119 g. (0.30 mole) of 2,4,6-triphenylpyrylium tetrafluoroborate (Note
1),
21 ml. (24 g., 0.39 mole) of nitromethane (Note
2), and
350 ml. of absolute ethanol (Note
3).
Triethylamine (70 ml., 51 g.) (Note
4) is added rapidly from the dropping funnel to the well-stirred suspension. The reaction mixture becomes reddish brown immediately, and the solid dissolves. After all the
triethylamine has been added, the mixture is heated under reflux for 3 hours, cooled, and allowed to stand overnight in a refrigerator. The crystalline product that separates is collected on a Buchner funnel and washed with two
50-ml. portions of ice-cold methanol. The product (
75–80 g.; m.p.
142–144°) is recrystallized from
200–250 ml. of glacial acetic acid to yield
70–75 g. (
67–71%) of
2,4,6-triphenylnitrobenzene as slightly yellow crystals, m.p.
144–145° (Note
5).
2. Notes
2.
Nitromethane is dried over anhydrous
calcium sulfate (Drierite) or
calcium chloride for 1 day and distilled; the fraction with b.p.
101.5–102.5° is used.
3. Commercial absolute
ethanol is used without additional drying.
5. The n.m.r. spectrum (CDCI
3) shows singlets at 7.45 p.p.m. (15 H) and 7.65 p.p.m. (2 H) (downfield from internal tetramethylsilane reference).
3. Discussion
This procedure illustrates a general method for converting substituted pyrylium salts to nitrobenzene derivatives. The reaction has been the subject of several reviews.
6,7,8 The yields are generally high, and under these conditions only a single product is formed, in contrast to the nitration of
1,3,5-triphenylbenzene. The preparation of
2,4,6-triphenylnitrobenzene from the corresponding pyrylium salt eliminates isomer separation problems, which are encountered when the direct nitration procedure is used. Also, labeled compounds can readily be prepared by this method.
9
This preparation is referenced from:
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
calcium sulfate (Drierite)
ethanol (64-17-5)
calcium chloride (10043-52-4)
acetic acid (64-19-7)
methanol (67-56-1)
sodium hydroxide pellets (1310-73-2)
Nitromethane (75-52-5)
triethylamine (121-44-8)
2,4,6-Triphenylnitrobenzene,
Benzene, 2-nitro-1,3,5-triphenyl- (10368-47-5)
2,4,6-Triphenylpyrylium tetrafluoroborate (448-61-3)
1,3,5-triphenylbenzene (612-71-5)
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