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Aryldiimine as an Intermediate during Reduction of Diazonium Salts with Tetrahydridoborate.

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Aryldiimine as an Intermediate during Reduction
of Diazonium Salts with Tetrahydridoborate
By M . Bloch, H. Musso, and U.I . Zahorszkyi*I
About 10% of 3,3'-difluorobimesityl containing no deuterium
and 8 % of 3-fluorobimesityl (also deuterated) were obtained
as by-products. Decomposition of (1) in boiling methanol
affords 75 % of 3,3'-difluorobimesityl and 13% of 3-flUOr0-3'methoxybimesityl (oil), but n o bimesityl.
Dedicated to Professor H. Bredereck on the occnsion of his
65th birthday
Aryldiazonium salts may be reduced to the corresponding
hydrocarbons by complex hydrides in methanol at 0 OC [l].
The hydrogen introduced can be transferred from various
sources as anion, radical, or proton. To discriminate between
these possibilities, we treated bimesityl-3,3'-bis(diazonium)
bis(tetrafluorob0rate) ( I ) in methanol with sodium tetrahydridoborate; in each case the reducing agent o r the solvent
was deuterated in a different manner (98-99% D). The
deuterium content of the resulting bimesityl, which could be
isolated in approximately 22% yield by chromatography,
was determined by mass spectrometry (* 1 %).
3,3'-Diaminobimesityl(270 mg) are diazotized in 20 % hydrochloric acid (1.1 g) with NaNO2 (138 mg) in water (1 ml) a t
0 'C; the filtered solution is treated with NaBF4 (330 mg) in
water (0.8 ml) and the resulting precipitate is filtered off after
30 min. The salt (350 mg), which is washed with 5 % NaBF4
solution (0.2 ml), cold methanol (0.2 ml), and ether (1 ml)
successively and dried over PzO5 for 5 h under vacuum, is
suspended in methanol (2 ml) and treated at 0 "C, with stirring,
with NaBH4 (80mg); heat is evolved during the reaction.
After 2 h the reaction mixture is extracted with n-hexane and
the residue obtained after evaporation of the solvent is separated by layer chromatography on silica gel (Merck PF 254)
with light petroleum (60-70 "C) into three zones. After
scraping the compounds off the plate they are eluted and
finally sublimed under high vacuum: bimesityl (40mg, 22 %
caic. w.r.t. ( I ) , -Rr = 0.57, m.p. 101"C);3,3'-difluorobimesityl
(20mg, lo%, Rf = 0.67, m.p. 127OC); 3-fluorobimesityl
(15 mg, 8 % , Rf = 0.61, m.p. 109°C).
Received: February 12, 1969
[Z 950 IE]
German version: Angew. Chem. 81, 392 (1969)
Publication delayed at the authors' request
[*I Dr. M. Bloch, Prof. Dr. H. Musso, and
( %)
( %)
% D
1 2
1 3
As is evident from Table 1, it is not the hydrido hydrogen
that appears in the product but the acidic proton of the
methanol to the extent of about 85 %.
Since the diazonium tetrafluoroborate ( I ) is stable for two
hours in methanol at room temperature, the tetrahydridoborate must first reduce the diazonium salt to the aryldiimine.
The latter compound then rapidly exchanges the N-H
proton with the solvent and decomposes mainly via the diimine anion to give nitrogen and an aryl anion, which abstracts a proton or deuteron from the solvent, thus yielding
the hydrocarbon [2,31.
+ Ar-N=NH
New Rearrangements during the Reaction of
Pyrylium Salts with Nitromethane
By K. Dimroth and G . Laubertt*l
Dedicated to Professor H. Bredereck on the occasion of his
65th birthday
+ A r e + Nz -+
In acetonitrile, the bimolecular decomposition of phenyldiimine was formulated as a free-radical process and an isotope
effect KH/KD = 4-5 was observed [41, The difference between
the third and fourth columns of Table 1 shows that with
hydride in methanol only about 1 5 % of aryl radicals are
formed that abstract hydrogen from the C-H bond. In the
reaction with 4 mol. equiv. of non-deuterated sodium methoxide the percentage of radicals increases markedly (columns
5 and 6).
We attribute incomplete deuteration (75 and 21 %) in [Ddmethanol with NaBD4 and with sodium methoxide, respectively, to the isotope effect [3,41 and to water present in compound ( I ) . If the salt is prepared in D20 up to 85 % deuteraation is achieved.
Dr. U. 1. Zahorszky
Abteilung fur Chemie der Ruhr-Universitat Bochum und
Institut fur Organische Chemie der Universitat Marburg [**I
[**I Present Address: Institut fur Organische Chemie
der Universitat
75 Karlsruhe, Richard-Willstatter- Allee (Germany)
[l] J. B. Hendrickson, J. Amer. chem. SOC. 83, 1251 (1961).
[2] J. F. Bunnett and H. Takayama, J. Amer. chem. SOC.90,5173
[3] R . W. Hoffmannand G. Guhn, Chem. Ber. 100, 1474 (1967).
[4] P. C. Huang and E. M. Kosower, I. Amer. chem. SOC.87,
4645 (1965); 90,2367 (1968).
Whereas 2,4,6-trisubstituted pyrylium salts, and those carrying a greater number of substituents. are converted smoothly
into nitrobenzene derivatives by nitromethane and a n excess
of base 111, reaction with only one mol. equiv. of base adopts a
different course, especially in chlorobenzene, dichloroethane,
or ethanol. Thus, 2,4,6-triphenylpyryiium tetrafluoroborate
( l a ) affords 2,4,6-triphenylphenoI (50). 2,3,5-triphenylphenol ( 8 a ) , and 2,4,6-triphenylnitrobenzene ( 6 a j . In a
similar manner, 2,3,4,6-tetraphenylpyrylium tetrafluoroborate ( I b ) gives ( 5 b ) , ( 8 b ) , and (6b).
If it is assumed that primary attack of the nitromethyl-anion
o n C-2 or C-6 to give (2) is followed by a n intramolecular
aldol condensation to yield the intermediate (3). then the
experimental findings may be interpreted as follows: Formation of the phenols (5) proceeds via 1.3-nitro-nitroso ester
rearrangement to ( 4 ) and aromatization with subsequent
hydrolysis of the phenol nitroso ester. The phenols (8)arise
by a migration of the aryl group R1 from C-2 to C-1 with
Angew. Chem. internat. Edit.
/ VoI. 8 (1969) / No. 5
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salt, diazonium, reduction, intermediate, tetrahydridoborate, aryldiimine
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