вход по аккаунту


Biradicals Quinones and Semiquinones of the Imidazole Series.

код для вставкиСкачать
Excited-State Reactions of 1,2,4,5-Tetramethyl-3,5cyclohexadiene-cis-1,2-dicarboxylicAnhydride
Biradicals, Quinones, and Semiquinones of the
Imidazole Series
By Dipl.-Chem. U. Mayer, Dr. H. BdumgLrtel, and
Prof. Dr. H. Zimmermann
By Dr. R. N. Warrener and J. B. Bremner
Deuartment of Chemistry, Australian National University,
Canberra, A.C.T. (Australia)
Ultraviolet irradiation111 of the title diene ( I a) [*I in ether
gave the products (2a)-(5a) together with two unidentified
a) u p to 40%
a) u p to 12%
b) <O.l%
Institut fur Orgdnische Chemie der Universitiit
Munchen (Germany)
Biradicals of thc imidazolyl series can occur as paramagnetic
biradicals ( I ) or as diamagnetic quinonoid molecules (2).
+ co + coz
a) R = CH,
b) R = II
6 ' g 2 H z H 3
The homologous series of compounds (2) can be considered
as derived from tetraazafulvalene. The first representative of
this class of compound has now been prepared. When the
disodium salt (3) of 1,4-bis-(4,5-diphenyl-2-imidazolyl)benzene "1 is treated with bromine in methylene chloride,
o +
We consider that the novel fragmentation of ( I a) into durene
(2u), CO, and C 0 2 is due to a multicenter valence-electron
redistribution 131 of the excited-state diene, rather than a consequence of the anhydride group itself. This accounts for the
photolytic stability of the reduction product (30). The source
of the hydrogen atoms in (30) is not certain, although solvent
participation seems unlikely since (30) is also formed with
carbon tetrachloride as solvent.
Disrotatory electrocyclic ring closure, a favorcd photoreaction in the diene system of (Ia)[41, accounts for the
formation of both valence isomers (4a) (m.p. 150-151 " C )
and (5a) (m.p. 100-101 "C). These structures are supported
by thermal reconversion into thc initial diene ( l a ) . The
stereochemical assignments were based o n the N M R spectra
of (40) and (50).
These results are in contrast with the photolysis pattern of the
parent dienedicarboxylic anhydride ( I b) 151. In this case,
photoaromatization is a minor reaction (< 0.1 %), no reduction product ( 3 b ) is observed and only a single valence
isomer ( 6 ) is formed of unknown stereochemistry.
Vapour phase pyrolysis (800 15 "C at 0.1 mm) of the diene
( l a ) also yielded durene (40 X), CO, and COz. This rcaction
may, however, be due to simple thermal anhydride fragof the mass
mentation 15al. A preliminary investigation
spectrum of ( l a ) showed that a unimolecular fragmentation
of the parent ion (mass 206) to the durenc molecular ion
COz) is a major decomwith loss of 72 mass units (CO
position pathway.
Received: December ZOth, 1965
[Z 137/967 IE]
German version: Angew. Chem. 78, 305 (1966)
[I] At 15-20°C in dilute solution (1-2%) under argon; Hanovia 450 watt high-pressure Hg lamp, water cooled quartz cylinder
with a Vycor filter.
[2] Prepared after the method of K . Ziegler, German Patent
875 573 (1953).
[3] For further details see R. N. Warrener and J. B. Bremner,
Rev. pure appl. Chem., in the press.
[4] R . B. Woodwardand R . Hofftnnnu, J. Amcr. chem. S O C . 87,
395 (1965).
[S] E . E. van Tamelen and S . P. Pappas, J. Amer. chem. SOC.85,
3297 (1963).
[5al F. 0. Rice and M . T. Murphy, J. Amer. chem. SOC.64, 896
(1 942).
[6] We thank Dr. C. S. Barnes, C.S.R. Research Laboratories,
Roseville, N.S.W. (Australia), for the mass spectra.
Angew. Chem. Internat. Edit. 1 Vol. 5 (1966)
No. 3
sodium bromide separates and 3,6-bis(4,5-diphcnyl-2imidazolylidcae)-l,4-~yclohexadiene
(2) (n = 1 , R = phcnyl),
blue crystals, long wavelength absorption in CHCI3 at
1~ = 602 mi*, log c = 4.95) is formed. This product contains
same of the prainagnetic form but consists mainly of the
diamagnetic quinonoid form.
Treatment of (2) with the disodium salt (3) in acetone affords
the red, unstable semiquinone ( 4 ) , whose paramagnetism was
detected by ESR spectroscopy in a stop-flow apparatus.
We have also prepared 4-(4,5-diphenyl-2-imidazolylidene)2,5-cyclohexadienone (5) by dehydrogenation of p-(4,5-diphenyl-2-imidazoly1)phenol ( 6 ) 121 with lead dioxide in anhydrous ether. This compound forms red crystals, deconip.
at 155 O C , having long-wavelength absorption i n CHC13 at
h = 440 mp, log i = 4.64.
Treatment of (5) with the dipotassium salt of the phenol ( 6 )
in dioxane also gives a short-lived, green semiquinone (7),
which was detected by ESR spectroscopy.
The quinonoid molecules (2) and ( 5 ) were titrated with
hydroquinone, which regenerated the starting materials ( 3 )
and (5).
Received: December 20th, 1965 [ Z 1311962 IE]
German version: Angew. Chem. 78, 303 (1966)
[I] T. Drrfnhl and W. Schmrririer, J . prakt. Chem. [4] 23, 225
[2] Sh. Kori and Sh. Norimivo, Asahi Garasu Kcnkyu Hokoku
12, 55 (1962); Chem. Abstr. 59, 1622a (1963).
31 1
Без категории
Размер файла
115 Кб
biradicals, imidazole, series, semiquinones, quinone
Пожаловаться на содержимое документа