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New Aroxyls Mono- Di- and Triradicals.

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New Aroxyls :Mono-, Di-, and Triradicals.
Detection of a Triplet State
By Gerd Kothe, Claudia Nowuk, Karl-Heinr Denkel, Ernst
Ohmes, and Herbert Zimmermann [*I
Dedicnted to Professor Eugen Muller on the occasion of his
65th birthday
Isolation of the chalcone is unnecessary if methoxyacetophenone and benzaldehyde are used as starting materials [I].
The pyridine ( I ) can also be prepared by Weiss's modification
of the Chichibabin synthesis [21.
The methyl ether ( I ) (colorless needles from ligroin, m.p.
130-1 31 "C) is cleaved by pyridinium chloride o r hydrobromic acid/glacial acetic acid t o give the 3-hydroxypyridine
(2) (yellow needles, m.p. 138-139 "C). Dehydrogenation of
(2) with potassium hexacyanoferrate(iI1) or potassium nitrosobissulfate in sodium hydroxide/carbon tetrachloride affords
a violet-brown solution of the radical 2,4,6-triphenyl-3pyridyloxyl ( 3 ) . Concentration of the carbon tetrachloride
solution yields (3) as a crystalline yellow dimer (containing
1 molecule of cc14) which we formulate as the quinol.
Figure. High resolution ESR spectrum of the radical 2,4,6-triphenyl3-pyridyloxyl (3) in benzene.
In organic solvents the dimer dissociates to the free radical
and shows the expected temperature-dependent equilibrium
(color deepening on warming). Characteristic quinol absorption bands are present in the IR spectrum of the solid (1707,
1623; 1210 cm-1). The free pyridyloxyl ( 3 ) gives a UV spectrum containing more bands than the spectrum of triphenylphenoxyl131 (Amax = 665, 550, 415, 358, 318 nm, in chloroform). The radical, which, unlike 2,4,6-tri-tert-butylphen0~~1141,
is completely stable towards oxygen, reacts preferentially as a dehydrogenating agent; it slowly attacks such
solvents as acetone, methanol, and chloroform. I t obviously
has a higher oxidation potential than triphenylphenoxyl. The
comparatively low symmetry of ( 3 ) can be deduced from the
ncreased number of lines in the ESR spectrum (see Figure) 151.
HC1 condensation of 4-acetyl-2,6-di-tert-butylphenol[11
dry ethanol at room temperature leads to a small yield of the
new triphenol 1,3,5-tris(3,5-di-rert-butyl-4-hydroxyphenyl)benzene ( I ) [colorlesscrystals,m.p. 313-314 'C (uncorrected):
= 271 nm, log E = 4.74 (ethanol)], from which a number
of radicals of the aroxyl series can be derived. The structure
of ( I ) follows from elemental analysis, the molecular weight,
I R spectrum [ v o ~
= 3639cm-1 (sharp)], and N M R spectrum [Ha 8 = 5.27 ppm (3), Hb 1.49 (54), H c 7.46 (6), Hd 7.57
(3); 6 in CDCl3 with TMS as internal standard].
Dehydrogenation of the triphenol ( I ) with a deficit of lead
dioxide in benzene or toluene yields the violet monoradical
= 529 nm (in benzene). The color of (2) closely
resembles that of the radical (31, Amax = 491 nm (in benzene) 121, which was isolated pure by Eugen Miiller et al. [31.
At low resolution the ESR spectrum of (2) in toluene shows
six equidistant lines at g = 2 in the intensity ratio 1 : 5 : 10 :
10 : 5 : 1, coupling constant U H = 1.71 G. The splitting
pattern of the signal can be attributed to coupling of the
radical electron with five equivalent protons [asterisked
aromatic positions in (211. Similar observations were made
in the case of (3) [4,51.
Received: May 8, 1970
[ Z 222 IE]
German version: Angew. Chem. 82, 522 (1970)
[*I Prof. Dr. Dr. H.-J. Teuber, Dip1.-Chem. G. Schiitz, and
Dip1.-Chem. H.-J. Gross
Institut fur Organische Chemie der Universitat
6 Frankfurt/Main, Robert-Mayer-Strasse 7-9 (Germany)
[l] F. Krohnke and W. Zecher, Angew. Chem. 74, 811 (1962);
Angew. Chem. internat. Edit. I , 626 (1962); Chem. Ber. 94, 690
[2] M. Weiss, J. Amer. chem. SOC.74,200 (1952).
[3] K . Dimroth, F. Knlk, and G. Neubauer, Chem. Ber. 90, 2058
(1957); K . Dimroth, F. Kalk, R . Sell, and K . Schlomer, Liebigs
Ann. Chem. 624, 51 (1959); K . Dimroth and A . Berndf, Angew.
Chem. 76, 434 (1964); Angew. Chem. internat. Edit. 3, 385
(I 964).
[4] E. Miiller, K . Ley, K . Schefller, and R . Mayer, Chem. Ber.
91, 2682 (1958); see also A . L . Buchachenko: Stable Radicals.
Consultants Bureau, New York 1965, p. 63.
[5] The interpretation of the ESR spectrum will be given
If the triphenol is dehydrogenated with a n excess of lead dioxide in benzene or toluene the orange-brown radical 14) is
formed whose chemical behavior is consistent with a triradical [Amax = 478 nm with a shoulder at 577 nm (in
On iodometric titration[61 of ( 4 ) , 2.4 to 2.5 equivalents of
iodine are liberated. After titration, 96 % of the triphenol can
be recovered, thus indicating the absence of fragmentation
Angew. Chem. internat. Edit.
Vol. 9 (1970) / N O . 7
[ * ] Dipl.-Chem. G . Kothe, C. Nowak, K.-H. Denkel,
Dip1.-Phys. E. Ohmes, and Prof. Dr. H. Zimmermann
Institut fur physikalische Chemie der Universitiit
78 Freiburg, Albertstrasse 21 (Germany)
[l] C. D . Cook and N . D . Gilmour, J . org. Chemistry 25, 1429
[2] G. N . Bogdanow and W . W . Jerschow, Izvest. Akad. Nauk
SSSR, Otdel. chim. Nauk 1963, 1516.
during dehydrogenation. Osmometric molecular-weight
determinations in benzene show the triradical (4) to be
monomeric in solution.
The ESR spectrum of ( 4 ) in toluene (-70OC) contains 16
lines at g = 2 (Fig la). The spectrum can be simulated with
two sets of three and six equivalent protons (Fig. l b ) [aHi =
1.75 (3), U H 2 = 0.60 G ( 6 ) ;half-width of the absorption curve
A H = 0.50 G ; 100% Gaussian curve].
131 E. Miiller, A. Schick, and K . Scheffer, Chem. Ber. 92, 414
141 A . Rieker and K . Schef/er, Liebigs Ann. Chem. 689, 78
151 C. D. Cook and M . Frmer, J. org. Chemistry 29, 3716
[6] E. Miifler and K . Ley, Chem. Ber. 87, 922 (1954).
Preparation of Cyclic and Acyclic Vinyl
By Thomas E. Dueber, Peter J . Stang, Wolf D. Pfeifer,
Richard H . Summerville, Michael A . Imhof, Paul v. Rague
Schleyer, Klaus Hummel, Siegfried Bocher, Charles E. Harding, and Michael Hunuck [*]
We have found that cyclic vinyl trifluoromethanesulfonates
(Table) can be prepared in a simple manner by treating
cycloalkanones with trifluoromethanesulfonic anhydride.
Method 1 (without solvent): Trifluoromethanesulfonic anhydridetll and a n equimolar amount of the cycloalkanone
are mixed with stirring a t a temperature of-15 to 0°C. The
temperature of the reaction mixture is kept at-5
C for 2 h
and then a t 0 “C for 2 to 14 days (Table). For work-up, the
reaction mixture is poured on t o ice, neutralized with aqueous
sodium bicarbonate, and extracted with ether. The ether
solution is washed with water, dried, and after removal of the
ether the residue is vacuum distilled. The trifluoromethanesulfonate can easily be separated from the distillate by
preparative gas chromatography (3 m column, diethylene
glycol succinate, l l ( t 1 4 0 “C).
Fig. 1. ESR spectrum of 4,4’,4”-(1,3,5-benzenetriyl)tri(2,6-di-tertbutylphenoxyl) (4) in toluene at -70OC.
a) Experimental.
b) Simulated.
Combination of toluene solutions of the triphenol ( I ) and
the triradical (4) in the molar ratio 1 : 2 affords a radical
solution containing not only (2) and (4)but also the diradical ( 5 ) which is present in the triplet state. The triplet state
of ( 5 ) can be deduced from its half-field transition (Am = 2)
at 1621 G and the corresponding Am = 1 lines in the ESR
spectrum of a toluene solution frozen t o a rigid glass. The
high-field ESR spectrum of this glass contains a number of
Am = 1 lines arranged almost symmetrically with respect to
the g = 2 transition of (2) and ( 4 ) at 3246 G (Fig. 2).
Method 2 (with methylene chloride or pentane as solvent):
Trifluoromethanesulfonic anhydride (15 mmoles) in methylene chloride (15 ml) is added dropwise within 30 min t o a
stirred, cold solution (-70°C) of the ketone (or aldehyde,
see Table) (15 mmoles) in methylene chloride (or pentane).
Anhydrous sodium carbonate (8 mmoles) is added simultaneously in small portions. During addition of the anhydride a slight increase in temperature is observed. When
the reaction mixture has warmed to - 1 O O C it is yellow o r
red in color. After one day to three weeks the solution is
filtered, washed with water, and dried. The methylene
chloride is removed by means of a rotary evaporator and the
residue is vacuum distilled. Pure trifluoromethanesulfonate
can be obtained from the distillate by preparative gas
chromatography, fractional distillation, o r chromatography
o n silica gel (pentane solvent).
Acyclic trifluoromethanesulfonates can be prepared in like
manner from acyclic aldehydes or ketones (Table). I n
addition to the compounds listed in the Table, the corresponding vinyl trifluoromethanesulfonates were prepared from 2,5dimethylcyclopentanone, p-chloroacetophenone, adamantyl
methyl ketone, and 1,l-diphenylacetone.
Fig. 2. ESR spectrum of 4,4’-[5-(3,5-di-tert-butyl-4-hydroxyphenyl)1,3-phenyleneJbis(2,6-di-ferf-butylphenoxyl)
( 5 ) in toluene glass at
approx. - 180 OC.
Received: May 19, 1970
I2 225 1El
German version: Angew. Chem. 82, 521 (1970)
Angew. Chem. internur. Edit. / Vol. 9 (I970) No. 7
Alkyl trifluoromethanesulfonates solvolyze about 104-105
times faster than the corresponding alkyl arylsulfonates 121.
Using another method Stang and Summerville[31 as well as
Jones and Munessc41 prepared for the first time acyclic vinyl
trifluoromethanesulfonates. While the acyclic derivatives
which they prepared can form the energetically favorable
linear vinyl cations [51 o n solvolysis [61, it may be possible to
generate nonlinear vinyl cations by the solvolysis of the
cyclic vinyl trifluoromethanesulfonates described above [TI.
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aroxyle, mono, triradical, new
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