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Isomerism of 1 3 4-Oxadiazolines.

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carried out with (4) and ( 5 ) , occasionally under very mild
conditions. Furthermore, compounds of type (5) are, as far
:is we know, the first stable examples of so-called imidoyl
phosphates, which are assumed to be intermediates in the
phosphorylation of carbodiimides, trichloroacetonitrile,
cyanuric chloride, etc.
Received, May 24th, 1963
[Z 516/346 IE]
German version: Angew. Chem. 75, 641 (1963).
Isomerism of 1,3,4-Oxadiazolines
By Dr. E. Fahr, K. Doppert, and Dr. F. Scheckenbach
Cheniisches lnstitut der Universitat Wurzburg (Germany)
1,3,4-Oxadiazolines (2), which are accessible from diazofuorene and a+'-dicarbonylazo compounds [ 1,2] or by interaction of the silver salts ( I ) and acyl chlorides [3], exhibit a
new kind of ring-chain isomerism. Thus, for example, the
compounds derived from diazo-fluorene and azodicarboxylic
esters are present as oxadiazoles ( 2 a) in the solid state, but
form an equilibrium in solution (the position of which
depends on the dielectric constant of the solvent) between the
oxadiazoline ( 2 a ) and the azomethine-imine (3.) isomers [4].
With R ' = C2H5 in dichloromethane/n-heptane: isobestic
points at 242 and 263 mp; specific bands in the infrared
(CC14): oxadiazole form: 5.85 and 5.98 p; azomethine-imine
form: 5.62; 5.74; 6.08 p. With increasing dielectric constant
of the solvent, the position of the equilibrium is displaced
towards the azomethine-imine form, which may be trapped,
e . g . byydiphenylketene to yield ( 4 ) . Above 50 "C, irreversible
conversion into the N'-acyl-N'-alkoxycarbonylhydrazone
(5a) sets in, proceeding presumably by way of the intermediate
( 6 ) [5]. The oxadiazolines (26) and (Zc), which are accessible
from ( l a ) and acetyl chloride and from (Ib) and ethyl
chloroformate, respectively, also rearrange irreversibly into
the hydrazone (5b) which in turn is obtainable from (7.)
and ethyl chloroformate.
Isomerisation of (2d), which is obtained from (Zb) and
acetyl chloride, proceeds differently: on warming, it rear-
ranges reversibly via (3b) and ( 6 ) to the hydrazone (Su),
which may be itself obtained by the action of acetyl chloride
on (7a). At room temperature, (86) reverts slowly into (2d).
In the case of (2e), rearrangement proceeds via (3c) + (6) 3
(86)+ (9) + (3d) to yield ( 2 f ] , which does not isomerize, not
even in the molten state. Compound (2e) is obtainable from
(16) and benzoyl chloride, while (2f) results from (Ic) and
acetyl chloride. The hydrazone (Sb) is so labile that it rearranges immediately to (2f) when its preparation from (7b)
and acetyl chloride is attempted. The oxadiazoline (2g) is
very stable, and even in the melt yields only limited amounts
of the azomethine-imine form (3u). Diphenyl ketene adds onto
(2g) to afford (lo), from which (2g) is regenerated on
treatment with alkali.
The reason for the differing behavior of the silver and
potassium salts of the monoacylhydrazones on acylation is
revealed by infrared spectroscopic examination of these salts.
The silver salt is present in the lactim structure ( I ) - indicated
by the absence of carbonyl valence vibration bands [ 6 ] while thelpotassium sa1t:haqstructure (7), for the position of
the carbonyl valence vibration band is the same as for the
free monoacyl hydrazones.
Received, May 30th, 1963
[Z 523/351 IE]
German version: Angew. Chem. 75, 670(1963).
[I] E. Fuhr, Angew. Chem. 7.1, 536 (1961).
[2] R. Breslow, C. Yarosluvsk r, and S. Yaroslavsky, Chem. and
Ind. 1961, 1961.
[ 3 ] K . Doppert, Ph. D. Thesis, Universit2t Wurzburg (now being
completed). The synthesis of oxadiazolines (2) from the silver
salts ( I ) is generally preferable to the interaction of diazofluorene and a,cc'-dicarbonyl;izo compounds, as the latter are
often difficult to obtain in a pure state. Furthermore, unequivocal
synthesis of unsymmetrically substituted oxadiazolines [e.g. (2e)
or (Zf))] via dicarbonylazo compounds is not possible.
[4] Concerning the nomenclature of ( 3 ) , cf. R. Huisgen and A .
Eckell, letrahedron Letters 1960, 5 ; R . Huisgen, R. Grashey,
P. L a w , and H . Leitermann, Angew. Chem. 72, 416 (1960).
[ 5 ] Crossing experiments served to exclude the possibility of
intermolecular migration of the R-CO group.
[6] If the silver salt had an anionic oxadiazoline structure, bands
due to an -N=C-0grouping (part of a heterocycle) would be
expected at 5.95 and 6.10 (L.
Angew. Chem. internat. Edit.
/
Vol. 2 (1963) I No. 8
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