7th International Electronic Conference on Synthetic Organic Chemistry (ECSOC-7), http://www.mdpi.net/ecsoc-7, 1-30 November 2003
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Isomerization of N-Acylselenoureas into N-Acylisoselenoureas
Pavel Pazderaa, Jiří šiborb and jaromír marekc
aResearch Group for Chemical Syntheses and Technologies of the Sustainable Development, Department of Organic Chemistry, Masaryk University, CZ-611 37 Brno, Czech Republic;
bDepartment of Organic Chemistry, Masaryk University, CZ-611 37 Brno, Czech Republic;
cLaboratory for Functional Genomics and Proteomics, Masaryk University, CZ-611 37 Brno, Czech Republic;
Abstract
Observation that some ethyl-2-(3-acylselenoureido)thiophen-3-carboxylates and their benzoanalogues undergo a photoisomerization to corresponding ethyl-2-(3-acylisoseleno-ureido)thiophen-3-karboxylates or appropriate benzoates into solid state is clarified on the base of X-ray structural analysis of some starting substances and quantum chemistry calculations for both types of compounds.
Ethyl-2-(3-acylselenoureido)thiophen-3-carboxylates and their benzoanalogues
(acyl is benzoyl or pivaloyl) undergo a photoisomerization
to corresponding ethyl-2-(3-acylisoseleno-ureido)thiophen-3-carboxylates or
appropriate benzoates into solution of organic solvents (acetone, chloroform,
ethanol, etc.) or on TLC plates at wave length 340 – 400 nm. Title N-benzoylselenoureas
isomerize also in acetic acid
as the solvent, analogously. However, the same solvent evokes retroisomerization
of N-pivaloylisoselenoureas2 (Scheme 1).
Scheme 1. Preparation and isomerization of title N-acylselenoureas into N-acylisoselenoureas
Further it is known that presented isomerization proceed under controlled
heating in the solid state and melting points are for both types
of compounds the same. This observation was supported for
ethyl-2-(3-benzoylselenoureido) benzoate by thermal analysis methods (DTA, TG,
DTG)3 (Figure 1).
Figure 1. Thermal analysis of ethyl-2-(3-benzoylselenoureido) benzoate
and its isomer.
An explanation of the isomerization reaction evoked by heat or light in the solid state may be effected using results of the X-ray structural analysis (Figure 2).
N-Acylselenourea stands in the solid state – single crystal in dimer form with existence of relative strong interactions of H-bond types between selenium atom of selenoureido group in the first molecule and hydrogen atom on nitrogen N(3) of selenoureido group in second molecule (distance Se…H-N(3) is about 2.05 Ĺ).
As far as it is into system supplied energy (thermal or by means of photon) happen to increasing of the vibration mode as on C=Se so on N(3)-H. This effect leads to distance contraction between both discussed selenium and hydrogen atoms. Overall process culminates by the reordering of electrons (bonds) and the thermodynamic more stable N-acylisoselenourea is formed (Figure 3). The exothermicity of isomerization process is confirmed by the DTA method (Figure 1).
Figure 2. X-ray structural analysis of
ethyl-2-(3-pivaloylselenoureido)-4,5,6,7-tetrahydrobenzo[b]thiophen-3-carboxylate.
Quantum chemistry calculations2 (HF/6-31G** level) show that
energy difference between N-acylselenoureas
and N-acylisoselenoureas
is ca 25 – 30 kcal.mol-1.
Identity of obtained N-acylisoselenoureas was supported by CHNSe element analysis, FTIR spectroscopy and first of all by 1H, 13C, 15N, 77Se NMR spectral experiments2.
Figure 3. Molecular
structure of
ethyl-2-(3-pivaloylisoselenoureido)-4,5,6,7-tetrahydrobenzo[b]thiophen-3-carboxylate
according
HF/6-31G** quantum chemistry optimalization.
References