Thermal Isomerization of Bencylselenocyanate. Flash Vacuum Pyrolysis as Alternative Synthesis of Building Blocks towards Selenoheterocycles

CEBALLOS NOELIA M.; MOYANO, ELIZABETH L.

Newfoundland and Labrador

Congreso; 22nd International Congress on Heterocyclic Chemistry; 2009

INTERNATIONAL SOCIETY OF HETEROCYCLIC CHEMISTRY

It was reported by Suzuki et al. that Bencylselenocyanate (BSC) isomerizes to the corresponding benzylisoselenocyanate (BISC) by a photochemical reaction in diluted solutions of the starting material using a high pressure mercury lamp.1 In this work we report an alternative methodology to obtain BISC using Flash Vacuum Pyrolysis (FVP). The use of FVP for synthesis and mechanistic studies is well documented;2 however, the information on reactions employing selenoderivatives as starting materials is scarce, which makes the gas phase thermal study of these compounds an interesting challenge. In addition, the BISC conveniently synthesized by FVP was used in a subsequent simple reaction with aromatic amines to prepare selenoheterocycles in moderate to good yields. BSC was readily available by reaction of benzylbromide and KSeCN at room temperature. The crude was purified by sublimation (90%). FVP reactions of BSC were carried out between 300-500 ºC at pressures of 0,01 Torr using nitrogen as inert carrier and achieving short contact times of 10-2 s. Different conditions were evaluated to improve the yield of BISC and it was determined that the presence of glass Raschig rings reduced secondary reactions (such as formation of bibenzyl and Se elimination), obtaining 30% of BISC at 425ºC (Scheme 1). This heterogeneous procedure was compared also with other methods of thermolysis (solution or static gas phase thermolysis) showing better selectivity to the formation of BISC. The pyrolyzate just obtained was treated without further purification with benzylamine to prepare N,N'-selenocarbonyldibenzamide in good yields. This methodology could be extended to the synthesis of selenohydantoins and selenopurine through cyclization reactions.3 1 In this work we report an alternative methodology to obtain BISC using Flash Vacuum Pyrolysis (FVP). The use of FVP for synthesis and mechanistic studies is well documented;2 however, the information on reactions employing selenoderivatives as starting materials is scarce, which makes the gas phase thermal study of these compounds an interesting challenge. In addition, the BISC conveniently synthesized by FVP was used in a subsequent simple reaction with aromatic amines to prepare selenoheterocycles in moderate to good yields. BSC was readily available by reaction of benzylbromide and KSeCN at room temperature. The crude was purified by sublimation (90%). FVP reactions of BSC were carried out between 300-500 ºC at pressures of 0,01 Torr using nitrogen as inert carrier and achieving short contact times of 10-2 s. Different conditions were evaluated to improve the yield of BISC and it was determined that the presence of glass Raschig rings reduced secondary reactions (such as formation of bibenzyl and Se elimination), obtaining 30% of BISC at 425ºC (Scheme 1). This heterogeneous procedure was compared also with other methods of thermolysis (solution or static gas phase thermolysis) showing better selectivity to the formation of BISC. The pyrolyzate just obtained was treated without further purification with benzylamine to prepare N,N'-selenocarbonyldibenzamide in good yields. This methodology could be extended to the synthesis of selenohydantoins and selenopurine through cyclization reactions.3 -2 s. Different conditions were evaluated to improve the yield of BISC and it was determined that the presence of glass Raschig rings reduced secondary reactions (such as formation of bibenzyl and Se elimination), obtaining 30% of BISC at 425ºC (Scheme 1). This heterogeneous procedure was compared also with other methods of thermolysis (solution or static gas phase thermolysis) showing better selectivity to the formation of BISC. The pyrolyzate just obtained was treated without further purification with benzylamine to prepare N,N'-selenocarbonyldibenzamide in good yields. This methodology could be extended to the synthesis of selenohydantoins and selenopurine through cyclization reactions.3