Benzenes selenenylation

OXIDATIVE CYCLIZATION Benzene-selenenyl chloride. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone. Palladium(ll) chloride-CopperCH) chloride. 

Reaction of the 1,3,2-benzodiselenastannole (166) with selenium tetrachloride gives the triselenole (40) which is converted into the triselenolium hexafluorophosphate (20b) by reaction with nitrosyl hexafluorophosphate <94CC1593>. Benzo[l,2-rf][l,2,3]triselenolium chloride (20c) was obtained by reaction of 1,2-benzene-bis(selenenyl chloride) with selenium <92AG(E)779>. The structure of the trifluoromethanesulfonate (20a) has been established by x-ray crystallography (see Section 4.21.3.1). 

The versatile selenenylating agent N-(phenylseleno)phthalimide (NPSP) is an effective carbocyclization mediator, which is capable of effecting acid catalyzed cyclization reactions from open-chain olefins, including the formation of cyclopropanes1. This is demonstrated by the quantitative cyclization of 3-butenyltrimethyltin to (cyclopropyl-methylseleno)benzene upon treatment with 1.1 equivalents of NPSP in CH2C12 at 25 °C under acid (e.g. p-TsOH) catalysis (equation 47). 

Stereoselective selenenylation of ketones with chiral selenamines has been utilized as a facile entry to optically active 4-substituted 2-cyclohexenones 9, but with low optical purity6. Thus, the reaction of 4-te/T-butyl- or 4-methylcyclohexanone with chiral selenamines in benzene or tetrahy-drofuran at various temperatures and/or times affords diastereomeric 2-arylseleno-4-/< /7-butyl-and 2-arylseleno-4-methylcyclohexanone (8). Oxidation of these selenides with hydrogen peroxide produces (S)-4-/erf-butyl- or (.S )-4-methyl-2-cyclohexenone (9) in quantitative yield6. 

Aryl selenides have also proven to be excellent reagents in group transfer reactions. Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Various substituted radicals can be generated in this manner by using a-selenenyl derivatives of esters, nitriles, malonates, P-ketoesters, a-methoxyesters, and phosphonates. The resulting radicals undergo addition to alkenes to generate y-seleno derivatives. 

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