Optically active arsonium ions

In 1902, the unsuccessful fractional crystallization of (— )-aspartate and (-I- )-tartrate salts of the ethylmethyl(a-naphthyl)phenylarsonium ion was reported and, in 1912, the failure to resolve allylbenzylmethylphenylarsonium iodide by seeding solutions of the racemate with crystals of the corresponding optically active ammonium salt was reported . The first evidence of optical activity in arsonium compounds was published... 

An optically active arsonium ion containing an allyl or a benzyl group is quantitatively reduced at a mercury electrode in water or ethanol with retention of configuration at arsenic (and liberation of toluene or propene) to give an optically active tertiary arsine, free of functional groups on the organic substituents (equation 3). 

An aqueous solution of (K)-( —)-allylmethylphenyl(n-propyl)arsonium bromide, when treated with aqueous sodium hydroxide, decomposes into methylphenyl(n-propyl)-arsine (3) (not isolated) and allyl alcohol - . Quarternization of the arsine with benzyl bromide regenerates the arsonium bromide of 89.5% optical purity, based upon the rotation of the sample used to prepare the optically active allylarsonium salt. Thus, the hydrolysis of an allylarsonium ion is highly stereoselective giving the tertiary arsine with retention of configuration at arsenic (equation 7). 

The absolute configurations of a number of arsonium ions and arsines have been correlated with one another on the basis of the stereospecificity of cathodic cleavage of the benzyl group from an arsonium ion and the quaternization of an arsine by benzyl and other alkyl halides, both of which occur with retention of configuration. Scheme 1 gives the connection between the stereochemistries of various optically active arsonium ions and arsines related to (S)-(+)-62. 

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