ARSONIUM IONS AND ARSINES

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 

There is considerable evidence for this type of reaction in organoarsenic chemistry and, indeed dissociation-equilibrium was believed to be the cause of racemization of arsonium halides which, for synthetic reasons, usually contained at least one alkyl substituent (ofteri the benzyl group) on the arsenic atom. In 1939, fractional crystallization was achieved of the diastereomers of the diarsonium picrate 3. The individual diastereo-mers of the salt, racemic (R, R ) and meso (R, S ), were stable in boiling ethanol, a fact that should have dispelled concerns of dissociation equilibria in halide-free arsonium salts. In 1940, the tetrahedral structure of [AsPhJI was established by X-ray crystallography.  

In the years following, various stable heterocyclic arsonium salts were synthesized and resolved, including the isoarsinolinium iodide 4 and the spirocyclic arsonium iodides 5 and 6, in which dissociation was unlikely to occur the optically active iodides in each case were stable in chloroform. The attempted resolutions of a variety of tetra-arylarsonium salts, however, failed, which was attributed to manipulative difficulties, particularly that of crystallization . In 1961, (-h )-amylbenzylethylphenylarsonium bromide having [a]o +16.5° was isolated, but the salt was reported to racemize rapidly in solution  

The opening up of the various synthetic routes between resolved arsonium ions and arsines, particularly for simple non-cyclic compounds, and the laying down of firm stereochemical foundations for interconversions between them, marked the beginning of an era in which chiral tertiary arsines could be designed, synthesized and resolved for a variety of applications in organic synthesis and coordination chemistry. 

---

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. 

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). 

Acyclic arsines have been obtained in enantiomerically pure form and, on account of their high inversion barrier, these compounds hold configuration indefinitely. We illustrate two methods for their formation from enantiomerically pure arsonium salts, and these are shown for 41 and 43. Salt 41 is converted into 42 by cathodic reduction, and 43 is transformed into 44 by reaction with aqueous cyanide ion also formed in the latter case is 45, which accounts for the allyl (prop-2-enyl) group (see Wild9). 

Reaction of adsorbed inhibitors In some cases, the adsorbed corrosion inhibitor may react, usually by electro-chemical reduction, to form a product which may also be inhibitive. Inhibition due to the added substance has been termed primary inhibition and that due to the reaction product secondary inhibition " . In such cases, the inhibitive efficiency may increase or decrease with time according to whether the secondary inhibition is more or less effective than the primary inhibition. Some examples of inhibitors which react to give secondary inhibition are the following. Sulphoxides can be reduced to sulphides, which are more efficient inhibitorsQuaternary phosphonium and arsonium compounds can be reduced to the corresponding phosphine or arsine compounds, with little change in inhibitive efficiency . Acetylene compounds can undergo reduction followed by polymerisation to form a multimolecular protective film . Thioureas can be reduced to produce HS ions, which may act as stimulators of... 

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). 

---