Arsines arsonium salts

Horner has extended this reaction to the electrochemical reduction of optically active arsonium salts (68, X = As), which also undergo cleavage with retention of stereochemical configuration at arsenic M 65). This is a convenient synthetic route of optically active arsines of known configuration. 

In early work, certain seven-membered and larger cyclic arsines were characterized by preparation and isolation of their arsonium salts by quaternization with alkyl halides. Reaction of more elaborate seven-membered cyclic arsines with activated alkyl bromide is shown in Equation (1). The C2-symmetric chiral tertiary arsine 61 reacts with methyl bromoacetate in a pressure tube under microwave irradiation to give the arsonium salt 62 in 71%. The latter was employed in the enantioselective olefination of 4-substituted cyclohexanes via the corresponding stabilized ylide <2002TA2187>. 

Cathodic cleavage of benzyl groups from resolved arsonium salts have been used to produce optically active tertiary arsines (Table 4). Some examples of interconversions... 

TABLE. 4. Optically active tertiary arsines via arsonium salts... 

Benzyl- or cinnamyl-triphenylarsonium bromides, upon reaction with aqueous sodium hydroxide at 100 °C, decompose into triphenylarsine oxide and toluene or 2-methyl-styrene, respectively . Since tertiary arsine oxides spontaneously racemize in water , alkaline hydrolysis of arsonium salts of this type cannot be used for the preparation of optically active arsines. 

Equilibrium concentrations of semi-stabilized benzylidene ylides derived from optically pure arsonium salts and alkali metal ethoxides react with aromatic aldehydes to produce high yields of enantiomerically enriched tra s-2,3-diaryloxiranes and optically pure tertiary arsines with retention of configuration at arsenic (equation S)" -" . 

The salts (-l-)-[AsAmylBnEtPh]Br, (-)-[AsBn(n-Bu)EtPh]Cl and (-l-)-[AsBnEt(n-Pentyl)Ph]Br were reported to completely racemize over ca 1 h at 20 °C in chloroform. A dissociation-equilibrium between the salt and the tertiary arsine and alkyl halide was put forward as the mechanism of the racemization, where the alkyl halide was considered to attack the tertiary arsine from the side of the lone-pair (preferred) or from the side of the plane of the three substituents . It was stated that the racemization would be facilitated by raising the temperature due to inversion of the tertiary arsine in line with earlier calculations . This notion of facile dissociation-equilibrium in arsonium salts, first suggested in 1921 and carried through until 1964 , was finally put to rest in 1965, however, when it was shown that the salts (—)-[AsBn( -Bu)EtPh]X (where X = Cl or Br) in chloroform or ethyl acetate were completely stable for days . Moreover,... 

The pyrolysis of quaternary arsonium salts in general affords arsines with the elimination of MeX when possible ... 

A publication discussing the uses of reactive arsonium ylides for the stereospecific preparation of epoxides draws attention to the fact that arsonium salts are less readily prepared than phosphonium salts because of the poorer nucleophilicity of arsenic compared to phosphorus, and suggests methods for obtaining them. Primary salts were made from alkyl triflates, while a-branched salts were prepared from alkyldiphenylarsines, obtained from iodo compounds as, for example, in equation 23. Reaction of alkyl halides with arsines to form arsonium salts is also promoted by the presence of silver tetra-fluoroborate . 

The reductive cleavage of achiral and optically active quaternary phosphonium and arsonium salts with alkali metal amalgams to form tertiary phosphines and arsines succeeds in high yield with retention of configuration [124]. The reduction with the amalgams was found to give better yields than the conventional cathodic cleavage. 

The synthesis of the arsonium ylides most commonly involves deprotonation of arsonium salts utilizing a variety of bases. Simple arsonium salts are available via direct alkylation of triphenylarsine, but more highly substituted compounds require specialized methods. These include the use of highly electrophilic triflate salts, alkylation of triphenylarsonium methylide, and double alkylation of lithiodiphenyl-arsine. For some purposes, formation of the tetrafluoroborate salts is desirable and can be effected in good yield by cation exchange. ... 

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). 

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