Optically active arsines

Without additional reagents Arsine sulfides from arsines Optically active arsenic compds. w.a.r. As -V AsS... 

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. 

Elements which form pyramidal bonding also form optically active compounds provided the groups are different. To this class belong derivatives of trivalent nitrogen, phosphines, arsines or stibines. 

Cathodic deprotection of tosylates of chiral alcohols was achieved without racemization by cleavage of the O—SO2 bond [351]. Optically active quaternary arsonium [352, 353] and phosphonium salts [354] are cathodically cleaved to tertiary arsines and phosphines respectively, with retention of the configuration. The first enantiomer enriched chiral phosphines have been prepared this way. 

Mori, K., and Toda, F. (1990) Optical Resolution of Arsine Oxides by Complexation with Optically Active [1,1 -Binaphthyl]-2,2 -diol, Bull. Chem. Soc. Jpn., 63, 2127-2128. 

As with the phosphines optically active arsines were obtained by combination of reduction and alkylation 51 ° ... 

Tetrachloroauric(III) acid in aqueous ethanol reacted with an excess of the phosphine-arsine undergoing reduction and chelation to give the cation, LXVII, precipitated as the colorless, highly stable iodide. The 4-coordinate aurous atom in this compound has the tetrahedral configuration, and attempts were made to resolve the cation into optically active forms. Fractional recrystallization of eight salts having different optically-active anions failed to give any indication of resolution. 

Optically active arsines preparation, uses and chiroptical properties 89... 

Relatively few applications of optically active tertiary arsines to asymmetric synthesis have been reported by comparison with the extensive work with phosphines . Authoritative accounts of the synthesis and stereochemistry of compounds of Group V elements are available other reviews cover the subject up until 1979 . For general treatments of organoarsenic chemistry up until 1976, including optically active compounds, two important works are available . Of related interest is an article on stereochemical aspects of phosphorus chemistry and another published in this series on optically active phosphines preparation, uses and chiroptical properties . On matters concerning the intricacies of resolutions work, the reader should consult Reference 21, especially Chapter 7, which is entitled Experimental Aspects and Art of Resolutions. 

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). 

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

Whereas the lithium aluminium hydride reduction of an optically active benzylphos-phonium salt in tetrahydrofuran leads to a racemic or highly racemized tertiary phosphine and toluene, the reduction of (—)-benzyl(n-butyl)methylphenylarsonium bromide (68) or (S)-(-(-)-benzylmethylphenyl(n-propyl)arsonium bromide, (S)-(-I-)-64, with this reagent affords (—)-n-butyImethylphenylarsine, ( —)-12, or (S)-(-l-)-methyl-phenyl(n-propyl)arsine, (S)-(-)-)-65, of similar optical purity to the products obtained by cathodic reduction of the respective arsonium compounds (Table 4) . Moreover,... 

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

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). 

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. 

In a bridge-splitting reaction, (R, R )-15 reacts with (S)-(-l-)-77 in methanol to give a solution of the chloride salts [S,(i As> As)] and [ S. (Sas>S as)]-82 which, when treated with 1 equiv. NH4PFg, yields 95% of [S,(Sas,Sas)]-( + )-83 having [a]p -1- 297° (acetone) after recrystallization from acetone-benzene (Scheme Cyanide cleanly liberated R,R)- + )-75 from [S,(Sas,SaJ]-( + )-83 in a dichloromethane-water mixture the arsine was recovered from the organic phase and recrystallized from aqueous methanol (Table 5). It was found later that 1,2-diaminoethane displaced the optically active bis(tertiary... 

The highly functionalized arsine 89 was resolved by the fractional crystallization of the diastereomers (/ ,/ as)-( + )- and (R,Sas)-(—)-92a . Oxidation of the dia-stereomer with BaMn04 produced the corresponding aldehyde complex that was converted into a complex of an optically active trans-As2N2 macrocycle (see below). 

Although R, R )- )- and (R, S )-( + )-93 epimerize at phosphorus when heated at 140 °C for ca 2 h, epimerization at arsenic was not effected by the usual racemizing conditions for tertiary arsines (methanolic HCl) indeed, optically active salts protonated at phosphorus were isolated. 

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