Arsonium ylides, addition reactions

Indeed, in diethyl ether, lithium dimethylcuprate usually reacts with the a-enone group to give a methyl-substituted bromo ketone. Addition of hexamethylphosphoric triamide (HMPT), however, slows down this reaction to such an extent that displacement of the bromo substituent takes place [698], Another remarkable example of the influence of HMPT on chemoselectivity is the reaction of an arsonium ylide, Ph3As= CH-CH=CH-Ph, with benzaldehyde in tetrahydrofuran solution, yielding either an epoxide (in THE) or an alkene (in THF/HMPT) [699],... 

The reactions of substituted arsonium ylides with ciubonyl compounds can be carried out with high stereoselectivity in favor of rranr-disubstituted epoxides (equation 14). Equatorial attack is observed for addition to 4-r-butylcyclohexanone. Good stereoselectivity (S9 l) was observed for the addition of triphenylarsonium methylide to some (A A(-dibenzyl)amino aldehydes at -78 °C in THF. Interestingly, the initial hydroxy tetraalkylarsonium adducts were isolated under these conditions, and had to be cyclized under the action of sodium hydride in a separate step. 

Addition of organolithium reagents to diphenylarsenoethene provides another route to a-arseno anions. This reaction is general to the second and third row elements (Si, P, S, Ge, As and Se) but does not occur with elements from the fourth and fifth rows (Sn, Sb, Te, Pb and Bi). Unlike their phosphorus analogs, triphenylarsonium salts cannot be prepared by reaction of triphenylarsine with an alkyl halide. Their preparation can be achieved by reaction of triphenylarsine with the more electrophilic alkyl tri-flates. It is also possible to alkylate arsonium ylides. ° ° ... 

Methylene trimethylarsorane (the term trimethylarsonium methylide is equally correct) had already been formulated in 1953 by Wittig and Torssell (104) who studied the reaction of tetramethylarsonium salts with organolithium compounds. Although this method cannot be used for the preparation of the salt-free material due to the strong complexation by the lithium cations, it is clear from reactions of the product mixture that the ylide is present in solution. The formation of arsonium salts upon addition of alkyl halides is a typical example ... 

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