Arsenic compounds ylides

Among the heavy main group elements arsenic has received the most attention. Arsonium ylides react with carbonyl compounds to yield either the alkenes or epoxides, depending upon the structure of the ylide and the reaction conditions. )espite their toxicity, arsenic compounds can be handled safely. 

Arsenic-containing ylides are better reagents than the corresponding phosphorus compounds for additions to alkenylphosphonic diesters as a route to cyclopropylphosphonicdiesters. ... 

Numerous reactions of carbonyl compounds, alcohols, olefins, etc., with compounds bearing E14=X bonds in which the latter act as direct analogs of phosphorus and arsenic ylides have already been accomplished.17 Recently, an interest in reactivity of compounds with multiple E14-X bonds is increasing due to challenges of important practical applications (see,... 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

There is a comprehensive review of this area2 so only a few recent developments will be mentioned here. Organoarsenic intramolecular coordination compounds, e.g. (1), have also recently been reviewed,3 and organoarsenic chemistry is reviewed annually.4 There is a review containing 102 references on arsonium ylides.5 The canonical structures of the arsenic ylides are... 

Since this discovery a number of binuclear complexes of this type have been isolated, including the arsenic analog of the above (57) and compounds prepared from more exotic ylide precursors (58-60). A polymeric gold(I) complex was obtained by reaction of [AuCl(PMe3)] with the biden-tate ylide CH2=PMe2(CH2)6PMe2=CH2 (60). 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

Resonance (chemistry) The migration of electrons between atoms in a molecule or ion (compare with resonance compound and ylide). Figure B.10 shows an example of resonance in inorganic arsenate. 

When this method is applied to compounds of phosphorus and arsenic, the components undergo an entirely different reaction, and alkylidene trialkyl derivatives of these elements (ylides) are obtained 102, 104). Thus, instead of the introduction of an additional alkyl substituent at the central element, as observed with antimony, a deprotonation at one of the a-carbon atoms adjacent to phosphorus and arsenic occurs ... 

It has been pointed out previously that silylation of ylides leads to stabilized products and that this is only one example of the very general phenomenon of carbanion stabilization through silicon (34, 61, 72). This effect was also found for arsenic ylides (34, 73), and is the basis for the preparation of other compounds of this series. The influence of silicon is by no means solely an electronic effect. In many cases, where alkylsilyl substituents are introduced, a steric effect may well dominate, which may reduce lattice energies for salts in transylidation reactions, preventing intermolecular contacts in decomposition processes, and rendering the formation of salt adducts unfavorable. This steric effect is reduced to a minimum, but not eliminated, if simple SiH3 groups are employed (61). Even then, however, a pronounced silicon effect is found, which must be based on electronic influences (49, 60, 61). 

Ylides are neutral compounds characterized by internally compensating ionic centers, a carbanionic group and a neighboring onium unit, typically localized at phosphorus, arsenic, or sulfur, Ylidic carbanions are strong nucleophiles and show a high affinity for most metals in their various oxidation states. This can be exemplified by the reactions of a simple phosphorus ylide, like trimethylphos-phonium methylide (trimethylmethylenephosphorane), that are now known to lead to organometallic compounds with exceptionally stable carbon-to-metal bonds. 

The first report of such effects was in a study of the reactions of a series of tris(p-substituted phenyl)arsonium ylides with benzaldehyde all these ylides gave epoxides in high yield save for the tris(p-dimethylamino) compound, which gave instead the trans-alkene L In further experiments replacement of a triphenylarsonium group by a tris(p-methoxyphenyl)arsonium group was found to have little effect on the ratio of products and inclusion of the arsenic atom in a strained ring also had no effect. ... 

Reactions of arsonium ylides with carbonyl compounds take place much more readily than with phosphonium or sulphonium ylides. The nature of the products depends upon the character of the substituents on the ylide carbon atom, where electron-withdrawing substituents favour alkene formation, and of substituents on the arsenic atom, where... 

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 . 

Arsonium ylides were discovered near the turn of the century, but their reactions with carbonyl compounds did not become elucidated until the 1960s. In a broad sense, arsonium ylides are midway in chemical behavior between ylides of phosphorus and those of sulfur. Stabilized arsonium ylides react with carbonyl compounds to afford alkenes, whereas the unstabilized analogs give rise to epoxides. More subtly, the nature of the substituents on either the ylide arsenic or carbon atom can alter the course of the reaction the choice of solvent can exert a similar effect. ... 

The addition of a carbon atom to the 0=N double bond has been reported using sulfur (equation 35)133 and arsenic ylides, halogen-stabilized carbenes, and diazo compounds.i i-i The best substrates are imines in which the nitrogen is substituted with an aromatic ring, substituted oximes, and hydrazones. 

The arsenic ylide PhaAs CHj has now been isolated as an unstable solid from a reaction between PhaAsMeBr and sodium amide in THF, and the analogous methyl ylide Me3As==CH2 is the immediate precursor of an unusual five-coordinate tetra-alkylarsonium compound. Methanol adds to the ylide to give Mc4AsOMe, which from low-temperature n.m.r. spectra has the structure (120). 

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