Arsenic ylides structure

The pentavalent triphenylarsenic dihalides are of interest because they present structures that are dependent on the halogenand are convenient sources for the preparation of arsenic ylides. The chemistry of triphenylarsenic difluoride. 

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... 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

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). 

A variety of ylides (e.g. 5) of arsenic, antimony and bismuth have been prepared under mild conditions via the reaction of diazo compounds with the appropriate three valent organometalloid. Schmidbaur s group continues to produce new ylide structures, for example (6).5 The lithiated aza-ylide (8) has been prepared directly from the parent... 

However, certain phosphonium ylides, such as those with an electron-withdrawing substituent in the alkylidene moiety, are relatively unreac-tive toward certain substrates such as ketones (22, 77, 95). This led us to consider whether arsonium ylides might be preferable to phosphonium ylides in certain reactions (48, 94). The overlap of the p orbitals of carbon with d orbitals of arsenic is less effective than with d orbitals of phosphorus. Therefore the covalent canonical form (la) should make a smaller contribution to the overall structure of arsonium ylides than to that of the corresponding phosphonium ylides. 

In addition to the phosphorus ylide complexes, arsenic and snlfur yUde complexes of silver have been obtained, althongh no structural reports exist. While the arsenic derivatives match the phosphorus ylide complexes for stability of the Ag products, the sulfoxonium ylide complexes are somewhat less stable. 

Extremely efficient stabilization can be observed for dipolar ions if the atoms or groups of atoms bearing the opposite charges are in the close vicinity. Such is the case with ylides, dipolar ions in which the carbanionic center is stabilized by the positive charge located on a neighboring multiple-electron atom such as phosphorus, sulfur, arsenic, etc. (onium centers). Typical ylides are represented in the structures 37 and 38 ... 

Arsonium ylides may be represented as hybrids of pentacovalent arsenic (2) and dipolar (3) structures. 

Semi-stabilized arsonium ylides are intermediate in behaviour between stable and reactive ylides, and may provide alkenes and/or epoxides " °. In these cases factors such as the substituent groups on arsenic, and the nature of the solvent and of the base, may become important in determining the nature of the product this will be considered in more detail later (Section V.A.4). Small changes in the structure of the ylidic moiety may also have a marked effect for example, whereas triphenylarsonium jS-napththylmethylide reacts to give epoxide, the presence of a bromine atom at the adjacent a-position of the naphthalene ring results instead in the formation of alkenes . ... 

The difference between the ylides (39) and the tetraphenylcyclopentadienylide is that the former, but not the latter, have substituent groups, carbonyl or sulphonyl, which can interact intramolecularly with the antimony or arsenic atom. This interaction has been clearly shown by X-ray crystal structures ... 

These X-rays studies show that the interaction is greater in the case of antimony than in the case of arsenic. Despite the greater size of antimony than arsenic, the Sb — O distance is shorter than the As—O distance. In addition the Sb—C (ylidic) and As—C(ylidic) bond distances indicate that the Sb—C bond has more double-bond character than the As—C bond. In all cases these ylides take up Z,Z-conformations, both in solution (as observed by NMR spectra) and in the solid state, but in the solid state they are not symmetric, only one of the substituent groups being involved in the intramolecular interaction. This can be represented as in 40 and 41. In accord with such structures one of the bonds linking substituents to the ylidic carbon atom is shorter than the other. 

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. 

The structure determination of the arsonium ylide triphenylarsonium 2-acetyl-3,4,5-triphenylcyclopentadienide shows that the arsenic co-ordination... 

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