Ylide

An ylide is an inner salt with carbon as the anion ( ie a zwitterion ).

The carbon - heteroatom bond has both covalent ( " -yl ") and ionic ( " id" ) character. The carbanionic center is stabilized by adjacent hetero atom, there are two resonance-stabilized resonance structures, as the ylide ( zwitterion ) and ylene (C = Y double bond ), respectively.

Phosphorus ylides

General

With ylides are often phosphorus ylide (also called a phosphorane ) meant. It is to organophosphorus compounds, mostly in the form ylene R3P = CR'R * (where R = organic radical, phenyl times ) are shown in the literature, thus having a double bond between the phosphorus and carbon atom. The phosphorus atom has an oxidation state of V. Phosphorus ylides can be prepared by reaction of phosphonium salts of [ R3P - CHR'R *] X ( X = Cl, Br, I, etc.) with a strong base (eg n- butyl lithium ( n-BuLi ), potassium -tert- butoxide ( KO t Bu ), sodium ( NaNH2 ). For the formation of ylides is an acidic α -H atom necessary, so is [ Ph4P ] X (Ph = phenyl ) no ylide.

Example:

Electronic structure of ylides

Ylide called a resonance structure with charge separation, where the atoms involved have each octet configuration. In the nonpolar ylene resonance structure (see above) the phosphorus atom carries ten electrons, which is why high-energy d orbitals should be involved in the binding. Structural studies speak of the ylide structure, ie a zwitterionic with positively charged phosphorus atom and negatively charged carbon atom ( carbanion ) to the highest percentage. There are non- stabilized ylides having as an alkyl radical. With stabilized ylides, the negative charge is stabilized by electron-withdrawing groups such as COOR, C ( = O) R, CN. ...

Use

Phosphorus ylides used in the Wittig reaction. They are usually not isolated but generated in situ and reacted directly with an aldehyde, which ultimately receives the methylene group of the ylide (ie, from C = O C = CH2). These reaction and their variations (for example, the Horner- Wadsworth-Emmons reaction) are used in a variety of industrial processes, as the key step ( for example, the vitamin synthesis).

Schmidbaur first recognized that ylides because of their donor ( negative charge of the carbanion ) are suitable as ligands for metals. He also succeeded for the first time the isolation of the trimethyl- methylene- phosphorane ( Me3P = CH2), an extremely reactive compound.

More ylides

There are still more ylides of main group elements known, for example, arsenic ( R3AS = CR'R * ), nitrogen ( R3N -C - R'R * ) and sulfur ( R2S = CR'R *). Therefore, one uses the terms P- ylid, N- or S- ylide ylide, to distinguish the different ylides. Since the octet rule for nitrogen is strictly valid, the N- ylides can be formulated only in ionic resonance structure.

• Substance Group
• Salt
• Zwitterion