1λ<sup>5</sup>,3λ<sup>5</sup>‐Diphosphol‐Derivate aus Bis(diphenylphosphino)methan und Propargylbromiden

2004 C-Heterosubstituted Phosphorus Ylides carbon atom. Sometimes the distribution of the electron density in the ylide molecule is simultaneously affected by two or even three different factors, their relative contribution dependmg on the nature of the element and the surrounding ligands. C-substituted phosphorus ylides attract particular interest as reagents for organic synthesis, and thus find increasing application in laboratory practice and industry. For example, C-silicon phosphorus ylides have served as the basis for a series of preparative syntheses of medicinal, pheromone, and other types of phosphorus ylide. C-Phosphino phosphorus ylides are important ligands in transition-metal complexes, C-Halo phosphorus ylides are of interest for the preparation of haloalkenes. C-Nitrogen ylides are of considerable importance, especially in the synthesis of antibiotics. Numerous papers and patents on applications of C-substituted phosphorus ylides in various fields of organic synthesis have been generalized in several monograph^^^^'* and ,2,1O-I 3 . T h s present chapter summarizes data on the application in organic synthesis of phosphorus ylides substituted on the a carbon atom by atoms of elements of Groups I-VII Phosphorus Ylides Substituted on the a-Carbon by Atoms of Element Groups I-IVThe influence of the o-donor inductive and n-acceptor mesomeric effects of the elements is clearly exemplified by ylides containing metals and metalloids of the main subgroups of Groups I-IV of the periodic table on the u carbon atom. The elements with o-donor properties increase the electron density on the ylide carbon atom, thus changing the nucleophilicity and reactivity of the ylides, as for instance, in the case of C-lithiated ylides. In contrast, such elements as germanium and silicon reduce the electron density on the a carbon atom thus forming partial element-carbon double bonding by a p z d n mechanism that results in lowering of the nucleophilicity of the ylide. Often these two opposite effects act together, and their relative contribution depends on the nature of element and the surroundmg ligands. The chemistry of ylides containing lithium, boron, silicon, tin, and other elements on the u carbon atom has recently received considerable attention.

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