Phosphorus belongs to group 15 in the periodic table, the same as nitrogen, and it binds with carbon to form various organophosphate compounds. However, since nitrogen belongs to the second-period and phosphorus to the third, they differ in atomic size, ionization energy and electronegativity etc. Therefore, organophosphate compounds show a characteristic reactivity which organic nitrogen compounds do not have, and it is utilized as a versatile reaction reagent in organic synthesis. For example, phosphorus has a high affinity for oxygen: phosphorus ylide and phosphonate are utilized for the Wittig reaction and the Horner-Emmons reaction, respectively, and it extracts oxygen from aldehydes to produce olefins. Phosphine extracts oxygen from alcohols to produce esters in the Mitsunobu reaction.
Another feature of phosphine includes bearing a soft lone pair. Therefore, it forms comparatively stable complexes with transition metals such as palladium. This transition metal complex is used frequently as a catalyst in reactions such as the Suzuki-Miyaura cross-coupling reaction and the Heck reaction.1) Although phosphine also has a trigonal pyramidal structure like amines, its rotational barrier energy is considerably higher than the corresponding amine. Therefore, a phosphine which has three different substituents has optical isomers. This optically-active phosphine is utilized for asymmetric syntheses as a P-chiral ligand.2) In this section, we introduce tri-substituted alkyl and aryl phosphines, such as tricyclohexylphosphine and triphenylphosphine.
Literature
2) T. Imamoto, J. Watanabe, Y. Wada, H. Masuda, H. Yamada, H. Tsuruta, S. Matsukawa, K. Yamaguchi, J. Am. Chem. Soc., 1998, 120, 1635 [DOI].
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