Oxaphosphetane
An oxaphosphetane is a molecule containing a four-membered ring with one phosphorus, one oxygen and two carbon atoms. In a 1,2-oxaphosphetane phosphorus is bonded directly to oxygen, whereas a 1,3-oxaphosphetane has the phosphorus and oxygen atoms at opposite corners.
1,2-Oxaphosphetanes are rarely isolated but are important intermediates in the Wittig reaction and related reactions such as the Seyferth–Gilbert homologation and the Horner–Wadsworth–Emmons reaction. Edwin Vedejs's nuclear [magnetic resonance spectroscopy|NMR] studies first revealed the importance of oxaphosphetanes in the mechanism of the Wittig reaction in the 1970s.
In 2005 the first isolation of 1,2-Oxaphosphetanes was reported. One of the compounds was characterized by X-ray crystallography and nuclear magnetic [resonance spectroscopy|NMR]. Although relatively stable, thermal decomposition of these oxaphosphetanes gave a phosphonium salt, which slowly dissociated to the Wittig reaction starting materials, the carbonyl and olefin compounds.