Dibenzophosphole ylides

Application of the Wittig reaction of a nonstabilized ylide to the synthesis of an ( )-alkene is practically and effectively carried out by the Schlosser modification. Alternatively, the use of a trialkylphos-phonium ylide can produce high ratios of ( )-alkene." Recently, Vedejs has developed a reagent using dibenzophosphole ylides (110) to synthesize ( )-disubstituted alkenes (111) fixnn rddehydes (equation 24). The initial addition of ylide occurs at -78 C, but the intermediate oxaphosphetane must be heated to induce alkene formation. The stereoselectivity in the process is excellent, particularly for aldehydes with branched substitution a to the reacting center. Both the ethyl and butyl yli s have b n utilized. 

Oxaphosphetanes containing an unsaturated substituent R at Cj are exceptionally short-lived. Only the unusually stable dibenzophosphole derivatives 52c, 53c, and 53b can be detected, but not even this phosphorus environment provides sufficient stabilization to allow detection of 53d, the oxaphosphetane that would be formed from an ester-stabilized ylide (21c). Strongly electron-withdrawing substituents are required in addition to a five-membered ring, as in structure F (Tables 4 and 5), the first known... 

Oxaphosphetanes from semi-stable ylides have been detected spectroscopically only in the special case of the dibenzophosphole group [43]. Naked betaines, i.e. those that are not complexed with metal ions, have not been found because they cyclize too rapidly for detection by NMR [6]. 

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