STRUCTURE AND PROPERTIES OF PHOSPHONIUM YLIDES-BETAINES, DERIVATIVES OF 2-PHENYL-2-OXAZOLIN-5-ONE AND ITS THIO-ANALOGS AND SELENO-ANALOGS

The combined chemical, spectral, and X-ray structural investigation ha s shown that the structure and properties of mesomeric triphenylphosph onium ylides containing the 5-oxo-2-phenyl-4-oxazolidene moiety underg o considerable changes when the carbonyl oxygen atom is replaced by su lfur or selenium, This substitution leads to a clear decrease of the c ontribution of the ylide structure and an increased importance of the betaine structure, in contradiction with the electronegativity decreas e in the O>S>Se series, but relates to the different ability of double bond formation of these elements. The deactivation of the ylide site by the thio- or selenocarbonyl group has been applied to regioselectiv e syntheses.