L. Vanmeervelt et al., STRUCTURE AND PROPERTIES OF PHOSPHONIUM YLIDES-BETAINES, DERIVATIVES OF 2-PHENYL-2-OXAZOLIN-5-ONE AND ITS THIO-ANALOGS AND SELENO-ANALOGS, Tetrahedron, 51(5), 1995, pp. 1471-1482
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.