Reaction of the ylide (cyclopropyl) 3 P=CH 2 with Michler's 3Ј) or bis(p-methoxyphenyl) thioketone (to give 5), respectively. The oxaphosphetane series 3Јa-3Јc shows a thioketone S=C(C 6 H 4 NMe 2 ) 2 in [D 5 ]pyridine gave the thio Wittig intermediate [(cyclopropyl) 3 P + -CH 2 -CAr 2 -S -] (11; 31 P strong response of the 1 J PCipso (aryl) values (3a: 94 Hz; 3Јb: 74 Hz; 3Јc: 47 Hz) to this substituent perturbation, as it is NMR: δ = +27.7). Compound 11 was characterized by X-ray diffraction and shown to exhibit a gauche-betaine-type expected for a dynamic trigonal-bipyramidal situation at the phosphorus atom, whereas the thio Wittig intermediate is structure with an S-C-C-P dihedral angle of 52.9(3)°and a P•••S separation of 3.312(2) A ˚. The characteristic averaged only very little affected [ 1 J PCipso (aryl) of 5a: 92 Hz; 5b: 86 Hz; 5c: 82 Hz]. Thus, the thio Wittig intermediates 5 seem to 1 J PCipso (aryl) coupling constant was used to elucidate the characteristic structural properties of Wittig and thio Wittig exhibit a pronounced phosphonium character in polar solvents and may be regarded as thiabetaine-type intermediates derived by treatment of the series Ph 3 P=CH 2 (2a), Ph 2 MeP=CH 2 (2b), and PhMe 2 P=CH 2 (2c) with bis(p-intermediates in solution. methoxyphenyl) ketone to give 3 or benzophenone (to give is characterized by a 31 P-NMR resonance at a strongly [a]