Structural and Spectroscopic Evidence for the Occurrence of gauche-Betaine Intermediates in the Thio Wittig Reaction

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