Structural and spectroscopic evidence for the occurrence of gauche-betaineintermediates in the thio Wittig reaction

Reaction of the ylide (cyclopropyl)(3)P=CH2 with Michler's thioketone S=C(C 6H4NMe2)(2) in [D-5]pyridine gave the thio Wittig intermediate [(cyclopropy l)(3)P+-CH2-CAr2-S-] (11; P-31 NMR: delta = +27.7). Compound nl was charact erized by X-ray diffraction and shown to exhibit a gauche-betaine-type stru cture with an S-C-C-P dihedral angle of 52.9(3)degrees and a P...S separati on of 3.312(2) Angstrom. The characteristic averaged (1)J(PCipso)(aryl) cou pling constant was used to elucidate the characteristic structural properti es of Wittig and thio Wittig intermediates derived by treatment of the seri es Ph3P=CH2 (2a), Ph2MeP=CH2 (2b), and PhMe2P=CH2 (2c) with bis(p-methoxyph enyl) ketone to give 3 or benzophenone (to give 3') or bis(p-methoxyphenyl) thioketone (to give 5), respectively. The oxaphosphetane series 3'a3'c sho ws a strong response of the (1)J(PCipso)(aryl) values (3a: 94 Hz; 3'b: 74 H z; 3'c: 47 Hz) to this substituent perturbation, as it is expected for a dy namic trigonal-bipyramidal situation at the phosphorus atom, whereas the th io Wittig intermediate is only very little affected [(1)J(PCipso)(aryl) of 5a: 92 Hz; 5b: 86 Hz; 5c: 82 Hz]. Thus, the thio Wittig intermediates 5 see m to exhibit a pronounced phosphonium character in polar solvents and may b e regarded as thiabetaine-type intermediates in solution.