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

摘要: Reaction of the ylide (cyclopropyl)3PCH2 with Michler's thioketone SC(C6H4NMe2)2 in [D5]pyridine gave thio Wittig intermediate [(cyclopropyl)3P+–CH2–CAr2–S–] (11; 31P NMR: δ = +27.7). Compound 11 was characterized by X-ray diffraction and shown to exhibit a gauche-betaine-type structure an S–C–C–P dihedral angle 52.9(3)° P···S separation 3.312(2) A. The characteristic averaged 1JPCipso(aryl) coupling constant used elucidate structural properties intermediates derived treatment series Ph3PCH2 (2a), Ph2MePCH2 (2b), PhMe2PCH2 (2c) bis(p-methoxyphenyl) ketone give 3 or benzophenone (to 3′) 5), respectively. oxaphosphetane 3′a–3′c shows strong response values (3a: 94 Hz; 3′b: 74 3′c: 47 Hz) this substituent perturbation, as it is expected for dynamic trigonal-bipyramidal situation at phosphorus atom, whereas only very little affected [1JPCipso(aryl) 5a: 92 5b: 86 5c: 82 Hz]. Thus, 5 seem pronounced phosphonium character polar solvents may be regarded thiabetaine-type solution.