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Structural and Spectroscopic Evidence for the Occurrence of gauche ‐Betaine Intermediates in the Thio Wittig Reaction
Author(s) -
Puke Carsten,
Erker Gerhard,
Wibbeling Birgit,
Fröhlich Roland
Publication year - 1999
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199908)1999:8<1831::aid-ejoc1831>3.0.co;2-r
Subject(s) - wittig reaction , chemistry , ylide , phosphonium , aryl , thio , substituent , stereochemistry , benzophenone , betaine , medicinal chemistry , photochemistry , organic chemistry , alkyl
Reaction of the ylide (cyclopropyl) 3 PCH 2 with Michler's thioketone SC(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 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 1 J P Cipso (aryl) coupling constant was used to elucidate the characteristic structural properties of Wittig and thio Wittig intermediates derived by treatment of the series Ph 3 PCH 2 ( 2a ), Ph 2 MePCH 2 ( 2b ), and PhMe 2 PCH 2 ( 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 1 J P Cipso (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 [ 1 J P Cipso (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.