Stereospecific Synthesis of Polysubstituted E-Olefins by Reaction of Acyclic Nitrones with Free and Platinum(II) Coordinated Organonitriles

Free nitriles NCCH₂R (1a R = CO₂Me, 1b R = SO₂Ph, and 1c R = COPh) with an acidic α-methylene react with acyclic nitrones ^-O⁺N(Me)C(H)R‘ (2a R‘ = 4-MeC₆H₄ and 2b R‘ = 2,4,6-Me₃C₆H₂), in refluxing CH₂Cl₂, to afford stereoselectively the E-olefins (NC)(R)CC(H)R‘ (3a−3c and 3a‘−3c‘), whereas, when coordinated at the platinum(II) trans-[PtCl₂(NCCH₂R)₂] complexes (4a R = CO₂Me and 4b R = Cl), they undergo cycloaddition to give the (oxadiazoline)-Pt^II complexes (R = CO₂Me, Cl and R‘ = 4-MeC₆H₄, 2,4,6-Me₃C₆H₂) (5a-5d). Upon heating in CH₂Cl₂, 5a affords the corresponding alkene 3a. The reactions are greatly accelerated when carried out under focused microwave irradiation, particularly in the solid phase (SiO₂), without solvent, a substantial increase of the yields being also observed. The compounds were characterized by IR and ¹H, ¹³C, and ¹⁹⁵Pt NMR spectroscopies, FAB⁺-MS, elemental analyses and, in the cases of the alkene (NC)(CO₂Me)CC(H)(4-MeC₆H₄) 3a and of the oxadiazoline complex 5c, also by X-ray diffraction analyses.