[3,3] Sigmatropic Rearrangement Versus [2+2] Cycloaddition: A DFT Investigation of Formal SN2′ Substitution of Imido Metal Complexes with Allylic Electrophiles



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Is common sense always best? A DFT investigation of two SN2′ substitution reactions with different central metals (zirconium and titanium) and types of allylic sources revealed that in both cases, a [2+2]/retro-[2+2] pathway is energetically favorable. This is in contrast with the usually accepted [3,3] sigmatropic rearrangement mechanism (see picture; TMS=trimethylsilyl).