The remarkable cis effect in the ene reactions of nitrosocarbonyl intermediates.

Nitrosocarbonyls are fleeting and highly reactive intermediates that undergo ene reactions in a two-step fashion. The addition steps are rate and product determining and lead to polarized diradicals that readily enter the H-abstraction step yielding the ene products. The addition TSs are reached early, and the stabilizing CH...O contacts drive the reactions to the cis adducts. B3LYP calculations alone do not describe the correct ordering of addition TSs in the ene reaction with trimethylethylene and (E)- and (Z)-3-methyl-2-pentenes. Only at the MPWB1K level of treatment, medium-range noncovalent interactions are successfully recovered, accounting satisfactorily for the experimental selectivities. The more stable and isolable ArNOs exhibit late addition TSs, and distortion energies become dominant driving the reaction exclusively to the Markovnikov adducts.