Alder-ene reaction: aromaticity and activation-strain analysis. 
We have computationally explored the trend in reactivity of the Alder-ene reactions between propene and a series of seven enophiles using density functional theory at M06-2X/def2-TZVPP.  The reaction barrier decreases along the enophiles in the order H(2) C=CH(2) > HC≡CH > H(2) C=NH > H(2) C=CH(COOCH(3) ) > H(2) C=O > H(2) C=PH > H(2) C=S. Thus, barriers drop in particular, if third-period atoms become involved in the double bond of the enophile.  Activation-strain analyses show that this trend in reactivity correlates with the activation strain associated with deforming reactants from their equilibrium structure to the geometry they adopt in the transition state.  We discuss the origin of this trend and its relationship with the extent of synchronicity between H transfer from ene to enophile and the formation of the new C−C bond.