Generality and Strength of Transition Metal β‑Effects

Using computation, we examine the generality and strength of β-effects from transition metal centers on β-elimination. In particular, we find that a β-Pd­(II) substituent imparts over twice the stabilization to a carbocation as a Si substituent, representative of the well-known β-silicon effect. We established efficient and practical computational parameters to investigate the σσ conjugation in an experimentally relevant system: N,N-picolinamide vinyl metalacycles with β-substituents that can undergo elimination. We have found that the β-Pd effect depends on the nature of the Cβ substituent (X): This effect is negligible for X = H, Me, OH, and F, but is significant for X = Cl, Br, and I. We have also extended these studies to the β-effect in N,N-picolinamide vinyl metalacycles with β-substituents of other transition metalsFe­(II), Ru­(II), Os­(II), Co­(III), Rh­(III), Ir­(III), Ni­(II), Pd­(II), Pt­(II), Cu­(III), Ag­(III), and Au­(III). We found that the electronegativity of the metals correlates reasonably well with the relative β-effects, with first-row transition metals exerting the strongest influence. Overall, it is our anticipation that a more profound appreciation of transition metal β-effects will facilitate the design of novel reactions, including new variants of transition metal catalyzed C–H functionalization.