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.