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.