Periodic Trends in Electron-Withdrawing Ability: Effects on Carbon–Fluorine Reductive Elimination from Palladium as a Model System

Tri(1-adamantyl)phosphine (Ad3P) represents a prototypical trialkylphosphine with polarizable diamondoid substituents. While certain properties of Ad3P, such as its relatively high donicity, have been correlated to increased polarizability, a more systematic assessment of the electronic properties of adamantyl-type fragments and associated ligand effects in catalysis is desirable. In this work, a suite of isosteric trialkylphosphines featuring tert-butyl, 1-adamantyl, as well as related heteroadamantyl and (hetero)diamantyl fragments were parametrized and computationally evaluated to isolate electronic substituent effects on the energetics of a challenging model reaction─C(sp2)–F reductive elimination (RE) from (R3P)PdII(Ar)F complexes. The RE energy barriers correlated more strongly to electrostatic versus polarizability, dispersion, or inductive parameters. Ligands varying by substitution (N, O, S, or Se) at remote δ positions of the diamondoid fragment still manifested significant net electron-withdrawing effects, particularly for chalcogen-containing ligands. Surprisingly, the withdrawing effect was insensitive to chalcogen identity, which we attribute to the compensation between periodic trend (i.e., electronegativity and bond angle) and geometry-dependent local dipole alignment. Observation that substituent withdrawing ability and concomitant ligand donicity can be tuned by geometric constraints may provide new opportunities for ligand design in transition metal catalysis beyond typical notions of substituent electronic effects.