Aryl-Fluoride Reductive Elimination from Pd(II): Feasibility Assessment from Theory and Experiment

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Abstract
DFT methods were used to elucidate features of coordination environment of Pd(II) that could enable Ar−F reductive elimination as an elementary C−F bond-forming reaction potentially amenable to integration into catalytic cycles for synthesis of organofluorine compounds with benign stoichiometric sources of F-. Three-coordinate T-shaped geometry of PdIIAr(F)L (L = NHC, PR3) was shown to offer kinetics and thermodynamics of Ar−F elimination largely compatible with synthetic applications, whereas coordination of strong fourth ligands to Pd or association of hydrogen bond donors with F each caused pronounced stabilization of Pd(II) reactant and increased activation barrier beyond the practical range. Decreasing donor ability of L promotes elimination kinetics via increasing driving force and para-substituents on Ar exert a sizable SNAr-type TS effect. Synthesis and characterization of the novel [Pd(C6H4-4-NO2)ArL(μ-F)]2 (L = P(o-Tolyl)3, 17; P(t-Bu)3, 18) revealed stability of the fluoride-bridged dimer forms of the requisite PdIIAr(F)L as the key remaining obstacle to Ar−F reductive elimination in practice. Interligand steric repulsion with P(t-Bu)3 served to destabilize dimer 18 by 20 kcal/mol, estimated with DFT relative to PMe3 analog, yet was insufficient to enable formation of greater than trace quantities of Ar−F; C−H activation of P(t-Bu)3 followed by isobutylene elimination was the major degradation pathway of 18 while Ar/F- scrambling and Ar−Ar reductive elimination dominated thermal decomposition of 17. However, use of Buchwald's L = P(C6H4-2-Trip)(t-Bu)2 provided the additional steric pressure on the [PdArL(μ-F)]2 core needed to enable formation of aryl-fluoride net reductive elimination product in quantifiable yields (10%) in reactions with both 17 and 18 at 60° over 22 h.

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