Mechanisms and Origins of Switchable Regioselectivity of Palladium- and Nickel-Catalyzed Allene Hydrosilylation with N-Heterocyclic Carbene Ligands: A Theoretical Study

Abstract

The mechanisms and origins for the Pd- and Ni-catalyzed regioselective hydrosilylation of allene have been investigated by means of density functional theory (DFT) calculations. The free-energy profiles of Pd- and Ni-catalyzed reactions with small and bulky N-heterocyclic carbene (NHC) ligands are calculated to determine the mechanism for regioselectivities. The calculation results show that different metals (Ni vs Pd) lead to regiochemical reversals for the hydrosilylation of allene. The allylsilane is the major product via palladium catalysis with small NHC ligand, while the vinylsilane is the major product via nickel catalysis with bulky NHC ligand. Both electronic and steric factors play a key role in the regioselectivities for the hydrosilylation of allene via Pd and Ni catalysts. The calculation results are in good agreement with observed regioselectivities and could provide insights into the design of new catalysts for the regioselectivity of hydrosilylation reactions.

Mechanistic Insights into Palladium‐Catalyzed Silylation of Aryl Iodides with Hydrosilanes through a DFT Study

“…It is known that palladium‐catalyzed reactions of hydrosilanes are involved in a number of important synthetic methodologies, and the oxidative addition of palladium(0) species to the Si−H bond is considered to be an important fundamental step. Similar to palladium‐ or platinum‐catalyzed hydrosilylation of alkynes and alkenes with hydrosilane, hydrosilane is initially coordinated to the palladium(0) catalyst to form a characteristic Pd 0 ‐η 2 ‐hydrosilane complex ( Cplx1 ) with a stabilization energy of 8.9 kcal mol −1 . Then, oxidative addition of palladium(0) complex O to the Si−H bond of 2 a proceeds smoothly with a low free energy barrier of 2.5 kcal mol −1 via TS OA , leading to a R 3 Si−Pd II −H intermediate ( A ), in which the hydrogen atom is cis to the silyl group.…”

“…Then, oxidative addition of palladium(0) complex O to the Si−H bond of 2 a proceeds smoothly with a low free energy barrier of 2.5 kcal mol −1 via TS OA , leading to a R 3 Si−Pd II −H intermediate ( A ), in which the hydrogen atom is cis to the silyl group. Owing to the trans influence of the silyl and hydrogen to the phosphine ligand, the stability of A is relative low. Intermediate A adopts a planar geometry, the main geometric parameters of which are quite close to the first stable mononuclear silyl palladium hydride reported by Fink and co‐workers with r (Pd−Si)=2.312 Å and r (Pd−P)=2.332/2.408 Å …”

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…357 The steric bulk of the ligand was found to determine whether the allene first is silylated (with bulky IPr) or hydrogenated (with IMes). Computational results obtained at the CPCM (THF)// B3LYP/6-31G(d)(LANL2DZ+d,f) level of theory indicated that Ni lowers the barriers for all reactions when compared to Pd (Scheme 26).…”

Computational insights into the mechanism of iron carbonyl-catalyzed ethylene hydrosilylation or dehydrogenative silylation

“…For one-to-one or two-to-two transformation, no correction was made. For two-to-one (or one-to-two) transformation, a correction of -2.6 (or 2.6) kcal/mol was made at the temperature of 298.15 K. These corrections have been applied in several earlier theoretical studies [45][46][47][48]. In this work, the entropy-corrected relative free energies were adopted to analyze the reaction mechanisms.…”