Hydrosilylation with Bis(alkynyl)(1,5-cyclooctadiene)platinum Catalysts:  A Density Functional Study of the Initial Activation

Abstract

At elevated temperatures bis(alkynyl)(1,5-cyclooctadiene)platinum complexes catalyze the cross-linking of polyorganosiloxanes containing Si−H and vinyl groups. Density functional calculations with medium-size basis sets and effective core potentials are reported for reactions that may activate these precatalysts for hydrosilylation. For a model system consisting of the bis(ethynyl) complex, trimethylsilane, and ethylene, the computations provide two plausible pathways for gaining access to the Chalk−Harrod cycle. The first one involves a sequence of four oxidative additions and reductive eliminations, while the second one requires a reductive coupling that is induced by olefin coordination. In both cases, the initial step is rate-determining, with a computed barrier of 27 kcal/mol. Experiments for polysiloxane systems of industrial interest favor the first pathway and yield barriers of 25−30 kcal/mol. Substituents in the alkynyl groups affect the measured barriers and the barriers computed for the rate-determining initial step of the first pathway in a qualitatively similar manner. We propose that the activation of the precatalysts is initiated by oxidative addition of Si−H.

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.…”

Recent advances and actual challenges in late transition metal catalyzed hydrosilylation of olefins from an industrial point of view

“…Quantumchemical calculations using DFT methods studying the activation steps of these complexes came to the conclusion that the activation of the pre-catalyst (COD)Pt(C CR) 2 is initiated by oxidative addition of Si-H. The activation energy and activation temperature depend on the substituents R in the alkynyl groups [172].…”

Synthesis of cross‐linked polyethylene oxide‐acetal macrocycles for solid superbase catalysts

“…A more recent publication39 by Bartlett et al reported the results of a computational study of reductive elimination reactions from platinum complexes and provides references to other recent work in this specific field. Another example of the application of computational tools to the study of these reactions can be found in the article by Jagadeesh et al,40 which examines the initial platinum catalyst activation in hydrosilation reactions.…”