DFT Study of the Transition States and Products of Methyl Radical Addition to Olefins Coordinated in an Asymmetrical Mode to [Cp₂Zr(O^tBu)]⁺: Predictions of Reversed Regioselectivities Compared to the Noncoordinated Reactions

Abstract

The addition of free alkyl radicals to olefins is known to encounter a small activation energy and to be highly regioselective, typically favoring the less substituted carbon of the double bond. The ability to modify the reactivity and regioselectivity of the given reaction can be of interest from both fundamental and practical perspectives. Olefins are known to bind to the [d 0 -Cp 2 Zr(O t Bu)] þ fragment (Zr) relatively weakly, affording adducts having a nonclassical asymmetrical bond between Zr and the double bond of the olefins. In the present study electronic structure methods based on density functional theory (B3LYP) have been used to investigate how the kinetics and thermodynamics of methyl radical addition to a systematically varied series of mono-and 1,1-disubstituted ethylene may change when they are coordinated to Zr. In general, methyl addition to the unsubstituted carbon of the coordinated olefins is found to encounter an increased activation energy compared to the noncoordinated reactions. In contrast, the barriers of addition to the substituted carbon of the coordinated olefins are slightly smaller than the barriers in the noncoordinated reactions. These effects lead to opposite regioselectivities in the free and the coordinated reactions of several olefins. However, even when the kinetic regioselectivity reverses upon coordination, the thermodynamic preference for addition to the terminal carbon in the noncoordinated reactions remains unchanged. These results are discussed qualitatively on the basis of unconventional geometries calculated in the coordinated reactants, transition states, and products.

Theoretical Structure–Reactivity Study of Ethylene Insertion into Nickel–Alkyl Bonds. A Kinetically Significant and Unanticipated Role oftransInfluence in Determining Agostic Bond Strengths

“…This should be associated with an energetic cost, which is greater in the case of a trans -coordinated Ni–P bond, and contributes further toward a larger barrier in 2a-C 2 H 4 . Related effects of the ancillary ligands have been invoked to account for calculated large differences in the activation energies of alkyl to CO migratory insertion in isomeric octahedral complexes. , Also, previous experimental , and theoretical , studies have shown that asymmetric metal–olefin coordination polarizes the π-electron density of the olefin toward the metal center, presumably accounting for the high trans influence resulting from this coordination mode.…”