DFT-Based Explanation of the Effect of Simple Anionic Ligands on the Regioselectivity of the Heck Arylation of Acrolein Acetals

Abstract

The Heck arylation of acrolein acetal has been studied computationally and compared to the corresponding reaction with allyl ethers. The reaction can be controlled to give either cinnamaldehydes or arylpropanoic esters by addition of different coordinating anions, acetate, or chloride. The computational study reveals that coordinating acetate raises the energy of an intermediate sufficiently to block the access to an otherwise favorable β-hydride elimination. The reaction path is also compared to that of allyl ethers, which always give significant amounts of cinnamyl ether products under all reaction conditions. The difference between the two substrate classes could be rationalized in terms of relative hydride donating power of the two substrates.

Solvent effects in palladium catalysed cross-coupling reactions

“…The activated complex that occurs during a -hydride elimination exhibits some charge separation as bonds form and break, which like migratory insertion is consistent with the observation that dipolar solvents accelerate the reaction (Scheme 40). 223,224,225 226 However, on some occasions less polar electron donors give superior yields, e.g. ethers (Scheme 42).…”

Mechanism, Reactivity, and Selectivity in Palladium-Catalyzed Redox-Relay Heck Arylations of Alkenyl Alcohols

“… 19 , 20 , 42 − 44 In addition, the β-hydride elimination step that is an important feature of the current reaction has also been investigated. 30 , 31 For asymmetric Heck reactions, previous studies have focused on cyclic substrates. 32 , 45 However, to our knowledge, the more complex situation, where asymmetry is induced in an open-chain alkene, has not been studied theoretically.…”

tBu or not tBu?

“…Thus, it is necessary to investigate all possible TSs generating linear or branched product, irrespective of the energies of the initial complexes. [42] It will be shown that the crossover between opposite regiochemical preferences on the two major paths is a general phenomenon observed for all four P ligands. The selectivity is determined mainly by the Gibbs free-energy difference DG°between the two lowest TS giving linear and branched product, respectively.…”

“…However, along the reaction coordinate the energy curves of the different paths may cross and the relative energies for the different coordination modes will be interchanged. Thus, it is necessary to investigate all possible TSs generating linear or branched product, irrespective of the energies of the initial complexes 42. It will be shown that the crossover between opposite regiochemical preferences on the two major paths is a general phenomenon observed for all four P ligands.…”