Mechanism of Ketone Allylation with Allylboronates as Catalyzed by Zinc Compounds: A DFT Study

The mechanism of the allylation reaction between 4-chloroacetophenone and pinacol allylboronates catalyzed by ZnEt2 with alcohols was investigated using density functional theory (DFT) at the M05-2X/6-311++G(d,p) level. The calculations reveal that the reaction prefers to proceed through a double gamma-addition step-wise reaction mechanism rather than a Lewis acid-catalyzed concerted one. The intermediate with a four-coordinated boron center, which is formed through proton transfer from EtOH to the ethyl group of ZnEt2 mediated by the boron center, is the active species and an entrance for the catalytic cycle. The latter is composed of three elementary steps: 1) boron to zinc transmetalation leading to the formation of allylzincate species, 2) electrophilic addition of ketone to allylzincate species, and 3) generation of the final product with recovery of the catalyst. The boron to zinc transmetalation step has the largest energy barrier of 61.0 kJmol(-1) and is predicted to be the rate-determining step. The calculations indicate that the additive EtOH plays important roles both in lowering the activation free energy for the formation of the four-coordinated boron active intermediate and in transforming the low catalytic activity ZnEt2 into high activity zinc alkoxide species. The alcohols with a less sterically encumbering R group might be the effective additives. The substituted groups on the allylboronates might primarily affect the boron to zinc transmetalation, and the allylboronates with substituents on the C-gamma atom is poor in reactivity. The comparison of the catalytic effect between the zinc compounds investigated suggest that Zn(OEt)(2), Zn(OH)(2), and ZnF2 exhibit higher catalytic efficiency for the boron to zinc transmetalation due to the activation of the B C-alpha bond through orbital interactions between the p orbitals of the EtO, OH, F groups and the empty p orbital of the boron center.