DFT Studies on the Borylation of α,β-Unsaturated Carbonyl Compounds Catalyzed by Phosphine Copper(I) Boryl Complexes and Observations on the Interconversions between O- and C-Bound Enolates of Cu, B, and Si

Abstract:
The detailed mechanisms for the borylation of α,β-unsaturated carbonyl compounds, acrolein and methylacrylate, catalyzed by phosphine copper(I) boryl complexes were studied with the aid of density functional theory calculations. The results show that the catalyzed borylation occurs through CC insertion into Cu−B to give a β-borylalkyl C-bound Cu(I) enolate intermediate. In the borylation of acrolein, the C-bound Cu(I) enolate undergoes a keto-to-enol isomerization to give an O-bound enolate intermediate followed by a σ-bond metathesis with a diboron reagent. In the borylation of methylacrylate, a keto-to-enol isomerization does not occur due to the inertness of the ester group. Alcoholysis or hydrolysis is necessary to convert the C-bound Cu(I) enolate intermediate efficiently to the borylation product and Cu(I) alkoxide that can easily undergo σ-bond metathesis with a diboron reagent. The different borylation reaction mechanisms of acrolein and methylacrylate are closely related to the relative thermodynamic and kinetic stability of the C- and O-enolate intermediates involved in the reactions of the two different classes of substrates. The insertion reactions of ethene, formaldehyde, acrolein (2-propenal, CH2CH—CHO), and methylacrylate (CH2CH—CO2Me) into a Cu−B bond were compared. It was found that the insertion barriers correlate very well with the orbital energies of the LUMOs calculated for the four substrates, supporting the notion that the insertion mainly involves a nucleophilic attack of the Cu−B σ bond at the coordinated unsaturated substrate molecule. Although formaldehyde is more active than ethene in the insertion reaction, the reactivity of the CC bond is greater than that of the CO bond in α,β-unsaturated aldehyde and ester. In addition, the thermodynamics and kinetics of interconversions between O- and C-bound enolates of Cu, B, and Si have been investigated.