DFT Studies on the Mechanisms of the Platinum-Catalyzed Diboration of Acyclic α,β-Unsaturated Carbonyl Compounds

Detailed mechanisms of the diboration of the acyclic alpha,beta-unsaturated carbonyl compounds acrolein, methyl acrylate, and dimethyl fumarate (DMFU) catalyzed by Pt(0) complexes were studied with the aid of density functional theory by calculating the relevant intermediates and transition states. For acrolein and methyl acrylate, the results show that the catalyzed diboration occurs via oxidative addition of the diboron reagent to the Pt(0) complex having diimine and acrolein (or methyl acrylate) as the ligands, 1,4-conjugate addition of a Pt B bond to acrolein/methyl acrylate to give an O-bound boron enolate intermediate containing a Pt-C-C=C-O-B linkage, and subsequent acrolein/methyl acrylate coordination to the Pt(II) center followed by reductive elimination to obtain the 1,4-diboration product of acrolein/methyl acrylate, i.e., the beta-borylsubstituted O-bound boron enolate. For acrolein, the 1,4-diboration product is the final product, whereas for methyl acrylate, the 1,4-diboration product then isomerizes to the experimentally observed and thermodynamically favored 3,4-addition product, i.e., the beta-boryl-substituted C-bound boron enolate, via a 1,3-shift of the O-bonded boryl group. Slightly different from what we have seen in the catalyzed diboration of acrolein/methyl acrylate, the catalyzed diboration of DMFU takes place through oxidative addition of the diboron reagent to the Pt(0) complex having DMFU and diimine as the ligands, 1,6-conjugate addition of both of the two Pt B bonds to the coordinated DMFU ligand to give a 1,6-addition intermediate containing BegO-C(OMe)=C-C=C(OMe)-OBeg (eg = ethyleneglycolato = -OCH2CH2O) as a ligand, and then isomerization via two consecutive 1,3-shifts of the two O-bonded boryl groups to produce the experimentally observed 3,4-diborated diastereomeric products.