Density Functional Theory Study on the H₂-Acceptorless Dehydrogenative Boration of Alkenes Catalyzed by a Zirconium Complex

Abstract

For the synthesis of vinyl boronate esters, the direct catalytic H2-acceptorless dehydrogenative boration of alkenes is one of the promising strategies. In this paper, the density functional theory method was employed to investigate the reaction mechanism of dehydrogenative boration and transfer boration of alkenes catalyzed by a zirconium complex (Cp2ZrH2). There are two possible pathways for this reaction: the alkene insertion followed by the dehydrogenative boration (path A) and the alkene insertion after the dehydrogenative boration (path B). The calculated results showed that path A is more favorable than path B, and that the rate-determining step is the C–B coupling step with an energy barrier of 18.7 kcal/mol. The reaction modes of the C–B coupling assisted dehydrogenative boration and the alkene insertion were also discussed. These analyses reveal a novel hydrogen release behavior in dehydrogenative boration and the alkene insertion modes and sequences were proposed to be of importance in the chemoselectivity of this reaction. In addition, the X ligand effect (X = H, Cl) on the catalytic activity of the zirconium complex was explored, indicating that the H ligand could enhance the catalytic activity of the complex for styrene dehydrogenative boration.

Understanding the Mechanism and Selectivity of 1,1-Diborylalkanes from Alkenes Catalyzed by a Zirconium Complex

“…Using HBpin as the reducing agent was also investigated (path B), which finally obtained product 3 and catalyst B3 through the similar σ - bond metathesis mechanism to complete the catalytic cycle. Additionally, the B3 and 3a formation mechanism has been reported by the authors of this study previously, which will not be repeated here …”

“…Finally, the product elimination via TS2, with an energetic span of 21.4 kcal/mol, leads to the production of the end products 1,1-diborylalkanes ( 3 ) and generates the species Cp 2 ZrH 2 ( 1 ). Also, Cp 2 ZrH 2 ( 1 ) can form B3 via dehydrogenative boration to complete the catalytic cycle, which has been reported in our previous work …”

“…19−23 As shown in Figure S1a,b, the reaction mechanism of Cp 2 ZrH 2 -catalyzed dehydrogenative boration of alkene to species 3a and B3 has been reported by the authors of the current study previously. 24 Furthermore, mechanistic studies and detailed density functional theory (DFT) calculations are lacking in the literature about 1,1-diborylalkanes (3) and the zirconium boryl intermediate (B3) as the new boron source in the reaction forming 1,1-diborylalkane. In short, some critical issues in the reaction mechanism of this catalytic system, such as the controlling factors of the catalytic activity and the ratedetermining step, remain unresolved.…”

“…Theoretically, very little has been reported about the reaction mechanism of 1,1-diborylalkanes. The transition metal-catalyzed alkene diboration reaction consists of the following processes: oxidative addition, ligand dissociation, alkene coordination and insertion, isomerization, β-H elimination or reductive elimination, and hydroboration steps. − As shown in Figure S1a,b, the reaction mechanism of Cp 2 ZrH 2 -catalyzed dehydrogenative boration of alkene to species 3a and B3 has been reported by the authors of the current study previously . Furthermore, mechanistic studies and detailed density functional theory (DFT) calculations are lacking in the literature about 1,1-diborylalkanes ( 3 ) and the zirconium boryl intermediate ( B3 ) as the new boron source in the reaction forming 1,1-diborylalkane.…”

“…The reaction starting with the alkene insertion mode, alkene side 1,2-insertion into the Zr−B bond, and formation of the zirconium diboryl alkyl intermediate B6 then obtains the product 1,1-diborylalkanes (3) and Cp 2 ZrH 2 (1) through the σ-bond metathesis of H 2 activation. This mechanism of dehydrogenative boration of Cp 2 ZrH 2(1) to obtain B3 to complete the catalytic cycle has been reported in our previous study 24. Path A is the internal attack of the H 2 molecule on the Zr−C bond of intermediate6.…”