Abstract
Density functional theory (DFT) calculations have been performed to investigate the mechanism of alkaline-earth-metal-catalyzed hydroboration of pyridines with borane. In this reaction, the active catalytic species is considered to be an alkaline earth metal hydride complex when the corresponding alkaline earth metal is used as the catalyst. The theoretical results reveal that initiation of the catalytic cycle is hydride transfer to generate a magnesium hydride complex when β-diimine alkylmagnesium is used as a pre-catalyst. The magnesium hydride complex can undergo coordination of the pyridine reactant followed by hydride transfer to form a dearomatized magnesium pyridine intermediate. Coordination of borane and hydride transfer from borohydride to magnesium then give the hydroboration product and regenerate the active magnesium hydride catalyst. The rate-determining step of the catalytic cycle is hydride transfer to pyridine with a free energy barrier of 29.7 kcal/mol. Other alkaline earth metal complexes, including calcium and strontium complexes, were also considered. The DFT calculations show that the corresponding activation free energies for the rate-determining step of this reaction with calcium and strontium catalysts are much lower than with the magnesium catalyst. Therefore, calcium and strontium complexes can be used as the catalyst for the reaction, which could allow mild reaction conditions.
Highlights
As an important derivative of pyridine, dihydropyridine is an important raw material to synthesize natural products
Density functional theory (DFT) calculations were performed for hydroboration of pyridines catalyzed by magnesium (Scheme 2)
The mechanism of pyridine hydroboration catalyzed by a series of Ae metals has been systematically investigated by DFT calculations
Summary
As an important derivative of pyridine, dihydropyridine is an important raw material to synthesize natural products. In 2011, Nikonov’s group (Gutsulyak et al, 2011) realized hydrosilylation of pyridine with ruthenium as the catalyst at room temperature and obtained a mixture of 1,2- and 1,4-borohydropyridine Application of these reactions in synthetic chemistry is greatly limited by the expensive transition metal catalyst (Yaroshevsky, 2006; Dobereiner and Crabtree, 2010; Osakada, 2011; Huang and Xia, 2015; Li et al, 2015; Qi et al, 2016, 2018; Yang et al, 2016; Yu et al, 2016; Xing et al, 2017; Liu et al, 2018; Luo et al, 2018; Zhu et al, 2018)
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