Abstract
Hydroformylation of 1,2-disubstituted alkenes usually occurs at the α position of the directing heteroatom such as oxygen atom and nitrogen atom. By contrast, to achieve hydroformylation on the β position of the heteroatom is a tough task. Herein, we report the asymmetric rhodium-catalyzed hydroformylation of 1,2-disubstituted alkenylsilanes with excellent regioselectivity at the β position (relative to the silicon heteroatom) and enantioselectivity. In a synthetic sense, we achieve the asymmetric hydroformylation on the β position of the oxygen atom indirectly by using the silicon group as a surrogate for the hydroxyl. Density functional theory (DFT) calculations are carried out to examine energetics of the whole reaction path for Rh/YanPhos-catalyzed asymmetric hydroformylation and understand its regioselectivity and enantioselectivity. Our computational study suggests that the silicon group can activate the substrate and is critical for the regioselectivity.
Highlights
Hydroformylation of 1,2-disubstituted alkenes usually occurs at the α position of the directing heteroatom such as oxygen atom and nitrogen atom
We report a rhodium-catalyzed regioselective and stereospecific hydroformylation of 1,2-disubstituted alkenylsilanes: CO can be mainly incorporated at the β position, respectively, and the corresponding βaldehydesilanes are obtained with excellent enantioselectivity (up to 97% enantiomeric excesses)
Our initial studies focused on the asymmetric hydroformylation (AHF) of (Z)-trimethyl(styryl)silane (1a) to give the desired chiral aldehyde product 2a, with the expectations of achieving a highly regioselective and enantioselective transformation
Summary
Hydroformylation of 1,2-disubstituted alkenes usually occurs at the α position of the directing heteroatom such as oxygen atom and nitrogen atom. We attempted to use the silicon group as a surrogate for the hydroxyl (via Fleming-Tamao oxidation35) in a synthetic sense, and the β aldehyde product is more favorable due to the steric hindrance of the silicon group (Fig. 1b)[36,37], which achieves the AHF on the β position of the oxygen atom indirectly. Based on these ideas, a series of 1,2-disubstituted alkenylsilanes were designed and Z-alkenes were chosen instead of E-alkenes because of higher regio- and enantioselectivities and faster rates[7,11]. The products can be useful synthetic platforms based on the various transformations of the aldehyde group and silicon group (transformations of the silicon include Fleming-Tamao oxidation[35], Hiyama coupling[46], Brook and retro-Brook rearrangements[47,48])
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