Impact of Ligand and Silane on the Regioselectivity in Catalytic Aldehyde–Alkyne Reductive Couplings: A Theoretical Study
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文摘
The reaction mechanisms of the (NHC)Ni(0)-catalyzed aldehyde–alkyne reductive couplings with silanes as reducing agent have been theoretically investigated with the aid of DFT calculations. The impacts of N-heterocyclic carbene (NHC) ligands and silanes on the reversal of regioselectivity and the rate-limiting step alteration were rationalized. It is found that the steric effects play a dominant role. The reversal of the regioselectivity is found to be related to the switching of the steric effect, from the aldehyde phenyl hindrance with the adjacent alkyne substituent to the NHC ligand hindrance with the adjacent alkyne substituent, when the NHC ligand employed is changed from small to large. The rate-limiting step alteration caused by using bulkier silanes is due to the generated strong steric effect, which makes the σ-bond metathesis transition state relatively high in enthalpic energy, thus with the entropy penalty making the metathesis step rate-limiting instead of the oxidative cyclization step.