Oxidative cyclopalladation triggers the hydroalkylation of alkynes

This report describes the oxidative cyclopalladation activation of a C≡C bond during the Pd-catalyzed hydroalkylation of alkynes and analyzes potential reaction pathways based on density functional theory calculations. The more favorable pathway in-volves an oxidative cyclopalladation to generate a palladacyclopropene intermediate, which is rarely examined in Pd-catalyzed alkyne transformations. The reaction pathway proposed herein is kinetically favorable relative to the commonly proposed alkyne insertion mode. Furthermore, the Laplacians of the electron density, interaction region indicators, Mayer bond orders, and localized orbital bonding are evaluated to determine the reaction processes and characterize the key intermediates. Theoretical calculations indicate covalent bonding between a Pd(II) center and the two C-atoms in three-membered palladacycle species. Finally, electrostatic potential analysis reveals that the regioselectivity is governed by the charge distribution on the palladacycle moiety during the protonation step.

阴离子协助镍催化碳氟键官能团化反应机理研究

由于碳-氟键的化学惰性,其高效催化转化一直是有机化学中的难题之一.本文运用密度泛函理论(DFT)计算,研究了镍催化碳-氟/碳-氢键交叉偶联反应构建2-芳基噁唑衍生物的机理.计算结果表明,体系中氟负离子能够稳定零价镍金属催化剂,得到氟负离子配位的阴离子镍活性催化物种.在氟负离子协助下碳-氟键对阴离子镍氧化加成得到二价芳基镍物种,是催化循环的速控步骤.在碳氢键切断过程中,Br?nsted碱直接进攻去质子,得到二芳基镍中间体,最终通过还原消除得到交叉偶联产物.在这一过程中,阴离子镍活性催化物种的稳定性明显高于通常推测的中性镍催化物种,这一模型为理解惰性碳-氟键活化反应机理提供了新的思路.

Theoretical prediction on the reactivity of the Co-mediated intramolecular Pauson-Khand reaction for constructing bicyclo-skeletons in natural products

The Co_2(CO)_8-mediated intramolecular Pauson-Khand reaction is an efficient approach for constructing polycyclic skeletons. Recently, some of us reported a series of this type reactions involving stericallyhindered enynes for synthesizing natural products with reasonable reaction rates and yields. However,the reason for the high reactivity of the reaction remains unclear. We employed density functional theory calculations to clarify the mechanism and reactivity for this reaction. In contrast with chain olefin reactants, CO insertion is considered to be the rate-determining step for the overall Pauson-Khand reaction of cyclooctene derivatives. The reduced activation free energy for the alkene insertion step is attributed to: i) the electron-withdrawing group in close proximity to the C—C triple bond enhancing the reactivity of the alkyne moiety; ii) lower steric hindrance during alkene insertion when using the cyclooctene derivative. The effect of the substituent on the Co_2(CO)_8-mediated intramolecular PausonKhand reaction was then investigated. Internal alkenes exhibit lower reactivity than terminal alkenes because of the steric hindrance introduced by the substituted group. The cis internal alkene exhibits higher reactivity than the trans internal alkene. An ester group in close proximity to the C—C triple bond significantly enhances the reactivity.

Is the metal involved or not? A computational study of Cu(I)-catalyzed [4+1] annulation of vinyl indole and carbene precursor

The Cu(I)-catalyzed [4 + 1] annulation of vinyl indoles and a carbene precursor is a powerful method for constructing cyclopentaindole derivatives. Density functional theory(DFT) calculations were used to elucidate the mechanism and regioselectivity of this reaction. After Cu-assisted indole C3-alkylation, direct1,5-annulation was favored over the Cu-assisted annulation pathway. Furthermore, the regioselectivity for1,5-annulation was attributed to the generated five-membered-ring product being more stable than the three-membered-ring product from 1,3-annulation, which was the kinetically favored pathway.

Geometry, stability and aromaticity of β-diketiminate-coordinated alkaline-earth compounds

Alkaline-earth(Ae) metals have attracted a wealth of interdependent research from synthetic chemists.In Ae-catalyzed organometallic reactions,β-diketiminate is a typical ligand used to stabilize Ae catalysts by forming six-membered rings comprising Ae metals.Herein,studies focusing on the configuration of β-diketiminate-coordinated Ae compounds observed that the C-C and C-N bonds are homogeneous and unchanged.Furthermore,energetic studies observed that the formation of the Ae-incorporated sixmembered rings results in enhanced stability of>20 kcal/mol.The nucleus-independent chemical shifts,anisotropy of the induced current density,and molecular orbital analyses demonstrated the nonaromaticity of the β-diketiminate-coordinated Ae compounds.The improved stability of these compounds can be explained by the delocalization of the π electrons derived from the β-diketiminate moiety.

Theoretical insight into phosphoric acid-catalyzed asymmetric conjugate addition of indolizines to a,β-unsaturated ketones

Phosphoric acid catalysis is a powerful tool for the construction of new CààC bonds because of its unique LUMO-lowering activity. Theoretical calculations were employed to investigate phosphoric acidcatalyzed asymmetric conjugate addition of indolizines to a,b-unsaturated ketones. The calculation results showed that this transformation proceeds via a reaction pathway involving nucleophilic addition,deprotonation–aromatization, and tautomerization. The computational results showed that deprotonation–aromatization is the rate-determining step and the enantioselectivity-determining step.The S-configured product was preferentially generated via a pathway starting from the s-cis conjugated ketone, whereas the s-trans isomer led to a side product with the R configuration. Non-covalent interaction analysis showed that the enantioselectivity is mainly caused by bond-rotation strain in the transition states of the deprotonation–aromatization step.