Search

Silaspiranes have attracted particular attention due to their chiral spiro‐silicon center, which serves as an ideal carbon isostere and can endow spiro‐analogs with distinct properties. Distinct from previously reported cyclization or cycloaddition strategies to form 5/5‐silaspiranes, we report herein the asymmetric dual ring expansion of spirosilabicyclobutanes with alkynes to synthesize axially chiral spirosilabicyclohexenes bearing a novel 6/6‐silaspirane framework. DFT (density functional theory) calculations provide the deep insight into the origin of the high enantioselectivity controlled by the sterically demanding binaphthyl phosphoramidite ligand. Preliminary studies of chiroptical properties indicate that one of the spirosilabicyclohexene analogs exhibit fluorescence emission, Cotton effects and CPL (circularly polarized luminescence) activity. [ABSTRACT FROM AUTHOR]

A theoretical study of the mechanism and regioselectivity of 1,3‐dipolar cycloaddition of nitrile oxides to 3‐methylene indolone has been performed by density functional theory at the M06‐2X/6‐311++G(d,p) level of theory. The results indicate that the concerted pathway is more favorable than the stepwise pathway. Investigation of the regioselectivity of this reaction shows that the main product is predicted to be 3,5‐substituted isoxazoline derivatives, which is consistent with experimental observations. The substituent effect of nitrile oxides on the reactivity and regioselectivity were also investigated. The opposite regioselectivity was observed for nitrile oxides with electron‐donating groups (methyl and phenyl) compared with electron‐withdrawing groups (CN and CO2Me). Distortion–interaction energy analysis shows that the reactivity of 1,3‐dipolar cycloaddition of nitrile oxides to 3‐methylene indolone is controlled by both the distortion and interaction energies. Natural population analysis indicates that the carbon atom of the 1,3‐dipole prefers to bond to the terminal carbon atom of the exocyclic double bond of 3‐methylene indolone, forming the 3,5‐substituted isoxazoline product. Frontier molecular orbital analysis shows that the regioselectivity is controlled by the orbital interaction between the lowest unoccupied molecular orbital of the nitrile oxide and the highest occupied molecular orbital of 3‐methylene indolone. [ABSTRACT FROM AUTHOR]

A mixture of pathways including the use of catalyst was proposed for a dirhodium-mediated triazole activation–cycloaddition. This proposal was supported by the results of density functional theory (DFT) calculations, and was indicated to involve a non-catalyzed cleavage of the triazole N–N bond followed by a dirhodium-assisted denitrogenation. Electron-deficient carboxylates were found to be favourable for the rate-determining denitrogenation step. [ABSTRACT FROM AUTHOR]

Density functional theory calculations were performed to reveal the mechanisms of Pd‐catalyzed cascade carbocyclization‐borylation and arylation reactions. The computational results indicate that the reactions start with allylic C−H cleavage through concerted metalation‐deprotonation and an intramolecular exo‐type allene insertion to form a six‐membered carbocycle intermediate. The regioselectivity of insertion could be explained by frontier molecular orbital theory and natural population analysis calculation. In the absence of extra nucleophiles, η1/η3‐isomerization followed by acetate‐assisted deprotonation could yield polyene product. When nucleophile was added to the reaction system, transmetalation and subsequent reductive elimination could give the exo‐substituted triene as major product. Meanwhile, η1/η3‐isomerization, transmetalation, and reductive elimination could afford the endo‐isomer as side product. The regioselectivity of further functionalization is controlled by the competition of transmetalation and η1/η3‐isomerization. The computational results show that both exo‐ and endo‐boronation product could be observed when bis(pinacolato)diboron is added as nucleophile. However, only exo‐phenylation product is observed when phenylboronic acid is used as nucleophile because of the high free‐energy barrier for reductive elimination from aryl η3‐allylic palladium. Show me the way: DFT calculations were employed to study the mechanism and origin of regioselectivity of cascade carbocyclization‐functionalization reactions. [ABSTRACT FROM AUTHOR]

The first example of iron-catalyzed decarboxylative (4+1) cycloaddition reactions is described in this publication. By using this method, a wide range of functionalized indoline products were prepared from easily available vinyl benzoxazinanones and sulfur ylides in high yields and selectivities. A possible reaction pathway involving an allylic iron intermediate is discussed based on a series of control experiments and density-functional theory calculations. [ABSTRACT FROM AUTHOR]

Higher-order cycloadditions are a powerful strategy for the construction of polycycles in one step. However, an efficient and concise version for the induction of asymmetry is lacking. N-heterocyclic carbenes are widely used organocatalysts for asymmetric synthesis and could be an ideal choice for enantioselective higher-order cycloadditions. Here, we report an enantioselective [10 + 2] annulation between catalytically formed aza-benzofulvene intermediates and trifluoromethyl ketone derivatives. This protocol exhibits a wide scope, high yields, and good ee values, reflecting a robust and efficient higher-order cycloaddition. Density functional theory calculations provide an accurate prediction of the reaction enantioselectivity, and in-depth insight to the origins of stereocontrol. High-order cycloaddition reactions are useful for the construction of polycycles in a single step, but versions that induce asymmetry are limited. Here the authors report the construction of asymmetric polycycles via N-heterocyclic carbene-catalyzed hetero-[10 + 2] cycloaddition of indole-2-carbaldehydes with trifluoromethyl ketone derivatives. [ABSTRACT FROM AUTHOR]