Lewis Acid-Catalyzed Carbonyl-Ene Reaction: Interplay between Aromaticity, Synchronicity, and Pauli Repulsion

Exploring CO2 activation mechanisms with triphenylphosphine derivatives: insights from energy decomposition and deformation density analyses

This study focuses on the reaction mechanisms involving triphenylphosphine (PPh3) derivatives, benzyne, and CO2, giving mechanistic insights into two competing pathways: Path a, which involves direct C-P bond formation, and Path b, which progresses via a [2 + 2] cycloaddition. Comprehensive computational analysis by energy decomposition analysis (EDA) and deformation density insights was employed to elucidate the electronic and steric factors influencing the reactivity and selectivity of PPh3 derivatives. The results reveal that Path b is energetically and kinetically favored. In Path a, substantial repulsive interactions (ΔE rep), especially for electron-withdrawing substituents, hinder C-P bond formation, making this pathway unfavorable, while Path b benefits from compensatory effects between interaction energies, with electron-releasing para-substituents, such as NHMe and OMe, increasing stabilization by enhancing ΔE orb contributions. Substituents in meta positions show greater distortion energies (ΔE dist), which limit their stabilizing effects compared to para-substituents. The deformation density analysis of transition states (TS1(b) and TS2(b)) emphasizes the crucial role of Pauli deformation (Δρ Pauli) and orbital deformation (Δρ Orb) in modulating stability. Para-substituents exhibit stronger electronic effects, reducing ΔE int more effectively than meta-substituents, which increase ΔE dist. This positional dependence underscores the importance of substituent design in optimizing reactivity.

Influence of the Nature of Group 15 Element on [AuI]-C≡E/Azide 1,3-Dipolar Cycloaddition Reaction

The impact of the nature of the Group 15 element on both the bonding situation and the reactivity of gold(I)-C ≡ E (E = N to Bi) complexes has been studied quantum chemically within the density theory functional framework. For this purpose, the 1,3-dipolar cycloaddition reaction involving tBuN3 as dipole has been selected and its main features, including the regioselectivity of the transformation and the in-plane aromaticity of the corresponding transition structures, have been investigated. It is found that the reactivity of the complexes is increased as one moves down Group 15 (N ≪ P < As < Sb < Bi). This reactivity trend has been rationalized by using the combined activation strain model and energy decomposition analysis methods, which indicate that the process is mainly dominated by the strain energy required by the reactants to reach the corresponding transition state geometries.

Unique catalytic role of intermolecular electric fields that emanate from Lewis acids in a ring closing carbonyl olefin metathesis reaction

Electric field (EF) catalysis has evolved as an effective tool for controlling reactivity and selectivity of reactions. While EF catalysis brings precise control over reactivity, it also challenges the concept's practical realization due to the difficulties in juxtaposing reactants with directional EF. The present density functional theory (DFT) studies demonstrate the catalytic role of the inherent intermolecular EFs that originate from Lewis acids (LA) during a ring-closing carbonyl-olefin metathesis (RCCOM) reaction. The specificity of LA coordination to reactants generates specifically oriented intermolecular EF components along the reaction axis which is defined parallel to the direction of flow of electrons wherein the influence of the EF would be at maximum. By examining the thermal [2+2] cycloaddition and carbonyl-ene reaction steps in a RCCOM reaction as model systems, the results revealed the pivotal role of intermolecular EF in mixing some of the dormant ionic structures into normal covalent structures and facilitating a partial rotation of the nonbonding orbitals at the carbonyl oxygen to enhance an ionic pseudo-pericyclic pathway. The unique role of intermolecular EF is further verified by modelling the pristine reaction, in the absence of LAs, under oriented external EFs. The conspicuous intermolecular EF component adds to other modes of catalysis, such as conventional Lewis acidity, to result in the gross catalytic effect. The findings offer insights into the practical realization of EF catalysis by harnessing the intermolecular EFs and point out the need to include intermolecular EF as an inevitable factor for a holistic explanation of any catalytic mechanism.

Facile and Highly Selective Deprotection of Aryl Propionates/Acetates Using a Supported Lewis Acid Catalyst (20% InCl3/MCM-41)

The selective deprotection of substituted aryl esters like acetates and propionates in the presence of different electron-donating and -withdrawing functional groups to the corresponding phenols in good yields was reported using the Lewis acid supported solid acid catalyst 20% InCl3/MCM-41 prepared by the wet impregnation method. The textural and microscopic properties of the catalyst were studied, which revealed a high degree of dispersion of InCl3 over MCM-41, promising quantification of Lewis acidity, and well-ordered honeycomb structure. The methodology was further explored for the selective deprotection of acetates and propionates in the presence of substituted amides that remain unaltered. Reusability studies revealed the robust nature of the catalyst without losing the catalytic activity for up to six recycles corroborated with hot leaching test studies monitored by ICP-AES analysis, which was further authenticated with XPS studies of the catalyst before and after the reaction.

Domino dehydration/intermolecular (enantioselective) ketone-ene reactions catalysed by a simple solid in batch and in flow

The intermolecular carbonyl-ene reaction of ketones is still considered a challenge in organic chemistry, particularly with reusable solid catalysts, and implemented in a domino reaction. Herein, we show that the extremely cheap and non-toxic solid salt MgCl2 catalyzes the reaction of trifluoromethyl pyruvates not only during the conventional carbonyl-ene reaction with various aromatic and alkyl alkenes (in very high yields, up to >99%) but also in a domino reaction with the corresponding alcohols (precursors to the alkenes) in similar good yields. The solid can be reused in both cases without any erosion of the catalytic activity and can be employed in an in-flow process to maximize the reaction throughput. Besides, the reaction can be performed under solventless reaction conditions. Addition of a catalytic amount of chiral binaphthyl hydrogen phosphate allows carrying out the reaction with a reasonable enantiomeric excess (up to >70%) and in flow, in a rare example of enantioselective solid-catalyzed domino carbonyl-ene reaction using a cheap, simple, readily available and physically mixed catalytic solid. The MgCl2-catalytic system is also active in the industrially relevant citronellal-to-isopulegol carbonyl-ene reaction. These results pave the way to design sustainable domino carbonyl-ene reactions with extremely cheap solid catalysts.

Origin of the Felkin-Anh(-Eisenstein) model: a quantitative rationalization of a seminal concept

Quantum chemical calculations were carried out to quantitatively understand the origin of the Felkin-Anh(-Eisenstein) model, widely used to rationalize the π-facial stereoselectivity in the nucleophilic addition reaction to carbonyl groups directly attached to a stereogenic center. To this end, the possible approaches of cyanide to both (S)-2-phenylpropanal and (S)-3-phenylbutan-2-one have been explored in detail. With the help of the activation strain model of reactivity and the energy decomposition analysis method, it is found that the preference for the Felkin-Anh addition is mainly dictated by steric factors which manifest in a less destabilizing strain-energy rather than, as traditionally considered, in a lower Pauli repulsion. In addition, other factors such as the more favorable electrostatic interactions also contribute to the preferred approach of the nucleophile. Our work, therefore, provides a different, more complete rationalization, based on quantitative analyses, of the origin of this seminal and highly useful concept in organic chemistry.

Metal Influence on Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Aromaticity and Activation Strain

The factors governing 1,3-dipolar cycloaddition reactions involving C≡P-containing compounds are computationally explored in detail using quantum chemical tools. To this end, the parent process involving tBuN3 and tBuCP is analyzed and compared to the analogous reaction involving organometallic cyaphide complexes (metal=Au, Pt, Ge, Mg), in order to understand the role of the metal fragment in such transformations. It is found that while the metal fragment does not significantly influence the aromaticity of the corresponding concerted transition states or the regioselectivity of the transformation, it may modify the reactivity of the cyaphide complexes (i. e. Ge and Mg cyaphide complexes are comparatively more reactive). The computed reactivity trends and the factors behind the regioselectivity of the cycloaddition reaction are quantitatively analyzed with the help of the activation strain model in combination with the energy decomposition analysis method.

Mechanistic Insights into the DABCO-Catalyzed Cloke-Wilson Rearrangement: A DFT Perspective

The mechanism and selectivity patterns of the DABCO-catalyzed Cloke-Wilson rearrangement were computationally studied in detail using density functional theory calculations. Our computations suggest that the process occurs stepwise involving the initial ring opening of the cyclopropane promoted by a DABCO molecule followed by a ring-closure reaction of the readily formed zwitterionic intermediate. The regioselectivity of the initial nucleophilic ring-opening step strongly depends on the nature of the substituent attached to the cyclopropane moiety. The physical factors governing the preference for the more sterically hindered C2 (tertiary) position have been quantitatively analyzed by applying the combined activation strain model-energy decomposition analysis method. In addition, our calculations revealed a new mechanism for the analogous transformation involving vinylcyclopropanes consisting of an initial SN2' ring-opening process followed by a 5-exo-trig cyclization step, which proceeds without facial selectivity.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.