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The nature of the C−I⋅⋅⋅π halogen bonding and its mode of activation in organocatalysis have been quantitatively explored in detail by means of Density Functional Theory (DFT) calculations. To this end, the uncatalyzed homo‐[4+2]‐cycloaddition reaction involving 2‐alkenylindoles is compared to the analogous process mediated by iodine‐substituted azolium salts. It is found that the catalysts establish relatively strong C−I⋅⋅⋅π noncovalent interactions with the indole reactant, which are characterized by a high degree of covalency. This interaction results in a significant acceleration of the cycloaddition by lowering the activation barrier up to 6 kcal/mol with respect to the uncatalyzed reaction. The calculations predict that this barrier can be further reduced by increasing the electrophilicity of the catalyst. Our quantitative analyses reveal that the origin of the catalysis is found mainly in a significant reduction of the steric (Pauli) repulsion between the diene and dienophile, therefore confirming the generality of the recently introduced Pauli‐repulsion lowering concept. [ABSTRACT FROM AUTHOR]

The catalytic effect of various weakly interacting Lewis acids (LAs) across the periodic table, based on hydrogen (Group 1), pnictogen (Group 15), chalcogen (Group 16), and halogen (Group 17) bonds, on the Diels‐Alder cycloaddition reaction between 1,3‐butadiene and methyl acrylate was studied quantum chemically by using relativistic density functional theory. Weakly interacting LAs accelerate the Diels‐Alder reaction by lowering the reaction barrier up to 3 kcal mol−1 compared to the uncatalyzed reaction. The reaction barriers systematically increase from halogen

The selectivity and rate enhancement of bifunctional hydrogen bond donor‐catalyzed Diels–Alder reactions between cyclopentadiene and acrolein were quantum chemically studied using density functional theory in combination with coupled‐cluster theory. (Thio)ureas render the studied Diels–Alder cycloaddition reactions exo selective and induce a significant acceleration of this process by lowering the reaction barrier by up to 7 kcal mol−1. Our activation strain and Kohn–Sham molecular orbital analyses uncover that these organocatalysts enhance the Diels–Alder reactivity by reducing the Pauli repulsion between the closed‐shell filled π‐orbitals of the diene and dienophile, by polarizing the π‐orbitals away from the reactive center and not by making the orbital interactions between the reactants stronger. In addition, we establish that the unprecedented exo selectivity of the hydrogen bond donor‐catalyzed Diels–Alder reactions is directly related to the larger degree of asynchronicity along this reaction pathway, which is manifested in a relief of destabilizing activation strain and Pauli repulsion. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations of the alkaline‐earth oxides, imides and dihydrides of the alkaline‐earth atoms (Ae=Be, Mg, Ca, Sr, Ba) and the calcium cluster Ca6H9[N(SiMe3)2]3(pmdta)3 (pmdta=N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine) have been carried out by using density functional theory. Analysis of the electronic structures by charge and energy partitioning methods suggests that the valence orbitals of the lighter atoms Be and Mg are the (n)s and (n)p orbitals. In contrast, the valence orbitals of the heavier atoms Ca, Sr and Ba comprise the (n)s and (n−1)d orbitals. The alkaline‐earth metals Be and Mg build covalent bonds like typical main‐group elements, whereas Ca, Sr and Ba covalently bind like transition metals. The results not only shed new light on the covalent bonds of the heavier alkaline‐earth metals, but are also very important for understanding and designing experimental studies. [ABSTRACT FROM AUTHOR]

The factors controlling the oxidative addition of C−C and C−H bonds in arenes mediated by AlI have been computationally explored by means of Density Functional Theory calculations. To this end, we compared the processes involving benzene, naphthalene and anthracene which are promoted by a recently prepared anionic AlI‐carbenoid. It is found that this species exhibits a strong tendency to oxidatively activate C−H bonds over C−C bonds, with the notable exception of benzene, where the C−C bond activation is feasible but only under kinetic control reaction conditions. State‐of‐the‐art computational methods based on the combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis have been used to rationalize the competition between both bond activation reactions as well as to quantitatively analyze in detail the ultimate factors controlling these transformations. [ABSTRACT FROM AUTHOR]

The Lewis acid(LA)‐catalyzed Diels–Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled‐cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I2

A synthetic method to prepare tetrahydroquinoline‐4‐carboxylic acid esters has been developed through the transition‐metal‐catalyzed intramolecular aromatic C−H functionalization of α‐diazoesters. Both [{Pd(IMes)(NQ)}2] (IMes=1,3‐dimesitylimidazol‐2‐ylidene, NQ=1,4‐naphthoquinone) and the first‐generation Grubbs catalyst proved effective for this purpose. The ruthenium catalyst was found to be the most versatile, although in a few cases the palladium complex afforded better yields or selectivities. According to DFT calculations, Pd0‐ and RuII‐catalyzed sp2‐CAr−H functionalization proceeds through different reaction mechanisms. Thus, the Pd0‐catalyzed reaction involves a Pd‐mediated 1,6‐H migration from the sp2‐CAr−H bond to the carbene carbon atom, followed by a reductive elimination process. In contrast, electrophilic addition of the ruthenacarbene intermediate to the aromatic ring and subsequent 1,2‐proton migration are operative in the Grubbs catalyst promoted reaction. [ABSTRACT FROM AUTHOR]

The influence of the replacement of C=C bonds by isoelectronic B−N moieties on the reactivity of π‐curved polycyclic aromatic hydrocarbons has been computationally explored by means of density functional theory calculations. To this end, we selected the Diels–Alder cycloaddition reactions of the parent corannulene and its BN‐doped counterparts with either cyclopentadiene or maleic anhydride. In addition, the analogous reactions involving larger buckybowls, such as BN‐hemifullerene, BN‐circumtrindene, and BN‐fullerene, have been also considered. It has been found that whereas corannulene behaves as a dienophile, its BN counterpart better acts as a diene. In contrast, the larger BN‐curved systems cannot be used as dienes in Diels–Alder reactions, but undergo facile (i.e. low barrier) cycloaddition reactions with cyclopentadiene. The observed trends in reactivity, which cannot be directly explained by using typical frontier molecular orbital arguments, are quantitatively described in detail by means of state‐of‐the‐art computational methods, namely the activation strain model of reactivity combined with the energy decomposition analysis method. The results of our calculations highlight the crucial role of the curvature of the system on the reactivity and its influence on the strength of the orbital interactions between the deformed reactants during their transformations. [ABSTRACT FROM AUTHOR]

We have quantum chemically analyzed the catalytic effect of dihalogen molecules (X2=F2, Cl2, Br2, and I2) on the aza‐Michael addition of pyrrolidine and methyl acrylate using relativistic density functional theory and coupled‐cluster theory. Our state‐of‐the‐art computations reveal that activation barriers systematically decrease as one goes to heavier dihalogens, from 9.4 kcal mol−1 for F2 to 5.7 kcal mol−1 for I2. Activation strain and bonding analyses identify an unexpected physical factor that controls the computed reactivity trends, namely, Pauli repulsion between the nucleophile and Michael acceptor. Thus, dihalogens do not accelerate Michael additions by the commonly accepted mechanism of an enhanced donor–acceptor [HOMO(nucleophile)–LUMO(Michael acceptor)] interaction, but instead through a diminished Pauli repulsion between the lone‐pair of the nucleophile and the Michael acceptor's π‐electron system. [ABSTRACT FROM AUTHOR]

We report calculated activation barriers for the Diels–Alder reaction between substituted butadienes and ethenes with density functional theory at the M06–2X level using a basis set with TZ2P quality. The interactions between the dienes and dienophiles at the transition state are analyzed with the EDA‐NOCV method. There is a modest correlation between the activation barriers and the strength of the frontier‐orbital interactions. The calculations suggest that the effect of the substituents on the reactivity of the Diels–Alder reactions depends on both, the deformation ("strain") energies of the reactants and the intrinsic orbital interactions at the transition states. Surprisingly, the frontier‐orbital interactions exhibit a better correlation with the reaction barriers than the total orbital interactions. There is only a modest correlation between the strength of the frontier orbital interactions and the activation barrier for the Diels–Alder reaction between substituted butadienes and ethenes. [ABSTRACT FROM AUTHOR]

The influence of the nature of the acid/base pairs on the reactivity of geminal frustrated Lewis pairs (FLPs) (Me2E‐CH2‐E′Ph2) has been computationally explored within the density functional theory framework. To this end, the dihydrogen‐activation reaction, one of the most representative processes in the chemistry of FLPs, has been selected. It is found that the activation barrier of this transformation as well as the geometry of the corresponding transition states strongly depend on the nature of the E/E′ atoms (E=Group 15 element, E′=Group 13 element) in the sense that lower barriers are associated with earlier transition states. Our calculations identify the geminal N/Al FLP as the most active system for the activation of dihydrogen. Moreover, the barrier height can be further reduced by replacing the phenyl group attached to the acidic atom by C6F5 or 3,5‐(CF3)2C6H3 (Fxyl) groups. The physical factors controlling the computed reactivity trends are quantitatively described in detail by means of the activation strain model of reactivity combined with the energy decomposition analysis method. Activating H2: The influence of the nature of the acid/base pairs on the reactivity of geminal frustrated Lewis pairs (FLPs) (Me2E‐CH2‐E′Ph2; see scheme) is analyzed in detail by means of computational methods. The geminal N/Al FLP is identified as the most active system for the dihydrogen activation reaction. [ABSTRACT FROM AUTHOR]

The intramolecular carbene C–H insertion of α‐diazo‐α‐(methoxycarbonyl)acetamides leading to β‐lactams is effectively catalyzed by palladium complexes. It is found that although Pd0 catalysts typically produce mixtures of β‐lactams together with Buchner‐type reaction products, the use of PdII catalysts results in highly chemoselective transformations. According to DFT calculations, this insertion reaction occurs stepwise and involves an unprecedented PdII‐promoted Mannich‐type reaction through a metallacarbene‐induced zwitterionic intermediate. [ABSTRACT FROM AUTHOR]

Abstract: The hydrogenation reaction of multiple bonds that is mediated by geminal aminoborane‐based frustrated Lewis pairs (FLPs) has been explored by means of density functional theory calculations. It was found that the release of the activated dihydrogen occurred in a concerted, yet highly asynchronous, manner. The physical factors that control the transformation were quantitatively described in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. This approach suggested a cooperative double hydrogen‐transfer mechanism, which involves the initial migration of the protic (N)H followed by the nucleophilic attack of the (B)H hydride to the carbon atom of the multiple bond. The influence of both the substituents directly attached to the boron atom of the initial FLP and the nature of the multiple bond on the transformation was also investigated. [ABSTRACT FROM AUTHOR]

The Activation Strain Model of reactivity nowadays constitutes a powerful tool to aid quantitative understanding of chemical reactions, and also their design. This approach, combined with the Energy Decomposition Analysis method, has been really helpful for our current understanding of different fundamental transformations in chemistry. This Microreview illustrates the usefulness of this methodology in providing more insight into the chemistry of fullerenes. To this end, representative recent applications, ranging from the regioselectivity in Diels-Alder cycloaddition to the reactivity of endohedral fullerenes, are presented. [ABSTRACT FROM AUTHOR]

The parent thioketene S-oxide H2CCSO has been generated in the gas phase through flash vacuum pyrolysis (ca. 1000 K) of vinyl sulfoxide H2CC(Br)−S(O)CF3 via the intermediacy of a novel vinyl sulfinyl radical H2C=C(Br)−SO ( syn and anti conformers). Upon irradiation at 266 nm, H2CCSO decomposes into HCCH/SO and H2CS/CO in cryogenic Ar matrix. Whereas, visible-light irradiations result in syn↔ anti conformational interconversion in H2C=C(Br)−SO. The molecular structures of H2CCSO and isomers are computationally studied at the CCSD(T)-F12/VTZ-F12 level of theory, and the bonding properties of H2CCSO are analyzed with the EDA-NOCV method at the M06-2X/TZ2P level. [ABSTRACT FROM AUTHOR]

The influence of the encapsulation of an ion inside the C60 fullerene cage on its exohedral reactivity was explored by means of DFT calculations. To this end, the Diels-Alder reaction between 1,3-cyclohexadiene and M@C60 (M=Li+, Na+, K+, Be2+, Mg2+, Al3+, and Cl−) was studied and compared to the analogous process involving the parent C60 fullerene. A significant enhancement of the Diels-Alder reactivity is found for systems having an endohedral cation, whereas a clear decrease in reactivity is observed when an anion is encapsulated in the C60 cage. The origins of this reactivity trend were quantitatively analyzed in detail by using the activation strain model of reactivity in combination with energy decomposition analysis. [ABSTRACT FROM AUTHOR]

The influence of the nature of the transition-metal fragment on the Diels-Alder reactivity of metallaanthracenes has been explored computationally within the Density Functional Theory framework. It is found that the cycloaddition reactions with maleic anhydride become kinetically less favored for those processes involving metallaanthracenes compared with the analogous reaction involving the parent anthracene. The origins of this reduction in the Diels-Alder reactivity have been quantitatively analyzed in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. In general, the transition-metal fragment makes the interaction energy between the reactants significantly lower, particularly at the transition state region, which is translated into a higher activation barrier. In addition, the influence of the aromaticity strength of the metallabenzene present in the considered metallaanthracenes on the barriers of the cycloaddition reactions has also been assessed. [ABSTRACT FROM AUTHOR]

Sulfonates, sulfonamides, and phosphonates have proven useful nucleophiles for palladium-catalyzed intramolecular α-arylation reactions leading to tetrahydroisoquinolines. Although the sulfonate α-arylation reaction can be successfully combined in a domino process with a broad range of Michael acceptors, only vinyl sulfones can be used in Michael additions when starting from sulfonamides. No domino process was developed with the phosphonate derivative. DFT calculations were carried out to gain more insights into the experimental differences observed in the reactions involving these substrates. [ABSTRACT FROM AUTHOR]

H2 activation mediated by geminal aminoborane-based frustrated Lewis pairs (FLPs; R2N-CH2-BR′2) has been computationally explored within the density functional theory framework. It is found that the activation barrier of this process as well as the geometry of the corresponding transition states strongly depend on the nature of the substituents directly attached either to the acidic or the basic centers of the FLPs. The physical factors controlling the whole activation path are quantitatively described in detail by means of the activation strain model of reactivity combined with the energy decomposition analysis method. This methodology suggests a highly orbital-controlled mechanism where the degree of charge transfer cooperativity between the most important donor-acceptor orbital interactions, namely LP(N)→σ*(H2) and σ(H2)→pπ(B), along the reaction coordinate constitutes a suitable indicator of the reaction barrier. [ABSTRACT FROM AUTHOR]

The oxidative addition reaction of X−H σ-bonds to Group 13 (E=Al, Ga, In) containing compounds has been computationally explored within the density functional theory framework. These reactions, which proceed concertedly involving the EI→EIII oxidation, are exothermic and associated with relatively low activation barriers. In addition, the following trends in reactivity are found: (i) the activation barriers are lower for the X−H bonds involving the heavier element in the same group (Δ E≠: C>Si; N>P; O>S), (ii) the process becomes kinetically more favorable in going from left to right in the same period (Δ E≠: C>N>O; Si≈P>S), and (iii) the activation barrier systematically increases when heavier Group 13 elements are involved in the transformation (Δ E≠: Al

Controlled preparation of tri- and tetrasubstituted furans, as well as carbazoles has been achieved through chemo- and regioselective metal-catalyzed cyclization reactions of cumulenic alcohols. The gold- and palladium-catalyzed cycloisomerization reactions of cumulenols, including indole-tethered 2,3,4-trien-1-ols, to trisubstituted furans was effective, due to a 5- endo-dig oxycyclization by attack of the hydroxy group onto the central cumulene double bond. In contrast, palladium-catalyzed heterocyclization/coupling reactions with 3-bromoprop-1-enes furnished tetrasubstituted furans. Also studied was the palladium-catalyzed cyclization/coupling sequence involving protected indole-tethered 2,3,4-trien-1-ols and 3-bromoprop-1-enes that exclusively generated trisubstituted carbazole derivatives. These results could be explained through a selective 6- endo-dig cumulenic hydroarylation, followed by aromatization. DFT calculations were carried out to understand this difference in reactivity. [ABSTRACT FROM AUTHOR]

The Diels-Alder reactivity of maleic anhydride towards the bay regions of planar polycyclic aromatic hydrocarbons was explored computationally in the DFT framework. The process becomes more and more exothermic and the associated activation barriers become lower and lower when the size of the system increases. This enhanced reactivity follows an exponential behavior that reaches its maximum for systems having 18-20 benzenoid rings in their structures. This peculiar behavior was analyzed in detail by using the activation strain model of reactivity in combination with energy decomposition analysis. The influence of the change in the aromaticity of the polycyclic compound during the process on the respective activation barriers was also studied. [ABSTRACT FROM AUTHOR]

The multicomponent assembly of pharmaceutically relevant N-aryl-oxazolidinones through the direct insertion of carbon dioxide into readily available anilines and dibromoalkanes is described. The addition of catalytic amounts of an organosuperbase such as Barton's base enables this transformation to proceed with high yields and exquisite substrate functional-group tolerance under ambient CO2 pressure and mild temperature. This report also provides the first proof-of-principle for the single-operation synthesis of elusive seven-membered ring cyclic urethanes. [ABSTRACT FROM AUTHOR]

A palladium-catalyzed carbene insertion into C(sp3)−H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd0 and PdII, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium-catalyzed C(sp3)−C(sp3) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp3)−H bond functionalization reaction involves an unprecedented concerted metalation-deprotonation step. [ABSTRACT FROM AUTHOR]

The effect of Rh- and Ir-centers on the optical properties of the BODIPY core has been studied. To this end, novel metal complexes tethered to BODIPY have been prepared through an easy and versatile procedure using N-directed C-H activation reactions. The organometallic moiety has a tremendous influence on the emissive properties of the BODIPY fragment. A photoinduced electron transfer (PET) mechanism is suggested to be the main mechanism responsible for the suppression of the BODIPY fluorescence emission in the newly formed dyads. The efficiency of the PET depends on both the distance between the chromophores in the dyad and the nature of the transition metal (Rh vs. Ir). [ABSTRACT FROM AUTHOR]

The Diels-Alder reactivity of different bowl-shaped polycyclic aromatic hydrocarbons (namely, corannulene, cyclopentacorannulene, diindenochrysene, hemifullerene, and circumtrindene) has been explored computationally within the DFT framework. To this end, both the increase in reactivity with the size of the buckybowl and complete [6,6]-regioselectivity in the process have been analyzed in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. These results have been compared with the parent C60 fullerene, which also produces the corresponding [6,6]-cycloadduct exclusively. The behavior of the buckybowls considered herein resembles, in general, that of C60. Whereas the interaction energy between the deformed reactants along the reaction coordinate mainly controls the regioselectivity of the process, it is the interplay between the activation strain energy and the transition-state interaction that governs the reactivity of the system. [ABSTRACT FROM AUTHOR]

The controlled metal-free preparation of fused δ-sultone derivatives has been developed starting from hydroxyallenynes. The use of 2-(3,3-diethyltriaz-1-enyl)-4-methylbenzene-1-sulfonyl chloride in a sulfonylation/rearrangement sequence gives access to 1,3-dien-2-yl arenesulfonates. These functionalized enynes suffered a direct cyclization/desaturation radical cascade, allowing the synthesis of a variety of enynyl [1,2]oxathiine 1,1-dioxides. Stereoselective cyclization of the readily formed core through intramolecular Diels–Alder reaction has also been demonstrated, affording β-lactam- and glucofuranoside-fused δ-sultone polycycles. These selective reactions have been studied experimentally and additionally, their reaction mechanisms have been investigated computationally by means of density functional theory calculations. [ABSTRACT FROM AUTHOR]

The addition of Barton's base has a dramatic effect on the classic rhodium(III)-mediated hydrogenations promoted by Wilkinson′s catalyst. Following the initial oxidative addition, a barrierless reductive elimination of HCl from the traditional rhodium(III) intermediates instantly produces a rhodium(I) monohydride species, which is remarkably reactive in the hydrogenation of several internal alkynes and functionalized trisubstituted alkenes. The direct formation of this species is unprecedented upon addition of molecular hydrogen and its catalytic potential has been hitherto barely explored. [ABSTRACT FROM AUTHOR]

The different factors that control the alkene Prins cyclization catalyzed by iron(III) salts have been explored by means of a joint experimental-computational study. The iron(III) salt/trimethylsilyl halide system has proved to be an excellent promoter in the synthesis of crossed all- cis disubstituted tetrahydropyrans, minimizing the formation of products derived from side-chain exchange. In this iron(III)-catalyzed Prins cyclization reaction between homoallylic alcohols and non-activated alkenes, two mechanistic pathways can be envisaged, namely the classical oxocarbenium route and the alternative [2+2] cycloaddition-based pathway. It is found that the [2+2] pathway is disfavored for those alcohols having non-activated and non-substituted alkenes. In these cases, the classical pathway, via the key oxocarbenium ion, is preferred. In addition, the final product distribution strongly depends upon the nature of the substituent adjacent to the hydroxy group in the homoallylic alcohol, which can favor or hamper a side 2-oxonia-Cope rearrangement. [ABSTRACT FROM AUTHOR]

Trends in reactivity of β-chloride and β-hydride elimination reactions involving Group 10 transition-metal complexes have been computationally explored and analyzed in detail by DFT. These reactions do not require the initial formation of a vacant coordination site; they proceed concertedly without a prior ligand-dissociation step. Whereas β-chloride elimination is associated with relatively moderate activation barriers, the high barriers calculated for analogous β-hydride eliminations suggest that the latter process is unfeasible for this type of compounds. This differential behavior is analyzed within the activation strain model, which provides quantitative insight into the physical factors controlling these β-elimination reactions. The effects of the nature of the Group 10 transition metal (Ni, Pd, Pt), as well as the substituents attached to the β-eliminating fragment (R2CCR2X; R, X=H, Cl) on the transformation have also been considered and are rationalized herein. [ABSTRACT FROM AUTHOR]

The influence of the heteroatom (nitrogen, oxygen, and sulfur) on the course of the palladium-catalysed intramolecular reactions of aryl iodides and aldehydes having heteroatom-containing tethers has been explored by an extensive experimental-computational (DFT) study. Two series of substrates were considered, namely aldehydes bearing either the α-(2-iodobenzylheteroatom) or β-(2-iodophenylheteroatom) moieties. While some experimental differences were observed when changing from nitrogen to oxygen or sulfur in the 2-iodobenzyl series, the aldehydes in which the heteroatom is directly bonded to the aromatic ring showed common chemical behaviour regardless of the nature of the heteroatom. The different reaction pathways leading to the experimentally observed reaction products were studied by computational means. Our calculations suggest that in all cases the initial nucleophilic addition involving a σ-aryl-PdII intermediate is preferred over the competing concerted metallation-deprotonation (CMD) process. [ABSTRACT FROM AUTHOR]

The physical factors behind the reduced Diels-Alder reactivity of the Sc3N@C78 metallofullerene as compared with free C78 have been investigated in detail by means of computational tools. To this end, the reactions between 1,3-butadiene and free C78 and endohedral Sc3N@C78 have been analysed in terms of regioselectivity and reactivity by using the activation strain model of reactivity in combination with the energy decomposition analysis method. Additional factors such as the molecular orbital overlap or the aromaticity of the corresponding transition states have been also explored. Our results indicate that the lower reactivity of the metallofullerene finds its origin mainly in the less stabilizing interaction between the deformed reactants along the reaction coordinate induced by the triscandium nitride moiety. [ABSTRACT FROM AUTHOR]

The divergent behavior of two homologue allenals, namely, 2-(buta-2,3-dienyloxy)- and 2-(propa-1,2-dienyloxy)benzaldehydes, as cyclization substrates is described. 2-(Buta-2,3-dienyloxy)benzaldehydes suffers a formal allenic carbocyclization reaction to afford chromenes, whereas 2-(propa-1,2-dienyloxy)benzaldehydes react to yield chromones. The formation of chromenes is strictly a formal hydroarylation process divided into two parts, namely, allenic Claisen-type rearrangement and oxycyclization. An unknown N-heterocyclic carbene (NHC)-catalyzed allenic hydroacylation reaction must be invoked to account for the preparation of chromones. ortho-Allenylbenzaldehydes bearing either electron-donating substituents or electron-withdrawing substituents worked well to afford both the hydroarylation and hydroacylation products. This unexpected difference in reactivity can be rationalized by means of density functional theory calculations. [ABSTRACT FROM AUTHOR]

The trend in reactivity of the thermal cycloisomerization reactions of 1,3-hexadien-5-ynes, ABCDEF, were explored and analyzed by using density functional theory at the M06-2X/def2-TZVPP level. These reactions proceed through formally aromatic transition states to form a bent-allene intermediate with relatively high activation barriers. Activation-strain analyses show that the major factor controlling this Hopf cyclization is the geometrical strain energy associated with the rotation of the terminal [A] group. This rotation is necessary for achieving a favorable HOMO-LUMO overlap with the yne-moiety [F] associated with the formation of the new AF single bond. In addition, the relationship between the aromaticity of the corresponding cyclic transition states (all six-membered rings) and the computed activation barriers were analyzed. The calculations also indicate that the aromatization of the bent-allene structures takes place through two consecutive 1,2-hydrogen shifts, the second one exhibiting negligible energy barriers. [ABSTRACT FROM AUTHOR]

The reaction of β- and γ-haloamines with carbon dioxide to give pharmaceutically relevant 2-oxazolidinones and 1,3-dioxazin-2-ones, was found to proceed efficiently in the presence of a base and in the absence of catalyst. After optimization of reaction conditions, the system was successfully expanded to a variety of haloamines, even at multigram scale. The reaction was further studied in silico by DFT calculations. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and photophysical properties of novel BODIPY–Fischer alkoxy-, thio-, and aminocarbene dyads are reported. The BODIPY chromophore is directly attached to the carbene ligand by an ethylenic spacer, thus forming donor–bridge–acceptor π-extended systems. The extension of the p-conjugation is decisive in the equilibrium geometries of the dyads and is clearly reflected in the corresponding absorption and emission spectra. Whereas the BODIPY fragment is mainly isolated in aminocarbene complexes, it is fully conjugated in alkoxycarbene derivatives. The former thus exhibit the characteristic photophysical properties of BODIPY units, whereas complete suppression of the BODIPY fluorescence emission is observed in the latter, as a direct consequence of the strong electron-accepting character of the (CO)5M=C moiety. As the π-acceptor character of the metal–carbene group can be modified, the electronic properties of the conjugated BODIPY can be tuned. Density functional calculations have been carried out to gain insight into the photophysical properties. [ABSTRACT FROM AUTHOR]

The origin of the experimentally known preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C60 has been computationally explored. To this end, the Diels-Alder reaction between cyclopentadiene and C60 has been analysed by means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method. Other issues, such as the aromaticity of the corresponding transition states, have also been considered. These results indicate that the major factor controlling the observed regioselectivity is the more stabilising interaction between the deformed reactants in the [6,6] reaction pathway along the entire reaction coordinate. [ABSTRACT FROM AUTHOR]

A new molecular receptor ( 1) for ammonium recognition has been designed and constructed by using only carbon atoms. This molecular receptor can co-exist in two different isoenergetic conformations but, upon complexation, the conformers are no longer isoenergetic, and a basket-shaped conformation becomes clearly more stable. The pre-organised tetrahedral structure of this basket-shaped molecule favours the complexation of ammonium ions by NH⋅⋅⋅π interactions with the four phenyl groups of the host. A similar behaviour is not observed in a similar, but less pre-organised, reference molecule. ESI-MS competition experiments show that 1 is able to bind NH4+ over K+ selectively. This is the first example of a neutral molecular receptor that shows a remarkable NH4+/K+ selectivity. DFT-calculations provide insight into the nature of host-guest interactions of both 1⋅NH4+ and 1⋅K+ complexes as well as in the mechanism involved in multiple cation-π interactions and the influence of these interactions on the conformational stability and the selective binding of the host. [ABSTRACT FROM AUTHOR]

To understand the factors that control the activation barrier of type-I 1,2-dyotropic reactions (X-EH2-CH2-X*→X*-EH2-CH2-X, with E=C and Si, X=H, CH3, SiH3, F to I) and trends therein as a function of the migrating groups X, we have explored ten archetypal model reactions of this class using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity, which had to be extended from bimolecular to unimolecular reactions. Thus, the above type-I dyotropic reactions can be conceived as a relative rotation of the CH2CH2 and [X⋅⋅⋅X] fragments in X-CH2-CH2-X. The picture that emerges from these analyses is that reduced CX bonding in the transition state is the origin of the reaction barrier. Also the trends in reactivity on variation of X can be understood in terms of how sensitive the CX interaction is towards adopting the transition-state geometry. A valence bond analysis complements the analyses and confirms the picture emerging from the activation strain model. [ABSTRACT FROM AUTHOR]

Cover Feature: Bifunctional Hydrogen Bond Donor-Catalyzed Diels-Alder Reactions: Origin of Stereoselectivity and Rate Enhancement (Chem. Eur. J. 16/2021) Keywords: (thio)urea; activation strain model; density functional calculations; Diels-Alder reaction; hydrogen bond donor catalysis; Pauli repulsion EN (thio)urea activation strain model density functional calculations Diels-Alder reaction hydrogen bond donor catalysis Pauli repulsion 5054 5054 1 03/22/21 20210317 NES 210317 B (Thio)ureas b have found numerous applications as organocatalysts, yet their mode of action, that is, the mechanism by which they accelerate reactions is still not fully understood. (thio)urea, activation strain model, density functional calculations, Diels-Alder reaction, hydrogen bond donor catalysis, Pauli repulsion. [Extracted from the article]

The Pd-catalyzed intramolecular carbene C–H insertion of α-diazo-α-(methoxycarbonyl)acetamides to prepare oxindoles as well as β-lactams was studied. In order to identify what factors influence the selectivity of the processes, we explored how the reactions are affected by the catalyst type, using two oxidation states of Pd and a variety of ligands. It was found that, in the synthesis of oxindoles, ((IMes)Pd(NQ))2 can be used as an alternative to Pd2(dba)3 to catalyze the carbene CArsp2–H insertion, although it was less versatile. On the other hand, it was demonstrated that the Csp3–H insertion leading to β-lactams can be effectively promoted by both Pd(0) and Pd(II) catalysts, the latter being most efficient. Insight into the reaction mechanisms involved in these transformations was provided by DFT calculations. [ABSTRACT FROM AUTHOR]

Ion ‐ encapsulated fullerenes constitute a new family of endohedral fullerenes having potential applications in material science. Despite that, their reactivity is so far poorly understood. By means of the activation strain model of reactivity in combination with quantitative molecular orbital theory, the influence of the encapsulated ion on the exohedral reactivity of the C60 ‐ fullerene cage is analyzed in detail. More information can be found in the Full Paper by I. Fernández et al. on page 11030. [ABSTRACT FROM AUTHOR]

The cover page of the journal "Chemistry A European Journal" is presented.

The inside back cover of the journal is presented along with some information about endohedral metallofullerenes and the use of activation stress model.

The physical factors behind the reduced Diels-Alder reactivity of the Sc3N@C78 metallofullerene as compared with free C78 have been investigated in detail by means of computational tools. To this end, the reactions between 1,3-butadiene and free C78 and endohedral Sc3N@C78 have been analysed in terms of regioselectivity and reactivity by using the activation strain model of reactivity in combination with the energy decomposition analysis method. Additional factors such as the molecular orbital overlap or the aromaticity of the corresponding transition states have been also explored. Our results indicate that the lower reactivity of the metallofullerene finds its origin mainly in the less stabilizing interaction between the deformed reactants along the reaction coordinate induced by the triscandium nitride moiety. [ABSTRACT FROM AUTHOR]

The Diels-Alder reactivity of different bowl-shaped polycyclic aromatic hydrocarbons (namely, corannulene, cyclopentacorannulene, diindenochrysene, hemifullerene, and circumtrindene) has been explored computationally within the DFT framework. To this end, both the increase in reactivity with the size of the buckybowl and complete [6,6]-regioselectivity in the process have been analyzed in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. These results have been compared with the parent C60 fullerene, which also produces the corresponding [6,6]-cycloadduct exclusively. The behavior of the buckybowls considered herein resembles, in general, that of C60. Whereas the interaction energy between the deformed reactants along the reaction coordinate mainly controls the regioselectivity of the process, it is the interplay between the activation strain energy and the transition-state interaction that governs the reactivity of the system. [ABSTRACT FROM AUTHOR]