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The catalytic performance of a series of mono‐ and bimetallic Ir(I) and Rh(I) complexes bearing the (COC) bis‐triazolylidene ligand, coordinated in a bridging or chelating fashion, was evaluated in the cyclization of 4‐pentynoic acid. The chelated mononuclear cationic complex [Ir(cod)(COC)]PF6 (cod=1,5‐cyclooctadiene; COC=biscarbene ether) displayed the highest reaction rate for 4‐pentynoic acid to form γ‐methyl‐ene‐γ‐butyrolactone (5‐methylenedihydrofuran‐2(3H)‐one). Density Functional Theory calculations were carried out to rationalize the reaction mechanism which proceeds in three steps, namely coordination of the alkyne to the active catalyst followed by intramolecular cyclization and final reductive elimination. Its catalytic performance was further assessed against different substituted 4‐pentynoic acid derivatives. [ABSTRACT FROM AUTHOR]

While the synthesis of nanographenes has advanced greatly in the past few years, development of their atomically precise functionalization strategies remains rare. The ability to modify the carbon scaffold translates to controlling, adjusting, and adapting molecular properties. Towards this end, here, we show that mechanochemistry is capable of transforming graphitization precursors directly into chlorinated curved nanographenes through a Scholl reaction. The halogenation occurs in a regioselective, high-yielding, and general manner. Density Functional Theory (DFT) calculations suggest that graphitization activates specific edge-positions for chlorination. The chlorine atoms allow for precise chemical modification of the nanographenes through a Suzuki or a nucleophilic aromatic substitution reaction. The edge modification enables modulation of material properties. Among the molecules prepared, corannulene-coronene hybrids and laterally fully π-extended helicenes, heptabenzo[5]superhelicenes, are particularly noteworthy. The synthesis of functionalizable nanographenes remains challenging. Here, the authors report that mechanochemical Scholl reaction allows access to regioselectively modifiable curved nanographenes in a high-yielding and general manner. [ABSTRACT FROM AUTHOR]

Scholl oxidation has become an essential reaction in the bottom‐up synthesis of molecular nanographenes. Herein, we describe a Scholl reaction controlled by the electronic effects on the starting substrate (1 a, b). Anthracene‐based polyphenylenes lead to spironanographenes under Scholl conditions. In contrast, an electron‐deficient anthracene substrate affords a helically arranged molecular nanographene formed by two orthogonal dibenzo[fg,ij]phenanthro‐[9,10,1,2,3‐pqrst]pentaphene (DBPP) moieties linked through an octafluoroanthracene core. Density Functional Theory (DFT) calculations predict that electronic effects control either the first formation of spirocycles and subsequent Scholl reaction to form spironanographene 2, or the expected dehydrogenation reaction leading solely to the helical nanographene 3. The crystal structures of four of the new spiro compounds (syn2, syn9, anti9 and syn10) were solved by single crystal X‐ray diffraction. The photophysical properties of the new molecular nanographene 3 reveal a remarkable dual fluorescent emission. [ABSTRACT FROM AUTHOR]

The ambiguous bonding situation of σ‐E−H (E=Si, B) complexes in transition metal compounds has been rationalized by means of Density Functional Theory calculations. To this end, the combination of the Energy Decomposition Analysis (EDA) method and its Natural Orbital for Chemical Valance (NOCV) extension has been applied to representative complexes described in the literature where the possible η1 versus η2 coordination mode is not unambiguously defined. Our quantitative analyses, which complement previous data based on the application of the Quantum Theory of Atoms in Molecules (QTAIM) approach, indicate that there exists a continuum between genuine η1 and η2 modes depending mainly on the strength of the backdonation. Finally, we also applied this EDA‐NOCV approach to related main‐group species where the backdonation is minimal. [ABSTRACT FROM AUTHOR]

The bonding situation of Ag(I)CO complexes having a Scorpionate ligand directly attached to the transition metal has been analyzed in detail by means of relativistic density functional theory calculations. To this end, different experimentally characterized complexes together with other representative species have been considered to rationalize the observed shift of the corresponding ν(CO) stretching frequencies and the influence of the substituents in the Scorpionate ligand. With the help of the energy decomposition analysis method combined with the natural orbital for chemical valence it is found that the main contribution to the bonding comes from the electrostatic attractions between the LAg(I) and CO fragments. Despite that, the LAg → CO π‐backdonation is also significant in these species as well as in related LCu(I)CO complexes. [ABSTRACT FROM AUTHOR]

The reaction of propargyl esters with alkynylsilanes under gold catalysis provides vinylallene derivatives through consecutive [1,2]‐acyloxy/[1,2]‐silyl rearrangements. Good yields, full atom‐economy, broad substrate scope, easy scale‐up and low catalyst loadings are salient features of this novel transformation. Density Functional Theory (DFT) calculations suggest a reaction mechanism involving initial [1,2]‐acyloxy rearrangement to generate a gold vinylcarbene intermediate which upon regioselective attack of the alkynylsilane affords a vinyl cation which undergoes a type II‐dyotropic rearrangement involving the silyl group and the metal fragment. Preliminary results on the enantioselective version of this transformation are also disclosed. [ABSTRACT FROM AUTHOR]

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(I)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold. Besides, the bonding situation of this species has been investigated by means of state-of-the-art Density Functional Theory (DFT) calculations indicating that π-backdonation plays a minor role compared with tricoordinate analogues. [ABSTRACT FROM AUTHOR]

The so far poorly understood factors controlling the complete meta‐selectivity observed in the C−H activation reactions of alkylarenes promoted by aluminyl anions have been explored in detail by means of Density Functional Theory calculations. To this end, a combination of state‐of‐the‐art computational methods, namely the activation strain model of reactivity and energy decomposition analysis, has been applied to quantitatively unveil the origin of the selectivity of the transformation as well as the influence of the associated potassium cation. It is found that the selectivity takes place during the initial nucleophilic addition step where the key LP(Al)→π*(C=C) molecular orbital interaction is more stabilizing for the meta‐pathway, which results in a stronger interaction between the reactants along the entire transformation. [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 activation of a single C−F bond in di‐ and trifluoromethyl groups by frustrated Lewis pairs (FLPs) has been computationally explored by means of Density Functional Theory calculations. It is found that in this activation reaction the FLP partners exhibit a peculiar cooperative action, which is markedly different from related FLP‐mediated processes, and where non‐covalent interactions established between the Lewis base and the substrate play a decisive role. In addition, the process proceeds through the intermediacy of a hypervalent species featuring a pentacoordinate carbon atom, which is rare in the chemistry of FLPs. The physical factors controlling this process as well as the bonding situation of these hypervalent intermediates have been quantitatively analyzed in detail by using state‐of‐the‐art computational methods to not only rationalize the mechanism of the transformation but also to guide experimentalists towards the realization of these so far elusive hypervalent systems. [ABSTRACT FROM AUTHOR]

Two (thieno[3,2-b]thiophene) and three annulated thiophenes (dithieno[2,3-b;3′,2′-d]thiophene and dithieno[3,2-b;2′,3′-d]thiophene) were employed as building blocks to synthesize linear or semi-circular chelated mononuclear biscarbene and dinuclear tetracarbene complexes. The electronic properties of the annulated thienylene chelated carbene complexes were investigated by cyclic voltammetry experiments and compared to non-chelated Fischer-type monocarbene complexes. Density functional theory (DFT) calculations were used to assign the redox events and to probe the extent of electron delocalisation as well as the possibility of electronic (intramolecular metal–metal) communication as a result of intervalence. The differences of these electronic properties in the conjugated chelated carbene complexes are compared to chelated carbene compounds without a linear conjugated pathway. [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

The role of frustrated Lewis pairs (FLPs) as ligands in gold(I) catalyzed-reactions has been computationally investigated by using state-of-the-art density functional theory calculations. To this end, the nature of (P,B)-FLP–transition metal interactions in different gold(I)-complexes has been first explored in detail with the help of the energy decomposition analysis method, which allowed us to accurately quantify the so far poorly understood Au⋯B interactions present in these species. The impact of such interactions on the catalytic activity of gold(I)-complexes has been then evaluated by performing the Au(I)-catalyzed hydroarylation reaction of phenylacetylene with mesitylene. With the help of the activation strain model of reactivity, the factors governing the higher activity of Au(I)-complexes having a FLP as a ligand as compared to that of the parent PPh3 system have also been quantitatively identified. [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]

Grubbs catalysts are described as a useful alternative to promote intramolecular carbene C–H insertion from α-diazoesters. Moreover, no competition arises from the possible metathesis reactions on substrates bearing alkene and alkyne moieties. DFT calculations were also carried out to gain insight into the reaction mechanism involved in these transformations. [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]

Abstract: Heteropolynuclear complexes [(CNC)(PPh3)PtM(PPh3)]ClO4 [M=Au (1), Ag (2)] and [{Pt(CNC)(PPh3)}2M]ClO4 [M=Ag (3), Au (4); CNC=2,6‐diphenylpyridinate] were prepared and studied by X‐ray crystallography, spectroscopic techniques, and DFT calculations. The X‐ray crystal structures of 1, 3, and 4 confirmed the existence of Pt−M bonds and M⋅⋅⋅Cipso interactions involving one of the phenyl fragments of CNC. Their NMR spectra showed the persistence of the Pt−M interactions in solution and also revealed an intramolecular metronome‐like dynamic process consisting of back‐and‐forth motion of the acidic M fragments along the C‐Pt‐C axis. DFT calculations on these complexes identified two main orbital interactions between the [PtCNC] and [M]+ fragments, namely, donation from the former to a vacant orbital of the latter and much weaker backdonation from the acidic M to the Pt fragment. Overall, the strength of the [Pt]⋅⋅⋅M interactions is higher for the gold compounds than for their silver counterparts. The interaction between the acidic center (silver or gold) and the carbon atom of one of the phenylene rings in these heteropolymetallic complexes can be envisaged as the first step in a process of interchange of aryl ligands. However, the ligand exchange cannot progress further due to the polydentate nature of the CNC ligand, and therefore these structures can be considered as frozen snapshots of a transmetalation reaction that has been arrested at different stages of the process. [ABSTRACT FROM AUTHOR]

The Diels-Alder reactivity of 1,2-heteroborines (H 4 C 4 B(H)X, X = NH, PH, AsH; O, S, Se) has been computationally explored by means of Density Functional Theory (DFT) calculations. The influence of the HB = X fragment on the reactivity of the system has been quantitatively analyzed in detail by means of the so-called Activation Strain Model (ASM) of reactivity. It is found that the interaction between these species and the dienophile is significantly stronger than that computed for their all-carbon isoelectronic counterpart, benzene. In addition, the strain energy plays a key role in the observed reactivity trends. The role of the aromaticity strength of these heteroarenes on the reactivity is also assessed. [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]

Abstract: Indole‐tethered amino allenynes were chemodivergently cyclized for the controlled preparation of fused polycyclic indoles using gold catalysis. Double cyclization of terminal allenynes afforded hexacyclic 15 H‐indolo[1,2,3‐de]quinolino[3,2,1‐ij]quinoxalines, in which allenynes bearing a substituted alkyne at the terminal end generated 12,13‐dihydro‐7 H‐indolo[3,2‐c]acridines, which are 5‐membered cyclized adducts. Density functional theory calculations were performed to shed light on this difference in reactivity. [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]

Chiral fullerene-metal hybrids with complete control over the four stereogenic centers, including the absolute configuration of the metal atom, have been synthesized for the first time. The stereochemistry of the four chiral centers formed during [60]fullerene functionalization is the result of both the chiral catalysts employed and the diastereoselective addition of the metal complexes used (iridium, rhodium, or ruthenium). DFT calculations underpin the observed configurational stability at the metal center, which does not undergo an epimerization process. [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

Density Functional Theory calculations are used to explore the double hydrogen atom transfer from different alkanes to 1,2-benzyne. State-of-the-art calculations including the Activation Strain Model of reactivity, Energy Decomposition Analysis, and Valence Bond methods, reveal the origins of the relatively low activation barriers computed for these processes compared to the analogous reaction involving acetylene. In addition, the interplay between the in-plane aromaticity of the corresponding transition states and the variation of the π-aromaticity associated with the benzyne moiety as well as their influence on the barrier heights of the transformations are analyzed in detail. © 2016 Wiley Periodicals, Inc. [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]

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 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]

By means of density functional theory calculations, we computationally analyze the physical factors governing the oxidative addition of aryl halides to gold(I) complexes. Using the activation strain model of chemical reactivity, it is found that the strain energy associated with the bending of the gold(I) complex plays a key role in controlling the activation barrier of the process. A systematic study on how the reaction barrier depends on the nature of the aryl halide, ligand, and counteranion allows us to identify the best combination of gold(I) complex and aryl halide to achieve a feasible (i.e., low barrier) oxidative addition to gold(I), a process considered as kinetically sluggish so far. © 2014 Wiley Periodicals, Inc. [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]

Aromaticity is an essential concept in chemistry, employed to account for the unusual stability, reactivity, molecular structures, and other properties of many unsaturated organic compounds. This concept was later extended to inorganic molecules and to saturated systems with mobile electrons, as well as to transition structures, the focus of the present review. Although transition structures are inherently delocalized, not all exhibit aromaticity. We contrast here examples of pericyclic reaction transition structures (where aromaticity is significant) with those for illustrative pseudo-pericyclic reactions (where aromaticity is less or not important). Non-pericyclic reactions may also have aromatic transition structures. State-of-the-art computational methods to evaluate the aromaticity of transition structures are described briefly. [ABSTRACT FROM AUTHOR]

Detailed density functional theory calculations definitively rationalize the preference for the endo cycloadduct (also known as endo rule) in text-book thermal Diels-Alder reactions involving maleic anhydride and cyclopentadiene or butadiene. This selectivity is mainly caused by an unfavorable steric arrangement in the transition-state region of the exo pathway which translates into a more destabilizing activation strain. In contrast with the widely accepted, orbital-interaction-based explanation for the endo rule, it is found that neither the orbital interactions nor the total interaction between the deformed reactants contributes to the endo selectivity. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The [5+2] and [6+2] cycloaddition reactions of vinylaziridines and vinylazetidines with ketenes generated photochemically from chromium(0) and molybdenum(0) Fischer carbene complexes have been investigated. These processes constitute a straightforward and efficient route to azepanones and azocinones, respectively. The peculiar electronic properties of the metalated ketenes allow for the introduction of electron-rich substituents in the final cycloadducts, a difficult task using conventional organic chemistry procedures. The versatility of the process is demonstrated by using Cr0 Fischer bis(carbene) complexes as metalated bis(ketene) precursors. These species produce tethered bis(azepanone) s in a single step under mild reaction conditions. Density functional theory calculations point to a stepwise reaction pathway through the initial nucleophilic attack of the nitrogen atom of the aziridine on the metalated ketene, followed by ring closure of the zwitterionic intermediate formed. [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 synthesis and structure of rare acyclic alkoxy- and aminocarbene complexes of gold(I) are reported, including a novel ferrocenophane dinuclear biscarbene complex. X-Ray diffraction analyses and DFT calculations reveal that these complexes are stabilized by genuine aurophilic interactions. [ABSTRACT FROM AUTHOR]

We discuss a novel selenium-based reaction mechanism consisting in a selenoxide elimination-triggered enamine hydrolysis. This one-pot model reaction was studied for a set of substrates. Under oxidative conditions, we observed and characterized the formation of primary and secondary amines as elimination products of such compounds, paving the way for a novel strategy to selectively release bioactive molecules. The underlying mechanism was investigated using NMR, mass spectrometry and density functional theory (DFT). [ABSTRACT FROM AUTHOR]

The factors responsible for the enhancement of the halogen bond by an adjacent hydrogen bond have been quantitatively explored by means of state-of-the-art computational methods. It is found that the strength of a halogen bond is enhanced by ca. 3 kcal/mol when the halogen donor simultaneously operates as a halogen bond donor and a hydrogen bond acceptor. This enhancement is the result of both stronger electrostatic and orbital interactions between the XB donor and the XB acceptor, which indicates a significant degree of covalency in these halogen bonds. In addition, the halogen bond strength can be easily tuned by modifying the electron density of the aryl group of the XB donor as well as the acidity of the hydrogen atoms responsible for the hydrogen bond. [ABSTRACT FROM AUTHOR]

We herein report the synthesis, antioxidant power and neuroprotective properties of nine homo-bis-nitrones HBNs1–9 as alpha-phenyl-N-tert-butylnitrone (PBN) analogues for stroke therapy. In vitro neuroprotection studies of HBNs1–9 against Oligomycin A/Rotenone and in an oxygen-glucose-deprivation model of ischemia in human neuroblastoma cell cultures, indicate that (1Z,1′Z)-1,1′-(1,3-phenylene)bis(N-benzylmethanimine oxide) (HBN6) is a potent neuroprotective agent that prevents the decrease in neuronal metabolic activity (EC50 = 1.24 ± 0.39 μM) as well as necrotic and apoptotic cell death. HBN6 shows strong hydroxyl radical scavenger power (81%), and capacity to decrease superoxide production in human neuroblastoma cell cultures (maximal activity = 95.8 ± 3.6%), values significantly superior to the neuroprotective and antioxidant properties of the parent PBN. The higher neuroprotective ability of HBN6 has been rationalized by means of Density Functional Theory calculations. Calculated physicochemical and ADME properties confirmed HBN6 as a hit-agent showing suitable drug-like properties. Finally, the contribution of HBN6 to brain damage prevention was confirmed in a permanent MCAO setting by assessing infarct volume outcome 48 h after stroke in drug administered experimental animals, which provides evidence of a significant reduction of the brain lesion size and strongly suggests that HBN6 is a potential neuroprotective agent against stroke. [ABSTRACT FROM AUTHOR]

Two of the most popular rules to characterize the aromaticity of molecules are those due to Hückel and Baird, which govern the aromaticity of singlet and triplet states. In this work, we study how these rules fade away as the ring structure increases and an optimal overlap between p orbitals is no longer possible due to geometrical restrictions. To this end, we study the lowest-lying singlet and triplet states of neutral annulenes with an even number of carbon atoms between four and eighteen. First of all, we analyze these rules from the Hückel molecular orbital method and, afterwards, we perform a geometry optimization of the annulenes with several density functional approximations in order to analyze the effect that the distortions from planarity produce on the aromaticity of annulenes. Finally, we analyze the performance of three density functional approximations that employ different percentages of Hartree-Fock exchange (B3LYP, CAM-B3LYP and M06-2X) and Hartree-Fock. Our results reveal that functionals with a low percentage of Hartree-Fock exchange at long ranges suffer from severe delocalization errors that result in wrong geometrical structures and the overestimation of the aromatic character of annulenes. [ABSTRACT FROM AUTHOR]

Facile synthetic procedures to synthesize a series of difficult-to-obtain mercaptoalkylferrocenes, namely, Fc(CH2)nSH, where n = 1 (1), 2 (2), 3 (3), or 4 (4) and Fc = Fe(η5-C5H5)(η5-C5H4), are reported. Dimerization of 1-4 to the corresponding disulfides 19-22 was observed in air. Dimer 20 (Z = 2) crystallized in the triclinic space group P1?. Dimers 20-22 could be reduced back to the original Fc(CH2)nSH derivatives with LiAlH4 in refluxing tetrahydrofuran. Density functional theory (DFT) calculations showed that the highest occupied molecular orbital of 1-4 lies exclusively on the ferrocenyl group implying that the electrochemical oxidation observed at ca. -15 < Epa < 76 mV versus FcH/FcH+ involves exclusively an Fe(II) to Fe(III) process. Further DFT calculations showed this one-electron oxidation is followed by proton loss on the thiol group to generate a radical, Fc(CH2)nS, with spin density mainly located on the sulfur. Rapid exothermic dimerization leads to the observed dimers, Fc(CH2)n-S-S-(CH2)nFc. Reduction of the ferrocenium groups on the dimer occurs at potentials that still showed the ferrocenyl group ΔE = Epa,monomer - Epc,dimer ≤ 78 mV, indicating that the redox properties of the ferrocenyl group on the mercaptans are very similar to those of the dimer. ¹H NMR measurements showed that, like ferrocenyl oxidation, the resonance position of the sulfhydryl proton, SH, and others, are dependent on -(CH2)n- chain length. Self-assembled monolayers (SAMs) on gold were generated to investigate the electrochemical behavior of 1-4 in the absence of diffusion. Under these conditions, ?E approached 0 mV for the longer chain derivatives at slow scan rates. The surface-bound ferrocenyl group of the metal-thioether, Fc(CH2)n -S-Au, is oxidized at approximately equal potentials as the equivalent CH2Cl2-dissolved ferrocenyl species 1-4. Surface coverage by the SAMs is dependent on alkyl chain length with the largest coverage obtained for 4, while the rate of heterogeneous electron transfer between SAM substrate and electrode was the fastest for the shortest chain derivative, Fc-CH2-S-Au. [ABSTRACT FROM AUTHOR]

By means of density functional theory calculations, we have computationally explored the intimacies of the crucial step of Noyori hydrogrogenation reactions of multiple bonds. This process can be considered analogous to the so-called double group transfer reactions. Both kinds of transformations proceed concertedly via the simultaneous migration of two hydrogen atoms/groups in a pericyclic [σ2s + σ2s + π2s] reaction through six-membered transition structures. Despite the structural resemblances of both types of saddle points, significant differences are found in terms of synchronicity and in-plane aromaticity. In addition, the activation strain model has been used to get quantitative insight into the factors which control the corresponding barrier heights. It is found that the presence of a heteroatom in the acceptor moiety is responsible for a remarkable increase of the interaction energy between the reactants which can compensate the destabilizing effect of the strain energy associated with the deformation of the initial reagents leading to low reaction barriers. [ABSTRACT FROM AUTHOR]

The great catalytic performance of supported silver nitrate in silica gel for indole-tethered allene hydrofunctionalization together with a selectivity reversal in comparison with homogeneous gold catalysis have been shown. • Convenient silver-catalyzed heterogeneous method for the intramolecular allene hydroarylation of indoles. • Divergent and controlled preparation of three different final products: 9 H -carbazoles, 4,9-dihydro-1 H -carbazoles, and spiroindolenines. • The AgNPs-catalyzed allene reactions contrast with the reaction pattern previously reported in homogeneous gold-catalyzed reactions. Supported silver nitrate in silica gel has been demonstrated as an excellent heterogeneous catalytic system for the selective hydroarylation of indole-tethered allenes, with a selectivity reversal in comparison with homogeneous gold catalysis. In this way, the controlled preparation of 1,2-disubstituted-9 H -carbazoles, 1,2-disubstituted-4,9-dihydro-1 H -carbazoles, and 2,4-disubstituted-spiro[cyclopentane-1,3′-indol]-3-enes has been accomplished. Control experiments supported by Density Functional Theory calculations strongly suggest that indole-tethered allenes react through a 5- endo - dig -carbometallation/rearrangement sequence, which contrasts to the previously reported 6- endo - dig -carbometallation pathway promoted by [Au(I)]-catalysts in related systems. [ABSTRACT FROM AUTHOR]

A new, direct, and diastereoselective synthesis of activated 2,3,4,6-tetrasubstituted tetrahydro-2H-pyrans is described. In this reaction, iron(III) catalyzed an SN2'-Prins cyclization tandem process leading to the creation of three new stereocenters in one single step. These activated tetrahydro-2H-pyran units are easily derivatizable through CuAAC conjugations in order to generate multifunctionalized complex molecules. DFT calculations support the in situ SN2' reaction as a preliminary step in the Prins cyclization. [ABSTRACT FROM AUTHOR]