Gold(I)-Catalyzed Allenyl Cope Rearrangement: Evolution from Asynchronicity to Trappable Intermediates Assisted by Stereoelectronic Switching

Gold-Catalyzed Arylative Cope Rearrangement

Cope rearrangements have garnered significant attention owing to their ability to undergo structural reorganization in stereoselective manner. While substantial advances have been achieved over decades, these rearrangements remained applicable exclusively to parent 1,5-hexadienes. Herein, we disclose the gold-catalyzed arylative Cope rearrangement of 1,6-heptadienes via a cyclization-induced [3,3]-rearrangement employing ligand-enabled gold redox catalysis. Detailed mechanistic investigations including several control experiments, cross-over experiment, HRMS analysis, 31P NMR and DFT studies have been performed to underpin the mechanism.

Theoretical insight into the regioselective formation of pyrazolo[1,4]-oxazepine and -oxazines

AuCl-, AuCl3-, or AuClPEt3-catalyzed formation mechanisms of pyrazolo[1,4]oxazepines and the NaH-promoted mechanism of pyrazolo[1,4]oxazines were investigated computationally. The structural properties of the reactants were studied in various solvents and with different functionals. The hybrid functionals B3LYP, M06, M06-2X, PBEPBE, and wB97X-D in density functional theory were used to determine and discuss the energetics of the compounds. The electronic properties of groups (R = H or R ≠ H) attached to the alkyne moiety played an essential role in the corresponding 7-endo-dig cyclization or 6-exo-dig cyclization in the presence of a gold catalyst. The regioselectivities of the products were investigated, and the natural bond orbitals of the reactants were determined. Furthermore, a gold-catalyzed alternative mechanism is suggested for synthesizing pyrazolo[1,4]oxazines using a terminal alkyne (R = H) moiety as substrate.

Mechanistic duality of indolyl 1,3-heteroatom transposition

A novel mechanistic duality has been revealed from the indolyl 1,3-heteroatom transposition (IHT) of N-hydroxyindole derivatives. A series of in-depth mechanistic investigations suggests that two separate mechanisms are operating simultaneously. Moreover, the relative contribution of each mechanistic pathway, the energy barrier for each pathway, and the identity of the primary pathway were shown to be the functions of the electronic properties of the substrate system. Based on the mechanistic understanding obtained, a mechanism-driven strategy for the general and efficient introduction of a heteroatom at the 3-position of indole has been developed. The reaction developed exhibits a broad substrate scope to provide the products in various forms of the functionalised indole. Moreover, the method is applicable to the introduction of both oxygen- and nitrogen-based functional groups.

Modulating Escape Channels of Cycloheptatrienyl Rhodium Carbenes To Form Semibullvalene

We describe the various escape channels available to dirhodium carbene intermediates from cycloheptatrienyl diazo compounds located with density functional theory. An intramolecular cyclopropanation would, in principle, provide a new route to semibullvalenes (SBVs). A detailed exploration of the potential energy surface reveals that methylating carbon-7 suppresses a competing β-hydride migration pathway to heptafulvene products, giving SBV formation a reasonable chance. During our explorations, we additionally discovered unusual spirononatriene, spironorcaradiene, and metal-stabilized 9-barbaralyl cation structures as local minima.

Stereoelectronic Modulation of a Single-Molecule Junction through a Tunable Metal-Carbon dπ-pπ Hyperconjugation

Conjugated molecules play a critical role in the construction of single-molecule devices. However, most conventional conjugated molecules, such as hydrocarbons, involve only a pπ-pπ conjugation of light elements. While the metal d-orbitals can introduce abundant electronic effects to achieve novel electronic properties, it is very scarce for the charge transport study of dπ-pπ conjugated pathways with a metal involved. Here, we employed the single-molecule break junction technique to investigate the charge transport through dπ-pπ conjugated backbones with metal-carbon multiple bonds integrated into the alternative conjugated pathways. The involved dπ-pπ conjugation not only supports high conductivity comparable to that of conjugated hydrocarbons but also significantly enhances the tunable diversity in electronic properties through the metal-induced secondary interaction. Specifically, the introduction of the metal brings an unconventionally stereoelectronic effect triggered by metal-carbon dπ-pπ hyperconjugation, which can be tuned by protonation taking place on the metal-carbon multiple bonds, collectively modulating the single-molecule rectification feature and transmission mechanism. This work demonstrates the promise of utilizing the diverse electronic effect of metals to design molecular devices.

Zwitterionic Bergman cyclization triggered polymerization gives access to metal-graphene nanoribbons using a boron metal couple

With the rapid growth in artificial intelligence, designing high-speed and low-power semiconducting materials is of utmost importance. This investigation provides a theoretical basis to access covalently bonded transition metal-graphene nanoribbon (TM-GNR) hybrid semiconductors whose DFT-computed bandgaps were much narrower than the commonly used pentacene. Systematic optimization of substrates containing remotely placed boryl groups and the transition metals produced the zwitterions via ionic Bergman cyclization (i-BC) and unlocked the polymerization of metal-substituted polyenynes. Aside from i-BC, the subsequent steps were barrierless, which involved structureless transition regions. Multivariate analysis revealed the strong dependence of activation energy and the cyclization mode on the electronic nature of boron and Au(I). Consequently, three regions corresponding to radical Bergman (r-BC), ionic Bergman (i-BC), and ionic Schreiner-Pascal (i-SP) cyclizations were identified. The boundaries between these regions corresponded to the mechanistic shift induced by the three-center-three-electron (3c-3e) hydrogen bond, three-center-four-electron (3c-4e) hydrogen bond, and vacant p-orbital on boron. The ideal combination for cascade polymerization was observed near the boundary between i-BC and i-SP.

Computational analysis of the formation mechanisms of carbazoles

Gold-, platinum-, and silver-catalyzed formation mechanisms of carbazole alkaloids were investigated computationally. The structural properties of the reactants were studied in various solvents and with different functionals. The hybrid functionals B3LYP and M06-2X in density functional theory were used to determine and discuss the energetics of the compounds. The electronic properties of groups attached to the terminal alkyne played an essential role in the formation of carbazoles. The stereo- and chemoselectivity of the mechanism were investigated and the natural bond orbitals of the reactants were determined. Furthermore, this theoretical study has suggested a gold-catalyzed alternative mechanism for the synthesis of 1,4-dihydrocyclopenta[b]indole derivative.

The Role of Through-Bond Stereoelectronic Effects in the Reactivity of 3-Azabicyclo[3.3.1]nonanes

Hyperconjugation/conjugation through-bond stereoelectronic effects were studied with density functional theory in the context of 3-azabicyclo[3.3.1]nonanes to unravel puzzling differences in reactivity between a vinylogous chloride (4) and a vinylogous ester (5). These compounds─whose structures differ only by one substituent─were found to display strikingly different reactivities in hydrochloric acid by Risch and co-workers ( J. Am. Chem. Soc., 1991, 113, 9411-9412). Computational analyses of substituent effects, noncovalent interactions, natural bond orbitals, isodesmic reactions, and hydration propensities lead to a model for which the role of remote, through-bond stereoelectronic effects is the key to explaining 4 and 5's diverging reactivity.

Violations to the Principle of Least Motion: the Shortest Path is not Always the Fastest

The reaction between two molecules is usually envisioned as following a least-motion path with both molecules travelling minimum distances to meet each other. However, the reaction path of lowest activation...

Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine

Non-classical processes such as heavy-atom tunneling and post transition-state dynamics govern stereoselectivity for benzene imine ⇌ 1H-azepine.

Iron-Catalyzed Contrasteric Functionalization of Allenic C(sp2)-H Bonds: Synthesis of α-Aminoalkyl 1,1-Disubstituted Allenes

An iron-catalyzed C-H functionalization of simple monosubstituted allenes is reported. An efficient protocol for this process was made possible by the use of a newly developed electron-rich and sterically hindered cationic cyclopentadienyliron dicarbonyl complex as the catalyst and N-sulfonyl hemiaminal ether reagents as precursors to iminium ion electrophiles. Under optimized conditions, the use of a mild, functional-group-tolerant base enabled the conversion of a range of monoalkyl allenes to their allenylic sulfonamido 1,1-disubstituted derivatives, a previously unreported and contrasteric regiochemical outcome for the C-H functionalization of electronically unbiased and directing-group-free allenes.

External Electric Fields Interrupt the Concerted Cope Rearrangement of Semibullvalene

The topic of this paper is whether the mechanism of the degenerate Cope rearrangement of semibullvalene can be affected by the presence of electrostatic fields. Herein, we report that the shape of the energy surface, as demonstrated by an "interrupted" (stepwise) mechanism, is altered in the presence of a copper cation, Cu⁺. Natural bond-orbital and block-localized wave-function energy decomposition analyses suggest that orbital and electrostatic interactions play a major role in altering the shape of the energy surface. Applying additional external electric fields (EEFs) induces a significant change to the energy surface with Cu⁺ present but negligible effects in the absence of Cu⁺. These findings are consistent with recent studies that demonstrate that EEFs more readily stabilize/destabilize systems with larger, more polarizable, dipole moments.

Stereochemistry of Transition Metal Complexes Controlled by the Metallo-Anomeric Effect

The use of stereoelectronic interactions to control reactivity and selectivity has a long history in chemistry. The anomeric effect, one of the fundamental concepts in organic chemistry, describes the preferences of a substituent at the anomeric carbon in glycosides to adopt axial configuration when the anomeric group is an electronegative element such as oxygen or a halogen. The origin of the anomeric effect has been the subject of intense debate. Explanations capitalizing on either the delocalization of the endocyclic oxygen lone pair into the antibonding σ*_(C-X) orbital or the minimization of the dipole-dipole interactions are currently the two leading theoretical models. Although the majority of experimental and theoretical studies have focused on the elements from groups 6 and 7, little is known about conformational preferences of tetrahydropyran rings substituted with a transition metal at the anomeric carbon and the role of these interactions in stereoselective synthesis. Here, we report studies on conformational and configurational preferences of organometallic complexes stabilized by vicinal heteroatoms. We provide computational evidence that late transition metals adopt the axial position in heterocycles or synclinal geometry in acyclic systems. Furthermore, the anomeric preferences of late transition metals correlate with the oxidation state of the metal and can be explained by hyperconjugative interactions between endocyclic heteroatom and the σ* acceptor orbitals of the C-M bond. In a broader context, this discovery provides insight into the role of previously unanticipated stereoelectronic effects that can be harnessed in the design of stereoselective reactions, including chemical glycosylation and enantioselective catalysis.

Isonitriles as Stereoelectronic Chameleons: The Donor-Acceptor Dichotomy in Radical Additions

Radical addition to isonitriles (isocyanides) starts and continues all the way to the transition state (TS) mostly as a simple addition to a polarized π-bond. Only after the TS has been passed, the spin density moves to the α-carbon to form the imidoyl radical, the hallmark intermediate of the 1,1-addition-mediated cascades. Addition of alkyl, aryl, heteroatom-substituted, and heteroatom-centered radicals reveals a number of electronic, supramolecular, and conformational effects potentially useful for the practical control of isonitrile-mediated radical cascade transformations. Addition of alkyl radicals reveals two stereoelectronic preferences. First, the radical attack aligns the incipient C···C bond with the aromatic π-system. Second, one of the C-H/C-C bonds at the radical carbon eclipses the isonitrile N-C bond. Combination of these stereoelectronic preferences with entropic penalty explains why the least exergonic reaction (addition of the t-Bu radical) is also the fastest. Heteroatomic radicals reveal further unusual trends. In particular, the Sn radical addition to the PhNC is much faster than addition of the other group IV radicals, despite forming the weakest bond. This combination of kinetic and thermodynamic properties is ideal for applications in control of radical reactivity via dynamic covalent chemistry and may be responsible for the historically broad utility of Sn radicals ("the tyranny of tin"). In addition to polarity and low steric hindrance, radical attack at the relatively strong π-bond of isonitriles is assisted by "chameleonic" supramolecular interactions of the radical center with both the isonitrile π*-system and lone pair. These interactions are yet another manifestation of supramolecular control of radical chemistry.

Alkyne Origami: Folding Oligoalkynes into Polyaromatics

Do not bend the triple bonds! This familiar undergraduate mantra must be disobeyed if the alkyne group is used as a building block in molecular construction. This Account will describe our exploits in "alkyne origami", that is, folding oligoalkynes into new shapes via cyclization cascades. This research stems from a set of guidelines for the cyclizations of alkynes that we suggested in 2011 ( Gilmore Chem. Rev. 2011 , 111 , 6513 ; Alabugin J. Am. Chem. Soc. 2011 , 133 , 12608 ). The guidelines blended critical analysis of ∼40 years of experimental research with computations into the comprehensive predictions of the relative favorability of dig-cyclizations of anions and radicals. In this Account, we will show how this new understanding has been instrumental in building polyaromatics. In particular, we illustrate the utility of these stereoelectronic models by developing a toolbox of practical, selective, and efficient synthetic transformations. The high energy and high carbon content render alkynes the perfect precursors for the preparation of polyaromatic ribbons and other carbon-rich materials with precisely controlled structure and reactivity. Still, the paradox of alkyne reactivity (alkynes store a lot of energy but are protected kinetically by their relatively strong π-bonds) requires precise use of stereoelectronic factors for lowering the activation barriers for alkyne cyclizations. These factors are drastically different in the "all-exo" and the "all-endo" cyclization cascades of oligoynes. This Account will highlight the interplay between the stereoelectronics of bond formation and topology of acyclic precursor "folding" into a polycyclic ribbon. The topology of folding is simpler for the endo cascades, which are compatible with initiation either at the edge or at the center. In contrast, the exo cascades require precise folding of an oligoalkyne ribbon by starting the cascade exactly at the center of the chain. These differences define the key challenges in the design of these two types of alkyne cyclization cascades. For the endo processes, the folding is simple, but these processes require a strategy ("LUMO Umpolung") for inverting the usual stereoelectronic requirements of alkyne cyclizations. We also show how alkenes can be used as alkyne equivalents in cyclizations coupled with fragmentations and how one can make endo cyclization products without ever going through an endo cyclization. In contrast, each elementary step of the exo cascades benefits from the inherent exo preference for the radical attack, but these cascades require precise initiation by starting exactly at the central alkyne unit of the oligoyne. This strict selectivity requirement led to the development of traceless directing groups capable of supramolecular assistance to the initiation step and self-terminating departure at the end of the cascade. With attention to electronic effects that can stop radical cascades, oligoalkynes can be selectively converted into precisely shaped and functionalized polyaromatic products. The generality of these concepts is further illustrated by the development of radical "peri annulations" at the zigzag edge of acenes.

Unveiling the mechanisms and secrets of chemoselectivities in Au(i)-catalyzed diazo-based couplings with aryl unsaturated aliphatic alcohols

The mechanisms and origins of the unsaturated-aliphatic-alcohols-modulated chemoselectivity of Au-catalyzed carbene transfers from diazo compounds are disclosed by DFT studies.

Au(I)-Catalyzed Dimerization of Two Alkyne Units-Interplay between Butadienyl and Cyclopropenylmethyl Cation: Model Studies and Trapping Experiments

In recent years, Au(I)-catalyzed reactions proved to be a valuable tool for the synthesis of substituted cycles by cycloaromatization and cycloisomerization starting from alkynes. Despite the myriad of Au(I)-catalyzed reactions of alkynes, the mono Au(I)-catalyzed pendant to the radical dimerization of nonconjugated alkyne units has not been investigated by quantum chemical calculations. Herein, by means of quantum chemical calculations, we describe the mono Au(I)-catalyzed dimerization of two alkyne units as well as the transannular ring closure reaction of a nonconjugated diyne. We found that depending on the system and the method used either the corresponding cyclopropenylmethyl cation or the butadienyl cation represents the stable intermediate. This circumstance could be explained by different stabilizing effects. Moreover, the calculation reveals a dramatic (>10¹²-fold) acceleration of the Au(I)-catalyzed reaction compared to that of the noncatalyzed radical variant. Trapping experiments with a substituted 1,6-cyclodecadiyne using benzene as a solvent at room temperature as well as studies with deuterated solvents confirm the calculations. In this context, we also demonstrate that trapping of the cationic intermediate with benzene does not proceed via a Friedel-Crafts-type reaction.

Gold Redox Catalysis through Base-Initiated Diazonium Decomposition toward Alkene, Alkyne, and Allene Activation

The discovery of photoassisted diazonium activation toward gold(I) oxidation greatly extended the scope of gold redox catalysis by avoiding the use of a strong oxidant. Some practical issues that limit the application of this new type of chemistry are the relative low efficiency (long reaction time and low conversion) and the strict reaction condition control that is necessary (degassing and inert reaction environment). Herein, an alternative photofree condition has been developed through Lewis base induced diazonium activation. With this method, an unreactive AuI catalyst was used in combination with Na2 CO3 and diazonium salts to produce a AuIII intermediate. The efficient activation of various substrates, including alkyne, alkene and allene was achieved, followed by rapid AuIII reductive elimination, which yielded the C-C coupling products with good to excellent yields. Relative to the previously reported photoactivation method, our approach offered greater efficiency and versatility through faster reaction rates and broader reaction scope. Challenging substrates such as electron rich/neutral allenes, which could not be activated under the photoinitiation conditions (<5 % yield), could be activated to subsequently yield the desired coupling products in good to excellent yield.

Changing the path of least resistance, or access to endo-dig products via a sequence of three exo-trig transition states: electronic effects in homoallyic ring expansion cascades of alkenyl isonitriles

Substituent effects reshape the potential energy surfaces for radical homoallylic expansion/fragmentation cascades that transform alkenyl isonitriles into N-heteroaromatics