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.

[1,5]-Sigmatropic Shifts Regulated by Built-in Frustration.

Conceptually, many organic reactions involve a flow of electron density from electron-rich to electron-poor regions. When the direct flow of electron density is blocked, the innate "frustration" can provide a driving force for a reaction that removes the blockade. Herein, we show how this idea can be used for the design of molecular rearrangements promoted by remotely placed donor-acceptor pair of substituents. We evaluate effects of such "frustration" on the rates of competing [1,5]-hydrogen and [1,5]-halogen shifts in boron-substituted 1,3-pentadienes. As the sp3 hybridized carbon (C1) in these dienes interrupts the conjugation path between the donor to the acceptor, the system conceptually resembles a frustrated Lewis pair (FLP). Frustration is weakened when the formation of a new chemical bond in the TS opens communication between electron-rich and -poor regions and is removed completely when the resonance interaction between donor and acceptor develops fully in the rearranged product. Such relief of chemical frustration is directly translated into more favorable thermodynamic driving force and decreased intrinsic activation energies. Marcus theory separates thermodynamic contribution to the activation barriers and suggests that the electronic communication between electron rich and poor regions lowers the activation barrier via the formation of stabilizing 3-center contacts in the TS. Dramatic TS stabilization illustrates that the migrating groups function as an electronic relay between migration origin and terminus with properties fine-tuned by the boronyl acceptor. The combined effects of the C-X bond strength (X = migrating group), Lewis acidities of the acceptors, thermodynamic driving forces, and secondary orbital interactions control the observed barrier trends and selectivity of migration.

Controlled Evolution of the Cope Rearrangement: Transition from Concerted to Interrupted and Aborted Pericyclic Reactions Regulated by a Switch Built from an Intramolecular Frustrated Lewis Pair.

We show how synergy between properly placed acceptor and donor groups allows rational design of interrupted and aborted pericyclic reactions, using the Cope rearrangement as a model process. When placed at C2 and C5 carbons of 1,5-hexadienes, a frustrated Lewis pair made of two complementary groups assures that the C-C bond formation is assisted by the flow of electron density from a donor at C5 into an acceptor at C2 through the formation of the C1-C6 bond. If the electron excess at the accepting group is strongly stabilized by the electronic nature of substituents, the pericyclic transition state can become an energy minimum, leading to a switch from a concerted sigmatropic shift to its aborted or interrupted versions. Depending on the electronic nature of acceptors at C5 and the donor groups at C2, a range of possibilities from concerted to aborted pathways is accessible, including the first example of an aborted Cope rearrangement in the absence of a metal catalyst. Furthermore, the zwitter-ionic strategy stabilizes the usually unfavorable boat TS that can potentially evolve into rarely accessible bicyclohexane products.

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

Pericyclic reactions bypass high-energy reactive intermediates by synchronizing bond formation and bond cleavage. The present work offers two strategies for uncoupling these two processes and converting concerted processes into their "interrupted" versions by combining Au(I) catalysis with electronic and stereoelectronic factors. First, we show how the alignment of the C3-C4 bond with the adjacent π systems can control the reactivity and how the concerted scission of the central σ bond is prevented in the boat conformation. Second, the introduction of a fluorine atom at C3 also interrupts the sigmatropic shift and changes the rate-determining step of the interrupted cascade from the 6-endo-dig nucleophilic attack to the fragmentation of the central C3-C4 bond. Furthermore, this effect strongly depends on the relative orientation of the C-F bond toward the developing cationic center. The equatorial C-F bond has a much greater destabilizing effect on TS2 due to the more efficient through-bond interaction between the acceptor and the cationic π system. In contrast, the axial C-F bond is not aligned with the bridging C-C bonds and does not impose an equally strong deactivating stereoelectronic effect. These differences illustrate that the competition between concerted and interrupted pericyclic pathways can be finely tuned via a combination of structural and electronic effects modulated by conformational equilibria. The combination of Au(I) catalysis and C-F-mediated stereoelectronic gating delays the central bond scission, opening access to the interrupted Cope rearrangements and expanding the scope of this classic reaction to the design of new cascade transformations.

Proximity effect: insight into the fundamental forces governing chemical reactivity of aromatic systems.

The analysis of different layers of proximity effect in ortho-substituted aromatic compounds, using a DFT-level study, is reported. Polar and steric components of the proximity effect have been partitioned by applying multivariate regression analysis to an unusual six-electron heteroelectrocyclic reaction of the ortho-substituted nitrosostyrenes. The two pathways, 1,5- and 1,6-cyclizations, emanating from these substrates result into zwitterionic five-membered and neutral six-membered rings, respectively. The substituents at position 1, which are adjacent to the polar nitroso group, influenced the barrier primarily through electronic effect. Furthermore, a mechanistic shift from the 1,5 to 1,6 pathway, for certain substrates, is explained by the electronic repulsion. In contrast to position 1, the substituents on position 4 stereoelectronically interacted with a bulkier alkene moiety. Furthermore, unlike position 1, the position-4-substituted substrates are predicted to give only 1,5 products. A comparison of the two ortho positions with position 2, which is meta to the nitroso and para to the alkene, revealed an intriguing relationship between various electronic factors.

Rh(I)-catalyzed transformation of propargyl vinyl ethers into (E,Z)-dienals: stereoelectronic role of trans effect in a metal-mediated pericyclic process and a shift from homogeneous to heterogeneous catalysis during a one-pot reaction.

The combination of experiments and computations reveals unusual features of stereoselective Rh(I)-catalyzed transformation of propargyl vinyl ethers into (E,Z)-dienals. The first step, the conversion of propargyl vinyl ethers into allene aldehydes, proceeds under homogeneous conditions via a "cyclization-mediated" mechanism initiated by Rh(I) coordination at the alkyne. This path agrees well with the small experimental effects of substituents on the carbinol carbon. The key feature revealed by the computational study is the stereoelectronic effect of the ligand arrangement at the catalytic center. The rearrangement barriers significantly decrease due to the greater transfer of electron density from the catalytic metal center to the CO ligand oriented trans to the alkyne. This effect increases electrophilicity of the metal and lowers the calculated barriers by 9.0 kcal/mol. Subsequent evolution of the catalyst leads to the in situ formation of Rh(I) nanoclusters that catalyze stereoselective tautomerization. The intermediacy of heterogeneous catalysis by nanoclusters was confirmed by mercury poisoning, temperature-dependent sigmoidal kinetic curves, and dynamic light scattering. The combination of experiments and computations suggests that the initially formed allene-aldehyde product assists in the transformation of a homogeneous catalyst (or "a cocktail of catalysts") into nanoclusters, which in turn catalyze and control the stereochemistry of subsequent transformations.

Stereocontrolled synthesis of (E,Z)-dienals via tandem Rh(I)-catalyzed rearrangement of propargyl vinyl ethers.

A novel Rh(I)-catalyzed approach to functionalized (E,Z) dienals has been developed via tandem transformation where a stereoselective hydrogen transfer follows a propargyl Claisen rearrangement. Z-Stereochemistry of the first double bond suggests the involvement of a six-membered cyclic intermediate whereas the E-stereochemistry of the second double bond stems from the subsequent protodemetalation step giving an (E,Z)-dienal.

Overriding the alkynophilicity of gold: catalytic pathways from higher energy Au(I)-substrate complexes and reactant deactivation via unproductive complexation in the gold(I)-catalyzed propargyl Claisen rearrangement.

Computational and experimental analysis of unusual substituent effects in the Au-catalyzed propargyl Claisen rearrangement revealed new features important for the future development of Au(I) catalysis. Despite the higher stability of Au-alkyne complexes, they do not always correspond to the catalytically active compounds. Instead, the product emanates from the higher energy Au(I)-oxygen complex reacting via a low barrier cation-accelerated oxonia Claisen pathway. Additionally, both intra and intermolecular competition from other Lewis bases present in the system, for the Au(I) catalyst, can lead to unproductive stabilization of the substrate/catalyst complex, explaining hitherto unresolved substituent effects.

Gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers: missing transition states revealed through evolution of aromaticity, Au(I) as an oxophilic Lewis acid, and lower energy barriers from a high energy complex.

Curtin-Hammett analysis of four alternative mechanisms of the gold(I)-catalyzed [3,3] sigmatropic rearrangement of allenyl vinyl ethers by density functional theory calculations reveals that the lowest energy pathway (cation-accelerated oxonia Claisen rearrangement) originates from the second most stable of the four Au(I)-substrate complexes in which gold(I) coordinates to the lone pair of oxygen. This pathway proceeds via a dissociative transition state where the C-O bond cleavage precedes C1-C6 bond formation. The alternative Au(I) coordination at the vinyl π-system produces a more stable but less reactive complex. The two least stable modes of coordination at the allenyl π-system display reactivity that is intermediate between that of the Au(I)-oxygen and the Au(I)-vinyl ether complexes. The unusual electronic features of the four potential energy surfaces (PESs) associated with the four possible mechanisms were probed with intrinsic reaction coordinate calculations in conjunction with nucleus independent chemical shift (NICS(0)) evaluation of aromaticity of the transient structures. The development of aromatic character along the "6-endo" reaction path is modulated via Au-complexation to the extent where both the cyclic intermediate and the associated fragmentation transition state do not correspond to stationary points at the reaction potential energy surface. This analysis explains why the calculated PES for cyclization promoted by coordination of gold(I) to allenyl moiety lacks a discernible intermediate despite proceeding via a highly asynchronous transition state with characteristics of a stepwise "cyclization-mediated" process. Although reaction barriers can be strongly modified by aryl substituents of varying electronic demand, direct comparison of experimental and computational substituent effects is complicated by formation of Au-complexes with the Lewis-basic sites of the substrates.

Gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers; synthesis of substituted 1,3-dienes.

Synthesis of substituted 1,3-dienes was achieved via gold(I)-catalyzed Claisen rearrangement of allenyl vinyl ethers. The N-heterocyclic carbene gold chloride catalyst (IPrAuCl) was superior in terms of activity and selectivity and afforded the 3,3-product in excellent yields. A proposed cation-π inter-action played a significant role in affecting the reaction rate.

Solvent controlled mechanistic dichotomy in a Au(III)-catalyzed, heterocyclization triggered, Nazarov reaction.

Tandem Au(III)-catalyzed heterocyclization/Nazarov cyclizations leading to substituted carbocycle fused furans are described. An interesting dichotomy of reaction pathways as a function of solvent, confirmed by the isolation and trapping of reaction intermediates, provided a basis for computational studies that supported the experimental findings.