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

Abstract

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.

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

“…3B). The near-perfect alignment of σ*(C3-O) and σ(C2-Au) at the second transition state (TS2), which manifests as an inflection on the PES, corroborates this phenomenon [39][40][41][42][43] . Alternatively, the Cope rearrangement mediated by the FLPs can be interrupted by a zwitterionic intermediate in which the boryl group holds onto the electron density it acquired, thus slowing down or aborting σ(C3-C4) bond scission (Fig.…”

Metal‐Free Photochemical Olefin Isomerization of Unsaturated Ketones via 1,5‐Hydrogen Atom Transfer

“…A possible explanation for the observed final stereoselectivity can be the existence of secondary orbital interactions during the stereodetermining 1,5‐hydrogen atom rearrangement. While the more stable E isomer is obtained when the aromatic group is pointing away from the carbonyl group (preventing secondary orbital interactions), the more energetic Z isomer requires that the aromatic group be closer to the carbonyl; thus, this interaction could explain the unusual stereoselectivity observed in the process [9f] …”

Zwitterionic Bergman-Triggered Cascade Polymerization: A Plateau Reaction Leading to Metal-Graphene Nanoribbon Semiconductors Using Boron-Metal Couple

“…In these cases, the second transition state (TS2) revealed only as an inflection due to a near-perfect alignment of *(C3-O) and (C2-Au). 39,40,41,42,43 On the other hand, the FLP mediated Cope rearrangement is interrupted by a zwitterionic intermediate in which the boronyl group at C5 resists giving up the newly acquired electron density and delays (C3-C4) bond cleavage (Figure 3, C). This transient shift of electron density from donor to acceptor becomes permanent and the Cope rearrangement is aborted when the boronyl groups are substituted with strong acceptors that prevent (C3-C4) bond cleavage (Figure 3, D).…”