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Geminal hyperconjugation is a key electronic effect that modulates bond distances, barrier widths, and thermochemical driving forces in heavy-atom tunneling reactions involving opening and closing three-membered rings. 

Cation tuning is a simple yet powerful strategy to modulate the reactivity of polymerization catalysts but the design rules to achieve maximum cation effects are not well understood. In the present work, it was demonstrated that inserting a methylene spacer between a nickel phenoxyimine complex and an M-polyethylene glycol (PEG) (where M = Li+, Na+, K+, or Cs+) unit led up to >70-fold increase in ethylene polymerization activity and 6-fold higher polymer molecular weight relative to that of the first-generation catalysts. It is hypothesized that these effects are due to the exclusive formation of 1:1 over 2:1 nick-el:alkali species and closer proximity of the M-PEG moiety to the nickel center. These results suggest that the successful creation of cation-responsive catalysts requires an understanding of the cation binding stoichiometry as well as the structural and electronic changes associated with its host-guest interactions. 

We present a six-step cascade that converts 1,3-distyrylbenzenes (bis-stilbenes) into nonsymmetric pyrenes in 40–60% yields. This sequence merges photochemical steps, E,Z-alkene isomerization, a 6π photochemical electrocyclization (Mallory photocyclization); the new bay region cyclization, with two radical iodine-mediated aromatization steps; and an optional aryl migration. This work illustrates how the inherent challenges of engineering excited state reactivity can be addressed by logical design. An unusual aspect of this cascade is that the same photochemical process (the Mallory reaction) is first promoted and then blocked in different stages within a photochemical cascade. The use of blocking groups is the key feature that makes simple bis-stilbenes suitable substrates for directed double cyclization. While the first stilbene subunit undergoes a classic Mallory photocyclization to form a phenanthrene intermediate, the next ring-forming step is diverted from the conventional Mallory path into a photocyclization of the remaining alkene at the phenanthrene’s bay region. Although earlier literature suggested that this reaction is unfavorable, we achieved this diversion via incorporation of blocking groups to prevent the Mallory photocyclization. The two photocyclizations are assisted by the relief of the excited state antiaromaticity. Reaction selectivity is controlled by substituent effects and the interplay between photochemical and radical reactivity. Furthermore, the introduction of donor substituents at the pendant styrene group can further extend this photochemical cascade through a radical 1,2-aryl migration. Rich photophysical and supramolecular properties of the newly substituted pyrenes illustrate the role of systematic variations in the structure of this classic chromophore for excited state engineering. 

Abstract Asymmetric catalysis is an advanced area of chemical synthesis, but the handling of abundantly available, purely aliphatic hydrocarbons has proven to be challenging. Typically, heteroatoms or aromatic substructures are required in the substrates and reagents to facilitate an efficient interaction with the chiral catalyst. Confined acids have recently been introduced as tools for homogenous asymmetric catalysis, specifically to enable the processing of small unbiased substrates¹. However, asymmetric reactions in which both substrate and product are purely aliphatic hydrocarbons have not previously been catalysed by such super strong and confined acids. We describe here an imidodiphosphorimidate-catalysed asymmetric Wagner–Meerwein shift of aliphatic alkenyl cycloalkanes to cycloalkenes with excellent regio- and enantioselectivity. Despite their long history and high relevance for chemical synthesis and biosynthesis, Wagner–Meerwein reactions utilizing purely aliphatic hydrocarbons, such as those originally reported by Wagner and Meerwein, had previously eluded asymmetric catalysis. 

Substrates engineered to undergo a 1,4-C–H insertion to yield benzocyclobutenes resulted in a novel elimination reaction to yieldortho-quinone dimethide (o-QDM) products that undergo Diels–Alder or hetero-Diels–Alder cycloadditions.