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Abstract Traditionally, cross‐dehydrogenative coupling (CDC) leads to C−N bond formation under basic and oxidative conditions and is proposed to proceed via a two‐electron bond formation mediated by carbenium ions. However, the formation of such high‐energy intermediates is only possible in the presence of strong oxidants, which may lead to undesired side reactions and poor functional group tolerance. In this work we explore if oxidation under basic conditions allows the formation of three‐electron bonds (resulting in “upconverted” highly‐reducing radical‐anions). The benefit of this “upconversion” process is in the ability to use milder oxidants (e. g., O₂) and to avoid high‐energy intermediates. Comparison of the two‐ and three‐electron pathways using quantum mechanical calculations reveals that not only does the absence of a strong oxidant shut down two‐electron pathways in favor of a three‐electron path but, paradoxically, weaker oxidants react faster with the upconverted reductants by avoiding the inverted Marcus region for electron transfer. 

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. 

Bicyclo[3.2.1]oct-2-yne was generated from the Fritsch–Buttenberg–Wiechell rearrangement of 2-norbornylidene carbene. The rearrangement preferentially involves migration of a tertiary carbon over a secondary carbon, a trend that contrasts with rearrangements of acyclic carbenes and which may be attributable to hyperconjugative effects promoted by the bridged structure of the carbene. 

Metal-ion-linked molecular multilayers on metal oxide surfaces are promising for applications ranging from solar energy conversion to sensing. Most of these applications rely on energy and electron transfer between layers/molecules which can be envisioned to occur via intra-assembly (IA; between metal-ion-linked molecules) and interlayer (IL; between separate layers of nonlinked molecules) processes. Here, we describe our effort to differentiate between IL and IA energy transfer using a bilayer composed of ZrO2, a phosphonated anthracene derivative (A), a zinc(II) linking ion, and a Pt(II)porphyrin (P). Both time-resolved emission and transient absorption measurements show no impact of diluting the anthracene layer with a spectroscopically inert spacer on the rate of 1A* to P and 3P* to A, singlet, and triplet energy transfer, respectively. These results indicate that energy transfer within the metal-ion-linked assembly (i.e., ZrO2-A–Zn-P) is more rapid than with an adjacent, nonlinked A molecule, even for a P derivative capable of laying down on the surface. These insights are an important step toward structural design principles maximizing the efficiency/rate of energy transfer in multilayer assemblies. 

Although photoredox catalysis is complex from a mechanistic point of view, it is also often surprisingly efficient. In fact, the quantum efficiency of a puzzlingly large portion of photoredox reactions exceeds 100% (i.e., the measured quantum yields (QYs) are >1). Hence, these photoredox reactions can be more than perfect with respect to photon utilization. In several documented cases, a single absorbed photon can lead to the formation of >100 molecules of the product, behavior known to originate from chain processes. In this Perspective, we explore the underlying reasons for this efficiency, identify the nature of common catalytic chains, and highlight the differences between HAT and SET chains. Our goal is to show why chains are especially important in photoredox catalysis and where the thermodynamic driving force that sustains the SET catalytic cycles comes from. We demonstrate how the interplay of polar and radical processes can activate hidden catalytic pathways mediated by electron and hole transfer (i.e., electron and hole catalysis). Furthermore, we illustrate how the phenomenon of redox upconversion serves as a thermodynamic precondition for electron and hole catalysis. After discussing representative mechanistic puzzles, we analyze the most common bond forming steps, where redox upconversion frequently occurs (and is sometimes unavoidable). In particular, we highlight the importance of 2-center-3-electron bonds as a recurring motif that allows a rational chemical approach to the design of redox upconversion processes. 

Here, we present a new approach for the activation of donor–acceptor cyclopropanes in ring-opening reactions, which does not require the use of a Lewis or Brønsted acid as a catalyst. Donor–acceptor cyclopropanes containing a phenolic group as the donor undergo deprotonation and isomerization to form the corresponding quinone methides. This innovative strategy was applied to achieve (4 + 1)-annulation of cyclopropanes with sulfur ylides, affording functionalized dihydrobenzofurans. Additionally, the generated ortho- and para-(aza)quinone methides can be trapped by various CH-acids. 

Chiral oxygen-containing heterocyclic compounds are of great interest for the development of pharmaceuticals. Monoterpenes and their derivatives are naturally abundant precursors of novel synthetic chiral oxygen-containing heterocyclic compounds. In this study, acid catalyzed reactions of salicylic aldehydes with (−)-8-acetoxy-6-hydroxymethyllimonene, readily accessible from α-pinene, leads to the formation of chiral polycyclic products of various structural types. Three of the six isolated chiral heterocyclic products obtained from salicylic aldehyde contain previously unknown polycyclic ring types. Having carried out the reaction in the presence of Brønsted or Lewis acids (Amberlyst 15, trifluoromethanesulfonic acid, trifluoroacetic acid and boron trifluoride etherate) or aluminosilicates (montmorillonite K10, halloysite nanotubes), we found that the nature of products depends on the catalyst as well as the reaction conditions (reaction time, reactant ratio, presence or absence of solvent). Detailed mechanistic insight on the complex cascade reactions for product formation is provided with extensive experimental and quantum mechanical computational studies. 

Iodocyclization of Ar-iodoacetylenes yields 2,3-diiodoheteroindene substrates for C2-regioselective Sonogashira cross-couplings. The high selectivity observed for diiodobenzothiophene allows the one-pot synthesis of unsymmetrical enediynes.