Acenes are attractive molecular platforms with characteristic π-electron delocalization, resulting in exceptionally small HOMO–LUMO gaps, which makes them valuable for use in a variety of organic electronics. Incorporating heteroatoms into the backbones of acenes enables systematic tuning of their electronic structures and introduces well-defined sites for redox and protonation chemistry. However, the redox chemistry of boron-doped higher acenes remains poorly understood. Herein, we report the synthesis, structural characterization, and multistate redox and protonation chemistry of a series of diboraheptacenes. Starting from tetrahydrodiboraheptacene (1), the corresponding radical anion (1^•–) was generated via one-electron reduction. The fully aromatized diborataheptacene dianion (2^2–) was accessed via double deprotonation, which represents the first diboron-doped π-isostere of all-hydrocarbon heptacene. Two-electron oxidation of 2^2– gave a neutral quinodimethane (2) that is isoelectronic with the heptacene dication, demonstrating redox-interconversion between benzenoid and quinoidal structures. Dianion 2^2–, isoelectronic to heptacene, features NIR absorption at λ_abs = 951 nm, reflecting its exceptionally small HOMO–LUMO gap. In addition, 2 shows remarkable low-energy electronic transitions, displaying red fluorescence (λ_em = 682 nm). Furthermore, reactions of 2^2– with CO₂ or acrylonitrile yield cycloaddition products, while the reaction with H₂O affords a hydroxylated diboraheptacene dianion. These results establish redox- and protonation-state-dependent electronic structures, photophysical properties, and reactivity of an extended diboraacene platform.

The Rh-catalyzed conversion of olefins and arenes to alkenyl arenes using [(η²-C₂H₄)₂Rh(μ-OPiv)]₂ as the catalyst precursor and 12 ortho- and para-substituted benzoquinone derivatives as the in situ oxidant is reported. Included are comparative studies of the quinone derivatives for (1) rate of styrene production from benzene and ethylene, (2) Markovnikov to anti-Markovnikov selectivity for reactions of benzene and propylene, and (3) ortho/meta/para selectivity when using tert-butylbenzene as the arene. Cyclic voltammetry was utilized to measure reduction potentials for each quinone to determine any possible influence of the quinone redox potential on arene alkenylation rate and selectivity. While significant differences in selectivity are observed between ortho-quinone derivatives, such differences are minimal when para-substituted quinones are utilized. These results suggest that ortho-benzoquinone derivatives likely serve as bidentate ligands, which explains the stronger influence on catalyst activity of ortho-benzoquinone identity compared to para-benzoquinones. Although ortho-benzoquinones generally give styrene production rates faster than those of para-benzoquinones, 3,5-di-tert-butyl-ortho-benzoquinone and ortho-chloranil react with ethylene to form bicyclo[2.2.2]oct-5-ene-2,3-dione derivatives as a significant side product.

We describe the synthesis and characterization of a Cu(I) complex, {Q₃Sb(o-chlor)}Cu(OTf) (Q = 8-quinolinyl; OTf = trifluoromethanesulfonate; o-chlor = o-choranil), supported by the Sb(V) ligand Q₃Sb(o-chlor). The complex {Q₃Sb(o-chlor)}Cu(OTf) was experimentally characterized via ¹H, ¹³C{¹H}, and ¹⁹F{¹H} NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) as well as examined computationally with density functional theory (DFT) calculations. Variable temperature ¹H NMR spectroscopy (20 to −110 °C) indicates temperature-dependent fluxional processes for {Q₃Sb(o-chlor)}Cu(OTf) and uncoordinated Q₃Sb(o-chlor). The electron density of Cu for {Q₃Sb(o-chlor)}Cu(OTf) was probed by comparing Cu^II/Cu^I redox potential and Cu 2p electron binding energies, using XPS, with a related non-Sb-containing complex, (TMQA)Cu(OTf) (TMQA = tris(quinolin-2-ylmethyl)amine). The E_1/2 of the Cu^II/Cu^I redox of {Q₃Sb(o-chlor)}Cu(OTf) is shifted 670 mV more positive than that of (TMQA)Cu(OTf). XPS spectra of {Q₃Sb(o-chlor)}Cu(OTf) and (TMQA)Cu(OTf) indicate a 0.8 eV higher Cu 2p binding energy for {Q₃Sb(o-chlor)}Cu(OTf). Computational studies of the molecular orbitals and localized natural bonding orbitals (NBOs) are consistent with a weak Cu(I) → Sb(V) interaction for {Q₃Sb(o-chlor)}Cu(OTf), for which Sb(V) acts as a Z-type ligand.

Prior work with the dearomatization agent [WTp(NO)(PMe₃)] (Tp = trispyrazolylborate) has demonstrated the ability to synthesize cis, cis−3,4,6 trisubstituted cyclohexenes from phenyl sulfones. Herein, the compatibility of these methods with sulfur nucleophiles was investigated for the synthesis of sp³-enriched thioether- and thioacetate-functionalized cyclohexenes. Protonation of the WTp(NO)(PMe₃)(η²-PhSO₂Me) complex followed by addition of a sulfur nucleophile was only successful for a thioacetate salt. However, when the first nucleophile was an ester enolate, subsequent protonation and nucleophilic addition successfully yielded a range of cyclohexene complexes with thioacetate, thiol, and sulfonyl substituents. Oxidative decomplexation of these complexes liberated the corresponding cyclohexene products. This strategy was then applied to the synthesis of a thioglycolate ester functionalized tetrahydrophenanthridinone.

Heteropolycyclic frameworks are widely represented in biologically and pharmaceutically relevant compounds; however, methods to synthesize these frameworks often result in heterocycles containing predominantly sp²-hybridized carbons. Herein we describe a heteroannulation scheme featuring a double protonation of a tungsten η²-anisole complex. The resulting dicationic intermediate reacts with activated arenes through an electrophilic aromatic substitution reaction to form an oxocarbenium complex, which can be reduced to an allylic ether complex. Subsequent acidolysis results in a π-allyl complex that can react with alcohol or amine substituents of the activated arene reagent to form the desired heteropolycyclic core.

cis-Tetrahydro-2-oxindoles are valuable scaffolds in medicinal chemistry. Herein, we report a transition-metal-mediated dearomatization strategy to access these compounds. The process begins with the coordination of a phenyl sulfone by a tungsten complex designed to bind two carbons of the phenyl ring, rendering it dearomatized. This is followed by protonation of the η²-bound arene followed by the addition of an ester nucleophile. The resulting η²-diene complex then undergoes a second protonation in the ring, and a primary amine is introduced. Dihydro-2-oxindole complexes form spontaneously through the construction of a γ-lactam and the elimination of a sulfinic acid. Dihydro-2-oxindoles are practically unknown─presumably owing to their ability to form indolines─but coordination to tungsten stabilizes these intermediates. The terminal position of the coordinated diene (C4 of the 2-oxindole core) can then be protonated to generate an η²-allyl complex, which undergoes nucleophilic addition with C-, N-, or S-type nucleophiles to form the corresponding tetrahydro-2-oxindole complexes. Finally, the organic ligand is obtained through the oxidative decomplexation of the metal. This methodology provides a modular approach for accessing 1,2,5-functionalized cis-2-oxindole compounds.