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.

Despite advances in reactions such as hydrogen isotope exchange (HIE) and reductive deuteration, achieving controlled and selective deuteration remains challenging. Moreover, the difficulty of developing successful deuteration platforms is compounded by a lack of means to assess the stereoisotopic purity of deuterated products. We previously reported a highly regio- and stereoselective approach for generating semideuterated cyclohexenes via tandem protonation (H⁺/D⁺) and reduction (H^–/D^–) sequences of a dihapto-coordinate tungsten-benzene complex. While NMR and HRMS analyses suggested successful deuterium incorporation, molecular rotational resonance (MRR) spectroscopy identified numerous over-, under-, and mis-deuteration impurities. At the time of publication, these impurities were attributed to H/D scrambling that could occur during thermolysis of the tungsten-bound cyclohexene ligand prior to MRR analysis. In this work, we describe the analysis of semideuterated cyclohexenes using MRR spectroscopy with an improved thermolysis apparatus that eliminates deuterium scrambling during analysis. Quantitative analysis of both racemic and enantiopure samples enables the optimization of deuteration conditions by providing multiple mechanistic insights into the formation of impurities.

Once considered a “laboratory curiosity”, cyclic borenium ions have recently been shown to exhibit tunable emission and stimuli-responsive properties. Utilizing hexaphenylcarbodiphosphorane (CDP), a series of borafluorenium and 3,3′-dimethoxyborafluorenium ions were synthesized to determine the impacts of counteranion and substituent effects on the optical properties and stability of borafluorenium ions. Clear relationships were established between structure and properties, with emission wavelengths of the borafluorenium ions ranging from yellow (λ_em = 559 nm) to red (λ_em = 650 nm). By employing density functional theory, a possible mechanism for the observed luminescent behavior was proposed. These compounds were shown to exhibit aggregation-induced emission (AIE) properties and significant changes in solid-state emission color when compared to those observed in solution. The AIE properties of a CDP–borafluorenium ion with dimethoxy substitution were further explored by perturbing the temperature in solution, which resulted in a clear shift in emission wavelength from 563 nm (yellow, 20 °C) to 513 nm (green, −90 °C).

The Zincke reaction combines a pyridinium salt bearing an N-withdrawing group and a primary aliphatic amine to form an alkylated pyridinium salt through a ring-opening/ring-closing sequence. Herein, we explore the analogous reaction sequence for a pyridinium salt η²-bound to a transition metal. We find that the N-sulfonylated pyridinium ligand (pyR¹, where R¹ = mesyl or tosyl) of [WTp(NO)(PMe₃)(η²-pyR¹)]OTf selectively reacts with a primary amine, and the resulting 2-aminodihydropyridine complex then undergoes a tungsten-stabilized ring-scission to form the corresponding η²-azatriene complex. Subsequent ring-closure between the newly installed amine and the sulfonylated imine results in a new aminodihydropyridinium species. This dihydropyridinium resists rearomatization due to a stabilizing influence of the tungsten fragment. Subsequent displacement of the sulfonamide by pendent heteroatoms leads to the formation of new heterocyclic frameworks. Herein the syntheses of 30 heterocyclic complexes are described (3 characterized by SC-XRD) including 7 examples of multicyclic systems.