A Pd(0)/blue light catalyzed carboiodination reaction is reported. A simple, air-stable catalytic system, utilizing [Pd(allyl)Cl] 2 and DPEPhos, generated a variety of iodinated hetero-and carbocycles including oxindoles, dihydrobenzofurans, indolines, a chromane, and a tetrahydronaphthalene. This protocol was tolerant of sensitive functional groups including free carboxylic acids, phenols, and anilines, as well as pyridines, while delivering products in up to 94% yield. Support for a reversible C−I bond formation via a single electron mechanism was obtained using a deuterium labeled substrate and a stoichiometric neopentylpalladium species.T ransition-metal catalyzed halogenation protocols have often hinged on gaining a deeper understanding of the reversibility of carbon−halogen bond forming events. One powerful synthetic strategy that implements reversible C−X bond formation is the palladium-catalyzed carboiodination reaction, wherein halogenated heterocycles can be built from the intramolecular transfer of a C(sp 2 )−X group across a tethered π-system. 1−4 Traditionally, palladium catalyzed carboiodination reactions involve a 2-electron mechanistic cycle and are initiated by a ground-state Pd(0) catalyst undergoing an oxidative addition with an aryl halide. 1,2,4 Recently, the use of blue light in conjunction with palladium catalysis has offered a convenient route to access excited-state palladium species. 5−10 Unlike their thermal counterparts, photochemical reactions involving excited-state Pd(0) are typically thought to involve tandem palladium/radical intermediates. These reactions can mimic the reactivity of ground state palladium species, via a single electron mechanism in domino reactions. 6,8,10−13 Though the presence of radical intermediates has been well established, 5,6,14 a mechanism involving both discrete Pd(II) intermediates and alkyl radical/Pd(I) species has not been broadly explored.Thermally initiated carboiodination reactions typically employ a palladium catalyst bearing bulky electron-rich phosphines, or a nickel catalyst and a PPh 3 , P(OR) 3 or an N,N-ligand. 1,15−20 These reaction mechanism initiate via an oxidative addition to the C−I bond, followed by a migratory insertion across the π-system, and terminate with a ligandmediated 3-center 2-electron, 18 or an H-bonding initiated S N 2type reductive elimination. 21 The most successful conditions for the palladium carboiodination reactions utilize high temperatures and QPhos or t-Bu 3 P, costly and air-sensitive bulky phosphines (Scheme 1a). Often times, bulky 3°-amine

A simple and modular approach toward bis-heterocyclic spirocycles using palladium catalysis is reported. The enclosed methodology leverages a Mizoroki–Heck-type reaction to generate a neopentylpalladium species. This species can undergo intramolecular C–H activation on a wide array of (hetero)­aryl C–H bonds, generating a variety of [4.4] and [4.5] bis-heterocyclic spirocycles in up to 95% yield. A diverse range of bis-heterocyclic spirocycles were possible, with 24 examples and 18 different combinations of heterocycles were synthesized. Biologically relevant aza-heterocycles such as purine, pyrazole, (benz)­imidazole, (aza)­indole, and pyridine were readily incorporated into the spirocyclic core. The reaction was readily scalable to 1 mmol using a lower catalyst loading and number of base equivalents, and the product was purified without the use flash column chromatography.

DFT modelling predicts proximal boron groups can accelerate sulfenate alkylation reactions, depending on boron substituents and boron distance from the reaction site.

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