Hypervalent iodine (HVI) compounds are efficient reagents for the double oxidative dearomatization of electron-rich phenols to o-quinones. We recently reported that an underexplored class of iodine(V) reagents possessing bidentate bipyridine ligands, termed Bi(N)-HVIs, could dearomatize electron-poor phenols for the first time. To understand the fundamental mechanistic basis of this unique reactivity, density functional theory (DFT) was utilized. In this way, different pathways were explored to determine why Bi(N)-HVIs are capable of facilitating these challenging transformations while more traditional hypervalent species, such as 2-iodoxybenzoic acid (IBX), cannot. Our calculations reveal that the first redox process is the rate-determining step, the barrier of which hinges on the identity of the ligands bound to the iodine(V) center. This crucial process is composed of three steps: (a) ligand exchange, (b) hypervalent twist, and (c) reductive elimination. We found that strong coordinating ligands disfavor these elementary steps, and, for this reason, HVIs bearing such ligands cannot oxidize the electron-poor phenols. In contrast, the weakly coordinating triflate ligands in Bi(N)-HVIs allow for the kinetically favorable oxidation. It was identified that trapping in situ-generated triflic acid is a key role played by the bidentate bipyridine ligands in Bi(N)-HVIs as this serves to minimize the decomposition of the ortho-quinone product.

Hypervalent iodine­(V) reagents are a powerful class of organic oxidants. While the use of I­(V) compounds Dess–Martin periodinane and IBX is widespread, this reagent class has long been plagued by issues of solubility and stability. Extensive effort has been made for derivatizing these scaffolds to modulate reactivity and physical properties but considerable room for innovation still exists. Herein, we describe the preparation, thermal stability, optimized geometries, and synthetic utility of an emerging class of I­(V) reagents, Bi­(N)-HVIs, possessing datively bound bidentate nitrogen ligands on the iodine center. Bi­(N)-HVIs display favorable safety profiles, improved solubility, and comparable to superior oxidative reactivity relative to common I­(V) reagents. The highly modular synthesis and in situ generation of Bi­(N)-HVIs provides a novel and convenient screening platform for I­(V) reagent and reaction development.

Density functional theory calculations were used to investigate the mechanisms of established hydroalkylation reactions of styrenes with 1,3-diketones that are promoted by either AuCl 3 /AgOTf or AgOTf catalyst systems. In the former case, our studies led us to propose an original mechanism that is initiated by the generation of highly electrophilic Au(OTf) 3 , which then coordinates the enol tautomer of the 1,3-diketone substrate. The ensuing highly Brønsted acidic π-complex serves to protonate the styrene to generate a relatively low-energy benzylic carbocation. Notably, this suggests that this benzylic carbocation represents the true catalytic species in the reaction, and thus, the role of the gold complex is solely to generate this active catalyst. AuCl 3 alone does not serve as a good initiator for this process because it is not electrophilic enough to generate the relatively low-energy benzylic carbocation. Our investigation of the hydroalkylation facilitated by the slightly electron-deficient AgOTf catalyst revealed that an alternative mechanism predominates. Specifically, it is more likely that the reaction proceeds via a demetallation process directly mediated by the silver catalyst. We found a clear trend indicating that the electron deficiency of the metal center dictates which of these two mechanistic scenarios occurs. This article discusses these two mechanistic pathways in detail, providing key information for the experimental development of hydroalkylation processes.

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Hypervalent iodine(V) (HVI) compounds are highly efficient reagents for the double oxidative dearomatization of electron-rich phenols to o-quinones. We recently reported that an underexplored class of iodine(V) reagents possessing bidentate bipyridine ligands, termed Bi(N)-HVIs, could efficiently dearomatize electron-poor phenols for the first time. To better understand the fundamental mechanistic basis of this unique reactivity, density functional theory (DFT) was utilized. In this way, different pathways were explored to determine why Bi(N)-HVIs are capable of facilitating these challenging transformations while more traditional hypervalent species, such as IBX cannot. Our calculations reveal that the first redox process is the rate-determining step, the barrier of which hinges on the identity of the ligands bound to the iodine(V) center. This crucial process is composed of three steps: (a) ligand exchange, (b) hypervalent twist, and (c) reductive elimination. We found that strong coordinating ligands disfavour these elementary steps and, for this reason, HVIs bearing such ligands cannot oxidize the electron-poor phenols. In contrast, the weakly coordinating triflate ligands in Bi(N)-HVIs allow for the kinetically favorable oxidation of such phenols {e.g., G ‡ = ~22 kcal/mol where Bi(N) = Bi(4-CO2Etbipy)}. It was also identified that trapping triflic acid, which is generated in situ, is a key role played by the basic bidentate bipyridine ligands in Bi(N)-HVIs as this serves to minimize decomposition of the sensitive ortho-quinone product.