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We report an approach to the core of the manzamine alkaloid keramaphidin B that relies on the strain-promoted cycloaddition of an azacyclic allene with a pyrone trapping partner. The cycloaddition is tolerant of nitrile and primary amide functional groups and can be complemented with a subsequent retro-Diels-Alder step. These efforts demonstrate that strained cyclic allenes can be used to build significant structural complexity and should encourage further studies of these fleeting intermediates.

Introducing a tri-coordinate boron-based functional group (e.g., boronic ester) into an unactivated C-H bond in the absence of directing groups is an ongoing challenge in synthetic chemistry. Despite previous developments in transition metal-catalyzed and -free approaches, C-H borylation of sterically hindered arenes remains a largely unsolved problem to date. Here, we report a synthetic strategy of a two-step, precious metal-free electrophilic C-H borylation of sterically hindered alkyl- and haloarenes to generate aryl boronic esters. The first step relies on electrophilic aromatic substitution (EAS) induced by cage-opening of Cs 2 [closo-B 10 H 10 ], forming a 6-Ar-nido-B 10 H 13 product containing a B-C bond, followed by a cage deconstruction of arylated decaboranes promoted by diols. The combination of these two steps allows for the preparation of aryl boronic esters that are hardly accessible by current direct C-H borylation approaches. This reaction does not require any precious metals, highly-engineered ligands, pre-functionalized boron reagents, or inert conditions. In addition, the unique properties of a non-classical boron cluster electrophile intermediate, B 10 H 13 + , afford a regioselectivity with unique steric and electronic control without the undesirable side reactions., (© 2023. The Author(s).)

Sulfide oxidation is accomplished by a new class of dioxomolybdenum(VI) catalyst ( 1 ) that uses the tridentate 2,6-bis[hydroxyl(methyl)amino]-4-morpholino-1,3,5-triazine ligand to form a five-coordinate molybdenum(VI) center. Resonance Raman spectra show that the dioxo groups on the Mo(VI) oxygens readily exchange with water in an acetonitrile media that allows 18 O labeling of catalyst 1 . The model oxidation reaction was the conversion of thioanisole ( 2 ) to the corresponding sulfoxide with 4% of 1 using an equimolar amount of H 2 O 2 in MeCN- d 3 . Oxygen-18 labeling experiments with either 18 O-labeled 1 or 18 O-labeled H 2 O 2 are consistent with a sulfide oxygenation pathway that uses a η 1 -Mo(OOH) hydroxoperoxyl species ( 3 ). The hypothesized intermediate 3 is initially formed in a proton transfer reaction between 1 and H 2 O 2 . Oxidation is hypothesized via nucleophilic attack of the sulfide on 3 that is supported from a Hammett linear free-energy relationship for para-derivatives of 2 . A Hammett reactivity constant (ρ) of -1.2 ± 0.2 was obtained, which is consistent with other ρ values found in prior sulfide oxidation reactions by group 6 complexes. An Eyring plot of the 2 oxidation by 1 gives an E a of 63.0 ± 5.2 kJ/mol, which is slightly higher than that of a similar oxidation of 2 by the molybdenum(VI) complex, oxodiperoxo (pyridine-2-carboxylato)molybdate(VI) bis(pyridine-2-carboxylic acid) monohydrate ( 5 ). Computational modeling with density functional theory (DFT) of the complete reaction profile gave enthalpy and entropy of activations (64 kJ/mol and -120 J/mol·K, respectively) within 1 standard deviation of the experimental values, further supporting the hypothesized mechanism.