Expanding the Clip-and-Cleave Concept: Approaching Enantioselective C-H Hydroxylations by Copper Imine Complexes Using O2 and H2O2 as Oxidants.

Copper-mediated aromatic and aliphatic C-H hydroxylations using benign oxidants (O2 and H2O2) have been studied intensively in recent years to meet the growing demand for efficient and green C-H functionalizations. Herein, we report an enantioselective variant of the so-called clip-and-cleave concept for intramolecular ligand hydroxylations by the application of chiral diamines as directing groups. We tested the hydroxylation of cyclohexanone and 1-acetyladamantane under different oxidative conditions (CuI/O2; CuI/H2O2; CuII/H2O2) in various solvents. As an outstanding example, we obtained (R)-1-acetyl-2-adamantol with a yield of 37% and >99:1 enantiomeric excess from hydroxylation in acetone using CuI and O2. Low-temperature stopped-flow UV-vis measurements in combination with density functional theory (DFT) computations revealed that the hydroxylation proceeds via a bis(µ-oxido) dicopper intermediate. The reaction product represents a rare example of an enantiopure 1,2-difunctionalized adamantane derivative, which paves the way for potential pharmacological studies. Furthermore, we found that 1-acetyladamantane can be hydroxylated in a one-pot reaction under air with isolated yields up to 36% and enantiomeric ratios of 96:4 using CuII/H2O2 in MeOH.

Structural Snapshots, Trajectory and Thermodynamics of the Reversible µ-1,2-Peroxo/µ-1,1-Hydroperoxo Dicopper(II) Interconversion.

Hydrogen bonds involving the oxygen atoms of intermediates that result from copper-mediated O2 activation play a key role for controlling the reactivity of Cux/O2 active sites in metalloenzymes and synthetic model complexes. However, structural insight into H-bonding in such transient species as well as thermodynamic information about proton transfer to or from the O2-derived ligands is scarce. Here we present a detailed study of the reversible interconversion of a µ1,2-peroxodicopper(II) complex ([1]+) and its µ1,1-hydroperoxo congener ([2]+) via (de)protonation, including the isolation and structural characterization of several H-bond donor (HBD) adducts of [1]+ and the determination of binding constants. For one of these adducts a temperature-dependent µ1,2-peroxo/µ1,1-hydroperoxo equilibrium associated with reversible H+-translocation is observed, its thermodynamics investigated experimentally and computationally, and effects of H-bonding on spectroscopic parameters of the CuII2(µ1,2-O2) species are revealed. DFT calculations allowed to fully map and correlate the trajectories of H+-transfer and µ1,2-peroxo→µ1,1-peroxo rearrangement. These findings enhance our understanding of two key intermediates in bioinspired Cu2/O2 chemistry.

Rhenium-Mediated Conversion of Dinitrogen and Nitric Oxide to Nitrous Oxide.

Reductive splitting of N2 is an attractive strategy towards nitrogen fixation beyond ammonia at ambient conditions. However, the resulting nitride complexes often suffer from thermodynamic overstabilization hampering functionalization. Furthermore, oxidative nitrogen atom transfer of N2 derived nitrides remains unknown. We here report a ReIV pincer platform that mediates N2 splitting upon chemical reduction or electrolysis with unprecedented yield. The N2 derived ReV nitrides undergo facile nitrogen atom transfer to nitric oxide, giving nitrous oxide nearly quantitatively. Experimental and computational results indicate that outer-sphere ReN/NO radical coupling is facilitated by the activation of the nitride via initial coordination of NO.

Triplet carbenes with transition-metal substituents.

The extraordinary advances in carbene (R1-C-R2) chemistry have been fuelled by strategies to stabilize the electronic singlet state via π interactions. In contrast, the lack of similarly efficient approaches to obtain authentic triplet carbenes with appreciable lifetimes beyond cryogenic temperatures hampers their exploitation in synthesis and catalysis. Transition-metal substitution represents a potential strategy, but metallocarbenes (M-C-R) usually represent high-lying excited electronic configurations of the well-established carbyne complexes (M≡C-R). Here we report the synthesis and characterization of triplet metallocarbenes (M-C-SiMe3, M = PdII, PtII) that are persistent beyond cryogenic conditions, and their selective reactivity towards carbene C-H insertion and carbonylation. Bond analysis reveals significant stabilization by spin-polarized push-pull interactions along both π-bonding planes, which fundamentally differs from bonding in push-pull singlet carbenes. This bonding model, thus, expands key strategies for stabilizing the open-shell carbene electromers and closes a conceptual gap towards carbyne complexes.