Unlocking the Potential of Phenacyl Protecting Groups: CO2-Based Formation and Photocatalytic Release of Caged Amines

Silver-Catalyzed Coupling of Ethynylbenziodoxolones with CO2 and Amines to Afford O-β-Oxoalkyl Carbamates

A novel three-component coupling reaction of ethynylbenziodoxolones (EBXs) with CO2 and amines has been achieved via silver catalysis, thereby providing an efficient method for the construction of a range of structurally diverse and valuable O-β-oxoalkyl carbamates. The transformation proceeds under mild reaction conditions and exhibits a wide substrate scope and good functional group compatibility. In addition, this strategy could be extended to the synthesis of α-acyloxyketones using carboxylic acids as the nucleophiles to react with EBXs.

Solution-Phase Late-Stage Chemoselective Photocatalytic Removal of Sulfonyl and Phenacyl Groups in Peptides

Herein, BPC catalyzed visible-light-triggered target-specific late-stage solution phase desulfonylation from tryptophan in oligopeptides is portrayed by overcoming the isolation issue up to octamers. This robust and mild method is highly predictable and chemoselective, tolerating myriad of functional groups in aza-heteroaromatics and peptides. Interestingly, reductive desulfonylation is also amenable to biologically significant reactive histidine and tyrosine side chains, signifying the versatility of the strategy. Additional efficacy of BPC is demonstrated by solution phase phenacyl deprotection from C-terminal in peptides. Furthermore, excellent catalyst loading of 0.5 mol% and recyclability demonstrate the practical utility and applicability of this strategy.

Halogen-atom and group transfer reactivity enabled by hydrogen tunneling

The generation of carbon radicals by halogen-atom and group transfer reactions is generally achieved using tin and silicon reagents that maximize the interplay of enthalpic (thermodynamic) and polar (kinetic) effects. In this work, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunneling that uses the cyclohexadiene derivative γ-terpinene as the abstractor under mild photochemical conditions. This protocol activates alkyl and aryl halides as well as several alcohol and thiol derivatives. Experimental and computational studies unveiled a noncanonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom or group abstraction through the reactivity of an effective H atom. This activation mechanism is seemingly thermodynamically and kinetically unfavorable but is rendered feasible through quantum tunneling.

A Boron-Nitrogen Double Transborylation Strategy for Borane-Catalyzed Hydroboration of Nitriles

Organoborane-catalyzed hydroboration of nitriles provides N,N-diborylamines, which act as efficient synthons for the synthesis of primary amines and secondary amides. Known nitrile hydroboration methods are dominated by metal catalysis. Simple and metal-free hydroboration of nitriles using diborane [H-B-9-BBN]₂ as a catalyst and pinacolborane as a turnover reagent is reported. The reaction of monomeric H-B-9-BBN with nitriles leads to the hydrido-bridged diborylimine intermediate; a subsequent sequential double hydroboration-transborylation pathway involving B-N/B-H σ bond metathesis is proposed.

An Expedient Method for the Umpolung Coupling of Enols with Heteronucleophiles**

In this paper, we present an unprecedented and general umpolung protocol that allows the functionalization of silyl enol ethers and of 1,3‐dicarbonyl compounds with a large range of heteroatom nucleophiles, including carboxylic acids, alcohols, primary and secondary amines, azide, thiols, and also anionic carbamates derived from CO₂. The scope of the reaction also extends to carbon‐based nucleophiles. The reaction relies on the use of 1‐bromo‐3,3‐dimethyl‐1,3‐dihydro‐1λ³[d][1,2]iodaoxole, which provides a key α‐brominated carbonyl intermediate. The reaction mechanism has been studied experimentally and by DFT, and we propose formation of an unusual enolonium intermediate with a halogen‐bonded bromide.

Crystal structure of 2-oxo-1,2-di-phenyl-ethyl diiso-propyl-carbamate

The title compound, C21H25NO3, crystallized as a racemic twin in the Sohnke space group P21. In the mol-ecular structure of the title compound, both enanti-omers show a highly similar conformation with the urethane function and the benzoyl group showing an almost perpendicular arrangement [the dihedral angle is 72.46 (8)° in the S-enanti-omer and 76.21 (8)° in the R-enanti-omer]. In the crystal structure, mol-ecules of both enanti-omers show infinite helical arrangements parallel to the b axis formed by weak C-H⋯O hydrogen bonds between the phenyl ring of the benzoyl group and the carbamate carbonyl group. In case of the R-enanti-omer, this helix is additionally stabilized by a bifurcated hydrogen bond between the carbonyl function of the benzoyl group towards both phenyl groups of the mol-ecule.

Crystal structure of 2-oxo-2-phenylethyl diisopropylcarbamate

In the mol­ecular structure of the title compound, the urethane function and the benzoyl group are almost perpendicular to each other [dihedral angle 88.97 (5)°]. In the crystal structure, infinite supra­molecular layers in the bc plane are formed by weak C—H⋯O hydrogen bonds.

In the mol­ecular structure of the title compound, C₁₅H₂₁NO₃, the urethane function and the benzoyl group are almost perpendicular to each other [dihedral angle 88.97 (5)°]. In the crystal structure, infinite supra­molecular layers in the bc plane are formed by weak C—H⋯O hydrogen bonds.

Triplet stabilization for enhanced drug photorelease from sunscreen-based photocages

Recently, sunscreen-based drug photocages have been introduced to provide UV protection to photoactive drugs, thus increasing their photosafety. Here, combined experimental and theoretical studies performed on a photocage based on the commercial UVA filter avobenzone (AB) and on the photosensitizing non-steroidal anti-inflammatory drug ketoprofen (KP) are presented unveiling the photophysical processes responsible for the light-triggered release. Particular attention is paid to solvent stabilization of the drug and UV filter excited states, respectively, which leads to a switching between the triplet excited state energies of the AB and KP units. Most notably, we show that the stabilization of the AB triplet excited state in ethanol solution is the key requirement for an efficient photouncaging. By contrast, in apolar solvents, in particular hexane, KP has the lowest triplet excited state, hence acting as an energy acceptor quenching the AB triplet manifold, thus inhibiting the desired photoreaction.

A Toolbox Approach To Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor-Acceptor Cyanoarenes

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor-acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors' molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts' redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely organic photoredox catalysts were evaluated in three challenging, mechanistically distinct classes of benchmark reactions (either requiring balanced, highly oxidizing or strongly reducing properties) to demonstrate their enormous potential as customizable photocatalysts, that outperform and complement prevailing typical best photocatalysts.