Halide abstraction competes with oxidative addition in the reactions of aryl halides with [Ni(PMenPh(3-n))4]

Density functional theory (DFT) calculations have been used to study the oxidative addition of aryl halides to complexes of the type [Ni(PMenPh(3-n))4], revealing the crucial role of an open shell singlet transition state for halide abstraction. The formation of NiI versus NiII has been rationalised through the study of three different pathways: (i) halide abstraction by [Ni(PMenPh(3-n))3], via an open shell singlet transition state; (ii) SN2-type oxidative addition to [Ni(PMenPh(3-n))3], followed by phosphine dissociation; and (iii) oxidative addition to [Ni(PMenPh(3-n))2]. For the case of [Ni(PMe3)4], a microkinetic model was used to show that these data are consistent with the experimentally-observed ratios of NiI and NiII. Importantly, [Ni(PMenPh(3-n))2] complexes often have little if any role in the oxidative addition reaction because they are relatively high in energy. The behaviour of [Ni(PR3)4] complexes in catalysis is therefore likely to differ considerably from those based on diphosphine ligands in which two coordinate Ni0 complexes are the key species undergoing oxidative addition

Photogeneration of α‑Bimetalloid Radicals via Selective Activation of Multifunctional C1 Units

Light-driven strategies that enable the chemoselective activation of a specific bond in multifunctional systems are comparatively underexplored in comparison to transition-metal-based technologies, yet desirable when considering the controlled exploration of chemical space. With the current drive to discover next-generation therapeutics, reaction design that enables the strategic incorporation of an sp 3 carbon center, containing multiple synthetic handles for the subsequent exploration of chemical space would be highly enabling. Here, we describe the photoactivation of ambiphilic C1 units to generate α-bimetalloid radicals using only a Lewis base and light source to directly activate the C–I bond. Interception of these transient radicals with various SOMOphiles enables the rapid synthesis of organic scaffolds containing synthetic handles (B, Si, and Ge) for subsequent orthogonal activation. In-depth theoretical and mechanistic studies reveal the prominent role of 2,6-lutidine in forming a photoactive charge transfer complex and in stabilizing in situ generated iodine radicals, as well as the influential role of the boron p-orbital in the activation/weakening of the C–I bond. This simple and efficient methodology enabled expedient access to functionalized 3D frameworks that can be further derivatized using available technologies for C–B and C–Si bond activation

Accelerated Ru-Cu Trinuclear Cooperative C−H Bond Functionalization of Carbazoles : A Kinetic and Computational Investigation

The mechanism of a trinuclear cooperative dehydrogenative C−N bond-forming reaction is investigated in this work, which avoids the use of chelate-assisting directing groups. Two new highly efficient Ru/Cu co-catalyzed systems were identified, allowing orders of magnitude greater TOFs than the previous state of the art. In-depth kinetic studies were performed in combination with advanced DFT calculations, which reveal a decisive rate-determining trinuclear Ru-Cu cooperative reductive elimination step (CRE)

Work-hardening Photopolymer from Renewable Photoactive 3,3’-(2,5-Furandiyl)bisacrylic Acid

The design of a photopolymer around a renewable furan-derived chromophore is presented herein. An optimised semi-continuous oxidation method using MnO2 affords 2,5-diformylfuran from 5-(hydroxymethyl)furfural in gram quantities, allowing the subsequent synthesis of 3,3’-(2,5-furandiyl)bisacrylic acid in good yield and excellent stereoselectivity. The photoactivity of the diester of this monomer is confirmed by reaction under UV irradiation, and the proposed [2+2] cycloaddition mechanism supported further by TD-DFT calculations. Oligoesters of the photoreactive furan diacid with various aliphatic diols are prepared via chemo- and enzyme-catalysed polycondensation. The latter enzyme-catalysed (Candida antarctica lipase B) method results in the highest Mn (3.6 kDa), suggesting milder conditions employed with this protocol minimised unwanted side reactions, including untimely [2+2] cycloadditions, whilst preserving the monomer's photoactivity and stereoisomerism. The photoreactive polyester is solvent cast into a film where subsequent initiator-free UV curing leads to an impressive increase in the material stiffness, with work-hardening characteristics observed during tensile strength testing

Modular and Selective Arylation of Aryl Germanes (C-GeEt3 ) over C-Bpin, C-SiR3 and Halogens Enabled by Light-Activated Gold Catalysis.

Selective C sp 2 -C sp 2 couplings are powerful strategies for the rapid and programmable construction of bi- or multiaryls. To this end, the next frontier of synthetic modularity will likely arise from harnessing the coupling space that is orthogonal to the powerful Pd-catalyzed coupling regime. This report details the realization of this concept and presents the fully selective arylation of aryl germanes (which are inert under Pd0 /PdII catalysis) in the presence of the valuable functionalities C-BPin, C-SiMe3 , C-I, C-Br, C-Cl, which in turn offer versatile opportunities for diversification. The protocol makes use of visible light activation combined with gold catalysis, which facilitates the selective coupling of C-Ge with aryl diazonium salts. Contrary to previous light-/gold-catalyzed couplings of Ar-N2 + , which were specialized in Ar-N2 + scope, we present conditions to efficiently couple electron-rich, electron-poor, heterocyclic and sterically hindered aryl diazonium salts. Our computational data suggest that while electron-poor Ar-N2 + salts are readily activated by gold under blue-light irradiation, there is a competing dissociative deactivation pathway for excited electron-rich Ar-N2 + , which requires an alternative photo-redox approach to enable productive couplings

Rational Design and Synthesis of Efficient Sunscreens to Boost the Solar Protection Factor

Skin cancer incidence has been increasing in the last decades, but most of the commercial formulations used as sunscreens are designed to protect only against solar erythema. Many of the active components present in sunscreens show critical weaknesses, such as low stability and toxicity. Thus, the development of more efficient components is an urgent health necessity and an attractive industrial target. We have rationally designed core moieties with increased photoprotective capacities and a new energy dissipation mechanism. Using these scaffolds, we have synthesized a series of compounds with tunable properties suitable for their use in sunscreens, and enhanced properties in terms of stability, light energy dissipation, and toxicity. Moreover, some representative compounds were included in final sunscreen formulations and a relevant solar protection factor boost was measured

Mechanistic Studies on the Synthesis of Pyrrolidines and Piperidines via Copper-Catalyzed Intramolecular C–H Amination

We have recently developed a method for the synthesis of pyrrolidines and piperidines via intramolecular C–H amination of N-fluoride amides using [TpxCuL] complexes as precatalysts [Tpx = tris(pyrazolyl)borate ligand and L = THF or CH3CN]. Herein, we report mechanistic studies on this transformation, which includes the isolation and structural characterization of a fluorinated copper(II) complex, [(TpiPr2OH)CuF] [TpiPr = hydrotris(3,5-diisopropylpyrazolyl)borate], pertinent to the mechanistic pathway. The effects of the nature of the Tpx ligand in the copper catalyst as well as of the halide in the N–X amides employed as reactants have been investigated both from experimental and computational perspectives.We thank the Ministerio de Ciencia e Innovación (PID2020- 113797RB-C21, PID2020-112825RB-I00, CTQ2017-88496-R, and CEX2019-000925-S), COST Action CA15106 “C−H Activation in Organic Synthesis (CHAOS)”and Red Intecat (CTQ2016-81923-REDC), and Universidad de Huelva (P.O.Feder 2014-2020 UHU-1254043). We thank Universidad de Huelva/CBUA for funding for open access charge