Dominique Lastécouères

Copper Catalyst Activation Driven by Photoinduced Electron Transfer: A Prototype Photolatent Click Catalyst

PET cat. While the copper(II) tren ketoprofenate precatalyst 1 (see picture) is inactive at room temperature in methanol, it is quantitatively and rapidly reduced to its cuprous state upon light irradiation to provide a highly reactive click catalyst. By simply introducing air into the reaction medium the catalysis can be switched off and then switched on again by bubbling argon followed by irradiation.

A Photoreducible Copper(II)‐Tren Complex of Practical Value: Generation of a Highly Reactive Click Catalyst

A detailed study on the photoreduction of the copper(II) precatalyst 1 to generate a highly reactive cuprous species for the copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) click reaction is presented. For the photoactive catalyst described herein, the activation is driven by a photoinduced electron transfer (PET) process harnessing a benzophenone-like ketoprofenate chromophore as a photosensitizer, which is equally the counterion. The solvent is shown to play a major role in the Cu(II) to Cu(I) reduction process as the final electron source, and the influence of the solvent nature on the photoreduction efficiency has been studied. Particular attention was paid to the use of water as a potential solvent, aqueous media being particularly appealing for CuAAC processes. The ability to solubilize the copper-tren complexes in water through the formation of inclusion complexes with β-CDs is demonstrated. Data is also provided on the fate of the copper(I)-tren catalytic species when reacting with O2, O2 being used to switch off the catalysis. These data show that partial oxidation of the secondary benzylamine groups of the ligand to benzylimines occurs. Preliminary results show that when prolonged irradiation times are employed a Cu(I) to Cu(0) over-reduction process takes place, leading to the formation of copper nanoparticles (NPs). Finally, the main objective of this work being the development of photoactivable catalysts of practical value for the CuAAC, the catalytic, photolatent, and recycling properties of 1 in water and organic solvents are reported.

New copper(I) and iron(II) complexes for atom transfer radical macrocyclisation reactions

New Cu(I) and Fe(II) complexes have been synthesized and proved to be efficient catalysts for atom transfer radical addition (ATRA) reactions. The catalytic activity was found to be greater than existing atom transfer systems based upon CuCl(bipyridine) or RuCl2(PPh3)3, for example. The addition of a reducing agent considerably improved the efficiency of the usual procedures.

Effective ascorbate-free and photolatent click reactions in water using a photoreducible copper(II)-ethylenediamine precatalyst

Summary The search for copper catalysts able to perform effectively click reactions in water in the absence of sodium ascorbate is an active area of current research with strong potential for applications in bioconjugation. The water-soluble and photoreducible copper(II)–EDA (EDA = ethylenediamine) complex 1, which has two 4-benzoylbenzoates acting as both counterion and photosensitizer, has been synthesized and characterized by different techniques including single crystal X-ray diffraction. Highly efficient photoreduction was demonstrated when solutions of 1 in hydrogen atom donating solvents, such as THF or MeOH, were exposed to UVA radiation (350–400 nm) provided by a low pressure mercury lamp (type TLC = thin-layer chromatography, 365 nm), or by a 23 W fluorescent bulb, or by ambient/sunlight. In water, a much poorer hydrogen atom donating solvent, the photoreduction of 1 proved inefficient. Interestingly, EPR studies revealed that complex 1 could nonetheless be effectively photoreduced in water when alkynes were present in solution. The catalytic activity of 1 for click reactions involving a range of water-soluble alkynes and azides, in particular saccharides, was tested under various illumination conditions. Complex 1 was found to exhibit a photolatent character, the photogenerated copper(I) being very reactive. On irradiating aqueous reaction mixtures containing 1 mol % of 1 at 365 nm (TLC lamp) for 1 h, click reactions were shown to proceed to full conversion.

Atom transfer radical cyclisations of activated and unactivated N-allylhaloacetamides and N-homoallylhaloacetamides using chiral and non-chiral copper complexes

Activated N-tosyl-2,2,2-trichloroacetamide 6a, N-benzyl-2,2,2-trichloroacetamide 6d, 2,2-dichloroacetamides 6b–c and 6e–f and 2-monohaloacetamides 11a–g undergo efficient 5-exo atom transfer radical cyclisations at room temperature mediated by CuCl or CuBr in the presence of tris(N,N-dimethylaminoethylene)amine 3 (trien-Me6). The efficiency and stereoselectivity of these cyclisations was found to be greater than existing published atom transfer procedures based upon CuCl(bipyridine), RuCl2(PPh3)3 and CuCl(TMEDA)2. The product distribution for the cyclisation onto alkyne 11g was found to be solvent dependent. Attempts to make larger ring sizes by endo cyclisation of N-tosylacetamides 19a–c led to a competing 5-exoipso aromatic substitution into the N-tosyl group followed by re-aromatisation and loss of SO2 to furnish an amidyl radical. Cyclisation of N-homoallylacetamides 25a–d proceeded smoothly to give δ-lactams with a range of catalysts based upon ligands 2 and 26. The stereoselectivity of cyclisation to give γ lactams could be somewhat influenced by using chiral enantiopure copper complexes 28–30 suggesting that the reactions may involve metal-complexed radicals.

New and improved catalysts for transition metal catalysed radical reactions

New CuI and FeII complexes displayed considerable improvements in atom transfer radical addition reactions.