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Organic Electron Donors As Powerful Single-Electron Transfer Reducing Agents in Organic Synthesis Julie Broggi, Thierry Terme, Patrice Vanelle

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Organic Electron Donors As Powerful Single-Electron Transfer Reducing Agents in Organic Synthesis Julie Broggi, Thierry Terme, Patrice Vanelle Organic Electron Donors as Powerful Single-Electron Transfer Reducing Agents in Organic Synthesis Julie Broggi, Thierry Terme, Patrice Vanelle To cite this version: Julie Broggi, Thierry Terme, Patrice Vanelle. Organic Electron Donors as Powerful Single-Electron Transfer Reducing Agents in Organic Synthesis. Angewandte Chemie International Edition, Wiley- VCH Verlag, 2014, 53 (2), pp.384-413. 10.1002/anie.201209060. hal-01428063 HAL Id: hal-01428063 https://hal.archives-ouvertes.fr/hal-01428063 Submitted on 6 Jan 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution - NonCommercial - NoDerivatives| 4.0 International License Electron Transfers Organic E lectron D onors as P owerful Single - Electron Transfer R educing A gents in O rganic S ynthesis Julie Broggi,* Thierry Terme,* Patrice Vanelle* Keywords: Dedicated to Prof. John A. Murph y Organic Electron Donors, Reduction reactions, Electron - rich Olefins, Electr on Transfer. 0 - 1e N N - 1e N N +2e X 1 O ne - electron reduction is commonly used in organic chemistry to achieve radical formation via the stepwise transfer of one or two electrons from a donor to an organic substrate. Beyond metallic reagents, S ingle - Electron Transfer reducers based on neutral organic molecules have emerged as an attractive novel source of reducing electrons. The past twenty years have thus seen the blossom of a particular class of organic reducing agents, the electron - rich olefi ns, and the multiplication of their application s in organic synthesis. These powerful neutral ground - state organic electron donors have since showed several significant advantages in the reduction of numerous organic substrates. This review give s an overvi ew of the different types of organic donors and of their specific characteristics in organic transformations. 1. Introduction Investigation of chemical reactivity is the hobbyhorse of organic Numerous SET reducing agents [ 3 ] are used to promote the chemistry and has long been the realm of electron pair transfer formation of carbon - centered radicals or anions: m etals in low reactions. First considered uncontrollable , single - electron transfer oxidation states (mainly alkali metals) dominate the field of ET processes have attracted increasing atte ntion over the past forty reactions, particularly for thermodynamically difficult reductio ns, year s. [1] Molecular electrochemistry has since brought its share of such as Birch r eductions, acyloin condensations or aryl halides discoveries, recognition (Nobel Prizes in Chemistry 1 983, Taube reduction s . Other methods include reduction by solvated electrons and 1992, Marcus) and pharmaceutical or industrial applications. or by alkali metal salts of an organic radical anion, electrochemical New synthetic methodologies involving radical intermediates have reduction at a (usually metal) cathode or photochemi cally - assisted built up the chemist ’s toolbox with reactions such as substitutions, electron transfer. [ 2 ] Nowadays, the search for new processes and additions, cycli z ations, polymerizations or ca scade processes alternative reducers providing clean carbon - carbon bond formation leading to polycyclic carbon skeleton s of natural products. [ 2 - 3 ] by free radical chemistry stems from the well - known problems Radical reactions frequently appear to be a mild, selective and encountered with inorganic SET reducers and with widely used predictable alternative method where classic polar reactions fail. toxic and troublesome tin hydr ogen donors . [3 ] In this context, Among the different way s to effect radical f ormation, [ 3 ] one - electron electron - donating reagents based on neutral organic molecules have reduction in organic chemistry involves the stepwise transfer of one emerged as an attractive novel source of reducing electrons. Organic or two electron ( s ) from a donor to an organic substrate ( Scheme 1 ) . species can behave as electron donors through spontaneous reaction The electron transfer (ET) and the bond dissociatio n can take place with a substrate that has sufficient oxidizing power or they can be either simultaneously or as two successive steps. Reductive ET - induced to donate an electron photochemically, electrochemically , initiated bond cleavage leads to the dissociation of a large variety of or under exposure to irradiation conditions. [ 4 - 5 ] This review focuses chemical bonds including C - C, C - , N - , O - and S - heteroatom on the first class of organic reducers that are prone to oxidation by bonds . [2] The most thoroughly investigated red uction is probably intrinsic ET . [ 6 ] that of organic halides . T he fir st intermediate of a single - electron transfer (SET) reduction is often a radical anion [ RX] • - which Electron - rich olefins (ERO) are certainly the most representative spontaneously dissociate s into a free radical R • . Reactivity depends class of m ultistage organic redox systems. [ 7 ] Since the sixties, ERO on the activ ation barrier of the ini tial ET , itself correlated with the have attracted considerable attention in both organic and inorga nic stability of the radical anion. In radical - substitution, R • can either be chemistry, as a result of their unique properties as versatile and converted into an electrophile [ RD] + by c oupling with the radical - highly reactive reagents or reaction intermediates. [ 8 ] Notably, cation D • + of the dono r, or abstract a hydrogen atom. A second one - tetraaminoethylene derivatives played an important role in the e lectron trans fer can also occur giving the anion R ¯ which act s as a understanding of their equilibrium, so called Wanzlick equ ilibrium, nucleophile (generated overall by double - electron transfer ( DET ) ). with the corresponding diaminocarbenes ( Scheme 2 ). [ 9 ] R X D imerization of carbenes is most likely achieved via a proton - D catalyzed mechanism but is thermodynamically unfavo u rable for D + e unsaturated and/or sterica lly hindered carbenes. ERO are used as reducing agents, [ 10 ] nucleophiles, [ 11 ] precursors of carbene - ligands in [ R X ] D E T S E T metal complexes [ 12 ] or organocatalysts for acyloin type C - C X [ 1 3 ] E D D Y coupling reactions. Up to now their reducing abilities had been R E R R R D R Y + e mostly examined w ithin the framework of their chemoluminescent C o n v e r s i o n t o a n u c l e o p h i l e H - S o l v e n t C o n v e r s i o n t o a n e l e c t r o p h i l e properties and their capacity to form electrically conductive charge - [ 1 4 ] R H transfer systems or redox - active ligands on transition metal D = E l e c t r o n d o n o r complexes. [ 1 5 ] Nonetheless, the past twenty years have seen the R e d u c t i v e t e r m i n a t i o n blossom of t his particular class of organic reducers with the Scheme 1 . Radical substitutions via single - (SET) or double - electron multiplication of attracting applications in organic synthesis. These transfer (DET) . powerful neutral ground - state organic electron donors offer several 2 significant advantages in the SET reduction of organic substrates, a s these radical methodologies. Our aim is to provide chemists with an they: exhaustive guide to the properties and capabilities of organic - Undergo spontaneous sequential loss of one or two electrons and reducers in organi c chemistry. We hope it will emphasiz e the milder thus upon electron transfer , generate radicals or anions including and resourceful alternative offered by these electron sources to the aryl anions. use of inorganic reductants and encourage other researchers to - Present a large range of redox potentials and can be finely tuned. utilize them in radical chemistry. Their reactivity can therefore be m odulated by the appropriate structural modification. - Are highly selective and tolerant to other functional groups (nitro, carbonyl, ester, cyano … ). - Are available as pure organic liquid or solid and can be used in appropriate quantities. Scheme 2 . Carbene/Dimer equilibrium. - Operate under mild conditions compared to highly aggressive metal - based reducers and are soluble in organic solvents , hence shorten ing the induction period. 2. Tetrathiafulvalenes TTF - Avoid the use of expensive metal derivatives as well as the recycling of metal residues causing environmental and economic problems. 2 .1. Properties - Can be easily removed from the media via precipitation of their salt form and can be regenerated. The y can also be attached to Since the early 1970s, tetrathiafulvalene (TTF) [ 1 6 ] and its solid supports. derivatives are recognized as st rong π - electron organic donors of - Have wider applicability than photochemically - assisted reactions. great interest, certainly owing to the following features: (1) TTF is a - Avoid complications encountered with electrochemical reductions , planar nonaromatic 14 - π - electron system in which oxidation to the such as the fouling of electrodes, the use of specific glassware and cation radical TTF • + and dication TTF 2+ occurs sequentially
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