UvA-DARE (Digital Academic Repository) Nitrene Radical Intermediates in Catalytic Synthesis Kuijpers, P.F.; van der Vlugt, J.I.; Schneider, S.; de Bruin, B. DOI 10.1002/chem.201702537 Publication date 2017 Document Version Final published version Published in Chemistry-A European Journal License CC BY-NC-ND Link to publication Citation for published version (APA): Kuijpers, P. F., van der Vlugt, J. I., Schneider, S., & de Bruin, B. (2017). Nitrene Radical Intermediates in Catalytic Synthesis. Chemistry-A European Journal, 23(56), 13819-13829. https://doi.org/10.1002/chem.201702537 General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:02 Oct 2021 DOI:10.1002/chem.201702537 Concept & MetalloradicalCatalysis Nitrene Radical Intermediates in Catalytic Synthesis Petrus F. Kuijpers,[a] Jarl Ivar van der Vlugt,[a] Sven Schneider,[b] and Bas de Bruin*[a] Chem. Eur.J.2017, 23,13819 –13829 13819 2017 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Concept reactions are often associated with low selectivities.[2] Free or- Abstract: Nitrene radical complexes are reactive inter- ganic radicals indeedoften lead to radical disproportionation mediates with discrete spin density at the nitrogen-atom and other side reactions, generating insoluble materials. Never- of the nitrene moiety.These species have become impor- theless, many selectivereactions based on free N-centered rad- tant intermediates for organic synthesis, being invoked in icals have since been achieved, taking advantage of kinetic abroad range of C Hfunctionalization and aziridination control (desired reactions outcompeting undesired ones).[3] À reactions. Nitrene radical complexes have intriguing elec- Better control can be achieved in the coordination sphereofa tronic structures,and are best described as one-electron metal,and transitionmetal bound N-centered radicals are in- reduced Fischer type nitrenes. They can be generatedby creasingly recognized as important intermediates to enable intramolecular single electron transfer to the “redox non- controlled radical typeC Nbond formation reactions.[4] Transi- À innocent” nitrene moiety at the metal. Nitrene radicals tion metal-bound N-centered radicals can be catalytically gen- generated at open-shell cobalt(II) have thusfar received erated, in low and controlled amounts, therebygiving rise to most attention in terms of spectroscopic characterization, much higher selectivities than typically achieved with free or- reactivity screening, catalytic nitrene-transfer reactions ganic radicals. and (computational and experimental) mechanistic stud- Ligands surrounding the metal are used to fine-tune the re- ies, but someinteresting iron and precious metal catalysts activity of these intermediates, both sterically and electronical- have also been employed in related reactions involving ni- ly.[5] Transition metal-bound nitrene/imido-based nitrogen-cen- trene radicals.Insome cases, redox-active ligands are tered radicals M N·R (i.e. nitrene- and imidyl radicalcom- À used to facilitateintramolecular single electron transfer plexes; see Figure 1and Figure 2), have received quite some from the complex to the nitrenemoiety.Organicazides attention,asthey enable avariety of useful nitrene-insertion are among the most attractive nitrene precursors in this and nitrene-transfer reactions. Such reactions are typically field, typically requiring pre-activatedorganic azides (e.g. more selectivethan those involving free N-centered radicals or RSO2N3,(RO)2P(=O)N3,ROC(=O)N3 and alike) to achieveef- free nitrenes. ficient and selective catalysis. Challenging, non-activated aliphatic organic azides were recently added to the pa- lette of reagents useful in synthetically relevant reactions proceedingvia nitrene radicalintermediates.This concept article describes the electronic structure of nitrene radical complexes, emphasizesontheir usefulness in the catalytic synthesis of variousorganic products, and highlights the important developments in the field. Introduction The use of nitrogen-centered radicals in synthesis, although in- itially perhaps not recognized as such, dates back to the late Figure 1. Simplified frontier molecularorbital diagramsof: a) Schrock type th imido complex (metal–nitrogen p-interactions stabilizing in two directions, 19 century Hofmann–Lçffler–Freytag reactionfor the synthe- nitrogensp-hybridization and linear coordination modes favored). b) Schrock sis of pyrrolidines.[1] The free organic radicals involved in these type imidyl radicalcomplex (one-electron oxidized Schrock type imido). [a] Dr.P.F.Kuijpers, Dr.Ir. J. I. vander Vlugt, Prof. Dr.B.deBruin Van‘tHoffInstitute for Molecular Sciences (HIMS) University of Amsterdam (UvA) The scientific literature is highly inconsistent aboutthe elec- Science Park 904, 1098 XH Amsterdam tronic structure description and nature of M NR species in À (The Netherlands) general,and frequently confusing descriptions are presented E-mail:[email protected] that are based on formal oxidation state counting arguments. [b] Prof. S. Schneider As such, the NR ligand is most typicallyconsidered as an imido Institut fürAnorganische Chemie 2 fragment (R N À), and frequently even as aredoxinactive UniversitätGçttingen, Tammannstr.4 À 37077 Gçttingen (Germany) moiety. This description fails to reflect the electrophilic and The ORCID identification number(s) for the author(s) of this articlecan be radical-type reactivity observed for many late transition metal found under https://doi.org/10.1002/chem.201702537. M-NR species.[6] The metal-nitrogen p-interactionsofagenuine 2017 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA. imido complex are stabilizing in case of early transition metals This is an open access article under the terms of Creative Commons Attri- (electropositive,relativelyhigh-energy emptyd-type orbitals; bution NonCommercial-NoDerivs License, which permits use and distribu- p tion in any medium, provided the originalwork is properly cited, the use is see Figure 1a), in whichcase the imido moiety should have a non-commercial and no modifications or adaptations are made. tendency to bind in alinear manner due to the presence of Chem. Eur.J.2017, 23,13819 –13829 www.chemeurj.org 13820 2017 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Concept Obviously,the simplified Fischer and Schrock type MO dia- grams in Figures 1and 2are merely extremes of acontinuum of intermediate cases, and these simplified and generalized picturesget severelyblurred, with increasing covalency,inpar- ticular for second and third row transition metalswhere relativ- istic effects furthercomplicate the electronic structure via spin- orbit coupling. However, for most first row transition metals the diagrams should be quite useful to understand their reac- tivity,including the redoxactivity of the imido/nitrene moiety. With the frontier orbitals shown in Figure 1and Figure 2in mind, it should be clear that both Schrock type imido com- plexes (HOMO dominated by the nitrogen p-orbital) and Fisch- er type nitrene complexes (LUMOdominated by the nitrogen p-orbital) are potentially redox-active at the nitrogen atom, and hence they can easily form M N·Rradicals. Twodistinct À electronic structures are possible for such species: (1) Schrock type imidylradicals (Figure 1b), formed by 1e-oxidation of (Schrock type) p-stabilized imido species, or (2) Fischertype ni- trene radical complexes (Figure 2b). The latter can be formed either by 1e-reduction of (Fischer type) nitrene radicalspecies (Figure 2a to 2b), or by 1e-oxidation of a p-destabilizedimido complex(Figure 2c to 2b). Schrock type imidyl radicalspecies are distinctly different from Fischertypenitrene radicals. In the first case the singly occupied molecular orbital (SOMO) is a half-filled metal–nitrogen p-bonding orbital, while in the second it is ahalf-filled metal–nitrogen p-antibonding orbital. Few examples of Schrock typeimidyl radicals formed in stoi- chiometric reactions exist,[7] butthey are very scarce and, to Figure 2. Simplified frontier molecularorbitaldiagramsof: a) Fischer type ni- our best knowledge,nounequivocal examples of such species trene complex (nitrogen sp2 hybridizationand bentcoordination modes fa- involved in catalytic reactions have been reported to date. This voredfor late transitionmetal complexes);.b)Nitrene radicalcomplex (one- is perhaps not very surprising, as the strong M Nbonding in- electronreducedFischer type nitrene);c)p-destabilized imidocomplex. À teractions and the linear coordination mode of these species are likely to hamper nitrene-transfer reactivity (Figure1b). The situation is quite different for Fischertype nitreneradical