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Azido Radical

Brazier, Bernath, et al., 1988Brazier, C.R.; Bernath, P.F.; Burkholder, J.B.; Howard, C.J.,Fourier transform spectroscopy of the ν3 band of the N3 radical,J. Chem. Phys., 1988, 89, 4, 1762, [all data]

azido radical

Pahnke, Ashworth, et al., 1988Pahnke, R.; Ashworth, S.H.; Brown, J.M.,Detection of the N3 free radical by laser magnetic resonance at 6.08 μm,Chem. Phys. Lett., 1988, 147, 2-3, 179, -2614(88)85079-6. [all data]

The azide radical N3 reacts selectively with amino acids, in neutral solution preferentially with tryptophan (k (N3 + TrpH) = 4.1 X 10(9) dm3 mol(-1s-1) and in alkaline solution also with cysteine and tyrosine (k(N3 + CyS-) = 2.7 X 10(9) dm3 mol-1s-1) and k(N3 + TyrO-) equals 03.6 X 10(9) dm3 mol-1s-1). Oxidation of the enzyme yADH by N3 involves primary attacks, mainly at tryptophan residues, and subsequent slow secondary reactions. N3-induced inactivation of yADH is likely to occur upon oxidation of tryptophan residues in the substrate binding pocket (58-TrpH and 93-TrpH) since the substrate ethanol although unreactive with N3, protects yADH and since elADH, which does not contain tryptophan in the substrate pocket, is comparatively resistant against N3-attack. N3 exhibits low reactivity with nucleic acid derivatives and it is inert towards aliphatic compounds such as methanol and sodium dodecylsulphate.

Atom transfer radical polymerization (ATRP) was demonstrated to yield well-defined polyacrylates with halogen end groups. These halogen end groups were transformed to azide groups which were subsequently reduced into amino groups. The replacement of the halogen end groups by azide groups was obtained either by using sodium azide or by treatment with trimethylsilyl azide in the presence of both stoichiometric and catalytic amounts of tetrabutylammonium fluoride (TBAF). The azide end groups were reacted with triphenylphosphine in order to obtain iminophosphorane groups which were then hydrolyzed yielding amino terminated polyacrylates.

Phosphinoylazidation of alkenes is a direct method to build nitrogen- and phosphorus-containing compounds from feedstock chemicals. Notwithstanding the advances in other phosphinyl radical related difunctionalization of alkenes, catalytic phosphinoylazidation of alkenes has not yet been reported. Thus, efficient access to organic nitrogen and phosphorus compounds, and making the azido group transfer more feasible to further render this step more competitive remain challenging. googletag.cmd.push(function() googletag.display('div-gpt-ad-1449240174198-2'); ); Recently, a research team led by Prof. Hongli Bao from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS) reported the first iron-catalyzed phosphinoylazidation of alkenes under relatively mild reaction conditions affording the nitrogen- and phosphorus- containing compounds, which was disclosed with the unusually low activation energy 4.8 kcal/mol of radical azido group transfer from the PcFeIIIN3 to benzylic radical. The results were published in Chinese Journal of Catalysis.

Fe(OTf)2 is a good catalyst for carboazidation of alkenes in the previous work reported by Bao's group. However, it is not an efficient catalyst for the phosphinoylazidation reaction. The reason for this result could presumably be the deactivation of the iron catalyst by the coordinative product, azidophosphonates. Therefore, the catalyst iron(II) phthalocyanine (PcFeII) which has a tetradentate ligand was chosen because it presumably can maximally evade the deactivation of its iron center, and, fortunately, the results confirmed the authors' hypothesis with the yield of desired products up to 88%.

Mechanism experiments and density functional theory (DFT) calculations were also conducted to further investigate the reaction mechanism. Two radical clock experiments with different rate and radical trapping experiments have confirmed the radical nature of the reaction. Furthermore, the signal of PcFeIIIOH and PcFeIIIN3 were observed in mass spectrometry experiments. Theoretical study was then conducted based upon the experimental results. The results support the mechanism of iron-catalyzed azidation which is via the group transfer pathway rather than the reductive elimination from high-valent species. The azido transfer from PcFe(N3) to benzylic radical has the lowest energy transition state with an energy barrier of only 4.8 kcal/mol. This work may inspire other in-depth mechanism studies of metal catalyzed radical reactions and spur further synthetic applications. More information:Xiaoxu Ma et al, Iron phthalocyanine-catalyzed radical phosphinoylazidation of alkenes: A facile synthesis of β-azido-phosphine oxide with a fast azido transfer step, Chinese Journal of Catalysis (2021). DOI: 10.1016/S1872-2067(21)63847-0 Provided byChinese Academy Sciences

Trinitrogen also known as the azide radical is an unstable molecule composed of three nitrogen atoms. Two arrangements are possible: a linear form with double bonds and charge transfer, and a cyclic form. Both forms are highly unstable. More-stable derivatives exist, such as when it acts as a ligand, and it may participate in azido nitration, which is a reaction between sodium azide and ammonium cerium nitrate.[1][2]

Vicinal diamines are privileged synthetic motifs in chemistry due to their prevalence and powerful applications in bioactive molecules, pharmaceuticals, and ligand design for transition metals. With organic diazides being regarded as modular precursors to vicinal diamines, enormous efforts have been devoted to developing efficient strategies to access organic diazide generated from olefins, themselves common feedstock chemicals. However, state-of-the-art methods for alkene diazidation rely on the usage of corrosive and expensive oxidants or complicated electrochemical setups, significantly limiting the substrate tolerance and practicality of these methods on large scale. Toward overcoming these limitations, here we show a photochemical diazidation of alkenes via iron-mediated ligand-to-metal charge transfer (LMCT) and radical ligand transfer (RLT). Leveraging the merger of these two reaction manifolds, we utilize a stable, earth abundant, and inexpensive iron salt to function as both radical initiator and terminator. Mild conditions, broad alkene scope and amenability to continuous-flow chemistry rendering the transformation photocatalytic were demonstrated. Preliminary mechanistic studies support the radical nature of the cooperative process in the photochemical diazidation, revealing this approach to be a powerful means of olefin difunctionalization.

a Prevalence of diamine motifs in pharmaceuticals, natural products, and synthesis. b Previous works on azido radical generation via Outer-Sphere single electron transfer (OSET) pathway. c Ligand-to-Metal Charge Transfer (LMCT) enables radical generation. d Radical ligand transfer allows for facile delivery of orthogonal nucleophiles. e The synergistic cooperation of LMCT and RLT in alkene diazidation.

Early approaches to alkene diazidation are dependent on stoichiometric, highly oxidative oxidants and/or harsh reaction conditions such as high heat and strong acid, limiting the functional group tolerance of these transformations5,6,7,8. Further, many of these early methods are centered on the reaction of activated olefins such as styrenes, exhibiting low reactivity for unactivated alkyl olefins. Recent advances contributed by the groups of Greaney9, Loh10, Xu11, Bao12, Liu13, and others have significantly expanded the substrate tolerance of olefin diazidation under thermal conditions, allowing for unactivated alkenes to be diazidated in high yield at moderate temperatures and without strongly acidic additives. Importantly, recent endeavors by Bao and coworkers have showcased the thermal, enantioselective diazidation of styrene-type alkenes using perester oxidants, providing a valuable tool for direct synthesis (with simple reduction) of chiral vicinal diamine14. While powerful, these approaches still require highly oxidizing, energetic, and expensive hypervalent iodine-derivatives or corrosive perbenzoate stoichiometric oxidants, presenting functional group compatibility concerns with oxidatively-labile substrates. Further, many of these methods require catalysts supported with complex ligand frameworks to function, presenting a barrier to the widespread adoption of these methods. As an alternative to the traditional thermal chemical transformations, electrochemical methods have also offered a direct and appealing route to access these useful diazides motifs, with these methods garnering increasing interest in recent years due to their sustainability and high energy efficiency. Lin group reported an elegant electrochemical approach to diazidation of alkenes15,16, exploiting the ability to achieve strong oxidative potentials at anodes in synergy with manganese15 or aminoxyl16 electrocatalysts to achieve dual azido group transfer onto alkenes. In a similar approach, efficient diazidation has been achieved with ppm loading of copper by Xu and coworkers, alleviating concerns of high catalyst loading in previous electrochemical diazidation17. While electrocatalysis has led to exciting advances in olefin diazidation, the high complexity of electrochemical apparatuses and required multivariate optimization of factors such as electrode composition, morphology, and mass transport are current limitations of this approach. Taking these advances together, we imagined that a different, light-enabled mechanistic concept might allow us to eliminate the corrosive/expensive oxidants and/or complex electrochemical apparatuses previously needed in olefin diazidation and provide an efficient, easily accessible synthetic route to access vicinal diamine precursors with general functional group compatibility (Fig. 1b). 041b061a72

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