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    Home > Angelw. Chem. Int. ed.: photocatalysis reduction amination by polarity matched single atom transfer (HAT)

    Angelw. Chem. Int. ed.: photocatalysis reduction amination by polarity matched single atom transfer (HAT)

    • Last Update: 2018-01-26
    • Source: Internet
    • Author: User
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    Photoinduced electron transfer usually produces unstable radicals, which can react in many different ways It is difficult to control these free radicals and get the target products with good yields It is very important to understand the activity of free radical intermediates for the success of photo redox catalysis Recently, α - aminoalkyl radical intermediates have received considerable attention, especially as the single electron photooxidation products of amines These photoinduced α - aminoalkyl radicals can be used for the formation of C-C bond and the breaking process of C-H bond As early as 1980, giannotti and Whitten's research clearly pointed out the possibility of photoreduction of substrates to α - aminoalkyl radicals and further reduction to stable dielectronic products (J am Chem SOC 1980, 102, 5627 – 5631) However, at present, there is no photocatalytic method to realize complete double electron reduction by α - aminoalkyl radical Recently, Xingwei Guo and Oliver S Wenger from the University of Basel, Switzerland, reported a new method of reductive amination via photoinduced electron transfer and polarity matched single atom transfer (HAT) Compared with the traditional reductive amination method, the new photocatalytic oxidation reduction process is better controlled It is worth mentioning that the concept of polarity matching hat can also be applied to other photo redox reactions (angel Chem Int ed 10.1002 / anie 201711467) Fig 1 Photo oxidation reduction strategy for reducing amination source: in the early stage of research, the author took isobutyraldehyde 1a and aniline 2A as model substrates and [Ru (bpy) 3] Cl 2 as photo oxidation reduction catalyst, and optimized the reaction conditions under 470 nm blue LED light (Fig 2) The results showed that the reductive amination could not be carried out normally with 3-mercaptopropionic acid (MPA), p-Methylthiophenol (p-mepsh) and triethylamine (tea) as reductive agents The target product 3A can be successfully obtained by the combination of MPa and ascorbic acid (asch2) When methanol was used as solvent, the yield of the reaction could be further improved, while when the catalyst loading was reduced, the yield decreased slightly Control experiments prove the importance of light Figure 2 Optimization source of reaction conditions: after angelw Chem Int ed had the best reaction conditions, the author expanded the reaction substrate A series of fat and aromatic aldehydes, as well as fatty ketones, can react well with aniline On the other hand, aniline and aliphatic amines containing electron withdrawing and electron donating substituents can also be compatible with such redox reaction conditions In order to study the reaction mechanism, the free radical clock experiment was designed with methylcyclopropyl ketone as the reaction substrate The products of reductive amination and cyclopropyl ring opening were obtained at the same time The results show that there is a competitive relationship between the ring opening of α - aminoalkyl radical and the direct hydrogen atom transfer Then the H / D kinetic isotopic effects of tevalaldehyde and aniline were studied The mechanism proposed by the authors is shown in Fig 3 Under the optimized reaction conditions, the reaction rate of photoexcited α - aminoalkyl radical with MPa is much higher than that of Asch - / Asch 2 The hat between MPa and α - aminoalkyl radical is originally reversible The existence of Asch - / Asch 2 plays a role in blocking the free radicals, making the hat process irreversible, so that the free radicals can be successfully converted into reductive amination products, and dehydroascorbic acid (DHA) can be generated at the same time After that, DHA can be reduced to Asch - / Asch 2 by selective double electron oxidation (polar reaction) Fig 3 Hat source of polarity matching: angelw Chem Int ed in addition, the author uses fluorescent anthracene marker to mark activated cellulose carrier, and conducts the above-mentioned photo graphic reaction on it, further proving the value of photocatalytic reduction amination reaction, indicating that such reduction amination reaction under the control of time and space has unique characteristics (Fig 5) Fig 4 Source of light pattern on cellulose carrier: angelw Chem Int ed conclusion: Guo and Wenger developed the first reduction amination reaction between aldehydes and ketones and amines catalyzed by photo oxidation-reduction This kind of reaction has a wide range of substrate applicability and excellent yield The combination of MPa and Asch 2 enables the polarity matching hat to intercept the electron rich active free radical intermediates in an irreversible way, and this concept can be widely used in other photo redox and free radical processes Paper link: http://onlinelibrary.wiley.com/doi/10.1002/anie.201711467/abstract Wenger group: http://www.chemie.unibas.ch/ ~ Wenger / index.html Oliver S Wenger and Dr Xingwei Guo (right)
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