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    Home > Angelw: the brown group of Indiana University realizes the aromatization of alkenyl aromatic hydrocarbons catalyzed by nickel

    Angelw: the brown group of Indiana University realizes the aromatization of alkenyl aromatic hydrocarbons catalyzed by nickel

    • Last Update: 2019-07-18
    • Source: Internet
    • Author: User
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    The bifunctionalization of alkenes can rapidly expand the molecular complexity, which is an important strategy in organic synthesis Among them, carboboration of olefins has become an effective strategy, which can introduce a variety of groups on olefins At present, the aromatization of monosubstituted, 1,2-disubstituted and 1,1-disubstituted alkenyl aromatics has been widely studied (scheme 1a), while the functionalization of trisubstituted alkenyl aromatics is still challenging Recently, the M Kevin Brown team of Indiana University found a method of Ni catalyzed aromatization of unactivated alkenes, which prompted them to explore the application of Ni catalysis in highly substituted alkenyl aromatics Recently, the research group reported a method of aromatic boration of trisubstituted alkenyl aromatics for the synthesis of challenging boron substituted quaternary carbon or diaryl quaternary carbon (scheme 1b), and found that 1, The regionally boronization of 2-disubstituted alkenyl aromatics is solvent dependent, which further increases the practicability of the method (scheme 1c) The result was published in angelw Chem Int ed (DOI: 10.1002 / anie 201904861) (image source: angelw Chem Int ed.) under the previously reported Ni catalytic conditions, the aromatization of trisubstituted alkenyl aromatics was carried out in low to medium yields By optimizing the conditions, the author found that the yield of koet and (bpin) 2 premixed with DMA as solvent was higher before adding substrate and catalyst In addition, 5 mol% NiCl2 (DME) and aryl bromide (3.0 Eq.) have the best effect, but lower catalyst loading or less aryl bromide can also obtain acceptable yield (image source: angelw Chem Int ed.) under optimized conditions, aryl bromides containing donor, acceptor and sterically hindered substituents, as well as amines, alcohols, esters and amides, have good tolerance (scheme 2) However, when heterocycles are used, the yield is slightly lower The main limitation of the application scope of aryl bromides is that strong electron absorption substituents, such as CF 3, will reduce the product yield (picture source: angelw Chem Int ed.) on the scope of application of alkenes (scheme 3), it should be noted that: 1) for the synthesis of boron substituted quaternary carbon, the substituent mode of aryl can be changed by various substituents including electron absorption, electron feeding and steric hindrance; 2) both cyclic and acyclic substrates can participate in the reaction; 3) 30 and 37 4) for the substrates containing acyclic substituents, the yield decreases with the increase of substituent hindrance (compare 2 and 33); 5) the construction of diaryl quaternary carbon can be realized by acyclic and cyclic substrates (picture source: angelw Chem Int ed.) aromatization can be carried out on a gram scale (scheme 4) In addition, the C-B bond can be constructed by oxidation and Matteson homologation / oxidation; the alcohol 48 for the synthesis of glucocorticoid receptor modulator can be stereoselectively synthesized by aromatization only 3-4 times (image source: angelw Chem Int ed.) next, the author investigated 1,2-disubstituted alkenyl aromatics (Table 2), and obtained 51 with excellent selectivity under standard conditions When toluene was used as a cosolvent, the isomer 52 (> 20:1dr) was produced; when toluene was added, the reaction temperature was increased (60 ℃ vs RT), and kohe (vs koet) and excess olefins were added, only 52 was obtained Therefore, the regioselectivity of aromatization can be adjusted by changing the reaction conditions such as solvents Recently, song Qiuling's research group reported that styrene derivatives can carry out PD catalyzed aromatization with aryldiazonium salt and (bpin) 2, but 1,2-disubstituted alkenyl aromatics can't (picture source: angelw Chem Int ed.) according to the conditions of entry 1 and 12 in Table 2, acyclic and cyclic 1,2-disubstituted alkenyl aromatics can participate in the reaction with excellent regioselectivity and enantioselectivity (scheme 5) Although the aromatization of trisubstituted alkenes can be carried out in DMA (schemes 2 – 3), it can not be reacted in toluene (image source: angelw Chem Int ed.) in order to explain the different regioselectivity observed in different solvents, the author proposed two catalytic cycles (scheme 6) In the reaction of DMA, the author thinks that [Ni] - bpin complex 62 is formed and then reacts with olefins No matter what olefin substitution mode, the regioselectivity of migration insertion will produce benzyl - [Ni] - complex 63, and then the product will be obtained by oxidation addition and reduction elimination with arbr In the reaction of toluene, the author thinks that the migration and insertion of Ni II arbr complex 66 is determined by the regioselectivity of benzyl - [Ni] - complex 67, and then the product is obtained by metal transfer and reduction elimination with (bpin) 2 (image source: angelw Chem Int ed.) finally, the author supports the above-mentioned catalytic cycle (scheme 7) through experiments When MeOH was added to the reaction mixture, 69 was produced, indicating that bpin was introduced before the aryl group In addition, vinyl borate 71 is produced by the reaction of vinyl fluoride 70 At room temperature, the coupling reaction of vinyl fluoride is very slow, which shows that the product can be obtained by 1,2-elimination of alkyl - [Ni] - complex (picture source: angelw Chem Int ed.) conclusion: the M Kevin Brown research group realized the synthesis of challenging boron substituted quaternary carbon and diaryl quaternary carbon by nickel catalyzed aromatization of alkenyl aromatics In addition, the regioselectivity of aromatization of 1,2-disubstituted alkenyl aromatics can be adjusted by changing solvents, which provides a practical method to obtain various and versatile products.
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