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    Home > Angew, Tobias Ritter research group: olefin region and stereoselective thianthrene reaction to construct alkenyl electrophilic reagent

    Angew, Tobias Ritter research group: olefin region and stereoselective thianthrene reaction to construct alkenyl electrophilic reagent

    • Last Update: 2019-12-04
    • Source: Internet
    • Author: User
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    Olefin is a very powerful block in organic synthesis, but it is still a difficult problem to synthesize alkenyl electrophilic reagent directly from olefin Although alkenes have C (SP 2) - H bonds similar to aromatics, their activities are quite different from those of electrophilic reagents: aromatics usually undergo electrophilic substitution reaction, while alkenes usually undergo addition reaction Alkenyl halides, such as alkenyl bromide and alkenyl iodine, are widely used as alkenyl electrophilic reagents in cross coupling reactions and other reactions Their synthesis is usually complicated and demanding Alkynes can be synthesized in one step, but the source of alkynes is very limited compared with alkenes Sometimes z-alkenyl halides can be obtained in modern olefin metathesis reaction, while e-alkenyl halides can be obtained from raw materials containing large steric substituents Carreira reported the method of direct C (SP 2) - H functionalization to construct alkenyl iodide, which used pyridinamide as the guiding group At present, there is no report on the method of constructing various alkenyl electrophilic reagents by functionalization of simple alkene C (SP 2) - H Recently, a regioselective and stereoselective method has been reported by Professor Tobias Ritter of Max Planck Institute of coal research in Germany to obtain alkenylelectrophilic reagent, which is directly prepared by C (SP 2) - H substitution reaction of non activated alkenes (scheme 1) The results were recently published in angelw Chem Int ed (DOI: 10.1002 / anie 201914215) (picture source: angelw Chem Int ed.) the best reaction conditions given by the author are: alkene (1.00 equiv.) and thianthene sulfur oxide (1) (1.03 equiv.) dissolved in anhydrous acetonitrile, adding trifluoroacetic anhydride and trifluoromethylsulfonic acid at low temperature, the reaction can be completed in less than two hours at room temperature Subsequently, the range of substrate was investigated, as shown in Table 1 (picture source: angelw Chem Int ed.) from table 1, it can be seen that for all α - olefins, e-alkenyl sulfonium salt is given as the main product For internal alkenes, the products of conformation retention are given, such as e-configuration (2,22) and Z-configuration (3,33) alkenes Allylic substituents with large steric substituents can also react (6,7) The reaction has a wide range of functional group compatibility, such as imide (11), primary alkyl bromide (8), alcohol (9), ether (15, 16), amide (17), esters (26-33), ketone (29), piperidine (30), nitro (31), sulfonamide (32), sulfone (33) and nitrile (33), etc The electron rich aromatics tend to undergo aromatic substitution reactions, while the electron deficient (11, 13), electroneutral (12) and six membered heterocyclic aromatics (26, 27, 31) are compatible The reaction is even compatible with the C (SP 3) - H bond of dibenzyl and allylic, without double bond isomerization and allylic functionalization For some 1,2-disubstituted olefins and trisubstituted olefins (15), the regioselectivity decreased Subsequently, in order to explain the high selectivity of the reaction, the author carried out a mechanism experiment (scheme 2) Two intermediates, 2-Int and 3-int, were isolated without post-treatment These two intermediates are obtained by [4 + 2] cyclization However, the addition of alkenes via thioanthracene free radical cations, and the production of bicycles through single electron oxidation addition and cyclization cannot be excluded In this process, the speed of oxidation addition and cyclization is faster than that of single bond rotation In addition, the addition of thioanthracene radical cations to olefins can form a double addition product After the intermediate was treated with weak base, the corresponding products 2-tt and 3-tt of alkenyl sulfonium were obtained, and the geometry of alkene was maintained Considering the stereoelectricity, the elimination mechanism of E2 is excluded, because the leaving group and proton cannot be in the trans coplanar The E1 mechanism can not be excluded, but it is contrary to the result of high retention of the double bond configuration In conclusion, the process may be carried out by E1cb mechanism, and deproton is the rate determining step (image source: angelw Chem Int ed.) next, the author carried out a series of transformation experiments on the substrate of alkenyl sulfonium salt (Table 2) Palladium catalyzed cross coupling reaction can give the products (34, 35, 36) with double bond configuration Alkenyl sulfonium salts can also be used as electrophilic reagents in ruthenium catalyzed reactions to synthesize alkenyl halides (37, 38) and pseudo halides (39) with stereoscopic configuration (photo source: angel Chem Int ed.) Summary: Tobias Ritter group reported a method of region and stereo selection to synthesize alkenyl electrophilic reagent, which is suitable for all kinds of non activated alkenes The preliminary mechanism experiments show that the thianthrene reaction is carried out by cycloaddition process Alkenyl sulfonium salt is compatible with a large number of functional groups, and can be used as electrophilic reagent for palladium catalyzed C-C cross coupling reaction and ruthenium catalyzed C-X bond formation reaction.
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