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    Home > Angelw: CO catalyzed intermolecular [2 + 2] cycloaddition of alkynes and alkenes

    Angelw: CO catalyzed intermolecular [2 + 2] cycloaddition of alkynes and alkenes

    • Last Update: 2019-02-01
    • Source: Internet
    • Author: User
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    Cyclobutane and cyclobutene derivatives are important structural segments in natural products and bioactive compounds, and are also commonly used synthesis intermediates Among them, [2 + 2] cycloaddition is the most direct and atomic economic method to construct quaternary carbon ring The 3-alkylidene cyclobutene skeleton (scheme 1a) can be constructed by intramolecular [2 + 2] cycloaddition of propanediene derivatives containing alkynes under heating conditions or transition metal catalysis In contrast, intermolecular alkyne propanediene [2 + 2] cycloaddition is more practical, but the reaction is limited to activated alkynes or activated propanedienes For example, the method developed by tamaru group requires the use of excessive terminal alkynes (scheme 1b) At present, the selective intermolecular [2 + 2] cycloaddition of propadiene and alkyne without electron bias under the catalysis of transition metal is still pending Recently, the naohiko yoshikai research group of Nanyang University of Technology reported the intermolecular [2 + 2] cycloaddition (scheme 1c) of alkynes and alkenes catalyzed by CO on angelw.chem Int ed (DOI: 10.1002 / anie 201813283) (photo source: angelw Chem Int ed.) previously, hilt team reported the [2 + 2] cycloaddition of terminal alkynes and tension alkenes catalyzed by low-cost Co Recently, rajanbabu team has realized [2 + 2] cycloaddition and asymmetric hydroolefinization of 1,3-enyne and ethylene catalyzed by CO in series Inspired by this, the author first used the reaction of 4-octyne (1a) and cyclohexylpropadiene (2a) to optimize the reaction conditions (Table 1) Finally, the author determined the reaction conditions of cobr 2 (5 mol%) / dppf (5 mol%) / in (20 mol%) and DCE as solvent, and obtained the expected [2 + 2] cycloaddition product 3AA with 89% yield, which is a single region and stereoisomer (picture source: angelw Chem Int ed.) in the optimal catalytic system, the author explored the application range of alkynes in [2 + 2] cycloaddition reaction (Table 2) All kinds of dialkyl, diaryl alkyne and phenylalkyl alkyne can react with 2a by cycloaddition, and the corresponding products are obtained The reaction has high yield and excellent selectivity The regioselectivity of 1 - (1-naphthyl) - 1-propargyne is lower than that of 1 - (2-thiophenyl) - 1-hexyne Alkyne and alkynyl ester can also be cycloaddition of [2 + 2] to obtain products with different regioselectivity However, the reaction of terminal alkynes such as phenylacetylene or 1-octyne can produce a mixture that is difficult to deal with In addition, the reaction can tolerate hydroxyl, methoxy, allyloxy and ester groups (image source: angelw Chem Int ed.) next, the author screened the application scope of propylene diene (Table 3) All kinds of mono alkyl substituted propadiene can undergo [2 + 2] cycloaddition reaction to obtain corresponding products The reaction has a wide range of functional group tolerance, including hydroxyl group, siloxy group, allyloxy group, ester group, phthalimide group and chlorine group In addition, 1,1-disubstituted propanedienes, such as 5-vinylidene nonane and vinylidene cyclohexane, can be regioselectively cycloaddition on the terminal C = C bond; dipropylene can be cycloaddition with Diphenylacetylene (image source: angelw Chem Int ed.) later, the author also investigated the reactivity of 1,3-disubstituted propadiene (scheme 2) Under the standard conditions, 1,3-dihexylpropanediene 2O reacts with alkyne 1E to produce 3EO (yield of 68%) and alder ene type product 4a (yield of 22%) at the same time Similarly, 3 EP (72%) and 4 B (19%) were obtained by the reaction of 1,2-cyclononyldiene 2p with 1E (picture source: angelw Chem Int ed.) through monitoring the [2 + 2] cycloaddition of 1a and 2a, the author found that there was a significant induction period in the reaction: only trace amount of 3AA was generated in the first 2 hours, and then the reaction rate gradually increased in the next 9 hours until the completion of the reaction, because the CO (II) pre catalyst slowly reduced to active low-cost Co In addition, the control reaction catalyzed by CoCl (PPH 3) 3 / dppf was carried out, and 3AA (scheme 3a) was successfully obtained when the amount of catalyst InBr 3 was added The above results show that the catalytic system involves CO (I) species, which is produced by CO (II) reduction At last, the possible reaction pathway (scheme 3b) is proposed Alkyne 1 and propanediene 2 replace less C = C bond for regioselective oxidation and cyclization to produce cyclocobalt intermediate a, and then [2 + 2] cycloaddition product 3 can be obtained by reduction elimination; a can also be eliminated by β - h to produce intermediate B, and then B can be reduced to eliminate to produce triene 4 In this reaction, the regioselectivity of phenylalkylalkynes is consistent with the typical CO catalytic reduction coupling, while the low regioselectivity of 1 - (1-naphthyl) - 1-propargynes may be caused by the spatial repulsion between CO and naphthyl in the process of oxidation and cyclization (image source: angel Chem Int ed.) finally, the cycloaddition product obtained by this method was further transformed (scheme 4) For example, under the catalysis of Pd / C, 3fa is hydrogenated via the external C = C bond to obtain cyclobutylene 5, which is then borohydride oxidized to obtain alcohol 6 In addition, 3VA and 3xa can react with maleic anhydride by Diels alder to synthesize tricyclic product 7 and tetracyclic product 8, respectively Conclusion: naohiko yoshikai group has developed a co catalyzed selective intermolecular [2 + 2] cycloaddition reaction of alkynes and propadienes for the synthesis of various 3-alkylidene cyclobutenes The reaction is resistant to various internal alkynes and monosubstituted and disubstituted propadiene, and shows high regioselectivity to alkynes and propadiene.
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