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    Home > Angew: site selective 1,2- bifunctionalization of alkenylborates

    Angew: site selective 1,2- bifunctionalization of alkenylborates

    • Last Update: 2020-01-21
    • Source: Internet
    • Author: User
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    Organic borides are widely used in synthetic chemistry because of their adjustable reactivity, stable properties and easy operation Traditionally, the synthesis of organoborates is as follows: 1) the reaction of equivalent organometallic reagents with electrophilic boron reagents; 2) the hydroboration of unsaturated double bonds; 3) the boration of C-X bonds catalyzed by transition metals Recently, chemists use equivalent organic lithium reagent to react with alkenyl boron to form boric acid complex, and then through the single electron or double electron [1,2] - transfer reaction process, they can produce organic borate ester compounds Despite some progress, there are still some challenges to be addressed in this area For example, the catalytic method can avoid the use of equivalent organometallic reagents, which will further increase the practicability of this method Recently, on the basis of the study of electrophilic cross coupling reaction and olefin functionalization in the early stage of the laboratory, the Ruben Martin research group of the Catalan Institute of chemistry in Spain, combined with photocatalyst and Ni catalyst, realized the site selective bifunctional reaction of alkenylborate ester by using three electrophilic reagents This reaction does not need equivalent organometallic reagents, and has mild conditions and high selectivity It provides a new synthesis method (scheme 1) for the functionalized alkylborate compounds Relevant research results were published in angelw Chem Int ed (DOI: 10.1002/anie.201916279) (source: angelw Chem Int ed.) the author's idea is as follows (scheme 2): firstly, the alkyl halides generate nucleophilic alkyl radicals through a single electron transfer process, which can selectively add with alkenylborates to form intermediate I; on the other hand, zero valent metal Ni can oxidize with aryl halides to form bivalent Ni complex II Finally, free radical intermediate I was captured by complex II to form trivalent Ni intermediate III, which was eliminated by reduction to obtain the final product At the same time, the monovalent Ni can be regenerated by single electron transfer (source: angelw Chem Int ed.) firstly, the author used methyl 4-bromobenzoate 1a, alkenylborate 2a and tert butane 3A as template substrates to optimize the conditions (scheme 3) By optimizing the conditions of nickel source, ligand, photosensitizer and electron donor, the optimal reaction conditions were determined as follows: NiCl2 · DME as metal salt, L1 as ligand, 4czipn as photosensitizer, TMEDA as electron donor, substrate reacted for 20 h under blue light irradiation, and finally the target product was obtained with 73% separation yield (source: angelw Chem Int ed.) determined the optimal reaction conditions, and then the author expanded the range of substrates (scheme 4) Many aryl halides with different electrical and steric resistances can participate in the reaction It is worth mentioning that lithium chloride needs to be added to improve the yield in some examples The reaction has good functional group tolerance, and the ester group, nitro group, olefin group, sulfonamide group, sulfone group, acetal group, ketone group, arylborate group, aryl halide group and other groups are all compatible with the reaction conditions When the reaction was expanded to gram scale, the product was still obtained with similar yield The third-order alkyl bromine and olefin substrates were also extended to obtain the target product (scheme 5) in medium to good yields (source: angelw Chem Int ed.) (source: angelw Chem Int ed.) in order to further explain the practicability of the reaction, the author subsequently transformed the product, mainly including: 1) oxidation reaction; 2) C-C bonding such as alkenylation, aromatization, acylation and heteroarylation Finally, the author further verified the previous assumption through free radical inhibition test, free radical cyclization test and equivalence test Summary: Ruben Martin group has developed a modular method to realize the selective 1,2-difunctional reaction of alkenylborates with three different electrophilic reagents The reaction conditions are mild and the selectivity is high, which provides a new idea for alkylborate compounds.
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