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    Home > JACS: bimetallic catalytic isomerization of epoxide to allyl alcohol

    JACS: bimetallic catalytic isomerization of epoxide to allyl alcohol

    • Last Update: 2019-06-24
    • Source: Internet
    • Author: User
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    From nucleophilic substitution to transition metal catalyzed cross coupling, these transformations utilize the rich reactivity of electrophilic reagents Epoxides are the most important synthons in organic chemistry They can be rearranged into allylic alcohols under alkaline conditions This rearrangement is redox neutral However, various side reactions, such as nucleophilic ring opening and α - depolymerization, are inevitable due to the use of strong bases Therefore, the synthesis of chiral allyl alcohol from epoxide is limited Recently, Song Lin and others from Cornell University published a basic redox relay strategy for isomerization of epoxides to allylic alcohols under mild conditions using Ti and Co double catalysis In order to avoid the use of alkali accelerants, the author combined a pair of single electron oxidation and reduction events in the same catalytic cycle This strategy was named free radical redox relay (scheme 1a) Relevant research results were published in J am Chem SOC (DOI: 10.1021 / JACS 9b04993) (source: J am Chem SOC.) the author proposes to introduce co cocatalyst and utilize the good reactivity of Ti III to induce all the epoxides to split (ERO, scheme 1c) The free radicals at β - C induce the weakening of C-H bond at γ - position, which makes the hydrogen atom transfer (HAT) and co II catalyst provide the required olefin functional groups The catalyst intermediate can then be used for proton transfer / electron transfer (Pt / ET) with Ti IV alkoxide to transport the desired allyl alcohol and regenerate Ti III and co II catalysts Therefore, the catalyst is very important for the successful implementation of the reaction The catalyst will adapt to the thermodynamics and kinetics of the key ero, hat and Pt / ET steps in the same catalytic cycle First, the authors evaluated various combinations of Ti and Co catalysts (Table 1) It is found that the catalyst combination of CP 2 TiCl 2 (10 mol%) and [CO] 5 (5 mol%) can effectively promote the conversion of epoxide 1a to the desired product 2A under the conditions of Zn (20 mol%) and et 3 N · HCl (2 equivalent), with a yield of 99% (source: J am Chem SOC.) the isomerization of various epoxides (scheme2a) was investigated under the optimum conditions Single and four substituted epoxides are challenging substrates Three substituted epoxide 1K gives different proportion of regional isomeric olefins, [CO] 4 is propitious to the formation of Endo product (71:29), [CO] 5 is propitious to the formation of exo product (39:61) 2l-o is allowed to form without alkali, and there is no differential isomerization of acid C-H The epoxide 1s and 1t rich in enantiomers were converted into corresponding alcohols, and the configuration was completely preserved Ti / Co double catalytic system has also been successfully applied to the kinetic resolution (scheme 2b) of the diastereomer mixtures of several epoxides The author used epoxide 1AA and 1ab to identify the free radical intermediate (scheme 2C) The results show that the hat process has a faster rate in the presence of electron deficient [CO] 5, and it is a competitive process with cross cyclization Compared with the previous work on Ti mediated epoxide isomerization, this double catalytic system shows complementary chemical selectivity (scheme 2D) with several tested substrates (source: J am Chem SOC.) finally, the author developed a enantioselective scheme (scheme 3a) using chiral Ti complex Few methods can be used to synthesize cyclopropanols enriched with enantiomers Kharasch sosnovsky reaction can functionalize allyl C-H bond, but it usually has low enantioselectivity and functional group tolerance The alkali mediated strategy requires a strong base and a chiral amine catalyst The former eliminates the base sensitive substrate, while the latter usually requires multi-step synthesis When conjugated ketene was considered, Corey Bakshi Shibata reduction was not satisfactory Using epoxicocyclohexene (1a), Kagan's complex 11 reacts with [CO] 8 to produce 13A in 81% yield and 83% ee The range of enantioselective isomerization can be extended to various racemic epoxides (scheme 3b) (source: J am Chem SOC.) finally, the author proposed a synergetic double catalytic cycle reaction mechanism (scheme 4) for epoxide isomerization (source: J am Chem SOC.) conclusion: Song Lin et al Of Cornell University used Ti / CO redox relay catalyst to realize the redox neutral epoxide isomerization, and also found the resolution of non enantiomeric epoxide and the desymmetry of racemic epoxide This work represents a rare example of Ti redox catalysis without stoichiometric reductant.
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