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    Home > JACS: synthesis of z-olefins from terminal alkynes and terminal alkenes by silver catalyzed hydroalkylation of Alkynes

    JACS: synthesis of z-olefins from terminal alkynes and terminal alkenes by silver catalyzed hydroalkylation of Alkynes

    • Last Update: 2019-11-07
    • Source: Internet
    • Author: User
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    Olefins are important intermediates in organic synthesis, which are common in drugs and other bioactive compounds The π bond of alkenes not only hinders the rotation of C-C σ bond and makes it structurally rigid, but also leads to different stereoisomeric forms (E and z) of disubstituted alkenes However, the effective synthesis of z-alkenes with unstable thermodynamics is still challenging The synthesis target of z-olefin includes the following three points: 1 The formation of double bond; 2 The control of double bond geometry; 3 The construction of two small segments through the formation of new C-C σ bond At present, for the synthesis of z-alkenes, the common methods of alkyne reduction, cross coupling, cross decomposition of alkenes or isomerization of alkenes are difficult to achieve It is gratifying that the hydroalkylation of alkynes can achieve the above three goals to synthesize z-alkenes (scheme 1a), which is also suitable for the synthesis of 1,1-disubstituted alkenes (scheme 1b) However, there are few reports on the study of z-alkene by hydroalkylation In 2015, the Fe catalyzed free radical hydroalkylation reported by Hu Xile et al Can produce a mixture of E and z-olefins with different selectivity (scheme 1c), but this method is limited to arylacetylene (photo source: J am Chem SOC.) in order to realize the synthesis of pure z-olefin, a new method of alkyne hydroalkylation needs to be developed In 1975, H C Brown reported the research work on the reaction of lithium acetylene with alkyl borane to form borate complex (scheme 2a) Inspired by it, Gojko lalic group of University of Washington realized the synthesis of non enantiomeric pure z-alkenes through the coupling of terminal alkynes and alkyl boranes catalyzed by silver, which was recently published in J am Chem SOC (DOI: 10.1021 / JACS 9b09336) (photo source: J am Chem SOC.) the author thinks that in 1, In the migration of 2-Metal salts, the metal cation complex can be used instead of Br? Nsted Acid (scheme 2b), and the possible catalytic mechanism (scheme 3) is speculated: in the presence of alkoxy base, the metal catalyst promotes the formation of acetylene; then, the alkylborane is added, the metal ions coordinate with alkyne and promote 1, 2-Metal salt migration; finally, adding alcohol, the intermediate is debonded and demetallized to produce olefins (photo source: J am Chem SOC.) later, the author explored the reaction between alkyne 1 and alkylborane 2 in the presence of copper, silver or gold catalysts (Table 1), and found that silver catalyst had the best effect and only formed z-isomer 3 Through further optimization of the conditions, the author found that triazole carbene ligands and non-polar solvents can improve the yield of z-olefin products Under the optimum conditions, the complete conversion can be achieved by using an appropriate amount of alkylborane (1.3-1.5 equivalent) Catalyst, alkali and methanol are all necessary for the reaction (photo source: J am Chem SOC.) the reaction for the synthesis of z-alkenes has universal substrate applicability (Table 2) Functionalized alkynes and electron rich / electron deficient arylalkynes can participate in the reaction; various end alkenes, including 1,1-disubstituted alkenes, in-situ formed alkylboron, are also suitable substrates The reaction has a wide range of functional group tolerance, and can tolerate alcohol, aldehyde, ketone, ester, nitrile, ene and BOC The protected primary amines and secondary amines, anilines, alkyl bromides and chlorides, aryliodides and bromides, arylborates, epoxides, acetals and alkyl or methylsilyl ethers can also be compatible with heterocycles such as pyridine, thiazole, benzimidazole, benzopyrazine, pyrimidine, pyridazine, thiophene and furan In addition, the reaction can be enlarged to the scale of gram In the synthesis of olefins 3, 4, 17, 35, 39 and 41, the crude product Z: e > 300:1 (photo source: J am Chem SOC.) although the hydroalkylation reaction has a wide range of applications, it also has certain limitations: Although primary alkyl participates in the migration of 1,2-metal salts, secondary alkyl, tertiary alkyl, alkenyl and aryl are not involved; free amines, carboxylic acids and carboxylates are not resistant to reaction conditions In addition, in the alkylborane, when there are electron absorption groups between the two carbon and boron atoms, the yield of the reaction is significantly reduced Next, the author also proved the practicability of the method (scheme 4) We applied it to the synthesis of zucapsaicin (scheme 4a) and falcarindiol (scheme 4b) (photo source: J am Chem SOC.) finally, the author explored the speculated reaction mechanism (scheme 5) In the stoichiometric reaction, in the presence of alkoxy base, silver catalyst is easy to react with terminal alkyne 61 to form silver acetylene 62; alkyne and alkyl borane 2 react rapidly to form borate complex 63, its single crystal X-ray diffraction structure (Figure 1) shows the silver ionon coordination between alkyne and borate, so it is expected to promote the migration of 1,2-metal salt However, the disadvantageous orientation of the transalkylation is consistent with the stability of the intermediate observed The silver borate complex is a feasible catalyst for the hydroalkylation reaction (photo source: J am Chem SOC.) (photo source: J am Chem SOC.) the above experimental results provide direct support for the proposed reaction mechanism (M = AG, scheme 3) in some key aspects, but it is still necessary to further study the reaction mechanism, including the subsequent steps of 1,2-metallization and migration Conclusion: Gojko lalic group has developed a silver catalyzed hydroalkylation method of terminal alkynes (alkylborane as coupling spouse), which can be used to synthesize non enantiomeric pure z-alkenes efficiently This method has a wide range of functional group compatibility and substrate application It is found that the rearrangement of 1,2-metal salts is the key step to control the stereochemistry of the reaction.
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