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    Home > JACS: Iron catalyzed π - bond α - C-H functionalization: hydroxylation of alkynes and alkenes

    JACS: Iron catalyzed π - bond α - C-H functionalization: hydroxylation of alkynes and alkenes

    • Last Update: 2019-12-15
    • Source: Internet
    • Author: User
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    It is an important goal of organic synthesis to develop new functional groups of hydrocarbons by using abundant reagents and sustainable catalysts It is reported that high valent iron has been used to catalyze o - / n-functionalization of C-H bonds, but this method is rarely used to construct C-C bonds (scheme 1a) Recently, the oxidative coupling between iron catalyzed C-H bonds and organometallic reagents has become possible due to the metallization promoted by chelating guiding groups or 1,5-hydrogen transfer Although this method can introduce universal carbon based fragments, it needs good reactivity to realize the guiding group action (scheme 1b) At present, it is still a challenge to construct C-C bond by catalytic C-H functionalization without any guiding group The functionalization strategy of inactive C-H bonds usually involves the removal of hydrogen atoms (H ·) or h-equivalents As an alternative, the C-H bond can be broken by the removal of protons (H +), resulting in an organic metal complex equivalent to a metal stable propargylic anion, which reacts with an electrophilic reagent to form a-functional product (scheme 1c) (photo source: J am Chem SOC.) at present, there have been reactions of introducing functional groups into the allylic position, but the intermolecular construction of C-C bond is still challenging In addition, the functionalization of propargylic position of alkyne is still difficult to achieve when the C-C triple bonds are not converted at the same time Intermolecular reactions usually need to introduce guiding or activating groups on alkynes or their mates or depend on free radical chemistry, but their regioselectivity will be affected Due to the high acidity of C-H bond, the C-H bond functionalization of hydrogen as proton rather than hydrogen or leaving hydrogen atom is particularly suitable for the internal alkyne Rosenblum group's research on FP (η 2-olefin) + complex shows that coordination leads to a sharp increase in the acidity of allylic (PKA ~ 43 → < 10, scheme 1D) In addition, the neutral σ - allylic iron complex formed by deprotonation reacts with a series of carbon / heteroatom electrophilic reagents to obtain the α - functionalized product (scheme 1E) of olefins Although the chemical study of the FPR complex derived from alkynes is not enough, the stoichiometric reaction model provides the possibility for the development of the catalytic system for the functionalization of internal alkynes In addition to finding suitable cyclopentadienyl ligands, it is also necessary to select appropriate Lewis acids to activate aldehydes and bases to capture alkyne protons Recently, Wang Yi Ming group, University of Pittsburgh, USA, developed a simple catalytic method for the synthesis of high allylic alcohol from aldehydes and internal alkynes The result was published in J am Chem SOC (DOI: 10.1021 / JACS 9b11716) First of all, 1-phenyl-1-propargyne (1a) and 4-Bromobenzaldehyde (2a) were used as model substrates to explore the reaction (Table 1) It was found that the best pre catalyst was 2,2,6,6-tetramethylpiperidine (tmph) and [FP * (THF)] + BF 4 - (FP * = (c5me5) Fe (CO) 2 +) The results show that iron complex, Lewis acid and base are indispensable for the reaction (photo source: J am Chem SOC.) next, the author investigated the applicability of the catalyst system to two coupling spouses (Table 2, upper part) Aromatic aldehydes from weak electron rich to strong electron deficient can be used as coupling mates The reaction can tolerate 2,6-substituent, diaryl ketone, ester, difluoroacetal, pinacol ester and sulfonamide Alkylaldehydes, electron rich heteroarylaldehydes, simple or cyclic α, β - unsaturated aldehydes, which can not be enolated, can also be used as aldehyde coupling ligands For the starting materials with low reactivity, the yield can be increased by adding Zn (NTF 2) 2 Next, the author evaluated the range of application of alkynes (Table 2, bottom) From weak electron rich to electron deficient arylmethylacetylene and advanced arylalkylacetylene can be coupled, but the enantioselectivity is poor In addition, the catalytic system is also suitable for complex skeletons derived from cholesterol and tocopherol At higher catalyst loading, some hindered dialkyl acetylene can also participate in the reaction (photo source: J am Chem SOC.) later, the author investigated the practicability of the reaction on olefin substrate (Table 3) Allyl functionalization can be carried out on all the end olefin substrates to obtain the corresponding high allyl alcohol products, and even the non activated olefins can also be coupled to obtain the corresponding products (photo source: J am Chem SOC.) in order to prove the practicability of the method, the author derived two of the products (3AA, 3AK) by the method of literature, and obtained several derivatives (scheme 3), including tetrasubstituted trifluoromethylolefins, tetrahydrofuran derivatives and two alkenyl alcohols (photo source: J am Chem SOC.) finally, the author proposed the following catalytic cycle (scheme 4): firstly, amines capture propargylic protons of alkyne iron π - complex cation I (CP r = substituted cyclopentadienyl) to obtain neutral allylic iron σ - complex II Then, II was electrophilically added to obtain the alkyne iron complex cation III, which then exchanged with alkyne to release the coproduct and complete the catalytic cycle (photo source: J am Chem SOC.) in order to confirm the above catalytic cycle hypothesis, the author studied each catalytic step (scheme 5) First, the stoichiometric reactivity of FP * - alkyne complex 1K - I ([Cp * Fe (CO) 2 (alkyne)] + [BF 4] –) was studied Then, the σ - allylic iron complex 1K - II was obtained by treating FP * - alkyne complex 1K - I with ET 3N Finally, in the presence of BF 3 · et 2O, 1K - II reacted with aldehydes 2a to obtain 3KA In addition, each of the above catalytic intermediates can be identified by spectroscopy (photo source: J am Chem SOC.) conclusion: Wang Yi Ming research group has developed an iron catalyzed functionalization method of C-H bond of allynyl and allynyl, which is used to synthesize a series of unsaturated alcohol coupling products, opening up a new way for the functionalization strategy of C-H bond of allynyl and allynyl.
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