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    Home > Angelw: the research group of the student of the University of Minster in Germany realizes the deoxidization and boronization of secondary alcohol and tertiary alcohol

    Angelw: the research group of the student of the University of Minster in Germany realizes the deoxidization and boronization of secondary alcohol and tertiary alcohol

    • Last Update: 2019-06-01
    • Source: Internet
    • Author: User
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    As an important structural unit in organic synthesis, alkyl borate is not only a good substrate in the transition metal catalyzed C-C coupling reaction, but also the C-B bond is easy to be converted into other useful functional groups Recently, the boronization of halides, carboxylic acids and amines with diboron by free radical chemistry or transition metal catalysis has been reported (scheme 1, A-C) However, most of the alcohols are cheap and easy to obtain, but their deoxidization and boration need to be developed Although the metal catalyzed boration of primary / secondary alkylates of p-toluenesulfonate has been reported, the boration of tertiary alcohols has not been reported In the presence of Lewis base, B 2cat 2 is very effective for the boration of alkyl radicals Inspired by this, a similar alcohol activation strategy has been designed by the research group of armido student, University of Minster, Germany recently Using silane as a free radical medium, xanthate can be converted into corresponding scheme 1D This achievement was published on angew.chem Int ed (DOI: 10.1002 / anie 201904028) First, the author studied the boration of xanthate The author found that the conversion rate of the boration of carbon radicals with B 2cat 2 was faster than that of the reduction of carbon radicals with silane Therefore, silane is used as the free radical medium in the targeted deoxidization and boration In the presence of different silanes, the author used xanthate 1A as a model substrate to react with B 2cat 2 (method a, scheme 2) It was found that ttmss (trimethylsilyl) silane) was the best reductant, and xanthate functional group was the best carbon free radical precursor The competitive reduction of alkyl radicals can be avoided by using too much diboron (4.0 Eq.) Reducing the amount of B 2cat 2 to 2 Eq will lead to a decrease in the yield Therefore, 4 Eq diborane is used in all subsequent experiments (picture source: angelw Chem Int ed.) under the optimized conditions, the author successfully realized the boration of xanthate 1b, 1D and 1E by using method a Moreover, the deoxidization and boration of menthol and isoborneol have excellent stereoselectivity It is found that the method B can be obtained by irradiating the reaction mixture with blue LED without AIBN In addition, if the synthesis of xanthate derivatives is difficult, the corresponding o-thiocarbamate must be selected as the precursor of carbon radicals The reaction has a wide range of functional group tolerance, such as BOC protected amines, esters, ketones, acetaldehyde of haicosapogenin, and acetaldehyde found in tropine and epiandrosterone (image source: angel Chem Int ed.) next, the author looks for a suitable active group for the deoxyboronization of tertiary alcohols Oxalate is a suitable precursor of TERT alkyl radical rather than xanthate It has been reported that oxalate can be reduced and cracked at high temperature by activated zinc or strong reducing photocatalyst Then, we use methyl oxalate 4A as a model substrate to study the formation of carbon radicals and obtain the expected product 5A (scheme 3) By optimizing the conditions, the author found that DMF was the best solvent All kinds of TERT alkyl borates can obtain the corresponding products with good to extremely good yields In addition, the reaction is also resistant to olefins, benzylamines, esters, etc The reaction of benzylmethyloxalate is good, but the reaction of secondary alkylmethyloxalate without activation is poor It is also found that the third-order propargyl alcohol can be regioselectively converted into allyl borate through this strategy, which is the first method to synthesize these compounds through free radical pathway Among them, branched propargyl alcohol has a better reaction, which can also be used for the bioactive merestriol However, the reaction of substituted alkynes is complex Finally, the author proposed the mechanism of photoreduction process (scheme 4a): through the visible light induction, excited IR (PPy) 3 catalyst and methyl oxalate 4 undergo electron transfer and C-O bond cleavage to produce alkyl radical a, adding B 2cat 2 to obtain the adduct radical B, which is captured by the solvent to produce intermediate C The weak B-B single electron σ bond is easy to be split into 5 and D; the free radical D is oxidized by photocatalyst (IR (IV)) to produce cation e, which regenerates IR (III) In addition, the reaction mechanism of non chain free radical ttmss process (scheme 4b, method a): free radical a and (me 3Si) 3siscosme are obtained by the addition of ttmss free radical produced by hydrogen transfer and xanthate 1; free radical a is boronized by B and C Free radical D is oxidized by 2-cyano radical to produce E At present, the mechanism of boron formation (m ethod b) of ttmss is not clear (picture source: Angel W chem Int ed.) conclusion: the research group of armido student has developed two different methods of deoxidizing free radical boration of secondary alcohol and tertiary alcohol, that is, by using xanthate or methyl oxalate as free radical precursor and passing through nonmetal ttmss As a free radical mediated pathway or photocatalytic oxidation-reduction catalysis, this method not only has mild reaction conditions, but also has a wide range of substrate application and functional group tolerance.
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