In recent years interestint he development of protocols that facilitate the oxidativea ddition of gold to access mild cross-coupling processes mediated by this metal hasincreased. In this context, we report herein that ascorbic acid, an atural and readily accessible antioxidant, can be used to accelerate the oxidative addition of aryldiazonium chlorides onto Au I .T he aryl-Au III speciesg enerated in this way,h as been used to prepare 3-arylindoles in ao ne-pot protocol startingf rom anilines and para-, meta-, and ortho-s ubstituted aryldiazonium chlorides. The mechanism underlying the oxidative addition has been examined in detail based on EPR analyses, cyclic voltammetry,a nd DFT calculations. Interestingly, we have found that in this protocol, the chloride atom induces the Au II /Au III oxidation step.
In the last decade, aryldiazoniums alts have attracted interest as coupling partnersi nc ross-couplingr eactions mediated by gold. Initially,t he presence of ap hotocatalyst and al ight sourcew as neededt oa chieve gold oxidation with these electrophiles.H owever, recently,i th as been shown that in some instances just heating, light irradiation, or the addition of certain bases and/or nucleophiles is enough. In this review,t he transformations developed so far using aryldiazonium salts as electrophiles are summarized with special emphasis on mechanistic studies.T he information gained by different authors, indicatest hat the specific steps of gold oxidation with aryldiazonium salts depends upon the activation mode of the diazonium salt.
A convenient two-step, one-pot synthesis of 4-chloro-2-(trichloromethyl)pyrimidines starting from 2-(trichloromethyl)-1,3-diazabutadienes is described. These nitrogen heterocycles were prepared by a sequential acylation/intramolecular cyclization reaction between 2-(trichloromethyl)-1,3-diazabutadienes and acyl chlorides in the presence of triethylamine followed by treatment with POCl3. This is the first report for the synthesis of this type of 4-chloro-2-(trichloromethyl)pyrimidine derivatives and serves as a source for a wide variety of other substituted pyrimidines by nucleophilic substitution reactions.
We
describe, herein, arylative carbocyclization of alkynes catalyzed
by gold. In this process, Au(I) is oxidized to Au(III) with aryldiazonium
tetrafluoroborates following a photosensitizer-free and irradiation-free
protocol. Ascorbic acid acts as a radical initiator, generating aryl
radicals. According to DFT calculations, these radicals are added
to Au(I), leading to a Au(II) species that is further oxidized to
Au(III) with the assistance of a tetrafluoroborate anion. The overall
arylative carbocyclization process is very energetically favorable,
transforming arylpropargyl ethers into valuable 3,4-diaryl-2H-chromenes in a completely regio- and stereoselective fashion.
Furthermore, we show that one of the synthesized 3,4-diaryl-2H-chromenes exhibits polymorphism with marked differences
in the color of its crystals, a property that could lead to the development
of colored derivatives in the future.
A gold-catalysed coupling of aryldiazonium salts with arylboronic acids is described. The reactions proceed in satisfactory yields under irradiation with blue LEDs in the presence of KF and a catalytic amount of ascorbic acid. Notably, 4-nitrobenzendiazonium tetrafluoroborate is sufficiently reactive to undergo the coupling with a variety of arylboronic acids in the absence of aryl radical initiators. The coupling is applicable for electron-donating and electron-withdrawing groups present at the para, ortho, and meta positions of both substrates.
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