The synthetic utility of aryl radicals has been established in the last century, however, their broad applications were hampered by ineffective generation methods. It was in the last decade, that a ...rapid development of various redox systems took place, thus triggering a renaissance of aryl radical chemistry. This tutorial review focuses on the start-of-the-art methods for generation of aryl radicals. Primarily, various light-induced systems, including photoredox catalysis, visible light transition metal catalysis, and chemistry of electron donor-acceptor complexes, are reviewed. The main current precursors of aryl radicals are evaluated together with the selected examples of their modern applications.
Aryl radicals have been long recognized as useful synthetic intermediates. However, their broad application was hampered due to the lack of mild and general methods for their generation. This tutorial review summarizes recent techniques for mild and efficient generation of aryl radicals.
The first example of direct C−H alkynylation of electron-rich heteroaromatics has been demonstrated. This mild, simple, and general method allows for the efficient synthesis of diverse alkynyl ...heterocycles. Kinetic isotope effect studies support an electrophilic substitution pathway for this transformation.
An efficient synthesis of densely substituted 2-aroylindolizines via the palladium-catalyzed carbonylative cyclization/arylation is reported. This transformation proceeds via the 5-endo-dig ...cyclization of 2-propargylpyridine triggered by an aroyl Pd complex. It produced diversely substituted 2-aroylindolizines in good to excellent yields.
A new general and easily installable silicon-tethered pyridyl-containing directing group (PyDipSi) that allows for highly efficient and regioselective Pd-catalyzed ortho C−H acyloxylation of arenes ...has been developed. It has also been demonstrated that this directing group can efficiently be removed as well as converted into a variety of other valuable functional groups. In addition, the installation of the PyDipSi directing group along with pivaloxylation and quantitative conversion of the PyDipSi group into a halogen functionality represents a formal three-step ortho oxygenation of haloarenes.
An efficient two-component palladium-catalyzed arylation/cyclization cascade approach toward a variety of N-fused pyrroloheterocycles has been developed. This transformation proceeds via the ...palladium-catalyzed coupling of aryl halides with propargylic esters or ethers followed by the 5-endo-dig cyclization leading to highly functionalized pyrroloheterocycles in good to excellent yield.
An efficient method for the synthesis of fused heteroaromatic dihydrosiloles via Ni-catalyzed hydrosilylation/intramolecular Ir-catalyzed dehydrogenative coupling of the Si–H bond with the ...heteroaromatic C–H bond has been developed. The method is efficient for both electron-deficient and -rich heterocycles. It exhibits high functional group tolerance and good regioselectivity. Fused heteroaromatic dihydrosiloles can be smoothly halogenated and then oxidized or arylated. Application of these transformations allows obtaining highly functionalized heteroaromatic structures. A gram-scale synthesis of dihydropyridinosilole has also been accomplished using reduced amounts of Ni- and Ir-catalysts.
And the world is your oyster…︁ Sequential halogenation/oxygenation reactions of 2‐diisopropylsilylpyrimidine‐substituted arenes provide a general and efficient synthesis of substituted ...meta‐halophenols from simple aryl iodides (see scheme; Piv=pivaloyl). The products are poised to undergo diverse CC, CN, and CO bond‐forming reactions that enable the transformation of their framework and the introduction of valuable functionalities.
Selective 1,2-iodine, bromine, and chlorine migration in haloallenyl ketones in the presence of Au catalyst has been demonstrated. It was found that, depending on the nature of the Au catalyst used, ...either selective bromine migration or hydrogen shift occurs, leading to the formation of 3- or 2-bromofurans, respectively. Halirenium intermediate was proposed for the unusual 1,2-halogen migration. This cascade transformation allows for mild and efficient synthesis of various types of 3-halofurans.
Transition metal catalyzed denitrogenative transannulation of a triazole ring has recently received considerable attention as a new concept for the construction of diverse nitrogen‐containing ...heterocyclic cores. This method allows a single‐step synthesis of complex nitrogen heterocycles from easily available and cheap triazole precursors. In this Minireview, recent progress of the transition metal catalyzed denitrogenative transannulation of a triazole ring, which was discovered in 2007, is discussed.
One two three: Transition‐metal catalyzed denitrogenative transannulation of 1,2,3‐triazole rings has recently received increasing attention as a new concept for the construction of diverse nitrogen‐containing heterocyclic cores in a single step (see scheme).
A modifiable or removable pyrimidyldiisopropylsilyl (PyrDipSi) directing group for double-fold symmetrical and unsymmetrical C–H functionalizations of arenes has been developed. The PyrDipSi ...directing group can be efficiently installed on arenes via the Rh-catalyzed cross-coupling reaction of an aryl iodide with 2-(diisopropylsilyl)pyrimidine (PyrDipSiH). This directing group allows for a highly efficient Pd-catalyzed C–H oxygenation and halogenation reaction of various arenes to produce a variety of symmetrically and unsymmetrically substituted arenes, including resorcinols, m-halophenols, and 1,3-dihaloarenes. Importantly, the PyrDipSi group can easily be removed or efficiently converted into valuable functionalities, which opens an access to densely substituted arene products from aryl iodides. Hence, the generality of this strategy was highlighted by the synthesis of up to hexasubstituted arene products from simple iodobenzene via iterative C–H functionalization reactions.
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