DDQ-mediated direct oxidative coupling of amides with benzylic and allylic sp3 C–H bonds under metal-free conditions

A General Photocatalytic Strategy for Nucleophilic Amination of Primary and Secondary Benzylic C-H Bonds

We report a visible-light photoredox-catalyzed method that enables nucleophilic amination of primary and secondary benzylic C(sp3)-H bonds. A novel amidyl radical precursor and organic photocatalyst operate in tandem to transform primary and secondary benzylic C(sp3)-H bonds into carbocations via sequential hydrogen atom transfer (HAT) and oxidative radical-polar crossover. The resulting carbocation can be intercepted by a variety of N-centered nucleophiles, including nitriles (Ritter reaction), amides, carbamates, sulfonamides, and azoles, for the construction of pharmaceutically relevant C(sp3)-N bonds under unified reaction conditions. Mechanistic studies indicate that HAT is amidyl radical-mediated and that the photocatalyst operates via a reductive quenching pathway. These findings establish a mild, metal-free, and modular protocol for the rapid diversification of C(sp3)-H bonds to a library of aminated products.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Selectfluor®-enabled photochemical selective C(sp3)-H(sulfonyl)amidation

Transition metal- and photosensitizer-free C(sp³)-H (sulfonyl)amidation reactions have been realized by employing Selectfluor® as a versatile reagent, functioning as a photoactive component, a HAT precursor and an oxidant. Various toluene derivatives, cycloalkanes, natural products and bioactive molecules can be converted into N-containing products under mild conditions in good yield and with high chemo- and site-selectivity.

Traceless Benzylic C-H Amination via Bifunctional N-Aminopyridinium Intermediates

C-H amination reactions provide the opportunity to streamline the synthesis of nitrogen-containing organic small molecules. The impact of intermolecular C-H amination methods, however, is currently limited the frequent requirement for the amine precursors to bear activating groups, such as N-sulfonyl substituents, that are both challenging to remove and not useful synthetic handles for subsequent derivatization. Here, we introduce traceless nitrogen activation for C-H amination-which enables application of selective C-H amination chemistry to the preparation of diverse N-functionalized products-via sequential benzylic C-H N-aminopyridylation followed by Ni-catalyzed C-N cross-coupling with aryl boronic acids. Unlike many C-H amination reactions that provide access to protected amines, the current method installs an easily diversifiable synthetic handle that serves as a lynchpin for C-H amination, deaminative N-N functionalization sequences.

Selective carboxylation of reactive benzylic C-H bonds by a hypervalent iodine(III)/inorganic bromide oxidation system

An oxidation system comprising phenyliodine(III) diacetate (PIDA) and iodosobenzene with inorganic bromide, i.e., sodium bromide, in an organic solvent led to the direct introduction of carboxylic acids into benzylic C–H bonds under mild conditions. The unique radical species, generated by the homolytic cleavage of the labile I(III)–Br bond of the in situ-formed bromo-λ³-iodane, initiated benzylic carboxylation with a high degree of selectivity for the secondary benzylic position.

Nickel-catalyzed product-controllable amidation and imidation of sp3 C–H bonds in substituted toluenes with sulfonamides

A nickel-catalyzed product-controllable imidation and amidation of sp³ C–H bonds in substituted toluenes with sulfonamides were developed.

Exploiting Alkylquinone Tautomerization: Amine Benzylation

A general protocol for the synthesis of benzylic amines via side-chain amination of alkylquinones is reported. The reactions are initiated by the tautomerization of an alkylquinone to the corresponding quinone methide, which is subsequently trapped in situ by an amine nucleophile. This process is promoted by tertiary amines in protic solvents under mild conditions and is compatible with many functional groups. 1,2- and 1,4-benzoquinones, as well as naphthoquinones, participate in this reaction using a wide range of primary and secondary amines/anilines. The synthetic utility of this transformation is also explored.

Benzylic C(sp(3))-H Functionalization for C-N and C-O Bond Formation via Visible Light Photoredox Catalysis

A visible light mediated highly selective benzylic C-H bond functionalization for intermolecular C-N and C-O bond formation is reported. This cross-dehydrogenative coupling reaction demonstrates a straightforward protocol for incorporating the heteroaromatics to the benzylic position. Benzylic oxidation of various alkyl aryls to corresponding carbonyl compounds has also been reported.

Metal-free TEMPO-promoted C(sp³)-H amination to afford multisubstituted benzimidazoles

An efficient TEMPO-air/cat. TEMPO-O2 oxidative protocol was developed to synthesize multisubstituted or fused tetracyclic benzimidazoles via a metal-free oxidative C-N coupling between the sp(3) C-H and free N-H of readily available N(1)-benzyl/alkyl-1,2-phenylenediamines.