Sulfonamides as N-Centered Radical Precursors for C-N Coupling Reactions To Generate Amidines.

Nitrogen-centered radicals (NCRs) are valuable intermediates for the construction of C-N bonds. Traditional methods for the generation of NCRs employ toxic radical initiators, transition metal catalysts, photocatalysts, or organometallic reagents. Herein, we report a novel strategy for the generation of NCRs toward the construction of C-N bonds under transition-metal-free conditions. Thus, super-electron-donor (SED) 2-azaallyl anions undergo single-electron transfer (SET) with sulfonamides, forming aminyl radicals (R2N•, R = alkyl) and culminating in the generation of amidines bearing various functional groups (33 examples, up to 96% yield). Broad substrate scope and gram-scale telescoped preparation demonstrate the practicality of this method. Radical clock and electron paramagnetic resonance (EPR) experiments support the proposed radical coupling pathway between the generated N-centered radical and the C-centered 2-azaallyl radical.

Radical C(sp3)-S Coupling for the Synthesis of α-Amino Sulfides.

A unique transition-metal-free radical thiolation of 2-azaallyl anions has been developed. Easily accessible thiosulfonates and 2-azaallyls undergo the tandem process of single-electron transfer and radical-radical coupling to construct C(sp3)-S bonds. This robust protocol enables a mild and chemoselective coupling between 2-azaallyl anions and thiosulfonates to access α-amino sulfides in 50-92% yields (25 examples). The scalability of this protocol was demonstrated by telescopic gram-scale experiments. Mechanistic studies provide significant evidence for this radical thiolation reaction.

Catalytic Chemo- and Regioselective Radical Carbocyanation of 2-Azadienes for the Synthesis of α-Amino Nitriles.

α-Amino nitriles are versatile structural motifs in a variety of biologically active compounds and pharmaceuticals and they serve as valuable building blocks in synthesis. The preparation of α- and ß-functionalized α-amino nitriles from readily available scaffolds, however, remains challenging. Herein is reported a novel dual catalytic photoredox/copper-catalyzed chemo- and regioselective radical carbocyanation of 2-azadienes to access functionalized α-amino nitriles by using redox-active esters (RAEs) and trimethylsilyl cyanide. This cascade process employs a broad scope of RAEs and provides the corresponding α-amino nitrile building blocks in 50-95 % yields (51 examples, regioselectivity >95 : 5). The products were transformed into prized α-amino nitriles and α-amino acids. Mechanistic studies suggest a radical cascade coupling process.

α-Branched amines through radical coupling with 2-azaallyl anions, redox active esters and alkenes.

α-Branched amines are fundamental building blocks in a variety of natural products and pharmaceuticals. Herein is reported a unique cascade reaction that enables the preparation of α-branched amines bearing aryl or alkyl groups at the ß- or γ-positions. The cascade is initiated by reduction of redox active esters to alkyl radicals. The resulting alkyl radicals are trapped by styrene derivatives, leading to benzylic radicals. The persistent 2-azaallyl radicals and benzylic radicals are proposed to undergo a radical-radical coupling leading to functionalized amine products. Evidence is provided that the role of the nickel catalyst is to promote formation of the alkyl radical from the redox active ester and not promote the C-C bond formation. The synthetic method introduced herein tolerates a variety of imines and redox active esters, allowing for efficient construction of amine building blocks.

Super-Electron-Donor 2-Azaallyl Anions Enable Construction of Isoquinolines.

Herein is introduced the application of "super-electron-donor"(SED) 2-azaallyl anions in a tandem reduction/radical cyclization/radical coupling/aromatization protocol that enables the rapid construction of isoquinolines. The value of this transition-metal-free method is highlighted by the wide range of isoquinoline ethyl amines prepared with good functional group tolerance and yields. An operationally simple gram scale synthesis is also conducted, confirming the scalability.

Transition-metal-free allylation of 2-azaallyls with allyl ethers through polar and radical mechanisms.

Allylation of nucleophiles with highly reactive electrophiles like allyl halides can be conducted without metal catalysts. Less reactive electrophiles, such as allyl esters and carbonates, usually require a transition metal catalyst to facilitate the allylation. Herein, we report a unique transition-metal-free allylation strategy with allyl ether electrophiles. Reaction of a host of allyl ethers with 2-azaallyl anions delivers valuable homoallylic amine derivatives (up to 92%), which are significant in the pharmaceutical industry. Interestingly, no deprotonative isomerization or cyclization of the products were observed. The potential synthetic utility and ease of operation is demonstrated by a gram scale telescoped preparation of a homoallylic amine. In addition, mechanistic studies provide insight into these C(sp3)-C(sp3) bond-forming reactions.

Nickel-catalyzed enantioselective vinylation of aryl 2-azaallyl anions.

A unique enantioselective nickel-catalyzed vinylation of 2-azaallyl anions is advanced for the first time. This method affords diverse vinyl aryl methyl amines with high enantioselectivities, which are frequently occurring scaffolds in natural products and medications. This C-H functionalization method can also be extended to the synthesis of enantioenriched 1,3-diamine derivatives by employing suitably elaborated vinyl bromides. Key to the success of this process is the identification of a Ni/chiraphos catalyst system and a less reducing 2-azaallyl anion, all of which favor an anionic vinylation route over a background radical reaction. A telescoped gram scale synthesis and a product derivatization study confirmed the scalability and synthetic potential of this method.

Transition-Metal-Free C(sp3)-H Coupling of Cycloalkanes Enabled by Single-Electron Transfer and Hydrogen Atom Transfer.

Here we report a unique transition-metal-free C(sp3)-H/C(sp3)-H coupling of cycloalkanes at room temperature. Unactivated cycloalkanes and 2-azaallyls underwent the combination process of single-electron transfer (SET) and hydrogen atom transfer (HAT) to deliver a wide variety of cycloalkane-functionalized products. This expedient approach enables C(sp3)-H/C(sp3)-H coupling of cycloalkanes under mild conditions without transition metals, initiators, and oxidants.

Synthesis and antitumor activity of aza-brazilan derivatives containing imidazolium salt pharmacophores.

The synthesis of a series of novel aza-brazilan derivatives containing imidazolium salt pharmacophores is presented. The biological activity of such imidazolium salts was further evaluated in vitro against a panel of human tumor cell lines. The results suggest that the electron-withdrawing group on the aza-brazilan moiety, substituted 5,6-dimethyl-benzimidazole ring and substitution of the imidazolyl-3-position with a 4-methylbenzyl group were essential for modulating the cytotoxic activity. Compounds 55 and 39, bearing a 4-methylbenzyl substituent at position-3 of 5,6-dimethyl-benzimidazole, were found to be the most potent compounds with IC50 values of 0.52-1.30 µM and 0.56-1.51 µM against four human tumor cell lines investigated. Particularly, compound 57 exhibited inhibitory activity against the MCF-7 cell line with an IC50 value of 0.35 µM and was 56-fold more sensitive than DDP. Moreover, compound 55 inhibited cell proliferation through inducing G0/G1 cell cycle arrest and apoptosis in SMMC-7721 cells.

Synthesis of Benzofuran Derivatives through Cascade Radical Cyclization/Intermolecular Coupling of 2-Azaallyls.

Benzofurans are among the most popular structural units in bioactive natural products, however, the synthesis of such structures by radical cyclization cascade reactions is rare. Herein, we report a mild and broadly applicable method for the construction of complex benzofurylethylamine derivatives through a unique radical cyclization cascade mechanism. Single-electron transfer (SET) from 2-azaallyl anions to 2-iodo aryl allenyl ethers initiates a radical cyclization that is followed by intermolecular radical-radical coupling. This expedient approach enables the synthesis of a range of polycyclic benzofurans that would otherwise be difficult to prepare.

Synthesis and cytotoxic activity of novel tetrahydrobenzodifuran-imidazolium salt derivatives.

The synthesis of a series of novel 4-substituted 2,3,6,7-tetrahydrobenzo [1,2-b;4,5-b']difuran-1H-imidazolium salts is presented. The biological properties of the compounds were evaluated in vitro against a panel of human tumor cell lines. Results suggest that the 5,6-dimethyl-benzimidazole or 2-methyl-benzimidazole ring, and substitution of the imidazolyl-3-position with a 2-naphthylmethyl substituent or 2-naphthylacyl substituent, were important to the cytotoxic activity. Notably, 3-(2-Naphthylmethyl)-1-((2,3,6,7-tetrahydrobenzo[1,2-b;4,5-b']difuran-4-yl)methyl)-1H-5,6-dimethyl-benzimidazol-3-ium bromide (42) was found to be the most potent derivative against five human tumor cell lines with IC50 values of 1.06-4.34µM and more selective towards SMMC-7721, A549 and SW480 cell lines. 3-(2-Naphthylacyl)-1-((2,3,6,7-tetrahydrobenzo[1,2-b;4,5-b']difuran-4-yl)methyl)-1H-2-methyl-benzimidazol-3-ium bromide (37) showed higher selectivity to SMMC-7721 and MCF-7 cell lines with IC50 values 2.7-fold and 8.4-fold lower than DDP. Study regarding to the antitumor mechanism of action showed that compound 37 could induce cell cycle G1 phase arrest and apoptosis in SMMC-7721 cells.