Design of Hemilabile N,N,N-Ligands in Copper-Catalyzed Enantioconvergent Radical Cross-Coupling of Benzyl/Propargyl Halides with Alkenylboronate Esters.

The enantioconvergent radical C(sp3)-C(sp2) cross-coupling of alkyl halides with alkenylboronate esters is an appealing tool in the assembly of synthetically valuable enantioenriched alkenes owing to the ready availability, low toxicity, and air/moisture stability of alkenylboronate esters. Here, we report a copper/chiral N,N,N-ligand catalytic system for the enantioconvergent cross-coupling of benzyl/propargyl halides with alkenylboronate esters (>80 examples) with good functional group tolerance. The key to the success is the rational design of hemilabile N,N,N-ligands by mounting steric hindrance at the ortho position of one coordinating quinoline ring. Thus, the newly designed ligand could not only promote the radical cross-coupling process in the tridentate form but also deliver enantiocontrol over highly reactive alkyl radicals in the bidentate form. Facile follow-up transformations highlight its potential utility in the synthesis of various enantioenriched building blocks as well as in the late-stage functionalization for drug discovery.

Copper-Catalyzed Asymmetric Coupling of Allenyl Radicals with Terminal Alkynes to Access Tetrasubstituted Allenes.

In contrast to the wealth of asymmetric transformations for generating central chirality from alkyl radicals, the enantiocontrol over the allenyl radicals for forging axial chirality represents an uncharted domain. The challenge arises from the unique elongated linear configuration of the allenyl radicals that necessitates the stereo-differentiation of remote motifs away from the radical reaction site. We herein describe a copper-catalyzed asymmetric radical 1,4-carboalkynylation of 1,3-enynes via the coupling of allenyl radicals with terminal alkynes, providing diverse synthetically challenging tetrasubstituted chiral allenes. A chiral N,N,P-ligand is crucial for both the reaction initiation and the enantiocontrol over the highly reactive allenyl radicals. The reaction features a broad substrate scope, covering a variety of (hetero)aryl and alkyl alkynes and 1,3-enynes as well as radical precursors with excellent functional group tolerance.

Copper-Catalyzed Enantioconvergent Cross-Coupling of Racemic Alkyl Bromides with Azole C(sp2 )-H Bonds.

The development of enantioconvergent cross-coupling of racemic alkyl halides directly with heteroarene C(sp2 )-H bonds has been impeded by the use of a base at elevated temperature that leads to racemization. We herein report a copper(I)/cinchona-alkaloid-derived N,N,P-ligand catalytic system that enables oxidative addition with racemic alkyl bromides under mild conditions. Thus, coupling with azole C(sp2 )-H bonds has been achieved in high enantioselectivity, affording a number of potentially useful α-chiral alkylated azoles, such as 1,3,4-oxadiazoles, oxazoles, and benzo[d]oxazoles as well as 1,3,4-triazoles, for drug discovery. Mechanistic experiments indicated facile deprotonation of an azole C(sp2 )-H bond and the involvement of alkyl radical species under the reaction conditions.

Copper-Catalyzed Enantioconvergent Radical Suzuki-Miyaura C(sp3)-C(sp2) Cross-Coupling.

A copper-catalyzed enantioconvergent Suzuki-Miyaura C(sp3)-C(sp2) cross-coupling of various racemic alkyl halides with organoboronate esters has been established in high enantioselectivity. Critical to the success is the use of a chiral cinchona alkaloid-derived N,N,P-ligand for not only enhancing the reducing capability of copper catalyst to favor a stereoablative radical pathway over a stereospecific SN2-type process but also providing an ideal chiral environment to achieve the challenging enantiocontrol over the highly reactive radical species. The reaction has a broad scope with respect to both coupling partners, covering aryl- and heteroarylboronate esters, as well as benzyl-, heterobenzyl-, and propargyl bromides and chlorides with good functional group compatibility. Thus, it provides expedient access toward a range of useful enantioenriched skeletons featuring chiral tertiary benzylic stereocenters.

Synthesis of Fullerotetrahydroquinolines via [4 + 2] Cycloaddition Reaction of [60]Fullerene with in Situ Generated Aza-o-quinone Methides.

An efficient [4 + 2] cycloaddition reaction of [60]fullerene with the in situ generated aza-o-quinone methides from N-(o-chloromethyl)aryl sulfonamides with the assistance of Cu2O has been developed to afford a series of fullerotetrahydroquinolines. This strategy exhibits a broad substrate scope and excellent functional group tolerance. A tentative reaction pathway for the formation of fullerotetrahydroquinolines is proposed on the basis of the experimental results.

Copper-Promoted Synthesis of 2-Fulleropyrrolines via Heteroannulation of [60]Fullerene with α-Amino Ketones.

A Cu(OAc)2-promoted heteroannulation of [60]fullerene with α-amino ketones has been exploited for the efficient synthesis of 2-fulleropyrrolines containing a trisubstituted or tetrasubstituted C═C bond via the formation of C-C and C-N bonds. Mechanistic study indicates that a radical process should be involved in this transformation. Furthermore, theoretical computations show that the process via the attack of the carbon radical generated from the employed α-amino ketone to [60]fullerene should be the preferred pathway. The electrochemical properties of the synthesized 2-fulleropyrrolines have also been investigated.

A general asymmetric copper-catalysed Sonogashira C(sp3)-C(sp) coupling.

Continued development of the Sonogashira coupling has made it a well established and versatile reaction for the straightforward formation of C-C bonds, forging the carbon skeletons of broadly useful functionalized molecules. However, asymmetric Sonogashira coupling, particularly for C(sp3)-C(sp) bond formation, has remained largely unexplored. Here we demonstrate a general stereoconvergent Sonogashira C(sp3)-C(sp) cross-coupling of a broad range of terminal alkynes and racemic alkyl halides (>120 examples) that are enabled by copper-catalysed radical-involved alkynylation using a chiral cinchona alkaloid-based P,N-ligand. Industrially relevant acetylene and propyne are successfully incorporated, laying the foundation for scalable and economic synthetic applications. The potential utility of this method is demonstrated in the facile synthesis of stereoenriched bioactive or functional molecule derivatives, medicinal compounds and natural products that feature a range of chiral C(sp3)-C(sp/sp2/sp3) bonds. This work emphasizes the importance of radical species for developing enantioconvergent transformations.

Cascade Radical Reaction of N-Sulfonyl-2-allylanilines with [60]Fullerene: Synthesis and Functionalization of (2-Indolinyl)methylated Hydrofullerenes.

The copper-promoted cascade radical reaction of N-sulfonyl-2-allylanilines with [60]fullerene has been developed to efficiently provide novel and scarce (2-indolinyl)methylated hydrofullerenes, featuring a broad substrate scope and excellent functional group tolerance. A plausible reaction mechanism for the formation of (2-indolinyl)methylated hydrofullerenes is proposed on the basis of the experimental results. In addition, further transformation into other carbocyclic derivatives of [60]fullerene as well as their applications in organic photovoltaic devices of the obtained products has also been explored.

Highly efficient synthesis of [60]fullerene oxides by plasma jet.

Atmospheric pressure nonequilibrium plasma jet has been applied to the synthesis of [60]fullerene oxides (C60On) for the first time. C60O and C60O2 were produced and isolated in high yields up to 44% and 21%, respectively. The structural assignment of C60O was confirmed by comparison with the reported spectroscopic data. Theoretical calculations of 13C NMR chemical shifts for eight isomers of C60O2 were performed and compared with the experimental data to assign the most possible structure for the obtained C60O2 dominantly as an e isomer.

Facile access to novel [60]fullerenyl diethers and [60]fullerene-sugar conjugates via annulation of diol moieties.

A general and facile annulation of various diol motifs to [60]fullerene has been developed. This protocol can afford not only 6- to 10-membered-ring fullerenyl diethers in one step from simple acyclic diols but also directly couple [60]fullerene with a variety of structurally diverse sugars. The [60]fullerene-sugar conjugates formed do not require any linker moiety and maintain their inherent structural integrity. The electrochemistry of the fullerenyl diethers and [60]fullerene-sugar conjugates has also been investigated.

Copper(I)-catalyzed heteroannulation of [60]fullerene with ketoxime acetates: preparation of novel 1-fulleropyrrolines.

A cuprous bromide-catalyzed heteroannulation reaction of [60]fullerene with ketoxime acetates has been exploited to prepare novel 1-fulleropyrrolines through the cleavage of N-O and C-H bonds and formation of C-C and C-N bonds under thermal conditions. A plausible mechanism for the formation of 1-fulleropyrrolines is proposed on the basis of the experimental results. The electrochemistry of the obtained products has also been investigated.