Palladium-Catalyzed Direct Cyclopropylation of Heterocycles

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

Catalytic enantioselective alkenylation-heteroarylation of olefins: stereoselective syntheses of 5-7 membered azacycles and oxacycles

Catalytic enantioselective domino alkenylation-heteroarylation of nonconjugated iododienes proceeded with excellent stereoselectivity and broad scope of substrates. The reaction enables stereoselective syntheses of substituted azacycles such as piperidine, pyrrolidine azepine and dihydropyrans carrying new quaternary stereocenters. Mechanistically, C-H bonds of heterocycles were activated by lithium alkoxides via reversible deprotonation, rather than conventional palladium(ii)-assisted metalation processes. Many types of heteroarenes can be used, including not only azoles (such as thiazoles, oxazoles, imidazoles and oxadiazoles), but also nonazoles (thiophene, furan and azine N-oxides).

Synthesis of Naphtho[2,3-d]oxazoles via Ag(I) Acid-Mediated Oxazole-Benzannulation of ortho-Alkynylamidoarylketones

A cascade oxazole-benzannulation for the synthesis of naphtho[2,3-d]oxazoles has been developed employing ortho-alkynylamidoarylketones as substrates. This procedure provides the advantage of preparing a wide variety of substituents on naphtho[2,3-d]oxazole structures. In addition, o-alkynylamidoarylketones could be prepared from easily accessible and a wide variety of commercially available starting materials. Therefore, this method is a judicious choice of strategy to synthesize naphtho[2,3-d]oxazoles with a great variety of substituents. In this work, 27 examples were demonstrated to provide the desired products in moderate to good yields.

Cationic Charge-Appended Abnormal Carbenes: Synthesis and Study of Electronically Modified Abnormal N-Heterocyclic Carbenes

A range of palladium complexes featuring electronically modified, imidazole-based abnormal N-heterocyclic carbene (aNHC) ligands have been prepared in the hopes of accessing a new class of cationic aNHC ligands electronically distinct from normal NHCs and aNHCs. These palladium complexes represent the first examples of transition metal-ligated aNHC complexes featuring a cationic moiety adjacent to the abnormal carbene center. It was anticipated that these design principles could facilitate electron transfer between the imidazolinylidene and the cationic heterocycle, thus reducing the electron density at the abnormal carbene center. However, this case study suggests that greater conformational restrictions that allow for heterocycle coplanarity are necessary to achieve significant electron transfer and enable access to a new class of cationic charge-appended aNHCs with unique electronic properties.

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.

Recent Developments in Transition-Metal Catalyzed Direct C-H Alkenylation, Alkylation, and Alkynylation of Azoles

The transition metal-catalyzed C–H bond functionalization of azoles has emerged as one of the most important strategies to decorate these biologically important scaffolds. Despite significant progress in the C–H functionalization of various heteroarenes, the regioselective alkylation and alkenylation of azoles are still arduous transformations in many cases. This review covers recent advances in the direct C–H alkenylation, alkylation and alkynylation of azoles utilizing transition metal-catalysis. Moreover, the limitations of different strategies, chemoselectivity and regioselectivity issues will be discussed in this review.

The Cyclopropane Ring as a Reporter of Radical Leaving-Group Reactivity for Ni-Catalyzed C(sp3)-O Arylation

The ability to understand and predict reactivity is essential for the development of new reactions. In the context of Ni-catalyzed C(sp³)-O functionalization, we have developed a unique strategy employing activated cyclopropanols to aid the design and optimization of a redox-active leaving group for C(sp³)-O arylation. In this chemistry, the cyclopropane ring acts as a reporter of leaving-group reactivity, since the ring-opened product is obtained under polar (2e) conditions, and the ring-closed product is obtained under radical (1e) conditions. Mechanistic studies demonstrate that the optimal leaving group is redox-active and are consistent with a Ni(I)/Ni(III) catalytic cycle. The optimized reaction conditions are also used to synthesize a number of arylcyclopropanes, which are valuable pharmaceutical motifs.

Regioselective Copper-Catalyzed Oxidative Coupling of α-Alkylated Styrenes with Tertiary Alkyl Radicals

A radical-mediated oxidative cross-coupling of readily accessible α-alkylated styrenes with 1,3-dicarbonyl compounds utilizing a combination of Cu(OAc)₂ and air as a catalytic system is described. Rather than requiring α-halocarbonyl compounds, this efficient approach enables direct installation of tertiary functionalized alkyl motifs to olefins with simple carbonyl derivatives. The novel protocol is characterized with high allylic selectivities via a competing β-H elimination. Both radical-clock and -trapping experiments provided clear-cut evidence for the intermediacy of an α-keto carbon-centered radical.

Heterogeneous Co-catalyzed direct 2-alkylation of azoles with ethers

The direct 2-alkylation of oxazoles and thiazoles with ethers through cross-dehydrogenative coupling reaction using Co-containing mesoporous zeolite ETS-10 as the heterogeneous catalyst is described. The basic Co-containing mesoporous zeolite ETS-10 catalyst facilitates this cross-dehydrogenative coupling reaction through metal-base synergy catalytic principle.

The direct 2-alkylation of oxazoles and thiazoles with ethers through cross-dehydrogenative coupling reaction using Co-containing mesoporous zeolite ETS-10 as the heterogeneous catalyst is described.

Spectroscopic characterization of (diiodomethyl)zinc iodide: application to the stereoselective synthesis and functionalization of iodocyclopropanes

Herein are described more efficient methodologies for the synthesis of highly substituted cyclopropanes as well as the first characterization of a diiodomethylzinc carbenoid.

Silver-catalyzed C2-selective direct alkylation of heteroarenes with tertiary cycloalkanols

A silver-catalyzed regioselective C2-alkylation of heteroarenes with primary radicals produced by tertiary cycloalkanols was developed.

Palladium-catalyzed difluoromethylthiolation of heteroaryl bromides, iodides, triflates and aryl iodides

A palladium-catalyzed difluoromethylthiolation of heteroaryl halides and triflates under mild conditions was described.

A palladium-catalyzed difluoromethylthiolation of heteroaryl halides and triflates under mild conditions was described. A vast range of heteroaryl halides such as pyridyl, quinolinyl, benzothiazolyl, thiophenyl, carbazolyl and pyazolyl halides could be difluoromethylthiolated efficiently, thus providing medicinal chemists with new tools for their search of new lead compounds for drug discovery. Likewise, aryl iodides were difluoromethylthiolated in high yields under a modified reaction condition.

Palladium-catalyzed cross-coupling of enamides with sterically hindered α-bromocarbonyls

Palladium-catalyzed intermolecular alkylation of enamides with α-bromo carbonyls was developed. Under mild reaction conditions, various cyclic and acyclic enamides reacted well with α-bromo carbonyls to afford the corresponding multi-substituted alkene products in good yields. The coupling products could be converted into very useful γ-amino acid, δ-amino alcohol, 1,4-dicarbonyl and γ-lactam derivatives.

Oxidative coupling between C(sp2)–H and C(sp3)–H bonds of indoles and cyclic ethers/cycloalkanes

Cross-dehydrogenative-coupling (CDC) between C–H/C–H bonds of indoles and cyclic ethers/cycloalkanes is made viable through a simple transition-metal-free pathway.