The Nickel/Copper-Catalyzed Direct Alkylation of Heterocyclic CH Bonds

A Tandem Dehydrogenation-Driven Cross-Coupling between Cyclohexanones and Primary Amines for Construction of Benzoxazoles

Herein, we report a transition metal-free, operationally simple, general method for straightforward syntheses of 2-substituted benzoxazoles from readily available cyclohexanones and aliphatic primary amines by an imine α-oxygenation-initiated cascade reaction sequence. The key to achieving high selectivity and excellent functional-group tolerance is the use of TEMPO as a mild oxidant that selectively oxidizes the reaction intermediates through its multiple reactivity modes, thus facilitating the individual steps to proceed in succession. More than 70 substrate combinations are disclosed, demonstrating the reliability of this protocol to synthesize structurally diverse products, including marketed drugs, drug candidate, and natural products that are unattainable by the existing methods.

Accessing C2-Functionalized 1,3-(Benz)azoles through Transition Metal-Catalyzed C-H Activation

The skeletal presence of 1,3-azoles in a variety of bioactive natural products, pharmacophores, and organic materials demands the derivatization of such heteroarenes regioselectively. Plenty of cross-coupling as well as cyclocondensation reactions have been performed to build up these skeletons but remained commercially unrealizable. A couple of severe drawbacks are faced by these traditional protocols that require a more straightforward strategy to obviate them. Transition metal-catalyzed C-H functionalization has emerged as a superior alternative in that context. 1,3-Azoles and their benzo counterparts have been extensively functionalized exploiting both noble and earth-abundant transition metals. Lately, C-2 functionalization have gained much traction due to the ease of attaining high regioselectivity and installation of synthetically manipulative functionalities. This critical review presents a bird's eye view of all major C-2 functionalization of (benz)azoles catalyzed by a diverse set of metals performed over the past 15 years.

Nickel-Catalyzed C-H Bond Functionalization of Azoles and Indoles

Direct C-H functionalization of privileged and biologically relevant azoles and indoles represents an important chemical transformation in molecular science. Despite significant progress in the palladium-catalyzed regioselective C-H functionalization of azoles and indoles, the use of abundant and less expensive nickel catalyst is underdeveloped. In the recent past, the nickel-catalyzed regioselective C-H alkylation, arylation, alkenylation and alkynylation of azoles and indoles have been substantially explored, which can be applied to the complex organic molecule synthesis. In this Account, we summarize the developments in nickel-catalyzed regioselective functionalization of azoles and indoles with a considerable focus on the reaction mechanism.

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.

Decarbonylative Silylation of Esters by Combined Nickel and Copper Catalysis for the Synthesis of Arylsilanes and Heteroarylsilanes

An efficient nickel/copper-catalyzed decarbonylative silylation reaction of carboxylic acid esters with silylboranes is described. This reaction provides access to structurally diverse silanes with high efficiency and excellent functional-group tolerance starting from readily available esters.

Synthesis and characterization of a family of POCOP pincer complexes with nickel: reactivity towards CO2 and phenylacetylene

A cyclohexyl-based POC sp 3OP pincer ligand (POC sp 3OP=cis-1,3-bis(di-tert-butylphosphinito)cyclohexyl) cyclometalates with nickel to generate a series of new POC sp 3OP-supported Ni(II) complexes, including the halide, hydride, methyl, and phenyl species. trans-[NiCl{cis-1,3-bis(di-tert-butylphosphinito)cyclohexane}], [(POC sp 3OP)NiCl] (1 a) and the analogous bromide complex (1 b) were synthesized and fully characterized by NMR spectroscopy and X-ray crystallography. Cyclic voltammetry measurements of 1 a and 1 b alongside their bis(phosphine) analogues [(PC sp 3P)NiCl] (2 a) and [(PC sp 3P)NiCl] (2 a) (PC sp 3P=cis-1,3-bis(di-tert-butylphosphino)cyclohexyl) indicate a reduced electron density at the metal center upon introducing electron-withdrawing oxygen atoms in the pincer arms. The methyl [(POC sp 3OP)NiMe] (3) and phenyl [(POC sp 3OP)NiPh] (4) complexes were formed from 1 a by reaction with the corresponding organolithium reagents. 1 a also reacts with LiAlH4 to give the hydride complex [(POC sp 3OP)NiH] (5). The methyl complex 3 reacts with phenyl acetylene to give the acetylide complex [(POC sp 3OP)NiCCPh] (6). The reactivity of compounds 3-5 towards CO2 was studied. The hydride complex 5 and the methyl complex 3 both underwent CO2 insertion to form the formate species [(POC sp 3OP)NiOCOH] (7) and acetate species [(POC sp 3OP)NiOCOCH3 ] (8), respectively, although with a higher barrier of insertion in the latter case. Compound 4 was unreactive towards CO2 even at elevated temperatures. Complexes 3-8 were all characterized by NMR spectroscopy and X-ray crystallography.