Ruthenium-catalyzed oxidative coupling of vinylene carbonate with isoxazoles: access to fused anthranils

Urazoles as Heterocyclic Directing Groups in Ru(II)-Catalyzed Annulation of N-H/C-H Bonds with Vinylene Carbonate

The development of novel directing groups is a valuable strategy to secure the advancement of catalytic C-H functionalization. To illustrate the feasibility of urazoles as heterocyclic directing groups, we herein present the ruthenium(II)-catalyzed annulation of N-H/C-H bonds on N-aryl urazoles with vinylene carbonate. This transformation provides the rapid construction of triazolocinnoline derivatives as hemiaminal and dehydrated forms. A series of mechanistic investigations and post-transformations highlighted the amenability of the developed method.

Ruthenium(II)-Catalyzed C-H/C-H (4+2) Annulation of 2-Aryl-N-heterocycles with Vinylene Carbonate

A ruthenium(II)-catalyzed direct C-H/C-H (4+2) annulation of 2-aryl-N-heterocycles such as 2-aryl-4H-pyrido[1,2-a]pyrimidin-4-ones, 2-arylimidazo[1,2-a]pyridines, 2-aryl-2H-indazoles and 2-arylquinolin-4(1H)-ones with vinylene carbonate has been described. This one-pot cascade strategy provided the diversely substituted fused-polyheterocycles such as 7H-benzo[h]pyrido[2,1-b]quinazolin-7-ones, naphtho[1',2':4,5]imidazo[1,2-a]pyridines, indazolo[2,3-a]quinolines and benzo[c]acridin-7(12H)-ones in moderate to excellent yields. The developed protocol exhibited a broad substrate scope with good functional group tolerance and acid/base-free conditions. Based on a preliminary mechanistic investigation, a tentative mechanism of Ru(II)-catalyzed (4+2) annulation reaction has been proposed.

Engaging vinylene carbonate in ruthenium-catalyzed regioselective C-4 methylenation and C-8 formylmethylation of isoquinolinones

Herein, we disclose the first reports on the utilization of vinylene carbonate as a C1 methylene source in ruthenium-catalyzed additive controlled regioselective C4-methylenation and weak chelation-assisted C8-formylmethylation of isoquinolinones. Adopting vinylene carbonate as both a C2 and C1 synthon is an important highlight of this work. Amide carbonyl acts as the traceless directing group in C8-formylmethylation. Remarkably, in this reaction, two C-C bonds form in one-pot producing dimeric isoquinolinones by employing vinylene carbonate as a methylene surrogate in the presence of copper acetate as an additive. Importantly, unsymmetrical dimers were achievable, albeit in low yield. Extensive control experiments are conducted to decipher the reaction mechanism. It is evident from the mechanistic studies, that the formation of a formylmethyl group in situ at the C4-carbon undergoes dimerization, oxidation and subsequent decarboxylation to produce methylene-bridged isoquinolinone dimers. This protocol is scalable and compatible with a repertoire of substrates and shows high functional group tolerance. Harnessing vinylene carbonate as both a C2 and C1 synthon is an important highlight of this work.

Harnessing reductive BF2-complexation via Ru(II)-catalyzed N-O cleavage of isoxazoles

Developed here is a highly fluorescent organic N,O bidentate BF2 complex from isoxazole in the presence of a Ru(II) catalyst. Herein, the complexation proceeds via a selective N-O cleavage of the isoxazole ring. The complex shows absorption (λmax,abs) in the range of 352-363 nm with an extinction coefficient (ε) in the range of 8000-64 000 M-1 cm-1, and fluorescence emission (λmax,em) in the range of 413-485 nm with a Stokes shift of 61-125 nm having quantum yield up to 33%. Apart from the solution state, the solid BF2 complex 2u exhibits absorption at 405 nm and strong fluorescence emission at 550 nm with a quantum yield of 26.9%.

DLPNO-CCSD(T) and DFT study of the acetate-assisted C-H activation of benzaldimine at [RuCl2(p-cymene)]2: the relevance of ligand exchange processes at ruthenium(II) complexes in polar protic media

To gain mechanistic insights into the acetate-assisted cyclometallations of arylimines promoted by [RuCl2(p-cymene)]2 in polar protic media, DFT geometry optimizations (with M06 and ωB97X-D3 functionals and the cc-pVDZ-PP[Ru] basis set) followed by DLPNO-CCSD(T)/CBS energy evaluations were performed using benzaldimine as a model substrate and methanol as the solvent (with CPCM or SMD models). The calculation results show that coordination of the imine to an acetate ruthenium precursor is followed by anion (chloride or acetate) dissociation as the rate-determining step of the process. H-Bonding of two explicit MeOH to the anion reduces the calculated activation energy to ca. 23 kcal mol-1, in good agreement with the experimental half-life at room temperature. Subsequent AMLA/CMD C-H activation of the intermediate cationic complexes is a faster, reversible process. Alternative reaction pathways involving neutral diacetate ruthenium complexes offer AMLA/CMD transition state structures of lower energy but are precluded due to higher energy barriers for the initial ligand exchange processes at ruthenium. Solvent assistance accelerates the final chloride/acetate exchange processes on the cycloruthenate intermediates, particularly when compression in the condensed phase is taken into consideration. The performance of six DFT functionals (with the aug-pVTZ-PP[Ru] basis set) was assessed using the DLPNO-CCSD(T)/CBS reference energies. Neutral diacetate ruthenium complexes were incorrectly predicted as being kinetically relevant when using hybrid DFT methods (PBE0-D3(BJ), M06-2X or ωB97M-V). Good agreement between the calculated barrier heights and our benchmark energy results was obtained by using double-hybrid DFT methods. PWPB95 with D3(BJ) or D4 dispersion energy corrections was found to be the most accurate (ΔG≠ MUE of ca. 1 kcal mol-1). This study may aid our understanding of and help with further experimental investigations of synthetically useful carboxylate-assisted C-H bond functionalizations involving (N,C)-cyclometallated (p-cymene)Ru(II) intermediate complexes in sustainable polar protic solvents.

Palladium-catalyzed denitrogenation/vinylation of benzotriazinones with vinylene carbonate

Herein, a novel Pd-catalyzed denitrogenation/vinylation of benzotriazinones using vinylene carbonate as the vinylation reagent is reported. This transformation demonstrates an unprecedented skeletal editing approach, effectively converting NN to CC fragments in situ and synthesizing a collection of isoquinolinones with broad-spectrum functional group tolerance. Moreover, the quite concise reaction system and late-stage modification of bioactive molecules comprehensively underscore the practical potential of this protocol.

Synthesis of Fluoren-9-ones via Pd-Catalyzed Annulation of 2-Iodobiphenyls with Vinylene Carbonate

A palladium-catalyzed reaction for intermolecular selective C-H cyclocarbonylation of 2-iodobiphenyls is described. Intriguingly, the vinylene carbonate acts as a carbon monoxide transfer agent to enable the annulation reaction. Moreover, as a versatile synthon, fluoren-9-one can be transformed into a variety of functionalized organic molecules, such as [1,1'-biphenyl]-2-carboxylic acid, 1'H,3'H-spiro[fluorene-9,2'-perimidine] and N-tosylhydrazones.

Rhodium-catalyzed annulation of hydrazines with vinylene carbonate to synthesize unsubstituted 1-aminoindole derivatives

Herein, we describe rhodium-catalysed C-H bond activation for [3 + 2] annulation using hydrazide and vinylene carbonate, providing an efficient method for synthesising unsubstituted 1-aminoindole compounds. Characterised by high yields, mild reaction conditions, and no need for external oxidants, this transformation demonstrates excellent regioselectivity and a wide tolerance for various functional groups.

Rhodium-catalyzed divergent dehydroxylation/alkenylation of hydroxyisoindolinones with vinylene carbonate

Herein, a novel organic transformation involving rhodium-catalyzed divergent dehydroxylation/alkenylation of hydroxyisoindolinone with vinylene carbonate is reported, and a series of architecturally rigid and widely used spirolactams are obtained with excellent functional group tolerance and high selectivity. Remarkably, the promising vinylene carbonate reagent presents a distinct chemical reactivity as a vinyl-oxygen cyclic synthon and first transfers the C-H bond to spiroheterocycle scaffolds. Moreover, another chemoselectivity, direct dehydrogenative coupling with vinylene carbonate, is also presented. This protocol is compatible with green chemistry and only releases H2O and CO2 as byproducts.

Rhodium-catalyzed C-H carboxymethylation of anilines with vinylene carbonate

A rhodium-catalyzed synthesis of phenylacetate has been realized by direct C-H carboxymethylation of anilines bearing removable directing groups. The reaction occurred most efficiently in air, without any external base or oxidant. This methodology is expected to provide a facile and general access to various bioactive 2-amino aromatic acetic acid derivatives.

Rhodium(III)-Catalyzed C-H/N-H Activation for Direct Synthesis of Pyrimidoindolones under Mild Conditions

Pyrimidoindolones are an important structural motif found in many natural products and are essential to the pharmaceutical and agrochemical industry. Direct synthesis of 3,4-unsubstituted pyrimidoindolones is not easily accessible. Here we report a rhodium(III)-catalyzed C-H/N-H activation and annulation approach for obtaining pyrimidoindolones from N-carbamoylindoles and vinylene carbonate. The reaction occurs at room temperature and does not require any external oxidants. A diverse spectrum of indoles were demonstrated to be viable substrates capable of producing the desired pyrimidoindolones in high yields. In addition, the reaction scope has been expanded to include pyrrole substrate. Furthermore, detailed mechanistic studies have been performed to delineate the working mode of the reaction.

Isoxazole as a nitrile synthon: en routes to the ortho-alkenylated isoxazole and benzonitrile with allyl sulfone catalyzed by Ru(II)

A Ru(II) catalyzed regioselective Heck-type C-H olefination of isoxazole with unactivated allyl phenyl sulfone is revealed. The solvent DCM offers dual sp2-sp2 C-H activation via an N-directed strategy, leading to ortho-olefinated isoxazoles with exclusive E-selectivity. On the other hand, in DCE solvent, isoxazole serves as the nitrile synthon and leads to o-olefinated benzonitrile. At a higher temperature (110 °C) in DCE, after the ortho-olefination Ru(II) mediated cleavage of isoxazoles delivered the nitrile functionality.

Transition metal-free annulative vinylene transfer via the 1,3-dipolar reaction of N-ylides: access to benzo-fused indolizines

An efficient metal-free annulative vinylene transfer protocol for the synthesis of benzo-fused indolizines via 1,3-dipolar cycloadditions of N-ylides with vinylene carbonate has been developed. Vinylene carbonate serves as an acetylene surrogate without any external oxidant involved. This transformation leads to the direct construction of versatile benzo-fused indolizine derivatives in moderate to good yields.