New highlights in the synthesis and reactivity of 1,4-dihydropyrazine derivatives

Visible-Light-Driven [2 + 2] Photocycloaddition for Constructing Dimers of N,N'-Diacyl-1,4-dihydropyrazines: Experimental and Theoretical Investigation

In this study, the visible-light-driven [2 + 2] photocycloaddition of 1,4-dihydropyrazines in solution was reported. The N,N'-diacyl-1,4-dihydropyrazines with different substituents showed completely different reactivity under the irradiation of a 430 nm blue light-emitting diode (LED) lamp. N,N'-Diacetyl-1,4-dihydropyrazine and N,N'-dipropionyl-1,4-dihydropyrazine were the only compounds capable of undergoing a [2 + 2] photocycloaddition reaction, yielding syn-dimers and cage-dimers (known as 3,6,9,12-tetraazatetraasteranes) with overall yields of 76 and 83%, correspondingly. The substituent-reactivity effect on [2 + 2] photocycloaddition of N,N'-diacyl-1,4-dihydropyrazines was investigated by density functional theory calculations. The results show that the substituents have little influence on Gibbs free energy for the [2 + 2] photocycloaddition and mainly affect the excited energy, reaction sites, and the triplet excited-state structures of 1,4-dihydropyrazines, which are closely related to whether the reaction occurs. The results offer insights into the photochemical reactivity of 1,4-dihydropyrazines and an approach for constructing dimers of N,N'-diacyl-1,4-dihydropyrazines through a solution-based visible-light-driven [2 + 2] photocycloaddition, especially for the construction of 3,6,9,12-tetraazatetraasteranes. Compared with the solid-state [2 + 2] photocycloaddition of 1,4-dihydropyrazine, this photocycloaddition will be an efficient and environmentally friendly method for synthesizing tetraazatetraasteranes with the advantages of milder reaction conditions, simple operation, adjustable reaction amounts by omitting the cocrystal growth step, etc.

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Acceptorless Dehydrogenative Coupling Using Ammonia: Direct Synthesis of N-Heteroaromatics from Diols Catalyzed by Ruthenium

The synthesis of N-heteroaromatic compounds via an acceptorless dehydrogenative coupling process involving direct use of ammonia as the nitrogen source was explored. We report the synthesis of pyrazine derivatives from 1,2-diols and the synthesis of N-substituted pyrroles by a multicomponent dehydrogenative coupling of 1,4-diols and primary alcohols with ammonia. The acridine-based Ru-pincer complex 1 is an effective catalyst for these transformations, in which the acridine backbone is converted to an anionic dearomatized PNP-pincer ligand framework.

Synthesis of Pyrazines and Quinoxalines via Acceptorless Dehydrogenative Coupling Routes Catalyzed by Manganese Pincer Complexes

Base-metal catalyzed dehydrogenative self-coupling of 2-amino alcohols to selectively form functionalized 2,5-substituted pyrazine derivatives is presented. Also, 2-substituted quinoxaline derivatives are synthesized by dehydrogenative coupling of 1,2-diaminobenzene and 1,2-diols. In both cases, water and hydrogen gas are formed as the sole byproducts. The reactions are catalyzed by acridine-based pincer complexes of earth-abundant manganese.

Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

A molecularly defined SNS-cobalt(ii) catalyst for the acceptorless dehydrogenative coupling (ADC) of unprotected amino alcohols with secondary alcohols leading to pyrrole and pyridine derivatives is reported.

Copper-catalyzed cyclization of 2-cyanobenzaldehydes and 2-isocyanoacetates: an efficient strategy for the synthesis of substituted 1-aminoisoquinolines

An efficient strategy for the synthesis of substituted 1-aminoisoquinoline has been developed.

Crystal structure of dimethyl 2,5-bis[(diphenoxyphosphoryl)oxy]cyclohexa-1,4-diene-1,4-dicarboxylate

In the title compound, C₃₄H₃₀O₁₂P₂, which was synthesized via the esterification of dimethyl 2,5-dioxo-1,4-cyclo­hexa­nedi­carboxyl­ate with diphenyl chloro­phosphate, the mol­ecule has crystallographic inversion symmetry. The dihedral angles between the plane of the cyclo­hexa-1,4-diene ring and those of the two benzene rings of the substituent phosphate groups are 41.0 (1) and 89.5 (1)°, while that with the ester group is 3.1 (3)°. In the crystal, only weak inter­molecular C—H⋯O hydrogen bonds are present.

Diversity-oriented synthesis of polycyclic diazinic scaffolds

An efficient and versatile synthesis of a polycyclic diazinic system starting from oxazine has been developed using a two-step Michael/retro Michael and cyclization sequence. The substrates were synthesized with good to high yields giving rapid access to molecular diversity.

Transition metal-catalysed cross-coupling reactions of P-activated enols

Transition metal catalysed cross-coupling reactions are ubiquitous in organic chemistry providing an impressive technique for C-C bond formation. Whilst many electrophilic partners have been described for these reactions, aryl and vinyl phosphates, phosphonates and phosphonites can offer advantages in terms of preparation, stability and reactivity profile. This critical review summarises the advances made to date utilising P-activated enols in metal-catalysed cross-coupling reactions (97 references).