Chemodivergent Organolanthanide Catalyzed C-H a-Mono-1 Borylation of Azines

C-H activation and functionalization of pyridinoid azines is a key transformation forthe synthesis of many natural products, pharmaceuticals, and materials. Reflecting the azinyl nitrogen lone-pair steric repulsion, tendency to irreversibly bind to metal ion catalysts, and the electron-deficient nature of pyridine, C-H functionalization at the important a-position remains challenging. Thus, the development of earth abundant catalysts for the a-selective mono-functionalization of azines is a crucial hurdle for modern chemical synthesis. Here, the selective organolanthanide catalyzed a-mono-borylation of a diverse series of pyridines is reported, affording a valuable precursor for cross-coupling reactions. Experimental and theoretical mechanistic evidence support the formation of a C-H activated η2-lanthanide-azine complex, followed by intermolecular a-mono-borylation via σ-bond metathesis. Notably, varying the lanthanide identity and substrate electronics promotes chemodivergence of the catalytic selectivity: smaller/more electrophilic lanthanide3+ ions and electron-rich substrates favor selective a-C-H functionalization, whereas larger/less electrophilic lanthanide3+ 1 ions and electron poor substrates favor selective B-N bond-forming 1,2-dearomatization. Such organolanthanide series catalytic chemodivergence is, to our knowledge, unprecedented.

Acidic Stabilization of the Dual-Aromatic Heterocyclic Anions

Recently, we discovered that the delocalization of nitrogen lone-pair electrons (NLPEs) in five-membered nitrogen heterocycles created a second σ-aromaticity in addition to the prototypical π-aromaticity. Such dual-aromatic compounds, such as the pentazole anion, were proved to have distinct chemistry in comparison to traditional π-aromatics, such as benzene, and were surprisingly unstable, susceptible to electrophilic attack, and relatively difficult to obtain. The dual-aromatics are basic in nature, but prefer not to be protonated when confronting more than three hydronium/ammonium ions, which violates common sense understanding of acid−base neutralization for a reason that is unclear. Here, we carried out 63 test simulations to explore the stability and reactivity of three basic heterocycle anions (pentazole anion N5¯, tetrazole anion N4C1H1¯, and 1,2,4-triazole anion N3C2H2¯) in four types of solvents (acidic ions, H3O+ and NH4+, polar organics, THF, and neutral organics, benzene) with different acidities and concentrations. By quantum mechanical calculations of the electron density, atomistic structure, interatomic interactions, molecular orbital, magnetic shielding, and energetics, we confirmed the presence of dual aromaticity in the heterocyclic anions, and discovered their reactivity to be a competition between their basicity and dual aromaticity. Interestingly, when the acidic ions H3O+/NH4+ are three times more in number than the basic heterocyclic anions, the anions turn to violate acid−base neutralization and remain unprotonated, and the surrounding acidic ions start to show a significant stabilization effect on the studied heterocyclic anions. This work brings new knowledge to nitrogen aromatics and the finding is expected to be adaptable for other pnictogen five-membered ring systems.

Structure and Stability of Aromatic Nitrogen Heterocycles Used in the Field of Energetic Materials

Understanding the stabilization of nitrogen heterocycles is critical in the field of energetic materials and calls for innovative knowledge of nitrogen aromatics. Herewith, we report for the first time that nitrogen lone pair electron (NLPE) delocalization in five-membered nitrogen heterocycles creates a second σ-aromaticity in addition to the prototypical π-aromaticity. The NLPE delocalization and the attendant dual-aromaticity are enhanced as more carbon atoms in the ring are substituted by unsaturated nitrogen atoms. The presence of adjacent nitrogen atoms in the ring can enhance the aromaticity of the nitrogen heterocycles and improve in-crystal intermolecular binding strength but will decrease the firmness of the individual molecular architecture. Notably, such σ-aromaticity is not present in six-membered nitrogen heterocycles, probably due to the longer bonds and broader regions of their rings; therefore, six-membered heterocycles present overall lower aromaticity than five-membered heterocycles. This work brings new knowledge to nitrogen aromatics and is expected to inspire broad interest in the chemistry community.

Co-crystals of an organic triselenocyanate with ditopic Lewis bases: recurrent chalcogen bond interactions motifs

Organic selenocyanates R–Se–CN can act as an amphoteric chalcogen bond (ChB) donor (through the Se atom) and acceptor (through the N atom lone pair). Co-crystallization of tri-substituted 1,3,5-tris(selenocyanatomethyl)-2,4,6-trimethylbenzene (1) is investigated with different ditopic Lewis bases acting as chalcogen bond (ChB) acceptors to investigate the outcome of the competition, as ChB acceptor, between the nitrogen lone pair of the SeCN group and other Lewis bases involving pyridinyl or carbonyl functions. In the presence of tetramethylpyrazine (TMP), benzoquinone (BQ) and para-dinitrobenzene (pDNB) as ditopic Lewis bases, a recurrent oligomeric motif stabilized by six ChB interactions is observed, involving six SeCN groups and the ChB acceptor sites of TMP, BQ and pDNB in the 2:1 adducts (1)2·TMP, (1)2·BQ and (1)2·pDNB.

Dinuclear manganese alkoxide complexes as catalysts for C–N bond cleavage of simple tertiary N,N-dialkylamides to give esters

Amide bonds are stable due to the resonance between the nitrogen lone pair and the carbonyl moiety, and therefore the chemical transformation of amides, especially tertiary amides, involving C–N bond fission is considered one of the most difficult organic reactions, unavoidably requiring harsh reaction conditions and strong acids or bases.

Synthesis and Structural Investigation of Some Electron-Rich Nitroaromatics

2,4-Difluoro-, 2,4,6-trifluoro-, and 2,3,4,6,tetrafluoronitrobenzenes undergo nucleophilic aromatic substitution, once, twice, and three times with a variety of amine substituents with a high degree of regiochemical control to provide a range of electron-rich nitrobenzene derivatives. In these structures the nitro group proves a useful structural probe to reveal the varying extents of electron donation from the varying number of amino substituents onto the nitro group as revealed by accurate low temperature X-ray crystal structure analysis, thus increasing electron donation manifests in a decrease in the Ar–NO2 distance consistent with increased double bond character, while the N–O bond distance increases as the oxygens accept the electron density. The effect of delocalization of the aniline nitrogen lone pair onto the nitro group impacts on the geometry and hybridization of the nitrogen substituent and also impacts on the ability of the nitrogen lone pair electrons to participate in other competing electronic interactions, such as the nN–σ*C–S anomeric effect as demonstrated by the thiazolidine substituted derivatives 3c, 4c, and 6c.

Comparison crystal structure conformations of two structurally related biphenyl analogues: 4,4′-bis[3-(pyrrolidin-1-yl)prop-1-yn-1-yl]-1,1′-biphenyl and 4,4′-bis{3-[(S)-2-methylpyrrolidin-1-yl]prop-1-yn-1-yl}-1,1′-biphenyl

The title compounds, C26H28N2, (I), and C28H32N2, (II), were designed based on the structure of the potent α9α10 nicotinic acetylcholine receptor antagonist ZZ161C {1,1′-[[1,1′-biphenyl]-4,4′-diylbis(prop-2-yne-3,1-diyl)]bis(3,4-dimethylpyridin-1-ium) bromide}. In order to improve the druglikeness properties of ZZ161C for potential oral administration, the title compounds (I) and (II) were prepared by coupling 4,4′-bis(3-bromoprop-1-yn-1-yl)-1,1′-biphenyl with pyrrolidine, (I), and (S)-2-methylpyrrolidine, (II), respectively, in acetonitrile at room temperature. The asymmetric unit of (I) contains two half molecules that each sit on sites of crystallographic inversion. As a result, the biphenyl ring systems in compound (I) are coplanar. The biphenyl ring system in compound (II), however, has a dihedral angle of 28.76 (11)°. In (I), the two independent molecules differ in the orientation of the pyrrolidine ring (the nitrogen lone pair points towards the biphenyl rings in one molecule, but away from the rings in the other). The torsion angles about the ethynyl groups between the planes of the phenyl rings and the pyrrolidine ringNatoms are 84.15 (10) and −152.89 (10)°. In compound (II), the corresponding torsion angles are 122.0 (3) and 167.0 (3)°, with the nitrogen lone pairs at both ends of the molecule directed away from the central biphenyl rings.