Mechanistic Insight into the NN Bond‐Cleavage of Azo‐Compounds that was Induced by an Al?Al‐bonded Compound [L2−AlII?AlIIL2−]

An α‐diimine‐stabilized Al? Al‐bonded compound [L^2?Al^II? Al^IIL^2?] (L=[{(2,6‐iPr₂C₆H₃)NC(Me)}₂]; 1 ) consists of dianionic α‐diimine ligands and sub‐valent Al²⁺ ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo‐compounds afforded phenylimido‐bridged products [L^?Al^III(μ₂‐NPh)(μ₂‐NAr)Al^IIIL^?] ( 2 – 4 ). During the reaction, the dianionic ligands and Al²⁺ ions were oxidized into monoanions and Al³⁺, respectively, whilst the [NAr]^2? imides were produced by the four‐electron reductive cleavage of the N?N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L^2?)₂Al^III₂(μ₂‐NPh)₂Na₂(thf)₄] ( 5 ). Interestingly, when asymmetric azo‐compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4 ). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet‐state monoanionic ligands could result from an exothermic singlet‐to‐triplet conversion during the reaction process.     

Synthesis and Antimalarial‐Activity Evaluation of Tetraoxane?Triazine Hybrids and Spiro[piperidine‐4,3′‐tetraoxanes]

A series of tetraoxane? triazine hybrids and spiro[piperidine‐4,3′‐tetraoxanes] have been synthesized, and all the compounds were screened for in vitro antimalarial activity against chloroquine‐sensitive (D6) and chloroquine‐resistant (W2) strains of Plasmodium falciparum. Most of the spiro[piperidine‐4,3′‐tetraoxanes] exhibited moderate to good antimalarial activities, and two compounds have shown good antimalarial activity with IC₅₀ values in the range of 0.30 to 0.70 μM against both the strains with high selectivity index and no cytotoxicity towards mammalian kidney cell line.     

Homolytic S?S Bond Dissociation of 11 Bis(thiocarbonyl)disulfides R‐C(S)?S?S?C(S)R and Prediction of A Novel Rubber Vulcanization Accelerator

The structures and energetics of eight substituted bis(thiocarbonyl)disulfides (RCS₂)₂, their associated radicals RCS₂^., and their coordination compounds with a lithium cation have been studied at the G3X(MP2) level of theory for R=H, Me, F, Cl, OMe, SMe, NMe₂, and PMe₂. The effects of substituents on the dissociation of (RCS₂)₂ to RCS₂^. were analyzed using isodesmic stabilization reactions. Electron‐donating groups with an unshared pair of electrons have a pronounced stabilization effect on both (RCS₂)₂ and RCS₂^.. The S? S bond dissociation enthalpy of tetramethylthiuram disulfide (TMTD, R=NMe₂) is the lowest in the above series (155 kJ mol^?1), attributed to the particular stability of the formed Me₂NCS₂^. radical. Both (RCS₂)₂ and the fragmented radicals RCS₂^. form stable chelate complexes with a Li⁺ cation. The S? S homolytic bond cleavage in (RCS₂)₂ is facilitated by the reaction [Li(RCS₂)₂]⁺+Li⁺→2 [Li(RCS₂)]^.+. Three other substituted bis(thiocarbonyl) disulfides with the unconventional substituents R=OSF₅, Gu¹, and Gu² have been explored to find suitable alternative rubber vulcanization accelerators. Bis(thiocarbonyl)disulfide with a guanidine‐type substituent, (Gu¹CS₂)₂, is predicted to be an effective accelerator in sulfur vulcanization of rubber. Compared to TMTD, (Gu¹CS₂)₂ is calculated to have a lower bond dissociation enthalpy and smaller associated barrier for the S? S homolysis.     

Methylidene Rare‐Earth‐Metal Complex Mediated Transformations of CN,NN and N?H Bonds: New Routes to Imido Rare‐Earth‐Metal Clusters

Three new patterns of reactivity of rare‐earth metal methylidene complexes have been established and thus have resulted in access to a wide variety of imido rare‐earth metal complexes [L₃Ln₃(μ₂‐Me)₃(μ₃‐Me)(μ ‐ NR)] (L=[PhC(NC₆H₃iPr₂‐2,6)₂]^?; R=Ph, Ln=Y ( 2 a ), Lu ( 2 b ); R=2,6‐Me₂C₆H₃, Ln=Y ( 3 a ), Lu ( 3 b ); R=p‐ClC₆H₄, Ln=Y ( 4 a ), Lu ( 4 b ); R=p‐MeOC₆H₄, Ln=Y ( 5 a ), Lu ( 5 b ); R=Me₂CHCH₂CH₂, Ln=Y ( 6 a ), Lu ( 6 b )) and [{L₃Lu₃(μ₂‐Me)₃(μ₃‐Me)}₂(μ ‐ NR′N)] (R′=(CH₂)₆ ( 7 b ), (C₆H₄)₂ ( 8 b )). Complex 2 b was treated with an excess of CO₂ to give the corresponding carboxylate complex [L₃Lu₃(μ‐η¹:η¹‐O₂CCH₃)₃(μ‐η¹:η²‐O₂C‐CH₃)(μ‐η¹:η¹:η²‐O₂CNPh)] ( 9 b ) easily. Complex 2 a could undergo the selective μ₃‐Me abstraction reaction with phenyl acetylene to give the mixed imido/alkynide complex [L₃Y₃(μ₂‐Me)₃(μ₃‐η¹:η¹:η³‐NPh)(μ₃‐C?CPh)] ( 10 a ) in high yield. Treatment of 2 with one equivalent of thiophenol gave the selective μ₃‐methyl‐abstracted products [L₃Ln₃(μ₂‐Me)₃(μ₃‐η¹:η¹:η³‐NPh)(μ₃‐SPh)] (Ln=Y ( 11 a ); Lu ( 11 b ). All new complexes have been characterized by elemental analysis, NMR spectroscopy, and most of the structures confirmed by X‐ray diffraction.     

A Straightforward Synthesis of 2‐[1‐Alkyl‐5,6‐bis(alkoxycarbonyl)‐1,2,3,4‐tetrahydro‐2‐oxopyridin‐3‐yl]acetic Acid Derivatives via Domino Michael Addition?Cyclization Reaction

A new and efficient synthesis of 2‐[1‐alkyl‐5,6‐bis(alkoxycarbonyl)‐1,2,3,4‐tetrahydro‐2‐oxopyridin‐3‐yl]acetic acid derivatives by a one‐pot three‐component reaction between primary amine, dialkyl acetylenedicarboxylate, and itaconic anhydride (=3,4‐dihydro‐3‐methylidenefuran‐2,5‐dione) is reported. The reaction was performed without catalyst and under solvent‐free conditions with excellent yields. Notably, the ready availability of the starting materials, and the high level of practicability of the reaction and workup make this approach an attractive complementary method to access to unknown 2‐[1‐alkyl‐5,6‐bis(alkoxycarbonyl)‐1,2,3,4‐tetrahydro‐2‐oxopyridin‐3‐yl]acetic acid derivatives. The structures were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this type of domino Michael addition? cyclization reaction is proposed (Scheme 2).     

[Fe]‐Hydrogenase and Models that Contain Iron?Acyl Ligation

[Fe]‐hydrogenase is a newly characterized type of hydrogenase. This enzyme heterolytically splits hydrogen in the presence of a natural substrate. The active site of the enzyme contains a mono‐iron complex with intriguing iron? acyl ligation. Several groups have recently developed iron? acyl complexes as synthetic models of [Fe]‐hydrogenase. This Focus Review summarizes the studies of this enzyme and its model compounds, with an emphasis on our own research in this area.     

Characterization of the N?H···ON and N?H···NO H‐bonds in nitrosamine dimers

Ab initio molecular orbital and DFT calculations have been carried out for three most stable dimers of parent nitrosamine (NA) in order to elucidate the structures and energetics of the dimers. The structures were optimized using HF, B3LYP, and MP2 methods with 6‐311+G(d,p) and 6‐311++G(2d,2p) basis sets. At the optimized geometries obtained at MP2/6‐311++G(2d,2p) level of theory, the energies were evaluated at QCISD/aug‐cc‐pVDZ and CCSD/aug‐cc‐pVDZ levels. The most stable dimer has two N? H···O?N hydrogen bonds and the least stable dimer has two N? H···N?O hydrogen bonds. The natural bond orbital analysis showed that the lpO(N) → BD*(N? N) and lpO(N) → BD*(N? H_b) interactions play a decisive role in the stabilization of the NH···O(N) hydrogen bonds in dimers. The atoms in molecules results reveal that the intermolecular N? H···O(N) H‐bonds in dimers have electrostatic character. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Bi?Zn Bond Formation in Liquid Ammonia Solution: [Bi?Zn?Bi]4−, a Linear Polyanion that is Iso(valence)‐electronic to CO2

Reactions of the zinc(I) complex [Zn₂(Mesnacnac)₂] (Mesnacnac=[(2,4,6‐Me₃C₆H₂)NC(Me)]₂CH) with solid K₃Bi₂ dissolved in liquid ammonia yield crystals of the compound K₄[ZnBi₂]⋅(NH₃)₁₂ ( 1 ), which contains the molecular, linear heteroatomic [Bi Zn Bi]⁴⁻ polyanion ( 1 a ). This anion represents the first example of a three‐atomic molecular ion of metal atoms being iso(valence)‐electronic to CO₂ and being synthesized in solution. The analogy of the discrete [Bi Zn Bi]⁴⁻ anion and the polymeric [(ZnBi_4/2)⁴⁻] unit to monomeric CO₂ and polymeric SiS₂ is rationalized.     

All‐Carbon [3+3] Oxidative Annulations of 1,3‐Enynes by Rhodium(III)‐Catalyzed C?H Functionalization and 1,4‐Migration

1,3‐Enynes containing allylic hydrogens cis to the alkyne function as three‐carbon components in rhodium(III)‐catalyzed, all‐carbon [3+3] oxidative annulations to produce spirodialins. The proposed mechanism of these reactions involves the alkenyl‐to‐allyl 1,4‐rhodium(III) migration.     

Ruthenium‐Catalyzed Oxidative Coupling/Cyclization of Isoquinolones with Alkynes through C?H/N?H Activation: Mechanism Study and Synthesis of Dibenzo[a,g]quinolizin‐8‐one Derivatives

The mechanism of [{RuCl₂(p‐cymene)}₂]‐catalyzed oxidative annulations of isoquinolones with alkynes was investigated in detail. The first step is an acetate‐assisted C? H bond activation process to form cyclometalated compounds. Subsequent mono‐alkyne insertion of the Ru? C bonds of the cyclometalated compounds then takes place. Finally, oxidative coupling of the C? N bond of the insertion compounds occurs to afford Ru⁰ sandwich complexes that undergo oxidation to regenerate the catalytically active Ru^II complex with the copper oxidant and release the desired dibenzo[a,g]quinolizin‐8‐one derivatives. All of the relevant intermediates were fully characterized and determined by single crystal X‐ray diffraction analysis. The [{RuCl₂(p‐cymene)}₂]‐catalyzed C? H bond functionalization of isoquinolones with alkynes to synthesize dibenzo[a,g]quinolizin‐8‐one derivatives through C? H/N? H activation was also demonstrated.     

Investigation on the individual contributions of N?H···OC and C?H···OC interactions to the binding energies of β‐sheet models

In this article, the binding energies of 16 antiparallel and parallel β‐sheet models are estimated using the analytic potential energy function we proposed recently and the results are compared with those obtained from MP2, AMBER99, OPLSAA/L, and CHARMM27 calculations. The comparisons indicate that the analytic potential energy function can produce reasonable binding energies for β‐sheet models. Further comparisons suggest that the binding energy of the β‐sheet models might come mainly from dipole–dipole attractive and repulsive interactions and VDW interactions between the two strands. The dipole–dipole attractive and repulsive interactions are further obtained in this article. The total of N? H···H? N and C?O···O?C dipole–dipole repulsive interaction (the secondary electrostatic repulsive interaction) in the small ring of the antiparallel β‐sheet models is estimated to be about 6.0 kcal/mol. The individual N? H···O?C dipole–dipole attractive interaction is predicted to be ?6.2 ± 0.2 kcal/mol in the antiparallel β‐sheet models and ?5.2 ± 0.6 kcal/mol in the parallel β‐sheet models. The individual C^α? H···O?C attractive interaction is ?1.2 ± 0.2 kcal/mol in the antiparallel β‐sheet models and ?1.5 ± 0.2 kcal/mol in the parallel β‐sheet models. These values are important in understanding the interactions at protein–protein interfaces and developing a more accurate force field for peptides and proteins. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Mixed (PS/PO)‐Stabilized Geminal Dianion: Facile Diastereoselective Intramolecular C?H Activations by a Related Ruthenium–Carbene Complex

A new unsymmetrical geminal dianion that contained both a phosphine oxide moiety and a phosphine sulfide moiety has been synthesized. Its reactivity towards Ru^II was explored, which led to the formation of a highly reactive carbene complex that evolved at room temperature to yield a kinetic orthometalated Ru^II complex through C? H activation of the phenyl group of the phosphine oxide moiety. This insertion was found to be thermally reversible and a second C? H insertion occurred at a phenyl group of the phosphine sulfide moiety to form the thermodynamic orthometalated Ru^II complex in a diastereospecific manner. DFT calculations fully rationalized the experimental findings in terms of the relative energies of the kinetic and thermodynamic products and allowed the mechanism of this process to be fully understood.     

Pyrazolato Ligands towards Gold(I) Derivatives with Aurophilic Interactions: X‐Ray Structure of Bis[3,5‐(dibutoxyphenyl)‐1H‐pyrazolato‐κN1]bis(triphenylphosphine)digold(Au⋅⋅⋅Au)([Au(pz)(PPh3)]2; RC4H9?O?C6H4)

A crossed ‘torch' structure and a short Au⋅⋅⋅Au contact was established by X‐ray analysis for the dimeric complex [Au(pz)(PPh₃)]₂ (pz=3,5‐disubstituted pyrazolato, RC_nH_2n+1 O C₆H₄, n=4; 1 ). The complex is a representative member of a new well‐characterized family of derivatives containing the pyrazolato ligand in an uncommon monodentate coordination form. In addition, 1 is luminescent in the solid state at 77 K.