[Law-making "pioneers" of scientometrics]. / A tudománymetria törvényalkotó "úttöroi".

The three much cited laws of scientometrics are the laws of Bradford, Lotka and Zipf. The authors briefly review the scientific career of the men behind the names, and that how they discovered the laws named after them. An outline is also given of the scientific aftermath of the laws and of the oeuvre of their eponym.

Theoretical studies on the bifunctionality of chiral thiourea-based organocatalysts: competing routes to C-C bond formation.

The mechanism of enantioselective Michael addition of acetylacetone to a nitroolefin catalyzed by a thiourea-based chiral bifunctional organocatalyst is investigated using density functional theory calculations. A systematic conformational analysis is presented for the catalyst, and it is shown that both substrates coordinate preferentially via bidentate hydrogen bonds. The deprotonation of the enol form of acetylacetone by the amine of the catalyst is found to occur easily, leading to an ion pair characterized by multiple H-bonds involving the thiourea unit as well. Two distinct reaction pathways are explored toward the formation of the Michael product that differ in the mode of electrophile activation. Both reaction channels are shown to be consistent with the notion of noncovalent organocatalysis in that the transition states leading to the Michael adduct are stabilized by extensive H-bonded networks. The comparison of the obtained energetics for the two pathways allows us to propose an alternative mechanistic rationale for asymmetric C-C bond forming reactions catalyzed by bifunctional thiourea derivatives. The origin of enantioselectivity in the investigated reaction is also discussed.

Mechanism of formation of organic carbonates from aliphatic alcohols and carbon dioxide under mild conditions promoted by carbodiimides. DFT calculation and experimental study.

Dicyclohexylcarbodiimide (CyN=C=NCy, DCC) promotes the facile formation of organic carbonates from aliphatic alcohols and carbon dioxide at temperatures as low as 310 K and moderate pressure of CO2 (from 0.1 MPa) with an acceptable rate. The conversion yield of DCC is quantitative, and the reaction has a very high selectivity toward carbonates at 330 K; increasing the temperature increases the conversion rate, but lowers the selectivity. A detailed study has allowed us to isolate or identify the intermediates formed in the reaction of an alcohol with DCC in the presence or absence of carbon dioxide. The first step is the addition of alcohol to the cumulene (a known reaction) with formation of an O-alkyl isourea [RHNC(OR')=NR] that may interact with a second alcohol molecule via H-bond (a reaction never described thus far). Such an adduct can be detected by NMR. In alcohol, in absence of CO2, it converts into a carbamate and a secondary amine, while in the presence of CO2, the dialkyl carbonate, (RO)2CO, is formed together with urea [CyHN-CO-NHCy]. The reaction has been tested with various aliphatic alcohols such as methanol, ethanol, and allyl alcohol. It results in being a convenient route to the synthesis of diallyl carbonate, in particular. O-Methyl-N,N'-dicyclohexyl isourea also reacts with phenol in the presence of CO2 to directly afford for the very first time a mixed aliphatic-aromatic carbonate, (MeO)(PhO)CO. A DFT study has allowed us to estimate the energy of each intermediate and the relevant kinetic barriers in the described reactions, providing reasonable mechanistic details. Calculated data match very well the experimental results. The driving force of the reaction is the conversion of carbodiimide into the relevant urea, which is some 35 kcal/mol downhill with respect to the parent compound. The best operative conditions have been defined for achieving a quantitative yield of carbonate from carbodiimide. The role of temperature, pressure, and catalysts (Lewis acids and bases) has been established. As the urea can be reconverted into DCC, the reaction described in this article may further be developed for application to the synthesis of organic carbonates under selective and mild conditions.

Behaviour of [PdH(dppe)2]X (X=CF3SO3-, SbF6-, BF4-) as proton or hydride donor: relevance to catalysis.

The synthesis, characterization and properties of [PdH(dppe)(2)](+)CF(3)SO(3) (-).0.125 THF (1; dppe=1,2-bis(diphenylphosphanyl)ethane) and its SbF(6) (-) (1') and BF(4) (-) (1") analogues, the missing members of the [MH(dppe)(2)](+)X(-) (M=Ni, Pd, Pt) family, are described. The Pd hydrides are not stable in solution and can react as proton or hydride donors with formation of dihydrogen, [Pd(dppe)(2)](2+) and [Pd(dppe)(2)]. Complexes 1-1" react with carbocations and carbanions by transferring a hydride and a proton, respectively. Such H(-) or H(+) transfer occurs also towards unsaturated compounds, for example, hydrogenation of a C=C double bond. Accordingly, 1 can hydrogenate methyl acrylate to methyl propionate. Complex 1" is an effective (hourly turnover frequency=16) and very selective (100 %) catalyst for the hydrogenation of cyclohexen-2-one to cyclohexanone with dihydrogen under mild conditions. Density functional calculations coupled with a dielectric continuum model were carried out to compute the energetics of the hydride/proton transfer reactions, which were used to rationalize some of the experimental findings. Theory provides strong support for the thermodynamic and kinetic viability of a tetracoordinate Pd complex as an intermediate in the reactions.

Acrylate formation via metal-assisted C-C coupling between CO2 and C2H4: reaction mechanism as revealed from density functional calculations.

The reaction path for the formation of a binuclear hydrido-acrylate complex in a CO(2)-C(2)H(4) coupling process is explored in detail by locating the key intermediates and transition states on model potential energy surfaces derived from density functional calculations on realistic models. The formation of the new C-C bond is shown to take place via oxidative coupling of coordinated CO(2) and C(2)H(4) ligands resulting in a metalla-lactone intermediate, which can rearrange to an agostic species allowing for a beta-hydrogen shift process. The overall reaction is predicted to be clearly exothermic with all intermediates lying below the reactants in energy, and the highest barrier steps correspond to C-C coupling and beta-hydrogen transfer. The phosphine ligands are found to play an important role in various phases of the reaction as their dissociation controls the coordination of CO(2), the formation of the agostic intermediate, and the dimerization process; furthermore, their presence facilitates the oxidative coupling by supplying electrons to the metal center. Our results provide a theoretical support for the reaction mechanism proposed from experimental observations. The effect of the solvent medium on the relative energy of reaction intermediates and transition states is examined and found important in order to predict reliable energetics.

Synthesis, characterization, and reactivity of cationic hydride [HPd(diphosphine)(2)](+)CF(3)SO(3)(-), the missing member of the family [HM(dppe)(2)](+)X(-) (M = Ni, Pd, Pt). DFT QM/MM structural predictions for the [HPd(dppe)(2)](+) moiety.

The synthesis, characterization, and properties of the cationic hydride [HPd(dppe)(2)](+)CF(3)SO3(-).1/8THF, the missing member of the family [HM(dppe)(2)](+)X(-) (M = Ni, Pd, Pt), are described. The Pd hydride is not stable in solution and may react as either a proton or a hydride donor. DFT QM/MM calculations of the [HPd(dppe)(2)](+) moiety have allowed us to predict its structure and reactivity.

Determination of the retention behavior of barbituric acid derivatives in reversed-phase high-performance liquid chromatography by using quantitative structure-retention relationships.

Retention parameters of 45 barbituric acid derivatives were determined on an amide embedded RP silica column using non-buffered water-dioxan eluent systems. Linear correlations were calculated between the logarithm of the capacity factor and the dioxan concentration in the eluent. Six different retention parameters of each barbituric acid derivative were correlated with different conventional and quantum chemical structural descriptors using quantitative structure-retention relationship (QSRR). The different parameters were: intercept (log k0) and slope (b) values of the linear, the combined retention parameter (log k0/b), asymmetry factor (AF5) and theoretical plate values (N(USP) and N(JP), according to the United States and Japanese Pharmacopoeia calculations). Stepwise regression analysis (SRA) and principal component analysis (PCA) followed by two-dimensional nonlinear mapping were used to determine the retention behavior of barbituric acid derivatives. SRA and PCA led to similar results. The results indicated that the retention of barbituric acid derivatives are mainly governed by the polaric and steric parameters of the substituents.

Ab initio conformational space study of model compounds of O-glycosides of serine diamide.

Relative stabilities of rotamers of the N-acetyl-O-(2-acetamido-2-deoxy-alpha-D-galactopyranosyl)-L-seryl-N'-methyl amide (1) and eleven analogous molecules containing beta-galactose, alpha- and beta-mannose, alpha- and beta-glucose, and L-threonine were calculated to learn whether they could explain the natural preference for 1 in linkages between the carbohydrate and protein in glycoproteins. The lowest energy rotamers of four O-glycoside models of serine diamide were identified with a Monte Carlo search coupled with molecular mechanics (MM2*). These rotamers were further optimized with an ab initio level of theory (HF/6-31G(d)). Subsequently, B3LYP/6-31 + G(d) single point energies were calculated for the most stable HF structures. The most favorable interactions are present in 1 and its glucose analogue. The monosaccharide for the carbohydrate antenna is anchored to the serine residue with an AcNH...O=C-NHMe hydrogen bond in the most stable rotamers. The mannose analogue and the beta-anomers are considerably less stable according to the MM2* and especially to the ab inito energy values. The three analogues have HF/6-31 G(d) energies which are 4-6 kcal mol-1 higher; the single point B3LYP/6-31 + G(d)//HF/6-31 G(d) calculations yield preferences of 3-5 kcal mol-1 for 1. The most stable L-threonine analogues show a behaviour very similarly to the corresponding serine analogues. The ZPE and thermal correction components of the calculated delta H298 and delta G298 values are relatively small (< 0.4 kcal mol-1). However, the T delta S298 term can be as large as 2.6 kcal mol-1. The entropy terms stabilize the alpha-anomers relative to beta-anomers, and ManNAc relative to GalNAc. The largest stabilization effect is observed for one of the rotamers of the alpha-anomer of ManNAc.