Polymerization of hexamethylcyclotrisiloxane with a biscatecholsiliconate. II. Kinetics of polymerization

The kinetics of polymerization were investigated for the polymerization of hexamethylcyclotrisiloxane (D₃) in toluene with methanol or water as an initiator, benzyltrimethylammonium bis(o-phenylenedioxy)phenylsiliconate as a catalyst, and dimethyl sulfoxide (DMSO) as a promoter. The rate of initiation was found to be comparable with both water and methanol. Addition of catechol drastically reduces the rate of initiation. The rate of propagation was found to be dependent upon the catalyst, DMSO, catechol and the aging of the catalyst solution. Two types of functional groups were postulated to be present during the propagation reaction, i.e., ?SiOH (dormant form) and ?SiONR₄ (living form). The former can be converted to the latter by R₄NOH derived from hydrolysis of catalyst. A postulated mechanism of polymerization with biscatecholsiliconate is presented.     

Bromination of aromatic molecules with polymer supported reagents

Crosslinked co-poly/styrene-4-vinyl(N-hexylpyridinium bromide) was converted with bromine or chlorine to insoluble polymer supported complexes or respectively, and their reactivity studied in reactions with various aromatic molecules. Reagent was found in all cases to be milder than reagent and regiospecifically transformed alkoxy and amino substituted benzenes ( ) into 4-bromo derivatives, while corresponding reactions with resulted in dibromo derivatives. Several benzoheterocyclic molecules were converted with to substitution or addition products, i.e. 2,3-dibromo-N-methylpyrrole, 3-bromobenzo/b/thiophene, and 2,3-dibromo-2,3-dihydrobenzofuran. In the series of ortho-alkyl disubstituted benzene derivatives, i.e. o-xylene, indane, and tetraline, where the Mills-Nixon effect was established with various electrophilic reagents, bromination reactions with showed higher β-selectivity than the corresponding reactions with bromine. The rate of bromination in various alkyl substituted benzenes with reagent depended on the magnitude of the alkyl group, as well as the para/ortho regioselectivity, amounting to 100% in the case of tert-butylbenzene.     

Die Oxydation von Blei in Wasser bei Gegenwart von atmosphärischem Sauerstoff, 2. Mitt.

Zusammenfassung Der zeitliche Verlauf der Oxydation von granuliertem Blei ist unregelmäßig, weil sich dessen Oberfläche während der Reaktion nicht konstant vermindert. Die Reaktion verläuft an der Luft mit und ohne CO₂ mit gleicher Geschwindigkeit, weil auch an der Luft nur eine Mischung von Bleioxyd und Hydroxydcarbonat entsteht. Auf die geometrische Oberfläche bezogen, hat die Oxydationsgeschwindigkeit unter den beschriebenen Bedingungen eine Größenordnung von 0,6 g/dm²h. Auch die Abhängigkeit der oxydierten Bleimenge vom Partialdruck des Sauerstoffes (0,00–0,21) wurde bestimmt. Die Bleikonzentration hat in breitem Intervall praktisch keinen Einfluß. Der Lichteinfluß ist positiv, auf die Reaktion in der Dunkelheit bezogen, ist die oxydierte Bleimenge 1,2mal höher. Die Bedingungen für die Herstellung von tetragonalem Bleioxyd, dessen Bildung über das rhombische Bleioxyd verläuft, werden wiedergegeben.Mit 3 Abbildungen     

Microwave-mediated intramolecular Diels-Alder cyclization of biodihydroxylated benzoic acid derivatives

The synthetic utility of biodihydroxylated benzoic acid derivatives for the construction of bridge bicyclo scaffolds was investigated. Biodihydroxylation of benzoic acid using Ralstonia eutropha B9 gave (1S,2R)-1,2-dihydroxycyclohexa-3,5-diene-1-carboxylic acid (DCD) in high optical purity (>95% ee). Protection of the intermediate and subsequent functional group transformation gave the required cyclization precursors in moderate to excellent overall yields. Subsequent intramolecular Diels-Alder cyclization of biodihydroxylated benzoic acid derivatives was carried out using either thermal or microwave conditions. Enantiomerically pure products with five chiral centers were obtained in 4-6 steps from achiral starting material.     

Conformationally tailored N-[(2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl)carbonyl]proline templates as molecular tools for the design of peptidomimetics. Design and synthesis of fibrinogen receptor antagonists

The proline peptide bond was shown by 2D proton NMR studies to exist exclusively in the trans conformation in benzyl (2S)-1-[[(2S)-2-methyl-6-nitro-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl]carbonyl]-2-pyrrolidinecarboxylate [(S,S)-11], benzyl (2S)-1-[[(2S)-2-methyl-7-nitro-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl]carbonyl]-2-pyrrolidinecarboxylate [(S,S)-9], and in the corresponding 6-amino and 7-amino carboxylic acids (S,S)-3 and (S,S)-4. On the other hand, the diastereomers (R,S)-11 and (R,S)-9 containing an (R)[2-methyl-6/7-nitro-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl]carbonyl moiety, and the diastereoisomers (R,S)-3 and (R,S)-4 incorporating an (R)[6/7-amino-2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl]carbonyl moiety were found to exist as equilibria of trans(63-83%) and cis(17-37%) isomers. These conformationally defined templates were applied in the construction of RGD mimetics possessing antagonistic activity at the platelet fibrinogen receptor.     

Diastereoselective Synthesis of <Emphasis Type="Italic">cis</Emphasis>-2,6-Disubstituted Perhydro-4-pyranones Using Elevated Pressure Hydrogenation

Summary. A diastereoselective strategy for the synthesis of γ-pyrons was developed, starting from the Mg diacetonedicarboxylate complex. Initial cyclization with suitable anhydrides or acid chlorides, followed by hydrolytic decarboxylation leads to 2,6-disubstituted pyrans. Elevated pressure hydrogenation using Pd/C affords the title compounds in high diastereoselectivity. Scope and limitations of the method are outlined on selected examples.     

Three methods for calculation of the hyper-Wiener index of molecular graphs

The hyper-Wiener index WW of a graph G is defined as WW(G) = (summation operator d (u, v)(2) + summation operator d (u, v))/2, where d (u, v) denotes the distance between the vertices u and v in the graph G and the summations run over all (unordered) pairs of vertices of G. We consider three different methods for calculating the hyper-Wiener index of molecular graphs: the cut method, the method of Hosoya polynomials, and the interpolation method. Along the way we obtain new closed-form expressions for the WW of linear phenylenes, cyclic phenylenes, poly(azulenes), and several families of periodic hexagonal chains. We also verify some previously known (but not mathematically proved) formulas.     

Regiospecifity of acetylene photocycloadditions to fluorosubstituted benzenes

Irradiation of a cyclohexane solution of hexafluorobenzene and 1-phenyl-2-alkylsubstituted acetylenes resulted in the formation of bicyclo ( 4.2.0) octatriene derivatives, the quantum yield of (2+2) photoaddition depending on the structure of the acetylene, the con- centration of hexafluorobenzene, and the solvent polarity. Reaction with phenylacetylene resulted in the formation of two isomeric cyclo- octatetraene derivatives. Regiospecific (2+2) photoaddition of 1- phenyl-2-tert-butylacetylene to position C-3 and C-4 in alkoxysubsti- tuted pentafluorobenzene derivatives was observed. (2+2) photoaddition of 1-phenylpropyne-1 to alkoxysubstituted pentafluorobenzenes resulted in 3-phenyl-4-methyl-7-alkoxy-1,2,5,6,8-pentafluorobicyclo (4.2.0) octa- triene derivatives. Replacement of the fluorine atom in hexafluoro- benzene with alkoxy substituents diminished quantum yields of (2+2) photocycloadditions.     

Generalized theoretical and practical treatment of the kinetics of an enzyme-catalyzed reaction in the presence of an enzyme equimolar irreversible inhibitor

We revisit a previous analysis of the classical Michaelis-Menten enzyme reaction for the case in which the free enzyme incurs the loss of its activity by an irreversible inhibitor concentration dependent but time unaltered rate constant (see Golicnik, M. J. Chem. Inf. Comput. Sci. 2002, 42, 157-161). We study the kinetic model of an enzyme-catalyzed reaction in the presence of an equimolar irreversible inhibitor showing a time dependent inactivation rate constant because of considerable inhibitor amount depletion during the course of the reaction. We show that an analytical solution containing the nonelementary Gauss hypergeometric function can be found for the reactants in equation Phi of an implicit type that precludes direct calculation of the extent of reaction at any time. The transformation theory of the hypergeometric function is used to obtain rapidly convergent power series, and for the root calculation of equation Phi the divergence-proof root bracketing algorithm according to Van Wijngaarden-Dekker-Brent is performed. Numerically generated data are analyzed according to this mathematical procedures, and the results are compared with ones obtained by the numerical integration treatment.