Rosana S. Montani

Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases

We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.

Synthesis of conjugated polymers. Polymerizability studies of bis-sulfonium salts

Although the bis-sulfonium salt precursor route is one of the most versatile procedures available for the synthesis of conjugated polymers, little effort has been made to explore the scope of this polymerization reaction. In this study, a series of bis-sulfonium salts were synthesized and in order to assess their polymerizability, isotopic exchange at the benzylic positions as well as their reactivity was studied by 13C-NMR spectroscopy. Meanwhile the generation of the reactive intermediate was analyzed by UV spectroscopy. In addition, the reaction heats of the equilibria leading to the intermediate polymerizable species from the corresponding precursors were calculated by computational methods. The relationship between the chemical structure of the bis-sulfonium salts, their ability to polymerize, and the theoretical results was then analyzed in an effort to establish a reactivity pattern that could help to predict the type of bis-sulfonium salt that can be polymerized by this synthetic route and to contribute to the rational design of new conjugated polymers.

4'-[2-(2-Ethoxyethoxy)ethoxy]biphenyl-4-carboxylic acid: correlation between its crystalline and smectic phases.

The crystal structure of the dimeric title compound, C(19)H(22)O(5), is dominated by a head-to-head hydrogen-bonding interaction between centrosymmetrically related carboxyl groups in each monomer. The result is a dimeric axis of unusual length (ca 34 A), but still shorter than what could be expected for a fully extended chain, owing to two turning points in the oligoethoxy ends. This allows for an explanation of the structure of the smectic mesophase exhibited by this compound and at the same time fully validates former geometric estimations based on PM3 calculations.

Triethylamine-induced Reactions of Methyl 2,3-Dibromo-2,3-diarylpropanoates in Methanol†

The title compounds undergo elimination with methanolic triethylamine to afford the corresponding debrominated olefins, most of them through an E2 stereoconvergent process.

Methyl 2,3-dibromo-2,3-diarylpropanoates. Debromination and dehydrobromination reactions

Rate and product studies of the iodide- and methoxide-mediated reactions of methyl (R,R)- and (R,S)-2,3-dibromo-2,3-diarylpropanoates and some of their 2- and 3-(4-substitutedphenyl)-derivatives have been carried out. The results indicate that the reaction of iodide with most of the substrates affords only debrominated olefins, whereas the course of the methoxide-promoted reactions seems to depend mainly on the configuration of the starting substrate. Thus, in the eliminations induced by methoxide ion most of the (R,R)-compounds led exclusively to dehydrobrominated olefins. On the other hand, the dehydrobrominations of most of the (R,S)-isomers can be accompanied by debromination. Mechanistic aspects are discussed.

Synthesis of Poly(m-pyridylene-1,2-diphenylvinylene)

The synthesis by dehalogenating polycondensation and characterization of a new soluble conjugated polymer, poly(m-pyridylene-1,2-diphenylvinylene), DP-PPyV, is reported here. It shows good mechanical properties and a λmax = 330 nm. The maximum intensity peak of MALDI-TOF corresponds to 1.800 Da.

Isomerisation of Methyl (E) 2-Bromo-3-(4-XC6H4)-propenoates

The geometric isomerisation of the title compound induced by bromine or chlorine appears to involve ionic pair species resulting from the nucleophilic attack of the halogen.

Triethylamine-promoted Elimination from (R,R)-PhCHORCHClCO2Me (R=MeCO, PhCO, 4-MeC6H4, 4-MeOC6H4, MeSO2 and 4-MeC6H4SO2) in Various Solvents

The rates and stereochemistry of elimination from a number of (R,R)-PhCHORCHClCO2Me (R=MeCO, PhCO, 4-MeC6H4, 4-MeOC6H4, MeSO2 and 4-MeC6H4SO2) with triethylamine in tetrahydrofuran, acetone, methanol, dimethylformamide and acetonitrile as solvents led to the conclusion that the elimination process occurs through an (E1cB)I mechanism.

Methoxide-induced fragmentation of 2,2,3-trihalogeno- and 2,2-dihalogeno-3-methoxy-1,3-diphenylpropanones

The reaction of 2,2,3-trihalogeno-1,3-diphenylpropanones with sodium methoxide in methanol occurs with exclusive formation of fragmentation products. Kinetic and stereochemical evidence is interpreted in terms of a concerted intramolecular process promoted by initial attack of methoxide anion at the carbonyl carbon. In the case of 2,2-dihalogeno-3-methoxy-1,3-diphenylpropanones the reaction gives fragmentation, rearrangement, and elimination products. The former products are believed to be formed through competing reactions involving an intermediate formed on addition of methoxide to the substrate. For the fragmentation pathway the results are consistent with a carbanionic-intermediate mechanism.