Practical synthesis of 1,12-difunctionalized o-carborane for the investigation of polar liquid crystals.

Two isomerically pure 1,12-difunctionalized derivatives of o-carborane, 12-iodo-1-vinyl (1b) and 12-iodo-1-phenyl (1c), are conveniently obtained on a practical scale in yields of 22% and 32%, respectively, by monoiodination of the corresponding o-carborane derivatives (4b and 4c) followed by separation of the regioisomers by crystallization (1b) and chromatography (1c). Subsequent functional group transformations gave access to other derivatives, including two liquid-crystalline compounds, in which o-carborane is a linear structural element. Regioselectivity of substitution on the carborane cage and on the benzene ring correlates with the inductive effect parameter of the substituent. The preparation of analogous derivatives of m-carborane was also investigated.

Cyclization of substitued 2-(2-fluorophenylazo)azines to azino[1,2-c]benzo[d][1,2,4]triazinium derivatives.

Light-induced cyclization of several substituted 2-(2-fluorophenylazo)azines in the presence of Ca(2+) ions to the corresponding triazinium derivatives is investigated experimentally and computationally. The azo derivatives of 4-methylpyridine 4 undergo facile cyclization to the corresponding triazinium 1, and the rate of cyclization increases with increasing number of fluorine atoms at the benzene ring. No triazinium ions were obtained from azo derivatives of 4-cyanopyridine, pyrazine and pyrimidine, presumably due to their instability under the reaction conditions. The experimental results and mechanism are discussed with the aid of DFT computational results.

Functional group transformations in derivatives of 6-oxoverdazyl.

Transformations of functional groups, such as OCH2Ph, OCOPh, NO2 and I, in 1,3,5-triphenyl-6-oxoverdazyls 1a-1e were investigated in order to expand the range of synthetic tools for incorporation of the verdazyl system into more complex molecular architectures and to increase spin delocalization. Thus, Pd-catalyzed debenzylation of the OCH2Ph group or basic hydrolysis of the OCOPh group gave the phenol functionality, which was acylated, but could not be alkylated. Orthogonal deprotection of diphenol functionality was also demonstrated in radical 1c. Pt-catalyzed reduction of the NO2 group led to the aniline derivative, which was acylated. Attempted C-C coupling reactions to iodophenyl derivatives 1e and 5e were unsuccessful. Selected verdazyl radicals were characterized by EPR and electronic absorption spectroscopy, and results were analyzed with the aid of DFT computational methods.

Thermochromic discotic 6-oxoverdazyls.

6-Oxoverdazyls 1b[n], substituted with three 3,4,5-trialkoxyphenyl groups (n = 8 and 10), exhibit a columnar hexagonal phase (Col(h)) below 130 °C. They display reversible color change from red to green at the transition to the isotropic phase. XRD and magnetization data do not support dimerization of the radicals in the mesophase.

Synthesis of oleophilic electron-rich phenylhydrazines.

Phenylhydrazines 1 substituted with two or three long-chain alkyl, alkoxy or alkylsulfanyl groups were successfully prepared by acid-induced removal of the Boc group in hydrazides 2. The reaction is carried out with 5 equivalents of TfOH in CF(3)CH(2)OH/CH(2)Cl(2) at -40 °C for 1.5 min. Under these conditions, the deprotected hydrazine 1 is fully protonated, which increases its stability in the reaction medium. The hydrazines were isolated in 60-86% yields and purities >90%. The hydrazides 2 were obtained in 43-71% yields from aryl bromides 5, which were lithiated with t-BuLi and subsequently reacted with di-tert-butyl azodicarboxylate (DTBAD).

Photoconductive liquid-crystalline derivatives of 6-oxoverdazyl.

1,3,5-Triphenyl-6-oxoverdazyl radicals 1[n], in which each phenyl group is substituted with three alkylsulfanyl groups (n = 6, 8, 10), exhibit a monotropic columnar rectangular (Col(r)) phase below 60 °C. Detailed analysis of 1[n] revealed a broad absorption band in the visible region with maxima at 540 and 610 nm and redox potentials E(1/2)(0/+1) = +0.99 V and E(1/2)(0/-1) = -0.45 V vs SCE. Photovoltaic studies of 1[8] demonstrated a hole mobility of 1.52 × 10⁻³ cm² V⁻¹ s⁻¹ in the mesophase with an activation energy of 0.06 ± 0.01 eV. Magnetization studies of 1[8] revealed nearly ideal paramagnetic behavior in either the solid or fluid phase above 200 K and weak antiferromagnetic interactions at low temperatures.

The preparation of 3-substituted-1,5-dibromopentanes as precursors to heteracyclohexanes.

The methodology to prepare 3-substituted 1,5-dibromopentanes I and their immediate precursors, which include 3-substituted 1,5-pentanediols VII or 4-substituted tetrahydropyrans VIII, is surveyed. Such dibromides I are important intermediates in the preparation of liquid crystalline derivatives containing 6-membered heterocyclic rings. Four dibromides 1a-1d containing simple alkyl and more complex fragments at the 3-position were prepared. 3-Propyl- and 3-pentyl-pentane-1,5-diol (2a,b) were prepared starting from either glutaconate or malonate diesters, while tetrahydropyrans 3c and 3d were obtained from tetrahydro-4H-pyran-4-one. The advantages and disadvantages of each route are discussed. Dibromides 1c and 1d were used to prepare sulfonium zwitterions 11c and 11d.

4-Substituted 1-acyloxypyridine-2(1H)-thiones: experimental and computational studies of the substituent effect on electronic absorption spectra.

A series of eight 4-substituted 1-(adamantane-1-carbonyloxy)pyridine-2(1H)-thiones (1, X = H, OC(7)H(15), Me, CF(3), SC(3)H(7), CN, COOMe, and Cl) was prepared and characterized by UV-vis spectroscopy in MeCN and cyclohexane. The observed lowest energy transition, designated as pi(CS) --> pi*(ring), exhibits a substantial substituent effect and lambda(max) ranges from 333 (X = OC(7)H(15)) to 415 nm (X = CN). Experimental lambda(max) values for all esters except for 1b (X = OC(7)H(15)) correlate with the sigma(p)(-) parameter (rho = 0.41 +/- 0.03, r(2) = 0.95). In contrast, the energy of the absorption band at about 295 nm, designated as pi(CS) --> pi*(CS), is practically substituent independent. Both absorption bands exhibit a modest negative solvatochromic effect. The experimental absorption energies correlate better with excitation energies calculated for N-acetyloxy analogues 2 with the ZINDO//DFT than with the TD-DFT//DFT method. Calculations for a series of 12 N-acetates 2 predict the most blue-shifted pi --> pi* transition for the alkoxy substituent and most red-shifted for the NO(2) group relative to the parent 2a (X = H).

Ring-alkyl connecting group effect on mesogenic properties of p-carborane derivatives and their hydrocarbon analogues.

The effect of the phenyl-alkyl connecting group on mesogenic properties of several series of isostructural compounds containing p-carborane (A and B), bicyclo[2.2.2]octane (C), and benzene (D) was investigated using thermal and optical methods. Results demonstrated that mesophase stability in the series containing A-D follows the order (Alk)CH2CH2- < (Alk)OOC- < (Alk)CH2O- < (Alk)COO-. Surprisingly, the connecting groups (Alk)CH2CH2- and (Alk)OOC- destabilize the mesophase significantly stronger for carboranes (A and B) than for carbocyclic derivatives (C and D). Analysis indicates that this effect may have quadrupolar and conformational origin.

Reactivity of 13,13-dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward organolithiums: remarkable resistance to the DMS rearrangement.

Reactions of dibromocyclopropane 2a, containing two spiro-fused 1,3-dioxane rings, with MeLi gave only the methylation products 8 and 9 even at elevated temperatures. In contrast, the cyclohexane analogue 2b treated with MeLi underwent a smooth rearrangement to bicyclo[1.1.0]butane 11b at -78, -10, or +35 degrees C. Treatment of 2a with PhLi gave the alpha-Ph anion 13 as the only product, which underwent smooth methylation with MeI to give 14. Under the same conditions, 2b with PhLi gave bicyclo[1.1.0]butane 11b accompanied by bromophenyl derivative 8b. Treatment of either dibromide with t-BuLi gave a mixture of products including debrominated cyclopropanes 12. Experimental results were augmented with DFT calculations for salts 23 and MP2//DFT-level calculations for carbenes 22. They demonstrated a higher stability of the dioxane alpha-bromo anion with respect to alpha-elimination by 4.8 kcal/mol and also a lower tendency of the carbene 22a to undergo rearrangement by 4.0 kcal/mol than the cyclohexane analogues. These differences have been attributed to the inductive effect of the four oxygen atoms, which results in lower LUMO energy, the higher positive charge at the carbenic center, and the overall more electrophilic character of carbene 22a as compared to the cyclohexane derivative 22b. The rearrangement of carbenes 22 to the corresponding allenes 1, the thermodynamic products, requires a higher activation energy DeltaG(double dagger)(298) by 4.2 kcal/mol for dioxane and 6.4 kcal/mol for cyclohexane derivatives than for the formation of the bicyclo[1.1.0]butanes 11. The DeltaG(double dagger)(298) for intramolecular insertions to the CH bond is low and calculated as 6.0 kcal/mol for dioxane 22a and 2.0 kcal/mol for the formation of cyclohexane 22b.