Mesophases de Derives du Cyclohexane

The mesophases of four trans 4-alkyl-cyclohexane carboxylic acids and of 1,4-bis(′-heptyl-cyclohexyl)-cyclohexane “all trans” were identified by isomorphy as nematic and normal smec-tic B phases. The existence of mesogenic molecules without a permanent dipolar moment or a delocalizated electron is thus confirmed.     

Temperature-modulated selective C(sp3)–H or C(sp2)–H arylation through palladium catalysis

Transition metal-catalysed C–H bond functionalisations have been extensively developed in organic and medicinal chemistry. Among these catalytic approaches, the selective activation of C(sp³)–H and C(sp²)–H bonds is particularly appealing for its remarkable synthetic versatility, yet it remains highly challenging. Herein, we demonstrate the first example of temperature-dependent selective C–H functionalisation of unactivated C(sp³)–H or C(sp²)–H bonds at remote positions through palladium catalysis using 7-pyridyl-pyrazolo[1,5-a]pyrimidine as a new directing group. At 120 °C, C(sp³)–H arylation was triggered by the chelation of a rare [6,5]-fused palladacycle, whereas at 140 °C, C(sp²)–H arylation proceeded instead through the formation of a 16-membered tetramer containing four 7-pyridyl-pyrazolo[1,5-a]pyrimidine–palladium chelation units. The subsequent mechanistic study revealed that both C–H activations shared a common 6-membered palladacycle intermediate, which was then directly transformed to either the [6,5]-fused palladacycle for C(sp³)–H activation at 120 °C or the tetramer for C(sp²)–H arylation at 140 °C with catalytic amounts of Pd(OAc)₂ and AcOH. Raising the temperature from 120 °C to 140 °C can also convert the [6,5]-fused palladacycle to the tetramer with the above-mentioned catalysts, hence completing the C(sp²)–H arylation ultimately.

Unprecedented 16-membered tetramer or [6,5]-fused palladacycle, mutually shadowboxing-like transformed from the shared common intermediate, accomplishes the Pd-catalysed temperature-dependent selective arylation of C(sp²)–H or C(sp³)–H.     

Trioxatruxenes: A New Family of Disc-Like Mesogens with a Complex Polymorphism

A homologous series of hexa-n-alkanoyloxytrioxatruxenes has been prepared with n=7 t o n=15 in order to check the influence upon the mesomorphic properties exerted by the substitution of the three methylenes of the truxene core by oxygen atoms. Interphase transitions between solid, mesomorphic and isotropic phases were studied by hot stage microscopy and DSC. One of the homologs (R=C₁₃H₂₇CO-) was previously identified by X-Ray investigations and used as t h e reference substance to confirm the structure of the various mesophases by contact method. A new example of the socalled inverted sequence (crystal-N_D nematic-D columnar-isotropic) and several examples of the new oblique disordered columnar phase (Dobd) are reported. Trioxatruxenes can be quoted as able to exhibit the most complex polymorphism found in disc-like liquid crystals with the sequence: K - N_D - D_obd - D_rd - D_hd - I.     

Preliminary Communications. Benzotrisfuran derivatives A new series of disc-like liquid crystals

An homologous series of 2,3,5,6,8,9-hexa-n-alkanoyloxyphenylbenzo (1,2b; 3,4b';5,6b')trisfurans has been synthezised. The synthesis and mesomorphic properties are described and compared with those of other disc-like systems. These derivatives exhibit the relatively uncommon hexagonal ordered columnar phase D_ho. One hexa-n-alkoxybenzoyloxy derivative has also been prepared, it provides a new example of the N_D lenticular nematic phase.