New procedure for the preparation of highly sterically hindered alkenes using a hypervalent iodine reagent

New methodology is described to construct the olefinic bond in overcrowded alkenes using a hypervalent iodine reagent, and applied in the synthesis of molecular motors.     

Orbital-order-induced metal-insulator transition in La(1-x)Ca(x)MnO3

We present evidence that the insulator-to-metal transition in La(1-x)Ca(x)MnO3 near x approximately 0.2 is driven by the suppression of coherent Jahn-Teller distortions, originating from d-type orbital ordering. The orbital-ordered state is characterized by large long-range Q2 distortions below T(O'- O*). Above T(O'- O*) we find evidence for coexistence between an orbital-ordered and an orbital-disordered state. This behavior is discussed in terms of electronic phase separation in an orbital-ordered insulating and an orbital-disordered metallic state.     

Benzyl anion abstraction from a (beta-diiminato)Fe(II) benzyl complex

The 3-coordinate 12 VE iron(II) benzyl complex [(nacnac)-Fe(CH2Ph)] reacts with B(C6F5)3 to yield a paramagnetic contact ion pair with an eta 2-(o,m)-[PhCH2B(C6F5)3] anion, which was characterised by X-ray diffraction.     

Exploring the boundaries of a light-driven molecular motor design: new sterically overcrowded alkenes with preferred direction of rotation

Insight in the steric and electronic parameters governing isomerization processes in artificial molecular motors is essential in order to design more advanced motor systems. A subtle balance of steric parameters and the combination of helical and central chirality are key features of light-driven unidirectional rotary molecular motors constructed so far. In an approach to decrease the steric hindrance around the central olefinic bond (rotary axis) and thereby lowering the energy barrier for helix inversion resulting in an increased rotation rate, the boundaries of our molecular motor design are explored. In a new design of a light-driven molecular motor based on a sterically overcrowded alkene the methyl substituent adjacent to the stereogenic center, which is responsible for the control of the direction of rotation, is shifted one position away from the fjord region of the molecule compared to the second-generation motor systems. In contrast to previously developed light-driven molecular motors, there is a preference for the methyl substituent to adopt a pseudo-equatorial orientation. Nevertheless, this new type of motor is capable of functioning as a rotary molecular motor, albeit not with full unidirectionality. Under the combined influence of light and heat, there is a preferred clockwise rotation of one half of the molecule. Surprisingly, the effect of shifting the methyl substituent on the energy barrier for helix inversion is small and even a slight increase in the barrier is observed.     

Silsesquioxane models for geminal silica surface silanol sites. A spectroscopic investigation of different types of silanols

The incompletely condensed monosilylated silsesquioxanes (c-C5H9)7Si7O9(OSiRR'2)(OH)2 (SiRR'2 = SiMe3, SiMe2C(H)CH2, SiMePh2) were reacted with SiCl(4) in the presence of an amine which yielded the dichloro compounds (c-C5H9)7Si7O9(OSiRR'2)O2SiCl2 (1-3). These compounds could be hydrolyzed into the corresponding silsesquioxanes containing geminal silanols, (c-C5H9)7Si7O9(OSiRR'2)O2Si(OH)2 (4-6). At elevated temperatures, the geminal silsesquioxanes 4 and 5 undergo condensation reactions and form the closed-cage silsesquioxane monosilanol, (c-C5H9)7Si8O12(OH). The more sterically hindered geminal silsesquioxane 6 undergoes in solution intermolecular dehydroxylation, yielding the thermodynamically stable dimeric disilanol, [(c-C5H9)7Si7O9(OSiMePh2)(O2Si(OH)-)]2-(mu-O) (7). NMR and FT-IR studies show that the two silanols of the geminal silsesquioxanes 4-6 are different from each other with respect to hydrogen bonding, both in solution and in the solid state. Hydrogen bonding of the geminal silanol-containing silsesquioxanes was examined and compared to hydrogen bonding in silsesquioxanes possessing vicinal or isolated silanol groups. The relative Br?nsted acidity of the geminal silanols was determined using pK(ip) (ion-pair acidity) measurements in THF with UV-vis. These acidities were compared with those of other silsesquioxanes containing silanol groups. Acidities of 4-6 were found to be among the lowest known for silsesquioxanes.     

Photoresponsive rolling and bending of thin crystals of chiral diarylethenes

Dithienylhexafluorocyclopentene with (R)- or (S)-N-phenylethylamide substituents formed rod-like and 0.2-1.0 microm-thick platelike crystals by sublimation; upon UV irradiation, the crystals bent concavely to the incident light and finally rolled crystals were obtained; the bent crystals were reconverted to flat crystals by visible light irradiation.     

A redesign of light-driven rotary molecular motors

Structural modification of unidirectional light-driven rotary molecular motors in which the naphthalene moieties are exchanged for substituted phenyl moieties are reported. This redesign provides an additional tool to control the speed of the motors, and should enable the design and synthesis of more complex systems.     

The role of salicylic acid, L-ascorbic acid and oxalic acid in promoting the oxidation of alkenes with H2O2 catalysed by [MnIV2(O)3(tmtacn)2]2+

The role played by the additives salicylic acid, L-ascorbic acid and oxalic acid in promoting the catalytic activity of [MnIV2(O)3(tmtacn)2](PF6)2 (1(PF6)2, where tmtacn = N,N',N'-trimethyl-1,4,7-triazacyclononane) in the epoxidation and cis-dihydroxylation of alkenes with H2O2 and in suppressing the catalysed decomposition of H2O2 is examined. Whereas aliphatic and aromatic carboxylic acids effect enhancement of the catalytic activity of 1 through the in situ formation dinuclear carboxylato bridged complexes of the type [MnIII2(mu-O)(mu-RCO2)2(tmtacn)2]2+, for L-ascorbic acid and oxalic acid notable differences in reactivity are observed. Although for L-ascorbic acid key differences in the spectroscopic properties of the reaction mixtures are observed compared with carboxylic acids, the involvement of carboxylic acids formed in situ is apparent. For oxalic acid the situation is more complex with two distinct catalyst systems in operation; the first, which engages in epoxidation only, is dominant until the oxalic acid additive is consumed completely at which point carboxylic acids formed in situ take on the role of additives to form a second distinct catalyst system, i.e. that which was observed for alkyl and aromatic carboxylic acids, which yield both cis-diol and epoxide products.