No abstract

Formic acid (HCO 2 H) is an important potential hydrogen storage material, which, in the presence of appropriate catalysts can be selectively dehydrogenated to give H 2 and CO 2 . In this work, well defined N^C cyclometallated iridium(III) complexes based on 2-aryl imidazoline ligands are found to be excellent catalysts for the decomposition of HCO 2 H-NEt 3 mixtures to give H 2 and CO 2 under mild conditions with high turnover frequencies (up to 147 000 h À1 at 40 C) and essentially no CO formation. The modular structures of these catalysts have allowed for the construction of structure-activity relationships for the complexes, leading to the rational optimisation of the catalyst structure with respect to both the rate of H 2 production and catalyst lifetime. In particular, the presence of the remote g-NH unit in the ligand is shown to be essential for catalytic activity, without which no reaction occurs. Mechanistic studies suggest that the dehydrogenation is rate-limited by the step of hydride protonation, which is made feasible by the g-NH unit via an unusual form of long-range metal-ligand bifunctional catalysis involving formic acid-assisted proton hopping.

All-trans-retinoic acid (ATRA) and its associated analogues are important mediators of cell differentiation and function during the development of the nervous system. It is well known that ATRA can induce the differentiation of neural tissues from human pluripotent stem cells. However, it is not always appreciated that ATRA is highly susceptible to isomerisation when in solution, which can influence the effective concentration of ATRA and subsequently its biological activity. To address this source of variability, synthetic retinoid analogues have been designed and synthesised that retain stability during use and maintain biological function in comparison to ATRA. It is also shown that subtle modifications to the structure of the synthetic retinoid compound impacts significantly on biological activity, as when exposed to cultured human pluripotent stem cells, synthetic retinoid 4-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-ylethynyl)benzoic acid, 4a (para-isomer), induces neural differentiation similarly to ATRA. In contrast, stem cells exposed to synthetic retinoid 3-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-ylethynyl)benzoic acid, 4b (meta-isomer), produce very few neurons and large numbers of epithelial-like cells. This type of structure-activity-relationship information for such synthetic retinoid compounds will further the ability to design more targeted systems capable of mediating robust and reproducible tissue differentiation.

Conjugated boron heterocycles have emerged as attractive synthetic targets due to their potential in medicinal chemistry and as electronic materials. However, the development of unsaturated boracycles has been hampered by difficulties in their preparation. Recently, a new synthetic avenue to access these species has been developed that takes advantage of the high reactivity of boroles. These five-membered anti-aromatic heterocycles can react with substrates to furnish ring expansion products via the insertion of one, two, or three atoms into the boracyclic ring. The ring expansion can occur via two pathways, the first exploits the activated diene moiety of the heterocycle in Diels-Alder chemistry with the resulting bicyclic species undergoing further rearrangements. The second reaction pathway is initiated by the coordination of the Lewis basic site of a substrate to the highly Lewis acidic boron center rendering the endocyclic B-C bond of the borole nucleophilic, inducing the formation of larger boracycles via attack at the electrophilic site of the substrate. This review summarizes the current state of this chemistry and details the mechanisms leading to the products. The methodologies described herein could very well be extended to other substrates, as well as applied to other anti-aromatic heterocycles.