Large-scale energy storage and the utilization of biomass as a sustainable carbon source are global challenges of this century. The reversible storage of hydrogen covalently bound in chemical compounds is a particularly promising energy storage technology. For this, compounds that can be sustainably synthesized and that permit high-weight% hydrogen storage would be highly desirable. Herein, we report that catalytically modified lignin, an indigestible, abundantly available and hitherto barely used biomass, can be harnessed to reversibly store hydrogen. A novel reusable bimetallic catalyst has been developed, which is able to hydrogenate and dehydrogenate N-heterocycles most efficiently. Furthermore, a particular N-heterocycle has been identified that can be synthesized catalytically in one step from the main lignin hydrogenolysis product and ammonia, and in which the new bimetallic catalyst allows multiple cycles of high-weight% hydrogen storage.

The synthesis of pyrroles via acceptorless dehydrogenative condensation of secondary alcohols and 1,2-amino alcohols mediated by a robust and reusable catalyst based on nanometer-sized iridium particles †

The conservation of our global element resources is a challenge of the utmost urgency. Since aliphatic and aromatic alcohols are accessible from abundant indigestible kinds of biomass, first and foremost lignocellulose, the development of novel chemical reactions converting alcohols into important classes of compounds is a particularly attractive carbon conservation and CO2-emission reduction strategy. Herein, we report the catalytic condensation of phenols and aminophenols or aminoalcohols. The overall reaction of this synthesis concept proceeds via three steps: hydrogenation, dehydrogenative condensation and dehydrogenation. Reusable catalysts recently developed in our laboratory mediate these reactions highly efficient. The scope of the concept is exemplarily demonstrated by the synthesis of carbazoles, quinolines and acridines, the structural motifs of which figure prominently in many important natural products, drugs and materials.

Novel synthetic routes to the commonly encountered indole motif are highly sought after. Tetrahydro-1H-indoles were synthesized for the first time from secondary alcohols and 2aminocyclohexanol in the presence of a well-established iridium catalyst using a modified synthetic procedure recently developed for the synthesis of hydrocarbazoles. The catalyst is stabilized by an inexpensive and easy-to-synthesize triazine based PN5P pincer ligand. The reaction proceeds through acceptorless dehydrogenative condensation (ADC) and yields the title compound, dihydrogen, and water and can thus be classified as sustainable synthesis. Overall, five examples, three of which were previously unknown compounds, were prepared. The propitious isolated yields and the mild reaction conditions show the synthetic value of this approach. These tetrahydroindoles can be quantitatively dehydrogenated over a heterogeneous Pd catalyst to yield the corresponding indoles.