Synthesis of 1,4-Diazacycles by Hydrogen Borrowing.

We report the syntheses of 1,4-diazacycles by diol-diamine coupling, uniquely made possible with a (pyridyl)phosphine-ligated ruthenium(II) catalyst (1). The reactions can exploit either two sequential N-alkylations or an intermediate tautomerization pathway to yield piperazines and diazepanes; diazepanes are generally inaccessible by catalytic routes. Our conditions tolerate different amines and alcohols that are relevant to key medicinal platforms. We show the syntheses of the drugs cyclizine and homochlorcyclizine in 91% and 67% yields, respectively.

Pressurized Formic Acid Dehydrogenation: An Entropic Spring Replaces Hydrogen Compression Cost.

Formic acid is unique among liquid organic hydrogen carriers (LOHCs), because its dehydrogenation is highly entropically driven. This enables the evolution of high-pressure hydrogen at mild temperatures that is difficult to achieve with other LOHCs, conceptually by releasing the "spring" of energy stored entropically in the liquid carrier. Applications calling for hydrogen-on-demand, such as vehicle filling, require pressurized H2. Hydrogen compression dominates the cost for such applications, yet there are very few reports of selective, catalytic dehydrogenation of formic acid at elevated pressure. Herein, we show that homogenous catalysts with various ligand frameworks, including Noyori-type tridentate (PNP, SNS, SNP, SNPO), bidentate chelates (pyridyl)NHC, (pyridyl)phosphine, (pyridyl)sulfonamide, and their metallic precursors, are suitable catalysts for the dehydrogenation of neat formic acid under self-pressurizing conditions. Quite surprisingly, we discovered that their structural differences can be related to performance differences in their respective structural families, with some tolerant or intolerant of pressure and others that are significantly advantaged by pressurized conditions. We further find important roles for H2 and CO in catalyst activation and speciation. In fact, for certain systems, CO behaves as a healing reagent when trapped in a pressurizing reactor system, enabling extended life from systems that would be otherwise deactivated.