Ammonia borane (AB), which has a very high hydrogen content by weight (19.6%), is attracting attention as a means of achieving efficient chemical hydrogen storage.1,2 Heating AB in the solid state, 3 ionic liquids 4 or the presence of acid 5 has been shown to induce the release of >1.0 equiv of H 2 over the course of several hours, and transition metal complexes of Rh, 6 Ni, 7 and Ir 8 have been shown to catalyze H 2 release under mild conditions. Notably, the Ni catalysts described by Baker release up to 2.8 equiv of H 2 , 7 an observation that has prompted a more detailed evaluation of their reaction mechanism.9a When considering strategies for dehydrogenation, AB is often compared to ethane.10 Computational predictions provide little mechanistic support for this analogy, however, 9,11 and in terms of local dipoles and reactivity AB may better be compared to methanol. Drawing from this analogy, we have discovered that ruthenium catalysts, originally developed for alcohol redox processes, 12 will induce the release of 1 equiv of H 2 from AB (up to 1.0 system wt %) within 5 min at room temperature with as little at 0.03 mol% Ru. Furthermore, 2 equiv of H 2 is rapidly released from highly concentrated (11 M) MeAB solutions at 22°C (ambient temperature), providing 3.0 system wt % hydrogen release. We have also found that by using an AB/MeAB mixture at 50°C, up to 3.6 system wt % H 2 can be released with 0.1 mol % Ru.An evaluation of several alcohol oxidation/reduction catalysts provided promising leads with complexes 1 to 6 (Figure 1). 13 In these studies, the precatalysts (0.1 mol%) were activated via treatment with KO t Bu (3 mol%) 13c in THF under inert atmosphere to generate the active ruthenium amide species prior to the addition of AB to the reaction vessel.14 Complexes 1 and 2 are particularly reactive even at 0.03 mol% Ru.14 Optimal outcomes are observed at high [AB] (5.0 M), an important parameter for maximal gravimetric H 2 density. In reactions with AB, we note the formation of an insoluble precipitate, which has been identified as the same polymeric aminoborane species obtained by Goldberg 8 and Manners. 15 The dehydrogenation of methylammonia-borane (MeAB) was also evaluated. 6 Under conditions similar to those employed for AB dehydrogenation, 0.5 mol% 1 induced the release of up to 2.0 equiv of H 2 from MeAB in ∼10 min (Figure 2). 16 These reactions are characterized by a very rapid release of the first equivalent of H 2 (occurring in less than 10 s), generating poly(N-methylaminoborane).15 This is followed by a slower release of the second equivalent over ∼10 min. 17 Since no insoluble precipitate is observed, more concentrated conditions (11 M [MeAB]) may be employed, increasing the system % H 2 to 3% (1.9 equiv of H 2 released). To further maximize the gravimetric H 2 density, AB-MeAB mixtures were evaluated.18 Initial results using a 1:1 AB/MeAB mixture at 50°C indicate that 3.6 system wt % H 2 release can be achieved with 0.1 mol% 1 under solvent-free conditions. 16As a preliminary probe i...

A copper (I)-catalyzed, asymmetric method to directly functionalize pyridines, quinolines, and isoquinolines with terminal alkynes is described. The reaction is readily diversified to incorporate a range of pyridine-based heterocycles and electron-rich or electron-poor alkynes. This provides a straightforward alternative to nucleophilic or cross-coupling approaches to directly derivatize these heterocycles, and yields useful propargylcarbamates.

The use of imines in a metal-catalyzed coupling with alkynes and acid chlorides is described. This process proceeds rapidly with CuI as the catalyst and provides an efficient and general three-component coupling method to prepare propargylamides. The coupling can also be diversified to allow the formation of N-carbamate-protected propargylamines with the use of chloroformates.

The palladium-catalyzed coupling of imines, chloroformates, organotin reagents, and carbon monoxide leads to the one-pot formation of ketocarbamates in good yields. These products can further be converted to highly substituted imidazolones via a cyclocondensation reaction. Overall, this methodology provides an alternative approach to imidazolones from five simple and readily available building blocks via a one-pot, multicomponent process.

A copper-catalyzed Petasis-type reaction of imines, acid chlorides, and organoboranes to form α-substituted amides is described. This reaction does not require the use of activated imines or the transfer of special units from the organoboranes and represent a useful generalization of the Petasis reaction.