These results demonstrate that group 2 metal catalysts, in particular 1‐Ba, are well able to hydrogenate highly stable aromatic substrates provided the rings are conjugated. Hydrogenolysis of 1‐Ba was considerably faster (1 bar H2, 90 °C, 100 % conversion, 5 h). DOSY NMR measurements show that the much higher activity of superbulky Ae amide catalysts may be explained by the lower aggregation numbers for these bulky systems. Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University), William Reusch, Professor Emeritus (Michigan State U. Preparation of Alkynes from Alkenes. (Org. The higher aggregation for the Ba hydride species is in line with the fact that larger metal cations tend to form larger aggregates. While bulky 1‐Ca is inactive, the 1‐Sr and 1‐Ba catalysts quantitatively hydrogenated one of the rings in naphthalene with conversion times as short as two hours (entries 8–10). Question 2: What are the major products of the following reactions: a.) single position isomerization of alkenes, which effected the transformation of Lastly, we will briefly look at how to prepare alkynes from alkenes. Number of times cited according to CrossRef: Dibenzotropylidene Substituted Ligands for Early Main Group Metal‐Alkene Bonding. While PAH′s with extended π‐systems could be routinely reduced to products with at least one aromatic ring, hydrogenation of the remaining aromatic 6π‐electron system is difficult and also for transition metal catalysts a challenge.43 However, during hydrogenation catalysis using our most active catalyst 1‐Ba in benzene, generally traces of cyclohexane were found. Generic catalytic cycles appear below. Chem. Chem. Learn about our remote access options, Chair of Inorganic and Organometallic Chemistry, Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany. Catalyst 1‐Ba even converted acyclic, unactivated, internal alkenes (entries 34–36). A Cu-based homogeneous catalytic system was proposed for the preparation of imides from alkene-tethered amides. 697, found 515; 1‐Ba: calc. In this case the Ca catalyst with the bulkier amide (TRIP)2N is more reactive than that with N′′. The N(TRIP)(DIPP) ligand, however, should still be considered bulky: the Mg−N bonds in 2‐Mg (1.934(2)–1.946(2) Å) are significantly longer than those in Mg[N(SiMe3)(DIPP)]2 (1.919(2) Å).32b The larger metal ions in the 2‐Ae series have a strong tendency to interact with the DIPP π‐system (this is especially the case in 2‐Ba). Lactams intermediates by reductants such as triphenylphosphine or dimethylsulfide under Again, the bulkier amides 1‐Ae (Ca, Sr, Ba) are clearly more active (entries 4–6); the most active catalyst 1‐Ba gave after 2.5 hours nearly quantitative yield. Combined KH/alkaline-earth metal amide catalysts for hydrogenation of alkenes. One of the base molecules will pull off the terminal hydrogen instead of one of the halides like we want. This is partially due to the extreme solubility of these species induced by the multiple iPr substituents. The very high activity of 1‐Ba is further supported by rapid reduction of doubly substituted conjugated alkenes which were fully converted within 1–1.5 hours using only 1 mol % catalyst (entries 16–18); also diphenylacetylene was effectively doubly reduced (entry 19). Since styrene or butadiene type substrates react to resonance‐stabilized benzylic or allylic intermediates, hydrogenation of conjugated double bonds is facile. As expected, reduction of cis‐3‐hexene is faster than trans‐3‐hexene but 1,1‐Et2C=CH2, which should be reduced more facile, gave partial isomerisation to a trisubstituted alkene which could only be slowly reduced further. For reaction mixtures of 1‐Ba with H2 clusters up to a molecular weight of circa 3300 g mol−1 have been observed. chemoselective conversion of diene 3 to amide 4. Answer 4: Bromine or chlorine can be used with different inert solvents for the halogenation. In 2008 we reported the hydrogenation of activated (conjugated) alkenes with K, Ca, and Sr benzyl catalysts or a β‐diketiminate Ca hydride complex.3 This transformation follows a simple mechanism in which addition of the alkene to a metal hydride intermediate is a key step. You can locate carbonyl groups, alkenes, alkynes, and aromatics in the IR (infrared) spectrum, based on their shapes and relative locations. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. The barrier for hydride formation by reaction of 1‐Ca with H2 is higher (Ca2*: 22.9 kcal mol−1) than that for the reaction of CaN′′2 with H2 (17.6 kcal mol−1). Chem. This is analogous to the reaction of sodium with water to give sodium hydroxide, but since ammonia is 10 18 times weaker an acid than water, the reaction is less violent. A drawback for the bulky amide catalysts is the enhanced oligomerization found for activated alkenes like styrene. OEt2-Mediated Synthesis of Substituted Complexes in the series 1‐Ae are all monomeric featuring (nearly) linear two‐coordinated metal ions with N−Ae−N′ angles ranging from 171.9(1)° to 179.7(1)°. "13.8 Elimination of Organohalogens. ~~ \text{NaNH}_2}][\displaystyle{\text{2.} Okuda and co‐workers introduced cationic calcium hydride catalysts (I–II) and demonstrated hydrogenation of unactivated alkenes like 1‐hexene.6-8 As the dicationic catalyst (II) was found to be particularly active, it was concluded that the positive charge is critical in imparting sufficient electrophilicity to the metal center. Catalyst 1 ‐Ba can reduce internal alkenes like cyclohexene or 3‐hexene and highly challenging substrates like 1‐Me‐cyclohexene or tetraphenylethylene. E-mail: Energy profiles (ΔH in kcal mol−1) for a) the hydrogenation of ethylene by catalysts 1‐Ca (orange), 1‐Ba (black) and CaN′′2 (red), and b) benzene hydrogenation by 1‐Ba; B3PW91/def2tzvpp including correction for dispersion (GD3BJ) and solvent (PCM=benzene). an iron catalyst that hydrosilylates alkenes with anti-Markovnikov selectivity, as In contrast, reaction of bulky (TRIP)2NCaH with (TRIP)2NH is extremely difficult (Ca4 → Ca2*: +29.9 kcal mol−1) and ethylene insertion is facile. This is a brief review of the E2 reaction. It is also active in arene hydrogenation reducing anthracene and naphthalene (even when substituted with an alkyl) as well as biphenyl. In a first set of calculations we investigated the effect of amide bulk on aggregation by optimization of (R2NCaH)x species (x=1, 2 or 4) with increasing amide size Me2N < (Me3Si)2N < (TRIP)2N (Table S9 and Scheme S4). Even simple AeN′′2 complexes are able to reduce anthracene, the activity increasing from Ca to Ba (entries 1–3). Chem. The effect of ligand size on cluster size was investigated by DOSY NMR using the external calibration method described by Stalke and co‐workers (see Supporting Information).48 For 1‐Ca and 1‐Ba in benzene we found in both cases molecular weights lower than calculated for the monomers (1‐Ca: calc. The bulkier amide complexes 1 ‐Ae are by far the most active catalysts in alkene hydrogenation with activities increasing from Mg to Ba. The less active 2‐Ae catalysts gave much more isomerisation to internal alkenes which could not be reduced further. doi("10.1021/ja300798k")) doi("10.1002/anie.201200922")) procedures that require many hours. the utility of this approach was provided by the conversion of styrene 28 to the with anti-Markovnikov selectivity by a sequence of hydroboration and copper-catalyzed amination. [b] Reaction conducted with [arene]. doi("10.1021/op300079z")) Ring enlargement to cyclooctene increased the conversion rate (entry 33). Complex BaN′′2 was also able to reduce the more challenging substrate naphthalene (entry 7). The Mg−N bond in 1‐Mg (1.997(1) Å) is clearly longer than that in monomeric MgN′′2 (1.91(3) Å)39 illustrating the considerable steric stress in the 1‐Ae series. Although conversion of isolated aromatic rings was not observed, we found in longer runs with the most active hydrogenation catalyst 1‐Ba always minor quantities of cyclohexane. These results demonstrate that current state‐of‐the‐art group 2 metal hydrogenation catalysis starts to reach a similar level as traditional transition metal catalysis. This is in line with the experimentally very challenging reduction of aromatic substrates. The superb activity of the Ba catalyst was independently confirmed by the Cheng group who introduced a highly active barium hydride catalyst (IV) that operates at 30 °C.10, a) Catalytic cycle for alkene hydrogenation with AeN′′2 catalysts. Corresponding authors, a First of all, these catalysts fully suppress alkene oligomerization. by Corey R. J. Stephenson at Boston University, an example of which was the addition of bromodiethyl malonate (J. Now, if we apply this concept using 2 halides that are vicinal (meaning on adjacent carbons) or geminal (meaning on the same carbon), the E2 reaction will take place twice resulting in the formation of 2 $$\pi$$ bonds and thus an Alkyne. (computational work) reported Monomers 1‐Ae are highly linear while the monomers 2‐Ae are slightly bent. Addition of PMDTA enabled interception of the smaller cluster (N′′CaH)3(CaH2)3⋅(PMDTA)3 (Scheme 2 b); also small Sr and Ba clusters have been isolated.21 Use of simple AeN′′2 catalysts turned out to be highly advantageous.