Giovanni Bottari

The reaction between the bis(ethylene) complex [TpRh(C₂H₄)₂], 1, (Tp=hydrotris(pyrazolyl)borate), and dimethyl acetylenedicarboxylate (DMAD) has been studied under different experimental conditions. A mixture of products was formed, in which TpRh^I species were prevalent, whereas the presence of trapping agents, like water or acetonitrile, allowed for the stabilization and isolation of octahedral TpRh^III compounds. An excess of DMAD gave rise to a small amount of the [2+2+2] cyclotrimerization product hexamethyl mellitate (6). Although no catalytic application of 1 was achieved, mechanistic insights shed light on the formation of stable rhodium species representing the resting state of the catalytic cycle of rhodium‐mediated [2+2+2] cyclo(co)trimerization reactions. Metallacyclopentene intermediate species, generated from the activation of one alkyne and one ethylene molecule from 1, and metallacyclopentadiene species, formed by oxidative coupling of two alkynes to the rhodium centre, are crucial steps in the pathways leading to the final organometallic and organic products.

Copper-doped porous metal oxides catalyze the one-pot disassembly of biomass-derived lignin via C–O bond hydrogenolysis and hydrodeoxygenation in supercritical methanol. This catalytic system cleanly converts lignin as well as lignocellulose composites, such as sawdust, to organic liquids with little or no formation of intractable tars or chars. However, this catalyst based on Earth-abundant components also catalyzes less desirable aromatic ring hydrogenations and various methylations that contribute to the diversity of products. In this context, we undertook a quantitative experimental and computational evaluation of model reactions relevant to the reductive disassembly of lignin by this catalyst system in order to determine quantitatively the rates of desirable and less desirable chemical steps that define the overall product selectivities. Global fitting analysis methods were used to map the temporal evolution of key intermediates and products and to elucidate networks that provide guidelines regarding the eventual fates of reactive intermediates in this catalysis system. Phenolic compounds display multiple reaction pathways, but substrates such as benzene, toluene, and alkyl- and alkoxy-substituted aromatics are considerably more stable under these conditions. These results indicate that modifying this catalytic system in a way that controls and channels the reactivity of phenolic intermediates should improve selectivity toward producing valuable aromatic chemicals from biomass-derived lignin. To this end we demonstrate that the O-methylating agent dimethyl carbonate can intercept the phenol intermediate formed from hydrogenolysis of the model compound benzyl phenyl ether. Trapping the phenol as anisole thus gave much higher selectivity towards aromatic products.

Benzimidazoles and N-methylbenzimidazoles were synthesized by simply heating 1,2-diaminobenzenes in supercritical methanol over copper-doped porous metal oxides. These catalysts were derived from synthetic hydrotalcites that only contain earth-abundant starting materials. The carbon equivalents needed for the construction of the benzimidazole core originated from the solvent itself, which is known to undergo reforming to hydrogen and carbon monoxide through the formation of a formaldehyde intermediate. A variety of 1,2-diaminobenzenes were converted to the corresponding mixtures of benzimidazoles and N-methylated analogues in good yields. Interestingly, the more challenging, but readily available 2-nitroanilines, which require an additional reduction step prior to cyclization, could also be successfully converted to benzimidazoles in high selectivity. Furthermore, various other alcohols were applied besides methanol, to obtain 2-alkyl- and 1,2-dialkylbenzimidazoles. Preliminary mechanistic insights into the origins of N-alkylation as well as the reactivity of the nitro derivatives are discussed.

Noble‐metal‐free copper–zinc nanoalloy (2. It is also possible to convert 10 wt % HMF solutions in CPME, with an excellent DMF yield of 90 %. Milder temperatures favor selective (95 %) formation of 2,5‐furandimethanol (FDM). The one‐pot conversion of fructose to valuable furan‐ethers was also explored. Recycling experiments for DMF production show remarkable catalyst stability. Transmission electron microscopy (TEM) characterization provides more insight into morphological changes of this intriguing class of materials during catalysis.

Tunable and selective hydrogenation of the platform chemical 5‐hydroxymethylfurfural into valuable C₆ building blocks and liquid fuel additives is achieved with copper‐doped porous metal oxides in ethanol. A new catalyst composition with improved hydrogenation/hydrogenolysis activity is obtained by introducing small amounts of ruthenium dopant into the previously reported Cu_0.59Mg_2.34Al_1.00 structure. At a mild reaction temperature (100 °C), 2,5‐furandimethanol is obtained with excellent selectivity up to >99 %. Higher reaction temperatures (220 °C) favor selective deoxygenation to 2,5‐dimethylfuran and minor product 2,5‐dimethyltetrahydrofuran with a combined yield as high as 81 %. Notably, these high product yields are maintained at a substrate concentration up to 10 wt % and a low catalyst loading. The influence of different alcohol solvents on product selectivity is explored. Furthermore, reaction intermediates formed at different reaction temperatures are identified. The composition of these product mixtures provides mechanistic insight into the nature of the reduction pathways that influence product selectivity. The catalysts are characterized by elemental analysis, TEM, and BET techniques before and after the reaction. Catalyst recycling experiments are conducted in batch and in a continuous‐flow setup.

Catalytic dehydrogenation of ammonia-borane (NH₃·BH₃, AB) and dimethylamine borane (NHMe₂·BH₃, DMAB) by the Pd^II complex [(^tBuPCP)Pd(H₂O)]PF₆ [^tBuPCP = 2,6-C₆H₃(CH₂P^tBu₂)₂] leads to oligomerization and formation of spent fuels of general formula cyclo-[BH₂–NR₂]_n (n = 2,3; R = H, Me) as reaction byproducts, while one equivalent of H₂ is released per amine-borane equivalent. The processes were followed through multinuclear (³¹P, ¹H, ¹¹B) variable temperature NMR spectroscopy; kinetic measurements on the hydrogen production rate and the relative rate constants were also carried out. One non-hydridic intermediate could be detected at low temperature, whose chemical nature was explored through a DFT modeling of the reaction mechanism, at the M06//6-31+G(d,p) computational level. The computational output was of help to propose a reliable mechanistic picture of the process.

Mildly does it: A highly enantioselective catalyst for the hydrogenation of N-aryl imines is described (see scheme). This catalyst offers practical advantages because it operates under very mild conditions and is based on an Ru complex with a diamine as the sole chiral ligand.

The synthesis of new imine‐amine and diamine ligands, based on both the atropisomeric (S_a)‐ or (R_a)‐1,1′‐binaphthyl and (R)‐bornyl backbones, and incorporating an ethylenediamino spacer, are reported. In addition, analogue ligands in which one chiral arm is replaced by the achiral NMe₂ group were synthesized. These N,N‐ligands when coordinated to a palladium metal centre form highly enantioselective catalysts for the asymmetric allylic alkylation of rac‐2‐acetoxy‐1,3‐diphenylpropene by dimethyl malonate. In one case, the synergic effect of the chirality elements in the palladium catalyst afforded the (S) substitution product with an enantiomeric excess (ee) of >99 %. Based on NMR studies of the active species in solution, a reliable explanation for the origin of the enantioselectivity of these palladium catalysts is also provided.