Current Green Chemistry - Volume 6, Issue 2, 2019

Palladium-catalyzed carbonylation in the presence of organic and organometallic nucleophiles serves as a powerful tool for the conversion of aryl/alkenyl halides or halide equivalents to carbonyl compounds and carboxylic acid derivatives. To circumvent the difficulties in product separation and recovery and reuse of the catalysts, associated with homogeneous reactions, supported counterparts of the homogeneous palladium catalysts were developed. The review intends to summarize the huge development that has been witnessed in recent years in the field of heterogeneous carbonylation. A great plethora of supports, organic modifiers on solid surfaces stabilizing metal particles, transition metal precursors, as well as alternative sources for CO was investigated. In most cases, careful optimization of reaction conditions was carried out. Besides simple model reactions, the synthesis of carbonyl compounds and carboxylic acid derivatives from substrates with different functionalities was performed. In some cases, causes of palladium leaching were clarified with detailed investigations. The advantages of immobilized catalysts were shown by several examples. The possibility of catalystrecycling was proved besides proving that metal contamination of the products could often be kept below the detection limit. At the same time, detailed investigations should be carried out to gain a better insight into the real nature of these processes.

Solvent accounts for majority of the waste derived from synthetic transformations. This implies that by making changes to the solvent used by either switching to greener options, reducing the volume of solvent used, or even better avoiding the use of solvent totally will have a positive impact on the environment. Herein, the focus will be on the use of bio-based-green-solvents in C-C crosscoupling reactions highlighting the recent developments in this field of research. Emphasis in this review will be placed on developments obtained for Mizoroki-Heck, Hiyama, Stille, and Suzuki- Miyaura cross-couplings. For these cross-coupling reactions, good reaction conditions utilizing green solvents are now available.

Propargylamines are versatile compounds for heterocyclic synthesis, some of which are current drugs prescribed to treat patients with Parkinson’s disease. There are different methods to synthesize propargylamines, however, modern chemistry has moved progressively to rely on new strategies that meet the principles of Green Chemistry. In this context, propargylamines are readily accessible by the cross-dehydrogenative coupling (CDC) of two C-H bonds (i.e., NCsp3-H and Csp-H bonds); surely, CDC can be considered the most atom-economic and efficient manner to form C-C bonds. The aim of this review is to provide a comprehensive survey on the synthesis of propargylamines by the CDC of amines and terminal alkynes from three fronts: (a) transition-metal homogeneous catalysis, (b) transition-metal heterogeneous catalysis and (c) photoredox catalysis. A section dealing with the asymmetric synthesis of chiral propargylamines is also included. Special attention is also devoted to the proposed reaction mechanisms.

Hydrogenation of furfural (FUR) to furfuryl alcohol (FFA) is a key step and one of the representative examples for comprehensive utilization of biomass, while relatively harsh conditions are typically required to achieve satisfactory results using molecular hydrogen, formic acid, or alcohol as H-donor over expensive metal catalysts. In this work, a new and benign reaction system, composed of green and cheap tetraethylammonium fluoride and polymethylhydrosiloxane (PMHS), is developed to be efficient for transfer hydrogenation of bio-based FUR to high-value FFA under mild conditions. After reacting at 35 132;ƒ for 0.5 h, 94.9% FUR conversion and 92.3% yield of FFA could be achieved. This protocol is also widely applicable to the selective reduction of various aromatic aldehydes, giving relevant alcohols in high yields of 81.0-99.9% at 35-60 °C within 30-120 min. Moreover, the mechanism of fluoride-activated hydrosilylation was demonstrated to be responsible for the efficient transfer hydrogenation process.

Benzaldehyde and benzoic acid are high-value aromatic chemicals and important intermediates in chemical industry, and the catalytic conversion of biomass-based sources to these aromatic chemicals is of great significance in both academic and industrial fields. This work demonstrated that bio-oil was directionally converted into benzaldehyde and benzoic acid by three-step process under atmospheric pressure and moderate temperatures. The process included the catalytic cracking of biooil into aromatics over 1% Ga/HZSM-5 catalyst, followed by the dealkylation of heavier alkylaromatics to toluene over Re/HY catalyst and the liquid-phase oxidation of toluene-rich aromatics to the targeted chemicals over CoCl2/NHPI (CoCl2/N-Hydroxyphthalimide) catalyst. The production of benzaldehyde and benzoic acid from the bio-oil-derived aromatics, with the overall selectivity of 86.8%, was achieved using CoCl2/NHPI catalyst at 100 °C. Furthermore, adding a small amount of methanol into the feed would efficiently suppress the coke formation, and thus, enhance the yield of aromatics. Potentially, the novel synthesis route offers a green way for the production of higher value-added aromatic chemicals using renewable and environmentally friendly biomass-based sources.

A convenient and cost-effective method for selective deuteration of rutin using biologically compatible bases and D2O both as a deuterium source and a solvent is herein reported. The protocol is benign and inexpensive affording good results in very mild conditions allowing to reduce the required amount of deuterium oxide. The position of the C-H/C-D exchange and the level of deuteration can be conveniently followed by 1H-NMR.