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In the last 25 years, the influence of microwave (MW) irradiation in chemical synthesis has intrigued chemists for the exceptional results obtained in shortening reaction times. Inspired by recent papers that demonstrate the existence of a non‐thermal effect in some MW assisted reactions, we examine herein a series of reactions catalysed by homogeneous and heterogeneous metals occurring under MW irradiation. The review contains examples of hydroformylation, hydrogenation (including enantioselective versions with 98 % ee), hydrogenolysis, hydrogen transfer and borrowing hydrogen reactions carried out under MWs and metal catalysis. An increase in the influence of MW irradiation was often observed in reactions performed with heterogeneous catalysts such as Pd/C. In these cases, a "microwave catalysis" can be possible. The increase in the (apparent) reaction rate requires the contribution of MW irradiation that modifies the catalyst surface, driving the reaction progress as occurring in other kinds of catalysis. [ABSTRACT FROM AUTHOR]

Starting from 2-aminobenzyl alcohols and 1,2-amino alcohols, 2,3,4,5-tetrahydro-1 H-1,4-benzodiazepines (TH-BDZ) can be prepared through a one-pot ruthenium-catalyzed reaction encompassing two consecutive borrowing hydrogen cycles. First, benzyl alcohol oxidation, condensation with the amino alcohol, and imine reduction occurs. This is followed by oxidation of the other alcohol functionality and reductive cyclocondensation to give the TH-BDZ derivatives. A single catalyst does the entire job, with two molecules of water being the only waste product. Many new substituted 2,3,4,5-tetrahydro-1 H-1,4-benzodiazepines have been prepared to demonstrate the versatility of this method, which can be used for the preparation of privileged scaffolds for drug discovery. [ABSTRACT FROM AUTHOR]

Hydroformylation is a powerful reaction that has suffered for some negative prejudices related to the use of gaseous H2 and CO. Now it is possible to carry out hydroformylation and different cyclohydrocarbonylations, even on complex substrates, using aqueous formalin as H2 and CO surrogate in few minutes under microwave irradiation. The catalytic system developed by Morimoto (Rh/BINAP for decomposition of formaldehyde and Rh/Xantphos for hydroformylation) is compatible with microwave dielectric heating and with complex substrates containing ligand atoms allowing rapid domino hydroformylation cyclization reactions without using the external supply of gaseous H2 and CO (gas cylinder) and without any particular safety limitation or device. [ABSTRACT FROM AUTHOR]

The synthesis of 2-substituted indoles starting from the corresponding unprotected 2-alkynylanilines was made possible in 3% TPGS-750-M water using Pd(OAc)2 alone as the catalyst. The reaction was sensitive to the heating mode respect to the nature of the starting material as, in many cases, convectional heating was better than microwave dielectric heating. The MW (microwave) delivery mode had also an influence in the formation of by-products and, consequently, product yields. A tandem Sonogashira-cyclisation reaction was also accomplished using Pd(OAc)2/Xphos in the nanomicellar water environment. [ABSTRACT FROM AUTHOR]