Transition-metal-promoted oxidative cross-coupling reactions involving two different carbon–hydrogen bonds for the construction of a new carbon–carbon ó bond are very at- tractive synthetic processes, since the preactivation of the reactants by, for example, hal- ogenation or metalation, is not necessary.[1–10] As a consequence, new carbon–carbon ó bonds may be constructed adopting effective atom- and step-economical strategies, and avoiding the generation of stoichiometric amounts of potentially toxic waste. An oxidative cross coupling, as the name suggests, requires an oxidant acting as a hy- drogen acceptor in the step that formally closes the catalytic cycle (vide infra). Among all the possible oxidants, molecular oxygen is the most ecologically and economically desir- able reagent,[11–14] given its availability and the generation of water as the only byproduct. The adjective “aerobic” is employed in this chapter to refer to transformations in which molecular oxygen acts as a stoichiometric oxidant, regardless of whether it is applied in the form of air or as pure gaseous dioxygen (O2). A substoichiometric amount of a different oxidizing agent may also be present that acts as an oxidation catalyst, i.e. by performing the required oxidation and then in turn being regenerated with the concomitant reduc- tion of dioxygen. In the following sections, the preeminent results on intermolecular aerobic oxidative Heck coupling reactions are discussed, in which alkenyl-substituted (het)arenes 1 are ob- tained by the transition-metal-mediated activation of a (hetero)aromatic and an sp2 olefin- ic C—H bond (Scheme 1).[10] The related aerobic oxidative intermolecular (het)arylation and alkynylation reactions of (het)arenes to give bi(het)aryls 2 and alkynyl(het)arenes 3 is also discussed. These transformations fit the requirements of “green” chemistry, and it is expected that their use in synthesis will increase in the future until they are considered a standard tool-of-the-trade of organic chemists.
12 Aerobic Oxidative Intermolecular Cross-Coupling and Heck Reactions
Bellina, F.
Primo
Writing – Original Draft Preparation
;
2017-01-01
Abstract
Transition-metal-promoted oxidative cross-coupling reactions involving two different carbon–hydrogen bonds for the construction of a new carbon–carbon ó bond are very at- tractive synthetic processes, since the preactivation of the reactants by, for example, hal- ogenation or metalation, is not necessary.[1–10] As a consequence, new carbon–carbon ó bonds may be constructed adopting effective atom- and step-economical strategies, and avoiding the generation of stoichiometric amounts of potentially toxic waste. An oxidative cross coupling, as the name suggests, requires an oxidant acting as a hy- drogen acceptor in the step that formally closes the catalytic cycle (vide infra). Among all the possible oxidants, molecular oxygen is the most ecologically and economically desir- able reagent,[11–14] given its availability and the generation of water as the only byproduct. The adjective “aerobic” is employed in this chapter to refer to transformations in which molecular oxygen acts as a stoichiometric oxidant, regardless of whether it is applied in the form of air or as pure gaseous dioxygen (O2). A substoichiometric amount of a different oxidizing agent may also be present that acts as an oxidation catalyst, i.e. by performing the required oxidation and then in turn being regenerated with the concomitant reduc- tion of dioxygen. In the following sections, the preeminent results on intermolecular aerobic oxidative Heck coupling reactions are discussed, in which alkenyl-substituted (het)arenes 1 are ob- tained by the transition-metal-mediated activation of a (hetero)aromatic and an sp2 olefin- ic C—H bond (Scheme 1).[10] The related aerobic oxidative intermolecular (het)arylation and alkynylation reactions of (het)arenes to give bi(het)aryls 2 and alkynyl(het)arenes 3 is also discussed. These transformations fit the requirements of “green” chemistry, and it is expected that their use in synthesis will increase in the future until they are considered a standard tool-of-the-trade of organic chemists.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
