Hydroelementation reactions

The addition of E-H bonds to unsaturated substrates is one of the most useful ways to prepare materials containing carbon-heteroelement bonds. This class of reactions also represents a very efficient use of reagents since every atom in the reagents is incorporated into the products. Although atom efficient in terms of substrate, a number of these processes require the use of additives or catalysts. This section will focus on how those additives and solvents can be minimized or eliminated through microwave-assisted hydroelementation reactions. 

In related work, the addition of diphenylphosphine oxide to alkynes was investigated under solvent-free conditions. Similar to the chemistry involving alkenes, the alkynes needed to contain an electron-withdrawing group for a successful addition 

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Scheme 10. Hydroelementation reactions in the functionalization of hexa-olefin stars hydrosilylation, hydrozirconation, and hydroboration.

Nickel catalysts promoted the addition of nitrogen nucleophiles to internal alkynes [138] TMS-protected alkynes are excellent snbstrates for the base-assisted anti-Markovnikov selective hydroamination reaction [143]. No need to remove the protecting group prior to the hydroelementation reaction Tetrahydropyridines have been generated through the treatment of dihydropyrans with aniline precursors [149]... 

The development of transition metal-free hydroelementation reactions has been a goal for the synthetic community for many years. As mentioned in the previous sections on Michael addition reactions, A-heterocyclic carbenes have been shown to be efficient catalysts for these addition reactions. One of the challenges that must be overcome when using these catalysts is the sensitivity of the catalysts to electrophiles. To circumvent this problem, carbon dioxide adducts have been synthesized and used as precatalysts for the phospha-Michael addition of secondary phosphine oxides to activated alkenes (Scheme 4.45) [96]. The success of this approach hinged on the observation that these adducts underwent cleavage in solution to generate the free carbene and carbon dioxide. The concentration of the free carbene was found to be dependent upon the solvent, and THF was found to be an effective solvent for the... 

Similar to the sections previously on hydroelementation reactions, the addition of secondary phosphites to alkynes is a popular approach to the synthesis of vinylphosphonates. Many of these reactions are problematie due to the formation of mixtures containing... 

The addition of sulfur-hydrogen bonds to alkynes (hydrothiolation) is a valuable and popular approach to the synthesis of vinyl sulfides [23], Some of the earliest reports of this chemistry involved the use of molybdenum [77] and palladium complexes [78]. While discussions of the challenges encountered when trying to design an effective cross-coupling reactions often focus on concerns about reactivity and substrate scope, hydroelementation reactions introduce further complications due to regio- and stereoselectivity issues, hi many... 

Hydroelementation a reaction in which an element-hydrogen bond adds across an unsaturated molecule... 

The term hydrosilation (or hydrosilylation) refers to the addition of a molecule containing a Si-H bond across the multiple bond of a substrate, usually an alkene, alkyne, or carbonyl compound (equation 1). The reaction can be promoted by UV-light, radiation (y- and X rays), radical initiators, Lewis acids, nucleophiles, or, most importantly, transition metal catalysts. Hydrosilation is related to the important processes of hydrogenation (see Hydrogenation) and hydroboration (see Hydroboration), all of which belong to the general reaction class of hydroelementation. 

Organolanthanide complexes are known to be highly active catalysts for a variety of organic transformations, which can be either intramolecular or intermolecular in character. Successful intramolecular transformations include hydroelementation processes, which is the addition of a H-E (E = N, O, P, Si, S, H) bond across unsaturated C-C bonds, such as hydroamination, hydroalkoxylation, and hydrophosphination. Intermolecular transformations include a series of asymmetric syntheses, the amidation of aldehydes with amines, Tishchenko reaction, addition of amines to nitriles, aUcyne dimerization, and guanylation of terminal aUcynes, amines, and phosphines with carbodiimides. 

The insertion of alkenes into M-H bonds has been examined in Chap. 4. This reaction is very important because, it leads to the dimerization, oligomerization and polymerization of alkenes. It is broad and concerns not only transition metals, but also main-group metals (group 13 Lewis acids), lanthanides and actinides. For instance, AlEt3 is an excellent initiator of olefin polymerization. This reaction can also be considered as the hydrometallation or the hydroelementation of an olefin, and stoichiometric examples have been shown. If the element E does not have the property of a Lewis acid allowing olefin pre-coordination onto a vacant site and thus facilitating insertion, the insertion reaction is not possible without a catalyst. 

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