Insertion into element-hydride bonds

Insertion into element-hydrogen bonds tend to be less favored thermodynamically than insertions into other bonds (e.g., element-carbon). This is often attributed to the high element-hydride bond strength, which is lost upon insertion. Since the insertion reaction is also entropically disfavored, the reverse deinsertion of the unsaturated moiety to generate an element-hydride bond can be thermodynamically favored. When the hydride exists in the P position of the inserted product, this process is commonly referred to as /S-hydride elimination. Nevertheless, there are many examples of insertions into element-hydride bonds that generate stable compounds, and when this insertion reaction is an uphill process, chelation to the element or subsequent chemistry (i.e., catalytic cycles) can be employed to facilitate the initial insertion step. 

Addition of element-element compounds to alkynes has been reviewed.Other insertions of alkynes into palladium-hydride bonds have been identified in Drent s palladium-catalyzed alkoxycarbonylation of alkynes palladium(ii)-alkenyl complexes have been invoked to account for the observed H/D exchange when conducted in GH3OD and to identify the pathway (i.e., through migratory insertion into Pd-H and formation of acyl species by carbonylation) of the overall reaction. ... 

In aerobic oxidations of alcohols a third pathway is possible with late transition metal ions, particularly those of Group VIII elements. The key step involves dehydrogenation of the alcohol, via -hydride elimination from the metal alkoxide to form a metal hydride (see Fig. 4.57). This constitutes a commonly employed method for the synthesis of such metal hydrides. The reaction is often base-catalyzed which explains the use of bases as cocatalysts in these systems. In the catalytic cycle the hydridometal species is reoxidized by 02, possibly via insertion into the M-H bond and formation of H202. Alternatively, an al-koxymetal species can afford a proton and the reduced form of the catalyst, either directly or via the intermediacy of a hydridometal species (see Fig. 4.57). Examples of metal ions that operate via this pathway are Pd(II), Ru(III) and Rh(III). We note the close similarity of the -hydride elimination step in this pathway to the analogous step in the oxometal pathway (see Fig. 4.56). Some metals, e.g. ruthenium, can operate via both pathways and it is often difficult to distinguish between the two. 

Oxidative additions are a special class of insertion reactions. In addition to the categories mentioned in Section 10, which covers this topic, insertions of alkylidenes, silylenes, etc., into M-H bonds fall into an ambiguous domain they are insertion reactions of the unsaturated species into the M-H bond, yet oxidative additions at the C, Si, etc., atom. A similar ambiguity exists regarding the reverse reactions, namely /i-hydride and a-hydride eliminations from element-alkyls compounds to yield hy-drido-olefin and hydrido-alkylidene complexes, respectively. The former reaction is a reverse insertion if the product is viewed as an olefin complex, but an oxidative addition if it is viewed as a three-membered metallocycle. The latter reaction is a reverse insertion if the alkylidene is viewed as neutral, but an oxidative addition of a C-H bond to the metal centre. The tautomerization of phosphorous acid and of dialkylphosphites ... 

Morton MS, Lachicotte RJ, Vicic DA, Jones WD. Insertion of elemental sulfur and SO2 into the metal-hydride and metal-carbon bonds of platinum compounds. Organometallics 1999 18(2) 227-234. 

The aerobic oxidation of alcohols catalysed by low-valent late-transition-metal ions, particularly those of group VIII elements, involves an oxidative dehydrogenation mechanism. In the catalytic cycle (Fig. 5) ruthenium can form a hydridometal species by jS-hydride elimination from an alkoxymetal intermediate, which is reoxidized by dioxygen, presumably via insertion of O2 into the M-H bond with formation of H2O2. Alternatively, an alkoxymetal species can decompose to a proton and the reduced form of the catalyst (Fig. 5), either directly or via the intermediacy of a hydridometal intermediate. These reactions are promoted by bases as cocatalysts, which presumably facilitate the formation of an alkoxymetal intermediate and/or jS-hydride elimination. 

In addition to the aforementioned reaction, the insertion of olefins (ethylene is polymerized in fairly good yield) into H-Ln and C-Ln bonds as well as the reverse -hydride and /5-alkyl elimination were also observed (7). The proposed concerted mechanism in the absence of valence change of the metal suggests different behaviour of f- with respect to d-elements in polymerization. 

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