Alkynes insertions into metal hydrides

Although relatively few systematic studies of alkyne insertions into transition metal hydride bonds have been reported, representative reactions of all the transition groups are now known. 

The two reactions depicted in Scheme 19 show alkyne insertions into a metal-H bond of a dinuclear complex. In Scheme 19a,[65] the insertion of the alkyne takes place even though the vacancy for alkyne coordination and the hydride ligand are situated on different metal centers of the starting complex 8.[28] In the second example (Scheme 19b), it was proposed that the incoming substrate initially coordinates to the complex as a bridging m-ri Ti ligand, and then undergoes subsequent insertion. [66]... 

I.2. Insertions of Alkynes into Metal-Hydride Bonds... 

The insertion of alkynes into metal-hydride bonds occurs during a number of catalytic processes, including alkyne hydrogenation, hydrosilylation, silylformylation, hydroesterification and dimerization. This insertion chemistry is more complex mechanistically than the insertions of olefins into metal hydrides. In some cases, ds addition products have... 

The insertions of alk5mes into metal-carbon o-bonds are less common than either the insertions of olefins into metal-carbon bonds or the insertions of alkynes into metal-hydride bonds. Nevertheless, several examples of this reaction have been studied, and many examples are part of catalytic processes. Most of the insertions of alkynes into metal-carbon bonds occur by concerted migratory insertion pathivays and provide products from cis addition of the metal and hydrocarbyl group across the carbon-carbon multiple bond, as predicted on theoretical groimds by Thom and Hoffmann. In some cases, the products from trans addition are observed, but these kinetic products are thought to result from isomerization of the vinyl group in reaction intermediates formed by cis addition. 

The most common insertion reactions involve the insertions of carbon monoxide into metal-hydrocarbyl ligands and the insertions of olefins and alkynes into metal-hydride... 

Alkynes show the same reaction and again the product obtained is the anti isomer. After a suitable elimination from the metal the alkene obtained is the product of the anti addition. Earlier we have seen that insertion into a metal hydride bond and subsequent hydrogenation will afford the syn product. If we use BH4 as the nucleophile we can accomplish anti addition of a hydride. Thus, with the borohydride methodology and the hydrogenation route either isomer can be prepared selectively. 

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. 

In certain other systems, there is compelling evidence for the insertion into a metal-caiboxylate complex (equation 37). For example, in the synthesis of a-methylene-y-lactones from alkynic alcohols,70,71 no double bond rearrangement to a butenolide occurs, a reaction shown to take place in the presence of transition metal hydrides. The source of the vinyl proton (deuterium) on the a-methylene group is indeed the alcohol function. Finally, palladium carboxylate complexes containing alkynic (equation 40) or vinyl tails (equation 41) can be isolated and the corresponding insertion reaction can be observed. 

Conjugated dienes were thus selectively obtained by hydrovinylation of alkynes catalyzed by a cationic ruthenium alkylidene complex [43] (Eq. 31). This reaction is thought to be promoted by the ruthenium hydride species resulting from the deprotonation of the <5-methyl group of the metallic precursor, followed by the sequential insertion of alkyne and ethylene into the metal-hydride and metal-vinyl bonds. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

The reactivity of the cationic Zr complexes is a direct consequence of their Lewis acidity see Lewis Acids Bases) (i) various substitution reactions can occur into the Zr-solvent weak bond, (ii) unsatnrated substrates (CO, alkenes, alkynes, or ketones) insert into the Zr-C bond, potentially leading to polymerization reactions (see Section 8.2), (iii) new organic ligands obtained after reaction in the coordination sphere of the metal can be spontaneously released by /3-H elimination see -Hydride Elimination), or (iv) C-H bond activation of suitable ligands can occur. 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

The many reactions that involve insertion of alkenes or alkynes into metal-carbon or metal-hydrogen bonds provide further examples of hypercoordination of carbon atoms during reactions. For example, an alkene may coordinate to the coordinatively unsaturated metal atom of a metal hydride complex prior to inserting into the metal-hydrogen bond [Eq. (1.9)] ... 

S.8.2.7.3. ri (7-Vinyl Complexes by Addition of Alkynes to Metal Hydrides The stereochemistry of the acetylene insertion into the M—H bond ... 

Alkyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles see Electrophilic Reaction). Hydrogenation see Hydrogenation) of niobium or tantalum M-R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta-C bonds are common. The unsaturated reagents include alkenes, alkynes, CO, NO, RN=C=NR, CNR, and others. 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

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