Transition-metal derivatives alkyne insertion into

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. 

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... 

An interesting cyclization reaction was reported that involved the reaction of dienes, diynes, or ene-ynes with transition metals to form cyclopentenone derivatives in the presence of carbon monoxide.363 in a simple example, ene-yne 444 was heated with dicobalt octacarbonyl and CO to give a 68% yield of 445.364 jjjj transformation has become an important synthetic tool known as the Pauson-Khand reaction.365 jhe mechanism probably involves insertion of the alkene (or alkyne) into the transition metal bond, which is why it is presented in this section. Formally, it is a [2+2+l]-cycloaddition, but the accepted mechanism is the one proposed by Magnus,364 and shown in Figure 13.8.366 n has been stated that further study is required to... 

When coordinated to palladium, the rr-indenyl ligand tends to slip from the if - to the 77 -coordination mode, and most of the complexes synthesized show severe distortions or clear -indenyl coordination. Thus, although being a cyclopentadienyl analog, it is rare to find a true Tj -indenyl coordination to palladium, however common for other transition metals. Metathesis with Li[indenide] and ligand-substitution reactions are common preparative routes for indenyl derivatives. The insertion of an alkyne into Pd-G bonds of vinyl substituted aryls, followed by intramolecular alkene insertion, also leads to highly substituted indenyl palladium complexes. Equation (66) shows one of these examples. ... 

Several transition-metal-catalyzed transformations of alkynes are rooted in the carbothiolation strategy. For example, transition-metal-catalyzed insertion of alkynes into sulfur-heteroatom bonds has been of particular interest from the synthetic chemist viewpoint. The first Pd-catalyzed thioboration of alkynes using 9-(alkylthio)-9-borabicyclo[3.3.1]nonane derivatives 106 was reported... 

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