Metal-carbon bonds protonolysis

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. 

The simplest reaction of zirconacyclopentadienes is their hydrolysis. It is characteristic for organo-early transition metal compounds the metal—carbon bond is easily hydrolyzed with acids to give free organic compounds. Similarly, deuterolysis of zirconacyclopentadienes, rather than protonolysis, affords deuterated compounds as expected (Eq. 2.9). The position of the deuterium is indicative of the position of the metal—carbon bond in the organozirconium compound. 

Syntheses of organoantimony and organobismuth compounds 771 6. Protonolysis of metal-carbon bonds. 

This is a reaction with high atom economy and without the use of organometallic reagents, additives, or redox systems. Such an acetoxypalladation-initiated carbon-heteroatom multiple bond insertion-protonolysis system may extend the scope of transition metal-catalysed reactions pertaining to the insertion of carbon-heteroatom multiple bonds into metal-carbon bonds, and provide a new methodology in organic synthesis. The generality of the present catalytic system is shown in Table 10.2.[3]... 

The mechanism of homogeneous catalysis invoives the same steps as heterogeneous catalysis. An initial tt complex is formed with the reactant. Metal-hydride bonds then react with the complexed alkene to form a C-H bond and a bond between the metal and alkyl group. There can be variation in the timing of formation of the M—H bonds. The metal carbon bond can be broken by either reductive elimination or protonolysis. Note that reductive elimination changes the metal oxidation state, whereas protonolysis does not. The catalytic cycle proceeds by addition of alkene and hydrogen. 

Mechanism of ProtonolysIs of Metal-Carbon Bonds in Complexes Possessing d-Electrons... 

Although Pd-catalyzed intramolecular hydroamination reactions of alkynes have been known for ten years, analogous transformations of unactivated alkenes have only recently been developed [33]. Key to the success of these studies was the use of a cationic palladium complex bearing a pyridine-derived P-N-P pincer ligand (29). For example, treatment of 26 with catalytic amounts of 29, AgB F4, and Cu(OTf)2 led to the formation of pyrrolidine 27 in 88% yield with 4 1 dr (Eq. (1.13)). Detailed mechamstic studies have indicated these transformations proceed via alkene coordination to the metal complex followed by outer-sphere aminopaUadation to provide 28. Protonolysis ofthe metal-carbon bond with acid generated in situ leads to formation of the product with regeneration of the active catalyst. 

The protic cleavage of the carbon-metal a-bond ranks among the simplest of all electrophilic substitution processes. As organomercurials are readily prepared in high purity, can be manipulated with ease and are monomeric in solution, most mechanistic studies of the protonolysis of carbon-metal o-bonds have focused on the protic cleavage of organomercurials. Reviews and a book have been published on this subject. 

This implies that the other elementary steps in cycle B (Scheme 1), i.e., Pd-carbomethoxy formation and protonolysis of the palladium-alkenyl species, must even be considerably faster than the observed overall high reaction rate. A high rate of Pd-carbomethoxy formation (at equilibrium) could be expected for the strongly electrophilic metal center. However, the latter step, protonolysis of the Pd-alkenyl bond in l-palladium-2-carbomethoxypropene and 2-palladium-1-car-bomethoxypropene, respectively, is expected to be a slow reaction, because the proton has to overcome a relatively high barrier of (electrostatic) repulsion by the cationic palladium center on its way to the palladium-carbon bond. 

The rich nucleophilic reactivity of square-planar platinum(II) and palladium(II) complexes is well established. One of the most documented examples is the stepwise oxidative addition of aUcyl halides to organoplatinum(II) [1] and organopalladium (II) [2,3] complexes via SN2-type substitution at the sp carbon center. Additionally, electron-rich Pt centers are subject to protonation at the metal to generate Pt hydrides as the first step in the protonolysis of many platinum-carbon bonds [4—7]. With a less reactive Lewis acid such as SO2, reversible adduct formation is observed [8], and this reaction has been used in the development of sensors [9-11],... 

Insertion of alkynes can also be used to form a carbon-carbon bond at the same time as the carbon-zirconium bond. This process, carbometallation, is useful for the stereospecific construction of trisubstituted alkenes (Scheme 5.63). The active reagent is formed by mixing zirconocene dichloride and trimethylaluminium. Both metals are required for the reaction to proceed. The vinyl organometallic 5.221 produced may be subjected to protonolysis or halogenation. An application with iodination can be found in Scheme 8.121. 

The formation of metal vinylidene complexes directly from terminal alkynes is an elegant way to perform anti-Markovnikov addition of nucleophiles to triple bonds [1, 2], The electrophilic a-carbon of ruthenium vinylidene complexes reacts with nucleophiles to form ruthenium alkenyl species, which liberate this organic fragment on protonolysis (Scheme 1). 

The formation of methane from methyl complexes of Pt(ll) and H+ received considerable attention as the reverse step of methane activation by Pt(ll) salts [130]. This protonolysis similarly proceeded via methyl(hydride)platinum(lV) intermediate, the existence of which has actually been confirmed spectrally [131]. Stereochemistry at the a-carbon during SO2 insertion into alkyl-metal bond is diverse, both retention and inversion of configuration having been observed [29,128,132]. 

A study has been reported regarding the ruthenium-catalysed reaction of benza-mides with alkynes, which yields ort/io-alkenylated derivatives. Here, the mechanism is likely to involve rate-limiting metalation, followed by alkyne insertion to form intermediates such as (63) which on protonolysis yield the alkenylated products. An allylic carbon-carbon double bond has also been used as a coordination site in palladium-catalysed alkenylation reactions, as shown in Scheme 3. Here measurement of kinetic isotope effects suggests that coordination of the palladium with the allylic double bond occurs before palladation to give (64). Insertion of the alkene into the carbon-palladium bond gives (65) and -hydride elimination " leads to the product... 

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