Ligand substitutions abstractions

Formation and characterization of zirconacyclopropanes and zirconacyclopropenes In addition to / -H abstraction of dialkylzirconocenes discussed earlier (Schemes 5 and 6), several other methods are also available for the preparation of three-membered zirconacycles as summarized in Scheme 35. From the viewpoint of cyclic carbozirconation reactions, especially those under Zr-catalyzed conditions, /3-H abstraction, 7t-ligand substitution, and decarbometallative ring contraction are particularly important. As such, these three-membered zirconacycles are generally unstable, but they can be stabilized with phosphines, for example, PMe3, and other bases, and are fully identified. Some of the well-identified examples are shown in Scheme 36.206-209... 

The results presented in Section IV.D.l show that the net loss of the dioxorhenium(VII) species, and the ultimate formation of the phosphate R3PO, must occur in two stages because the rate of reaction (17) shows a direct first-order phosphine dependence. That said, the chemical mechanism is still open to discussion does the first step entail abstraction of an oxo oxygen or addition to it If the former, the cycle is completed by PR3 coordination to a four-coordinate rhenium intermediate if the latter, the addition step is then followed by yet another ligand substitution reaction. The alternatives are presented in Schemes 1 and 2. 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

The radical OH reacts with many inorganic anions at near-diffusion-controlled rates, but with metal ions there seems to be an upper limit of about 3 x 10 dm mol s , regardless of the reduction potential of M " ". There is no correlation between the measured values of 3.13 and the rates of exchange of water molecules coordinated to M" , which rules out ligand substitution as a general mechanism other possibilities are abstraction of H from a coordinated water molecule and OH entering the coordination shell to increase the coordination number by one. 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

On the next page is the catalytic cycle associated with the Heck olefina-tion reaction. For each step marked with a letter, attach the name of one of the fundamental types of organometallic reactions (e.g., nucleophilic abstraction or ligand substitution). 

Ligand Substitutions by Radical Chains Initiated by Atom Abstractions... 

Ligand substitutions of 18-electron complexes can also occur by radical-chain processes initiated by atom abstraction. These radical chains occur through 17-electron intermediates that imdei o facile associative substitutions. Substitutions of metal carbonyl hydrides, halides, and stannyl complexes by this mechanism are all known. These reactions are particularly prevalent in first-row metal hydrides because the M-H bond is weaker than the M-H bond in second- and third-row metal complexes, and hydrogen atom abstraction is one step of the radical chain. However, they have also been proposed to occiu with third-row metal-hydride complexes... 

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