Organometallic intermediates transfers from

Subsequently, the following experiments were carried out to find out whether this radical reduction occurred through hydrolysis of an organometallic intermediate or from a hydrogen-atom transfer process. 

Thus, deuterium transfer from D20 was faster than the mixed disproportionation leading to the allylic alcohol 29, which in turn had proved to be much faster than the potential radical trapping leading to the organometallic intermediate 34. This strongly suggests that deuterium incorporation in 35 cannot occur via hydrolysis of 34 [73, 74],... 

The lack of deuterium labeling in 37 indicated that the reduction of the primary radical took place by hydrogen-atom transfer from 1,4-CHD and not by hydrolysis of a potential organometallic intermediate. Then 36 was treated with Cp2TiCl in the presence of 1,4-CHD and D20. A mixture of the normal products 37 and 38 as well as the deuterium-labeled 39 and 40 could be isolated (Scheme 27). 

The 57% of deuterium incorporation in 40 demonstrates that deuterium transfer from D20 was slightly faster than the hydrogen transfer from 1,4-CHD, which had been shown to be much faster than the radical trapping by a second equivalent of Cp2TiCl. As before, deuterium incorporation in 40 can therefore not occur via protonation of an organometallic intermediate, such as 41 (Scheme 28). 

Alkyl Transfers from Organometallic Intermediates in Catalytic Process ... 

Divalent samarium complexes can also catalyze ethylene polymerization, initially through one-electron transfer from the Sm(II) species to an ethylene molecule to form a Sm(III)-carbon bond, which is the active intermediate that induces ethylene polymerization. The less reducing divalent organometallic ytterbium and europium complexes are generally inert [143]. 

A common pathway in palladium-catalyzed oxidation reactions is that the 7r-olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme l) (i) an intramolecular hydride shift leads to ketone formation (ii) a )6-elimination results in the formation of a vinyl functionalized olefin (iii) an oxidative cleavage of the palladium-carbon bond produces a 1,2-functionalized olefin and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium-carbon bond, which may be cleaved according to one of the previous processes ()6-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated, in this way a palladium(II)-catalyzed oxidation is realized. 

Part V is devoted to the study of H transfers in organic and organometallic reactions and systems. In Ch. 18 Koch describes kinetic studies of proton abstraction from CH groups by methoxide anion, of the reverse proton transfer from methanol to hydrogen bonded carbanion intermediates, and of proton transfer associated with methoxide promoted dehydrohalogenation reactions. Substitutent effects, kinetic isotope effects and ah initio calculations are treated. Of great importance is the extent of charge delocalization in the carbanions formed which determine the kinetic and thermodynamic acidities. 

Mixtures of alkynes and cyclopentadiene with Co atoms yield a variety of air-stable organometallics . These result from hydrogen transfer from cyclopentadiene, dimerization or trimerization of alkyne, clustering of Co atoms and simple addition. The nature of the products implies that a variety of reactive intermediates must be formed en route. Table 1 shows the major products that are formed. 

Copper-catalysed aerobic oxidations (Scheme 2) implicating multiple possible mechanisms ranging from single-electron-transfer pathways to the involvement of organometallic intermediates and supported by recent smdies were discussed in a review article. ... 

The distinguishing feature of this mechanism is the second step, in which an electron is transferred from the organometallic reagent to the carbonyl compound to give the radical anion of the carbonyl compound. Subsequent collapse of the ion pair gives the same product as is formed in the normal mechanism. The electron transfer mechanism would be expected to be favored by structural features that stabilize the radical anion. Aryl ketones and diones fulfill this requirement, and much evidence for the electron transfer mechanism has been accumulated for such ketones. In several cases, it is possible to observe the intermediate radical anion by EPR spectroscopy. ... 

Another difficulty which is related to the potentially catalytic use of organometallics concerns the often enormous substitutional labilization of such systems after heterogeneous or homogeneous electron transfer. Typical textbook cases are the ligand exchange reactions of 18 valence electron (VE) complexes which can be accelerated by many orders of magnitude via one-electron oxidation (17 VE intermediates) or reduction (19 VE intermediates) [10,11]. It is shown below (Section 6) how even partial intramolecular electron transfer from ligands to metals can activate these for CO substitution. 

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