C—M bonding

In order to understand why the notoriously water-sensitive metal-mediated reactions can proceed in water at all, it is necessary to look at three basic aspects (1) the fundamental bonding in organometallic compounds, (2) the kinetics of the hydrolysis of C—M bonds, and (3) the kinetics of carbonyl addition (and other nucleophilic reactions) by C—M bonds. 

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Hydrolysis of the a-keto acid imine by nucleophilic addition of water to the C=M bond gives the transamination products pyridoxamine phosphate (PMP) and a-keto acid. 

In contrast to the open-chain and dipolar models, which are based on conformations of the carbonyl compound not representing energy minima, Karabatsos proposed a different model assuming an early, reactant-like transition state in which the most stable conformation of the free carbonyl compound is preserved1314. Thus, the C-M bond eclipses the carbonyl double bond and, in order to minimize the energy of the transition state, the nucleophile approaches close to the small substituent on the stereogenic center (Figure 5). 

The value of CM has been determined by a number of groups as 6x 10 5 (Table 6.14.1. " However, the mechanism of transfer has not been firmly established. A mechanism involving direct hydrogen abstraction seems unlikely given the high strength of vinylic and aromatic C-M bonds. The observed value of Cy is only slightly lower than Ctr for ethylbenzene ( 7x 10"5). w... 

There is a large and growing field of transition metal chemistry in which silicon-containing ligands are involved. The object of this review is to provide a guide to the literature on those aspects of the subject described by the title and to deal in detail with topics not treated specifically elsewhere. Section II is concerned with complexes having Si-transition metal (M) bonds, Section III with the role of transition metal complexes in hydrosilylation, and Section IV with complexes having Si—C—M bonds. 

B. The Synthesis of Compounds with Silicon-Carbon-Transition Metal (RR MeSi—C—M) Bonds... 

Because of the slightly acidic nature of the sp C-H bonds, the reaction of metal acetylides with various electrophiles is one of the most general strategies in organic transformations.1 Traditionally, such reactions are carried out by using alkali metal acetylides which are air and water sensitive. On the other hand, there is much interest in developing transition-metal catalyzed terminal alkyne reactions involving soft and more stable C-M bonds as reaction intermediates, because many such reactions can tolerate water. 

The nature of the C-M bond varies widely, ranging from bonds that are essentially ionic to those that are primarily covalent. 

The structure of the organic portion of the organometallic compound has some effects on the nature of the C-M bond, the identity of the metal itself is of far greater importance. 

The reactivity of organometallic compounds increases with the percent ionic character of the C-M bond. 

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