Metal alkyl, transition states leading

Based on these observations a stereochemical model for a transition state leading to the metal alkyl intermediate was developed with the following assumptions ... 

Studies of similar reactions under a variety of conditions had been previously reported. Thus, electroreductive, photoreductive, as well as metal-induced ketyl-olefm cyclizations had all been explored prior to our investigations. Many of these cyclization reactions using simple unsaturated ketones took place with reasonably high diastereoselectivity at two stereocenters (Eq. 3). This feature of the transformation was ascribed to favorable secondary orbital interactions between (he developing methylene radical center and the alkyl group of the ketyl, and/or to electrostatic interactions in the transition state leading to product. ... 

Bergman et al. [12] reported one of the first studies of C—H bond activation with a transition metal system capable of intermolecular oxidative addition. The reaction involved the photolysis of [Ir(ri-Cp )(PMe3)(H)J in different hydrocarbon solvents. Figure 25.5 shows that the reaction likely proceeds via loss to form a very reactive IF 16 electron metal intermediate. The C—H activation proceeds via a 3-centered transition state leads to an Ir hydrido-alkyl complex in a high yield at room temperature. The process is well described as an oxidative addition of the alkane. 

As the second selectivity issue, the catalyst should usually favor w-aldehyde over iso-aldehyde formation. This task is mainly addressed by the right choice of ligand. The ligand influences both the electronics and sterics of the catalyst in the step of the catalytic cycle that determines regioselectivity (see -II versus iso-11 in Scheme 6.14.4). Note that the transition state leading to the linear hydroformylation product involves a linear alkyl chain attached to the metal center that requires less space compared to the branched counterpart. Moreover, the electronic properties of the ligand influence the hydride transfer from the metal complex to Cl versus C2 during formation of the metal-carbon bond. 

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.24 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. For these halides, the overall mechanism probably consists of two steps an oxidative addition to the metal, after which the oxidation state of the copper is +3, followed by combination of two of the groups from the copper. This process, which is very common for transition metal intermediates, is called reductive elimination. The [R 2Cu] species is linear and the oxidative addition takes place perpendicular to this moiety, generating a T-shaped structure. The reductive elimination occurs between adjacent R and R groups, accounting for the absence of R — R coupling product. 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

In spite of the many known silicon-transition metal complexes (15,16), little systematic work has appeared on the reaction mechanisms of silyl metal complexes. This state of affairs is in marked contrast to the current work on cr-alkyl transition metal complexes, where much emphasis is placed on determining detailed decomposition mechanisms (75-76). The reason for the interest in decomposition mechanisms is that the products of a transition metal-catalyzed reaction are released by the decomposition of the product-metal complex. Thus, to understand a catalytic process, one must have knowledge not only of the interaction of the reactants with the metal (leading to a substrate-metal complex), but also of the mechanisms whereby substrates are transformed on the metal and the manner in which the products are released. 

It is well known that in conventional catalyst systems a chemical interaction between the catalyst and the metal-alkyl takes place, which essentially leads to a variation of the transition metal oxidation state. This is likewise true with MgCl2 catalysts however, in this case there are many more possible reactions, given the greater complexity of the system. Thus, besides modifying the Ti valence, the metal-alkyl may interact with the Lewis base incorporated in the catalyst. The Lewis base added to the cocatalyst can, in turn, interact both with the support and with the TiCl4, as can the byproducts originating from the reaction between Al-alkyl and Lewis base. The situation appears to be quite complex. However, detailed knowledge about these processes is absolutely necessary for any attempt to rationalize the polymerization behavior of these catalytic systems. 

It is especially remarkable that optically active homoaldoi adducts can be obtained when enantiomeri-cally pure 2-alkenyl carbamates (47 R = alkyl) are employed. Apparently the deprotonation occurs with retention of configuration and leads to configurationally stable liAium derivatives, which, after metal exchange with Ti(OPr )4, again with retention, add to aldehydes with efficient 1,3-chirality transfer coupled with enantiofacial differentiation at the carbonyl group, indicating a rigid six-membered transition state. Recently even an asymmetric homoaldoi reaction by enantioselective lithiation of prochiral primary alkenyl carbamates in the presence of (-)-sparteine was reported. ... 

The counterion of an enolate has a pronounced influence on competing transition states of enolate reactions. The effect is often the result of cation chelation by the carbonyl oxygen atom and one or more additional basic portions of the reactants. For example, alkylation of chiral enolates may lead to more or less diastereomerically pure products and selectivity often depends on the countercation. The importance of the countercation in controlling enolate reaction product distributions requires that the synthetic chemist has at hand stereoselective methods for the preparation of enolate anions with a wide variety of counterions. This chapter is divided into several sections. The 10 following sections describe important current methods for preparing Li, Mg, B, Al, Sn, Ti, Zr, Cu, Zn and other transition metal enolates. 

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