Carbometallation reaction

In the case when 3 has a similar abihty to react with an unsaturated substrate 

for a synthetically useful reaction, both regioselectivity and diastereoselectivity of the carbometallation process must be controlled, where spatial arrangement and reactivity of the sp organometallic species determine the diastereoselection. Since the earlier editions of this book [3], an increasing number of reports have appeared in the literature that have been summarized in excellent reviews and book chapters. In this chapter, the focus will be on the most important and recent advances in this field since the preceding edition of this book. 

Metal-Catalyzed Cross-Coupling Reactions and More, First Edition. 

Edited by Armin de Meijere, Stefan Erase, and Martin Oestreich. 

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The term carbometallation was most probably coined only about a quarter of a century ago.1 However, the history of those reactions that can be classified as carbometallation reactions is much older. If one includes not only the Ziegler-Natta-type organometallic alkene polymerization reactions2 but also various types of organometallic conjugate addition reactions,3 carbometallation collectively is easily more than a century old. In its broadest definition, carbometallation may be defined as a process of addition of a carbon-metal bond to a carbon-carbon multiple bond. As such, it may represent either a starting material-product relationship irrespective of mechanistic details or an actual mechanistic microstep of carbon-metal bond addition to a carbon-carbon metal multiple bond irrespective of the structure of the product eventually formed. 

Scheme 1 Some representative patterns of the carbometallation reactions.

It has become increasingly clear that carbometallation reactions are mechanistically diverse. Although most of the synthetically interesting carbometallation reactions of organotransition metals appear to involve concerted four-centered processes in which the presence or ready availability of a low-lying metal-empty orbital is critically important (Scheme 3), many other processes including radical and polar processes are also known. 

Scheme 3 HOMO-LUMO interaction schemes for concerted four-centered carbometallation reactions.

Carbometallation Reactions of Organotitanium Compounds 10.06.2.2.1 Controlled monocarbometallation... 

Aside from the Ziegler-Natta polymerization, alkene and alkyne metathesis, and other reactions of Ti-methylene complexes, carbometallation reactions induced by alkyltitanium compounds have been dominated by those involving... 

As already indicated, the carbometallation reactions of zirconacyclopropanes and zirconacyclopropenes with alkenes and alkynes are in many ways similar to the corresponding reactions of titanacycles developed more recently. At the same time, however, there are a number of significant differences, as detailed in Section 10.06.2.2. At the present time, synthetically useful carbotitanation reactions are predominantly cyclic and stoichiometric in Ti and more so than the corresponding chemistry of Zr. It seems reasonable to state that Ti and Zr are complementary to each other more often than not. The cyclic carbozirconation may be either stoichiometric or catalytic. Frequently, the difference between the two is that the stoichiometric reactions lack one or more microsteps for completing catalytic cycles. Otherwise, they often share same stoichiometric microsteps. With this general notion in mind, many stoichiometric carbozirconation reactions have indeed been developed into Zr-catalyzed reactions, as discussed later. 

Carbometallation Reactions of Group 5-7 Metals 10.06.3.1 General Remarks about Carbometallation of Group 5-7 Metals... 

All of the reactions discussed above are cyclic carbometallation reactions of metallacycles. Very recently, an interesting Cr-catalyzed carboalumination of propargyl derivatives producing allenes via a carbometallation-elimina-tion sequence has been studied. This reaction provides an asymmetric synthesis of chiral allenes (Scheme 57). 

Not surprisingly, little, if any, appears to be known about the carbometallation reactions of Tc. What is somewhat surprising is that similarly little appears to be known about the carbometallation of Re. Its high cost (cf. Table 1) might be one of the reasons for the apparent paucity of its carbometallation chemistry. So, the discussion in this section is limited to the carbometallation of Mn. 

Controlled single-stage carbometallation reactions of alkenes and alkynes with group 4—7 metals are discussed with emphasis on regio-, stereo-, and chemoselectivity including clarification and understanding of factors governing these synthetically important aspects. 

By far, the most extensively investigated, developed, and used are those carbometallation reactions involving Zr. Some of the representative examples of this class include ... 

The intermolecular carbometallation reaction catalyzed by transition metals from groups 8 to 11 has been fully investigated during the last decade. Many examples have been reported in the literature concerning different metals in order to create C-C bonds. Overview of this powerful method will be given as exhaustively as possible and carbometallation reactions will be classified according to unsaturated systems. 

Chinkov, N. Tene, D. Marek, I. Carbometalation Reaction. In Metal-Catalyzed Cross-Coupling Reactions, 2nd ed. Diederich, F., de Meijere, A., Eds. Wiley-VCH Weinheim, 2004 pp 395M78. 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

Certain transition metal complexes can serve as templates for the synthesis of chelating NHC ligands. For example, 1-phenylphosphole complexes of pal-ladium(II) are attacked in a Diels-Alder reaction by 1-vinylimidazole. If 1,2-dichloroethane is used as the solvent the imidazole is alkylated in situ and then subjected to a spontaneous carbometallation reaction [Eq. (37)]. 

Intramolecular reactions from organolithium reagents (Scheme 37) have also been reported. The organolithium reagent is produced through iodine-lithium exchange", through a carbometalation reaction or by deprotonation at a position activated by an aryl. 

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