Carbometalation Process

In conclusion, the carbonylation of allenes provides useful methods for the synthesis of functionalized unsaturated carbonyl compounds and cycUc carbonyl compounds. 

1 (a) N. Krause and S. Hashmi (eds) (2004) Modem AUene Cherrdstty, Wiley-VCH Verlag GmbH, New York, (b) E. Negishi (ed.) (2002) Handbook of OrganopaUadium Chemistry for Organic Synthesis, John Wiley Sons, Inc, New York, (c) Brandsma, L.(2004) Synthesis of Acetylenes, Allenes and Cumulenes, Elsevier, Oxford. 

3 Alper, H., Hartstock, E.H. and Despeyroux, B. (1984) Journal of the Chemical Society, Chemical Communications, 905. 

8 Murakami, M., Itami, K. and Ito, Y. (1998) Angewandte Chemie-Intemational Edition, 

and Sonoda, N. (1992) Journal of the American Chemical Society, 114, 5902. 

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In a series of late transition metal catalyzed processes the first step in the catalytic cycle is the coordination of the reagent to the metal atom, which is in a positive oxidation state, followed by its covalent attachment through the concomitant breaking of an unsaturated carbon-carbon bond or a carbon-hydrogen bond. These processes usually require a highly electrophilic metal centre and are frequently carried out in an intramolecular fashion. The carbometalation processes that follow a similar course, but take place only at a later stage in the catalytic cycle, will be discussed later. 

Carbometallation is a term coined for describing chemical processes involving net addition of carbon-metal bonds to carbon-carbon Jt-bonds [1] (Scheme 4.1). It represents a class of insertion reactions. Whereas the term insertion per se does not imply anything chemical, the term carbometallation itself not only explicitly and clearly indicates carbon-metal bond addition but also is readily modifiable to generate many additional, more specific terms such as carboalumination, arylpalladation, and so on. In principle, carbometallation may involve addition of carbon-metal double and triple bonds, that is, carbene- and carbyne-metal bonds, as well as those of metallacycles. Inasmuch as alkene- and alkyne-metal Jt-complexes can also be represented as three-membered metallacycles, their ring expansion reactions via addition to alkenes and alkynes may also be viewed as carbometallation processes (Scheme 4.1). 

Despite its inherent difficulties, carbometallation has, in fact, played important roles in catalytic asymmetric carbon-carbonal bond formation. Isotactic and syndiotactic alkene polymerization involving both heterogeneous and homogeneous Ti and Zr catalysts must involve a series of face-selective carbometallation processes, although the main stereochemical concern in poly(alkene) formation is diastereoselectivity rather than enantioselectivity. This fascinating topic, however, is outside the scope of this chapter, and the readers are referred to Chapter 11 and other previous reviews [6]. 

Chiral ligands are generally expensive. It is therefore highly desirable to use chiral ligands as catalyst components rather than those of stoichiometric regents. In the desired Zr-catalyzed enantioselective carboalumination of alkenes, for example, chiral ligands should be part of the Zr catalysts. Furthermore, it appears desirable to devise Zr-centened carbometallation processes rather than Al-centered ones. This factor can be potentially serious as both Zr- and Al-centered... 

Despite many such promising results, development of Zr-catalyzed enantioselective reactions based on these Zr-promoted or Zr-catalyzed cyclic carbometallation processes has not been straightforward. Thus, there has been no report of Zr-catalyzed cyclic carbometallation of simple, unactivated alkenes exhibiting enantioselectivity >33% [9,29]. 

Interesting consequence of the reversibility of the carbozir-conation see Carbometalation) process includes stereo and regioselectivity backbone rearrangement (equation 54). ... 

Negishi, E.-i., Kondakov, D. Y., Choueiry, D., Kasai, K., Takahashi, T. Multiple Mechanistic Pathways for Zirconium-Catalyzed Carboalumination of Alkynes. Requirements for Cyclic Carbometalation Processes Involving C-H Activation. J. Am. Chem. Soc. 1996,118, 9577-9588. 

The key step in nearly all of the catalytic processes to be discussed is olefin insertion into a metal hydride [Eq. (2)] or organometallic species [Eq. (3)]. These hydrometallation and carbometallation processes also form the basis for the polymerization of alkenes. Olefin insertions generally occur with the same regiose-lectivity as hydroboration reactions [9], with the bulky metal and associated ligands residing at the least hindered site of the two carbon reactive unit. 

An evaluation of the electronic, steric, and stereochemical factors involved in the kinetics and thermodynamics of this kind of process may be most helpful in elucidating the course of the Ziegler carbometallation process [Eq. (5)], central to the industrial production of long-chain alcohols and olefins from ethylene and to the stereoregular polymerization of a-olefms (18, 21). 

Thus, 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. 

Reaction of the resulting cyclopropylmetal derivatives with different electrophiles provided diastereomerically enriched products 123 in good yields [34]. Catalytic processes promoted by transition metals are powerful synthetic tools for the selective formation of carbon-carbon bonds including carbometallation processes. Thus, a highly diastereoselective iron-catalyzed carbometallation of oxa- and azabicycloalkenes with arylzinc reagents was reported recently by Nakamura et al. 

Another example of the iron-catalyzed carbometallation reaction of unactivated alkynes was reported for the aryknagnesiation of alkyl(aryl)acetylenes [64b]. The addition of arylmagnesium bromide to the disubstituted aryl(alkyl)alkynes 234 was found to proceed efficiently in the presence of Fe(acac)j as a catalyst and a substoichiometric amount of an N-heterocyclic carbene (NHC) ligand (IPr) as an additive, which significantly increased the yield of this carbometallation process (Scheme 10.79). 

Stereochemical assignment of all aforementioned reactions by NMR spectroscopy revealed a high level of ds addition, and a regioselectivity with the copper atom being attached to the sp carbon atom a to the fluorinated alkyl group. On the basis of the stereochemical outcome of the considered reaction, the authors proposed a mechanism for this carbometallation process (Scheme 10.113). 

The application of ynamides in various organic transformations, including carbometallation processes has recently been summarized in an excellent review Evano, G., Coste, A., and Jouvin, K. (2010) Angew. Chem. Int. Ed., 49, 2840-2859. 

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