Stoichiometric Reactivity of Allenylidenes

Preparation and Stoichiometric Reactivity of Metal Allenylidene Complexes... 

Stabilization of organic vinylidene and allenylidene species via coordination to a ruthenium centre is now well established, and the stoichiometric reactivity of these highly unsaturated ligands is still under intense investigation [ 1-4], and theoretical studies are being carried out [5,6]. Most of the chemical properties of cumulenylidene structures arise from the alternate electronic distribution along the carbon chain (Fig. 1). 

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established [26 0]. They are governed by the electron deficient character of the and Cy carbon atoms in the cumulenic chain, the Cp being a nucleophilic center. This can also be rationalized by considering the mesomeric forms depicted in Fig. 14. However, it should be noted that, as commented on previously, only contribution from the alkynyl resonance form is supported by X-ray diffraction. 

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established, being governed by the electron-deficient... 

The content of this book gathers in the same volume all aspects of vinylidene- and allenylidene-metal complexes, including the preparation of these organometallic carbon-rich systems with a metal-carbon double bond, their stoichiometric reactivity and theoretical aspects, and their applications in catalysis for the production of fine chemicals, mainly in the field of selective transformations of functional terminal alkynes. It provides essential general information on catalytic transformations of alkynes and their use in synthesis. 

In this chapter we have tried to give a general presentation of the most efficient synthetic routes, the main characteristic structural features and a discussion of the reactivity patterns. Special attention is devoted to the chemistry of mononuclear derivatives containing linear allenylidene groups as they are most frequently the active species in catalytic processes. Illustrative examples of the synthetic applications via stoichiometric reactions will also be covered. For more comprehensive information we refer the reader to the reviews and accounts mentioned above [3, 4, 7]. 

The metal-ligand fragment L M, the number of carbon atoms x, and the substituents at the terminal sp -carbon may vary considerably and, correspondingly, the properties and reactivities. The early members of the series of cumulenylidene complexes (x=l, 2, 3 carbene, vinylidene and allenylidene complexes) have established themselves as invaluable building blocks in stoichiometric synthesis and as highly potent catalyst precursors. The higher members might potentially be very useful candidates for application as one-dimensional wires and in opto-electronic devices. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

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