Processes Involving Metal-Multiple Bond Interactions

PROCESSES INVOLVING METAL-MULTIPLE BOND INTERACTIONS 

The final type of process which is to be dealt with here is the interaction of a vacant metal orbital with an unsaturated organic groups, such as a double or triple bond. The interactions possible are of two types. The first involves the interaction of a filled multiple bond on an organic moiety with the vacant nonbonding orbitals generated on formation of the three-centered MO scheme described in Section II. This type of interaction has been confirmed by X-ray structural data for the trans-t-butylvinyl-, cyclopropyl-, and phenyl-bridged aluminum derivatives. 

The second class of metal-multiple bond interactions has been postulated both for interaction of olefins (10, II, 13, 14, 20, 21, 45) and for acetylenes (12, 15-18) as the initial step in metal-carbon or metal-hydrogen addition across the multiple bond. The complexes proposed are illustrated in 24 and 25. 

Eisch has developed a substantial amount of evidence based on the kinetics of reaction and on the stereochemistry of the products for the aluminum-triple bond interaction 12, 15-18). Further, structure of the phenylethynyl bridged dimer, recently determined by X-ray crystallography (52), provides strong evidence for this metal — triple bond interaction with bridging groups clearly oriented appropriately for complex formation, as seen in 26. 

If we now turn our attention to the interaction of the metal center with olefinic derivatives, we find two classes of reactions. The first of these is the basis for the olefin polymerization or simple addition reactions, as illustrated in Eq. (13). A wide variety of data have been provided with regard to this reaction, which supports the formation of the complex 13, 14, 21. 35). 

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Processes Involving Metal-Multiple Bond Interactions. 126... 

PROCESSES INVOLVING METAL-MULTIPLE BOND INTERACTIONS... 

Alkene metathesis catalysis involves intermediates in which a transition metal is multiply bonded to carbon. These species are often referred to as nucleophilic caibenes when the carbon atom is negatively polarized. A more functional description is to name these compounds as alkylidene complexes, since they react to transfer an alkylidene moiety from a transition metid to a substrate carbon atom. Previous sections of this chapter have focused on a common example of this chemistry the process of metathesis that involves transition metal mediated interaction of carbon-carbon multiple bonds. 

The Pauson-Khand reaction provides another new approach to the metal-catalyzed synthesis of heterocycles. This reaction involves the interaction of the multiple bonds of an alkyne with an alkene and carbon monoxide in the presence of dicobalt octacarbonyl (Co2(CO)g), or with just this reagent as a source of CO. The overall process has been described as a [2 -h 2 -h 1] cycloaddition. Only a few applications to heterocyclic synthesis have been reported so far. A 2008 paper that is illustrative of the process describes the use of this reaction for the construction of a heterocyclic ring that is part of an azabicy-clo[3.3.1]nonane derivative. This ring system is present in the alkaloid (-)-alstonerine (4.37), which prompted this study. 

The foniiation of 4, therefore, involved several different noncovalent interactions. The cyclization step was brought about by the formation of Pd—N coordinate bonds that is, by a metal-mediated process. The interlocking step involved n- and hydrophobie/hydrophilic-mediated processes, along with an entropic effect. Catenane 4 can. therefore, be considered an example of a multi-mediated,"" multiple-interaction self-assembly. Fujita referred to such processes as ""double-molecular recognition"" procedures, in which the two interlocking molecules bind each other in their cavities. 

But whether the result of metal-ligand covalent bonding or a more subtle polarizability effect, extractants and complexants containing soft donor atoms are central to most ion exchange and solvent extraction separations of lanthanides from actinides. To generalize, those materials with the greatest potential for increased covalent interactions provide the most significant opportunity for successful lanthanide/ actinide separations. As discussed below, the sheer multiplicity of reactions involved in separations processes offer many opportunities to exploit this difference in soft donor interactions. 

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