Group 16 Centred Ligands

Few organoytterbium(II) alkyls have been synthesized [15,16]. The trimethyl-silyl group features prominently in ligands such as the bis(trimethylsilyl)methyl and tris(trimethylsilyl)methyl. These compounds, whose reactivity is currently 

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An example of a high coordination number for cadmium is found in aquabis(4-aminobenzoato)cadmium(II),793b which involves seven-coordinate cadmium, with two chelating carboxylate groups, a water molecule and axial positions filled by amino groups from ligands bound to the neighbouring metal centres.794... 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

Many complexes have more than one coordination mode of BH4 featured in their structure, e.g. [U ()9 -BH4)()9 -BH4)2(dmpe)2]. Likewise, whereas [M(BH4)4] are monomeric 12-coordinate complexes for M = Zr, Hf, Np, Pu, they are polymeric for M = Th, Pa, U the coordination number rises to 14 and each metal centre is coordinated by two r) -BH4 and four bridging r) -BH4 groups. It is clear that among the factors which determine the mode adopted are the size of the metal atom and the steric requirements of the co-ligands. Many of the complexes... 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

The three-fold degenerate set of p orbitals are labelled tiu t for three-fold, u for odd under inversion through the centre of symmetry). As shown in Fig. 6-6, each metal p orbital matches symmetry with ligand group orbitals comprising just two... 

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