Early transition metals, syntheses

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

Ph2P(NH2)NP(NH2)Ph2]+Cl, 19, which is prepared by the reaction of Ph2PCl3 and ammonia (136). This synthon has a preformed N-P-N-P-N unit and can be readily cyclized to a six or higher membered ring upon reaction with an appropriate reagent (137). An important application of the use of the Bezman s salt approach has been the synthesis of metallocyclophosphazenes of the type 20, which contain an early transition metal in the framework of the cyclophosphazene skeleton [Fig. 15(A)] (138). A modification of the Bezman s salt to design a chiral synthon allowed the synthesis of an optically pure cyclophosphazene (139). 

Whilst hydrogenation catalysts based on early transition metals are as active and selective as those based on late transition metals, they are usually not as compatible with functional groups, and this represents the major difficulty for their use in organic synthesis. Nonetheless, titanocene derivatives have been used in industry to hydrogenate unsaturated polymers. 

There has been great interest in the area of chiral acid catalysts in organic synthesis over the past few decades. This topic has been the subject of several previous reviews. For example, the book Lewis Acids in Organic Synthesis (edited by Hisashi Yamamoto) was published by Wiley-VCH in 2000. In this chapter, successful and significant chiral Brpnsted acid catalysts, chiral Lewis acid catalysts [typical Lewis acidic elements main group elements, B(III) and Al(III), and early transition metal, Ti(IV)], and Lewis acid-assisted chiral Brpnsted acid catalysts developed after 2000 are discussed. Chiral acid/base catalysts wdl be discussed in Chapter 13 by Shibasaki and Kanai. 

When early transition metals are considered, the synthesis is essentially accomplished by dissolving the appropriate metal oxide in aqueous solutions of hydrogen peroxide. 

The extensive chemistry of amido complexes, and, more particularly, of alkylamido complexes, reveals that the planar form is almost invariably found, along with bridging amides (221). Much attention has been paid to the synthesis of metal amido complexes of early transition metals, lanthanides and actinides. The amido group, particularly where it is bulky, confers unusual low coordination numbers on the metals and can also produce materials with considerable kinetic stability toward attack by nucleophiles (42, 67). However, the relevance of this extensive and fascinating chemistry to nitrogen fixation is somewhat problematic. 

Martinez, L.E. et al. Highly Efficient and Enantioselective Synthesis of Carbocyclic Nucleoside Analogs Using Selective Early Transition Metal Catalysis. 2.4 1996 [148]... 

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

Early transition metal allyl complexes have an enormous practical importance as either catalytic precursors or stoichiometric reagents in organic synthesis [103-108]. In the majority of the Group 4 complexes containing the allyl moiety, the metals exhibit the higher oxidation state (+4). Very few of these compounds are available in the literature with a +3 oxidation state, presumably because of their paramagnetic nature (reactivity) and difficulty in their handling. 

Although some Ti(III)-allyl complexes have been fully characterized spectroscopically [ 109,110], well-characterized Zr(III)-allyl compounds in the solid state have not been reported in the literature. On the basis of previous results obtained in our laboratory, it was very attractive and conceptually important to find a route to synthesize simple monomeric Group 4 early transition metal allyl complexes and to compare their catalytic activity to that of the well-characterized heteroallylic octahedral early transition metal compounds. Here we report the synthesis and solid-state X-ray structural characteristics of a Zr(III) bulky bis-allylic complex, and its catalytic activity in the polymerization of a-olefins [111]. 

Whereas Trost and Takano established the usefulness of late transition metals and their complexes for the construction of the dendrobine skeleton, Mori et al. demonstrated the advantage of the more stable early transition-metal complexes in the synthesis of dendrobine (82) (167-169). They presented their formal EPC-synthesis centered on Negishi s (Taber s) zirconium-assisted cyclization in several publications (189,190). Mori et al. tried to test their key step with a model synthesis (191). 

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