Organo-rare-earth metal complexes

Catalytic applications of organo-rare-earth metal complexes reported prior to 2002 are summarized in two excellent reviews [19,20] and, therefore, will not be discussed unless being relevant for understanding of key reaction details. A recent comprehensive review on theoretical analyses of organo-rare-earth metal-mediated catalytic reactions is available [17], Although o-bond metathesis plays a pivotal role in many rare-earth metal-catalyzed polymerizations, the discussion of these processes is beyond the scope of this review and the interested reader may consult one of the pertaining reviews [21-24],... 

The ability of organo-rare-earth metal complexes to undergo alkene or alkyne insertion provides the possibility to perform polyene cyclizations, producing metal-alkyl species which can then undergo o-bond metathesis with an appropriate reagent to produce a cyclic compound. Thus, termination via protonolysis (6) results in cycloalkane derivatives however, termination via silylation is more desirable as a functionalized cyclic framework is formed (Fig. 9). 

Cui, D. M. Nishiura, M. Hou, Z. M. Alternating Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by Organo Rare Earth Metal Complexes. Macromolecules 2005, 38, 4089 095. 

There are a number of synthetic pathways to trivalent rare earth siloxides and their Lewis base adducts such as salt metathesis, transesterification, acid-base chemistry, and silanolysis. Less frequently used are insertions of organo rare earth complexes into cychc and linear sUoxanes and CO2 insertion into rare earth silylamides. Transesterification is a common route for the preparation of stericaUy less-hindered early transition metal trialkyl siloxides and involves... 

Since grafting stabilizes species that would be too reactive in solution (in general, by allowing for isolated sites), and supported species may present catalytic properties unknown to the molecular chemistry, a lack of data concerning d(0) metal complexes derived from Nb and Cr is astonishing. Hopefully, the knowledge accumulated so far will be an incentive to develop the surface organo-metallic chemistry of these elements as well as the surface chemistry of the rare earths. 

Synthetic routes include anionic, cationic, zwitterionic, and coordination polymerization. A wide range of organometallic compounds has been proven as effective initiators/catalysts for ROP of lactones Lewis acids (e.g., A1C13, BF3, and ZnCl2) [150], alkali metal compounds [160], organozinc compounds [161], tin compounds of which stannous octoate [also referred to as stannous-2-ethylhexanoate or tin(II) octoate] is the most well known [162-164], organo-acid rare earth compounds such as lanthanide complexes [165-168], and aluminum alkoxides [169]. Stannous-2-ethylhexanoate is one of the most extensively used initiators for the coordination polymerization of biomaterials, thanks to the ease of polymerization and because it has been approved by the FDA [170]. 

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