Lewis acid transition organometallic

The catalyst may be homogeneous or heterogeneous. The former usually consists of a transition metal compound such as tungsten hexachloride with a Lewis acid or organometallic compound such as ethylaluminium chloride, tetramethyl or tetrabutyltin, or triethylboron. Heterogeneous catalysts are usually oxides or carbonyls of molybdenum or tungsten on alumina or silica. 

Transition metal compounds can be divided into two broad categories classical complexes and organometallics. The former have the metal in a high oxidation state, and the metal center can be considered to have Lewis acid character. Organometallic complexes of the transition metals, however, usually have the metal in a low oxidation state, and the metal center can be considered to have Lewis base character. 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

An important focus of organometallic research has been the development and study of transition metal complexes as catalysts for transformations of organic compounds. These systems enable chemical reactions to occur under conditions that are often milder and more environmentally benign than more traditional routes, such as Lewis acid-catalyzed reactions, Grignard... 

Organometallic crown ethers have also been synthesized.96-98 Recently, a crown-cation group was shown to interact with an appended transition metal acyl ligand (29)." Complexes of this type have potential applications in Lewis acid-accelerated alkyl migration to coordinated carbonyls. 

Library of Congress Cataloging in Publication Data. Main entry under title Theoretical inorganic chemistry. (Topics in current chemistry 56). Bibliography p, Includes index. CONTENTS Jorgensen, C. K. Continuum effects indicated by hard and soft antibases (Lewis acids) and bases. - Brunner, H. Stereochemistry of the reactions of optically active organometallic transition metal compounds, [etc.]. 1. Chemistry, Physical and theoretical- Addresses, essays, lectures. I. Series. 

Scheme I further indicates the tendency of the Ln(III) cations to form the mofe unusual oxidation states in solution [73]. Hitherto, organometallic compounds of Ce(IV), Eu(II), Yb(II) and Sm(II) have been isolated. Charge-dependent properties, such as cation radii and Lewis acidity, significantly differ from those of the trivalent species (Table 4). Ln(II) and Ce(IV) ions show very intense and ligand-dependent colors which is attributed to Laporte-allowed 4/-+ 5d transitions [65b]. Complexes of Ce(IV) and Sm(II) have acquired considerable importance in organic synthesis due to their strong oxidizing and reducing behavior, respectively their reaction patterns have been reviewed in detail [40, 44-47, 74], Catalytic amounts of compounds containing the hot oxidation states also initiate substrate transformations as a rule this implies switch to the more stable, catalytically-acting Ln(III) species [75],...

There are also catalysts that lack any apparent source of metal-carbon bonds. These catalysts include the aforementioned alumina- and silica-sup-ported transition metal oxides (which, in principle, do not demand any activation by organometallic compounds), and also several group 6-8 transition metal chlorides (soluble in hydrocarbons or chlorohydrocarbons), most typically RuC13. Some of these transition metal halides require activation by a cocatalyst of the Lewis acid type (e.g. A1C13, GaBr3, TiCU) [66,67], Noble metal chlorides may be used in alcoholic solvents or in water containing emulsifiers [68]. 

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