Ziegler method

A) Ziegler method Catalyst systems can be prepared by reacting a nickel(II) salt with an organometallic compound of the main group metals but aluminum is preferred. 

The details of Ziegler method and its mechanism are given in the following paragraphs. 

Problem 9.32 Account for the fact that intramolecular cyclizations to rings with more than six C s are effected at extremely low concentrations (Ziegler method). 

ZIEGLER METHOD. Cydization of dinitriles at high dilution in dialkyl ether in the presence of ether-soluble metal alkylamlide and hydrolysis of the resultant imino-nitril with formation of macrocyclic ketones in good yields. 

Bromophthalide has previously been prepared by direct bromination of phthalide over a period of 10-13 hours in yields of 82-83%. The procedure above, a modification of the Wohl-Ziegler method, appears to be preferable since it may be completed in 3-4 hours, is applicable to the preparation of small samples, and gives comparable yields. 

Ziegler bromination is an important method for the synthesis of allylic and benzylic bromides (equation 8) (26). Critical features to the success of this reaction include the use of N-bromosuccinimide (NBS) as the brominating agent and CCI4 as reaction solvent. The advantage of the Ziegler method over direct bromination with Br2 is that competing electrophilic processes are virtually eliminated. An appreciation as to why electrophilic processes are minimized can be derived from an analysis of the reaction mechanism. 

The hydroxy-group of cholesterol is protected either as the acetate or benzoate. Bromination in the allylic position by Karl Zieglers method using N-hro-mosuccinimide or l,3-dibromo-5,5-dimethylhydantoin, then dehydrobromina-tion with 2,4,6-collidine and hydrolysis gives 7-dehydrocholesterol. Photolysis of 7-dehydrocholesterol and ergosterol, respectively, with a mercury vapour lamp in an inert solvent (e.g. peroxide-free diethyl ether, methanol, cyclohexane... 

Zervas (see Bergmann-Zervas Carbobenzoxy Method) Ziegler (see Thorpe-Ziegler Method)... 

Ziegler Method Ziegler-Natta Polymerization Zimmermann Reaction Zincke Disulfide Cleavage Zincke Nitration Zincke-Suhl Reaction... 

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