Primary carbon atom chain extensions

Cyanide ion is a good small nucleophile and displaces tosylate from primary carbon atoms and adds one carbon atom Lo the chain. As the cyanide (nitrile) group can be converted directly to a carboxylic acid or ester (Chapter 14) this sequence is a useful chain extension. 

The primary step consists in the formation of a n-complex between ethylene and the catalytic active center (Cat) that was already alkylated by the activator (R-M). This complexed monomer is then inserted into the metal-carbon bond with chain extension of two C-atoms. The propagation reaction consists of consecutive insertion steps. Termination occurs by -elimination or by reaction with hydrogen. In both cases the active center Cat is maintained. 

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

Biosynthesis of Esters. Fatty acids are naturally occurring, long, straight-chain, C12-C40 carboxylic acids most contain an even number of carbon atoms.Their biosynthesis provides an important and interesting example of a primary metabolic pathway in which a special type of ester is the essential link between the enzyme and the substrate (acetic acid).The enzyme-bound substrate grows by repeated addition of two-carbon (C2) units and, when eventually released from the enzyme, has undergone an extension of the fatty acid hydrocarbon chain. 

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