Open-Chain Alkenes and Dienes

Carbon-13 chemical shifts of some representative alkenes [90, 232 236] are collected in Table 4.10. Inspection of the data shows that the shift value increases with increasing alkylation of the observed olefinic carbon atom. 

The a alkylation effect on an alkene carbon is similiar to the values reported for alkanes ( 10 ppm), but two ft effects must be considered Across a a bond, the / effect deshields 

2-Methyl-1,3-butadiene (C)-2-Methyl-l,3-pcn tad icnc ( )-3-Mcthyl-l, 3-pcntadicnc 

Similarly to alkanes, an increment system has been proposed in order to predict carbon-13 shifts of olefinic carbons from the reference value of ethene (122.1 ppm) [237] and the increments A, for a, f , y, and 6 alkylation, as well as multiple substitution corrections S (Table 4.11). Accuracy is about 1 ppm. 

In (Z)-( )-isomeric pairs of alkenes, the olefinic carbons of the cis isomers are slightly shielded when compared with those in the trans compounds. A much stronger shielding is experienced by the carbon atoms a to the double bond in the cis isomers due to the 

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