Electrophilic Attack on Pentadienyl Compounds

13C-NMR spectra show that the negative charge in the pentadienylmetal derivatives of groups I, II, and III resides preferentially at the terminal and central positions (21). Electrophiles can therefore attack either at the terminal or at the central position Z and E isomers can result from terminal addition. For the reactions to be useful synthetically, high regiospecificity at one or other site is desirable. 

The nature of the counter ion and the solvent medium is also significant as these additions are reversible. The terminal adduct is thermodynamically the more stable, and the initial product mixture can in some cases be converted to it. Thus the zinc and lithium derivatives rearrange on prolonged heating in thf or thf-HMPA (91). The more ionic potassium compounds, however, which can be obtained by addition of potassium hydride, isomerize rapidly, especially in the presence of crown ethers and in polar solvents (92,93) [Eq. (4)]. 

Products from such reactions have been employed in the synthesis of terpenes via intramolecular Diels-Alder cyclizations (94-96). If the bulk of the carbonyl reagent is increased by inclusion of a large adjacent protecting group, much improved regioselectivity toward terminal attack is acheived (97,98). A complementary approach has been adopted by Oppolzer who 

Terminal attack occurs with water, methyl iodide, and trimethylchlorosilane, whereas central attack was preferred with alkenyl halides, aldehydes, and ketones at low temperatures under kinetic control [Eq. (5)]. The Et3SiO group is readily removed from 6 by potassium fluoride in isopropanol to give the vinyl ether RCH2CH2COCH=CH2 (61). Some of these reactions have also been used in elegant syntheses of terpenes (99-102). 

By introducing a thiomethyl substituent at the terminal position of 5 it was possible to direct attack to this position exclusively (61,103). Introduction of bulky 1,5-trimethylsilyl substituents into a pentadienyllithium provides a way of getting selective attack at C-3 (62). This approach worked with acetone, cyclohexanone, acetaldehyde, and 2-methylpropanal. With diisopropyl ketone, however, addition at C-3 still occurred, followed by Peterson elimination, giving a novel synthesis of 1,3,5-trienes (104) [Eq. (6)]. 

---