Lithiated C4-Hydrocarbons

According to ab initio calculations by Schleyer et al. 1,4-Dilithiobutane 28 has a similar doubly bridged structure as 1,3-dilithiopropane 16 although somewhat twisted (Cj symmetry), 28 is 34.6 kcal/mol (144.8 kJ/mol) more stable than the extended zick-zack form of 1,4-dilithiobutane, the cyclization energy being 10 kcal/mol (41.8 kJ/mol) higher than in the case of 16. 

Due to our engagement in carbanion rearrangements we were especially interested in the homoallylic system 4,4-dilithio-l-butene 33. In the case of homoallyllithium 29 ring-chain equilibrium strongly favors the open-chain lithiumorganic compound and the introduction of four methyl groups into the a- and -positions is necessary 

For the cyclopropylcarbinyl species 34 this time two isomers have to be discussed. While the trans isomer 34a (trans) is more or less classical comparable with 30a, the cis isomer 34 a (cis) was found to prefer a lithium doubly bridged structure, 22 kcal/mole (92 kJ/mol) more stable than 34a (trans) and even 26 kcal/mol (108.8 kJ/ mol) more stable than the open-chain starting material 33a (Fig. 1). 

Another 29 kcal/mol (121.3 kJ/mol) are gained by going to the most stable end-product, l,4-dilithio-2-butene 35, which again was found by MNDO calculations to show double lithium bridging 35 a 

Schleyer has recently shown by ab initio calculations (3-2IG) 35 to be even more stable by 6.7 kcal/mol (28 kJ/mol) in a conformer with the terminal methylene groups in plane with the carbon skeleton. According to these calculations 35 is also 17.6 kcal/mol (73.6 kJ/mol) more stable than 1,4-dilithiobutane 28. 

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