Conjugative Interaction in Styrenes

Conjugative interactions of the carbon−carbon double bond are fundamental in organic chemistry. In this work, equilibria are established among conjugated and unconjugated isomers of two β-substituted styrenes, 1-phenylbut-1-ene and 1-phenyl-3-methylbut-1-ene, and one α,β-disubstituted styrene, 2-phenyl-5-methylhex-2-ene, over a range of temperatures (the van't Hoff method) in hexamethylphosphoric triamide and potassium <i>tert</i>-butoxide. From the trans styrenes of the first two sets, an enthalpy of conjugative interaction of phenyl <i>vis</i>-<i>à</i>-<i>vis</i> alkyl (ΔΔ<i>H</i><sub>ConjInter/Alk</sub>) = −2.5 ± 0.2 kcal/mol [−5.1 kcal/mol defined as phenyl <i>vis</i>-<i>à</i>-<i>vis</i> hydrogen (ΔΔ<i>H</i><sub>ConjInter/H</sub>)] is observed, while the cis styrenes reveal an attenuated ΔΔ<i>H</i><sub>ConjInter/Alk</sub> of −1.1 kcal/mol (ΔΔ<i>H</i><sub>ConjInter/H</sub> = −2.7 kcal/mol). The α-methyl group in the third set also leads to a reduced conjugative interaction. Entropy plays an important role in determining positions of equilibrium. Free energies of conjugation are reported for several sterically hindered <i>o</i>-methyl-substituted styrenes.