Carbon-13 Magnetic Resonance of Hydroaromatics. 3. Conformation of 1,2,3,4-Tetrahydrophenanthrene and 9,10-Dihydrophenanthrene and Their Methyl Derivatives

Article abstract of DOI:10.1021/jo00218a027

13C chemical shift data have been obtained for 27 methylated 1,2,3,4-tetrahydro- and 9,10-dihydrophenanthrenes.It has been found that methyl-substituent parameters previously determined for the saturated ring of tetralin and 1,2,3,4-tetrahydroanthracene show a remarkable ability to predict the chemical shifts in spite of the significant structural differences in the basic structure of the hydrophenanthrenes, exemplified by the proximity of the C-4 and C-5 positions.Analogous to the tetralins, 1-methyltetrahydrophenanthrene was determined to prefer slightly the conformer with a pseudoaxial methyl, while, in contrast to the tetralins, the C-4 methyl derivatives were forced to exist entirely with the C-4 methyl pseudoaxial.The conformation of the cis-2,4 compound is ambiguous.Variable-temperature 13C and 1H NMR has provided ΔH^* and ΔS^* of 10.3 kcal mol^-1 and -3.3 cal K^-1*mol^-1, respectively, for the conformational inversion of cis-9,10-dimethyl-9,10-dihydrophenanthrene.It is hypothesized that significantly slower methyl group rotation in the transition state compared to the ground state is responsible for the negative ΔS^*.The trans-9,10 compound is shown to exist exclusively as the diaxal conformer, and 9,9,10-trimethyldihydrophenanthrene is present as an equilibrium mixture where the conformer with the C-10 methyl axial dominates to better than 95percent.