Journal of Organic Chemistry p. 3380 - 3388 (1985)
Update date:2022-08-11
Topics:
Morin, Frederick G.
Horton, Walter J.
Grant, David M.
Pugmire, Ronald J.
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.
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(1981)