THE 2 CONFORMATIONS OF HINDERED PHOTOCHROMIC 4A,4B-DIHYDROPHENANTHRENES

We analyze conformational stereoisomerism in the sterically hindered 4 a,4b-dihydrophenanthrenes of C-2 symmetry. In these labile photointerm ediates, steric hindrance brought about by disubstitution at the 4 and 5 positions or by benzoannelation at the [c] and [g] bonds allows obs ervation of two interconvertible intermediates denoted as L and S. In such molecules, theoretical analysis suggests two low-energy structure s (C and T). These differ in the chirality of the polyene perimeter he lix for one given configuration of the 4a,4b-ethane unit. C and T can be distinguished by the position of atom pair H-4a, X-4, or Of the equ ivalent atom pair H-4b, X-5 With respect to the the mean molecular pla ne. In C, atoms H-4a and X-4 are on one side of the mean molecular pla ne, while in T atoms H-4a and X-4 are on opposite sides of this plane. The same holds for the atom pair H-4b, X-5. The C conformation is ass igned to the primary photocyclization product (L modification) in whic h the intersubstituent distance X-4-X-5 is short and where the angular hydrogens are quasi-antiperiplanar. This is the regular conformation of the unhindered 4a,4b-dihydrophenanthrenes. The T conformation is as signed to the secondary conformers (S modification) formed spontaneous ly from the primary L product. Here the distances H-4a-X-5 and H-4b-X- 4 are short, and in the case of severe steric hindrance (X = t-Bu) the angular hydrogens are anticlinal. The observed stability order (usual ly S > L at around room temperature, except in the 4,5-dimethyl, 4,5-d i-tert-butyl, and 4,5-dichloro molecules) is the outcome of steric hin drance opposing skeletal deformations. X-4-X-5 steric interactions are dominant in the C forms while the H-4a-X-5 and H-4b-X-4 interactions (and corresponding interactions in the [c] and [g] dibenzoannelated sy stems) play a foremost role in the T forms. Computed strain energies f or C and T reproduce the observed stability trends of L and S modifica tions. Differences in visible absorption band energies are traced to d ifferent extents of departure from planarity of the polyene perimeter. The L intermediates assigned to the more planar C structure are thus predicted to absorb at longer wavelengths. The low intensity of the vi sible band in the L modifications is attributed to symmetry-imposed ca ncelling-out of one-ethylene-bond transition moments. The present anal ysis explains also the simultaneous existence and the observed range o f values of potential barriers separating the two modifications. The p resent analysis changes the previous structure assignment for the S an d L modifications of the dibenzo system, leading to full agreement wit h the observed trend of the electronic spectra of the S and L modifica tions. Similar considerations applied to the 4a,4b-dimethyl system sug gest that its labile low-temperature intermediate (lambda(max) = 360 n m) could have a T-like conformation.