Energy Profiles for Ring Formation‐ and Ring Opening‐Processes in the cis‐Stilbene‐4a,4b‐Dihydrophenanthrene System. An example of the feasibility of a process forbidden by the rules of orbital symmetry conservation

Abstract

Summary. An Extended Hiickel MO treatment of the system cis-stilbene (1)-4a,4b-dihydrophenanthrene (11) indicates that the concerted photocyclisation is a conrotatory process of the first excited electronic state. The activation energies for the photocyclisation and for the thermal ring opening and the ground state energy difference between I and 11, as predicted by the present calculation, are in good agreement with the experimental findings. The observed effects of substituents on the photocyclisation quantum yields are satisfactorily accounted for by a general qualitative treatment and thus indicate that the photocyclisation takes place in the first excited state.We have carried out an MO calculation of the energy profiles for the electrocyclic concerted [l] cyclisation and ring opening processes in the cis-stilbene (I)-4a, 4b-dihydrophenanthrene (11) system3). Scheme A. Initial geometries of cis-stilbene and of 4 a , 4b-dirhydrophenanthrene and numbering system I IIOur calculations indicate that in the lowest excited electronic state, the conrotatory cyclisation of I and the reverse ring cleavage of I1 (photocyclisation and photochemical ring opening) are allowed processes1). As expected [2], this behaviour is entirely parallel to that predicted for the excited state processes in the hexatriene (111)-cyclohexadiene (IV) system by the rules of orbital symmetry conservation laid down by Woodward & Hoffnzann [l]. The similar behaviour in the systems of schemes I n the present context the terms 'allowed' and 'forbidden' refer to the magnitudes of the Boltzmann factors for the respective rate constants.

Chemistry, Stability and Bioavailability of Resveratrol

“…3). The formation of the ketonic intermediate gives indirect evidence that the 4a and 4b hydro-gens were in a trans configuration, according to Woodward-Hoffman orbital symmetry rules predicting an electron conrotatory movement cyclization [28]. 4 and 5).…”

Polymorphism Versus Thermochromism: Interrelation of Color and Conformation in Overcrowded Bistricyclic Aromatic Enes

“…[2,31,56] The photocyclization of BAEs is analogous to that of (Z)-stilbene to give 4a,4b-dihydrophenanthrene. [8,57] Substitution with bulky substituents at the 1-and 1'-positions prevents thermochromism. [2,35,53,58,59] Because substituents in the fjord region would cause prohibitive strain in planar conformations, this has been interpreted as indicating a planar thermochromic B form.…”