Regioselectivity of the Photoisomerization of Acyclic 1,3-Dienes and Stable Trans Cycloheptenes

Abstract

There is still no complete understanding of how the visual system controls the regioselectivity of the photoisomerization of the C11=C12 double bond in the visual chromophore retinal. In this work we used a series of acyclic 1,3-dienes, as models of the visual system. We investigated effects of substituents on the dienes photochemistry. The substituents were chosen to reveal the relative importance of inertial mass, charge stabilizing capability and steric size in controlling which of the two bonds of the diene preferentially photoisomerized. The photochemical experiments were conducted in solvents of different polarity to examine the extent of charge development in the excited state pathways that were traversed. The dienes that were chosen, synthesized and purified were isomers of 6,6- dimethyl-2,4-heptadiene (DMH), 2,4-nonadiene (ND) and 1,1,1-trifluoro-2,4-hexadiene (TFHD). In addition, we attempted to synthesize 1,2-dideutero-1,3-pentadiene (DDPD). To explain the observed substituent effects, a hybrid mechanism was proposed, where the charge stabilizing capability of the substituents determined which specific conical intersection was preferred and thus which double bond was preferentially photoisomerized. Previously in our group it was determined that the decay of trans-cycloheptene to its cis isomer does not occur via “direct twisting” about the double bond. Rather it occurs through a dimerization of two trans molecules to produce a 1,4-biradical which quickly geometrically ii relaxes and cleaves to form two cis-cycloheptene molecules. If this bimolecular mechanism could be inhibited sterically, there is a possibility that one could produce room temperature stable trans-cycloheptenes. To test this possibility, we chose to examine the stability of three allylically substituted trans-cycloheptenes, 3,3,7,7-tetramethyl (TMCHP) and syn and anti 3,7-di-t-butyl (DTBCHP) trans-cycloheptene. Using a variety of NMR techniques and high level ab initio calculations, we were able to prove that we could photochemically produced these stable trans-cycloheptenes which should now be considered the smallest trans cycloalkenes to be persistent at room temperature.