Photochemistry of cyclobutenes and related constrained s-cis dienes

Abstract

<p>This thesis is concerned with the development of a new qualitative model to explain the photochemistry of cyclobutene and 1,3-butadiene, in which the $\rm 2A\sb{g}{\to}1A\sb{g}$ decay channel of the van der Lugt-Oosterhoff mechanism does not actually correspond to a $\rm 2A\sb{g}$ avoided crossing pericyclic minimum. In fact, decay from the $\rm 2A\sb{g}$ excited state to the ground $\rm(1A\sb{g})$ state occurs at a conical intersection point (crossing between the excited state and ground state potential energy surfaces). The study described pertains to the photochemical ring opening of a series of alkyl-substituted bicyclic cyclobutenes (structure I), and the cis,trans isomerization of related constrained s-cis dienes (structure II). The high degree of disrotatory stereospecificity observed in the ring opening of I is not related to ring strain factors induced by the ancillary ring, but rather to the decreased flexibility in the isomeric 1,3-diene products or the rotational flexibility of the substituents on the cyclobutene double bond. These results strongly suggest that orbital symmetry selection rules are important in these reactions. The effects of constraining the C-C central bond in 1,3-dienes on the cis,trans photoisomerization process and motions around C=C bond in cyclobutene photochemical ring opening are two of the direct implications of the results obtained from these experiments.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) The results from the photochemical ring opening of alkyl-substituted monocyclic cyclobutenes provide evidence that orbital symmetry does play a significant role in the reaction, as indicated by the decrease in the quantum yields of ring opening upon syn-dimethyl substitution. In addition, the same results are exclusive of the presence of operative non-concerted pathways in the ring opening reaction. The energy requirements for the photochemical ring opening of cyclobutenes have been clearly established. Thus, the inability of 1-phenylcyclobutenes to undergo ring opening reactions has been found to be related to energetic considerations rather than polarizability of their excited states.</p>