Forward and Backward Pericyclic Photochemical Reactions Have Intermediates in Common, Yet Cyclobutenes Break the Rules

Abstract

Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes.

Predistortion amplified in the excited state

“…They involve forces connected with the excitation of the p system, for which the s bonds are a minor perturbation at most, whereas the Woodward-Hoffmann (WH) rules are based on forces and interactions that involve both p and s electrons. These rules and forces become active only later, when the distortion is sufficient that a suitable s bond can interact with the p system [34,56,97] (by contrast, one can read often that the motion follows the WH rules from the beginning. But this opinion would also be in conflict with the cases, where both conrotatory and disrotatory alternative reactions are possible, such as in several examples of Section 3.2.…”

“…In the case of polyenes, however, the minima and CIs for forward and backward reactions typically do not coincide [21,97], in contrast to pericyclic reactions, where they are in common for the two processes according to theory and experiments [56,112].…”

“…The initial acceleration is always in the direction of the FCactive coordinates, as repeatedly pointed out (e.g., in [33,56,97], in particular in [34]). This is already implied by their definition.…”

“…This includes -electrocyclic ring opening of cyclohexadiene and derivatives (even large molecules -although with relatively localized excitation -such as 7-dehydrocholesterol [40,[51][52][53]), as described above; ring closure of dienes to form cyclobutenes [54,55] and the corresponding ring opening [56], ring opening of cyclooctatriene to octatetraene [57], -formation of bicyclo [110]alkanes from dienes [48,55,58]. Also formation of bicyclo[3.1.0]alkenes from trienes must be ultrafast, as it can compete with cis-trans isomerization [28,41,42,48,49,59], -[2 + 2]-cycloaddition to cyclobutanes [60][61][62], including also the dimerization of cyclohexadiene, which at high concentration can compete with the ultrafast ring opening [2] ([2 + 2]-cycloaddition in a divinylcyclophane takes longer -13.5 ps [63] -perhaps due to unfavourable geometry and/or too much delocalization of the excitation), -sigmatropic hydrogen 1,3-and 1,7-migration [55,64-68] and a suggested antarafacial H 1,5-migration (Scheme 4 and context in [69], proposed geometry confirmed in Fig.…”

“…But this opinion would also be in conflict with the cases, where both conrotatory and disrotatory alternative reactions are possible, such as in several examples of Section 3.2. The initial motion along FC active coordinates was also crucial for explaining the partially anti-WH behavior of photochemical ring opening of cyclobutenes [56]; for some details, see Section 4). When the WH forces are activated, the direction of the slopes change, which also influences the motion of the wave packet.…”