Conservation of Orbital Symmetry can be Circumvented in Mechanically Induced Reactions

Abstract

In first-principles molecular dynamics simulations of the mechanically induced ring-opening of substituted benzocyclobutene we observe both con- and disrotatory ring-opening reactions. We show that this finding does not contradict the fundamental principle that the orbitals develop continuously in time. However, it constitutes an exception from the principle of the conservation of orbital symmetry and thus is indeed an exception from the Woodward-Hoffmann rules. In contrast, the ring-opening of unsubstituted cyclobutene proceeds in a conrotatory fashion. This shows that the breaking of the Woodward-Hoffmann rules is significantly facilitated by the substituents.

Theoretical Approaches for Understanding the Interplay Between Stress and Chemical Reactivity

“…Five independent trajectories were evaluated at each pulling rate. All AIMD simulations showed that ring opening of cyclobutene occurred exclusively through the conrotatory pathway, which is in agreement with the previous study using similar methods [33], but is inconsistent with EFEI calculations [29,31,34], previous SMD simulations [30], and experiments performed under mechanochemical conditions [19]. Structures observed during the simulation performed with a pulling rate of 0.5 Å/ps are shown in Fig.…”

“…The mechanochemical ring opening of cyclobutene was also investigated using a time-dependent distance constraint to mimic the application of F during MD simulations [33]. In those simulations, the constraint was applied between hydrogen atoms bonded to the carbon atoms in the scissile bond of cyclobutene that were in a cis arrangement with respect to the ring in an attempt to induce disrotatory opening.…”

Should the Woodward–Hoffmann Rules be Applied to Mechanochemical Reactions?

“…Thus, the WH “demon” has been demonstrated to be successful in redirecting the course of electrocyclic reactions to yield products that formally violate the WH rules, yet the mechanistic reason remained open. With this in mind, ab initio molecular dynamics simulations have been carried out for cyclobutene derivatives by pulling them with a constant velocity 16. According to frontier orbital analysis along WH‐forbidden trajectories (see Figure 4 b therein), it has been concluded that “Conservation of Orbital Symmetry can be Circumvented in Mechanically Induced Reactions”, which leads to what has been called “anti‐Woodward–Hoffmann products” 16.…”

“…With this in mind, ab initio molecular dynamics simulations have been carried out for cyclobutene derivatives by pulling them with a constant velocity 16. According to frontier orbital analysis along WH‐forbidden trajectories (see Figure 4 b therein), it has been concluded that “Conservation of Orbital Symmetry can be Circumvented in Mechanically Induced Reactions”, which leads to what has been called “anti‐Woodward–Hoffmann products” 16. A distinctly different point of view has been adopted in a study of cyclobutene and cyclohexadiene where is has been concluded lucidly “…︁ that applying F ext can activate the forbidden reaction without changing the electronic state.”15 (where F ext is a constant external force).…”

Force‐Transformed Free‐Energy Surfaces and Trajectory‐Shooting Simulations Reveal the Mechano‐Stereochemistry of Cyclopropane Ring‐Opening Reactions

“…Gathering information on the possibilities offered by the use of forces in the realm of reactivity is not only extremely valuable for a fundamental understanding of chemical reactions under external mechanical forces, but also has the potential to dramatically influence both synthetic chemistry [16] and materials science. [2,22,23] Cyclobutene-based mechanophores have received a lot of attention [12,13,17,20,[24][25][26][27][28][29][30] whereas cyclopropane systems, such as gem-dihalogencyclopropane derivatives, [10,15,19] are under-researched in the realm of CMC. The force-induced electrocyclic ring-opening of these compound classes has furnished striking results.…”