Substituent Effects in Mechanochemical Allowed and Forbidden Cyclobutene Ring-Opening Reactions

Abstract

Woodward and Hoffman once jested that a very powerful Maxwell demon could seize a molecule of cyclobutene at its methylene groups and tear it open in a disrotatory fashion to obtain butadiene (Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry. Angew. Chem., Int. Ed. 1969, 8, 781−853). Nearly 40 years later, that demon was discovered, and the field of covalent polymer mechanochemistry was born. In the decade since our demon was befriended, many fundamental investigations have been undertaken to build up our understanding of force-modified pathways for electrocyclic ring-opening reactions. Here, we seek to extend that fundamental understanding by exploring substituent effects in allowed and forbidden ring-opening reactions of cyclobutene (CBE) and benzocyclobutene (BCB) using a combination of single-molecule force spectroscopy (SMFS) and computation. We show that, while the forbidden ring-opening of cis-BCB occurs at a lower force than the allowed ring-opening of trans-BCB on the time scale of the SMFS experiment, the opposite is true for cis-and trans-CBE. Such a reactivity flip is explained through computational analysis and discussion of the so-called allowed/forbidden gap.

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…22 Substituent effects can differ depending on the reaction pathway; for example, cyclobutene-based polymers containing trans-alkyl handles provided more mechanical leverage than trans-ester handles in conrotatory reactions, whereas cis-ester handles gave more mechanical leverage than cis-alkyl handles in disrotatory reactions. 23 The enhanced mechanical leverage of dialkyl relative to diester attachments was consistent with quantum chemical calculations reported previously on trans-cyclobutene derivatives. 24 In similar polymers, replacing a methylene in the pulling attachment with a phenyl group dropped the force necessary to achieve a given rate constant in SMFS experiments by a factor of three, which was attributed to a combination of electronic stabilization and mechanical leverage effects.…”

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…19 This effect has now been observed for several types of polymer with different substituents. [20][21][22] However, the lever arm has not yet been detected for molecular or organometallic systems, such as ZDDP.…”

The Contributions of Model Studies for Fundamental Understanding of Polymer Mechanochemistry

“…Examples include the force-accelerated dissociation of Diels-Alder adducts of anthracene and maleimide, 52 for which computations reveal the minimum-energy mechanism that switches from concerted at <0.2 nN to stepwise at higher force and isomerization of cis-3,4-disubstituted cyclobutenes to butadienes, where an open-shell singlet transition state becomes more stable than the closed-shell analog as force increases and is responsible for the domination of the "orbitally-forbidden" isomerization path at tensile force >1.5 nN as evidenced both by computation and by sonication experiments on cis-3,4-disubstituted cyclobutene containing chains (Figure 5). 45,53 Figure 5-Tensile changes the reaction mechanism of the dissociation of anthracene-maleimide adduct and of the isomerization of cis-3,4-substituted cyclobutenes. In anthracene-maleimide adducts the crossover occurs at low force (~200 pN), whereas in the isomerization the two mechanisms remain kinetically competitive over a ~1.5 nN range.…”

“…Examples include the force-accelerated dissociation of Diels–Alder adducts of anthracene and maleimide, 52 for which computations reveal the minimum-energy mechanism that switches from concerted at <0.2 nN to stepwise at higher force and isomerization of cis -3,4-disubstituted cyclobutenes to butadienes, where an open-shell singlet transition state becomes more stable than the closed-shell analogue as force increases and is responsible for the domination of the ‘orbitally forbidden’ isomerization path at tensile force >1.5 nN as evidenced both by computation and by sonication experiments on cis -3,4-disubstituted cyclobutene containing chains (Figure 5 ). 45 53…”