Mechanochemical Synthesis of Elusive Fluorinated Polyacetylene

Abstract

<p>Polymer mechanochemistry has traditionally been employed to study the effects of mechanical force on one or two chemical bonds within a polymer. It is underexploited for the scalable synthesis of wholly new materials by activating bonds along the entire polymer, especially products inaccessible by other means. Herein we utilize polymer mechanochemistry to synthesize fluorinated polyacetylene, a long-sought-after air-stable polyacetylene that has eluded synthesis by conventional means. Our synthetic approach proceeds via ultrasonication of a force-responsive precursor polymer that was synthesized in five steps on gram scale. The synthesis is highlighted by rapid incorporation of fluorine in an exotic photochemical cascade whose mechanism and exquisite diastereoselectivity were elucidated by computation. </p>

Role of the Perfluoro Effect in the Selective Photochemical Isomerization of Hexafluorobenzene

“…This wavelength was selected to match the 255 nm lamps employed in gram scale reactors for these syntheses. 4 The transient absorption spectrum is rendered at a series of pump−probe delays denoted by the color-coordinated legend. The stick spectrum represents the computed gas-phase excited state absorption spectrum at the S 1 minimum energy geometry.…”

“…In the demonstration of this reaction by Burns et al, the ratio of photons to molecular reagents was 168:1 and achieved a 41% synthetic yield. 4 Interpreting this through the lens of these results shows that although most absorption events result in the reformation of the D 6h HFB, provided there is a small but exclusive pathway leading to Dewar-HFB, constant upcycling of the HFB by using a high molar excess of photons still yields an effective reaction.…”

“…3 Not only is difluoroacetylene an explosive and pyrophoric gas, but polymerization results in a highly irregular material that contains a mixture of −CF, −CF 2 , and −CF 3 functional groups. Burns and co-workers 4 recently achieved this elusive synthesis by leveraging the mechanochemical "unzipping" of highly strained ladderene polymers to form partially fluorinated polyacetylenes. The key step in their synthetic scheme is the formation of a fluorinated ladderene, achieved through a photochemical cascade reaction, involving a hexafluorobenzene (HFB) [2 + 2]-cycloaddition, followed by a disrotatory 4πelectrocyclic ring-closing of the hexafluorinated diene intermediate (Figure 1b).…”

“…This work utilizes ultrafast time-resolved spectroscopic studies, multireference calculations, and nonadiabatic molecular dynamics simulations to clarify the photophysics and origin of the low yielding, chemoselective isomerization to Dewar-HFB. While this photochemical reaction is synthetically useful, 4 this work shows why this photochemical reaction is inefficient and emphasizes the difficulty in tracking such reaction pathways.…”

Heterocyclic Mechanophores in Polymer Mechanochemistry

“…71 Covalent bonds in strained ring structures have lower BDEs than the analogous linear structures and can respond to mechanical forces. Strained homocyclic rings, including gem -dihalocyclopropane, 11 , 72 73 74 75 76 77 78 79 80 81 82 cyclobutane, 24 , 83 84 85 86 87 88 89 90 polyladderene, 30 , 91 92 93 94 95 benzocyclobutene, 32 74 , 96 97 98 99 100 cyclopentadiene-maleimide Diels–Alder (DA) adduct, 101 and norbornene, 27 have been reported to be mechanophores (Figure 2c ). Likewise, heterocycles with ring strains or particular electronic features can be efficient mechanophores.…”