The week's hardest bonds: fluoroalkynes isolated, chloride oxidized, C-H coupled without metal

Four hard bonds, four new answers

Every subfield of chemistry has bonds it treats as off-limits — too unstable to isolate, too oxidizing to reach, too inert to break without a precious metal. Four preprints each take one of those bonds down. A family of molecules declared too unstable to exist gets isolated. Chloride gets oxidized to chlorine radicals by light. A C-H bond couples to a C-F bond with no metal at all. And a copper catalyst gets a ligand that outruns its own oxidation. m:…7836 m:…0880 m:…2915 m:…1281

None of the four is a finished method ready for a process chemist's shelf; all four are preprints with the usual gaps in scope and scale. But as a group they show how the frontier moves now: not by discovering new elements, but by finding the precise conditions under which the 'impossible' bond gives way. m:…7836 m:…0880 m:…2915 m:…1281

Fluoroalkynes leave the impossible list

The authors report the first successful synthesis and characterization of a diverse range of fluoroalkynes — molecules with a carbon-carbon triple bond bearing fluorine that the literature had written off as too unstable to isolate. m:…7836

The new molecules are not curiosities. The team reports six distinct classes of reactions for fluoroalkynes, including the formation of difluorocyclobutadiene dimers — the beginnings of a reaction chemistry for a functional group that previously had none. This is an opening, not yet a toolkit. m:…7836

Chlorine radicals from chloride

The authors built a photoredox system around bathophenanthroline activated by the Lewis acid ZrCl4, and used it to oxidize chloride in a single-electron step, at +2.03 V versus SCE, generating chlorine radicals. Lewis acid activation sharply increases the photocatalyst's excited-state oxidizing power. The same system promotes Giese-type alkylation of electron-deficient alkenes via chlorine radical-mediated hydrogen atom transfer. m:…0880

The radicals are put to work in Giese-type alkylation of electron-deficient alkenes via chlorine-radical-mediated hydrogen atom transfer — a route into C(sp3)-H alkylation that runs on chloride, about as cheap a reagent as chemistry offers. The +2.03 V figure is the number that matters: it puts high-potential oxidations within reach of a photoredox setup. m:…0880

C-H activation without a metal

The authors report a direct cross-coupling of methylpyridines' C(sp3)-H bonds with aryl fluorides' C(sp2)-F bonds — using silylboronate and KOtBu, and no transition metal at all. Mechanistic studies point to an ionic pathway: a heterobenzylic nucleophile and a concerted SNAr substitution. The method also works on alkyl fluorides, extending it to C(sp3)-H/C(sp3)-F coupling. m:…2915

The obvious attraction is the absence of metal: for pharmaceutical synthesis, where residual metals are a regulatory headache, a metal-free C-H activation is worth more than its yield numbers suggest. m:…2915

A ligand that outruns oxidation

The authors solved a narrower but commercially relevant problem: making enantioenriched alpha-stereogenic alkyl phosphonates from alkylboron reagents without the reaction being derailed by H-phosphonate oxidation. The trick is an anionic pincer ligand that renders Cu(II) less oxidizing than its neutral counterparts, suppressing Cu(II)-P homolysis. DFT calculations identify C(sp3)-P reductive elimination as the enantiodetermining step. m:…1281

The result is an enantioconvergent deborylative C(sp3)-P coupling: boron reagents in, enantioenriched alpha-stereogenic alkyl phosphonates out, with the side reaction outrun. Ligand design beating a competing oxidation is a small story next to isolating fluoroalkynes — but it is the kind of small story that ends up in a process. m:…1281