Probing Radical Addition to 1‐Phosphabutadienes by Employing Muonium as a “Light Isotope” of Hydrogen

• Published in: Chemistry : a European journal Vol. 30; no. 2; pp. e202302869 - n/a
• Main Authors: Walsgrove, Henry T. G., Percival, Paul W., Gates, Derek P.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 08-01-2024
• Subjects: Addition polymerization; Beta decay; Cationic polymerization; Chemistry; Electron capture; Free radicals; Intermediates; Isoprene; main group elements; multiple bonds; muon spin spectroscopy; Muonium; Muons; NMR; Nuclear magnetic resonance; phosphaalkenes; Phosphorus; Polymerization; radicals; Spectroscopy; phosphaalkenes; radicals; multiple bonds; main group elements; muon spin spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202302869

Understanding free radical addition to multiple bonds is important to elucidating the mechanistic details of addition polymerization reactions, albeit the fleeting radical intermediates are very difficult to detect by conventional methodologies. Muon spin spectroscopy (μSR) is a highly sensitive method that can detect radical species at 106 spins (cf. EPR: 1012 spins, NMR: 1018 spins). Herein, we employ μSR to detect the radical‐addition products from three 1‐phosphabutadiene monomers, P‐analogues of isoprene. We show that muonium (Mu), a “light” H‐atom surrogate, adds predominantly at the C4 position of the P1=C2−C3=C4 moiety to give unprecedented 1‐phosphaallyl radicals as the major products. Our structural assignments are supported by assignment of muon, phosphorus and proton hyperfine coupling constants using DFT‐calculations. A minor radical product is also detected that is tentatively assigned to an PC3‐heterocyclic free radical. On the basis of DFT‐predictions, we speculate that its formation may involve initial addition of Mu+ at the C3 position followed by electron capture. These studies provide rare insights into the prospective radical (or cationic) polymerization of 1‐phosphabutadienes, which have previously been polymerized using anionic initiation.
The radical addition to 1‐phosphabutadienes, Mes*P=CMe−CR=CH2 (R=H, Me) using muonium (Mu) as an H‐atom analogue affords unprecedented, delocalized phosphaallylic radicals: Mes*P⋅−CMe=CR(CMuH2)↔Mes*P=CMe−⋅CR(CMuH2). Structural elucidation was accomplished using muon spin spectroscopy (μSR) which takes advantage of the nuclear spin properties of the Mu (Mu=μ++e−). This study provides important insights into the radical reactivity of low‐coordinate phosphorus species, which are actively being investigated for their uses in functional macromolecule formation.