Laser Flash Photolysis and Time-Resolved ESR Study of Phosphinoyl Radical Structure and Reactivity

Abstract

The photochemistry of two phosphine oxides and the rate constants of reaction of their daughter radicals with several alkenes, halocarbons, and oxygen have been determined. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) in each case affords a phosphinoyl and a benzoyl radical. The phosphinoyl radicals are readily detected by laser flash photolysis and exhibit absorption maxima at 325 and 450 nm for the diphenylphosphinoyl (3) and 2,6-dimethoxybenzoyl-2,4,4-trimethylpentylphosphinoyl (6) radicals, respectively. The rate constants for reaction of the phosphinoyl radicals with alkyl halides, alkenes, and oxygen range from 104 to 109 M-1 s-1. Radical 3 is 2−6 times more reactive than radical 6. For example, 3 adds to methyl methacrylate with a rate constant of (11 ± 2) × 107 M-1 s-1 whereas 6 has an addition rate constant for the same reaction of (2.3 ± 0.3) × 107 M-1 s-1. The rate constants for reaction with alkyl halides decrease with increasing C−X bond strength, while the rate constants for quenching by acrylates decrease with increasing methyl substitution on the β-carbon. The 2,6-dimethoxybenzoyl (5) and phosphinoyl (6) radicals derived from 4 are readily detected by time-resolved ESR (TR ESR); benzoyl radical 5 appears as a singlet and phosphinoyl radical 6 appears as a doublet of triplets (A(P) = 285 G, A(H) = 4.8 G). The CIDEP patterns of 5 and 6 indicate that the radicals are formed from α-cleavage of the triplet excited state of 4. TR ESR has also proved useful in the direct detection of the polarized benzyl radicals formed from addition of phosphinoyl radicals 3 and 6 to styrene and 2,4,6-trimethoxystyrene. The lower reactivity of 6 compared to 3 is attributed to its more planar structure and lower degree of spin localization in a s-orbital on phosphorus.

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

“…36 The transient absorption decayed in the microsecond time scale. 37 This transient absorption was assigned to the phosphinoyl radical 1c. 36 Addition of alkenes (A, e.g., 5a, 6a, and 7a) reduce the lifetime of 1c.…”

“…We report here a systematic study using standard photoinitiators (Scheme 1) of the addition of two well-studied radicals (phosphinoyl [35][36][37] and 2-hydroxy-2-propyl [38][39][40] ) to the bifunctional monomer, vinyl acrylate. The radicals were photochemically generated from precursors that are known to give strong spin polarized radicals.…”

Direct and Sensitized Photolysis of Phosphine Oxide Polymerization Photoinitiators in the Presence and Absence of a Model Acrylate Monomer: A Time Resolved EPR, Cure Monitor, and PhotoDSC Study

“…These constants were used in the simulation of TR EPR spectra of spin-adducts ( Figure 3) and are in agreement with the reported values for radical adducts to double bonds. 4,24,25 The line width of the TR EPR of the spin adducts shows an alternation (Figure 3). In particular, the outermost components of the multiplets are narrower than the inner components.…”

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

“…73,74,93,103,104 Figure 3 shows the ESE signal intensity of the diphenylphosphinoyl radical 1a vs time in the absence (Figure 3a) and in the presence (Figure 3b,c) of n-butyl acrylate 6 at different concentrations. Fits to eq 3 are superimposed over the data, and the bottom traces are fit residuals.…”

“…A direct comparison with our results is difficult, because the absolute rates from the literature were determined for different alkenes and/or in different solvents, and therefore, we repeated the measurements for the addition of 4a to the same set of alkenes 5-10 in toluene by using the well-established line width method with TR CW ESR detection. 35 In contrast to radicals 1a-3a, with their huge phosphorus coupling constants of a couple of tens of millitesla, which are extending the spectral range of the adduct radicals, 73,74,93 the dimethyl ketyl 4a has a much smaller one of a(6H ) ) 1.933 ( 0.005 mT, resulting in an interference of its ESR spectrum with the resonance lines of the adduct radicals resulting from the addition of 4a to the double bond (for examples see refs 48, 54, 74, 102, and 116). The overlapping of ESR lines complicates the line width analysis.…”