97142-01-3Relevant academic research and scientific papers
The Photodynamic Covalent Bond: Sensitized Alkoxyamines as a Tool to Shift Reaction Networks Out-of-Equilibrium Using Light Energy
Herder, Martin,Lehn, Jean-Marie
supporting information, p. 7647 - 7657 (2018/06/26)
We implement sensitized alkoxyamines as "photodynamic covalent bonds" - bonds that, while being stable in the dark at ambient temperatures, upon photoexcitation efficiently dissociate and recombine to the bound state in a fast thermal reaction. This type of bond allows for the photochemically induced exchange of molecular building blocks and resulting constitutional variation within dynamic reaction networks. To this end, alkoxyamines are coupled to a xanthone unit as triplet sensitizer enabling their reversible photodissociation into two radical species. By studying the photochemical properties of three generations of sensitized alkoxyamines it became clear that the nature and efficiency of triplet energy transfer from the sensitizer to the alkoxyamine bond as well as the reversibility of photodissociation crucially depends on the structure of the nitroxide terminus. By employing the thus designed photodynamic covalent bonding motif, we demonstrate how to use light energy to shift a dynamic covalent reaction network away from its thermodynamic minimum into a photostationary state. The network could be repeatedly switched between its minimum and kinetically trapped out-of-equilibrium state by thermal scrambling and selective photoactivation of sensitized alkoxyamines, respectively.
Kinetics of nitroxide radical trapping. 2. Structural effects
Bowry,Ingold
, p. 4992 - 4996 (2007/10/02)
Laser flash photolysis and kinetic competition product demonstrated that in isooctane at ambient temperature the rate constant for coupling of carbon-centered with presistent nitroxides, kT, upon the of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of kT are observed for changes in both the structure of the nitroxide the structure of the carbon radical. Thus, for any particular carbon kT is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the "usual" di-tert-alkyl nitroxides kT decreases along the series, 1,1,3,3-tetra-methylisoindoline-2-oxyl ≥ 2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo) > di-tert-butyl nitroxide, i.e., kT decreased on going from a five-membered ring to a six-membered ring to a noncyclic structure. Cyclopropyl triphenylmethyl are trapped at the fastest and slowest rates, respectively, the corresponding kT value 3.0 × 109 and 1.2 × 108 M-1 s-1 for ABNO and 2.1 × 109 6 M-1 s-1 for Tempo. Steric effects in the carbon radicals are for Tempo than for ABNO. For example, the ratio of kT's for the trapping of nonyl tert-butyl is 1.7 for Tempo but 1.3 for ABNO, while for the trapping of benzyl cumyl the ratio of kT's is 4.1 for Tempo 0.9 for ABNO. The effect of resonance stabilization can be illustrated by the kT values for three sterically unhindered primary radicals, n-nonyl, benzyl, 2-naphthyhmethyl, viz., 1.2 × 109, 4.8 × 108, and 5.7 × 107 M-1 s-1, respectively, for Tempo and 2.2 × 109, 1.2 × 109, and 8.1 × 108 M-1 s-1, respectively, for ABNO.
RECTIONS OF HYDROXYL RADICALS WITH POLYMERIZABLE OLEFINS
Grant, Richard D.,Rizzardo, Ezio,Solomon, David H.
, p. 379 - 384 (2007/10/02)
The reactions of hydroxyl radicals with methyl acrylate, methyl methacrylate, styrene, and α-methylstyrene have been investigated by a trapping technique which uses 1,1,3,3-tetramethylisoindolin-2-yloxyl (1) as a radical scavenger.The major reaction pathway in each case was addition to the unsubstituted (tail) end of the monomer, although addition to the substituted (head) end and reaction with the double-bond substituents were also observed.Absolute rates of reaction were estimated by means of competition experiments using cyclohexane as reference substrate.The implications of the results for the structure of polymers initiated by hydroxyl radicals are discussed.
