Radical Addition Rate Constants to Acrylates and Oxygen: α-Hydroxy and α-Amino Radicals Produced by Photolysis of Photoinitiators

Laser flash photolysis of α-hydroxy and α-amino ketones, which are used as photoinitiators in free radical polymerization, lead to the generation of a series of nucleophilic α-hydroxy and α-amino radicals. Absolute addition rate constants of these radicals to n-butylacrylate and oxygen were measured by laser flash photolysis employing an indirect probe technique. Crystal violet and N,N‘-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide were used as selective probe molecules for these nucleophilic initiator radicals to measure the addition rate constants to n-butylacrylate and oxygen, respectively. High acrylate addition rate constants of some initiator radicals were found in acetonitrile solution, e.g., dimethylketyl radical (kacrylate = 1.3 × 107 M-1 s-1) and 2-morpholino propan-2-yl radical (kacrylate = 2.9 × 107 M-1 s-1)

Highly Diastereoselective Dioxetane Formation in the Photooxygenation of Enecarbamates with an Oxazolidinone Chiral Auxiliary: Steric Control in the [2 + 2] Cycloaddition of Singlet Oxygen through Conformational Alignment

The photooxygenation of oxazolidinone-substituted enecarbamates leads to diastereomerically pure dioxetanes. The high diastereoselectivity is rationalized in terms of effective π-facial control achieved by shielding one side of the double bond with the chiral auxiliary. The absolute configuration of the dioxetanes is assigned by derivatization to diols

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

Addition of two radicals (diphenyl phosphinoyl and 2-hydroxy-2-propyl) to the bifunctional alkene, vinyl acrylate, was studied by both time-resolved (TR) and steady-state (SS) ESR and laser flash photolysis (LFP). The adduct radicals are predominately a result of tail addition (addition to the unsubstituted carbon atom) of the acrylate double bond. Chemical structures of the adducts were established by comparison of the observed ESR spectra with those of adducts of the same reactive radicals to structurally related alkenes, tert-butyl acrylate and vinyl pivalate, which have only one type of double bond. Adducts of bulky phosphinoyl radicals to the acrylates demonstrate hindered rotation and a cis−trans isomerization at room temperature. The structure of the adduct radicals and the reactivity of the two radicals are discussed. Absolute rate constants for the addition of the phosphinoyl radical to the alkenes were measured by LFP in ethyl acetate at 296 K. A rate constant of kadd = (33 ± 1) × 106 M-1s-1 was found for vinyl acrylate. The latter value is ∼1.5 times higher than that for tert-butyl acrylate (kadd = (22 ± 1) × 106 M-1s-1) and ∼17 times higher than that for vinyl pivalate (kadd = (2.0 ± 0.1) × 106 M-1s-1). These rate constants are consistent with conclusions derived from the ESR data. The results provide some insights into free radical polymerization of vinyl acrylate and vinyl ethers

Control of the Mode Selectivity (Ene Reaction versus [2 + 2] Cycloaddition) in the Photooxygenation of Ene Carbamates: Directing Effect of an Alkenylic Nitrogen Functionality

The geometry of the double bond in oxazolidinone-substituted ene carbamates controls the mode selectivity (ene reaction versus [2+2] cycloaddition) of singlet oxygen through stereoelectronic effects, whereas the chiral auxiliary provides high diastereoselectivity through steric shielding

Spectroscopic Characterization of the Surface of Iron Oxide Nanocrystals

The surface of highly monodisperse magnetic iron oxide (γ-Fe2O3) nanocrystals was thoroughly investigated by FTIR, NMR, and mass spectroscopy. The nanocrystals were prepared by a thermal decomposition method now widely used for the preparation of magnetic metal and metal oxide nanocrystals. This method takes advantage of oleic acid as a means to passivate the surface and render the particles stable with respect to aggregation or grain growth and keeps them highly dispersed in a variety of organic media. The nature of this surface in terms of ligand structure and the role of oleic acid during the synthesis remained somewhat undetermined until this report. We provide spectroscopic evidence of an oleic acid ligand structural change during γ-Fe2O3 nanocrystal synthesis

Time Resolved CW-EPR Spectroscopy of Powdered Samples: Electron Spin Polarization of a Nitroxyl Radical Adsorbed on NaY Zeolite, Generated by the Quenching of Excited Triplet Ketones

Chemically induced dynamic electron polarization (CIDEP) generated in a faujasite zeolite (NaY) by the interaction between a stable free radical (4-oxo-TEMPO) and the triplet state of benzophenone was investigated by time-resolved electron spin resonance spectroscopy (TR-CW-EPR). The TR-CW-EPRs were performed by either pulling a long tube containing powdered zeolite through the EPR cavity during the laser irradiation, or by flowing a liquid transport medium (polydimethylsiloxane) for the zeolite powder, through a flat cell in the EPR cavity. CIDEP was observed for intermolecular triplet quenching (benzophenone triplets with 4-oxo-TEMPO) and intramolecular triplet quenching using a covalently linked TEMPO-benzophenone molecule. The identification of the polarized nitroxide structure was confirmed by employing both 14N and 15N 4-oxo-TEMPO isotopomers. The kinetics of the triplet quenching inside the zeolite were studied by diffuse reflection laser flash photolysis

Rh(III)-Photosensitized Interconversion of Norbornadiene and Quadricyclane

The utility of two Rh(III) diimine complexes, Rh(phen)33+ and Rh(phi)2(phen)3+ (phen = 1,10-phenanthroline, phi = 9,10-phenanthrenequinone diimine), as sensitizers for the interconversion of norbornadiene (N) and quadricyclane (Q) has been investigated using steady-state photochemical and laser flash photolysis (LFP) techniques. Irradiation of acetonitrile solutions of Rh(phen)33+ and N causes slow conversion to Q. The reaction is reversible; irradiation of Rh(phen)33+ in the presence of Q leads to N. Irradiation of acetonitrile solutions of Rh(phi)2(phen)3+ and Q yields N. However, this reaction is irreversible; irradiation of the Rh(III) complex in the presence of N fails to afford Q. Irradiation of methanol solutions of either Rh(III) complex in the presence of N or Q affords minor amounts of two methanol-C7 adducts but fails to quench the N−Q interconversion reaction. The results are consistent with N−Q interconversion via an exciplex intermediate. The Rh(III)-sensitized deazatization of two cyclic azoalkane derivatives (Azo-N, Azo-Q) of N and Q was also investigated. Deazatization was achieved by Rh(phen)33+ but not Rh(phi)2(phen)3+ sensitization. The results are consistent with a mechanism involving triplet energy transfer, but the involvement of exciplex intermediates cannot be ruled out. Bimolecular rate constants for quenching of the Rh(III) excited states by N, Q, Azo-N, and Azo-Q were determined by LFP

A Time-Resolved Electron Paramagnetic Resonance Investigation of the Spin Exchange and Chemical Interactions of Reactive Free Radicals with Isotopically Symmetric (¹⁴N−X−¹⁴N) and Isotopically Asymmetric (¹⁴N−X−¹⁵N) Nitroxyl Biradicals

Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N−X−N) were studied by time-resolved electron paramagnetic resonance (TR-EPR). Reactive spin-polarized free radicals (r#), with non-Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6-trimethylbenzoyl)diphenyl phosphine oxide (the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals (14N−X−14N) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing 15N and the other nitroxyl bearing 14N (14N−X−15N), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r# with the asymmetric ortho-nitroxyl biradical (14N−O−15N), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (14N−O−15N)# (product of spin exchange with r#) and two polarized monoradicals (r14N−O−15N)# and (14N−O−15Nr)# (products of chemical reaction with r#) were observed. The latter three species possess three distinct TR-EPR spectra with different line splittings. The relative cross sections for spin exchange (Rex) and chemical reaction (Rrxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, Rex, between r# and (N−X−N) biradical is estimated to be 4−6 times larger than the cross section of chemical reaction, Rrxn, between r# and (N−X−N). The para-nitroxyl biradical (14N−P−15N) exists in weak spin exchange, and behaves as an equimolar mixture of 14N and 15N mononitroxyls

Mechanistic Studies of Photoinitiated Free Radical Polymerization Using a Bifunctional Thioxanthone Acetic Acid Derivative as Photoinitiator

A bifunctional photoinitiator for free radical polymerization, thioxanthone catechol-O,O′-diacetic acid, was synthesized, characterized, and compared to photoinitiator parameters of the monofunctional analogue, 2-(carboxymethoxy)thioxanthone. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerizations of methyl methacrylate show that the bifunctional photoinitiator is more efficient in polymer generation than the monofunctional derivative. These studies suggest that initiator radicals are generated from a π−π* triplet state in an intramolecular electron transfer, followed by proton transfer and decarboxylation to generate alkyl radicals, which initiate polymerization. The initial electron transfer is faster for the bifunctional photoinitiator than the monofunctional derivative, which is based on laser flash photolysis studies. Because of the relatively fast intramolecular radical generation from the triplet state (triplet lifetime = 490 ns), quenching by molecular oxygen is insignificant and polymerization of methyl methacrylate proceeds efficiently without deoxygenation. At higher concentrations of initiator (∼5 mM) intermolecular electron transfer competes with intramolecular electron transfer. Both processes, inter- and intramolecular processes, yield initiating alkyl radicals

CIDEP from a Polarized Ketone Triplet State Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution

Thioxanthone and benzil derivatives were incarcerated into an octa acid nanocapsule. Photoexcitation of these ketones generated electronic triplet excited states, which become efficiently quenched by positively charged nitroxides adsorbed outside on the external surface of the negatively charged nanocapsule. Although the triplet excited ketone and quencher are separated by a molecular wall (nanocapsule), quenching occurs on the nanosecond time scale and generates spin-polarized nitroxides, which were observed by time-resolved EPR spectroscopy. Because opposite signs of spin polarization of nitroxides were observed for thioxanthone and benzil derivatives, it is proposed that the electron spin polarization transfer mechanism of spin-polarized triplet states to nitroxides is the major mechanism of generating nitroxide polarization