Aminoalkyl Radicals Research Articles

Deconstructive fluorination of cyclic amines by carbon-carbon cleavage.

Deconstructive functionalizations involving scission of carbon-carbon double bonds are well established. In contrast, unstrained C(sp3)-C(sp3) bond cleavage and functionalization have less precedent. Here we report the use of deconstructive fluorination to access mono- and difluorinated amine derivatives by C(sp3)-C(sp3) bond cleavage in saturated nitrogen heterocycles such as piperidines and pyrrolidines. Silver-mediated ring-opening fluorination using Selectfluor highlights a strategy for cyclic amine functionalization and late-stage skeletal diversification, establishing cyclic amines as synthons for amino alkyl radicals and providing synthetic routes to valuable building blocks.

Synthesis of Dihydrophenanthridines by Dehydrogenative Coupling

Key words cross-dehydrogenative coupling - dihydro-phenanthridines - aminoalkyl radicals - biaryl amines

One-component thioxanthone-based polymeric photoinitiators

The photochemical and photophysical properties of new polymeric photoinitiators (PPIs) containing pendant thioxanthone (TX) and amine moieties are studied. The PPIs are synthesized by copolymerization of tert-butyl 2-((9-oxo-9H-thioxanthen-2-yloxy)methyl)acrylate (TX1) with N,N-dimethylaminoethyl methacrylate (DMAEM) at two different ratios using free radical polymerization. UV–vis spectra indicate that PPIs possess similar absorption characteristics to TX1 in the violet range (∼400 nm; absorption red-shift 20 nm). The photochemical mechanisms are studied by electron spin resonance (ESR), steady state photolysis, laser flash photolysis, and cyclic voltammetry. ESR studies indicate formation of two different aminoalkyl radicals on the hydrogen donor amine. The triplet state of the PPIs is short-lived compared to isopropyl thioxanthone and TX1, due to the built-in amine functionality. Photopolymerization of trimethylolpropane triacrylate (TMPTA) initiated by these photoinitiators under LED exposure at 385 and 405 nm using real-time FTIR spectroscopy shows that they exhibit higher efficiency than TX/N-methyldiethanolamine (MDEA) and TX1/MDEA systems with the advantage of a much higher molecular weight that can be very helpful to overcome migration issues. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3370–3378

ChemInform Abstract: Visible Light Photoredox Catalysis: Regioselective Radical Addition of Aminoalkyl Radicals to 2,3‐Allenoates.

AbstractUnsaturated γ‐aminobutyric ester derivatives are efficiently obtained by a simple method.

Visible light photoredox catalysis: regioselective radical addition of aminoalkyl radicals to 2,3-allenoates

Visible-light-mediated regioselective addition of α-aminoalkyl radicals to 2,3-allenoates catalyzed by a photoredox catalyst was developed. The scope and generality of the substrates were broad.

Enhanced photopolymerization of dimethacrylates with ketones, amines, and iodonium salts: The CQ system

AbstractThe kinetics and mechanism of the photopolymerization of dimethacrylates using three‐component initiation systems consisting of camphorquinone (CQ), diphenyliodonium hexafluorophosphate (Ph2IPF6), and either N,N,3,5‐tetramethylaniline (TMA) or N,N‐dimethylbenzylamine or triethylamine were studied by photo‐DSC and UV‐visible spectroscopy. The effect of monomer structure on the curing kinetics and photobleaching were also investigated. Photo‐DSC studies showed fivefold increases in polymerization rate when all three components were present and the kinetics followed the trend: CQ/amine/Ph2IPF6 ≫ CQ/amine > CQ/Ph2IPF6.. For both CQ/amine/Ph2IPF6 and CQ/amine systems, the CQ was rapidly photobleached during the photo‐DSC timescale but for the systems without amine there was an induction stage before CQ photobleaching was evident. Studies of the effect of monomer type on the photobleaching rate show that the photobleaching behavior was independent of monomer structure. Three photoinitiation mechanisms have been proposed. The reaction mechanism of the CQ/amine/Ph2IPF6 system involves the reduction of the excited CQ molecule by the amine to form ketyl and aminoalkyl radicals, followed by the irreversible oxidation of the amine, and to a lesser extent, the ketyl radical by the iodonium salt, to form an initiating radical. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Reaction between aminoalkyl radicals and akyl halides: Dehalogenation by electron transfer?

Aminoalkyl radicals, such as Et2NCHCH3, have low oxidation potentials and are therefore powerful reducing agents. We have found that Et2NCHCH3 reacts with CCl4 and CBr4 in di-tert-butyl peroxide with bimolecular rate constants (measured by LFP) close, or equal, to the diffusion-controlled limit. For the less reactive halide, CH2Br2, the reaction rate is increased substantially by the addition of acetonitrile as a co-solvent. It is tentatively concluded that these reactions occur by electron-transfer from the aminoalkyl to the organohalide with formation of the iminium ion, Et2N+CHCH3 (NMR detection), halide ion and a halomethyl radical, e.g., CCl3 and CHCl2 (ESR, spin-trapping detection).

On the chain length effect: Comparative reactivity of a macroradical and a radical

The reactivity of two aminoalkyl radicals formed from ethyldimethylaminobenzoate (EDB) and a polymeric derivative (PoEDB) toward different additives such as oxygen, TEMPO, methylacrylate MA, phenols, an onium salt, CBr 4 is investigated through Laser Flash Photolysis. The transient absorption spectra of the radicals are in a good agreement with the spectra calculated by using molecular orbitals calculations. The interaction rate constants with the different additives are measured. The reactivity of EDB and PoEDB radicals appears as quite similar. Quantum mechanical calculations support the experimental results.

Halogen abstraction reaction between aminoalkyl radicals and alkyl halides: Unusual high rate constants

The very high reactivity of aminoalkyl radicals toward the halogen abstraction reaction is reported for the first time. Reaction rate constants with CCl 4 and CBr 4 are close to the diffusion limit: they are about 4–5 orders of magnitude higher than those previously determined for typical alkyl radicals. A better understanding of this unusual behavior is obtained using molecular orbitals (MO) calculations. The participation of polar effects is directly evidenced. This approach can be useful for the design of new reducing agents.

Chemical incorporation of thioxanthone into β-cyclodextrin and its use in aqueous photopolymerization of methyl methacrylate

Photoinitiated free radical polymerization of methyl methacrylate (MMA) in aqueous solution via hydrogen abstraction mechanism was described. For this purpose, thioxanthone (TX) chromophoric group was chemically incorporated into β-cyclodextrin (β-CD) by a simple esterification process. The resulting thioxanthone photoinitiator (TX–β-CD) exhibited similar spectral characteristics and photoactivity to that of the parent TX molecule. Host guest complexes of MMA with TX–β-CD in water, in the presence of N-methyldiethanol amine (NMDEA) as a hydrogen donor, facilitated photoinitiated free radical polymerization in aqueous medium. The postulated mechanism is based on the intermolecular reaction of photoexcited triplet TX moiety of TX–β-CD with NMDEA. The resulting amino alkyl radicals initiate the polymerization.

Interaction of monomer radicals with nitroxides: A new access to the radical-radical combination rate constants

Acrylate, methacrylate and styrene monomer radicals produced by the addition of an aminoalkyl radical to the corresponding monomer unit were directly observed by laser flash photolysis. These radicals are characterized by a strong absorption in the near UV/visible part of the spectrum allowing to directly follow their interaction with different nitroxides usable in nitroxide mediated polymerization (NMP). The recombination rate constants were measured: they are in agreement with the literature data when available; other values are new. This procedure is fast and reliable.

Aminoalkyl Radicals: Direct Observation and Reactivity toward Oxygen, 2,2,6,6-Tetramethylpiperidine-N-oxyl, and Methyl Acrylate

The reactivity of 11 aminoalkyl radicals toward different additives [oxygen, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and methyl acrylate (MA)] has been investigated through laser flash photolysis and quantum mechanical calculations. The transient absorption spectra of the radicals were recorded: good agreement was found with the spectra calculated by using quantum mechanical calculations. All the interaction rate constants were measured. A large range of values are obtained: (0.04-3) x 10(9) M(-1) s(-1) for O2, (0.002-5) x 10(8) M(-1) s(-1) for TEMPO, and (<0.004-2) x 10(7) M(-1) s(-1) for MA. Generation of the decarboxylated aminoalkyl radical derived from N-phenylglycine was unambiguously demonstrated. It was clearly found that both the addition to oxygen and the recombination with TEMPO were strongly governed by the reaction exothermicity. On the other hand, both polar and enthalpy factors have a large influence on the rate constants of the addition reaction to the acrylate unit, which were ranging over at least 4 orders of magnitude. This paper provides a set of new data to characterize the structure/reactivity relationships of aminoalkyl radicals.

Comparative reactivity of aminyl and aminoalkyl radicals

The reactions of aminyl and aminoalkyl radicals, derived from primary or secondary amines toward two reactants – a double bond (methylacrylate MA) and a phenol (hydroquinone methylether HQME) – were investigated for the first time through Laser Flash Photolysis (LFP). The aminoalkyl radicals were directly observed and studied by LFP; their structure was confirmed by ESR spin trapping experiments. The reactivity of the aminyl radical was also monitored by LFP using an indirect method. Compared to aminyl radicals, the addition of aminoalkyl radicals to MA is very efficient (rate constant: 0.7–1.7 × 10 7 M −1s −1 vs. <10 5 M −1s −1) whereas the interaction with HQME is rather low (<5 × 10 5 M −1s −1 vs. 1.1 × 10 8 M −1s −1). Quantum mechanical calculations allow a discussion of the aminyl vs. aminoalkyl radical reactivity.

Nuclear‐Spin Polarization in Electron‐Transfer Reactions of Amines

AbstractChemically induced dynamic nuclear polarization (CIDNP) observed during electron transfer (ET) reactions of tertiary amines such as DABCO (1) or Et3N (2) with a wide range of electron acceptors support the involvement of amine radical‐cations (e.g., 1.+ or 2.+) as key intermediates. Radical ions such as 2.+ may be deprotonated, generating neutral aminoalkyl radicals (e.g., 2.). When generated by reaction with an electron acceptor of energetically low triplet state such as naphthalene (1Naph*), the resulting pair 2.+/Naph.− reacts mostly by reverse electron transfer (RET) from triplet pairs populating the naphthalene triplet state.

Search for High Reactivity and Low Selectivity of Radicals toward Double Bonds: The Case of a Tetrazole-Derived Thiyl Radical

The search for new radical structures having both low selectivity and high reactivity toward the addition reaction onto alkenes can be of interest in organic synthesis or polymer chemistry and has led us to propose a new tetrazole-derived thiyl radical. The reactivity of this sulfur-centered structure is compared to that of an aminoalkyl radical also efficient for alkene addition Worthwhile results are obtained; the new structure is more reactive on the complete range of alkenes with addition rate constants higher than 107 M(-1) s(-1) for both electron-deficient (acrylonitrile, ...) or electron-rich (vinylether, ...) double bonds. Quantum mechanical calculations have allowed a better understanding of this unique feature.

Photoinitiated Free Radical Polymerization Using Benzoxazines as Hydrogen Donors

AbstractSummary: Free radical polymerization of methyl methacrylate (MMA) is initiated upon irradiation at λ &gt; 350 nm in CH2Cl2 that contains benzoxazine (P‐a) and one of the following photosensitizers: benzophenone (BP), thioxanthone (TX), 2‐chlorothioxanthone (CTX), 2‐isopropyl thioxanthone (ITX), and camphorquinone (CQ). The postulated mechanism is based on the intermolecular reaction of the excited photosensitizer with the tertiary amino moiety of the ground state P‐a and a subsequent hydrogen abstraction reaction. The resulting aminoalkyl radicals initiate the polymerization. The incorporation of P‐a groups into polymers is demonstrated by spectroscopic methods. The possibility of deep curing using the described photoinitiating system followed by the thermal ring opening of the incorporated P‐a groups is also demonstrated.Schematic of the photoinitiated free radical polymerization using benzoxazines as hydrogen donors.magnified imageSchematic of the photoinitiated free radical polymerization using benzoxazines as hydrogen donors.

Direct measurements of the addition and recombination of acrylate radicals: Access to propagation and termination rate constants?

AbstractAcrylate radicals produced by the addition of an aminoalkyl radical to five acrylate monomers were directly observed by transient absorption spectroscopy, which allowed us to easily follow their chemical reactivity. It was possible (1) to characterize their absorption in the visible part of the spectrum, (2) to calculate their absorption properties, (3) to determine the energy barriers of the addition through quantum mechanical calculations, (4) to monitor the kinetics of the subsequent addition to another monomer unit, and (5) to follow the recombination of two acrylate radicals. These two latter points could mimic the propagation and termination reactions of polymerization‐propagating acrylate radicals. Methacrylate and acrylonitrile radicals were also studied. The obtained results were in good agreement with the propagation rate constants determined by the well‐established pulsed laser polymerization techniques. Our method could likely provide rapid access to both the propagation and termination rate constants in suitable systems and appears to be powerful and promising for studying and comparing the reactivities of different acrylate monomer structures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3577–3587, 2006

Degradation of alkyl radicals via C-H bond dissociation: Kinetic parameters and transition state geometry

Experimental data on the decomposition of alkyl radicals in the gas phase of the type RCH2C.H2 → RCH=CH2 + H. were analyzed in terms of the method of crossing parabolas. The parameters characterizing such decomposition were calculated. The activation energy Ee0 for the thermoneutral degradation reaction was shown to depend on the radical structure. The degradation of the radicals RCH2C.H2, CH2=CRC.H2, PhC.HCH2R, and CH2=C.CH2R is characterized by Ee0 = 81.6, 85.9, 85.9, and 115.2 kJ mol−1, respectively. The semiempirical algorithm for calculation of geometric parameters of the transition state for reactions of hydrogen atom addition to olefins was extended to the degradation reactions of alkyl, aminoalkyl, and ketyl radicals. The kinetic (activation energies) and geometric (interatomic distances in the transition state) reaction parameters were calculated for the decomposition reactions of various alkyl, alkylaromatic, aminoalkyl, and ketyl radicals.

Acrylate radicals: Direct observation and reactivity

A new procedure for the direct study of the chemical reactivity of acrylate radicals is developed: the acrylate radicals are produced by the addition of an aminoalkyl radical to an acrylate double bond and directly observed by transient absorption spectroscopy. These acrylate radicals are characterized by a strong absorption in the visible part of the spectrum allowing to easily probe their chemical reactivity. This provides a new access to their interactions with different additives. Moreover, the subsequent addition to another acrylate double bond as well as the recombination of two acrylate radicals are also investigated.

Thermal decomposition of O-benzyl ketoximes; role of reverse radical disproportionation.

Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, R(1)R(2)C[double bond, length as m-dash]NOCH(2)Ph, have been examined in three hydrogen donor solvents: tetralin, 9,10-dihydrophenanthrene, and 9,10-dihydroanthracene. All the oxime ethers gave the products expected from homolytic scission of both the O-C bond (viz., R(1)R(2)C[double bond, length as m-dash]NOH and PhCH(3)) and N-O bond (viz., R(1)R(2)C[double bond, length as m-dash]NH and PhCH(2)OH). The yields of these products depended on which solvent was used and the rates of decomposition of the O-benzyl oxime ethers were greater in 9,10-dihydrophenanthrene and 9,10-dihydroanthracene than in tetralin. These results indicated that a reverse radical disproportionation reaction in which a hydrogen atom was transferred from the solvent to the oxime ether, followed by [small beta]-scission of the resultant aminoalkyl radical, must be important in the latter two solvents. Benzaldehyde was found to be an additional product from thermolyses conducted in tetralin. This, and other evidence, indicated that another induced decomposition mode involving abstraction of a benzylic hydrogen atom, followed by [small beta]-scission of the resulting benzyl radical, became important for some substrates. Participation by minor amounts of enamine tautomers of the oxime ethers was shown to be negligible by comparison of thermolysis data for the O-benzyloxime of bicyclo[3.3.1]nonan-9-one, which cannot give an enamine tautomer, with that of the O-benzyloxime of cyclohexanone.