Electron‐transfer Reactions of Amines

Das, Suresh; Suresh, V.

doi:10.1002/9783527618248.ch21

Cited by 15 publications

(13 citation statements)

References 291 publications

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“…The transient absorption spectra obtained in these experiments are seen to be very similar to the CH • radicals obtained in 2PNL solution and there is no absorption band in the 600−800 nm region corresponding to the C • - form of the radicals. These results are along with our expectations as the TEA • + radical cations formed by a reaction similar to 19 are strong proton donors. − Thus the C • - radicals initially formed by reaction 20 in ACN solution will quickly undergo a fast proton-transfer reaction with TEA • + (reaction 22) resulting in the formation of the CH • radicals in the solution. Typical transient absorption spectra obtained for C151 and C500 in ACN solution in the presence of 1 mol dm -3 of TEA are shown as spectra 4 in Figure after normalization at the 500−530 nm peaks for a comparison with the spectra obtained in ACN in the presence of AN.…”

Section: Resultssupporting

confidence: 80%

Reduced Radical Characteristics of 7-Aminocoumarin Dyes Studied by Pulse Radiolysis Technique

Nad

Pal

2002

J. Phys. Chem. A

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Characteristics of the reduced radicals of a series of 7-aminocoumarin dyes (C) have been investigated in different solvents using pulse radiolysis (PR) technique. Since these coumarin dyes are almost insoluble in water, a mixed solvent (MS) system containing 5.0 mol dm-3 of 2-propanol (2PNL) and 1.0 mol dm-3 of acetone (ACT) in water has been used in the present work to substitute the aqueous solvent. In MS, for a wide pH range of ∼1−9, the reduced coumarin radicals are formed in the neutral form, CH•. These radicals show two absorption bands, one in the 300−400 nm region and the other in the 500−600 nm region. As the coumarin dyes undergo hydrolysis in strongly alkaline solutions, PR studies in MS could not be extended beyond pH ∼9. In 2PNL, the reduced coumarin radicals are also formed in the CH· form, even in the presence of ∼1 mol dm-3 of a strong proton acceptor like triethylamine (TEA). Since the pK b value of TEA is 3.19, it is indicated that the acid dissociation constant of the CH· radicals (pK a R) must be >11. The PR results in MS and 2PNL clearly indicate that the anionic form of the reduced coumarin radicals (C•-) formed by initial one-electron reduction of the dyes undergo very fast protonation to give the CH• form in the solution. To avoid protonation of C•-, PR experiments were carried out in acetonitrile (ACN) solutions in the presence of ∼1 mol dm-3 of aniline (AN). With this high concentration, AN efficiently scavenges the cation radicals from the solvent ion pairs (ACN+···e-) formed following the electron pulse, and thus the equivalent amount of electrons become available to reduce C to C•-. It is seen that the C•- radicals of all the coumarin dyes studied have unusually weak absorption bands in the 500−800 nm region. Since the coumarin dyes are good electron acceptors, present results on CH• and C•- radicals will be useful in elucidating the ET mechanism using coumarin dyes as the electron acceptors.

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Section: Resultssupporting

confidence: 80%

Reduced Radical Characteristics of 7-Aminocoumarin Dyes Studied by Pulse Radiolysis Technique

Nad

Pal

2002

J. Phys. Chem. A

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“…Yet, it is known that photoexcitation of electron donors as well as acceptors changes their redox properties, and they act as sensitizers (as illustrated in Supporting Information Figure S4). ,− Excitation of a sensitizer molecule transfers an electron from the highest occupied molecular orbital (HOMO) into the lowest unoccupied molecular orbital (LUMO). The electron transfer from the LUMO requires less energy than from the HOMO.…”

Section: Resultsmentioning

confidence: 99%

“…Therefore, a sensitizer in its excited state is a far better reducing agent than in its ground state, ,, explaining why amines can serve as reducing agents for Cu(II) toward Cu(I) and therefore accelerate the polymerization under irradiation with light. It is known that amines and their derivatives undergo photoinduced electron transfer reactions. ,,− This process results in the formation of aminium radicals (cation radicals of amines), that preferably form α-aminoalkyl radicals, via deprotonation at an α-carbon, which can subsequently react, for instance, with olefinic substrates. The preference for hydrogen abstraction follows the order of tertiary > secondary > primary > aromatic amines, provided no steric hindrance is involved. , The low preference of aromatic amines to abstract a hydrogen in α-position thus explains why all attempts to carry out a photoRDRP with aromatic ligands such as 2,2′-bipyridine remained unsuccessful .…”

Section: Resultsmentioning

confidence: 99%

Enlightening the Mechanism of Copper Mediated PhotoRDRP via High-Resolution Mass Spectrometry

et al. 2015

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The initiation mechanism of photochemically mediated Cu-based reversible-deactivation radical polymerization (photoRDRP) was investigated using pulsed-laser polymerization (PLP) and high-resolution mass spectrometry. The variation of the catalyst composition and ESI-MS analysis of the resulting products provided information on how initiator, ligand, copper species, and monomer are interacting upon irradiation with UV light. A discussion of the results allows for a new postulation of the mechanism of photoRDRP and-for the first time-the unambiguous identification of the initiating species and their interactions within the reaction mixture. One pathway for radical generation proceeds via UV light-induced C-Br bond scission of the initiator, giving rise to propagating radicals. The generation of copper(I) species from copper(II) can occur via several pathways, including, among others, via reduction by free amine ligand in its excited as well as from its ground state via the irradiation with UV light. The amine ligand serves as a strong reducing agent and is likely the main participant in the generation of copper(I) species.

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“…The important role played by the electron-transfer reactions of amines in a wide variety of synthetic, biological, and technological applications has been documented in several recent reviews. − In this contribution, we extend our EPR studies of primary alkylamine radical cations described in the previous paper to examine the radiolytic oxidation of propargylamine, allylamine, and closely related compounds. These amines possess highly reactive functional groups, and together with cyclopropylamine, they and their derivatives feature prominently as efficient inactivators of enzymes such as monoamine oxidase and cytochrome P-450. 3b-d Consistent with an enzymatic one-electron oxidation process, the aminium radical cations that are formed in this case are considered to undergo a “mechanism-based rearrangement” to form highly reactive xenobiotic species 3c.…”

Section: Introductionmentioning

confidence: 82%

EPR Studies of Amine Radical Cations. Part 2. Thermal and Photo-Induced Rearrangements of Propargylamine and Allylamine Radical Cations in Low-Temperature Freon Matrices

et al. 2006

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Matrix EPR studies and quantum chemical calculations have been used to characterize the consecutive H-atom shifts undergone by the nitrogen-centered parent radical cations of propargylamine (1b*+) and allylamine (5*+) on thermal or photoinduced activation. The radical cation rearrangements of these unsaturated parent amines occur initially by a 1,2 H-atom shift from C1 to C2 with pi-bond formation at the positively charged nitrogen; this is followed by a consecutive reaction involving a second H-atom shift from C2 to C3. Thus, exposure to red light (lambda > 650 nm) converts 1b*+ to the vinyl-type distonic radical cation 2*+ which in turn is transformed on further photolysis with blue-green light (lambda approximately 400-600 nm) to the allene-type heteroallylic radical cation 3*+. Calculations show that the energy ordering is 1b*+ > 2*+ > 3*+, so that the consecutive H-atom shifts are driven by the formation of more stable isomers. Similarly, the parent radical cation of allylamine 5*+ undergoes a spontaneous 1,2-hydrogen atom shift from C1 to C2 at 77 K with a t1/2 of approximately 1 h to yield the distonic alkyl-type iminopropyl radical cation 6*+; this thermal reaction is attributed largely to quantum tunneling, and the rate is enhanced on concomitant photobleaching with visible light. Subsequent exposure to UV light (lambda approximately 350-400 nm) converts 6*+ by a 2,3 H-shift to the 1-aminopropene radical cation 7*+, which is confirmed to be the lowest-energy isomer derived from the ionization of either allylamine or cyclopropylamine. Although the parent radical cations of N, N-dimethylallylamine (9*+) and N-methylallylamine (11*+) are both stabilized by the electron-donating character of the methyl group(s), the photobleaching of 9*+ leads to the remarkable formation of the cyclic 1-methylpyrrolidine radical cation 10*+. The first step of this transformation now involves the migration of a hydrogen atom to C2 of the allyl group from one of the methyl groups (rather than from C1); the reaction is then completed by the cyclization of the generated MeN + (=CH2) CH2CH2CH2* distonic radical cation, possibly in a concerted overall process. In contrast to the ubiquitous H-atom transfer from carbon to nitrogen that occurs in the parent radical cations of saturated amines, the alternate rearrangements of either 1b*+ or 5*+ to an ammonium-type radical cation by a hypothetical H-atom shift from C1 to the ionized NH2 group are not observed. This is in line with calculations showing that the thermal barrier for this transformation is much higher (approximately 120 kJ mol-1) than those for the conversion of 1b*+ --> 2*+ and 5*+--> 6*+ (approximately 40-60 kJ mol-1).

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Electron‐transfer Reactions of Amines

Cited by 15 publications

References 291 publications

Reduced Radical Characteristics of 7-Aminocoumarin Dyes Studied by Pulse Radiolysis Technique

Reduced Radical Characteristics of 7-Aminocoumarin Dyes Studied by Pulse Radiolysis Technique

Enlightening the Mechanism of Copper Mediated PhotoRDRP via High-Resolution Mass Spectrometry

EPR Studies of Amine Radical Cations. Part 2. Thermal and Photo-Induced Rearrangements of Propargylamine and Allylamine Radical Cations in Low-Temperature Freon Matrices

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