Direct observation of carbon-carbon bond fragmentation in .alpha.-amino alcohol radical cations

Lucia, Lucian A.; Burton, Richard D. E.; Schanze, Kirk S.

doi:10.1021/j100138a002

Cited by 23 publications

(25 citation statements)

References 14 publications

(11 reference statements)

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“…The spectra produced by photolysis of e -1a and e -1b exhibit absorption bands with maxima at 365 and 425 nm, respectively. These absorption bands are attributed to the α-amino radical chromophore, RC 6 H 4 NHĊHC 6 H 5 , 13b,, which is present in metal complex radicals 10a and 10b . The absorption maximum of the RC 6 H 4 NHĊHC 6 H 5 chromophore is red-shifted substantially in 10b compared to 10a because of the effect of conjugation between the pyridine and aniline rings.…”

Section: Discussionmentioning

confidence: 99%

“…This energetic barrier may be due to free energy costs associated with the stereoelectronic demands of the transition state 36,37 and solvent repolarization that occurs along the reaction coordinate. Indeed, the fragmentation kinetics of diastereomeric aminoalcohol cation radicals (eq 1, X = OH) imply that in the transition state for fragmentation there is a stereoelectronic requirement for maximization of orbital overlap between the p orbital on the heteroatom of the electrofugal group (X in eq 1) and the singly occupied p orbital on N. , Overlap is maximized if the two heteroatoms are oriented either syn or anti with respect to rotation about the 1,2-CC bond. It is plausible that the intrinsic barrier for fragmentation of the 1,2-diamine cation radical is due to an unfavorable enthalpy or probability factor (activation entropy) associated with achievement of the proper orbital alignment in the transition state. , …”

Section: Discussionmentioning

confidence: 99%

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Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

and

Schanze

1996

J. Phys. Chem.

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Two transition metal complexes of the type fac-(bpy)ReI(CO)3(DA)+ (where bpy = 2,2‘-bipyridine and DA is a pyridine ligand that is substituted with a 1,2-diamine electron donor) have been prepared. The 1,2-diamine serves as a “reactive donor ligand” owing to its propensity to undergo rapid C−C bond fragmentation when activated by single electron transfer oxidation. Photoexcitation of the diamine complexes affords a ligand-to-ligand charge transfer (LLCT) state via intramolecular electron transfer quenching of a metal-to-ligand charge transfer (MLCT) state, [(bpy)ReI(CO)3(DA)]+ + hν → [(bpy•-)ReII(CO)3(DA)]+*(MLCT) → [(bpy•-)ReI(CO)3(DA•+)]+*(LLCT). Photochemical product and quantum efficiency studies indicate that the diamine reactive donor ligand undergoes photoinduced C−C bond fragmentation with high efficiency, presumably via the radical cation (DA•+) which is present in the LLCT excited state. Laser flash photolysis allows direct detection of the metal complex based radicals that are formed by C−C bond fragmentation. Quantitative kinetic information gathered through luminescence, laser flash photolysis, and quantum yield studies allows estimation of the rates for formation of the LLCT state by forward electron transfer (k FET), decay of the LLCT state by back electron transfer (k BET), and the rate of diamine radical cation bond fragmentation in the LLCT state (k BF). The relationship between these kinetic parameters and the driving force for electron transfer and bond fragmentation as well as the structure of the reactive donor ligands is discussed.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

and

Schanze

1996

J. Phys. Chem.

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“…Some of the first photofragmentation reactions studied in our laboratory and others involved amino alcohols as donors. [46][47][48][49][50][51][52] The initially formed radical cations selectively fragment at the C-C bond between the amine and alcohol functions to yield a ketone and an R-amino radical (Figure 3). The close relationship between these reactions and the Grob fragmentation [53][54][55] of even-electron species has been noted earlier.…”

Section: 2-disubstituted Ethanesmentioning

confidence: 99%

“…We have observed that the lifetime of the radical cation decreases linearly with increasing concentration of an added base such as pyridine. Schanze et al 48 have suggested that deprotonation of the amino alcohol radical cation is the rate determining step in these reactions.…”

Section: 2-disubstituted Ethanesmentioning

confidence: 99%

Photoinduced Electron Transfer Bond Fragmentations

1996

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“…Another possibility is that a limiting factor in the fragmentation of 2 •+ following its formation by SET quenching is base-assisted deprotonation of the ion-radical. We and others have previously shown that the oxidative fragmentation of amino alcohols can be accelerated substantially by assistance of either the acceptor ion-radical or other bases added to the medium. 5a, In fact the relatively rapid reaction in the films of pure 2 in the presence of Ru(bpy) 3 2+ might be attributed in part to an enhanced activity of OH - at the film−water interface due to the concentration of the ruthenium cations. Examination of the compression isotherms of unirradiated and irradiated mixed films of 2 with AA indicates that selective removal of 2 occurs upon irradiation; however, unlike the mixed films of 1 / 2 /AA examined in the intralayer studies, addition of AA to the interfacial system 2 /Ru(bpy) 3 2+ does not result in elimination of reaction.…”

Section: Resultsmentioning

confidence: 99%

Electron Transfer Photofragmentation Reactions in Monolayer Films at the Air/Water Interface

et al. 1998

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A series of photoinduced electron-transfer fragmentation reactions have been studied in compressed monolayer films at the air/water interface. The reactions investigated involve amphiphilic and polymeric derivatives of fragmentable amino alcohol, 1,2-diamine, and pinacol donors and light-absorbing acceptors, which are reactive in solution-phase studies from their triplet states. For intralayer studies a surfactant anthraquinone derivative was the light-absorbing acceptor. For comparable "interfacial" studies, the water soluble cation tris(2,2′-bipyridine)ruthenium(II) 2+ (Ru(bpy)3 2+ ) was the photoactive acceptor from the subphase. The fragmentation reactions all involve oxidative cleavage of a relatively strong C-C bond in the donor. Reaction was followed in each case by monitoring changes in surface pressure that occur when the compressed film is irradiated and maintained at a constant area. Reaction was readily observed in most cases where the donor and light-absorbing substrate are present; however the consequences were found to be quite dependent upon the specific donor substrate. Thus for simple single-chain amphiphiles containing either amino alcohol or 1,2-diamine donor sites, both intralayer and interfacial reactions result in rapid decrease in surface pressure, consistent with destruction of the film as the more hydrophilic redox products are solubilized into the subphase. For a polymeric diamine, much more complex behavior is observed, consistent with a situation where single fragmentation events do not lead to removal of material from the film but multiple fragmentation reactions culminate in film solubilization. Finally, a doublechain amphiphilic pinacol was found to undergo interfacial fragmentation with Ru(bpy)3 2+ in the subphase with a concurrent increase in surface pressure to form stable films that do not "dissolve" into the subphase. The isotherms observed following irradiation, decompression, and recompression are consistent with an expansion that occurs as the two-chain amphiphile undergoes redox fragmentation to produce two equivalents of a single-chain amphiphile.

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Direct observation of carbon-carbon bond fragmentation in .alpha.-amino alcohol radical cations

Cited by 23 publications

References 14 publications

Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

Photoinduced Electron Transfer Bond Fragmentations

Electron Transfer Photofragmentation Reactions in Monolayer Films at the Air/Water Interface

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