scholarly journals The photochemistry of 1-naphthylmethyl carbonates and carbamates

1994 ◽  
Vol 72 (5) ◽  
pp. 1254-1261 ◽  
Author(s):  
T. Parman ◽  
J.A. Pincock ◽  
P.J. Wedge
Keyword(s):  
Electron Transfer ◽  
Singlet State ◽  
Radical Pair ◽  
Ion Pair ◽  
Excited Singlet State ◽  
Excited Singlet ◽  
Homolytic Cleavage ◽  
Product Yields ◽  
Oxygen Bond

The photochemistry in methanol of 1-naphthylmethyl phenyl carbonate (3) and 1-naphthylmethyl benzyl carbonate (4) has been studied. Products resulting from both the 1-naphthylmethyl cation and the 1-naphthylmethyl radical are obtained for 3, but only from the cation for 4. Similar results were obtained for the corresponding 1-naphthylmethyl derivatives 5 and 6 of N-phenyl and N-benzyl carbamic acids. The product yields for all four compounds can be explained by a mechanism of initial homolytic cleavage of the 1-naphthylmethyl carbon–oxygen bond from the excited singlet state. The radical pair generated then partitions between the two pathways: electron transfer to form the ion pair or decarboxylation. For PhO-CO-O• and PhNH-CO-O•, decarboxylation is rapid and competitive with electron transfer. For PhCH2O-CO-O• and PhCH2NH-CO-O•, decarboxylation is slower, electron transfer dominates, and only products from the ion pair are obtained.

Photolysis of the 1-naphthylmethyl ester of substituted phenylacetic acids: intramolecular charge transfer and rates of decarboxylation of arylacyloxy radicals

1992 ◽  
Vol 70 (3) ◽  
pp. 992-999 ◽  
Author(s):  
James W. Hilborn ◽  
James A. Pincock
Keyword(s):  
Electron Transfer ◽  
Singlet State ◽  
Intramolecular Charge Transfer ◽  
Radical Pair ◽  
Product Distribution ◽  
Intramolecular Electron Transfer ◽  
Naphthalene Ring ◽  
Excited Singlet ◽  
Homolytic Cleavage ◽  
Oxygen Bond

The photolysis of esters 6 and 8 in methanol leads to products resulting from both naphthylmethyl cations and radicals. The product distribution is nearly independent of X for the esters 6 except when X equals methoxy. A mechanism involving initial homolytic cleavage of the carbon–oxygen bond in the excited singlet state of the ester is proposed. Competition between electron transfer in the radical pair to form the ion pair and decarboxylation of the arylacyloxy radical allows calculations of the rates for this decarboxylation process. The ρ value versus σ is close to zero. When X equals methoxy, intramolecular electron transfer occurs with the naphthalene ring serving as the acceptor and the methoxyaromatic as the donor. This exciplex fragments to carbon dioxide and 1-(1-naphthyl)-2-arylethane. Keywords: acyloxy radical, decarboxylation, photolysis of benzylic esters.

The design of radical clocks to probe the reactivity of the intermediates in arylmethyl ester photochemistry

1997 ◽  
Vol 75 (2) ◽  
pp. 232-247 ◽  
Author(s):  
S.M. Nevill ◽  
J.A. Pincock
Keyword(s):  
Singlet State ◽  
Radical Pair ◽  
Ion Pair ◽  
Excited Singlet ◽  
Homolytic Cleavage ◽  
Radical Pairs ◽  
Photochemical Rearrangement ◽  
Radical Clocks ◽  
Radical Clock ◽  
Oxygen Bond

The photochemistry in methanol of the esters 1–6was examined. These reactions normally proceed through radical pairs that result from homolytic cleavage of the carbon–oxygen bond in the excited singlet state. Each of the esters was designed to probe the intervention and reactivity of the substituted arylmethyl radical by incorporating a potential radical clock at the carbon of the reactive bond. For esters 1–5, the products isolated indicated that the radical clock was not reactive enough to compete with the very rapid alternate processes of the radical pair, namely, electron transfer to form the corresponding ion pair and decarboxylation of the phenylacyloxy radical (k = 4.6 × 109 s−1). Ester 6, which incorporates the extremely rapid fluorenylcyclopropylcarbinyl clock, showed very unusual reactivity. On thermal solvolysis in methanol, 6 rearranged quantitatively to the ester 20. No methyl ethers were detected. In contrast, photolysis of 6 in benzene resulted in an alternate rearrangement to the cyclobutyl ester, 22, resulting from the aryl version of the cyclopropyl-π-methane photochemical rearrangement. No ester cleavage occurred on excitation. A rationale for the latter conversion was based on stereoelectronic arguments provided by a crystal structure of 6. Keywords: photochemistry of arylmethyl esters, radical clocks.

Intramolecular electron transfer in the photochemistry of substituted 1-naphthylmethyl esters of benzoic acids

1992 ◽  
Vol 70 (7) ◽  
pp. 1879-1885 ◽  
Author(s):  
D. P. DeCosta ◽  
J. A. Pincock
Keyword(s):  
Electron Transfer ◽  
Singlet State ◽  
Bond Cleavage ◽  
Ion Pair ◽  
Excited Singlet State ◽  
Intramolecular Electron Transfer ◽  
Excited Singlet ◽  
Transfer Rates ◽  
Exciplex Formation ◽  
Direct Excitation

Direct excitation of the esters 5 in methanol solvent leads to rapid intramolecular exciplex formation (kex = 1010 s−1 for X = CH3O, Y = CN) with electron transfer from the naphthalene to the benzoate ring. This process dominates the usual fluorescence and reaction of the excited singlet state. The rate of this process can be varied over 103 by suitable change in the substituents X and Y. The electron-transfer rates can be correlated with the two-parameter Hammett equation: log kex = 8.48 − 1.5σ+ + 0.77σ. For cases where the rate of exciplex formation is slow, the usual homolytic carbon–oxygen bond cleavage occurs from the excited singlet state. The eventual products result from the ion pair since the rate of electron transfer in the radical pair to form the ion pair is considerably faster than the rate of decarboxylation of the benzoyloxy radical.

Photochemistry of Nitracrine and its Analogues: Acridine Derivatives as Electron Donors

1984 ◽  
Vol 39 (2) ◽  
pp. 195-200 ◽  
Author(s):  
J. Biedrzycki ◽  
J. Błażejowski ◽  
J. Szychliński ◽  
A. Ledòchowski
Keyword(s):  
Electron Transfer ◽  
Excited State ◽  
Fluorescence Quenching ◽  
Triplet State ◽  
Uv Radiation ◽  
Electron Acceptor ◽  
Singlet State ◽  
Electron Donors ◽  
Excited Singlet State ◽  
Excited Singlet

Studies on the electron transfer in the excited state from electron donors: acridine, 9- aminoacridine and its derivatives, including nitracrine and its isomers, to CCl4 as a suitable electron acceptor, were undertaken. UV radiation (λ= 366 nm), absorbed only by acridine bases, generates CI- ions. The fluorescence quenching measurements indicate that no interaction appears between molecules of acridine bases in the excited singlet state and CCl4 molecule. A scheme of photochemical processes is proposed which assumes that the triplet state of the base molecule is the reactive one.

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