Through-space interactions of double bonds by photoelectron spectroscopy

Lambert, Joseph B.; Xue, Liang; Bosch, Richard J.; Taba, Kalulu M.; Marko, Dale E.; Urano, Shigeyuki; Lebreton, Pierre

doi:10.1021/ja00284a022

Cited by 14 publications

(2 citation statements)

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“…The charge is less shared as the reaction proceeds, and at the product region 90% of the charge is localized only over cyclohexene. This is in agreement with the lower ionization energy of cyclohexene as compared to 6-azauracil measured experimentally by photoelectron spectroscopy [ 27 , 28 ].…”

Section: Resultssupporting

confidence: 91%

Theoretical Study on the Photo-Oxidation and Photoreduction of an Azetidine Derivative as a Model of DNA Repair

et al. 2021

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Photocycloreversion plays a central role in the study of the repair of DNA lesions, reverting them into the original pyrimidine nucleobases. Particularly, among the proposed mechanisms for the repair of DNA (6-4) photoproducts by photolyases, it has been suggested that it takes place through an intermediate characterized by a four-membered heterocyclic oxetane or azetidine ring, whose opening requires the reduction of the fused nucleobases. The specific role of this electron transfer step and its impact on the ring opening energetics remain to be understood. These processes are studied herein by means of quantum-chemical calculations on the two azetidine stereoisomers obtained from photocycloaddition between 6-azauracil and cyclohexene. First, we analyze the efficiency of the electron-transfer processes by computing the redox properties of the azetidine isomers as well as those of a series of aromatic photosensitizers acting as photoreductants and photo-oxidants. We find certain stereodifferentiation favoring oxidation of the cis-isomer, in agreement with previous experimental data. Second, we determine the reaction profiles of the ring-opening mechanism of the cationic, neutral, and anionic systems and assess their feasibility based on their energy barrier heights and the stability of the reactants and products. Results show that oxidation largely decreases the ring-opening energy barrier for both stereoisomers, even though the process is forecast as too slow to be competitive. Conversely, one-electron reduction dramatically facilitates the ring opening of the azetidine heterocycle. Considering the overall quantum-chemistry findings, N,N-dimethylaniline is proposed as an efficient photosensitizer to trigger the photoinduced cycloreversion of the DNA lesion model.

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Section: Resultssupporting

confidence: 91%

Theoretical Study on the Photo-Oxidation and Photoreduction of an Azetidine Derivative as a Model of DNA Repair

et al. 2021

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“…We have noticed an unusual reaction of 1-methoxy-1,3-cyclohexadiene, 1 , in methanol with catalytic p -toluenesulfonic acid . Isolation of the major product by gas chromatography and identification by 1- and 2-dimensional NMR spectroscopy and comparison with literature demonstrated that the major product was the dimethyl ketal of cyclohexa-3-enone, 5 . The direct formation of this product involves protonation to form the less stable cation 3 , followed by capture of the cation by the methanol solvent, Scheme .…”

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confidence: 97%

Unexpected Anti-Markovnikov Addition to 1-Methoxy-1,3-cyclohexadiene

et al. 1999

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The concepts of carbocation stability and resonance stabilization are central tenets in organic chemistry, and much regiochemistry can be explained in terms of formation of the more stable carbocationic intermediate. Thus Markovnikov addition and the role of carbon substitution on S N 1 and E1 chemistry are promptly taught to first year organic chemistry students.We have noticed an unusual reaction of 1-methoxy-1,3-cyclohexadiene, 1, in methanol with catalytic p-toluenesulfonic acid. 1 Isolation of the major product by gas chromatography and identification by 1-and 2-dimensional NMR spectroscopy and comparison with literature demonstrated that the major product was the dimethyl ketal of cyclohexa-3-enone, 5. 2 The direct formation of this product involves protonation to form the less stable cation 3, followed by capture of the cation by the methanol solvent, Scheme 1. The cationic precursor to 5 is stabilized only through resonance with the ether oxygen, while protonation at carbon 4 of 1 would have generated a carbocation 2 which was stabilized by both the ether oxygen and the adjacent double bond. On the basis of simple resonance theory, the observed product appears to be formed from the less stable cation. We have explored this reaction in further detail to determine the cause of this unexpected regiochemistry.We have investigated the relative stability of the diconjugated cation intermediate 2 and the mono-conjugated intermediate cation 3 by ab initio theory using the Gaussian 98 program. 3 Even at the MP2//6-31G(d) and B3LYP/ 6-31G(d)// B3LYP/6-31G(d) levels of theory, the di-conjugated cation is more stable than the mono-conjugated cation by approximately 14 kcal/mol, Table 1. This energy difference should completely eliminate the formation of product 5 by a direct mechanism.It is conceivable, however, that the reaction proceeds through the more stable cation 2 and then undergoes a rapid rearrangement under the acidic conditions to form the observed product. We have therefore calculated the relative energies of the two ketals 4 and 5, Table 1. Again, several levels of theory predict that the observed ketal is indeed the more stable isomer. This difference, however, is slight: approximately 0.6 kcal/mol. Assuming complete equilibration, one would expect a product ratio of approximately 70:30; however, none of the conjugated ketal 4 is observed. Were the actual energy difference 1.7 kcal/mol (a typical uncertainty for such calculations), the expected product ratio would be 95:5. This obviously does not rule out an equilibrium process.Kinetics and labeling studies are often valuable tools for differentiating between alternative reaction mechanisms. Thus we have followed this reaction by 500 MHz NMR in methanol-d 4 . These experiments demonstrated that any rearrangement from 4 to 5 must occur very rapidly, as no new products or intermediates were observed during the reaction (Scheme 1). More importantly, the position of deuterium incorporation is a strong indication of the pathway for formation of product 5 (Sc...

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The predominance of axial conformers fortrans-4-substituted cyclohexene oxides

Samoshin

Yartseva

Svyatkin

et al. 1996

J. Phys. Org. Chem.

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A 'H N M R conformational study of cis-and trans-4-substituted cyclohexene oxides revealed an increased predominance, as compared with the parent 4-substituted cyclohexenes, of the equatorial conformer for cisisomers and a preference of the axial conformer for trans-isomers. These conformational shifts can be rationalized in terms of intramolecular dipole-dipole and/or steric interactions. However, molecular mechanics calculations failed to reproduce the relative stability of the axial conformer in trans-4-substituted cyclohexene oxides.

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Through-space interactions of double bonds by photoelectron spectroscopy

Cited by 14 publications

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Theoretical Study on the Photo-Oxidation and Photoreduction of an Azetidine Derivative as a Model of DNA Repair

Theoretical Study on the Photo-Oxidation and Photoreduction of an Azetidine Derivative as a Model of DNA Repair

Unexpected Anti-Markovnikov Addition to 1-Methoxy-1,3-cyclohexadiene

The predominance of axial conformers fortrans-4-substituted cyclohexene oxides

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