53273-14-6Relevant academic research and scientific papers
The importance of conformational effects on the carbon-carbon bond cleavage of β-phenethyl ether radical cations
Perrott,De Lijser,Arnold
, p. 384 - 397 (2007/10/03)
The photosensitized (electron transfer) bond cleavage of some β-phenylethyl ether radical cations has been investigated. In previous studies the feasibility of the bond cleavage was thought to depend on the bond dissociation energy (BDE). However, this simple hypothesis led to several incorrect predictions and therefore additional criteria, conformational effects, were added to the hypothesis. This study has now been extended and additional examples of the importance of the conformation on the carbon-carbon bond cleavage of radical cations are provided. The four β-phenylethyl ethers studied are 2-methoxy-3-phenylbutane (9, both diastereomers), and cis- and trans-2-methyl-3-phenyltetrahydropyran (10c, 10t). Generally, bond cleavage will occur if the (calculated) BDE in the radical cation is less than 55 kJ/mol, and if there is significant overlap between the singly occupied molecular orbital (SOMO) and the vulnerable (C - C or C - H) bond. In the case of a β-phenylethyl ether radical cation, the alkoxy group must also be oriented so that an oxygen lone pair of electrons can overlap with the C - C sigma antibonding (σ*) orbital. The calculated BDE values of the vulnerable C - C bond in the radical cations of the four ethers studied here are well below the threshold value, 55 kJ/mol, and C - C cleavage will therefore be governed by conformational effects. Molecular mechanics (MM3) calculations were used to identify the most stable conformers of the neutral molecules. Based on the calculated angles and overlap between orbitals it was predicted that the global-minimum conformers of the ethers 9 and 10c would not give C - C bond cleavage products or deprotonation to any significant extent. The global minimum of ether 10t is well oriented for C - C cleavage but not for deprotonation. Irradiation of an electron-accepting photosensitizer, 1,4-dicyanobenzene (2), in the presence of the ethers showed that the ethers 9 did not cleave efficiently; no deprotonation or isomerization was observed. This is in good agreement with the predictions based on the MM3 calculations. Both ethers 10c and 10t gave reasonable yields of the C - C cleavage products; in fact, ether 10c cleaved more efficiently than 10t. This can be explained by the fact that a conformer of 10c, only 4.35 kJ/mol higher in energy than the global minimum, is perfectly aligned for cleavage. Ether 10t did not show any evidence for deprotonation whereas 10c did. This is also in good agreement with the calculations.
Gas-Phase Acid-Induced Nucleophilic Displacement Reactions. 7. Structural and Stereochemical Evidence for the Existence and the Relative Stability of Alkylenebenzenium Ions in the Gas Phase
Fornarini, Simonetta,Sparapani, Cinzia,Speranza, Maurizio
, p. 34 - 41 (2007/10/02)
A comprehensive investigation on the existence and relative stability of gaseous 2,3-butylene- and 1,2-propylenebenzenium ions was carried out by establishing the structural features and the stereochemistry of acid-induced displacement by CH3OH on isomeric 3-phenylbutyl-2 onium and β-phenylpropyl onium intermediates.The latter were obtained in the gas phase from the reaction of radiolytically formed GA+ (GA+ = D3+, CnH5+ (n=1,2), i-C3H7+, and CH3FCH3+) acids with isomeric 3-phenyl-2-chlorobutanes and β-phenyl-Y-propanes (Y = Cl, OH).The analysis of the isomeric distribution of the neutral substitution products allows the establishment of extensive phenyl-group participation in the displacement process, occuring in competition with methyl and hydrogen 1,2-transfers.The participating ability of a phenyl moiety adjacent to the substitution center is found to depend essentially upon the configuration of the precursor and to be related to its gas-phase nucleophilicity.The occurence of relatively stable cyclic alkylenebenzenium ions as static intermediates in these displacement reactions is suggested by the particular isomeric and stereoisomeric distribution of the products and by its comparison with that obtained from open-chain isomeric ions.The results obtained from the present gas-phase experiments are discussed in the light of those from related gas-phase and solution studies.
